WO2016183589A1 - Metal micro-grid electrode for highly efficient si microwire solar cells with over 80% fill factor - Google Patents

Metal micro-grid electrode for highly efficient si microwire solar cells with over 80% fill factor Download PDF

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WO2016183589A1
WO2016183589A1 PCT/US2016/032774 US2016032774W WO2016183589A1 WO 2016183589 A1 WO2016183589 A1 WO 2016183589A1 US 2016032774 W US2016032774 W US 2016032774W WO 2016183589 A1 WO2016183589 A1 WO 2016183589A1
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micro
grid
solar cells
electrode
junction
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PCT/US2016/032774
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French (fr)
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Han-Don Um
Inchan HWANG
Namwoo Kim
Young-June Yu
Kwanyong Seo
Munib Wober
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Zena Technologies, Inc.
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Publication of WO2016183589A1 publication Critical patent/WO2016183589A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/02—Details
    • H01L31/0224—Electrodes
    • H01L31/022408—Electrodes for devices characterised by at least one potential jump barrier or surface barrier
    • H01L31/022425—Electrodes for devices characterised by at least one potential jump barrier or surface barrier for solar cells
    • H01L31/022433—Particular geometry of the grid contacts
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/0352—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their shape or by the shapes, relative sizes or disposition of the semiconductor regions
    • H01L31/035209—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their shape or by the shapes, relative sizes or disposition of the semiconductor regions comprising a quantum structures
    • H01L31/035227—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their shape or by the shapes, relative sizes or disposition of the semiconductor regions comprising a quantum structures the quantum structure being quantum wires, or nanorods
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/04—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
    • H01L31/06—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices characterised by at least one potential-jump barrier or surface barrier
    • H01L31/068—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices characterised by at least one potential-jump barrier or surface barrier the potential barriers being only of the PN homojunction type, e.g. bulk silicon PN homojunction solar cells or thin film polycrystalline silicon PN homojunction solar cells
    • 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
    • Y02E10/547—Monocrystalline silicon PV cells

Definitions

  • a photovoltaic device also called a solar cell is a solid state device that converts the energy of sunlight directly into electricity by the photovoltaic effect. Assemblies of cells are used to make solar modules, also known as solar panels. The energy generated from these solar modules, referred to as solar power, is an example of solar energy.
  • the photovoltaic effect is the creation of a voltage (or a corresponding electric current) in a material upon exposure to light. Though the photovoltaic effect is directly related to the photoelectric effect, the two processes are different and should be distinguished. In the photoelectric effect, electrons are ejected from a material's surface upon exposure to radiation of sufficient energy. The photovoltaic effect is different in that the generated electrons are transferred between different bands (i.e. from the valence to conduction bands) within the material, resulting in the buildup of a voltage between two electrodes.
  • Photovoltaics is a method for generating electric power by using solar cells to convert energy from the sun into electricity.
  • the photovoltaic effect refers to photons of light- packets of solar energy-knocking electrons into a higher state of energy to create electricity. At higher state of energy, the electron is able to escape from its normal position associated with a single atom in the semiconductor to become part of the current in an electrical circuit.
  • These photons contain different amounts of energy that correspond to the different wavelengths of the solar spectrum. When photons strike a PV cell, they may be reflected or absorbed, or they may pass right through. The absorbed photons can generate electricity.
  • the term photovoltaic denotes the unbiased operating mode of a photodiode in which current through the device is entirely due to the light energy. Virtually all photovoltaic devices are some type of photodiode.
  • Silicon microwires have been extensively investigated as a potential means for developing highly efficient and low-cost solar cells thanks to their superior optical absorption and effective carrier separation.
  • Vertically-aligned Si MW arrays have shown drastically enhanced light trapping with ultimately reduced surface reflection of less than 4 %.
  • wire-based solar cells employing a radial p-n junction are beneficial because light absorption and carrier collection are decoupled through their orthogonal orientation.
