WO2007060465A2 - Dispositif photovoltaique - Google Patents

Dispositif photovoltaique Download PDF

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Publication number
WO2007060465A2
WO2007060465A2 PCT/GB2006/004426 GB2006004426W WO2007060465A2 WO 2007060465 A2 WO2007060465 A2 WO 2007060465A2 GB 2006004426 W GB2006004426 W GB 2006004426W WO 2007060465 A2 WO2007060465 A2 WO 2007060465A2
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layer
electrode
blocking layer
electron
photovoltaic
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PCT/GB2006/004426
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WO2007060465A3 (fr
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Hazel Elaine Assender
Bernard Martin Henry
Victor Burlakov
Hannah Elaine Smith
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Isis Innovation Limited
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    • 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
    • H10K30/10—Organic devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation comprising heterojunctions between organic semiconductors and inorganic semiconductors
    • H10K30/15—Sensitised wide-bandgap semiconductor devices, e.g. dye-sensitised TiO2
    • H10K30/151—Sensitised wide-bandgap semiconductor devices, e.g. dye-sensitised TiO2 the wide bandgap semiconductor comprising titanium oxide, e.g. TiO2
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K2102/00—Constructional details relating to the organic devices covered by this subclass
    • H10K2102/10—Transparent electrodes, e.g. using graphene
    • H10K2102/101—Transparent electrodes, e.g. using graphene comprising transparent conductive oxides [TCO]
    • H10K2102/103—Transparent electrodes, e.g. using graphene comprising transparent conductive oxides [TCO] comprising indium oxides, e.g. ITO
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K85/00—Organic materials used in the body or electrodes of devices covered by this subclass
    • H10K85/10—Organic polymers or oligomers
    • H10K85/111—Organic polymers or oligomers comprising aromatic, heteroaromatic, or aryl chains, e.g. polyaniline, polyphenylene or polyphenylene vinylene
    • H10K85/114—Poly-phenylenevinylene; Derivatives thereof
    • 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

  • the present invention relates to photovoltaic heterojunction devices having an electron acceptor material and an electron donor material between two electrodes.
  • the invention concerns hybrid devices in which the electron donor material is a photoactive conjugated organic polymer and the electron acceptor material is an inorganic semiconductor, for instance a metal oxide such as TiO 2 .
  • Hybrid polymer/metal oxide photovoltaic cells have been intensively investigated. In such cells three key processes need to occur: light absorption, charge separation of the exciton, and transport of the separated charges to the electrodes. Light absorption is reliant on the optical density of the photoactive polymer, and charge separation is achieved by blending the electron acceptor with the polymer film. Charge separation occurs at the interface between electron acceptor and polymer. Following charge separation, the electrons and holes are transported, respectively, to one electrode via the electron acceptor material, and to the other electrode via the polymeric electron donor. In conjugated polymer/Ti ⁇ 2 photovoltaic cells, it is generally the case that electrons are transported to a transparent electrode which is in contact with the TiO 2 electron acceptor. Similarly, holes are generally transported to the other, top electrode via the conjugated polymer.
  • Each of the devices had four layers on top of an ITO-coated glass substrate deposited in the following order: first, a dense TiO 2 blocking layer; second, a porous TiO 2 layer; third, a dip-coated polymer layer; and finally, a spin-coated polymer layer.
  • the blocking layer was deposited using a spray pyrolysis technique. Improved power conversion efficiencies of up to 0.4 % (under one sun) were observed in these devices. Interfacial recombination is another notable factor detrimental to photovoltaic device performance. This can take the form of electron back-transfer from the conduction band of the electron acceptor material to the HOMO of the electron donor material.
  • the present inventors have found that the incorporation of a blocking layer not at the electrode interface, but at the charge separation interface, between the electron donor and electron acceptor layers, leads to a surprisingly improved stability in photovoltaic devices, hi particular, the inventors have observed that devices including such a layer maintain a constant and good device performance. Without such a layer, transient effects are pronounced, with measured parameters such as the open circuit voltage (Voc) falling soon after illumination.
  • Voc open circuit voltage
  • the invention provides a photovoltaic device comprising a first electrode; a second electrode; and a photovoltaic region disposed between the first electrode and the second electrode, the photovoltaic region comprising an electron acceptor layer, an electron donor layer, and a blocking layer, wherein the blocking layer does not contain alumina and is disposed between the electron acceptor layer and the electron donor layer.
  • the blocking layer comprises a metal oxide.
  • the metal oxide is typically selected from titania, ZnO, SnO 2 , In 2 O 3 , Y 2 O 3 , WO 3 , GeO 2 , MgO 5 BaTiO 3 and ZrO 2 .
  • the invention provides a photovoltaic device comprising a first electrode; a second electrode; and a photovoltaic region disposed between the first electrode and the second electrode, the photovoltaic region comprising an electron acceptor layer, an electron donor layer, and a blocking layer, which blocking layer is disposed between the electron acceptor layer and the electron donor layer and is the only blocking layer in the photovoltaic device.
  • the blocking layer comprises a metal oxide.
  • the metal oxide is typically selected from titania, alumina, ZnO, SnO 2 , In 2 O 3 , Y 2 O 3 , WO 3 , GeO 2 , MgO, BaTiO 3 and ZrO 2 .
