WO2019067900A1 - Titanium (iv)-based halide double-perovskites with tunable 1.0 to 1.8 ev bandgaps for photovoltaic applications - Google Patents

Titanium (iv)-based halide double-perovskites with tunable 1.0 to 1.8 ev bandgaps for photovoltaic applications Download PDF

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WO2019067900A1
WO2019067900A1 PCT/US2018/053426 US2018053426W WO2019067900A1 WO 2019067900 A1 WO2019067900 A1 WO 2019067900A1 US 2018053426 W US2018053426 W US 2018053426W WO 2019067900 A1 WO2019067900 A1 WO 2019067900A1
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based double
cs2tibr6
thin film
double perovskite
perovskite
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French (fr)
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Nitin P. Padture
Min Chen
Yuanyuan Zhou
Xiao CHENG ZENG
Ming-gang JU
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Brown University
The Board Of Regents Of The University Of Nebraska
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    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
    • C01G23/00—Compounds of titanium
    • C01G23/002—Compounds containing, besides titanium, two or more other elements, with the exception of oxygen or hydrogen
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2002/00—Crystal-structural characteristics
    • C01P2002/30—Three-dimensional structures
    • C01P2002/34—Three-dimensional structures perovskite-type (ABO3)
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K30/00—Organic devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation
    • H10K30/50—Photovoltaic [PV] devices
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/549—Organic PV cells