  • photo carriers can be separated along the relatively short radial direction (up to wire radius, ⁇ 10 ⁇ ) compared to conventional solar cells that require the diffusion length of - 100 ⁇ .
  • PCEs power conversion efficiencies
  • Si MW solar cells are still relatively low compared to that of conventional crystalline Si solar cells.
  • TCEs transparent conducting electrodes
  • ITO indium tin oxide
  • silver nanowires network have been attempted to form a contact between Si wires and TCEs, but some reports have shown relatively low cell efficiencies caused by low values of experimental FF; e.g., 28% for integration of ITO and metal grids as bus bars and 55% for Ag nanowire networks with Ni nanoparticles.
  • the serious degradation of FF is mainly due to high contact resistances at the interface between Si and TCEs.
  • the charge carriers can be collected from Si MWs despite the point-contact with only micro-scale area, as a degenerately-doped emitter layer is relatively conductive for the carrier diffusion in the small cells requiring shorter diffusion length.
  • robust Ohmic contact was
  • edge electrode selectively-patterned Al electrode onto planar area
  • highest FF value was obtained without bus/finger bars
  • the PCE of MWs solar cell is still not comparable with that of the conventional cell due to a serious Auger/surface recombination in the radial junction MWs with larger junction and surface areas, resulting from a highly- conductive emitter with deep junction (sheet resistance, R Sh : ⁇ 30 ⁇ /sq).
  • a device comprising Si microwire solar cells with over 80% fill factor.
  • the device could further comprise a micro-grid electrode on a grid pattern having a grid spacing of 1 to 1000 microns, wherein the micro-grid electrode is attached to the Si microwire solar cells via a junction.
  • the grid spacing is 10 to 500 microns.
  • the grid spacing is 50 to 400 microns.
  • the micro-grid electrode allows the junction to function properly without optical and electrical losses.
  • the junction has a sheet resistance of 10-500 ⁇ /sq.
  • the junction has a sheet resistance of approximately 100 ⁇ /sq.
  • power conversion efficiency of the Si microwire solar cell is enhanced by collecting photocarriers through the micro-grid electrode without substantial Auger/surface recombination.
  • the Si microwire solar cells with areas of 1 cm 2 show a conversion efficiency of up to 16.5 %, with an open-circuit voltage of 565.2 mV and a short-circuit current density of 35.9 mA/cm 2 .
  • the fill factor is 81.2 %.
  • the micro-grid electrode comprises a Ni electrode of about 1 ⁇ thickness.
  • J-V density-voltage
  • FIG. 1 Optical microscope images of metal micro-grids on glass substrates with a line width of 2 ⁇ and different grid spacings of 50, 100, 200, and 400 ⁇ .
  • (e) Optical transmission (black solid symbol), averaged over the main spectral range from 400 to 1000 nm, and sheet resistance (blue solid symbol) of micro-grid as a function of grid spacing.
  • the red solid line represents the calculated sheet resistance of the micro-grid electrode.
  • the sheet resistance of deep junction emitter layer blue dashed line.
  • the inset shows the optical image of the micro-grid on glass substrate, showing the symbol mark (UNIST) in the
  • FIG. 3 (a) J-V curves of planar junction Si solar cells with different spacing of metal micro-grid; 50 (black), 100 (red), 200 (green), and 400 (blue solid line and symbol) ⁇ . (b) Open-circuit voltage (black) and short-circuit current density (blue solid line and symbol) extracted from the J-V curves plotted in panel a. (c) Fill factor (red) and efficiency (green solid line and symbol) are plotted as a function of grid spacing. The efficiency was improved by increasing the spacing of micro-grid due to higher transmittance of wider micro-grid.
  • Figure 4. (a) Schematic illustration, (b) optical, and (c-d) SEM images of Si MWs solar cells with micro-grid electrode. The vertically-aligned Si MW arrays were formed between grid lines.