  • the invention provides a process for producing a photovoltaic device, the device comprising a first electrode; a second electrode; and a photovoltaic region disposed between the first electrode and the second electrode, the photovoltaic region comprising an electron acceptor layer, an electron donor layer, and a blocking layer, wherein the blocking layer is disposed between the electron acceptor layer and the electron donor layer, and comprises a metal oxide; wherein the process comprises forming said blocking layer using a precursor metal peroxide compound.
  • the invention provides the use of a metal peroxide compound for fabricating a blocking layer in a photovoltaic device.
  • the invention provides a process for producing a photovoltaic device, the device comprising a first electrode; a second electrode; and a photovoltaic region disposed between the first electrode and the second electrode, the photovoltaic region comprising an electron acceptor layer, an electron donor layer, and a blocking layer, wherein the blocking layer does not contain alumina and is disposed between the electron acceptor layer and the electron donor layer; wherein the process comprises: (i) forming the electron acceptor layer on a substrate comprising the first electrode; (ii) forming the blocking layer on the electron acceptor layer; (iii) forming the electron donor layer on the blocking layer; and (iv) forming the second electrode on the electron donor layer.
  • the invention provides a process for producing a photovoltaic device, the device comprising a first electrode; a second electrode; and a photovoltaic region disposed between the first electrode and the second electrode, the photovoltaic region comprising an electron acceptor layer, an electron donor layer, and a blocking layer, which blocking layer is disposed between the electron acceptor layer and the electron donor layer and is the only blocking layer in the photovoltaic device; wherein the process comprises: (i) forming the electron acceptor layer on a substrate comprising the first electrode; (ii) forming the blocking layer on the electron acceptor layer; (iii) forming the electron donor layer on the blocking layer; and (iv) forming the second electrode on the electron donor layer.
  • the invention provides the use of an oxide other than alumina, a nitride or carbon as a blocking layer in a photovoltaic device, the device comprising a first electrode; a second electrode; and a photovoltaic region disposed between the first electrode and the second electrode, the photovoltaic region comprising an electron acceptor layer, an electron donor layer, and said blocking layer, wherein the blocking layer does not contain alumina and is disposed between the electron acceptor layer and the electron donor layer.
  • an oxide other than alumina is used as the blocking layer.
  • the oxide other than alumina is a metal oxide.
  • the metal oxide is typically selected from titania, ZnO, SnO 2 , In 2 O 3 , Y 2 O 3 , WO 3 , GeO 2 , MgO, BaTiO 3 and ZrO 2 .
  • the invention provides the use of an oxide, a nitride or carbon as a blocking layer in a photovoltaic device, the device comprising a first electrode; a second electrode; and a photovoltaic region disposed between the first electrode and the second electrode, the photovoltaic region comprising an electron acceptor layer, an electron donor layer, and a blocking layer, which blocking layer is disposed between the electron acceptor layer and the electron donor layer and is the only blocking layer in the photovoltaic device.
  • an oxide is used as the blocking layer.
  • the oxide is a metal oxide.
  • the metal oxide is typically selected from titania, alumina, ZnO, SnO 2 , In 2 O 3 , Y 2 O 3 , WO 3 , GeO 2 , MgO, BaTiO 3 and ZrO 2 .
  • the blocking layer provides a stepped workfunction which decreases the probability of recombination of holes with electrons.
  • the blocking layer may reduce the recombination of holes with electrons.
  • the blocking layer may limit the injection of minority charge carriers.
  • the blocking layer reduces transient effects due to minority carriers.
  • the blocking layer may reduce transient effects observed on exposure of the device to light.
  • the blocking layer typically accepts injection of, and conducts, electrons to the electron acceptor layer, or accepts injection of, and conducts, holes to the electron donor layer.
  • the blocking layer may accept injection of, and conduct, majority charge carriers.
  • Figure 1 shows the effect of incorporating a peroxide-fabricated titania layer on transient variations in the short circuit current (Jsc) and open circuit voltage (Voc) of non-heat-treated devices under illumination.
  • Graphs (a) to (d) are of Jsc in units of ⁇ A/pixel (left hand y axis) versus illumination time in units of minutes (x axis) and of Voc in units of volts (right hand y axis) versus illumination time in units of minutes (x axis); the circular data points represent Voc values and the diamond-shaped data points represent Jsc values.
  • Graph (a) is for the device of Comparative Example 2 (Solaronix doctor blade twice); graph (b) for the device of Example 5 (Solaronix doctor blade + peroxide drop coat overlayer); graph (c) for the device of Example 4 (Solaronix doctor blade + peroxide spin coat overlayer); and graph (d) for the device of Comparative Example 3 (peroxide spin coat underlayer + solaronix doctor blade).
  • Figure 2 shows transient variations in the short circuit current and open circuit voltage of heat treated devices under illumination.
  • Graphs (a) to (c) are of Jsc in units of ⁇ A/pixel (left hand y axis) versus illumination time in units of minutes (x axis) and of Voc in units of volts (right hand y axis) versus illumination time in units of minutes (x axis); the circular data points represent Voc values and the diamond-shaped data points represent Jsc values.
  • Graph (a) is for the device of Example 4 (Solaronix doctor blade + peroxide spin coat overlayer); graph (b) for the device of Comparative Example 3 (peroxide spin coat underlayer + solaronix doctor blade); and graph (c) for the device of Comparative Example 2 (Solaronix doctor blade twice).