Definitions

  • Halide perovskites are a family of semiconductor materials that have shown great promise as light-absorbers in the emerging technology of perovskite solar cells (PSCs). See, Yang, W. S.; Park, B.-W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C; Lee, D. U.; Shin, S. S.; Seo, J.; Kim, E. K.; Noh, J. H.; et al. Iodide management in formamidinium-lead-halide-based perovskite layers for efficient solar cells. Science 2017, 356, 1376-1379.; Eperon, G. E. ; Leijtens, T.; Bush, K.
  • Perovskite solar cells go lead free.
  • Joule 2017, 1, 1-6 which are highly hygroscopic and volatile which makes them intrinsically unstable, with inadequate tolerance for environmental stresses such as heat and moisture.
  • Efforts have been made to address these issues. See, Li, Z.; Yang, M. ; Park, J.-S.; Wei, S.-H.; Berry, J. J.; Zhu, K.
  • Pb 2+ can be replaced by other cations, such as tin (Sn 2+ ) (see, Li, Z. ; Yang,
  • Giustino, F. Cs2lnAgCl6 A new lead-free halide double perovskite with direct band dap. /.
  • Air-stable molecular semiconducting iodosalts for solar cell applications Cs2Snl6 as a hole conductor. /. Am. Chem. Soc. 2014, 136, 15379-15385.
  • Cs2Snl6 is found to exhibit insufficient stability, and it contains intrinsically deep defects that are detrimental to the PSC performance. See, Saparov, B.; Sun, J. -P.; Meng, W.; Xiao, Z.; Duan, H.-S.; Gunawan, O.; Shin, D.; Hill, I. G. ; Yan, Y. ; Mitzi, D. B.
  • the present disclosure provides a transition metal-based double perovskite of the formula A2TX6 wherein A is one or more of K + , Rb + , Cs + , In + , MA + , or FA + ; wherein T is Ti, Zr, Hf, V, Nb, Mo, or W; and wherein X is selected from one or more of CI, Br, or I. According to one aspect, T is Ti.
  • transition metal-based double perovskites as thin films or powders are disclosed herein.
  • Methods of making photovoltaic devices or solar cells including the transition metal-based double perovskites described herein are provided.
  • Fig. 1A shows a schematic crystal structure of A2T1X6 HPs.
  • Fig. IB shows computed bandgaps of various HPs using HSE06 functional. The range of optimal bandgaps for solar- cell materials is highlighted by the horizontal band.
  • Fig. 1C shows computed optical absorption spectra (based on the HSE06 functional) of several predicted HPs, compared with absorption spectra of Si and MAPM3 HP. The absorption coefficient of MAPM3 is computed using the PBE functional without considering the SOC effect.
  • Fig. ID shows computed band structure of CS2T1I6 basing on primitive unit cell (Fig. 2A) using PBE functional; here ⁇ (0.0, 0.0, 0.0), W (0.5, 0.25, 0.75), L (0.5, 0.5, 0.5), X (0.5, 0.0, 0.5) and K (0.375, 0.375, 0.75) refer to the high-symmetry special points in the first Brillouin zone.
  • Fig IE shows computed DOS and projected DOS of CS2T1I6 using HSE06 functional.
  • Fig. 2A shows a primitive unit cell of CS2T1X6.
  • Fig. 2B shows a computed band structure of CS2T1I6 using PBE functional.
  • the four solid black circles labeled i, ii, iii and iv correspond to highest VB and lowest CB at the ⁇ and X, respectively.
  • Fig. 2C shows charge density distribution of the highest VB and lowest CB of a periodic slab model of CS2T1I6 HP at the ⁇ and X.
  • Fig. 3A shows computed bandgaps of a series of A2T1I6 HP, using the HSE06 functional with SOC. The optimal range of bandgaps is highlighted by two light-blue dotted lines.
  • Fig. 3B shows computed DOS and PDOS of Cs 2 TiI 6 using the HSE06 functional with SOC.
  • Fig. 4 shows computed AH corresponding to different decomposition pathways for CS2T1I6, Rb2Til6, K2T1I6, and In2Til6 HPs based on the PBE functional.
  • Figs. 5A-D depict AIMD for A 2 TiI 6 HP at 300K and 500 K.
  • AIMD ab initio molecular dynamic
  • Fig. 6A shows allowed values of Ti and I chemical potentials (gray shaded region), which define the thermodynamic stability of CS2T1I6.
  • the chemical potentials ⁇ , ⁇ , and uc s are limited by the formation of the secondary phases CsLt, Csh, Csl, T1I3, and T1I4.
  • Figs. 7A-E show details the fabrication of the Cs2TiBr6 HP thin film and PSCs, and photovoltaic performance.
  • Fig. 7A depicts a achematic illustration showing the formation of a uniform Cs2TiBr6 thin film via reaction of CsBr solid-precursor thin film with TiBr4 vapor precursor.
  • Fig. 7B depicts an XRD pattern of the as-prepared Cs2TiBr6 HP thin film. The inset is a SEM image showing the surface morphology of the Cs2TiBr6 HP thin film.
  • Fig. 7C depicts current-voltage (J-V) characteristics of the champion PSC based on Cs2TiBr6 HP thin film.
  • the inset is a cross-sectional SEM image of the PSC.
  • Fig. 7D depicts stabilized PCE output of the PSC at the maximum-power-point voltage extracted from Fig. 7C.
  • Fig. 7E depicts PCE statistics of PSCs made using Cs2TiBr6 HP thin films.
  • Fig. 8 shows a low magnification SEM image showing the surface morphology of the Cs 2 TiBr 6 HP thin film.
  • Fig. 9 shows a schematic illustration of the experimental setup for performing the reaction of as-deposited CsBr thin films with TiBr4 vapor for conversion to phase-pure Cs2TiBr6 thin films.
  • Fig. 10 shows the J-V hysteresis of the champion PSC.
  • the extracted PV parameters for reverse scans 2.36% PCE; 0.97 V Voc; 4.06 mA cm 2 Jsc; 0.600 FF.
  • the extracted PV parameters for forward scans 2.22% PCE; 0.96 V Voc; 3.94 mA cm 2 Jsc; 0.588 FF.
  • Fig. 11A depicts experimentally measured XRD patterns of CS2T1I6, Cs2TiLtBr2, Cs2Til2Br4, and Cs2TiBr6 HPs (insets: photographs of the as-synthesized materials).
  • Fig. 11B depicts corresponding higher resolution XRD patterns.
  • Fig. 11 C depicts absorption spectra of Cs 2 TiI 6 , Cs 2 TiI 4 Br 2 , Cs 2 TiI 2 Br 4 , and Cs 2 TiBr 6 HPs.
  • Fig. 12A shows the calculated XRD patterns of CS2T1I6 with Fm-3m space group.
  • Fig. 12B shows the calculated XRD patterns of Cs2TiBr6 with Fm-3m space group.
  • Fig. 13A shows a Tauc plot of Cs2TiBr6
  • Fig. 13B shows a Tauc plot of Cs2Til2Br4
  • Fig. 13C shows a Tauc plot of Cs2TiLtBr2
  • Fig. 13D shows a Tauc plot of Cs2Til6HPs, giving bandgaps of 1.78 eV eV, 1.38 eV, 1.15 eV, and 1.02 eV, respectively.
  • Fig. 14 shows a tauc plot of the MAPM3 HP giving a bandgap of -1.51 eV.
  • Fig. 16A is a schematic crystal structure of mixed-I/Br Cs2TiI x Br6 x HPs.
  • Fig. 16A is a schematic crystal structure of mixed-I/Br Cs2TiI x Br6 x HPs.
  • Fig. 16B depicts computed bandgaps of a series of Cs2TiI x Br6- x HPs, using the HSE06 functional
  • 16D depicts computed band structure of Cs2Til2Br4 HP with space group I4/mmm (139) using PBE functional; here ⁇ (0.0, 0.0, 0.0), Z (0.5, 0.5, -0.5), X (0.0, 0.0, 0.5), P (0.25, 0.25, 0.25), and N (0.0, 0.5, 0.0) refer to the high-symmetry special points in the first Brillouin zone.
  • Fig. 17A depicts primitive unit cell of Cs2Til2Br4 with different arrangement orders of I and Br sites.
  • Fig. 17B depicts computed band structure of Cs2Til2Br4 HP with space group Imm2 (44) using PBE functional; here T (0.5, -0.5, 0.0), W (0.75, -0.25, -0.25), R (0.5, 0.0, 0.0), ⁇ (0.0, 0.0, 0.0) and X (0.5, -0.5, 0.5), refer to the high-symmetry special points in the first Brillouin zone.
  • Figs. 18A-D show the crystal orbital Hamilton population (COHP) analysis of Cs2TiBr6 (Fig. 18A), Cs 2 TiI 6 (Fig. 18B), and Cs2TiI 2 Br 4 HPs (Figs. 18C-D) in which the density of states is partitioned for Ti-Br and Ti-I interactions, with the sign indicating bonding or antibonding character, and the magnitude related to the strength of the interaction.
  • the VBM is set to 0 eV.
  • Figs. 19A-D provide stability study results for the most representative composition Cs2TiBr6 and Cs2Til2Br4 HPs.
  • the samples were annealed at 473 K for 1 h.
  • For the moisture stability testing the samples were stored at 298 K for 4 h under 70% RH.
  • Figs. 20A-B show experimentally measured XRD patterns of MAPbbBr (Fig. 20A) and MAPbb (Fig. 20B) HP samples before and after environmental (thermal, moisture) stresses.
  • thermal stability testing the samples were annealed at 473 K for 1 hr.
  • moisture stability testing the samples were stored at 298 K for 4 h under 70% relative humidity (RH).
  • Both MAPbbBr and MAPbb samples are prepared dissolving of the precursor solids in the DMF solvent, followed by precipitation.
  • Fig 21A depicts a schematic illustration of the vapor-based synthesis of Cs2TiBr6 HP thin film.
  • Fig. 21B depicts XRD patterns and Fig. 21C depicts UV-vis spectra (inset: photograph of the final Cs2TiBr6 thin film).
  • Fig. 21D depicts the corresponding SEM surface morphologies of the thin films.
  • FIG. 21 E depicts Ti content as a function of depth in the intermediate thin film after 12-h annealing at 200 °C (Film-II).
  • Fig. 21F depicts the proposed mechanism of the formation of the Cs2TiBr6 HP thin film.
  • Figs. 22A-D depict surface morphology at top surface (Fig. 22A) and 40 nm from top surface of the intermediate Film-II (Fig. 22C).
  • Corresponding EDS Ti elemental maps of Ti: top surface (Fig. 22B) and 40 nm from film top surface (Fig. 22D). Scale bars 5 ⁇ .
  • Fig. 23 depicts Ti content as a function of depth in the Cs2TiBr6 HP thin film after 24- h annealing at 200 °C (Film-Ill).
  • Fig. 27 depicts an AFM scan of top-surface of a Cs2TiBr6 HP thin film on TiC -coated glass substrate.
  • Fig. 28A depicts a Tauc plot and PL spectrum of the Cs2TiBr6 HP thin film.
  • Fig. 28B depicts AFM film topography with overlaid PL intensity map of the Cs2TiBr6 HP thin film.
  • Fig. 28C depicts with and without TiC electron-quencher and
  • Fig. 28D depicts with and without P3HT hole-quencher.
  • Fig. 28E depicts UPS spectrum of the Cs2TiBr6 HP thin film.
  • Fig. 28F depicts a schematic illustration of the energy-level diagram of the Cs2TiBr6 HP thin film.
  • Figs. 29A-B depict PL spectra from Cs2TiBr6/glass, Cs2TiBr6/Ti02/glass, and Cs2TiBr 6 /C60/TiO 2 /glass (Fig. 29 A) and glass/Cs 2 TiBr 6 and glass/Cs 2 TiBr 6 /P3HT (Fig. 29B).
  • Insets schematic illustrations of the layers.
  • Figs. 30A-B depict computed properties of Cs2TiBr6 HP using the PBE functional: band structure (Fig. 30A) and DOS and PDOS (Fig. 30B).
  • Fig. 31 A depicts a schematic illustration of PSC architecture using the Cs2TiBr6 HP thin film as the light absorber.
  • Fig. 3 IB depicts energy levels diagram in the PSC.
  • Fig. 31C depicts J-V curves at both forward (hollow circles) and reverse (solid circles) scans of the best PSCs without and with the presence of the C60 interfacial layer.
  • Fig. 3 ID depicts stabilized PCE output at the maximum power points of the PSCs without and with the presence of the C6o interfacial layer.
  • Fig. 3 IE depicts EQE spectrum of the best PSC with the presence of the C6o interfacial layer.
  • Fig. 31F depicts PCE statistics of the PSC with the presence of the C6o interfacial layer.
  • Fig. 32 depicts absorbance spectra for Cs2TiBr6, P3HT and Cs2TiBr6/P3HT thin films.
  • Cs2TiBr6 HP thin films were prepared on a glass substrate, and the P3HT layer was deposited by spin-coating a solution of P3HT in toluene (10 mg/ml) at 3000 rpm, 30 s.
  • Figs. 33A-C depict photoconductive-AFM (pcAFM) mapping of Cs2TiBr6 HP and P3HT thin films.
  • Fig. 33A depicts a schematic illustration of the basic setup for pcAFM measurements. The LED source of 567 nm wavelength is used to stimulate photocurrent in the thin film. Topology (left) and photo-response (right) of thin films with light on and light off:
  • Fig. 35 depicts an AFM scan of top surface of a Cs2TiBr6 HP thin film on C6O/T1O2- coated glass substrate.
  • Fig. 36 depicts thermogravimetric analysis (TGA) of Cs2TiBr6 HP powder sample.
  • Figs. 37A-C depict XRD patterns of before and after heat (200 °C, 6 h, N2 atmosphere), light (one-sun, encapsulated), and moisture (23 °C, 80% RH, 6 h) stresses for Cs2TiBr6 HP thin film (Fig. 37A) and reference MAPbbBr OIHP thin film (Fig. 37B).
  • Fig. 37C depicts evolution of PCE of the best Cs2TiBr6-based PSC (unencapsulated) as function of the storage time under environmental stress (70 °C, 30% RH, ambient light).
  • Fig. 38 depicts XRD patterns of Cs2TiBr6 HP thin film after 24-hour annealing (200 °C, N2 atmosphere).
  • Fig. 39 depicts evolution of PCE of the Cs2TiBr6-based PSC (unencapsulated) as a function of the storage time under environmental stress (70 °C, 30% RH, one-sun AM 1.5G illumination).
  • Fig. 40A is an XRD pattern of a thin film (red) of the Cs 2 TiBr 6 HP.
  • Fig. 40B is an XRD pattern of a bulk powder (control sample; blue) of the Cs2TiBr6 HP.
  • Fig. 41 depicts a typical cross-sectional SEM micrograph of the in the Cs2TiBr6 HP thin film.
  • Fig. 42 depicts an XPS spectrum for Ti 2p in an as-prepared Cs2TiBr6 HP thin film.
  • aspects of the present disclosure are directed to titanium (Ti)-based perovskite materials useful in perovskite solar cells and other applications and for methods of making such perovskite materials and solar cells.
  • the titanium (Ti)-based perovskite materials described herein have a desirable band gap and other optoelectronic properties making them useful in the manufacture of solar cells.
  • the present disclosure describes titanium-based double perovskite compounds or materials, such as vacancy-ordered halide double perovskites based on Ti(IV) for use in perovskite solar cells.
  • the titanium-based double perovskite compound is represented by the formula
  • Exemplary titanium-based double perovskites include CS2T1I6, Rb2Til6, K2T1I6, and In2Til6. Exemplary titanium-based double perovskites lack an organic moiety. Exemplary titanium-based double perovskites are inorganic.
  • Such titanium-based double perovskites of Cs2TiI x Br6- x (0 ⁇ x ⁇ 6) can be tuned continuously from 1.02 eV to 1.78 eV with exemplary bandgaps of 1.78 and 1.38 eV, Exemplary titanium-based double perovskites lack an organic moiety. Exemplary titanium-based double perovskites are inorganic.
  • the titanium-based double perovskite compounds or materials described herein exhibit desirable optical and/or electronic and/or stability properties as visible- light absorber materials for PV applications.
  • the band structures of titanium-based double perovskites described herein show fairly dispersive conduction.
  • the titanium-based double perovskites described herein exhibit a continuously tuned band gap of between 0.9 eV to 1.8 eV. Desirable bad gaps of between 1.0 eV to 1.8 eV can be achieved such as bandgaps of -1.38 eV and -1.78 eV, which are useful for single -junction perovskite solar cell and tandem photovoltaic application.
  • the titanium- based double perovskites described herein exhibit high intrinsic/environmental stability, superior to the Pb-containing halide perovskites, while avoiding the toxicity issues of Pb- containing halide perovskites.
  • the titanium-based double perovskites described herein exhibit stable, respectable power conversion efficiencies and open circuit voltages useful for perovskite solar cells.
  • double perovskite compounds or materials where the titanium is substituted by a transition metal such as Zr, Hf, V, Nb, Mo, or W.
  • a transition metal such as Zr, Hf, V, Nb, Mo, or W.
  • Such transition metal-based double perovskite compounds and materials have desirable band gap or other optical or electronic or other properties for photovoltaic or other applications.
  • Such transition metal based double perovskite compounds and materials are represented by the formula
  • A is selected from one or more of K + , Rb + , Cs + , In + , MA + , or FA + ; wherein T is selected from Ti, Zr, Hf, V, Nb, Mo, or W; and wherein X is selected from one or more of CI, Br, or I.
  • transition metal-based double perovskites such as titanium-based double perovskites are made into powders or are made into thin films through inter-diffusion reaction of their precursors. Such powders have uses for coatings. Such thin films are incorporated into optical or photovoltaic devices. Exemplary photovoltaic or solar devices include electron-transporting layers sandwiched between the thin film perovskite and cathode, and hole-transporting layers sandwiched between the thin film perovskite and anode.
  • the A2BX6 halide perovskites may be viewed as a derivative structure of the conventional ABX3 halide perovskites in which every other B 2+ cation is missing.
  • the interrupted solid-state framework results in isolated [ ⁇ ] 2" octahedra and discrete anions, akin to molecular salts.
  • the bandgaps for titanium-based double perovskites described herein were computed (see Fig. IB).
  • MAPbX3 MAPbX3
  • the bandgap becomes wider with increasing A-site cation radius, suggesting that the tilting of the [ ⁇ ] 2" octahedron dictated by the A-site cation can markedly affect the electronic structures of the titanium-based double perovskites.
  • the bandgap of the titanium-based double perovskites can be tuned via substitution of both the A- site and the X-site elements to meet the desired value.
  • the titanium-based double perovskites can include an organic cation, MA + or FA + , to occupy the A-site in A2T1I6 halide perovskites, which are oriented in [001] and [100] direction, respectively.
  • MA2T1I6 and FA2T1I6 halide perovskites exhibit bandgaps of 1.67 eV and 1.70 eV, respectively.
  • Other inorganic ⁇ 2 ⁇ 6 halide perovskites such as K2T1I6, Rb2Til6, CS2T1I6, and In2Til6, exhibit bandgaps of 1.55 eV, 1.62 eV, 1.65 eV, and 1.40 eV, respectively.
  • Rb2Til6 and CS2T1I6 possess slightly larger bandgap beyond the optimal region (0.9-1.6 eV), both halide perovskites are useful for certain photovoltaic applications.
  • the optical absorption spectra of the titanium-based double perovskites described herein were computed, and compared with the absorption spectra of two photovoltaic materials, Si and MAPM3 halide perovskite, as shown in Fig. 1C.
  • MAPM3 two photovoltaic materials
  • all the Pb- free halide perovskite displayed relatively lower absorption intensity in the visible region, but higher absorption in the lower-energy range.
  • Titanium-based double perovskites described herein show favorable absorption behavior (with absorption coefficient >1.4xl0 5 cm 1 ) making them useful in perovskite solar cells.
  • Figs. ID and IE show the computed band structures and density of state (DOS) of CS2T1I6.
  • CS2T1I6 possesses an indirect fundamental bandgap between ⁇ (VBM) and X (CBM) and a direct bandgap at X, slightly larger than the indirect gap by about 30 meV.
  • the band structure is similar to that of Cs2PdBr6, (See, Sakai, N. ; Haghighirad, A. A. ; Filip, M. R.; Nayak, P. K.; Nayak, S.; Ramadan, A.; Wang, Z.; Giustino, F.; Snaith, H. J. Solution-processed cesium hexabromopalladate(IV), Cs2PdBr6, for optoelectronic applications. /. Am. Chem. Soc.
  • spin-orbit coupling can lower the computed bandgap significantly. See, Mosconi, E.; Amat, A.; Nazeeruddin, M. K.; Gratzel, M.; De Angelis, F. First-principles modeling of mixed halide organometal perovskites for photovoltaic applications. /. Phys. Chem. 2013, 117, 13902-13913.); Even, J. ; Pedesseau, L.; Jancu, J. ; Katan, C. Importance of spin-orbit coupling in hybrid organic/inorganic perovskites for photovoltaic applications. J Phys. Chem. Lett. 2013, 4, 2999-3005.
  • the effect of SOC is quite small on the computed bandgap of CS2T1I6 (see Fig. 3A-B): the computed bandgap is only narrowed slightly to 1.50 eV when SOC is considered.
  • the computed bandgaps are reduced slightly in titanium-based double perovskites.
  • the bandgap decreases in the order In2Til6 ⁇ K2Til6 ⁇ Rb2Til6 ⁇ Cs2Til6.
  • the highest VB and lowest CB are contributed mainly by cation B and anion X.
  • the A-site element does not contribute directly to the electronic properties of titanium-based double perovskites, but can indirectly affect the electronic properties via altering the tilting of the [ ⁇ ] 2" octahedra and the distance between adjacent octahedra.
  • MAPbI 3 , CsSnI 3 , and CsPbI 3 halide perovskites can be synthesized successfully using the corresponding binary halides (MAI, Pb , Csl, Sn , etc.), the same approach was used to synthesize titanium-based double perovskites.
  • the predominant decomposition pathways are the reverse reactions of the corresponding synthesis routes, while there may be ternary products involved in other decomposition pathways.
  • the decomposition enthalpy of different decomposition pathways is defined as:
  • a positive value of AH represents energy gain from the formation of decomposition products from the corresponding titanium-based double perovskites.
  • Fig. 4 shows the calculated AH values for the four titanium-based double perovskites with different decomposition processes.
  • the titanium-based double perovskites exhibit good stability with positive AH values.
  • CS2T1I6 and Rb2Til6 possess robust thermodynamic stability due to the fairly large positive values of AH>50 meV/atom.
  • ab initio molecular dynamic (AIMD) simulations were performed to examine the thermal stability of the four titanium- based double perovskites.
  • Fig. 5A-D shows snapshots of the four titanium-based double perovskites at initial time and 5 ps after the AIMD simulations, respectively.
  • the temperature is maintained at 300°K or 500°K.
  • the overall framework of the titanium-based double perovskites is sustained in the final configuration at 300°K or 500°K.
  • the [ ⁇ ] 2" octahedra in the final configuration appear to have higher degrees of freedom, such as tilting.
  • Titanium-based double perovskites have good intrinsic stability up to very high temperatures, consistent with the analyses based on the decomposition energies.
  • CS2T1I6 HP 12 possible intrinsic point defects were considered in CS2T1I6 HP, including three types of vacancies (Vi, V-n, and Vcs), three types of interstitials (I;, Ti;, and Cs;), two cation substitutions (Tics and CSTO, and four antisite substitutions (hi, Ics, Tii, and Csi).
  • the formation energies of point defects are mostly dependent on the chemical potentials of the host elements, ⁇ , ⁇ , and uc s -
  • a moderate chemical potential region is identified for achieving thermodynamically stable CS2T1I6.
  • the identified chemical-potential region for CS2T1I6 is highlighted in gray in Fig. 6A.
  • Fig. 6A To calculate the formation energies, two extreme cases marked by two solid black circles in Fig. 6A were considered: (a) I-rich/Ti-lean and ( ⁇ ) I-lean/Ti-rich. Under the I-rich and Ti-lean conditions, Vi has the lowest formation energy, consistent with acting as a recombination center for a photo-generated electron-hole pair (see Fig. 6B).
  • the acceptors In, Ii, and Vcs also have relatively low formation energies, which due to the neutral charge state in the bandgap, do not affect the electronic and photovoltaic properties significantly.
  • the Fermi level induced by compensation of charged acceptor and donor defects is pinned slightly below the CB minimum (CBM), resulting in «-type conductivity, as is in the case of T1O2.
  • CBM CB minimum
  • the Cs; and Ti have the lowest formation energy, and are the dominant defects.
  • both defects may be more detrimental to photovoltaic performance than the defects under I-rich/Ti-lean conditions (see Fig. 6C).
  • the Fermi level is pinned above the CBM, indicating degenerate «-type conductivity in CS2T1I6.
  • the deep-level defects can be suppressed in the synthesis of CS2T1I6 HP.
  • aspects of the present disclosure are directed to methods of making transition metal- based double perovskite materials, such as titanium-based double perovskite compounds and materials in powder form, such as for use in making coatings, or thin film form, such as for use in making photovoltaics or solar cells.
  • transition metal-based double perovskite materials such as titanium-based double perovskite compounds and materials in powder form, such as for use in making coatings, or thin film form, such as for use in making photovoltaics or solar cells.
  • titanium-based double perovskites as described herein were synthesized into thin films and photovoltaic devices using the melt-crystallization method.
  • a sequential-deposition method was used to deposit high-quality Cs2TiBr6 HP thin films.
  • a uniform CsBr layer was first deposited by vacuum evaporation.
  • the CsBr layer was then annealed in TiBr4 vapor at 473°K for 24 hours to form the Cs2TiBr6 thin film.
  • the XRD pattern of the as-fabricated Cs2TiBr6 thin film confirms the phase purity, and the inset SEM image shows the excellent uniformity of the thin film.
  • Fig. 8 shows a lower-magnification SEM image of the Cs2TiBr6 HP thin film, indicating its pinhole- free nature over large areas.
  • a CsBr layer was deposited on the substrate by thermal evaporation.
  • the CsBr thin film was then placed in a closed chamber filled with TiBr4 vapor for 24 hours as illustrated in Fig. 9.
  • the TiBr4 vapor was generated by heating TiBr4 powders at 473°K. After full conversion of CsBr to phase-pure Cs2TiBr6, the thin film was washed by toluene for subsequent characterization and device fabrication.
  • Cs2TiBr6 perovskite solution was synthesized by reaction between mixed solid powder precursors CsBr:TiBr4 (2:1 molar ratio) dissolved in gamma-butyrolactone (GBL) solvent.
  • GBL gamma-butyrolactone
  • suitable solvents can be determined by those of skill in the art such as dimethylformamide, dimethyl sulfoxide, N-methyl-2-pyrrolidone, and combinations thereof.
  • the mixture was stirred in GBL solvent for 12 hours at 150°C to obtain a clear solution.
  • This solution can be used for spin-coating, spraying, slot-die coating, or infiltrating into a porous preform, etc.
  • titanium-based double perovskite solar cells were made by sandwiching the Cs2TiBr6 thin films between a T1O2/FTO anode and poly(3-hexylthiophene) (P3HT)/Au cathodes.
  • Cross-section of the Ti-based PSC is shown in Fig. 7(C) inset, where each layer can be differentiated.
  • the current-voltage (J-V) curve of the champion perovskite solar cell is also shown in Fig. 7(C), which exhibits an overall power conversion efficiency of 2.36 % with an open circuit voltage (VOC) of 0.97 V, a short circuit current density (JSC) of 4.06 mA cm-2 and a fill factor (FF) of 0.600.
  • VOC open circuit voltage
  • JSC short circuit current density
  • FF fill factor
  • a mixed-cation lead mixed- halide perovskite absorber for tandem solar cells Science 351, 151-155 (2016); Jacobsson, J. T. et al. Exploration of the compositional space for mixed lead halogen perovskites for high efficiency solar cells. Energy Environ. Sci. 9, 1706-1724 (2016); Zhou, Y. et al. Exceptional Morphology-Preserving Evolution of Formamidinium Lead Triiodide Perovskite Thin Films via Organic-Cation Displacement. J. Am. Chem. Soc. 138, 5535-5538 (2016).
  • a compact-TiC electron selective layer (ESL) was first deposited on pre-patterned FTO-coated glass by spray pyrolysis at 450°C.
  • a titanium-based double perovskite layer was then deposited, followed by spin-coating a hole-transporting material (HTM) solution, which consisted of 10 mg P3HT and 1 ml toluene solvent.
  • HTM hole-transporting material
  • Au Au layer was deposited using a thermal evaporator and a shadow mask.
  • the external quantum efficiency (EQE) spectra were conducted in AC mode on a solar cell quantum efficiency measurement system (EQE-200, Oriel Instruments, USA).
  • the J-V characteristics of the PSCs were measured using a 2400 Sourcemeter (Keithley, USA) under simulated one-sun AM 1.5G 100 mW cm 2 intensity (Oriel Sol3A Class AAA, Newport, USA), under both reverse (from VOC to JSC) and forward (from JSC to VOC) scans.
  • the step voltage was 5 mV with a 10 ms delay time per step.
  • the maximum-power output stability of the PSCs was measured by monitoring the J output at the maximum-power V bias (deduced from the reverse-scan J-V curves) using the 2400 SourceMeter.
  • a shutter was used to control the one-sun illumination on the PSC.
  • a typical active area of 0.12 cm 2 was defined using a non-reflective mask for the J-V measurements.
  • Fig. 11 A and Figs. 12A-B show, respectively, the experimental and calculated X-ray diffraction (XRD) patterns of CS2T1I6 and Cs2TiBr6, confirming their phase purity.
  • the Tauc plot of the CS2T1I6 and Cs2TiBr6 samples are shown in Fig. 13D and 13 A.
  • the measured optical bandgaps of the phase-pure CS2T1I6 and Cs2TiBr6 HPs are -1.02 and -1.78 eV, respectively.