  • Figure 5. (a) J-V curves of Ni film electrode measured by a solar simulator (red) and a Suns- Voc (blue solid line). The inset shows a thick Ni film of micro-grid electrode deposited by the electroplating method, (b) Two J-V curves (red: solar simulator, blue solid line: Suns- Voc) and power output curve (red dashed line) of Al film electrode.
  • Figure SI (a) Optical and (b) SEM images of Si microwires solar cells with the edge electrode, (c) J-V curves of planar and microwires solar cells with various lengths of Si microwires. (d) Secondary ion mass spectrometry profile of the deep emitter with sheet resistance of - 30 ⁇ /sq. Increase in length of microwires intensifies the serious degradation of the power conversion efficiency due to increase in areal density of degenerately-doped region on microwire surfaces.
  • Figure S2 Secondary ion mass spectrometry profile of the shallow emitter with sheet resistance of -100 ⁇ /sq in which junction depth was estimated to be -220 nm and surface concentration was measured to be 6.2 x 10 19 cm -3 .
  • Figure S3. Transmittance spectra of the micro-grids onto the glass substrates. An overall increase in transmittance is observed for larger spacing, which is attributed to the decrease in the surface area that is covered by Al film as the spacing increases.
  • a photovoltaic device operable to convert light to electricity, comprising a substrate, a plurality of structures essentially perpendicular to the substrate, and one or more recesses between the structures.
  • photovoltaic device as used herein means a device that can generate electrical power by converting light such as solar radiation into electricity.
  • single crystalline as used herein means that the crystal lattice of the entire structures is continuous and unbroken throughout the entire structures, with no grain boundaries therein.
  • An electrically conductive material can be a material with essentially zero band gap. The electrical conductivity of an electrically conductive material is generally above 10 3 S/cm.
  • a semiconductor can be a material with a finite band gap upto about 3 eV and general has an electrical conductivity in the range of 10 3 to 10 ⁇ 8 S/cm.
  • An electrically insulating material can be a material with a band gap greater than about 3 eV and generally has an electrical conductivity below 10 ⁇ 8 S/cm.
  • structures essentially perpendicular to the substrate as used herein means that angles between the structures and the substrate are from 85° to 90°.
  • the term "recess” as used herein means a hollow space in the substrate and is open to a space outside the substrate.
  • the single crystalline semiconductor material could be selected from a group consisting of silicon, germanium, group III-V compound materials, group II- VI compound materials, and quaternary materials.
  • a group III-V compound material as used herein means a compound consisting of a group III element and a group V element.
  • a group III element can be B, Al, Ga, In, Tl, Sc, Y, the lanthanide series of elements and the actinide series of elements.
  • a group V element can be V, Nb, Ta, Db, N, P, As, Sb and Bi.
  • a group II- VI compound material as used herein means a compound consisting of a group II element and a group VI element.
  • a group II element can be Be, Mg, Ca, Sr, Ba and Ra.
  • a group VI element can be Cr, Mo, W, Sg, O, S, Se, Te, and Po.
  • a quaternary material is a compound consisting of four elements.
  • the structures could be cylinders or prisms with a cross-section selected from a group consisting of elliptical, circular, rectangular, and polygonal cross-sections, strips, or a mesh.
  • the term "mesh” as used herein means a web-like pattern or construction.
  • the structures could be pillars with diameters from 50 nm to 5000 nm, heights from 1000 nm to 20000 nm, a center-to- center distance between two closest pillars of 300 nm to 15000 nm.
  • electrode as used herein means a conductor used to establish electrical contact with the photovoltaic device.
  • the substrate could have a flat surface opposite the structures.
  • the flat surface could have a doped layer and optionally a metal layer metal layer disposed on and forming an Ohmic contact with the doped layer.
  • An Ohmic contact is a region a current-voltage (I-V) curve across which is linear and symmetric.