  • Figure 3 is a graph of Incident Photon to Charge Carrier Efficiency (IPCE) in , units of % (y axis) versus illumination wavelength in units of nm (x axis) for the heat- treated device of Comparative Example 2.
  • IPCE Incident Photon to Charge Carrier Efficiency
  • Figure 4 is a graph of IPCE in units of % (y axis) versus illumination wavelength in units of nm (x axis) for the heat-treated device of Example 4.
  • the square data points are IPCE values taken during a scan downwards in wavelength (from 650nm to 350nm) and the diamond-shaped data points are IPCE values recorded during a subsequent scan upwards in wavelength.
  • Figure 5 is a graph of IPCE in units of % (y axis) versus illumination wavelength in units of nm (x axis), recorded by scanning downwards in wavelength from 650nm to 350nm.
  • the filled square data points are for the heat-treated device of Example 4, and the open square data points are for the heat-treated device of Comparative Example 2.
  • Figure 6 is a graph of the ratio of the EPCEs of the samples of Comparative Example 2 and Example 4 (y axis) versus illumination wavelength in units of nm (x axis).
  • the diamond-shaped data points are the ratios of the downward scan IPCE values, and the square data points are the ratios of the upward scan EPCE values.
  • the graph shows a rising trend to low energy consistent with the peroxide acting as an electron-blocking layer.
  • Photovoltaic devices comprise an electron acceptor layer, an electron donor layer and a blocking layer, all of which are sandwiched between a first electrode and a second electrode.
  • the blocking layer is disposed between the electron acceptor and electron donor layers.
  • the first and second electrodes are an anode and a cathode, one of which is transparent to allow the ingress of light.
  • Such multilayer devices are often termed heterojunction devices.
  • a known bilayer heterojunction device has the structure Cathode/Electron acceptor/Electron donor/Anode.
  • the electron donor and electron acceptor layers of the devices of the present invention may each be selected from a wide range of either organic or inorganic materials, including p-type (hole transporting) semiconductors, and n-type (electron transporting) semiconductors, as would be appreciated by a person of skill in the art.
  • the electron donor layer of the devices of the present invention may be any p-type, or hole transporting, semiconducting material.
  • the electron acceptor layer may be any n-type, or electron transporting, semiconducting material.
  • the present invention particularly concerns "hybrid" organic/inorganic devices in which the electron donor material is a photoactive conjugated organic polymer and the electron acceptor material is an inorganic semiconductor.
  • the structures of hybrid polymer/metal oxide photovoltaic cells are well known and general descriptions of the device types and method of working can be found for example in K.M Coakley and Michael D. McGehee, Chem. Mater. 2004, 16, 4533-4542; and P. Ravirajan et al, Adv. Fund. Mater. 2005, 15, No. 4, April.
  • the choice of the first and second electrodes of the photovoltaic devices of the present invention may depend on the structure type. Typically when a metal oxide is used as the electron acceptor the metal oxide is deposited onto indium-tin oxide (ITO), the first electrode, and the second electrode is a high work function metal such as gold. If the device contains organic materials, e.g.
  • ITO is often used as the transparent (first) electrode in combination with a low work function metal as the second electrode.
  • Suitable high work function materials for use as an electrode in the present invention may be selected from the group comprising ITO, tin oxide, aluminum or indium doped zinc oxide, gallium doped zinc oxide, magnesium-indium oxide, cadmium tin-oxide, gold, silver, nickel, palladium and platinum.
  • ITO is used as the transparent electrode in the claimed photovoltaic devices.
  • conducting polymers such as PANI (polyaniline) or PEDOT can also be used.
  • the electrode material is deposited by sputtering or vapour deposition as appropriate.
  • Suitable low work function materials for use as an electrode in the present invention may be selected from the group including Li, Na, K, Rb, Be, Mg, Ca, Sr, Ba, Yb, Sm and Al.
  • suitable low work function electrode materials maybe an alloy of such metals or an alloy of such metals in combination with other metals, for example the alloys MgAg and LiAl.
  • the electrode may thus comprise multiple layers, for example Ca/ Al, Ba/ Al, or LiF/ Al.
  • the device may further comprise a layer of dielectric material between the cathode and the emitting layer, such as is disclosed in WO 97/42666.
  • an alkali or alkaline earth metal fluoride may be used as a dielectric layer between the cathode and an organic semiconductor.
  • photovoltaic region pertains to the whole of the structure of the photovoltaic device between the first electrode and the second electrode, including both transparent and photoactive materials.
  • a "photoactive" material is one that absorbs photons to make a significant contribution to the photocurrent of the device.
  • the photovoltaic region includes at least one photoactive layer which may be, for example, the electron donor layer or the electron acceptor layer or both.
  • the electron donor layer is typically an organic layer.
  • the organic electron donor layer is photoactive, i.e. photons absorbed by that layer make a contribution to the photocurrent of the device, typically each photon resulting in one electron and one hole.
  • the electron acceptor layer is typically transparent, e.g. titania.
  • CdSe is an example of a suitable photoactive electron acceptor layer.
  • the electron donor layer comprises an organic photoactive compound.
  • Suitable organic compounds that may be used as the electron donor layer are well known in the art.