  • the bandgap of CS2T1I6 is within the optimal bandgap range (0.9-1.6 eV) for single-junction perovskite solar cells. See, Loferski, J. J. Theoretical considerations governing the choice of the optimum semiconductor for photovoltaic solar energy conversion. /. Appl. Phys. 1956, 27, 777-784.
  • the optical bandgap (-1.51 eV) of MAPbL was also measured independently as a benchmark (see Fig. 14). The measured bandgaps are slightly smaller than the computed HSE06 bandgaps.
  • the hybrid functional HSE06 includes a fraction parameter a (default value 0.25) for the screened short-range Hartree-Fock exchange to improve the derivative discontinuity of the Kohn-Sham potential for integer electron numbers.
  • a parameter should be adjusted to yield more accurate bandgap.
  • T1O2 when the value of a is set at 0.20 or 0.21, the computed bandgap is in agreement with measured bandgap. See, Janotti, A.; Varley, J. B.; Rinke, P.; Umezawa, N. ; Kresse, G.; Van de Walle, C. G. Hybrid functional studies of the oxygen vacancy in T1O2. Phys. Rev.
  • the Shockley-Queisser limit suggests a bandgap of -1.3 eV for achieving maximum power conversion efficiency in a single p-n junction solar cell.
  • titanium-based double perovskites are described where the bandgap is tuned using doping or alloying strategies. See, Zong, Y. ; Wang, N.; Zhang, L.; Ju, M.-G.; Zeng, X. C; Sun, X. W.; Zhou, Y.; Padture, N. P.
  • the simulated XRD results for CS2T1I6 and Cs2TiBr6 confirm the shift, and are in good agreement with the experimental results (see Figs. 12A-B).
  • the lattice parameter, a, of CS2T1I6 Cs2TiLtBr2, Cs2Til2Br4, and Cs2TiBr6 are estimated at 11.67 A, 11.43 A, 11.25 A, and 10.92 A, respectively, and all crystals possess Fm-3m space group.
  • the insets in Fig. 11A are photographs of the corresponding as- synthesized Cs2TiI x Br6-x titanium-based double perovskites.
  • Fig. 16A depicts schematically the crystal structure of the mixed-I/Br Cs2TiI x Br6 x titanium-based double perovskites.
  • the calculated bandgaps are in reasonable agreement with the experimental results (see Fig. 16B).
  • Table 1 below shows computed bandgaps of a series of Cs2TiI x Bri-x HPs using HSE06 functional without and with spin-orbital coupling (SOC). Experimentally measured bandgaps are also included.
  • the bandgaps can be reduced to about 0.1 eV considering the SOC effect.
  • the I/Br ratio exceeds 0.5, the calculated bandgap values of the titanium-based double perovskites no longer decrease linearly. This is due to the different arrangement orders of I and Br sites.
  • the Cs2Til2Br4 titanium-based double perovskite has a direct bandgap of 1.38 eV, which is closest to the ideal bandgap (1.34 eV), making it an exemplary candidate for the light- absorber layer in single -junction perovskite solar cells.
  • the Cs2TiBr6 titanium-based double perovskite can be an optimal large-bandgap material (1.8 eV) for application in the tandem photovoltaics in conjunction with the low-bandgap (1.1-1.2 eV) materials such as Si. See, Eperon, G. E. ; Leijtens, T.; Bush, K.
  • Fig. 16C shows computed absorption spectra of a series of titanium-based double perovskites, compared with computed absorption spectra of Si and MAPM3 halide perovskites.
  • the experimental UV-v « absorbance spectra and Tauc plots (see Figs. 11C and Figs. 13A-D) also confirm this result.
  • the maximum absorption coefficient in the visible- light range decreases, proportional to the decrease of I-content in the HPs.
  • I-based titanium-based double perovskites tend to yield better absorption than Br-based titanium-based double perovskites.
  • the band structures of Cs2Til2Br4 with different arrangement of the I and Br sites were computed (see Fig. 16D and Fig. 17A-B). Two structures were considered with space groups l4/mmm and lmm2. The structure with 14/mmm space group is slightly more stable than that with Imm2 space group (-30 meV). The structure with 14/mmm space group possesses a direct bandgap. In contrast, the structure with Imm2 space group shows a quasi-direct bandgap, which is found to be similar to that in CS2T1I6 (see Fig. 17B). From the computed DOS of Cs2Til2Br4, it can be seen that the highest VBs are mostly contributed by the I 5p orbitals, while the lowest CBs are mostly contributed by the Ti 3d orbitals (see Fig. 16E).
  • the electrons have effective mass of 1.58 m e . Although the effective masses are larger than those of Pb-based halide perovskites, both materials have reasonable carrier mobilities for photovoltaic application.
  • COHP crystal Hamilton population
  • the average integrated COHP values for Ti-I and Ti-Br bond are -2.77 and -2.53 for CS2T1I6 and Cs2TiBr6, respectively, indicating that the Ti-I bond exhibits more covalent characteristic than the Ti-Br bond.
  • the Born effective charges Z * for the Ti 4+ in CS2T1I6 and Cs2TiBr6 are 5.29 and 4.36, respectively, confirming the higher degree of covalency in the Ti- I bonds (see Table 1).
  • the value of the Born effective charges for the Ti 4+ is between CS2T1I6 and Cs2TiBr6, due to the alloying of Ti-I and Ti-Br bonds.
  • the increased covalency is in the order Cs2TiBr6 ⁇ Cs2Til2Br4 ⁇ Cs2Til6, which likely prevents the formation of halide vacancies in these HPs.
  • the calculated high-frequency dielectric tensors 6 have the same trend, .e. in the order Cs2TiBr6 ⁇ Cs2Til2Br4 ⁇ Cs2Til6.
  • Table 3 shows dielectric properties of Cs2TiBr6, CS2T1I6 and Cs2Til2Br4 HPs. including the High-Frequency Dielectric Tensors (e °° ij) and Born Effective charge tensors (Z * ; Id) on Ti 4+ .
  • pristine titanium-based double perovskite samples were annealed at high temperature (473 °K) in N2 atmosphere to evaluate thermal tolerance, and were then exposed to room-temperature (298°K) humid environment (70% RH) to assess moisture stability.
  • the XRD patterns shown in Fig. 19B and Fig. 19D indicate no degradation in both Cs2TiBr6 and Cs2Til2Br4 titanium-based double perovskite samples after the applications of the thermal/moisture stresses.
  • MAPbbBr and MAPM3 halide perovskites with bandgaps close to those of Cs2TiBr6 and Cs2Til2Br4, respectively show severe decomposition under the same conditions (see Fig. 20A-B) indicating superior properties of the titanium-based double perovskites based on the inorganic nature of the titanium-based double perovskites and the highly stable covalent/ionic interaction between Ti (IV) and halide ions in the Ti-based HPs.
  • Hybrid functionals based on a screened Coulomb potential. /. Chem. Phys. 2003, 118, 8207-8215) and with a cutoff energy of 400 eV, and the ion cores are described by using the projector augmented wave (PAW) method.
  • PAW projector augmented wave
  • Kresse, G From ultrasoft pseudopotentials to the projector augmented- wave method.
  • Grimme's DFT-D3 correction is adopted to describe the long-range van der Waals interaction. See, Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H.
  • a 3x3x3 k- point grid is used for the Ti vacancy-ordered HPs.
  • the optical absorption coefficient is given by " TM ; " ⁇ 3 ⁇ 4 , where ⁇ and 82 are real and imaginary part of dielectric function, respectively.
  • the initial configurations of A 2 Til6 with ⁇ 2 X 3 ⁇ 4 2 supercell (144 atoms) are adopted for ab initio molecular dynamics (AIMD) simulations. Each 5-ps AIMD simulation is performed in the constant- volume and constant- temperature (NVT) ensemble.
  • the time step is 1.0 fs, and the temperature (300 K or 500 K) is controlled using the Nose-Hoover method. See, Martyna, G. J.; Klein, M. L.; Tuckerman, M. Nose-Hoover chains: The canonical ensemble via continuous dynamics. J. Chem. Phys. 1992, 97, 2635-2643.
  • X-ray diffraction (XRD) of the synthesized powder was performed using an X-ray diffractometer (D8-Advance, Bruker, Germany) with CuKoc radiation (1.15406 A) in the 2 ⁇ range of 10°-60°.
  • UV-vis spectra were obtained using a spectrophotometer (UV-2600, Shimadzu, Japan) equipped with Integrating Sphere attachment (ISR-2600, Shimadzu, Japan).
  • the standard BaSC (Nacalai Tesque, Inc., Japan) pellet was used as the reference.
  • Surface morphology of films and cross-sections (fractured) of the whole PSCs were observed in a scanning electron microscope (SEM; LEO 1530VP, Carl Zeiss, Germany).
  • Fig. 21 A illustrates schematically the vapor-based method that is used for the deposition of Cs2TiBr6 HP thin films.
  • a uniform thin film of CsBr is first deposited via thermal evaporation on the substrate. This is followed by annealing the CsBr thin film in a TiBr4-vapor atmosphere at between 100°C and 300°C, such as 200°C.
  • Cs2TiBr6 HP thin films were first deposited on the substrate by thermal evaporation.
  • the as- deposited CsBr thin film was placed in a chamber (see Fig. 9) filled with TiBr4 vapor.
  • the TiBr4 vapor was slowly generated by heating TiBr4 powder at between 100°C and 300°C, such as 200°C.
  • the typical reaction time for the complete conversion of CsBr to pure -phase Cs2TiBr6 is 24 hours when a TiC /FTO-glass substrate is used.
  • the resulting Cs2TiBr6 thin films were washed using toluene to remove any possible excess TiBr4 on the film surface for subsequent characterization and device fabrication.
  • the hot TiB3 ⁇ 4 vapor is expected to interact with the as-deposited CsBr, forming a uniform Cs2TiBr6 thin film as below:
  • the TiBr4 vapor can be produced by simply heating solid TiBr4 at ambient pressure.
  • the thin film before annealing (pure CsBr phase), after 12-h annealing (pure CsBr phase contacted with TiBr4 vapor for 12 hours), and after 24-h annealing in TiBr4 vapor are denoted as Film-I, -II and -III, respectively.
  • Figs. 21B and 21C show indexed X-ray diffraction (XRD) patterns of the three thin films and their corresponding UV-vw spectra. As seen in Fig. 21B, Film-I contains pure CsBr phase of high crystallinity.
  • FIG. 21C While the expected absorption features of CsBr and Cs2TiBr6 in Film-I and Film-Ill are respectively shown, Film-II has the typical mixed absorption feature of the CsBr and Cs2TiBr6 phases, which is consistent with the XRD results.
  • the inset in Fig. 21C is a photograph of the final Cs2TiBr6 HP thin film showing even, dark reddish-brown color and good transparency, confirming its uniformity.
  • Fig. 21D shows scanning electron microscope (SEM) images of the surface morphologies of Film-I, Film-II, and Film- Ill.
  • Film-I is polycrystalline, compact, and pinhole-free, showing a typical microstructure comprising crystalline equiaxed grains and grain boundaries.
  • SEM scanning electron microscope
  • Film III exhibits uniform grains of Cs2TiBr6 HP with well-defined grain boundaries.
  • the Ti content in the intermediate Film-II was measured as a function of depth.
  • the Film-II sample (film thickness -200 nm) was etched locally using focused ion beam (FIB) for different durations, and the Ti content was measured using energy dispersive spectroscopy (EDS) at each depth.
  • FIB focused ion beam
  • FIG. 21E shows that the Ti content decreases with depth into Film-II, which indicates that the reaction progression is driven by the concentration gradient of Ti 4+ in the thin film.
  • the curve in Fig. 21E is a Gauss error-function fit using Fick's second law which is consistent with thermally-activated non-steady-state diffusion of Ti 4+ in the solid state.
  • the Ti elemental EDS maps at the top surface and surface at 40-nm depth of Film-II presented in Fig. 22-A-D are highly homogenous indicating the uniform progression of the diffusion front into the thin film during the TBr4-vapor-annealing step.
  • Fig. 23 plots the Ti content as a function of depth in the final Cs2TiBr6 thin film (Film-Ill), showing homogeneous Ti distribution which is consistent with its phase-pure nature.
  • the proposed formation mechanism of the Cs2TiBr6 HP thin film is depicted schematically in Fig. 21F.
  • the Ti 4+ and Br are transported from the film top surface to the bottom of the film bottom via solid-state diffusion, resulting in the progression of the conversion-reaction front into the thin film.
  • the compact nature of the initial CsBr thin film and the uniform TiBr4 vapor atmosphere no obvious in-plane reaction heterogeneity is observed in the thin film.
  • the volatile TiBr4 causes a high vapor pressure that immediately damages the CsBr thin film (see Fig. 24).
  • a lower-annealing temperature 150 °C
  • Reaction 1 remains incomplete, resulting in the presence of CsBr in Cs2TiBr6 thin film (Fig. 25A-B).
  • the average grain size in this final Cs2TiBr6 HP thin film is -270 nm, as measured using image analysis (see Fig. 26), and the root-mean-square (RMS) roughness is 24.5 nm, as measured using atomic force microscopy (AFM) (Fig. 27).
  • Fig. 28A presents Tauc plot and photoluminescence (PL) spectrum of the Cs2TiBr6 HP thin film. Linear fitting of the absorption band edge results in a bandgap of 1.82 eV, which is near-ideal for top-cell application in tandem PVs with conventional Si-based or CIGS-based bottom-cells. (See, McMeekin et al, Science 351, 151-155 (2016).
  • Fig. 28B is a PL map of the Cs2TiBr6 HP thin film overlaid on a AFM film-morphology scan, showing highly uniform PL-intensity over the whole film area.
  • Photo-generated charge-carrier diffusion length in the light-absorber material in a PSC is another key optoelectronic property.
  • the PL decay dynamics in the Cs2TiBr6 HP thin film with and without quencher layers were studied.
  • the PL spectra in Figs. 29A-B confirm that T1O2 and P3HT effectively quench the PL in the Cs2TiBr6 HP thin film, and, thus, T1O2 and P3HT were chosen as the electron- and hole-quenching layers, respectively.
  • the distributions of photo-generated electrons or holes n(x, t) in the Cs2TiBr6 thin film are described according to the following diffusion-based equation:
  • D is the diffusion coefficient for electrons or holes and k ⁇ t) is the PL decay rate, respectively.
  • k(f) is the PL decay rate, respectively.
  • ⁇ is the characteristic lifetime extracted at when the PL intensity falls to l/e th of its initial intensity without the quencher.
  • Fig. 28C shows time-resolved PL decay for a Cs2TiBr6 HP thin film with and without T1O2 quencher layer, where the extracted PL lifetimes are 24.0 ns and 2.5 ns, respectively.
  • the L for electrons is estimated at 121 nm.
  • the L for holes is estimated at 103 nm based on the results in Fig. 28D.
  • Cs2TiBr6 possesses an indirect fundamental bandgap between ⁇ (valence band maximum, VBM) and X (conduction band minimum, CBM) and a direct bandgap at X, slightly larger than the indirect gap by -30 meV.
  • the energy levels of photovoltaic materials match with the other layers in the solar cell device, such as the electron- transporting layer (ETL) and hole-transporting layer (HTL).
  • ETL electron- transporting layer
  • HTL hole-transporting layer
  • the CBM and VBM energy levels were determined using ultraviolet photoelectron spectroscopy (UPS), and the results are presented in Fig. 28E.
  • the 4.5 eV work function of Cs2TiBr6 HP was calculated by subtracting the cutoff (16.75 eV) located in the higher binding-energy region from the energy of He ions (21.2 eV). The turning point located at lower binding-energy region indicates the energy gap between the VBM energy level and Fermi level of Cs2TiBr6 HP.
  • the VBM is estimated at -5.9 eV for the Cs2TiBr6 thin film with respect to the vacuum energy level.
  • the CBM energy level was deduced to be -4.1 eV based on the optical bandgap (1.82 eV) of the Cs2TiBr6 thin film.
  • the Fermi level of the Cs2TiBr6 thin film is relatively closer to the CBM, indicative of the «-type self-doping nature of the as-prepared Cs2TiBr6 HP thin film.
  • the energy level diagram is shown schematically shown in Fig. 28F.
  • the VBM/CBM values match favorably with the state-of-the-art ETL materials (e.g.
  • T1O2, Ceo T1O2, Ceo
  • HTL materials e.g. Spiro-OMeTAD, PEDOT, P3HT, NiO
  • the present disclosure contemplates Cs2TiBr6-based perovskite solar cells with many different device architectures for achieving desirable photovoltaic performance.
  • perovskite solar cells are fabricated by sandwiching the Cs2TiBr6 thin film in-between T1O2 ETL and P3HT HTL.
  • Fluorine-doped tin oxide (FTO) and Au are used as the electrodes. More particularly,
  • FTO Fluorine-doped tin oxide
  • a compact-TiC ETL was first deposited on pre- patterned FTO-coated glass by spray pyrolysis at 450°C.
  • an interfacial layer of C60 layer was deposited before the deposition of the Cs2TiBr6 thin film.
  • This C60 layer was deposited by spin-coating a solution of C60 in chlorobenzene (2 mg/ml) at 3000 rpm for 40 s on the as-prepared T1O2 substrate, followed by annealing at 100 °C for 30 min.
  • the Cs2TiBr6 thin film was then deposited based on the procedure described above, followed by spin-coating the HTL solution, which consisted of 10 mg P3HT and 1 ml toluene solvent.
  • the Au layer was deposited using thermal evaporator and a shadow mask.
  • the J-V characteristics of the PSCs were measured using a 2400 Sourcemeter (Keithley, USA) under simulated one-sun AM1.5G 100 mW.cm "2 intensity (Oriel Sol3A Class AAA, Newport, USA), under both reverse (from Voc to Jsc) and forward (from Jsc to Voc) scans.
  • the step voltage was 5 mV with a 10 ms delay time per step.
  • the maximum-power output stability of PSCs was measured by monitoring the / output at the maximum-power-point V bias (deduced from the reverse-scan J- V curves) using the 2400 SourceMeter.
  • a typical active area of 0.12 cm 2 was defined using a non-reflective mask for the J-V measurements.
  • a shutter was used to control the one-sun illumination on the PSC.
  • the EQE spectra were obtained using a quantum- efficiency measurement system (Oriel IQE 200B, Newport, USA) consisting of a Xenon lamp, a monochoromator, a lock-in amplifier and a calibrated silicon photodetector.
  • the PSC stability was evaluated by measuring the J-V characteristics of PSCs after storing the cells under constant environmental stress for a certain period of time in an environmental climate chamber (HPP110, Memmert, Germany).
  • the perovskite solar cell shows a useful overall PCE of 2.26% in reverse scan with small hysteresis (2.05% in forward scan).
  • the stabilized PCE output is 2.15% at the maximum power point, which is reached immediately upon light illumination as shown in Fig. 3 ID.
  • This efficient photoresponse is mainly contributed by Cs2TiBr6 rather than P3HT, as the P3HT layer has negligible contribution to light absorption (see Fig. 32).
  • the photo-response of Cs2TiBr6 using conductive-AFM under light was further studied. As shown in Figs.
  • Cs2TiBr6 shows more than an order-of-magnitude higher photocurrent, which confirms that the measured PCE is mainly attributed to the Cs2TiBr6 thin film in the device.
  • Cs2TiBr6 HP is an efficient light-absorber material.
  • the 2.26% PCE was improved by incorporating a C6o interfacial layer between the Cs2TiBr6 HP thin film and the T1O2 ETL.
  • the overall PCE is improved to a maximum 3.28 % in reverse scan (see Fig. 31C, Table 4).
  • the stabilized PCE output at the maximum power point is 3.22% (see Fig. 3 ID).
  • the external quantum efficiency (EQE) spectrum of the same PSC shows an integrated current density (5.43 mA.cm 2 ), which is close to the short-circuit current density ( sc) extracted from the J-V curves in Fig. 31C (Table 4).
  • EQE spectrum shows a band edge at -680 nm, which is identical to the absorption edge shown in Fig, 28 A.
  • Fig. 3 IF presents the PCE statistics of the Cs2TiBr6 thin film based PSC with the C60 interfacial layer, which shows a tight PCE distribution with a mean value of 3%.
  • the C60 interfacial layer has two beneficial functions.
  • C60 molecules have a CBM commensurate with that of T1O2 ETL and Cs2TiBr6 HP, which can facilitate the electron transfer from Cs2TiBr6 to the T1O2 ETL. This is consistent with the fact that the PL is much more efficiently quenched with the presence of C60 (Fig. 29A-B).
  • the C60 layer influences the microstructure of the as-deposited CsBr thin film, and, thus, the formation of the Cs2TiBr6 HP thin film. As seen in Fig.
  • the as-deposited CsBr thin film under the same preparation conditions exhibits a reduced grain size and a higher-density grain-boundary network.
  • This modified CsBr microstructure facilitates Reaction 1, most likely due to the faster through-thickness ion diffusion along the grain boundaries. It is found that the Reaction 1 completion time is reduced by half (12 h) in this case.
  • the final Cs2TiBr6 HP thin film shows superior quality with a reduced RMS roughness of 14.6 nm (Fig. 35).
  • the achieved PCE exceeds several of the PCEs of PSCs based on other Pb-free HPs.
  • the best Voc for the Cs2TiBr6 thin film achieved is over 1.0 V, which is useful for perovskite solar cells.
  • the most widely studied all-inorganic Sn-based Pb-free HPs show a Voc less than 0.6 V. See, Hao et al., Nat. Photonics 8, 489-494 (2014); Ke et al., Sci. Adv. 3, el71029 (2017).
  • Cs2TiBr6 is not susceptible to the ambient degradation and conversion to titania.
  • the heat-tolerance experiment was conducted by annealing the film at between 100°C and 300°C, such as 200°C, for 6 hours in a N2-filled glovebox.
  • the moisture tolerance was tested by storing the film in a precisely-controlled climate chamber (23°C, 80% relative humidity (RH), 6 hours).
  • the tolerance of the film to sunlight was evaluated by placing the film under stimulated one-sun illumination for 24 h where the film is sealed in a poly(methyl methacrylate) coating to exclude other environmental effects. These test conditions are intended to separate the thermal, moisture, and light effects.
  • the Cs2TiBr6 thin films used in these tests were all deposited on compact-TiC coated glass substrates bearing in mind that the TiC -perovskite interfaces are regarded as an important factor of the PSC device stability.
  • the Cs2TiBr6 thin films show much higher tolerance compared to the reference thin film. Note that, while the time duration that is chosen to evaluate environmental stresses is until significant degradation occurs in the reference film, the Cs2TiBr6 thin films can maintain their phase purity for much longer durations. For example, in the thermal stability test, when the annealing time is prolonged to 24 h, there is still no obvious degradation (Fig. 38). Cs2TiBr6 thin films exhibit useful intrinsic/environmental stability.
  • X-ray diffraction (XRD) of the thin films was performed using a high-resolution diffractometer (D8 Advance, Bruker, Germany) with CuKa radiation. UV-v « spectra were obtained using a spectrophotometer (UV-2600, Shimadzu, Japan). The morphology and EDS elemental-distribution maps of the thin film were observed in a SEM (LEO 1530VP, Carl Zeiss, Germany) equipped with an EDS detector (Oxford Instruments, UK). The FIB (Helios 600, FEI, HiUisboro, OR) was used to etch the thin films, where the different depths were achieved by Ga + ions bombardment.
  • FIB Helios 600, FEI, HiUisboro, OR
  • a PHI5600 XPS system was used to acquire both XPS and UPS spectra.
  • the analysis chamber base pressures were ⁇ lxl0 -9 Torr prior to analysis.
  • the instrument utilized a monochromated K a Al source for X-ray radiation at 1486.7 eV and a UVS 40 A2 (PRE VAC, Tru) UV source and UV40A power supply provided by He la excitation (He I) for UPS at 21.22 eV.
  • Chamber pressure for UPS was maintained ⁇ 3xl0 "8 Torr.
  • the steady-state and time-resolved PL spectra were recorded with a Varian Cary Eclipse fluorescence spectrophotometer operating at 395 nm excitation.
  • the decay rate and lifetime was determined using the two-parameter decay function fitting method.
  • the fitted diffusion coefficients for electrons and holes are 0.61 cm 2 .s _1 and 0.44 cm 2 .s _1 , respectively, which are used to calculate the corresponding diffusion lengths.
  • the pc-AFM measurements were conducted in contact mode using an MFP-3D AFM (Asylum Research, USA) with a conducting platinum-coated silicon probe (Econo-SCM-PIC, Asylum Research, USA) and a LED light source.
  • the present disclosure provides a transition metal-based double perovskite of the formula A2TX6 wherein A is one or more of K + , Rb + , Cs + , In + , MA + , or FA + ; wherein T is Ti, Zr, Hf, V, Nb, Mo, or W; and wherein X is selected from one or more of CI, Br, or I. According to one aspect, T is Ti.
  • T is Ti.
  • the transition metal-based double perovskite is of the formula Cs2Til2Br4 or Cs2TiBr6. According to one aspect, the transition metal-based double perovskite lacks organic moieties. According to one aspect, the transition metal-based double perovskite is all inorganic. According to one aspect, the transition metal-based double perovskite is in the form of a powder. According to one aspect, the transition metal-based double perovskite is in the form of a thin film. According to one aspect, the transition metal-based double perovskite is in the form of a thin film between an anode and a cathode.
  • the transition metal-based double perovskite has a bandgap that can be tuned continuously from 0.9 eV to 1.82 eV. According to one aspect, the transition metal-based double perovskite has a bandgap that can be tuned continuously from 1.02 eV to 1.78 eV. According to one aspect, the transition metal-based double perovskite has the formula Cs2TiBr6 and a direct bandgap of 1.78 eV. According to one aspect, the transition metal-based double perovskite has the formula Cs2Til2Br4 and a direct bandgap of 1.38 eV.
  • the present disclosure provides a photovoltaic cell including the transition-metal based double perovskite as described herein in the form of a thin film having an anode and a cathode electrically connected thereto.
  • thin film is contacted on a first side by a hole-transporting layer which is contacted by an anode layer and wherein the thin film is contacted on a second side by an electron-transporting layer which is contacted by a cathode layer.
  • the present disclosure provides a solar cell including the transition-metal based double perovskite described herein in the form of a thin film having an anode and a cathode electrically connected thereto.
  • the thin film is contacted on a first side by a hole- transporting layer which is contacted by an anode layer and wherein the thin film is contacted on a second side by an electron-transporting layer which is contacted by a cathode layer.
  • the present disclosure provides a method of making a thin film of a transition-metal based double perovskite as described herein including vapor depositing a compound of formula AX on a substrate to form a first film wherein A is K + , Rb + , Cs + , or In + and wherein X is CI, Br, or I, contacting the first film with a compound of TX in vapor form wherein T is Ti, Zr, Hf, V, Nb, Mo, or W and wherein X is CI, Br, or I in a manner to form a thin film of the compound of formula A2TX6 wherein A is one or more of K + , Rb + , Cs + , or In + , MA + , or FA + , wherein T is Ti, Zr, Hf, V, Nb, Mo, or W; and wherein X is selected from one or more of CI, Br, or I.
  • the present disclosure provides a method of making a thin film of a transition-metal based double perovskite as described herein wherein the transition-metal based double perovskite is Cs2TiBr6 and the method includes vapor depositing CsBr on a substrate to form a first CsBr film, and contacting the first film with TiBr4 in vapor form in a manner to form a thin film of the Cs2TiBr6.
  • the present disclosure provides a method of making a thin film of a transition-metal based double perovskite as described herein wherein the transition-metal based double perovskite is Cs2TiBr6 and the method includes applying a solution of CsBr and TiBn in solvent to a substrate and heating the substrate to remove the solvent and form a thin film of Cs2TiBr6 on the substrate.
  • the solvent is gamma-butyrolactone, dimethylformamide, dimethyl sulfoxide, N-methyl-2-pyrrolidone, or combinations thereof.
  • the substrate is heated to between 100°C and 300°C.
  • the present disclosure provides a method of making a powder of a transition-metal based double perovskite as described herein wherein the transition-metal based double perovskite is Cs2TiBr6 and the method includes mixing CsBr powder and TiBn powder in a solvent in a 2: 1 molar ratio under conditions to form a solution, and removing the solvent to form a Cs2TiBr6 powder.
  • the solvent is gamma-butyrolactone, dimethylformamide, dimethyl sulfoxide, N-methyl-2-pyrrolidone, or combinations thereof.
  • the solvent is removed by heating the solution to remove the solvent.