  • the substrate could have a thickness of at least 50 microns.
  • the structures could have pillars arranged in an array; each structure is about 5 microns in height; a pitch of the structures is from 300 nm to 15 microns.
  • the strategies for improving the performance of MWs solar cells should include i) reducing the dopant concentration and junction depth of MW emitter so as to suppress Auger recombination, which leads to higher Voc and Jsc, ( ⁇ ) narrowing the distance between electrode to collect the charge carriers from non-conductive emitter (i.e., shallow junction) without the electrical loss, which increases FF, and (iii) minimizing the area of metal electrode on the cell surface for reducing shading loss, which directly enhances Jsc-
  • Applicants incorporate a novel metal micro-grid to the MWs solar cells as top electrode. Owing to superior optical transmittance and electrical conductivity of micro-grid electrodes, robust and reliable Ohmic contacts would be realized for effective collection of charge carriers through the top electrode while diminishing the loss of light absorption associated with shading by metal electrode. In addition, the electrical conductivity of the micro-grid electrode was significantly improved by electroplating a thick Ni film with thickness of about 1 ⁇ , leading to a FF of over 81% in the MWs solar cells. The remarkable improvement of FF could be originated from the decreased contact resistance between metal-grid and emitter layer as well as the low resistance of micro-grid electrode.
  • Applicants' MWs solar cells with metal micro-grid electrode exhibit a conversion efficiency of 16.5% which is 72% improved value compared to that with the edge electrode.
  • This novel electrode structure is therefore considered an important step toward the practical realization of highly efficient microwire solar cells in the future.
  • Figure la shows the experimental and simulated current density-voltage (J-F) curves for the edge electrode cells with deep (R Sh of -30 ⁇ /sq) and shallow (R Sh of -100 ⁇ /sq) junctions. Simulation results were obtained by using the one-dimensional solar cell modeling program (PC ID) and are well-matched with experimental data.
  • the J- V curve shows a relatively sudden increase of current density value and lower Voc compared to that of the deep junction.
  • the carriers generated in the shallow junction are ineffectively collected through the edge electrode with the high series resistance due to the insufficient conductivity of the emitter.
  • the value of Jsc was significantly increased from 20.2 to 25 mA/cm 2 owing to reduction of Auger recombination because depth of the highly-doped region (> 10 20 cm -3 ) is decreased (supporting information Figure S2).
  • FF value is significantly decreased with increase in the sheet resistances of emitter layer only except the case of extremely small cell size of 0.01 cm 2 , as shown in Figure lb.
  • the simulation results indicate that the design of the edge electrode would not be effective for the charge collection in the large-scale cells due to the serious degradation of FF. Therefore the top metal electrode should be re-designed to collect the charge carriers effectively while minimizing the optical and electrical losses.
  • Figure 2a-d show optical microscope images of novel metal micro-grids on glass substrates with a line width of 2 ⁇ and different grid spacings of 50, 100, 200, and 400 ⁇ , which are taken under the same magnification.
  • the micro-grids are successfully fabricated using a lithography process followed by thermal evaporation of 200-nm-thick Al film, resulting in the formation of continuous and uniform square pattern.
  • the optical and electrical properties of micro-grid electrodes were measured as shown in Figure 2e.
  • An averaged transmittance value increases with increasing the spacing, which is attributed to the decrease in the surface area covered by Al film as the spacing increases (supporting information Figure S3).
  • N, p, L, w, and t represent the number of lines in 1 cm width, resistivity of Al, gird spacing, width, and thickness of Al film, respectively, as the micro-grid electrode consisting of N x N lines with the square pattern follows Kirchhoff s rules.
  • the calculated R Sh is simply given by pLIwt because N/(l+N is close to unity.
  • the experimental R sh s are 3.1 + 1.7, 7.2 + 2.0, 14.5 + 1.9, and 24.9 + 2.8 ⁇ /sq for the 50, 100, 200, and 400 ⁇ spacing, respectively. These values are well-matched with the calculated R Sh , showing the good agreement with the model.