  • four organic semiconductors commonly used in photovoltaic cells are poly(2-methoxy-5-(2'-ethylhexoxy)-l,4-phenylenevinylene (MEH-PPV), poly(3-hexylthiophene) (P3HT), poly[2-methoxy-5-(3',7'- dimemyloctyloxy)-/?-phenylene-vinylene] (OC 1 C 1O -PPV) 5 poly[2-methoxy-5-(3',7'- dimethyloctyloxy)-l,4-phenylene vinylene] (MDMO-PPV) and copper phthalocyanine (CuPc).
  • MEH-PPV poly(2-methoxy-5-(2'-ethylhexoxy)-l,4-phenylenevinylene
  • P3HT poly(3-hexylthiophene)
  • the electron donor is a conjugated organic polymer.
  • Particularly suitable polymers include substituted and unsubstituted polyarylene, poly(arylene- vinylene), poly(arylene-acetylene) homo- and copolymers. Such polymers are described by J. L. Segura, Acta. Polym., 1998, 49, 319.
  • Particularly suitable substituents for these polymers include alkyl and alkoxy groups, which can impart polymer solubility in polar aprotic solvents such as toluene, chlorobenzene, tetrahydrofuran and chloroform. C 1-10 alkyl and C 1-10 alkoxy are examples of typical polymer substituents.
  • the electron donor layer may comprise MEH-PPV, P3HT, MDMO-PPV or OC 1 C 10 -PPV, for instance.
  • the polymer layer can consist entirely of a polymer comprising monomer units of a suitable polymer (e.g. MEH-PPV, P3HT, MDMO-PPV or OC 1 C 1O -PPV). Equally, the polymer layer can consist entirely of a copolymer, terpolymer, tetrapolymer or a polymer of any number of different monomer units from suitable polymers.
  • the polymer (or copolymer, terpolymer, tetrapolymer, etc.) layer may be blended with other polymers or small molecules to aid light absorption, charge separation and/or charge transport.
  • hole transporting materials such as TPD (N,N'-diphenyl-N,N I -bis(3-methylphenyl)[l,r-biphenyl]-4,4 l -diamine), NPD (4,4'- bis[N-naphthyl)-N-phenyl-amino]biphenyl) and MTDATA may be added.
  • the polymer layer comprises MEH-PPV.
  • the electron donor layer is from 5 run to 500 nm thick but is more typically from 10 nm to 150 nm thick, for instance from 10 nm to 75 nm thick. Even more typically, the electron donor layer has a thickness of from 20 nm to 50 nm, or, for instance, a thickness of from 25 nm to 35 nm.
  • the electron acceptor layer comprises an inorganic electron- transporting semiconductor such as titania, C 60 or CdSe. More typically, the electron acceptor layer comprises titania (TiO 2 ). Titania does not absorb light like CdSe, but it has certain potential advantages over CdSe as an electron accepting material.
  • titania As the electron acceptor layer and a conjugated polymer as the electron donor layer in a photovoltaic cell is the fact that the titania can be patterned into a continuous network for electron transport. This allows for a fairly high volume fraction of the conjugated polymer to be used in films, as long as the titania and polymer can be structured so that excitons can be dissociated and the resultant charges transported to their respective electrodes.
  • titania is that it is non- toxic, and many molecules can be attached to its surface, such as a dye for making a dye-sensitised photovoltaic cell.
  • the electron acceptor layer comprises porous titania.
  • porous titania means either nanoporous titania or columnar titania, as opposed to dense or “solid” TiO 2 .
  • Nanoporous titania contains pores with diameters in the nanometer scale, and can be formed synthetically, for example by sintering together titania nanocrystals, as is decribed in by K.M Coakley et al., Chem. Mater. 2004, 16, 4533-4542. Layers of such nanoporous titania can be deposited using doctor blade coating techniques, which are well known to the skilled person.
  • layers of nanoporous titania can be produced using Glancing Angle Deposition (GLAD), which is a technique for fabricating materials with controlled structure.
  • GLAD Glancing Angle Deposition
  • the use of GLAD to fabricate porous structures is outlined in US Patent 5,866,204.
  • GLAD may also be used to produce porous titania in columnar form, as is outlined in JP2000231943 and JP2002170557.
  • An advantage of using a porous electron acceptor layer such as nanoporous titania or columnar titania produced by GLAD) in conjunction with an electron donor layer of an organic conjugated polymer is that devices can be fabricated in which the polymer infiltrates into the pores of the titania, thus increasing the interfacial area between the two semiconductors for exciton splitting.
  • the electron-acceptor layer comprises columnar titania.
  • the electron acceptor layer has a thickness of from 25 nm to 2 ⁇ m, for instance from 50 nm to 1.5 ⁇ m. More typically, the electron acceptor layer has a thickness of from 150 nm to 1.2 ⁇ m.
  • blocking layer refers to a solid layer that is suitable for (a) accepting injection of, and conducting, the type of charge carrier that should be travelling through it, and (b) decreasing recombination (i.e. limiting the process of electron-hole recombination) at the interface at which the blocking layer is disposed.
  • a blocking layer disposed at the interface between an electron-acceptor layer and an ITO electrode would be a layer that accepts injection of and conducts electrons but reduces short-circuiting (i.e. limits holes recombining with electrons at the ITO electrode).
  • the present invention is concerned with devices in which a blocking layer is disposed at the interface between an electron acceptor layer and an electron donor layer.
  • These blocking layers are layers suitable for reducing interfacial recombination (i.e. limiting holes recombining with electrons) at the charge separation interface between the electron donor layer and electron acceptor layer.