Abstract

Disclosed is a family of all-inorganic Ti (IV)-based perovskite materials that can be used in photovoltaic applications. These materials exhibit a double-perovskite crystal structure with a general chemical formula of A2TiX6 (A is K, Rb, Cs and In; X = Cl, Br and I). Specific representative family-members have the formula Cs2TiIxBr6-x (x = 0, 2, 4 and 6). The bandgap of Cs2TiIxBr6-x (0?x?6) can be tuned continuously from 1.02 eV to 1.78 eV. The family members Cs2TiBr6 and Cs2TiI2Br4 exhibit direct bandgaps of 1.78 and 1.38 eV, which are useful for single-junction PSCs and tandem PSCs. The useful thermal/environmental stability of the Ti (IV)-based perovskite materials has been confirmed. Ti-based perovskite solar cells are described herein. The resulting perovskite solar cells show a stable PCE of 2.36 % with a VOC of 0.97 V.

Description

Titanium (IV)-Based Halide Double-Perovskites with Tunable 1.0 to 1.8 eV Bandgaps For Photovoltaic Applications
RELATED APPLICATION DATA
This application claims priority to U.S. Provisional Application No. 62/564,596 filed on September 28, 2017, which is hereby incorporated herein by reference in its entirety for all purposes.
STATEMENT OF GOVERNMENT INTERESTS
This invention was made with government support under DMR- 1420645, DMR- 1305913, and OIA-1538893 awarded by the National Science Foundation. The Government has certain rights in the invention.
BACKGROUND
Halide perovskites (HPs) are a family of semiconductor materials that have shown great promise as light-absorbers in the emerging technology of perovskite solar cells (PSCs). See, Yang, W. S.; Park, B.-W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C; Lee, D. U.; Shin, S. S.; Seo, J.; Kim, E. K.; Noh, J. H.; et al. Iodide management in formamidinium-lead-halide-based perovskite layers for efficient solar cells. Science 2017, 356, 1376-1379.; Eperon, G. E. ; Leijtens, T.; Bush, K. A.; Prasanna, R.; Green, T.; Wang, J. T.-W.; McMeekin, D. P.; Volonakis, G.; Milot, R. L.; May, R. Perovskite-perovskite tandem photovoltaics with optimized band gaps. Science 2016, 354, 861-865; Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. /. Am. Chem. Soc. 2009, 131, 6050-6051.; NREL efficiency chart, world wide website nrel.gov/pv/assets/images/efficiency-chart.jpg. Owing to the exceptional optoelectronic properties of HPs, the record power conversion efficiency (PCE) of PSCs has surpassed 22% within only a few years. See, NREL efficiency chart, world wide website nrel.gov/pv/assets/images/efficiency-chart.jpg. However, current HP materials in the high- PCE PSCs contain toxic lead (Pb) the use of which is restricted in many countries and regions. See, Giustino, F.; Snaith, H. J. Toward lead-free perovskite solar cells. ACS Energy Lett. 2016, 1, 1233-1240; Xiao, Z.; Zhou, Y.; Hosono, H.; Kamiya, T.; Padture, N. P. Bandgap Optimization of Perovskite Semiconductors for Photovoltaic Applications. Chemistry - A European Journal, 2018, DOI: 10.1002/chem.201705031 (in press). Also, current HPs in the highest-performing PSCs contain organic cations, such as formamidinium (FA+) or methylammonium (MA+), (See, Yang, W. S.; Park, B.-W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C; Lee, D. U.; Shin, S. S.; Seo, J.; Kim, E. K.; Noh, J. H.; et al. Iodide management in formamidinium-lead-halide-based perovskite layers for efficient solar cells. Science 2017, 356, 1376-1379.); Eperon, G. E.; Leijtens, T.; Bush, K. A.; Prasanna, R.; Green, T. ; Wang, J. T.-W.; McMeekin, D. P.; Volonakis, G.; Milot, R. L.; May, R. Perovskite -perovskite tandem photovoltaics with optimized band gaps. Science 2016, 354, 861-865.; Abate, A. Perovskite solar cells go lead free. Joule 2017, 1, 1-6) which are highly hygroscopic and volatile which makes them intrinsically unstable, with inadequate tolerance for environmental stresses such as heat and moisture. See, Rong, Y.; Liu, L.; Mei, A.; Li, X.; Han, H. Beyond efficiency: the challenge of stability in mesoscopic perovskite solar cells. Adv. Ener. Mater. 2015, 5, 1501066. Efforts have been made to address these issues. See, Li, Z.; Yang, M. ; Park, J.-S.; Wei, S.-H.; Berry, J. J.; Zhu, K. Stabilizing perovskite structures by tuning tolerance factor: formation of formamidinium and cesium lead iodide solid-state alloys. Chem. Mater. 2015, 28, 284-292; Hao, F.; Stoumpos, C. C; Cao, D. H.; Chang, R. P. H.; Kanatzidis, M. G. Lead-free solid-state organic-inorganic halide perovskite solar cells. Nat. Photon. 2014, 8, 489-494). Replacing MA+ and/or FA+ by cesium (Cs+) in HPs can significantly enhance their thermal/moisture stability. See, Noel, N. K.; Stranks, S. D.; Abate, A.; Wehrenfennig, C; Guarnera, S.;
Haghighirad, A.-A.; Sadhanala, A.; Eperon, G. E.; Pathak, S. K.; Johnston, M. B. Lead-free organic-inorganic tin halide perovskites for photovoltaic applications. Energy Environ. Sci.
2014, 7, 3061-3068. Pb2+ can be replaced by other cations, such as tin (Sn2+) (see, Li, Z. ; Yang,
M.; Park, J.-S.; Wei, S.-H.; Berry, J. J.; Zhu, K. Stabilizing perovskite structures by tuning tolerance factor: formation of formamidinium and cesium lead iodide solid-state alloys. Chem.
Mater. 2015, 28, 284-292; Hao, R; Stoumpos, C. C; Cao, D. H.; Chang, R. P. H.; Kanatzidis,
M. G. Lead-free solid-state organic-inorganic halide perovskite solar cells. Nat. Photon. 2014,
8, 489-494; Kumar, M. H.; Dharani, S.; Leong, W. L.; Boix, P. P.; Prabhakar, R. R.; Baikie,
T.; Shi, C; Ding, H.; Ramesh, R.; Asta, M.; et al. Lead-free halide perovskite solar cells with high photocurrents realized through vacancy modulation. Adv. Mater. 2014, 26, 7122-7127) germanium (Ge2+), (see, Stoumpos, C. C; Frazer, L.; Clark, D. J.; Kim, Y. S.; Rhim, S. H.;
Freeman, A. J.; Ketterson, J. B.; Jang, J. I.; Kanatzidis, M. G. Hybrid germanium iodide perovskite semiconductors: active lone pairs, structural distortions, direct and indirect energy gaps, and strong nonlinear optical properties. /. Am. Chem. Soc 2015, 137, 6804-6819), bismuth (Bi3+), (see, Park, B.; Philippe, B.; Zhang, X.; Rensmo, H. ; Boschloo, G.; Johansson,
E. M. J. Bismuth based hybrid perovskites A3B12I9 (A: methylammonium or cesium) for solar cell application. Adv. Mater. 2015, 27, 6806-6813), antimony (Sb3+), (see, Saparov, B.; Hong,
R; Sun, J.-P.; Duan, H.-S.; Meng, W.; Cameron, S.; Hill, I. G.; Yan, Y.; Mitzi, D. B. Thin-film preparation and characterization of Cs3Sb2¾: A lead-free layered perovskite semiconductor.
Chem. Mater 2015, 27, 5622-5632), indium (In+), (see, Volonakis, G. Haghighirad, A. A.;
Milot, R. L.; Sio, W. H.; Filip, M. R.; Wenger, B.; Johnston, M. B. ; Herz, L. M.; Snaith, H. J.;
Giustino, F. Cs2lnAgCl6: A new lead-free halide double perovskite with direct band dap. /.
Phys. Chem. Lett., 2017, 8, 772-778; Zhou, J.; Xia, Z.; Molokeew, M. S. ; Zhang, X.; Peng, D. ;
Liu, Q. Composition design, optical gap and stability investigations of lead-free halide double perovskite Cs2AgInCl6. /. Mater. Chem. A, 2017, 5, 15031-15037), and silver (Ag+). (See, McClure, E. T.; Ball, M. R.; Windl, W.; Woodward, P. M. Cs2AgBiX6 (X = Br, CI): New visible light absorbing, lead-free halide perovskite semiconductors, Chem. Mater., 2016, 28, 1348-1354; Greul, E.; Petrus, M. L.; Binek, A.; Docampo, P.; Bein, T. Highly stable, phase pure Cs2AgBiBr6 double perovskite thin films for optoelectronic applications. /. Mater. Chem. A 2017, 5, 19972-19981; Slavney, A. H.; Hu, T.; Lindenberg, A. M; Karunadasa, H. I. A bismuth-halide double perovskite with long carrier recombination lifetime for photovoltaic applications. /. Am. Chem. Soc. 2016, 138, 2138-2141). However, prior art Pb-free, all- inorganic HPs have bandgaps that are not suitable for single -junction and/or tandem PV applications with the exception of CsSnb HP with a bandgap of -1.3 eV. See Chung, I.; Song, J.-H.; Im, J.; Androulakis, J.; Malliakas, C. D.; Li, H.; Freeman, A. J.; Kenney, J. T.; Kanatzidis, M. G. CsSnb: semiconductor or metal? High electrical conductivity and strong near-infrared photoluminescence from a single material. High hole mobility and phase- transitions. /. Am. Chem. Soc. 2012, 134, 8579-8587; Wang, N.; Zhou, Y.; Ju, M. G.; Garces, H. F.; Ding, T.; Pang, S.; Zeng, X. C; Padture, N. P.; Sun, X. W. Heterojunction-depleted lead- free perovskite solar cells with coarse-grained B-y-CsSnb thin films. Adv. Energy Mater. 2016, 6, 1601130; Marshall, K. P.; Walker, M.; Walton, R. I.; Hatton, R. A. Enhanced stability and efficiency in hole-transport-layer-free CsSnb perovskite photo voltaics. Nat. Ener. 2016, 1, 16178). But, the development of efficient CsSnb-based PSCs is severely hampered by the metallic conductivity of the CsSnb HP. See, Saparov, B.; Hong, F.; Sun, J.-P.; Duan, H.-S.; Meng, W.; Cameron, S.; Hill, I. G.; Yan, Y.; Mitzi, D. B. Thin-film preparation and characterization of Cs3Sb2¾: A lead-free layered perovskite semiconductor. Chem. Mater 2015, 27, 5622-5632; Wang, N.; Zhou, Y.; Ju, M. G.; Garces, H. F.; Ding, T.; Pang, S.; Zeng, X. C; Padture, N. P.; Sun, X. W. Heterojunction-depleted lead-free perovskite solar cells with coarse-grained B-y-CsSnb thin films. Adv. Energy Mater. 2016, 6, 1601130; Marshall, K. P.; Walker, M.; Walton, R. I.; Hatton, R. A. Enhanced stability and efficiency in hole-transport- layer-free CsSnB perovskite photovoltaics. Nat. Ener. 2016, 1, 16178). Furthermore, Sn2+ is extremely sensitive to moisture and oxygen in the ambient. See, Chung, I.; Song, J.-H.; Im, J.; Androulakis, J.; Malliakas, C. D.; Li, H.; Freeman, A. J.; Kenney, J. T.; Kanatzidis, M. G. CsSnb: semiconductor or metal? High electrical conductivity and strong near-infrared photoluminescence from a single material. High hole mobility and phase -transitions. /. Am. Chem. Soc. 2012, 134, 8579-8587. Preventing CsSnb HP from degrading is much more challenging than its Pb-based HP counterparts. Furthermore, there is a certain level of toxicity in Sn-based HPs, as reported by Babayigit et al. See, Babayigit, A.; Ethirajan, A.; Muller, M.; Conings, B. Toxicity of organometal halide perovskite solar cells. Nature Mater. 2016, 15, 247-251. In this context, the compound Cs2Snl6 with a special double-perovskite crystal structure, containing ordered vacancies due to the +4 oxidation state of Sn, has also been proposed as a candidate for use in PSCs. See, Lee, B.; Stoumpos, C. C; Zhou, N.; Hao, F. ; Malliakas, C; Yeh, C.-Y.; Marks, T. J.; Kanatzidis, M. G.; Chang, R. P. H. Air-stable molecular semiconducting iodosalts for solar cell applications: Cs2Snl6 as a hole conductor. /. Am. Chem. Soc. 2014, 136, 15379-15385. However, Cs2Snl6 is found to exhibit insufficient stability, and it contains intrinsically deep defects that are detrimental to the PSC performance. See, Saparov, B.; Sun, J. -P.; Meng, W.; Xiao, Z.; Duan, H.-S.; Gunawan, O.; Shin, D.; Hill, I. G. ; Yan, Y. ; Mitzi, D. B. Thin- film deposition and characterization of a Sn-deficient perovskite derivative Cs2Snl6. Chem. Mater. 2016, 28, 2315-2322. In fact, many non-toxic transition- metals possess stable +4 oxidation state, and, thus, there are opportunities for finding new promising HP materials for PSCs by replacing Sn4+ in Cs2Snl6 by other transition-metal cations. This is confirmed by a recent study by Sakai et al. (see, Sakai, N.; Haghighirad, A. A.; Filip, M. R.; Nayak, P. K.; Nayak, S.; Ramadan, A.; Wang, Z.; Giustino, F. ; Snaith, H. J. Solution-processed cesium hexabromopalladate(IV), Cs2PdBr6, for optoelectronic applications. /. Am. Chem. Soc. 2017, 139, 6030-6033) who reported Cs2PdBr6 as a possible HP for use in PSCs. However, Cs2PdBr6-based PSCs has not been demonstrated as yet, and it contains the noble element Pd, making it potentially unaffordable.
Accordingly, new perovskite materials are needed that have sufficient performance capabilities to make them useful as perovskite solar cells and other applications.
SUMMARY
The present disclosure provides a transition metal-based double perovskite of the formula A2TX6 wherein A is one or more of K+, Rb+, Cs+, In+, MA+, or FA+; wherein T is Ti, Zr, Hf, V, Nb, Mo, or W; and wherein X is selected from one or more of CI, Br, or I. According to one aspect, T is Ti. According to one aspect, the transition metal-based double perovskite is of the formula A2TBxC6-x wherein A is one or more of K+, Rb+, Cs+, In+, MA+, or FA+; wherein T is Ti, Zr, Hf, V, Nb, Mo, or W; wherein each of B and C is selected from one or more of CI, Br, or I and wherein B and C may be the same or B and C may be different and wherein (x = 0, 1, 2, 3, 4, 5 or 6). According to one aspect, T is Ti.
Methods of making the transition metal-based double perovskites as thin films or powders are disclosed herein. Methods of making photovoltaic devices or solar cells including the transition metal-based double perovskites described herein are provided.
BRIEF DESCRIPTION OF THE DRAWINGS
This application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
The foregoing and other features and advantages of the present invention will be more fully understood from the following detailed description of illustrative embodiments taken in conjunction with the accompanying drawings in which: Fig. 1A shows a schematic crystal structure of A2T1X6 HPs. Fig. IB shows computed bandgaps of various HPs using HSE06 functional. The range of optimal bandgaps for solar- cell materials is highlighted by the horizontal band. Fig. 1C shows computed optical absorption spectra (based on the HSE06 functional) of several predicted HPs, compared with absorption spectra of Si and MAPM3 HP. The absorption coefficient of MAPM3 is computed using the PBE functional without considering the SOC effect. Note that the computed absorption spectrum of MAPM3 is coincidentally in good agreement with the experimental spectrum due to the cancellation of errors by using the PBE functional without considering SOC. Fig. ID shows computed band structure of CS2T1I6 basing on primitive unit cell (Fig. 2A) using PBE functional; here Γ (0.0, 0.0, 0.0), W (0.5, 0.25, 0.75), L (0.5, 0.5, 0.5), X (0.5, 0.0, 0.5) and K (0.375, 0.375, 0.75) refer to the high-symmetry special points in the first Brillouin zone. Fig IE shows computed DOS and projected DOS of CS2T1I6 using HSE06 functional.
Fig. 2A shows a primitive unit cell of CS2T1X6. Fig. 2B shows a computed band structure of CS2T1I6 using PBE functional. The four solid black circles labeled i, ii, iii and iv correspond to highest VB and lowest CB at the Γ and X, respectively. Fig. 2C shows charge density distribution of the highest VB and lowest CB of a periodic slab model of CS2T1I6 HP at the Γ and X.
Fig. 3A shows computed bandgaps of a series of A2T1I6 HP, using the HSE06 functional with SOC. The optimal range of bandgaps is highlighted by two light-blue dotted lines. Fig. 3B shows computed DOS and PDOS of Cs2TiI6 using the HSE06 functional with SOC.
Fig. 4 shows computed AH corresponding to different decomposition pathways for CS2T1I6, Rb2Til6, K2T1I6, and In2Til6 HPs based on the PBE functional.
Figs. 5A-D depict AIMD for A2TiI6 HP at 300K and 500 K. The initial structures (left panels) and snapshots (middle and right panels) of: Fig.5(A) CS2T1I6, Fig.5 (B) Rb2Til6, Fig.5 (C) K2T1I6, and Fig.5 (D) In2Til6, after 5 ps of ab initio molecular dynamic (AIMD) simulations with temperature controlled at 300 K and 500 K, respectively (in NVT ensemble).
Fig. 6A shows allowed values of Ti and I chemical potentials (gray shaded region), which define the thermodynamic stability of CS2T1I6. The chemical potentials μτί, μι, and ucs are limited by the formation of the secondary phases CsLt, Csh, Csl, T1I3, and T1I4. The two solid black circles labeled a and β correspond to the two extreme cases: a (I-rich/Ti-lean, μτι = -3.33 eV, μι = -0.035 eV, μ = -3.61 eV), β (I-lean/Ti-rich, μτί = 0 eV, μι = -1.06 eV, ncs = - 2.20 eV), both selected for calculating the formation energies of a point defect. Computed formation energies of various intrinsic point defects versus the Fermi level in two extreme cases in CS2T1I6: Fig. 6B shows I-rich/Ti-lean and Fig. 6C shows I-lean/Ti-rich.
Figs. 7A-E show details the fabrication of the Cs2TiBr6 HP thin film and PSCs, and photovoltaic performance. Fig. 7A depicts a achematic illustration showing the formation of a uniform Cs2TiBr6 thin film via reaction of CsBr solid-precursor thin film with TiBr4 vapor precursor. Fig. 7B depicts an XRD pattern of the as-prepared Cs2TiBr6 HP thin film. The inset is a SEM image showing the surface morphology of the Cs2TiBr6 HP thin film. Fig. 7C depicts current-voltage (J-V) characteristics of the champion PSC based on Cs2TiBr6 HP thin film. The inset is a cross-sectional SEM image of the PSC. Fig. 7D depicts stabilized PCE output of the PSC at the maximum-power-point voltage extracted from Fig. 7C. Fig. 7E depicts PCE statistics of PSCs made using Cs2TiBr6 HP thin films.
Fig. 8 shows a low magnification SEM image showing the surface morphology of the Cs2TiBr6 HP thin film.
Fig. 9 shows a schematic illustration of the experimental setup for performing the reaction of as-deposited CsBr thin films with TiBr4 vapor for conversion to phase-pure Cs2TiBr6 thin films. Fig. 10 shows the J-V hysteresis of the champion PSC. The extracted PV parameters for reverse scans: 2.36% PCE; 0.97 V Voc; 4.06 mA cm 2 Jsc; 0.600 FF. The extracted PV parameters for forward scans: 2.22% PCE; 0.96 V Voc; 3.94 mA cm 2 Jsc; 0.588 FF.
Fig. 11A depicts experimentally measured XRD patterns of CS2T1I6, Cs2TiLtBr2, Cs2Til2Br4, and Cs2TiBr6 HPs (insets: photographs of the as-synthesized materials). Fig. 11B depicts corresponding higher resolution XRD patterns. Fig. 11 C depicts absorption spectra of Cs2TiI6, Cs2TiI4Br2, Cs2TiI2Br4, and Cs2TiBr6 HPs.
Fig. 12A shows the calculated XRD patterns of CS2T1I6 with Fm-3m space group. Fig. 12B shows the calculated XRD patterns of Cs2TiBr6 with Fm-3m space group.
Fig. 13A shows a Tauc plot of Cs2TiBr6, Fig. 13B shows a Tauc plot of Cs2Til2Br4, Fig. 13C shows a Tauc plot of Cs2TiLtBr2, and Fig. 13D shows a Tauc plot of Cs2Til6HPs, giving bandgaps of 1.78 eV eV, 1.38 eV, 1.15 eV, and 1.02 eV, respectively.
Fig. 14 shows a tauc plot of the MAPM3 HP giving a bandgap of -1.51 eV.
Fig. 15A shows computed DOS of CS2T1I6 and Fig. 15B shows computed DOS of Cs2TiBr6 HPs using the HSE06 functional (with using HSE06 parameter a=0.16) and with consideration of SOC.
Fig. 16A is a schematic crystal structure of mixed-I/Br Cs2TiIxBr6 x HPs. Fig. 16B depicts computed bandgaps of a series of Cs2TiIxBr6-x HPs, using the HSE06 functional (a=0.16) with and without SOC, and experimental data. Fig. 16C depicts computed optical absorption spectra (based on the HSE06 functional, a=0.16) of CS2T1I6, Cs2TiLtBr2, Cs2Til2Br4, and Cs2TiBr6 HPs compared with absorption spectra of Si and MAPM3 HP. Fig. 16D depicts computed band structure of Cs2Til2Br4 HP with space group I4/mmm (139) using PBE functional; here Γ (0.0, 0.0, 0.0), Z (0.5, 0.5, -0.5), X (0.0, 0.0, 0.5), P (0.25, 0.25, 0.25), and N (0.0, 0.5, 0.0) refer to the high-symmetry special points in the first Brillouin zone. Fig. 16E depicts computed DOS and projected DOS of Cs2Til2Br4 HP using the HSE06 functional (0=0.16) with SOC.
Fig. 17A depicts primitive unit cell of Cs2Til2Br4 with different arrangement orders of I and Br sites. Fig. 17B depicts computed band structure of Cs2Til2Br4 HP with space group Imm2 (44) using PBE functional; here T (0.5, -0.5, 0.0), W (0.75, -0.25, -0.25), R (0.5, 0.0, 0.0), Γ (0.0, 0.0, 0.0) and X (0.5, -0.5, 0.5), refer to the high-symmetry special points in the first Brillouin zone.
Figs. 18A-D show the crystal orbital Hamilton population (COHP) analysis of Cs2TiBr6 (Fig. 18A), Cs2TiI6 (Fig. 18B), and Cs2TiI2Br4 HPs (Figs. 18C-D) in which the density of states is partitioned for Ti-Br and Ti-I interactions, with the sign indicating bonding or antibonding character, and the magnitude related to the strength of the interaction. The VBM is set to 0 eV.
Figs. 19A-D provide stability study results for the most representative composition Cs2TiBr6 and Cs2Til2Br4 HPs. The initial structures (left panels) and snapshots (right panels) of Cs2TiBr6 (Fig. 19A) and Cs2TiI2Br4 (Fig. 19C) after 5 ps of AIMD simulations 500 K. Experimentally measured XRD patterns of Cs2TiBr6 (Fig. 19B) and Cs2Til2Br4 HP (Fig. 19D) samples before and after exposure to thermal and moisture stresses. For the thermal stability testing, the samples were annealed at 473 K for 1 h. For the moisture stability testing, the samples were stored at 298 K for 4 h under 70% RH.
Figs. 20A-B show experimentally measured XRD patterns of MAPbbBr (Fig. 20A) and MAPbb (Fig. 20B) HP samples before and after environmental (thermal, moisture) stresses. For the thermal stability testing, the samples were annealed at 473 K for 1 hr. For the moisture stability testing, the samples were stored at 298 K for 4 h under 70% relative humidity (RH). Both MAPbbBr and MAPbb samples are prepared dissolving of the precursor solids in the DMF solvent, followed by precipitation. Fig 21A depicts a schematic illustration of the vapor-based synthesis of Cs2TiBr6 HP thin film. Vapor-deposited CsBr thin film before annealing (Film- 1), after 12-h annealing (Film- II), and after 24-h annealing (Film-Ill) at 200 °C: Fig. 21B depicts XRD patterns and Fig. 21C depicts UV-vis spectra (inset: photograph of the final Cs2TiBr6 thin film). (The XRD pattern of Film-Ill is compared with that of Cs2TiBr6 bulk powder in Fig. 40A-B.) Fig. 21D depicts the corresponding SEM surface morphologies of the thin films. (See Fig. 41for a cross-sectional SEM micrograph of Film-Ill.) Fig. 21 E depicts Ti content as a function of depth in the intermediate thin film after 12-h annealing at 200 °C (Film-II). Fig. 21F depicts the proposed mechanism of the formation of the Cs2TiBr6 HP thin film.
Figs. 22A-D depict surface morphology at top surface (Fig. 22A) and 40 nm from top surface of the intermediate Film-II (Fig. 22C). Corresponding EDS Ti elemental maps of Ti: top surface (Fig. 22B) and 40 nm from film top surface (Fig. 22D). Scale bars = 5 μπι.
Fig. 23 depicts Ti content as a function of depth in the Cs2TiBr6 HP thin film after 24- h annealing at 200 °C (Film-Ill).
Fig. 24 depicts surface morphology for the thin film annealed at a higher temperature (230°C) for 24 hours. Scale bar = 1 μιη.
Fig. 25A depicts XRD patterns and Fig. 25B depicts corresponding SEM micrographs of top surfaces of thin films annealed at a lower temperature (150 °C) for: as-deposited CsBr thin film (top), 12 h (middle), and 24 h (bottom). Scale bars = 1 μπι.
Fig. 26 depicts an SEM image of top surface of a Cs2TiBr6 HP thin film used for estimating the average grain size employing image analysis. Scale bar = 1 μπι.
Fig. 27 depicts an AFM scan of top-surface of a Cs2TiBr6 HP thin film on TiC -coated glass substrate.
Fig. 28A depicts a Tauc plot and PL spectrum of the Cs2TiBr6 HP thin film. Fig. 28B depicts AFM film topography with overlaid PL intensity map of the Cs2TiBr6 HP thin film. Time-resolved PL decay of the Cs2TiBr6 HP thin film: Fig. 28C depicts with and without TiC electron-quencher and Fig. 28D depicts with and without P3HT hole-quencher. Fig. 28E depicts UPS spectrum of the Cs2TiBr6 HP thin film. Fig. 28F depicts a schematic illustration of the energy-level diagram of the Cs2TiBr6 HP thin film.