  • R Sh of micro-grid electrode linearly scales with the grid spacing.
  • R Sh will be higher than 30 ⁇ /sq which is R Sh of deep junction emitter (horizontally-dashed line at left y-axis in figure 2e) if the grid spacing increases over 450 ⁇ . That is, the solar cell with larger grid spacing than 450 ⁇ would show severe FF degradation as explained with the simulation results in Figure lb.
  • the FF is not greatly influenced by the spacing of micro-grid electrode as shown in Figure 3c.
  • the photocarriers are effectively collected through top electrode without electrical loss if R Sh of emitter is less than 30 ⁇ /sq in 1 cm 2 cell size.
  • Table 1 shows the photovoltaic properties of solar cells with edge and micro-grid electrodes. Due to enhancement of Voc and Jsc, the conversion efficiency is improved from 6.7% (edge electrode) to 11.3% by using metal micro-grid electrode with grid spacing of 400 ⁇ .
  • micro-grid electrode with grid spacing of 400 ⁇ shown the highest PCE in the planar solar cells, was applied to the Si microwire solar cells.
  • the detailed device features are illustrated in Figure 4a.
  • Vertically-aligned Si MW arrays were designed between grid lines, so that the photo-generated carriers in wire arrays could be effectively collected through the top electrode of metal micro-grid owing to shorter diffusion length up to half of grid spacing (-200 ⁇ ).
  • the Si MWs and micro-grid were fabricated using photolithography followed by reactive ion etching (RIE), resulting in the formation of MW arrays in the square patterns of micro-grids as shown in SEM images of Figure 4b and 4c.
  • RIE reactive ion etching
  • Phosphorus doping was then performed using a spin-on-doping (SOD) technique to form shallow p-n junctions with R Sh of -100 ⁇ /sq in which junction depth was estimated to be -220 nm and surface concentration was measured to be 6.2 x 10 19 cm -3 based on the secondary ion mass spectrometry (SIMS) profile (supporting information Figure S2).
  • SOD spin-on-doping
  • SIMS secondary ion mass spectrometry
  • a thin SiN x layer 60-nm- thick) was deposited by plasma-enhanced chemical vapor deposition (PECVD) as a passivation layer as well as an anti -reflection layer.
  • PECVD plasma-enhanced chemical vapor deposition
  • the emitters of both MWs solar cells with the micro-grid electrode and the edge electrode have shallow junction with R Sh of - 100 ⁇ /sq.
  • the PCE increased from 9.6 to 15.4% along with the improved Voc and FF values (supporting information Figure S4).
  • FF (75.1%) of the micro-grid electrode was notably improved compared to the edge electrode (52.1%) which requires longer carrier diffusion length (up to half of cell size, ⁇ 5 mm) through relatively non- conductive channel of shallow emitter.
  • This is mainly due to decrease in the series resistance (R s ) of solar cells which is the sum of the resistance of the metal electrode (R m ), the contact (Rc), the emitter, and the base. Applicants can ignore the influence of the emitter and base resistances, since it is assumed that they do not change with the structure of front electrode.
  • the resistance parameters the Rc can be calculated using 18
  • R Sh is sheet resistance, L a grid length, p c contact resistivity, JFgrid width, L c unit cell length, s grid spacing.
  • This calculation enables the estimation of R s for different s values of the edge and micro-grid electrodes. Since the edge electrode is considered as millimeter-scale grid structure, the spacing of edge electrode (1 cm) is much longer than that of micro-grid electrode (400 ⁇ ). According to Eq. 3, the R c of micro-grid electrode can remarkably be reduced by 25 times compared to edge electrode. Hence, the improved FF could be explained by the fact that the sum of R s and Rc decreased with applying the micro-grid electrode.