  • these blocking layers accept injection of and conduct electrons to the bulk electron acceptor layer or accept injection of and conduct holes to the bulk electron donor layer.
  • these conditions may be satisfied if the bottom of the conduction band of the blocking layer is of a higher energy than that of the electron acceptor layer and the top of the valance band of the blocking layer is lower than that of the hole acceptor layer.
  • the energy difference in both cases is typically higher than kT, more typically 3kT and even more typically 5kT, but typically not more than 1OkT since this reduces charge injection (k is the Boltzmann constant and T is temperature).
  • the energy misfit can be measured from a combination of measuring the bandgap of the respective materials, for example by optical spectroscopy and measuring the ionisation potentials or electron affinities of the respective materials using photo-electron spectroscopy. As shown in Figure 6, IPCE measurements of devices with and without the blocking may be compared to give an indication of an offset.
  • the energy misfit of the blocking layer (a) suppresses electron / hole back-transfer from the conduction band / valance band of the electron acceptor / donor layer to the HOMO / LUMO level (or valence band / conduction band) of the electron donor / acceptor layer, but (b) does not excessively impede electron / hole injection from the photoexcited electron donor / acceptor layer into the conduction / valance band of the blocking layer (followed by passage of the electron / hole into the lower / higher energy band of the electron acceptor / donor layer).
  • the band gap of the blocking layer should be wider than the corresponding band gap of the electron acceptor / donor layer, to reduce interfacial recombination, but not so wide as to excessively impede electron / hole injection.
  • the size of the band gap of a material may be influenced by a number of factors, including the choice of material itself, the phase of the material, the density of the material, the thickness of the layer of the material and the level of impurities (e.g. dopant atoms) in the material. By manipulating these factors it is possible to prepare a blocking layer having a suitable band gap in order that interfacial recombination is reduced but electron / hole injection is not impeded excessively.
  • Electron extraction may be measured using IPCE and compared between devices with and without the blocking layer, to establish its effect. Interfacial recombination may show itself indirectly through transient variations in device performance, direct measurements may be made through measurement and comparison of overall light absorption, photo-luminescence quantum yield of the light absorbing material and current at short circuit conditions.
  • the blocking layer in the devices of the present invention typically accepts injection of and conducts electrons to the bulk electron acceptor layer or accepts injection of and conducts holes to the bulk electron donor layer.
  • the blocking layer typically accepts injection of, and conducts, majority charge carriers.
  • the blocking layer does not block the movement of majority charge carriers at the interface at which the blocking layer is disposed.
  • the blocking layer in the devices of the present invention limits the injection of minority charge carriers.
  • This has the advantages of (a) decreasing recombination (i.e. limiting the process of electron-hole recombination) at the interface at which the blocking layer is disposed, and (b) reducing transient (short- term reversible time-dependent effects) as a result of the minority carriers.
  • these conditions may be satisfied if the bottom of the conduction band of the blocking layer is of a higher energy than that of the electron acceptor layer and the top of the valance band of the blocking layer is lower than that of the hole acceptor layer.
  • the blocking layer thereby provides a "stepped" workfunction at the surface of the electrode, which means that any interfacial recombination would require a two-step energetically disfavoured process, making recombination much more improbable.
  • the blocking layer provides a stepped workfunction which decreases the probability of recombination of holes with electrons.
  • the blocking layer may reduce the recombination of holes with electrons.
  • the blocking layer may limit the injection of minority charge carriers.
  • the blocking layer reduces transient effects due to minority carriers.
  • the blocking layer may reduce transient effects observed on exposure of the device to light.
  • the blocking layer typically accepts injection of, and conducts, electrons to the electron acceptor layer, or accepts injection of, and conducts, holes to the electron donor layer.
  • the blocking layer may accept injection of, and conduct, majority charge carriers.
  • the blocking layer is not photoactive. Rather, the blocking layer is typically transparent.
  • the blocking layer comprises an oxide, a nitride or carbon. More typically, the blocking layer comprises a metal oxide.
  • suitable materials for use as blocking layers at the interface between the electron acceptor layer and the electron donor layer in the photovoltaic devices of the present invention may be selected from titania, alumina, ZnO, SnO 2 , In 2 O 3 , Y 2 O 3 , WO 3 , MgO, GeO 2 , BaTiO 3 , ZrO 2 , N 2 O 5 , SiO 2 , SiN and C.
  • the material for the blocking layer is selected from titania, ZnO, SnO 2 , In 2 O 3 , Y 2 O 3 , WO 3 , MgO, GeO 2 , BaTiO 3 , ZrO 2 , N 2 O 5 , SiO 2 , SiN and C.
  • the blocking layer comprises titania. More typically, the blocking layer consists of titania.
  • the blocking layer is disposed between the electron acceptor layer and the electron donor layer, at the charge separation interface. However, the blocking layer need not interpose the two layers across the whole area of the interface.
  • the blocking layer may interpose the acceptor and donor layers only partially, thus permitting direct contact between the acceptor and donor layers in the remaining, uncovered area or areas.
  • the blocking layer may interpose the acceptor and donor layers across only a certain percentage of the total area of the interface, wherein that percentage may be 90 %, 80 %, 70 %, 60 %, 50 %, 40 %, 30 %, 20 % or 10 %.