Figs. 29A-B depict PL spectra from Cs2TiBr6/glass, Cs2TiBr6/Ti02/glass, and Cs2TiBr6/C60/TiO2/glass (Fig. 29 A) and glass/Cs2TiBr6 and glass/Cs2TiBr6/P3HT (Fig. 29B). Insets: schematic illustrations of the layers.
Figs. 30A-B depict computed properties of Cs2TiBr6 HP using the PBE functional: band structure (Fig. 30A) and DOS and PDOS (Fig. 30B).
Fig. 31 A depicts a schematic illustration of PSC architecture using the Cs2TiBr6 HP thin film as the light absorber. Fig. 3 IB depicts energy levels diagram in the PSC. Fig. 31C depicts J-V curves at both forward (hollow circles) and reverse (solid circles) scans of the best PSCs without and with the presence of the C60 interfacial layer. Fig. 3 ID depicts stabilized PCE output at the maximum power points of the PSCs without and with the presence of the C6o interfacial layer. Fig. 3 IE depicts EQE spectrum of the best PSC with the presence of the C6o interfacial layer. Fig. 31F depicts PCE statistics of the PSC with the presence of the C6o interfacial layer.
Fig. 32 depicts absorbance spectra for Cs2TiBr6, P3HT and Cs2TiBr6/P3HT thin films. Cs2TiBr6 HP thin films were prepared on a glass substrate, and the P3HT layer was deposited by spin-coating a solution of P3HT in toluene (10 mg/ml) at 3000 rpm, 30 s.
Figs. 33A-C depict photoconductive-AFM (pcAFM) mapping of Cs2TiBr6 HP and P3HT thin films. Fig. 33A depicts a schematic illustration of the basic setup for pcAFM measurements. The LED source of 567 nm wavelength is used to stimulate photocurrent in the thin film. Topology (left) and photo-response (right) of thin films with light on and light off:
Cs2TiBr6 HP (Fig. 33B) and P3HT (Fig. 33C). Fig. 34 depicts an SEM image of top surface of an as-deposited CsBr thin film on a C6o-covered substrate. Scale bar = 1 μπι.
Fig. 35 depicts an AFM scan of top surface of a Cs2TiBr6 HP thin film on C6O/T1O2- coated glass substrate.
Fig. 36 depicts thermogravimetric analysis (TGA) of Cs2TiBr6 HP powder sample.
Figs. 37A-C depict XRD patterns of before and after heat (200 °C, 6 h, N2 atmosphere), light (one-sun, encapsulated), and moisture (23 °C, 80% RH, 6 h) stresses for Cs2TiBr6 HP thin film (Fig. 37A) and reference MAPbbBr OIHP thin film (Fig. 37B). Fig. 37C depicts evolution of PCE of the best Cs2TiBr6-based PSC (unencapsulated) as function of the storage time under environmental stress (70 °C, 30% RH, ambient light).
Fig. 38 depicts XRD patterns of Cs2TiBr6 HP thin film after 24-hour annealing (200 °C, N2 atmosphere).
Fig. 39 depicts evolution of PCE of the Cs2TiBr6-based PSC (unencapsulated) as a function of the storage time under environmental stress (70 °C, 30% RH, one-sun AM 1.5G illumination).
Fig. 40A is an XRD pattern of a thin film (red) of the Cs2TiBr6 HP. Fig. 40B is an XRD pattern of a bulk powder (control sample; blue) of the Cs2TiBr6 HP.
Fig. 41 depicts a typical cross-sectional SEM micrograph of the in the Cs2TiBr6 HP thin film.
Fig. 42 depicts an XPS spectrum for Ti 2p in an as-prepared Cs2TiBr6 HP thin film.
DETAILED DESCRIPTION
Aspects of the present disclosure are directed to titanium (Ti)-based perovskite materials useful in perovskite solar cells and other applications and for methods of making such perovskite materials and solar cells. The titanium (Ti)-based perovskite materials described herein have a desirable band gap and other optoelectronic properties making them useful in the manufacture of solar cells.
According to one aspect, the present disclosure describes titanium-based double perovskite compounds or materials, such as vacancy-ordered halide double perovskites based on Ti(IV) for use in perovskite solar cells. According to one aspect, the titanium-based double perovskite compound is represented by the formula
A2TiX6
wherein A is selected from one or more of K+, Rb+, Cs+, In+, MA+, or FA+; wherein X is selected from one or more of CI, Br, or I. Exemplary titanium-based double perovskites include CS2T1I6, Rb2Til6, K2T1I6, and In2Til6. Exemplary titanium-based double perovskites lack an organic moiety. Exemplary titanium-based double perovskites are inorganic.
Aspects of the present disclosure are directed to titanium-based double perovskite compounds or materials represented by the formula
A2TiBxC6-x
wherein A is selected from one or more of K+, Rb+, Cs+, In+, MA+, or FA+; wherein each of B and C is selected from one or more of CI, Br, or I and wherein B and C may be the same or B and C may be different and wherein x = 0, 1, 2, 3, 4, 5, 6). Exemplary titanium- based double perovskites are represented by Cs2TiIxBr6-x wherein x = 0, 1, 2, 3, 4, 5 or 6 and include Cs2Til2Br4 and Cs2TiBr6. Such titanium-based double perovskites of Cs2TiIxBr6-x (0 < x < 6) can be tuned continuously from 1.02 eV to 1.78 eV with exemplary bandgaps of 1.78 and 1.38 eV, Exemplary titanium-based double perovskites lack an organic moiety. Exemplary titanium-based double perovskites are inorganic.
According to one aspect, the titanium-based double perovskite compounds or materials described herein exhibit desirable optical and/or electronic and/or stability properties as visible- light absorber materials for PV applications. Despite the presence of ordered B-site vacancies, the band structures of titanium-based double perovskites described herein show fairly dispersive conduction. The titanium-based double perovskites described herein exhibit a continuously tuned band gap of between 0.9 eV to 1.8 eV. Desirable bad gaps of between 1.0 eV to 1.8 eV can be achieved such as bandgaps of -1.38 eV and -1.78 eV, which are useful for single -junction perovskite solar cell and tandem photovoltaic application. The titanium- based double perovskites described herein exhibit high intrinsic/environmental stability, superior to the Pb-containing halide perovskites, while avoiding the toxicity issues of Pb- containing halide perovskites. The titanium-based double perovskites described herein exhibit stable, respectable power conversion efficiencies and open circuit voltages useful for perovskite solar cells.
According to one aspect, double perovskite compounds or materials are provided where the titanium is substituted by a transition metal such as Zr, Hf, V, Nb, Mo, or W. Such transition metal-based double perovskite compounds and materials have desirable band gap or other optical or electronic or other properties for photovoltaic or other applications. Such transition metal based double perovskite compounds and materials are represented by the formula
A2TX6
wherein A is selected from one or more of K+, Rb+, Cs+, In+, MA+, or FA+; wherein T is selected from Ti, Zr, Hf, V, Nb, Mo, or W; and wherein X is selected from one or more of CI, Br, or I. Such transition metal based double perovskite compounds and materials are represented by the formula wherein A is selected from one or more of K+, Rb+, Cs+, In+, MA+, or FA+; wherein T is selected from Ti, Zr, Hf, V, Nb, Mo, or W; wherein each of B and C is selected from one or more of CI, Br, or I and wherein B and C may be the same or B and C may be different and wherein (x = 0, 1, 2, 3, 4, 5, or 6).
According to one aspect, transition metal-based double perovskites such as titanium-based double perovskites are made into powders or are made into thin films through inter-diffusion reaction of their precursors. Such powders have uses for coatings. Such thin films are incorporated into optical or photovoltaic devices. Exemplary photovoltaic or solar devices include electron-transporting layers sandwiched between the thin film perovskite and cathode, and hole-transporting layers sandwiched between the thin film perovskite and anode.
EXAMPLE I
Vacancy Ordered Titanium-based Double Perovskite Materials
Like Cs2Snl6, the optimized structures of vacancy-ordered A2BX6 halide perovskites described herein are K2PtCl6-type (see Fig. 1A). See, Lee, B.; Stoumpos, C. C; Zhou, N. ; Hao, F. ; Malliakas, C ; Yeh, C.-Y. ; Marks, T. J. ; Kanatzidis, M. G. ; Chang, R. P. H. Air-stable molecular semiconducting iodosalts for solar cell applications: Cs2Snl6 as a hole conductor. /. Am. Chem. Soc. 2014, 136, 15379-15385; Maughan, A. E.; Ganose, A. M. ; Bordelon, M. M. ; Miller, E. M. ; Scanlon, D. O. ; Neilson, J. R. Defect tolerance to intolerance in the vacancy- ordered double perovskite semiconductors Cs2Snl6 and Cs2Tel6. /. Am. Chem. Soc. 2016, 138, 8453-8464). The A2BX6 halide perovskites may be viewed as a derivative structure of the conventional ABX3 halide perovskites in which every other B2+ cation is missing. The interrupted solid-state framework results in isolated [ΒΧβ]2" octahedra and discrete anions, akin to molecular salts. See, Lee, B. ; Stoumpos, C. C ; Zhou, N. ; Hao, F.; Malliakas, C; Yeh, C- Y. ; Marks, T. J. ; Kanatzidis, M. G. ; Chang, R. P. H. Air-stable molecular semiconducting iodosalts for solar cell applications: Cs2Snl6 as a hole conductor. /. Am. Chem. Soc. 2014, 136, 15379-15385. As light-absorbing materials in photovoltaics, the bandgaps for titanium-based double perovskites described herein were computed (see Fig. IB). For the same A-site cation, the bandgap increases gradually in the order I<Br<Cl, which is consistent with the bandgap trend seen in MAPbX3 (X=I, Br, CI) HPs. See, Abate, A. Perovskite solar cells go lead free. Joule 2017, 1, 1-6. It is also observed that the bandgap becomes wider with increasing A-site cation radius, suggesting that the tilting of the [ΒΧβ]2" octahedron dictated by the A-site cation can markedly affect the electronic structures of the titanium-based double perovskites. The bandgap of the titanium-based double perovskites can be tuned via substitution of both the A- site and the X-site elements to meet the desired value. According to certain aspects, the titanium-based double perovskites can include an organic cation, MA+ or FA+, to occupy the A-site in A2T1I6 halide perovskites, which are oriented in [001] and [100] direction, respectively. MA2T1I6 and FA2T1I6 halide perovskites exhibit bandgaps of 1.67 eV and 1.70 eV, respectively. Other inorganic Α2ΊΊΙ6 halide perovskites such as K2T1I6, Rb2Til6, CS2T1I6, and In2Til6, exhibit bandgaps of 1.55 eV, 1.62 eV, 1.65 eV, and 1.40 eV, respectively. Although Rb2Til6 and CS2T1I6 possess slightly larger bandgap beyond the optimal region (0.9-1.6 eV), both halide perovskites are useful for certain photovoltaic applications.
EXAMPLE II
Optical Absorption Spectrum of Titanium-based Double Perovskite Materials
The optical absorption spectra of the titanium-based double perovskites described herein were computed, and compared with the absorption spectra of two photovoltaic materials, Si and MAPM3 halide perovskite, as shown in Fig. 1C. Compared with MAPM3, all the Pb- free halide perovskite displayed relatively lower absorption intensity in the visible region, but higher absorption in the lower-energy range. Titanium-based double perovskites described herein show favorable absorption behavior (with absorption coefficient >1.4xl05 cm 1) making them useful in perovskite solar cells. Figs. ID and IE show the computed band structures and density of state (DOS) of CS2T1I6. CS2T1I6 possesses an indirect fundamental bandgap between Γ (VBM) and X (CBM) and a direct bandgap at X, slightly larger than the indirect gap by about 30 meV. The band structure is similar to that of Cs2PdBr6, (See, Sakai, N. ; Haghighirad, A. A. ; Filip, M. R.; Nayak, P. K.; Nayak, S.; Ramadan, A.; Wang, Z.; Giustino, F.; Snaith, H. J. Solution-processed cesium hexabromopalladate(IV), Cs2PdBr6, for optoelectronic applications. /. Am. Chem. Soc. 2017, 139, 6030-6033) which may be considered as a quasi- direct band gap. An analysis of the orbital character of CS2T1I6 indicates that the highest valence bands (VBs) are contributed mostly by the I 5p orbital, while the lowest conduction bands (CBs) are contributed by the Ti 3d orbital at Γ and X (see Fig. 2A-C). Similar to MAPM3, the A-site cation Cs does not contribute to the highest VB and lowest CB at Γ and X.
EXAMPLE III
Spin Orbital Coupling Effects on Titanium-based Double Perovskite Materials
For Pb-containing HPs, spin-orbit coupling (SOC) can lower the computed bandgap significantly. See, Mosconi, E.; Amat, A.; Nazeeruddin, M. K.; Gratzel, M.; De Angelis, F. First-principles modeling of mixed halide organometal perovskites for photovoltaic applications. /. Phys. Chem. 2013, 117, 13902-13913.); Even, J. ; Pedesseau, L.; Jancu, J. ; Katan, C. Importance of spin-orbit coupling in hybrid organic/inorganic perovskites for photovoltaic applications. J Phys. Chem. Lett. 2013, 4, 2999-3005. In contrast, the effect of SOC is quite small on the computed bandgap of CS2T1I6 (see Fig. 3A-B): the computed bandgap is only narrowed slightly to 1.50 eV when SOC is considered. In fact, with SOC, the computed bandgaps are reduced slightly in titanium-based double perovskites. Without wishing to be bound by scientific theory, spin orbital coupling becomes less important in materials containing light elements (Ti, Z=22), as compared to those containing Pb (Z=82). As the radius of A-site atom decreases, the bandgap decreases in the order In2Til6<K2Til6<Rb2Til6<Cs2Til6. The highest VB and lowest CB are contributed mainly by cation B and anion X. The A-site element does not contribute directly to the electronic properties of titanium-based double perovskites, but can indirectly affect the electronic properties via altering the tilting of the [ΒΧβ]2" octahedra and the distance between adjacent octahedra.
EXAMPLE IV
Structural Stability of Titanium-based Double Perovskite Materials
To assess the structural stability of titanium-based double perovskites, decomposition energies were calculated with respect to several possible decomposition pathways. See, Zhao, X.-G. G. ; Yang, J.-H. H.; Fu, Y.; Yang, D. ; Xu, Q.; Yu, L. ; Wei, S.-H. H.; Zhang, L. Design of lead-free inorganic halide perovskites for solar cells via cation-transmutation. /. Am. Chem. Soc. 2017, 139, 2630-2638. Since MAPbI3, CsSnI3, and CsPbI3 halide perovskites can be synthesized successfully using the corresponding binary halides (MAI, Pb , Csl, Sn , etc.), the same approach was used to synthesize titanium-based double perovskites. The predominant decomposition pathways are the reverse reactions of the corresponding synthesis routes, while there may be ternary products involved in other decomposition pathways. The decomposition enthalpy of different decomposition pathways is defined as:
S¾,BL] or -
A positive value of AH represents energy gain from the formation of decomposition products from the corresponding titanium-based double perovskites. Fig. 4 shows the calculated AH values for the four titanium-based double perovskites with different decomposition processes. The titanium-based double perovskites exhibit good stability with positive AH values. CS2T1I6 and Rb2Til6 possess robust thermodynamic stability due to the fairly large positive values of AH>50 meV/atom. Moreover, ab initio molecular dynamic (AIMD) simulations were performed to examine the thermal stability of the four titanium- based double perovskites. Fig. 5A-D shows snapshots of the four titanium-based double perovskites at initial time and 5 ps after the AIMD simulations, respectively. In the AIMD simulations, the temperature is maintained at 300°K or 500°K. The overall framework of the titanium-based double perovskites is sustained in the final configuration at 300°K or 500°K. When the temperature increases from 300°K to 500°K, the [ΒΧβ]2" octahedra in the final configuration appear to have higher degrees of freedom, such as tilting. Titanium-based double perovskites have good intrinsic stability up to very high temperatures, consistent with the analyses based on the decomposition energies.
EXAMPLE V
Defect Formation Energies of Titanium-based Double Perovskite Materials
Defect formation energies (see, Walsh, A.; Zunger, A. Instilling defect tolerance in new compounds. Nat. Mater. 2017, 16, 964-967.); Yin,W.-J.; Shi, T.; Yan, Y. Unusual defect physics in CHsNHsPbfc perovskite solar cell absorber. Appl. Phys. Lett. 2014, 104, 63903) important in light absorbers in photovoltaics are calculated to assess the transport and optical properties of CS2T1I6. 12 possible intrinsic point defects were considered in CS2T1I6 HP, including three types of vacancies (Vi, V-n, and Vcs), three types of interstitials (I;, Ti;, and Cs;), two cation substitutions (Tics and CSTO, and four antisite substitutions (hi, Ics, Tii, and Csi). Considering the thermodynamic equilibrium growth conditions, the formation energies of point defects are mostly dependent on the chemical potentials of the host elements, μι, μπ, and ucs- A moderate chemical potential region is identified for achieving thermodynamically stable CS2T1I6. The identified chemical-potential region for CS2T1I6 is highlighted in gray in Fig. 6A. To calculate the formation energies, two extreme cases marked by two solid black circles in Fig. 6A were considered: (a) I-rich/Ti-lean and (β) I-lean/Ti-rich. Under the I-rich and Ti-lean conditions, Vi has the lowest formation energy, consistent with acting as a recombination center for a photo-generated electron-hole pair (see Fig. 6B). The acceptors In, Ii, and Vcs also have relatively low formation energies, which due to the neutral charge state in the bandgap, do not affect the electronic and photovoltaic properties significantly. The Fermi level induced by compensation of charged acceptor and donor defects is pinned slightly below the CB minimum (CBM), resulting in «-type conductivity, as is in the case of T1O2. Under I-lean/Ti-rich conditions, the Cs; and Ti; have the lowest formation energy, and are the dominant defects. However, due to the resulting deep defect states, both defects may be more detrimental to photovoltaic performance than the defects under I-rich/Ti-lean conditions (see Fig. 6C). The Fermi level is pinned above the CBM, indicating degenerate «-type conductivity in CS2T1I6. Under I-rich/Ti-lean growth conditions, the deep-level defects can be suppressed in the synthesis of CS2T1I6 HP.
EXAMPLE VI
Manufacture of Titanium-based Double Perovskite Materials
Aspects of the present disclosure are directed to methods of making transition metal- based double perovskite materials, such as titanium-based double perovskite compounds and materials in powder form, such as for use in making coatings, or thin film form, such as for use in making photovoltaics or solar cells. To support the theoretical predictions, a series of Cs2TiIxBr6-x (x=0, 2, 4, 6) titanium-based double perovskites as described herein were synthesized into thin films and photovoltaic devices using the melt-crystallization method. As shown in Fig. 7(A), a sequential-deposition method was used to deposit high-quality Cs2TiBr6 HP thin films. In general, a uniform CsBr layer was first deposited by vacuum evaporation. The CsBr layer was then annealed in TiBr4 vapor at 473°K for 24 hours to form the Cs2TiBr6 thin film. In Fig. 7(B), the XRD pattern of the as-fabricated Cs2TiBr6 thin film confirms the phase purity, and the inset SEM image shows the excellent uniformity of the thin film. Fig. 8 shows a lower-magnification SEM image of the Cs2TiBr6 HP thin film, indicating its pinhole- free nature over large areas.
According to a more particular aspect, all the raw chemicals were purchased from Sigma Aldrich (USA). For synthesizing Cs2TiIxBr6-x HP powders, stoichiometric amounts of Csl, CsBr, and T1I4, TiBr4 were loaded in quartz tubes and evacuated to -10-6 Torr and sealed using an oxy-methane torch. The evacuated tubes were heated to 700°C at 10oC.min_1 and held for 72 hours before slowly cooling them to room temperature at 10°C min 1. The sealed tubes were opened in a glovebox filled with nitrogen gas for further characterization/testing.
According to one aspect, for the fabrication of Cs2TiBr6 thin films, a CsBr layer was deposited on the substrate by thermal evaporation. The CsBr thin film was then placed in a closed chamber filled with TiBr4 vapor for 24 hours as illustrated in Fig. 9. The TiBr4 vapor was generated by heating TiBr4 powders at 473°K. After full conversion of CsBr to phase-pure Cs2TiBr6, the thin film was washed by toluene for subsequent characterization and device fabrication.
According to one aspect, methods are provided for the solution processing of Cs2TiBr6. Cs2TiBr6 perovskite solution was synthesized by reaction between mixed solid powder precursors CsBr:TiBr4 (2:1 molar ratio) dissolved in gamma-butyrolactone (GBL) solvent. Other suitable solvents can be determined by those of skill in the art such as dimethylformamide, dimethyl sulfoxide, N-methyl-2-pyrrolidone, and combinations thereof. The mixture was stirred in GBL solvent for 12 hours at 150°C to obtain a clear solution. This solution can be used for spin-coating, spraying, slot-die coating, or infiltrating into a porous preform, etc. to form a Cs2TiBr6 perovskite thin film on a substrate. It is heated to between 100°C and 300°C, such as 200°C, to form the perovskite thin film. Alternatively, the solution can be dried by distillation at between 100°C and 300°C, such as 200°C, to result in Cs2TiBr6 perovskite powder. EXAMPLE VII
Manufacture of Titanium-based Double Perovskite Solar Cells
According to one aspect, titanium-based double perovskite solar cells were made by sandwiching the Cs2TiBr6 thin films between a T1O2/FTO anode and poly(3-hexylthiophene) (P3HT)/Au cathodes. Cross-section of the Ti-based PSC is shown in Fig. 7(C) inset, where each layer can be differentiated. The current-voltage (J-V) curve of the champion perovskite solar cell is also shown in Fig. 7(C), which exhibits an overall power conversion efficiency of 2.36 % with an open circuit voltage (VOC) of 0.97 V, a short circuit current density (JSC) of 4.06 mA cm-2 and a fill factor (FF) of 0.600. Notably, the VOC value has surpassed reported lead-free perovskite solar cells. See, Marshall, K. P., Walker, M., Walton, R. I. & Hatton, R. A. Enhanced stability and efficiency in hole-transport-layer-free CsSnB perovskite photo voltaics. Nat. Energy 1, 16178 (2016); Noel, N. K. et al. Lead-Free Organic-inorganic tin halide perovskites for photovoltaic applications. Energy Environ. Sci. 7, 3061-3068 (2014); Hao, F., Stoumpos, C. C, Cao, D. H., Chang, R. P. H. & Kanatzidis, M. G. Lead-free solid- state organic-inorganic halide perovskite solar cells. Nat. Photon. 8, 489-494 (2014); Wang, N. et al. Heterojunction-depleted lead-free perovskite solar cells with coarse-grained Β-γ- CsSnB thin films. Adv. Energy Mater. 6, 1601130 (2016).
Furthermore, only very slight hysteresis is observed between reverse and forward J-V scans, as shown in Fig. 10, and, thus, the stabilized PCE output (see the inset of Fig. 7(D)) quickly reaches a stable PCE of -2.28%, a value very close to the extracted PCE from the J-V scan. These results clearly demonstrate the facile processability of Cs2TiBr6 HP thin films and perovskite solar cell device fabrication. According to certain aspects, other Ti-based double perovskite thin films or solar cells with a broad range of other compositions can be processed using ion-substitution strategies. See, McMeekin, D. P. et al. A mixed-cation lead mixed- halide perovskite absorber for tandem solar cells. Science 351, 151-155 (2016); Jacobsson, J. T. et al. Exploration of the compositional space for mixed lead halogen perovskites for high efficiency solar cells. Energy Environ. Sci. 9, 1706-1724 (2016); Zhou, Y. et al. Exceptional Morphology-Preserving Evolution of Formamidinium Lead Triiodide Perovskite Thin Films via Organic-Cation Displacement. J. Am. Chem. Soc. 138, 5535-5538 (2016).