  • the blocking layer completely covers the layer on which it was deposited (being either the electron acceptor layer or the electron donor layer), so that direct contact between the electron acceptor layer and the electron donor layer is not possible.
  • the blocking layer has a thickness of less than 200 nm.
  • Typical thickness ranges for the blocking layer are as follows: typical minimum thicknesses for the blocking layer are 3 nm, 5 nm, 8 nm, 10 nm, 15 nm, 20 nm and 25 nm; and typical maximum thicknesses for the blocking layer are 60 nm, 75 nm, 100 nm, 125 nm, 150 nm, 175 nm and 190 nm.
  • Typical ranges for the thickness of the blocking layer include all possible ranges formed by combining a typical minimum thickness from the above list with a typical maximum thickness from the above list.
  • typical rangesin include the following: from 3 nm to 190 nm; from 5 nm to 175 nm; from 8 nm to 150 nm; from 10 nm to 125 nm; from 15 nm to 100 nm; from 20 nm to 75 nm; and from 25 nm to 60 nm.
  • These thicknesses may be material specific. For example, if the band misfit is 0.5eV compared to 0.25eV in the current example, then the optimal thickness may be 5- 10 times smaller.
  • the blocking layer has a low porosity. More typically, the blocking layer is continuous.
  • the blocking layer is a layer of a metal oxide.
  • a suitable metal oxide blocking layer may be formed using a precursor peroxide compound of said metal.
  • precursor metal peroxide compound pertains to any compound containing one or more atoms or ions of the metal and one or more peroxide groups or ions, which compound can be utilised as a source for producing a layer of the corresponding metal oxide in a form suitable for use as a blocking layer.
  • precursor metal peroxide compounds which may be used to form suitable blocking layers of the respective metal oxide compounds are zinc peroxide, tin peroxide, zirconium peroxide and titanium peroxide. Fabricating a blocking layer from a peroxide precursor material has the advantages that no high temperature treatment is required for deposition of the blocking layer, and that a thin, compact, continuous coating is produced, ideally suited for the purpose of a blocking layer.
  • the blocking layer is of titania.
  • the titania has relatively low porosity compared to the porous titania described herein which may be used as the electron acceptor layer.
  • the titania blocking layer is continuous.
  • a suitable titania blocking layer may be formed using a precursor titanium peroxide compound.
  • precursor titanium peroxide compound or "titanium peroxide compound” pertain to any compound containing one or more titanium atoms or ions and one or more peroxide groups or ions, which compound could be utilised as a source for producing a layer of titania suitable for use as a blocking layer.
  • the precursor titanium peroxide compound may be a compound that could be decomposed, reacted or dried to yield a layer of titania.
  • a precursor titanium peroxide compound for use in the present invention is typically in the form of an aqueous solution or a gel.
  • suitable precursor titanium peroxide compounds are peroxo-polytitanic acid and peroxotitanate compounds.
  • typical precursor titanium peroxide compounds are selected from peroxo-polytitanic acid gel and aqueous peroxotitanate solution.
  • Zhong-chun Wang and co-workers describe the preparation of highly agglomerated TiO 2 at near room temperature from peroxo-polytitanic acid gel ⁇ Materials Letters 43, 2000, 87-90) and Yanfeng Gao et al.
  • the photovoltaic devices of the present invention may further comprise organic layers between the anode and cathode to improve charge extraction and device efficiency.
  • a layer of conductive or hole-transporting material may be situated over the anode. This layer serves to increase charge conduction through the device.
  • a typical anode coating in polymer devices is a conductive organic polymer such as polystyrene sulfonic acid doped polyethylene dioxythiophene (PEDOT:PSS) as disclosed in WO98/05187.
  • Other hole transporting materials such as doped polyaniline, TPD, NPD and MTDATA may also be used.
  • a layer of electron transporting material may be next to. the cathode as this can improve device efficiency. Suitable materials for electron transporting layers include BCP, TPBI and PBD.
  • the substrate of the photovoltaic device should provide mechanical stability to the device and may be required to act as a barrier to seal the device from the environment. Where it is desired that light enters the device through the substrate, the substrate should be transparent or semi-transparent. Glass is widely used as a substrate due to its excellent barrier properties and transparency. Other suitable substrates include ceramics, and plastics such as acrylic resins, polycarbonate resins, polyester resins, polyethylene terephthalate resins and cyclic olefin resins. Plastic substrates may require a barrier coating to ensure that they remain impermeable.
  • the substrate may comprise a composite material such as the glass and plastic composite. To provide environmental protection the device may be encapsulated.
  • Encapsulation may take the form of a glass sheet which is glass bonded to the substrate with a low temperature frit material.
  • a layer of passivating material may be deposited over the device.
  • Suitable barrier layers comprise a layered structure of alternating polymer and ceramic films and may be deposited by vacuum deposition.
  • Photovoltaic devices of the invention may be produced by any suitable process known to those skilled in the art, or by the processes described herein including in the examples below.
  • a process for producing a photovoltaic device of the invention comprises: (i) forming the electron acceptor layer on, or over, a substrate comprising the first electrode; (ii) forming the blocking layer on, or over, the electron acceptor layer; (iii) forming the electron donor layer on, or over, the blocking layer; and (iv) forming the second electrode on, or over, the electron donor layer.
  • the substrate comprising a first electrode is indium tin oxide (ITO) on glass.
  • the electron donor layer, electron acceptor layer and blocking layer may be deposited by evaporation, Glancing Angle Deposition (GLAD), sputtering, spin coating, doctor blading or solution processing techniques, as appropriate, providing that any subsequent solution processing step does not substantially remove the already deposited layers.
  • Doctor blading is a well-known process for producing a layer of material in which a slurry of the material is spread onto the substrate using the edge of a moving blade.
  • forming the electron acceptor layer, electron donor layer and/or blocking layer involves deposition of the layer, followed by drying the as-deposited layer. More typically, the step of forming the electron acceptor layer, electron donor layer and/or blocking layer involves deposition of the layer, followed by drying and annealing the deposited layer.
  • the annealing is carried out at a temperature of from 400 0 C to 550 0 C.
  • the electron donor layer is an organic conjugated polymer and this polymer is soluble it may be advantageously deposited by solution processing techniques.
  • Solution processing techniques include selective methods of deposition such as screen printing and ink-jet printing and non-selective methods such as spin coating and doctor blade coating.
  • the polymer is deposited by spin coating. If a precursor polymer is used, then after solution processing it is thermally converted under vacuum or in an inert atmosphere to the conjugated polymer.
  • the blocking layer is formed on the electron acceptor layer using a precursor metal peroxide compound.
  • the metal oxide is titania and the precursor metal peroxide compound is a titanium peroxide compound.
  • the blocking layer is formed by spin coating.
  • the material of the first and/or second electrode may be deposited by sputtering, vapour deposition or spin coating.
  • a process for producing a photovoltaic device in which the blocking layer is a metal oxide blocking layer and is disposed between the electron acceptor layer and the electron donor layer, comprises the step of forming said blocking layer using a precursor metal peroxide compound.
  • the metal oxide is titania and the precursor metal peroxide compound is a titanium peroxide compound.
  • the step of forming said blocking layer from said precursor metal peroxide compound comprises spin-coating a solution of the metal peroxide compound over the electron-acceptor layer.
  • spin-coating can be used to produce a titania layer that has a thickness which is particularly desirable for use as a blocking layer, as demonstrated in the Examples below.
  • the step of fabricating said blocking layer from said precursor compound may further comprise the step of annealing the spin-coated layer to form said blocking layer.
  • the process comprises (i) forming the electron acceptor layer on, or over, a substrate comprising the first electrode; (ii) forming the blocking layer on, or over, the electron acceptor layer; (iii) forming the electron donor layer on, or over, the blocking layer; and (iv) forming the second electrode on, or over, the electron donor layer.
  • the devices were prepared by depositing a titania layer (either from peroxide solution or from a paste) onto pre-cleaned patterned ITO on glass, followed by drying and annealing. A further titania layer was then deposited, if required, followed by drying and annealing again. A polymer (MEH-PPV) layer was then spin coated onto the uppermost titania layer. Finally, gold was evaporated onto the polymer surface, to yield devices having the general structure: ITO - Titania - Polymer - Gold
  • the standard porous titania layers were produced from Solaronix paste, through the doctor blade method as described herein. These layers were of the order of 1 micron thick.
  • Commercial TiO 2 paste (HT) was obtained from Solaronix SA (Switzerland). This paste has principally been designed for fabricating micron-thick porous layers to be used in dye-sensitised solar cells.
  • the colloidal paste is composed of anatase (1 lwt%), water, ethanol and a surfactant: polyethylene oxide.
  • a doctor blade method was used to apply layers.
  • a thin adhesive tape, 3M was used in the doctor-blade process to mask the end ITO contacts and control the thickness of the deposited layer.
  • a glass microscope slide was used as the blade. The deposited layers were allowed to dry at 100°C for a few minutes before annealing in air at 45O 0 C on a hotplate.
  • the peroxide-based titania layers were fabricated from a peroxide precursor solution: aqueous peroxotitanate solution.
  • the aqueous peroxotitanate solution was obtained from CCIC (Japan).
  • the solution was synthesized by dissolving metatitanic acid (H 2 TiO 3 ) in a mixture OfH 2 O 2 and H 2 O.
  • the resultant solution is yellow in appearance and probably contains numerous Ti complex species.
  • the water peroxide solution OfH 2 TiO 3 was supplied by CCIC and stored in a refrigerator. Before spin coating or drop coating, the peroxide solution was first filtered through a 0.45 ⁇ m filter to remove precipitates.
  • the thickness of the peroxide-based titania layers formed using spin coating was of the order of 50 nm or less, and the thickness of the peroxide-based titania layers formed using drop coating was of the order of 200 nm.
  • Spin coating was done at 2000 rpm for 60 s.
  • Spin coated layers were prepared by drop casting the substrate with a filtered titania peroxide solution and spinning was carried out at 2000 r.p.m. for 60 seconds on a Dynapert PRS 14E spinner for photoresists.
  • Drop coating involved allowing the substrate to soak under drops of the solution for an hour, during which the solution was not allowed to dry and was then rinsed in water. This was achieved by placing the drop covered substrate in a dry petri dish within a larger water filled dish which was then covered.
  • the MEH-PPV polymer layer was then fabricated by spin coating MEH-PPV from solution in chlorobenzene onto the titania layers, producing a polymer layer approximately 3 Onm thick.
  • Gold contacts were then evaporated under vacuum to a thickness of 45 nm.
  • the resulting devices had the following configuration: ITO/TiO x /MEH-PPV/Au, where the TiO x layer varied in composition as described below.