According to a more particular aspect for fabrication of perovskite solar cells, a compact-TiC electron selective layer (ESL) was first deposited on pre-patterned FTO-coated glass by spray pyrolysis at 450°C. A titanium-based double perovskite layer was then deposited, followed by spin-coating a hole-transporting material (HTM) solution, which consisted of 10 mg P3HT and 1 ml toluene solvent. Finally, a Au layer was deposited using a thermal evaporator and a shadow mask. The external quantum efficiency (EQE) spectra were conducted in AC mode on a solar cell quantum efficiency measurement system (EQE-200, Oriel Instruments, USA). The J-V characteristics of the PSCs were measured using a 2400 Sourcemeter (Keithley, USA) under simulated one-sun AM 1.5G 100 mW cm 2 intensity (Oriel Sol3A Class AAA, Newport, USA), under both reverse (from VOC to JSC) and forward (from JSC to VOC) scans. The step voltage was 5 mV with a 10 ms delay time per step. The maximum-power output stability of the PSCs was measured by monitoring the J output at the maximum-power V bias (deduced from the reverse-scan J-V curves) using the 2400 SourceMeter. The J output is converted to PCE output using the following relation: PCE = {J (mA cm-2) X V (V))/(100 (mW cm 2)}. A shutter was used to control the one-sun illumination on the PSC. A typical active area of 0.12 cm2 was defined using a non-reflective mask for the J-V measurements.
EXAMPLE VIII
Measured Bandgaps of Titanium-based Double Perovskite Materials
For the end members, CS2T1I6 and Cs2TiBr6 or formula Cs2TiIxBr6-x (x=6 or 0), Fig. 11 A and Figs. 12A-B show, respectively, the experimental and calculated X-ray diffraction (XRD) patterns of CS2T1I6 and Cs2TiBr6, confirming their phase purity. The Tauc plot of the CS2T1I6 and Cs2TiBr6 samples are shown in Fig. 13D and 13 A. The measured optical bandgaps of the phase-pure CS2T1I6 and Cs2TiBr6 HPs are -1.02 and -1.78 eV, respectively. The bandgap of CS2T1I6 is within the optimal bandgap range (0.9-1.6 eV) for single-junction perovskite solar cells. See, Loferski, J. J. Theoretical considerations governing the choice of the optimum semiconductor for photovoltaic solar energy conversion. /. Appl. Phys. 1956, 27, 777-784. The optical bandgap (-1.51 eV) of MAPbL was also measured independently as a benchmark (see Fig. 14). The measured bandgaps are slightly smaller than the computed HSE06 bandgaps. It is known that the hybrid functional HSE06 includes a fraction parameter a (default value 0.25) for the screened short-range Hartree-Fock exchange to improve the derivative discontinuity of the Kohn-Sham potential for integer electron numbers. For certain compounds, a parameter should be adjusted to yield more accurate bandgap. Using T1O2 as an example, when the value of a is set at 0.20 or 0.21, the computed bandgap is in agreement with measured bandgap. See, Janotti, A.; Varley, J. B.; Rinke, P.; Umezawa, N. ; Kresse, G.; Van de Walle, C. G. Hybrid functional studies of the oxygen vacancy in T1O2. Phys. Rev. B 2010, 81, 85212. When a value is set at 0.16, the computed bandgaps of CS2T1I6 and Cs2TiBr6 are -1.05 eV and -1.89 eV, respectively, which are in closer agreement with the measured bandgaps (see Figs. 15A-B).
EXAMPLE IX
Tunable Bandgaps of Titanium-based Double Perovskite Materials
The Shockley-Queisser limit (see, Shockley, W. ; Queisser, H. J. Detailed balance limit of efficiency of p-n junction solar cells. /. Appl. Phys. 1961, 32, 510-519) suggests a bandgap of -1.3 eV for achieving maximum power conversion efficiency in a single p-n junction solar cell. According to certain aspects of the present disclosure, titanium-based double perovskites are described where the bandgap is tuned using doping or alloying strategies. See, Zong, Y. ; Wang, N.; Zhang, L.; Ju, M.-G.; Zeng, X. C; Sun, X. W.; Zhou, Y.; Padture, N. P. The formation of homogenous FAPbl3-CsSnl3 alloys for efficient, ideal-bandgap perovskite solar cells. Angew. Chem. 2017, 56, 12658-12662; Xiao, J.; Liu, L.; Zhang, D.; De Marco, N.; Lee, J.; Lin, O.; Chen, Q.; Yang, Y. The emergence of the mixed perovskites and their applications as solar cells. Adv. Ener. Mater. 2017, 7, 1700491. Since B4+ and X" contribute to the CB and the VB, the bandgap can be tuned by replacing either B4+ or X". The replacement of X" may be achieved more easily experimentally. Mixed-I/Br titanium-based double perovskites Cs2Til2Br4 and Cs2TiLtBr2 were synthesized and their properties were investigated experimentally and computationally. As shown in Fig. 11 A, the XRD patterns for Cs2TiLtBr2 and Cs2Til2Br4, and the end members, CS2T1I6, and Cs2TiBr6, exhibit similar features because these titanium-based double perovskites are isostructural solid-solutions. The 222 reflection shifts to higher 2Θ with decreasing I/Br ratio (see Fig. 11B), as the gradual substitution of the larger I" with the smaller Br shrinks the lattice parameter. The simulated XRD results for CS2T1I6 and Cs2TiBr6 confirm the shift, and are in good agreement with the experimental results (see Figs. 12A-B). The lattice parameter, a, of CS2T1I6 Cs2TiLtBr2, Cs2Til2Br4, and Cs2TiBr6 are estimated at 11.67 A, 11.43 A, 11.25 A, and 10.92 A, respectively, and all crystals possess Fm-3m space group. The insets in Fig. 11A are photographs of the corresponding as- synthesized Cs2TiIxBr6-x titanium-based double perovskites. It can be seen that the color of the HPs becomes lighter as the I/Br ratio decreases. The experimentally measured bandgaps of the corresponding materials decrease almost linearly (see Figs. 13A-D). The absorption spectra of CS2T1I6, Cs2TiLtBr2, Cs2Til2Br4, and Cs2TiBr6 HPs in Fig. 11C show significant shift to higher energy levels in the visible region, consistent with the bandgap trend.
Fig. 16A depicts schematically the crystal structure of the mixed-I/Br Cs2TiIxBr6 x titanium-based double perovskites. The calculated bandgaps are in reasonable agreement with the experimental results (see Fig. 16B). Table 1 below shows computed bandgaps of a series of Cs2TiIxBri-x HPs using HSE06 functional without and with spin-orbital coupling (SOC). Experimentally measured bandgaps are also included.
Table 1
Figure imgf000028_0001
The bandgaps can be reduced to about 0.1 eV considering the SOC effect. When the I/Br ratio exceeds 0.5, the calculated bandgap values of the titanium-based double perovskites no longer decrease linearly. This is due to the different arrangement orders of I and Br sites. By calculating bandgaps of Cs2TiLtBr2 with different arrangement orders, there is a difference of about 0.25 eV in the bandgap values. Thus, based on the experimental measurements and calculations, the Cs2Til2Br4 titanium-based double perovskite has a direct bandgap of 1.38 eV, which is closest to the ideal bandgap (1.34 eV), making it an exemplary candidate for the light- absorber layer in single -junction perovskite solar cells. According to one aspect, the Cs2TiBr6 titanium-based double perovskite can be an optimal large-bandgap material (1.8 eV) for application in the tandem photovoltaics in conjunction with the low-bandgap (1.1-1.2 eV) materials such as Si. See, Eperon, G. E. ; Leijtens, T.; Bush, K. A.; Prasanna, R.; Green, T.; Wang, J. T.-W.; McMeekin, D. P.; Volonakis, G.; Milot, R. L.; May, R. Perovskite-perovskite tandem photovoltaics with optimized band gaps. Science 2016, 354, 861-865. This bandgap tunability also enables the realization of specific value of bandgap for extending the absorption of sunlight, compared to the current Pb-containing halide perovskite absorber layers.
Fig. 16C shows computed absorption spectra of a series of titanium-based double perovskites, compared with computed absorption spectra of Si and MAPM3 halide perovskites. With increasing I/Br ratio the absorption is continuously red-shifted, consistent with the trend of bandgap change. The experimental UV-v« absorbance spectra and Tauc plots (see Figs. 11C and Figs. 13A-D) also confirm this result. The maximum absorption coefficient in the visible- light range decreases, proportional to the decrease of I-content in the HPs. According to one aspect, I-based titanium-based double perovskites tend to yield better absorption than Br-based titanium-based double perovskites. The band structures of Cs2Til2Br4 with different arrangement of the I and Br sites were computed (see Fig. 16D and Fig. 17A-B). Two structures were considered with space groups l4/mmm and lmm2. The structure with 14/mmm space group is slightly more stable than that with Imm2 space group (-30 meV). The structure with 14/mmm space group possesses a direct bandgap. In contrast, the structure with Imm2 space group shows a quasi-direct bandgap, which is found to be similar to that in CS2T1I6 (see Fig. 17B). From the computed DOS of Cs2Til2Br4, it can be seen that the highest VBs are mostly contributed by the I 5p orbitals, while the lowest CBs are mostly contributed by the Ti 3d orbitals (see Fig. 16E).
EXAMPLE X
Isotropic Electron and Hole Effective Masses of Titanium-based Double Perovskites
The isotropic electron and hole effective masses at the X and Γ point, respectively, were calculated using PBE functional. Table 2 below shows effective masses calculated using PBE functional for Cs2TiBr6 and CS2T1I6 HPs. Both the calculated heavy (h) and light (I) hole effective masses are reported. The effective masses are isotropic. Table 2
Figure imgf000030_0001
Similar to Cs2PbBr6, (see, Sakai, N. ; Haghighirad, A. A.; Filip, M. R. ; Nayak, P. K. ; Nayak, S. ; Ramadan, A.; Wang, Z. ; Giustino, F. ; Snaith, H. J. Solution-processed cesium hexabromopalladate(IV), Cs2PdBr6, for optoelectronic applications. /. Am. Chem. Soc. 2017, 139, 6030-6033) for holes, there are light and heavy hole effective masses, -0.79(Z) and - 1.58(A) me at Γ point, respectively. The electrons have effective mass of 1.58 me. Although the effective masses are larger than those of Pb-based halide perovskites, both materials have reasonable carrier mobilities for photovoltaic application. Based on the crystal Hamilton population (COHP) analysis (see, Yang, D. ; Ming, W. ; Shi, H. ; Zhang, L. ; Du, M.-H. Fast diffusion of native defects and impurities in perovskite solar cell material C¾NH3Pbl3. Chem. Mater. 2016, 28, 4349-4357) of the Ti-X interaction in Cs2TiI6 and Cs2TiBr6 titanium-based double perovskites, we found that the states at the highest occupied band are primarily nonbonding, similar to the Sn-I interaction in Cs2Snl6 (see Fig. 18A-D). For Cs2Til2Br4, there are two interactions, Ti-Br and Ti-I and both possess similar profile as that of CS2T1I6 and Cs2TiBr6 titanium-based double perovskites. Due to the effect of cross-interaction, the bonding of Ti-I and antibonding of Ti-Br in Cs2Til2Br4 also contribute slightly to the highest occupied band. The average integrated COHP values for Ti-I and Ti-Br bond are -2.77 and -2.53 for CS2T1I6 and Cs2TiBr6, respectively, indicating that the Ti-I bond exhibits more covalent characteristic than the Ti-Br bond. The Born effective charges Z* for the Ti4+ in CS2T1I6 and Cs2TiBr6 are 5.29 and 4.36, respectively, confirming the higher degree of covalency in the Ti- I bonds (see Table 1). For Cs2Til2Br4 titanium-based double perovskite, the value of the Born effective charges for the Ti4+ is between CS2T1I6 and Cs2TiBr6, due to the alloying of Ti-I and Ti-Br bonds. The increased covalency is in the order Cs2TiBr6<Cs2Til2Br4<Cs2Til6, which likely prevents the formation of halide vacancies in these HPs. Additionally, the calculated high-frequency dielectric tensors 6 have the same trend, .e. in the order Cs2TiBr6<Cs2Til2Br4<Cs2Til6. Table 3 below shows dielectric properties of Cs2TiBr6, CS2T1I6 and Cs2Til2Br4 HPs. including the High-Frequency Dielectric Tensors (e°°ij) and Born Effective charge tensors (Z*; Id) on Ti4+.
Table 3
Figure imgf000031_0001
EXAMPLE XI
Stability of Titanium-based Double Perovskite Materials
The intrinsic stability and tolerance to environmental (thermal, moisture) stresses of titanium-based double perovskites as a photovoltaic material were studied. In this context, the stability of the most representative Cs2TiBr6 and Cs2Til2Br4 titanium-based double perovskites were assessed as representative titanium-based double perovskite materials. AIMD simulations were first performed to evaluate the intrinsic stability of the Cs2TiBr6 and Cs2Til2Br4 HP crystal structures. As shown in Fig. 19A and Fig. 19C, very small perturbation in the crystal structures of both titanium-based double perovskites is observed after 5 ps AIMD simulation, confirming the intrinsic stability of these materials up to 500°K. Experimentally, pristine titanium-based double perovskite samples were annealed at high temperature (473 °K) in N2 atmosphere to evaluate thermal tolerance, and were then exposed to room-temperature (298°K) humid environment (70% RH) to assess moisture stability. The XRD patterns shown in Fig. 19B and Fig. 19D indicate no degradation in both Cs2TiBr6 and Cs2Til2Br4 titanium-based double perovskite samples after the applications of the thermal/moisture stresses. For comparison, the widely-studied MAPbbBr and MAPM3 halide perovskites with bandgaps close to those of Cs2TiBr6 and Cs2Til2Br4, respectively, show severe decomposition under the same conditions (see Fig. 20A-B) indicating superior properties of the titanium-based double perovskites based on the inorganic nature of the titanium-based double perovskites and the highly stable covalent/ionic interaction between Ti (IV) and halide ions in the Ti-based HPs.
EXAMPLE XII
Computational Methods
All first-principles computations were performed based on density-functional theory (DFT) methods as implemented in the Vienna ab initio simulation package (VASP 5.4). See, Kresse, G.; Furthmuller, J. Efficiency of ab initio total energy calculations for metals and semiconductors using a plane- wave basis set. Comput. Mater. Sci. 1996, 6, 15-50. An energy cutoff of 520 eV is employed, and the atomic positions are optimized using the conjugate gradient scheme without any symmetric restrictions until the maximum force on each atom is less than 0.02 eVA 1. The electronic structures and the optical properties are computed using the HSE06 functional (see, Heyd, J. ; Scuseria, G. E. ; Ernzerhof, M. Hybrid functionals based on a screened Coulomb potential. /. Chem. Phys. 2003, 118, 8207-8215) and with a cutoff energy of 400 eV, and the ion cores are described by using the projector augmented wave (PAW) method. See, Kresse, G. From ultrasoft pseudopotentials to the projector augmented- wave method. Phys. Rev. B 1999, 59, 1758-1775. Grimme's DFT-D3 correction is adopted to describe the long-range van der Waals interaction. See, Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. /. Chem. Phys. 2010, 132, 154104. A 3x3x3 k- point grid is used for the Ti vacancy-ordered HPs. The optical absorption coefficient is given by "™ ;" ··· ¾ , where ει and 82 are real and imaginary part of dielectric function, respectively. The initial configurations of A2Til6 with ^2 X ¾ 2 supercell (144 atoms) are adopted for ab initio molecular dynamics (AIMD) simulations. Each 5-ps AIMD simulation is performed in the constant- volume and constant- temperature (NVT) ensemble. The time step is 1.0 fs, and the temperature (300 K or 500 K) is controlled using the Nose-Hoover method. See, Martyna, G. J.; Klein, M. L.; Tuckerman, M. Nose-Hoover chains: The canonical ensemble via continuous dynamics. J. Chem. Phys. 1992, 97, 2635-2643.
EXAMPLE XIII
Materials Characterization
X-ray diffraction (XRD) of the synthesized powder was performed using an X-ray diffractometer (D8-Advance, Bruker, Germany) with CuKoc radiation (1.15406 A) in the 2Θ range of 10°-60°. UV-vis spectra were obtained using a spectrophotometer (UV-2600, Shimadzu, Japan) equipped with Integrating Sphere attachment (ISR-2600, Shimadzu, Japan). The standard BaSC (Nacalai Tesque, Inc., Japan) pellet was used as the reference. Surface morphology of films and cross-sections (fractured) of the whole PSCs were observed in a scanning electron microscope (SEM; LEO 1530VP, Carl Zeiss, Germany). EXAMPLE XIV
Two-Step Vapor Method for Making Transition Metal-Based Double Perovskites
Aspects of the present disclosure are directed to a two-step vapor deposition method for making transition metal-based double perovskites as described herein on a substrate including titanium-based double perovskites on a substrate. Fig. 21 A illustrates schematically the vapor-based method that is used for the deposition of Cs2TiBr6 HP thin films. In general, a uniform thin film of CsBr is first deposited via thermal evaporation on the substrate. This is followed by annealing the CsBr thin film in a TiBr4-vapor atmosphere at between 100°C and 300°C, such as 200°C.
More particularly, all the raw chemicals were purchased from Sigma Aldrich (USA) and used as-received. For the fabrication of the Cs2TiBr6 HP thin films, a CsBr layer of thickness -100 nm was first deposited on the substrate by thermal evaporation. The as- deposited CsBr thin film was placed in a chamber (see Fig. 9) filled with TiBr4 vapor. The TiBr4 vapor was slowly generated by heating TiBr4 powder at between 100°C and 300°C, such as 200°C. The typical reaction time for the complete conversion of CsBr to pure -phase Cs2TiBr6 is 24 hours when a TiC /FTO-glass substrate is used. The resulting Cs2TiBr6 thin films were washed using toluene to remove any possible excess TiBr4 on the film surface for subsequent characterization and device fabrication.
The hot TiB¾ vapor is expected to interact with the as-deposited CsBr, forming a uniform Cs2TiBr6 thin film as below:
2CsBr(s) + TiBr4(g) Cs2TiBr6(s). Rxn 1
Since the boiling point of TiBr4 is very low (-230 °C), the TiBr4 vapor can be produced by simply heating solid TiBr4 at ambient pressure.
The thin film before annealing (pure CsBr phase), after 12-h annealing (pure CsBr phase contacted with TiBr4 vapor for 12 hours), and after 24-h annealing in TiBr4 vapor are denoted as Film-I, -II and -III, respectively. Figs. 21B and 21C show indexed X-ray diffraction (XRD) patterns of the three thin films and their corresponding UV-vw spectra. As seen in Fig. 21B, Film-I contains pure CsBr phase of high crystallinity. In Film-II, the intensity of XRD peaks for the CsBr phase is significantly reduced, and the XRD peaks associated with the Cs2TiBr6 HP evolve, which confirms the progression of Reaction 1 during the annealing step. The conversion reaction is complete after 24-h annealing, and only phase-pure Cs2TiBr6 HP is found in Film-Ill. X-ray photoelectron spectroscopy (XPS) showing a spectrum for Ti 2p in Film-Ill confirmed the +4 oxidation state of Ti. The absence of significant features ascribed to lower oxidation states in Ti in the XPS spectrum indicates that there are no side reactions {e.g. decomposition of TiBr4 into Ti metal and Br2, etc.). In Fig. 21C, while the expected absorption features of CsBr and Cs2TiBr6 in Film-I and Film-Ill are respectively shown, Film-II has the typical mixed absorption feature of the CsBr and Cs2TiBr6 phases, which is consistent with the XRD results. The inset in Fig. 21C is a photograph of the final Cs2TiBr6 HP thin film showing even, dark reddish-brown color and good transparency, confirming its uniformity.
In order to gain further insights into Reaction 1, the microstructural and compositional evolution of the thin film are studied as the reaction progresses. Fig. 21D shows scanning electron microscope (SEM) images of the surface morphologies of Film-I, Film-II, and Film- Ill. Film-I is polycrystalline, compact, and pinhole-free, showing a typical microstructure comprising crystalline equiaxed grains and grain boundaries. After 12-h annealing, the initial smooth film surface becomes relatively rough over the entire film, which implies the uniform occurrence of Reaction 1 with nucleation of Cs2TiBr6 HP in the parent CsBr thin film at the surface. Upon the completion of Reaction 1 (24 h), since there is no more interaction of the solid thin film with TiBr4 vapor, the surface morphology of the thin film is completely reconstructed, which may be due to the thermally induced coarsening of Cs2TiBr6 grains, along with the progression of Reaction 1. Thus, Film III exhibits uniform grains of Cs2TiBr6 HP with well-defined grain boundaries. The Ti content in the intermediate Film-II was measured as a function of depth. The Film-II sample (film thickness -200 nm) was etched locally using focused ion beam (FIB) for different durations, and the Ti content was measured using energy dispersive spectroscopy (EDS) at each depth. Fig. 21E shows that the Ti content decreases with depth into Film-II, which indicates that the reaction progression is driven by the concentration gradient of Ti4+ in the thin film. The curve in Fig. 21E is a Gauss error-function fit using Fick's second law which is consistent with thermally-activated non-steady-state diffusion of Ti4+ in the solid state. The Ti elemental EDS maps at the top surface and surface at 40-nm depth of Film-II presented in Fig. 22-A-D are highly homogenous indicating the uniform progression of the diffusion front into the thin film during the TBr4-vapor-annealing step. Fig. 23 plots the Ti content as a function of depth in the final Cs2TiBr6 thin film (Film-Ill), showing homogeneous Ti distribution which is consistent with its phase-pure nature.
Based on these results, the proposed formation mechanism of the Cs2TiBr6 HP thin film is depicted schematically in Fig. 21F. In the proposed mechanisms, the Ti4+ and Br are transported from the film top surface to the bottom of the film bottom via solid-state diffusion, resulting in the progression of the conversion-reaction front into the thin film. Owing to the compact nature of the initial CsBr thin film and the uniform TiBr4 vapor atmosphere, no obvious in-plane reaction heterogeneity is observed in the thin film. We have further studied the effect of annealing temperature on the formation of the Cs2TiBr6 thin film. When a higher annealing-temperature (230 °C) is employed, the volatile TiBr4 causes a high vapor pressure that immediately damages the CsBr thin film (see Fig. 24). Whereas, when a lower-annealing temperature (150 °C) is employed for the same annealing duration, Reaction 1 remains incomplete, resulting in the presence of CsBr in Cs2TiBr6 thin film (Fig. 25A-B). These results are consistent with the proposed diffusion-based film-formation mechanism. Thus, the annealing condition of between 100°C and 300°C, such as 200°C, for 24 h is suitable for producing highly uniform and phase-pure Cs2TiBr6 HP thin films of -200 nm thickness. The average grain size in this final Cs2TiBr6 HP thin film is -270 nm, as measured using image analysis (see Fig. 26), and the root-mean-square (RMS) roughness is 24.5 nm, as measured using atomic force microscopy (AFM) (Fig. 27).
EXAMPLE XV
Optoelectronic Properties of Titanium-Based Double Perovskites
For evaluating the potential of the as-fabricated Cs2TiBr6 thin films as a light-absorber material in PSCs, their optoelectronic properties that are closely related to the PV performance were measured. Fig. 28A presents Tauc plot and photoluminescence (PL) spectrum of the Cs2TiBr6 HP thin film. Linear fitting of the absorption band edge results in a bandgap of 1.82 eV, which is near-ideal for top-cell application in tandem PVs with conventional Si-based or CIGS-based bottom-cells. (See, McMeekin et al, Science 351, 151-155 (2016). Note that this bandgap is slightly higher than that of the Cs2TiBr6 HP bulk powder sample (1.78 eV). See Ju et al, ACS Energy Lett. 3, 297-304 (2018). The Cs2TiBr6 HP thin film also shows bright PL under excitation of 395 nm laser. The emission is centered at -704 nm (1.76 eV), which is reasonably close to the bandgap. The PL properties of Cs2TiBr6 HP indicates a useful photovoltaic material. Fig. 28B is a PL map of the Cs2TiBr6 HP thin film overlaid on a AFM film-morphology scan, showing highly uniform PL-intensity over the whole film area. These results confirm the uniform physical properties of the as-deposited Cs2TiBr6 HP thin films over its entirety, which are highly consistent with the composition, phase, and microstructure uniformity results.
Photo-generated charge-carrier diffusion length in the light-absorber material in a PSC is another key optoelectronic property. In order to estimate the electron/hole diffusion lengths in Cs2TiBr6, the PL decay dynamics in the Cs2TiBr6 HP thin film with and without quencher layers were studied. The PL spectra in Figs. 29A-B confirm that T1O2 and P3HT effectively quench the PL in the Cs2TiBr6 HP thin film, and, thus, T1O2 and P3HT were chosen as the electron- and hole-quenching layers, respectively. The distributions of photo-generated electrons or holes n(x, t) in the Cs2TiBr6 thin film are described according to the following diffusion-based equation:
dn(x,t) dzn(x,t) , , . , .
—— = D—^- - k(t)n(x, t), Eqn. 1
where D is the diffusion coefficient for electrons or holes and k{t) is the PL decay rate, respectively. The k(f), as a result of the total contribution from the monomolecular and bimolecular recombination, is determined by fitting a stretched exponential decay to the PL data measured from the bare Cs2TiBr6 HP thin film. All the photo-generated carriers that reach the Ti02/Cs2TiBr6 or P3HT/Cs2TiBr6 interface will be quenched, and, thus, the boundary conditions n(L,f) = 0 and n(x,t) = n0exp(-Ax/L) are used, where x = L means the interface of quencher/Cs2TiBr6 and AIL is the absorbance at 395 nm divided by Cs2TiBr6 thin film thickness. Finally, the characteristic diffusion lengths (L) of electrons and holes is calculated using:
Figure imgf000038_0001
where τ is the characteristic lifetime extracted at when the PL intensity falls to l/eth of its initial intensity without the quencher. Fig. 28C shows time-resolved PL decay for a Cs2TiBr6 HP thin film with and without T1O2 quencher layer, where the extracted PL lifetimes are 24.0 ns and 2.5 ns, respectively. Using Eqn. 2, the L for electrons is estimated at 121 nm. Using the same method, the L for holes is estimated at 103 nm based on the results in Fig. 28D. There are opportunities for optimizing the microstructures and compositions in the Cs2TiBr6 HP thin films for obtaining even longer diffusion lengths. Nevertheless, the estimated L values for electrons/holes here approach those found in the popular MAPM3 OIHP. Such long and balanced electron/hole diffusion-lengths make the Cs2TiBr6 a useful light-absorber material for planar-heterojunction PSCs. The band structure and DOS of Cs2TiBr6 have been calculated which reveals the quasi- direct nature of the bandgap of Cs2TiBr6. As shown in Fig. 30A-B, Cs2TiBr6 possesses an indirect fundamental bandgap between Γ (valence band maximum, VBM) and X (conduction band minimum, CBM) and a direct bandgap at X, slightly larger than the indirect gap by -30 meV. An analysis of the orbital character of Cs2TiBr6 reveals that the highest valence band levels are contributed mostly by the Br 4p orbital, while the lowest conduction band levels are contributed by the Ti 3d orbital at Γ and X points. The isotropic electron and hole effective masses at the X and Γ points are also computed, respectively. The holes have both light and heavy effective masses, -0.9 me and -1.79 me at Γ point, respectively. The electrons have effective mass of 1.79 me. The quasi-direct nature of band gap and the relatively low effective masses of carriers in Cs2TiBr6 are consistent with the outstanding charge-carrier diffusion.
EXAMPLE XVI
Compatibility of Titanium-Based Double Perovskites With Solar Cell Materials
Since solar cells are a multi-component device, it is desirable that the energy levels of photovoltaic materials match with the other layers in the solar cell device, such as the electron- transporting layer (ETL) and hole-transporting layer (HTL). To that end, the CBM and VBM energy levels were determined using ultraviolet photoelectron spectroscopy (UPS), and the results are presented in Fig. 28E. The 4.5 eV work function of Cs2TiBr6 HP was calculated by subtracting the cutoff (16.75 eV) located in the higher binding-energy region from the energy of He ions (21.2 eV). The turning point located at lower binding-energy region indicates the energy gap between the VBM energy level and Fermi level of Cs2TiBr6 HP. Thus, the VBM is estimated at -5.9 eV for the Cs2TiBr6 thin film with respect to the vacuum energy level. Correspondingly, the CBM energy level was deduced to be -4.1 eV based on the optical bandgap (1.82 eV) of the Cs2TiBr6 thin film. It can also be seen that the Fermi level of the Cs2TiBr6 thin film is relatively closer to the CBM, indicative of the «-type self-doping nature of the as-prepared Cs2TiBr6 HP thin film. The energy level diagram is shown schematically shown in Fig. 28F. The VBM/CBM values match favorably with the state-of-the-art ETL materials (e.g. T1O2, Ceo) and HTL materials (e.g. Spiro-OMeTAD, PEDOT, P3HT, NiO) that are widely used in PSCs. Accordingly, the present disclosure contemplates Cs2TiBr6-based perovskite solar cells with many different device architectures for achieving desirable photovoltaic performance.
EXAMPLE XVII
Design Considerations Increasing Efficiency of Titanium-Based Double Perovskites
To evaluate the photovoltaic performance of the Cs2TiBr6 thin film, perovskite solar cells are fabricated by sandwiching the Cs2TiBr6 thin film in-between T1O2 ETL and P3HT HTL. Fluorine-doped tin oxide (FTO) and Au are used as the electrodes. More particularly, For the fabrication of perovskite solar cells, a compact-TiC ETL was first deposited on pre- patterned FTO-coated glass by spray pyrolysis at 450°C. For some PSCs, an interfacial layer of C60 layer was deposited before the deposition of the Cs2TiBr6 thin film. This C60 layer was deposited by spin-coating a solution of C60 in chlorobenzene (2 mg/ml) at 3000 rpm for 40 s on the as-prepared T1O2 substrate, followed by annealing at 100 °C for 30 min. The Cs2TiBr6 thin film was then deposited based on the procedure described above, followed by spin-coating the HTL solution, which consisted of 10 mg P3HT and 1 ml toluene solvent. Finally, the Au layer was deposited using thermal evaporator and a shadow mask. The J-V characteristics of the PSCs were measured using a 2400 Sourcemeter (Keithley, USA) under simulated one-sun AM1.5G 100 mW.cm"2 intensity (Oriel Sol3A Class AAA, Newport, USA), under both reverse (from Voc to Jsc) and forward (from Jsc to Voc) scans. The step voltage was 5 mV with a 10 ms delay time per step. The maximum-power output stability of PSCs was measured by monitoring the / output at the maximum-power-point V bias (deduced from the reverse-scan J- V curves) using the 2400 SourceMeter. A typical active area of 0.12 cm2 was defined using a non-reflective mask for the J-V measurements. The stable output PCE was calculated using the following relation: PCE = / (mA cm 2) X V (V))/(100 (mW cm 2). A shutter was used to control the one-sun illumination on the PSC. The EQE spectra were obtained using a quantum- efficiency measurement system (Oriel IQE 200B, Newport, USA) consisting of a Xenon lamp, a monochoromator, a lock-in amplifier and a calibrated silicon photodetector. The PSC stability was evaluated by measuring the J-V characteristics of PSCs after storing the cells under constant environmental stress for a certain period of time in an environmental climate chamber (HPP110, Memmert, Germany).
The schematic illustrations of this PSC device architecture and the energy-level diagram are shown in Figs. 31A and 3 IB, respectively. The current density - voltage (J-V) curves of this perovskite solar cell in both forward and reverse scans are shown in Fig. 31C, and the extracted photovoltaic performance parameters are presented in Table 4 below which summarizes photovoltaic performance parameters of Cs2TiBr6 with and without C6o- Table 4
Scan j Voc (V) : Jsc FF PCE (%) Stable PCE (%) i Direction i (mA.cnr2)
Cs2TiBr6 with C6o i Reverse j 1.02 j 5.69 0.564 3.28 3.22
i Forward j 0.99 j 5.75 0.549 3.12
Cs2TiBr6 ; Reverse j 0.89 j 4.03 0.631 2.26 2.15
I Forward I 0.89 I 3.87 0.595 2.05
The perovskite solar cell shows a useful overall PCE of 2.26% in reverse scan with small hysteresis (2.05% in forward scan). The stabilized PCE output is 2.15% at the maximum power point, which is reached immediately upon light illumination as shown in Fig. 3 ID. This efficient photoresponse is mainly contributed by Cs2TiBr6 rather than P3HT, as the P3HT layer has negligible contribution to light absorption (see Fig. 32). The photo-response of Cs2TiBr6 using conductive-AFM under light was further studied. As shown in Figs. 33A-C, compared with P3HT, Cs2TiBr6 shows more than an order-of-magnitude higher photocurrent, which confirms that the measured PCE is mainly attributed to the Cs2TiBr6 thin film in the device. Cs2TiBr6 HP is an efficient light-absorber material.
The 2.26% PCE was improved by incorporating a C6o interfacial layer between the Cs2TiBr6 HP thin film and the T1O2 ETL. The overall PCE is improved to a maximum 3.28 % in reverse scan (see Fig. 31C, Table 4). The stabilized PCE output at the maximum power point is 3.22% (see Fig. 3 ID). The external quantum efficiency (EQE) spectrum of the same PSC shows an integrated current density (5.43 mA.cm 2), which is close to the short-circuit current density ( sc) extracted from the J-V curves in Fig. 31C (Table 4). The EQE spectrum shows a band edge at -680 nm, which is identical to the absorption edge shown in Fig, 28 A. Fig. 3 IF presents the PCE statistics of the Cs2TiBr6 thin film based PSC with the C60 interfacial layer, which shows a tight PCE distribution with a mean value of 3%.
The C60 interfacial layer has two beneficial functions. First, as shown in Fig. 3 IB, C60 molecules have a CBM commensurate with that of T1O2 ETL and Cs2TiBr6 HP, which can facilitate the electron transfer from Cs2TiBr6 to the T1O2 ETL. This is consistent with the fact that the PL is much more efficiently quenched with the presence of C60 (Fig. 29A-B). Second, the C60 layer influences the microstructure of the as-deposited CsBr thin film, and, thus, the formation of the Cs2TiBr6 HP thin film. As seen in Fig. 34, with the incorporation of the C60 layer, the as-deposited CsBr thin film under the same preparation conditions exhibits a reduced grain size and a higher-density grain-boundary network. This modified CsBr microstructure facilitates Reaction 1, most likely due to the faster through-thickness ion diffusion along the grain boundaries. It is found that the Reaction 1 completion time is reduced by half (12 h) in this case. As a result, the final Cs2TiBr6 HP thin film shows superior quality with a reduced RMS roughness of 14.6 nm (Fig. 35). The achieved PCE exceeds several of the PCEs of PSCs based on other Pb-free HPs. In particular, the best Voc for the Cs2TiBr6 thin film achieved is over 1.0 V, which is useful for perovskite solar cells. For comparison, the most widely studied all-inorganic Sn-based Pb-free HPs show a Voc less than 0.6 V. See, Hao et al., Nat. Photonics 8, 489-494 (2014); Ke et al., Sci. Adv. 3, el71029 (2017).
EXAMPLE XVIII
Additional Stability Studies of Titanium-Based Double Perovskites
Intrinsic and environmental stability of the Cs2TiBr6 thin films have been assessed. Thermogravimetric analysis (TGA) results in Fig. 36 show that the decomposition of Cs2TiBr6 starts at a temperature as high as 400 °C. For comparison, MAPbb, MAPbBr3, and MASnb OIHPs begin to decompose at 330°C, 340°C, and 200°C, respectively. See, Dang et al., Angew. Chem. Int. Ed. 55, 3447-3450 (2016); Dang et al., CrystEngComm 17, 665-670 (2015); Brunetti et al., Sci. Rep. 6, 31896 (2016). This is attributed to the all-inorganic nature of the Cs2TiBr6, and that the titanium cation exists in its preferred and stable +4 oxidation state. Note that, unlike other titanium halides such as TiCU, Cs2TiBr6 is not susceptible to the ambient degradation and conversion to titania.
The environmental stability of the Cs2TiBr6 HP thin films was further tested by examining their tolerance to heat, moisture, and light. A uniform MAPbbBr OIHP was used as the reference film, as it exhibits nearly the same bandgap as Cs2TiBr6, and it is the most popular choice for use in PSCs for tandem PV applications. Pb-based halide perovskites, in general, have higher environmental stability compared to their Pb-free counterparts. (See, Gratzel et al., Nat. Mater. 13, 838-842 (2014); Leijtens et al., Adv. Energy Mater. 5, 1500963 (2015)). The heat-tolerance experiment was conducted by annealing the film at between 100°C and 300°C, such as 200°C, for 6 hours in a N2-filled glovebox. The moisture tolerance was tested by storing the film in a precisely-controlled climate chamber (23°C, 80% relative humidity (RH), 6 hours). The tolerance of the film to sunlight was evaluated by placing the film under stimulated one-sun illumination for 24 h where the film is sealed in a poly(methyl methacrylate) coating to exclude other environmental effects. These test conditions are intended to separate the thermal, moisture, and light effects. The Cs2TiBr6 thin films used in these tests were all deposited on compact-TiC coated glass substrates bearing in mind that the TiC -perovskite interfaces are regarded as an important factor of the PSC device stability.
By comparing the XRD patterns from the thin films before and after being subjected to environmental (heat/moisture/light) stresses in Figs. 37A and 37B, the Cs2TiBr6 thin films show much higher tolerance compared to the reference thin film. Note that, while the time duration that is chosen to evaluate environmental stresses is until significant degradation occurs in the reference film, the Cs2TiBr6 thin films can maintain their phase purity for much longer durations. For example, in the thermal stability test, when the annealing time is prolonged to 24 h, there is still no obvious degradation (Fig. 38). Cs2TiBr6 thin films exhibit useful intrinsic/environmental stability.
In additional experiments, the PCE evolution of the perovskite solar cell device (unencapsulated) was monitored as a function of its storage time (70 °C, 30% RH, ambient light). As seen in Fig. 37C, after 14 days of storage, the PSC still shows a PCE of 3.03% (94 % PCE retention). The stability of the PSC device (unencapsulated) was further evaluated under continuous one-sun illumination, which still shows good PCE retention of 85 % (Fig. 39). Power conversion efficiencies for double perovskite materials described herein range between 1% and 4%.
EXAMPLE XIX
Materials Characterization
X-ray diffraction (XRD) of the thin films was performed using a high-resolution diffractometer (D8 Advance, Bruker, Germany) with CuKa radiation. UV-v« spectra were obtained using a spectrophotometer (UV-2600, Shimadzu, Japan). The morphology and EDS elemental-distribution maps of the thin film were observed in a SEM (LEO 1530VP, Carl Zeiss, Germany) equipped with an EDS detector (Oxford Instruments, UK). The FIB (Helios 600, FEI, HiUisboro, OR) was used to etch the thin films, where the different depths were achieved by Ga+ ions bombardment. A PHI5600 XPS system was used to acquire both XPS and UPS spectra. The analysis chamber base pressures were <lxl0-9 Torr prior to analysis. The instrument utilized a monochromated Ka Al source for X-ray radiation at 1486.7 eV and a UVS 40 A2 (PRE VAC, Poland) UV source and UV40A power supply provided by He la excitation (He I) for UPS at 21.22 eV. Chamber pressure for UPS was maintained < 3xl0"8 Torr. The steady-state and time-resolved PL spectra were recorded with a Varian Cary Eclipse fluorescence spectrophotometer operating at 395 nm excitation. The decay rate and lifetime was determined using the two-parameter decay function fitting method. The fitted diffusion coefficients for electrons and holes are 0.61 cm2.s_1 and 0.44 cm2.s_1, respectively, which are used to calculate the corresponding diffusion lengths. The pc-AFM measurements were conducted in contact mode using an MFP-3D AFM (Asylum Research, USA) with a conducting platinum-coated silicon probe (Econo-SCM-PIC, Asylum Research, USA) and a LED light source.
All first-principles computations are performed based on density-functional theory (DFT) methods as implemented in the Vienna ab initio simulation package (VASP 5.4). Briefly, an energy cutoff of 520 eV is employed, and the atomic positions are optimized using the conjugate gradient scheme without any symmetric restrictions until the maximum force on each atom is less than 0.02 eV»A_1. The ion cores are described using the projector augmented wave (PAW) method. Grimme's long-range van der Waals interaction is described using DFT- D3 correction. A 16x16x16 k-point grid is used. EXAMPLE XIX
Embodiments
The present disclosure provides a transition metal-based double perovskite of the formula A2TX6 wherein A is one or more of K+, Rb+, Cs+, In+, MA+, or FA+; wherein T is Ti, Zr, Hf, V, Nb, Mo, or W; and wherein X is selected from one or more of CI, Br, or I. According to one aspect, T is Ti. According to one aspect, the transition metal-based double perovskite is of the formula A2TBXC6-X wherein A is one or more of K+, Rb+, Cs+, In+, MA+, or FA+; wherein T is Ti, Zr, Hf, V, Nb, Mo, or W; wherein each of B and C is selected from one or more of CI, Br, or I and wherein B and C may be the same or B and C may be different and wherein (x = 0, 1, 2, 3, 4, 5 or 6). According to one aspect, T is Ti. According to one aspect, the transition metal-based double perovskite is of the formula Cs2TiIxBr6-x wherein X = 0, 2, 4 or 6. According to one aspect, the transition metal-based double perovskite is of the formula Cs2Til2Br4 or Cs2TiBr6. According to one aspect, the transition metal-based double perovskite lacks organic moieties. According to one aspect, the transition metal-based double perovskite is all inorganic. According to one aspect, the transition metal-based double perovskite is in the form of a powder. According to one aspect, the transition metal-based double perovskite is in the form of a thin film. According to one aspect, the transition metal-based double perovskite is in the form of a thin film between an anode and a cathode. According to one aspect, the transition metal-based double perovskite has a bandgap that can be tuned continuously from 0.9 eV to 1.82 eV. According to one aspect, the transition metal-based double perovskite has a bandgap that can be tuned continuously from 1.02 eV to 1.78 eV. According to one aspect, the transition metal-based double perovskite has the formula Cs2TiBr6 and a direct bandgap of 1.78 eV. According to one aspect, the transition metal-based double perovskite has the formula Cs2Til2Br4 and a direct bandgap of 1.38 eV. The present disclosure provides a photovoltaic cell including the transition-metal based double perovskite as described herein in the form of a thin film having an anode and a cathode electrically connected thereto. According to one aspect, thin film is contacted on a first side by a hole-transporting layer which is contacted by an anode layer and wherein the thin film is contacted on a second side by an electron-transporting layer which is contacted by a cathode layer.
The present disclosure provides a solar cell including the transition-metal based double perovskite described herein in the form of a thin film having an anode and a cathode electrically connected thereto. According to one aspect, the thin film is contacted on a first side by a hole- transporting layer which is contacted by an anode layer and wherein the thin film is contacted on a second side by an electron-transporting layer which is contacted by a cathode layer.
The present disclosure provides a method of making a thin film of a transition-metal based double perovskite as described herein including vapor depositing a compound of formula AX on a substrate to form a first film wherein A is K+, Rb+, Cs+, or In+ and wherein X is CI, Br, or I, contacting the first film with a compound of TX in vapor form wherein T is Ti, Zr, Hf, V, Nb, Mo, or W and wherein X is CI, Br, or I in a manner to form a thin film of the compound of formula A2TX6 wherein A is one or more of K+, Rb+, Cs+, or In+, MA+, or FA+, wherein T is Ti, Zr, Hf, V, Nb, Mo, or W; and wherein X is selected from one or more of CI, Br, or I.
The present disclosure provides a method of making a thin film of a transition-metal based double perovskite as described herein wherein the transition-metal based double perovskite is Cs2TiBr6 and the method includes vapor depositing CsBr on a substrate to form a first CsBr film, and contacting the first film with TiBr4 in vapor form in a manner to form a thin film of the Cs2TiBr6.
The present disclosure provides a method of making a thin film of a transition-metal based double perovskite as described herein wherein the transition-metal based double perovskite is Cs2TiBr6 and the method includes applying a solution of CsBr and TiBn in solvent to a substrate and heating the substrate to remove the solvent and form a thin film of Cs2TiBr6 on the substrate.
According to one aspect of the methods, the solvent is gamma-butyrolactone, dimethylformamide, dimethyl sulfoxide, N-methyl-2-pyrrolidone, or combinations thereof. According to one aspect of the methods, the substrate is heated to between 100°C and 300°C.
The present disclosure provides a method of making a powder of a transition-metal based double perovskite as described herein wherein the transition-metal based double perovskite is Cs2TiBr6 and the method includes mixing CsBr powder and TiBn powder in a solvent in a 2: 1 molar ratio under conditions to form a solution, and removing the solvent to form a Cs2TiBr6 powder. According to one aspect, the solvent is gamma-butyrolactone, dimethylformamide, dimethyl sulfoxide, N-methyl-2-pyrrolidone, or combinations thereof. According to one aspect, the solvent is removed by heating the solution to remove the solvent.