  • devices were fabricated in which the following layers were deposited onto patterned ITO on pre-cleaned glass substrates in the following combinations:
  • Table 1 shows the measured device parameters for the as-manufactured devices. Short circuit current (J sc ), open circuit voltage (V oc ), fill factor (FF) and power conversion efficiency (PCE) (standard deviation indicated in brackets) are shown. Measurements were taken after the indicated time (ilium, time) in hours and minutes under 78.4 mWcm "2 (0.78 sun intensity) simulated solar illumination, with all light below 420nm blocked. All values are averages over the given number of pixels.
  • Table 2 shows the device parameters measured following heat treatment under moderate vacuum at 200°C for 5 minutes.
  • Figure 4 shows IPCE curves taken from a scan down in wavelength (from 650nm to 350nm) immediately followed by a scan upwards in wavelength on the device of Example 4. Comparison of Figures 3 and 4 shows a reduced hysteresis in Figure 4; the device with the peroxide spin-coated overlayer is affected less by the exposure to higher frequency radiation, so there is a better overlap of the two curves in Figure 4.
  • Figure 5 shows a comparison of IPCE data from scans starting at long wavelengths on the two samples.
  • Figure 6 shows the ratio of the IPCE curves (shown in Figure 5) of the samples of Comparative Example 2 and Example 4 as a function of wavelength, illustrating a rising trend to low energy consistent with the peroxide acting as an electron blocking layer.
  • the peroxide layer can act as an electron blocking layer.
  • excitons dissociate, the electrons pass from the polymer into the peroxide layer, and from there into the lower energy state in the doctor-blade paste, away from the polymer interface.
  • this would limit the transient effects resulting from the recombination of electrons in the titania with holes in the polymer (as has been predicted by modelling work carried out by the present inventors) to limit device currents, particularly in structured interfaces. If the electron within an exciton has an energy lower than the acceptor level, it cannot be injected into the TiO x .

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Abstract

L'invention concerne un dispositif photovoltaïque comprenant une première électrode, une seconde électrode et une région photovoltaïque disposée entre la première et la seconde électrode. La région photovoltaïque comprend une couche accepteuse d'électrons, une couche donneuse d'électrons et une couche de blocage destinée à limiter la recombinaison des charges interfaciales. La couche de blocage est disposée entre la couche accepteuse d'électrons et la couche donneuse d'électrons. L'invention concerne également des procédés permettant de produire un dispositif photovoltaïque, notamment, des procédés mettant en oeuvre un composé peroxyde de métal précurseur pour former la couche de blocage, et les utilisations correspondantes dudit composé peroxyde de métal précurseur.
PCT/GB2006/004426 2005-11-25 2006-11-27 Dispositif photovoltaique WO2007060465A2 (fr)

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EP2243172A1 (fr) * 2008-02-12 2010-10-27 The Governors Of The University Of Alberta Dispositif photovoltaïque à base de revêtement enrobant de structures colonnaires
WO2010120393A3 (fr) * 2009-01-12 2011-05-19 The Regents Of The University Of Michigan Amélioration de la tension de circuit ouvert de cellules photovoltaïques organiques utilisant des couches de blocage d'excitons bloquant les électrons/trous
WO2011110869A3 (fr) * 2010-03-11 2011-11-03 Isis Innovation Limited Dispositif d'hétérojonction photosensible à semi-conducteur
CN102625956A (zh) * 2009-06-08 2012-08-01 牛津大学技术转移公司 固态异质结装置
WO2013078585A1 (fr) * 2011-11-28 2013-06-06 海洋王照明科技股份有限公司 Dispositif électroluminescent polymère et procédé de préparation de ce dispositif
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Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP2243172A1 (fr) * 2008-02-12 2010-10-27 The Governors Of The University Of Alberta Dispositif photovoltaïque à base de revêtement enrobant de structures colonnaires
EP2243172A4 (fr) * 2008-02-12 2012-03-28 Univ Alberta Dispositif photovoltaïque à base de revêtement enrobant de structures colonnaires
WO2010120393A3 (fr) * 2009-01-12 2011-05-19 The Regents Of The University Of Michigan Amélioration de la tension de circuit ouvert de cellules photovoltaïques organiques utilisant des couches de blocage d'excitons bloquant les électrons/trous
CN104835912A (zh) * 2009-01-12 2015-08-12 密歇根大学董事会 利用电子/空穴阻挡激子阻挡层增强有机光伏电池开路电压
CN102625956A (zh) * 2009-06-08 2012-08-01 牛津大学技术转移公司 固态异质结装置
WO2011110869A3 (fr) * 2010-03-11 2011-11-03 Isis Innovation Limited Dispositif d'hétérojonction photosensible à semi-conducteur
CN103119673A (zh) * 2010-03-11 2013-05-22 牛津大学技术转移公司 光敏固态异质结装置
WO2013078585A1 (fr) * 2011-11-28 2013-06-06 海洋王照明科技股份有限公司 Dispositif électroluminescent polymère et procédé de préparation de ce dispositif
CN104012179A (zh) * 2011-11-28 2014-08-27 海洋王照明科技股份有限公司 聚合物电致发光器件及其制备方法
GB2512796B (en) * 2012-02-10 2016-07-20 Bangor Univ Sintering of dye-sensitised solar cells using metal peroxide

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