Claims

What is claimed is:
1. A transition metal-based double perovskite of the formula
A2TX6
wherein A is one or more of K+, Rb+, Cs+, In+, MA+, or FA+;
wherein T is Ti, Zr, Hf, V, Nb, Mo, or W; and
wherein X is selected from one or more of CI, Br, or I.
2. The transition metal-based double perovskite of claim 1 wherein T is Ti.
3. The transition metal-based double perovskite of claim 1 of the formula wherein A is one or more of K+, Rb+, Cs+, In+, MA+, or FA+;
wherein T is Ti, Zr, Hf, V, Nb, Mo, or W;
wherein each of B and C is selected from one or more of CI, Br, or I and wherein B and C may be the same or B and C may be different and wherein (x = 0, 1, 2, 3, 4, 5 or 6).
4. The transition metal-based double perovskite of claim 3 wherein T is Ti.
5. The transition metal-based double perovskite of claim 1 of the formula Cs2TiIxBr6-x wherein X = 0, 2, 4 or 6.
6. The transition metal-based double perovskite of claim 1 of the formula Cs2Til2Br4 or Cs2TiBr6.
7. The transition metal-based double perovskite of claim 1 which lacks organic moieties.
8. The transition metal-based double perovskite of claim 1 being all inorganic.
9. The transition metal-based double perovskite of claim 1 in the form of a powder.
10. The transition metal-based double perovskite of claim 1 in the form of a thin film.
11. The transition metal-based double perovskite of claim 1 in the form of a thin film between an anode and a cathode.
12. The transition metal-based double perovskite of claim 1 having a bandgap that can be tuned continuously from 0.9 eV to 1.82 eV.
13. The transition metal-based double perovskite of claim 1 having a bandgap that can be tuned continuously from 1.02 eV to 1.78 eV.
14. The transition metal-based double perovskite of claim 1 having the formula Cs2TiBr6 and a direct bandgap of 1.78 eV.
15. The transition metal-based double perovskite of claim 1 having the formula Cs2Til2Br4 and a direct bandgap of 1.38 eV.
16. A photovoltaic cell comprising the transition-metal based double perovskite of claim 1 in the form of a thin film having an anode and a cathode electrically connected thereto.
17. The photovoltaic cell of claim 12 wherein the thin film is contacted on a first side by a hole-transporting layer which is contacted by an anode layer and wherein the thin film is contacted on a second side by an electron-transporting layer which is contacted by a cathode layer.
18. A solar cell comprising the transition-metal based double perovskite of claim 1 in the form of a thin film having an anode and a cathode electrically connected thereto.
19. The solar cell of claim 14 wherein the thin film is contacted on a first side by a hole-transporting layer which is contacted by an anode layer and wherein the thin film is contacted on a second side by an electron-transporting layer which is contacted by a cathode layer.
20. A method of making a thin film of a transition-metal based double perovskite of claim 1 comprising
vapor depositing a compound of formula AX on a substrate to form a first film wherein A is K+, Rb+, Cs+, or In+ and wherein X is CI, Br, or I,
contacting the first film with a compound of TX in vapor form wherein T is Ti, Zr, Hf, V, Nb, Mo, or W and wherein X is CI, Br, or I in a manner to form a thin film of the compound of formula A2TX6 wherein A is one or more of K+, Rb+, Cs+, or In+, MA+, or FA+, wherein T is Ti, Zr, Hf, V, Nb, Mo, or W; and wherein X is selected from one or more of CI, Br, or I.
21. A method of making a thin film of a transition-metal based double perovskite of claim 1 wherein the transition-metal based double perovskite is Cs2TiBr6 comprising
vapor depositing CsBr on a substrate to form a first CsBr film,
contacting the first film with TiBn in vapor form in a manner to form a thin film of the Cs2TiBr6.
22. A method of making a thin film of a transition-metal based double perovskite of claim 1 wherein the transition-metal based double perovskite is Cs2TiBr6 comprising
applying a solution of CsBr and TiBn in solvent to a substrate and heating the substrate to remove the solvent and form a thin film of Cs2TiBr6 on the substrate.
23. The method of claim 18 wherein the solvent is gamma-butyrolactone, dimethylformamide, dimethyl sulfoxide, N-methyl-2-pyrrolidone, or combinations thereof.
24. The method of claim 18 wherein the substrate is heated to between 100°C and
300°C.
25. A method of making a powder of a transition-metal based double perovskite of claim 1 wherein the transition-metal based double perovskite is Cs2TiBr6 comprising
mixing CsBr powder and TiBr4 powder in a solvent in a 2: 1 molar ratio under conditions to form a solution,
removing the solvent to form a Cs2TiBr6 powder.
26. The method of claim 1 wherein the solvent is gamma-butyrolactone, dimethylformamide, dimethyl sulfoxide, N-methyl-2-pyrrolidone, or combinations thereof.
27. The method of claim 18 wherein the solvent is removed by heating the solutionve the solvent.
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Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN110797456A (en) * 2019-10-18 2020-02-14 苏州大学 Lead-free double perovskite-based electric storage device and preparation method thereof
CN110887874A (en) * 2019-11-20 2020-03-17 苏州大学 Based on perovskite Cs2PdBr6Humidity-sensitive sensor and preparation method and application thereof
CN111933730A (en) * 2020-08-20 2020-11-13 西安电子科技大学 Nuclear radiation detector based on lead-free perovskite single crystal and preparation method thereof
CN112179957A (en) * 2020-11-12 2021-01-05 华北电力大学 Perovskite-like gas-sensitive sensing material and preparation method of gas-sensitive sensor
CN113526545A (en) * 2021-07-16 2021-10-22 辽宁科技大学 Method for preparing tin-based perovskite powder based on mechanochemical method
RU2775160C1 (en) * 2021-11-16 2022-06-28 Федеральное государственное автономное образовательное учреждение высшего образования "Национальный исследовательский технологический университет "МИСиС" Method for obtaining semiconductor thin-film photoconverters based on halide perovskites
CN114772943A (en) * 2022-04-13 2022-07-22 滨州裕能化工有限公司 Cs2TiBr6Lead-free double perovskite thin film and preparation method of solar cell

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20150122316A1 (en) * 2013-10-16 2015-05-07 OmniPV, Inc. Photovoltaic cells including halide materials
US20150144195A1 (en) * 2013-11-26 2015-05-28 Michael D. IRWIN Perovskite and other solar cell materials
US20170038483A1 (en) * 2015-08-06 2017-02-09 Lawrence Livermore National Security, Llc Scintillators having the k2ptcl6 crystal structure

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20150122316A1 (en) * 2013-10-16 2015-05-07 OmniPV, Inc. Photovoltaic cells including halide materials
US20150144195A1 (en) * 2013-11-26 2015-05-28 Michael D. IRWIN Perovskite and other solar cell materials
US20170038483A1 (en) * 2015-08-06 2017-02-09 Lawrence Livermore National Security, Llc Scintillators having the k2ptcl6 crystal structure

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
CHEN ET AL.: "Cesium Titanium(IV) Bromide Thin Films Based Stable Lead-free Perovskite Solar Cells", JOULE, vol. 2, no. 3, 13 February 2018 (2018-02-13), XP055585702, Retrieved from the Internet <URL:https://www.sciencedirect.com/ science /article/pii/S2542435118300370>> [retrieved on 20181119] *
MAUGHAN ET AL.: "Defect Tolerance to Intolerance in the Vacancy-Ordered Double Perovskite Semiconductors Cs2Snl6 and Cs2Tel6", J. AM. CHEM. SOC., vol. 138, 10 June 2016 (2016-06-10), pages 8453 - 8464, XP055585701 *

Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN110797456A (en) * 2019-10-18 2020-02-14 苏州大学 Lead-free double perovskite-based electric storage device and preparation method thereof
CN110887874A (en) * 2019-11-20 2020-03-17 苏州大学 Based on perovskite Cs2PdBr6Humidity-sensitive sensor and preparation method and application thereof
CN110887874B (en) * 2019-11-20 2022-06-07 苏州大学 Moisture-sensitive sensor based on perovskite and preparation method and application thereof
CN111933730A (en) * 2020-08-20 2020-11-13 西安电子科技大学 Nuclear radiation detector based on lead-free perovskite single crystal and preparation method thereof
CN111933730B (en) * 2020-08-20 2024-02-23 西安电子科技大学 Nuclear radiation detector based on leadless perovskite monocrystal and preparation method thereof
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CN113526545A (en) * 2021-07-16 2021-10-22 辽宁科技大学 Method for preparing tin-based perovskite powder based on mechanochemical method
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CN114772943A (en) * 2022-04-13 2022-07-22 滨州裕能化工有限公司 Cs2TiBr6Lead-free double perovskite thin film and preparation method of solar cell
CN114772943B (en) * 2022-04-13 2023-07-25 滨州裕能化工有限公司 Cs (cell lines) 2 TiBr 6 Preparation method of lead-free double perovskite thin film and solar cell

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