  • Voc was also improved from 501 mV to 564 mV, as the recombination is suppressed by transporting carriers to the top electrode with shorter diffusion length, leading to decrease in the reverse saturation current (Eq. 2).
  • Jsc slightly decreased from 37.0 to 36.3 mA/cm 2 in the micro-grid electrode due to the light reflection from the metal grid with only 1% coverage of the front surface
  • the cell efficiency of MWs solar cells is remarkably improved by effectively collecting the carriers with highly conductive micro-grid electrode.
  • the electroplating method enables the deposition of metal film with high deposition rate in the solution for cost-effective solar cells.
  • a ⁇ - ⁇ -thick Ni film deposited using the electroplating method is shown in the inset of Figure 5a. The highly dense, thick Ni layer was uniformly coated with the micro-grid pattern, resulting in the significant improvement of FF from 75.1 to 81.2% for the MWs solar cells.
  • the best performance of the MWs solar cells achieved the PCE of 16.5%, along with V oc and Jsc of 565 mV and 35.9 mA/cm 2 , respectively.
  • the R s values of MWs solar cells with thick Ni film ( ⁇ - ⁇ -thick, electroplating method) and Al electrode (200-nm-thick, thermal evaporation) were extracted by comparing the J-V curves obtained using a solar simulator (red lines) to those measured using a Suns- Voc (blue lines) as shown in Figure 5a and 5b.
  • the R s is calculated by dividing the difference between the voltages in two different J-V curves (AV) by the current density at the maximum power point (J mmp ) given in Figure 5b, as the Suns- Voc measurement provides an ideal J- curve in the absence of R s .
  • the real J-V curve (R s -included, solar simulator) of the ⁇ - ⁇ -thick Ni film is almost overlapped with the ideal J-V curve (R s -excluded, Suns-Foe) as shown in Figure 5a, while the 200-nm-thick Al film electrode has significant voltage gap between two J-V curves (Figure 5b).
  • the extracted R s of the Ni film electrode (0.645 ⁇ cm 2 ) is much lower than that of deposited Al film electrode (1.51 ⁇ cm 2 ).
  • the R Sh of Ni film decreases with increasing the Ni thickness easily controlled by the deposition time, as shown in Figure 5c. Due to the increased thickness, R Sh of the Ni film electrode decreases up to 3.7 ⁇ /sq compared to 24.9 ⁇ /sq of 200-nm-thick Al film electrode (blue symbol in Figure 5c).
  • the Rc value of the Ni film electrode significantly decreased compared to Al film electrode (black symbol) as shown in Figure 5d.
  • the Rc and contact resistivity (p c ) was measured by a transmission line model (TLM).
  • TLM transmission line model
  • the values of p c at the interface between various metals and Si were extracted by measuring the resistance of metal/Si excluding the intrinsic resistances of both metal and Si.
  • the resistances were measured by adjusting the number of pitches between two metal pads. At the zero-point (the intersection with the y-axis in the inset of Figure 5d) of the grid pitches, resistance is induced by the two grid contacts without the influence of the emitter.
  • the extracted p c of Ni/Si contact was 1.69 mQ cm 2 at the 1.35 x 10 ⁇ 3 cm 2 contact area while Al/Si contact achieved the p c of 2.63 mQ cm 2 .
  • the improvement of p c could be explained by new current paths, which were formed by filling pinholes of SiN x layer with Ni layer during electroplating.
  • the R c values of Ni (red line) and Al electrodes (black line) were calculated from Eq. 3 as a function of the grid width ( Figure 5d). In order to obtain lower Rc than that of Ni film electrode, the grid width of the Al film electrode is required to be wider than over 10 ⁇ , leading to the shading loss and then Jsc degradation.
  • the shading loss which is proportional to the grid width can be reduced by using narrower width of Ni film electrode with the same or even lower Rc, as the decrease in Rc of Ni film electrode is mainly due to the reduced p c at the interface between Ni and Si. Therefore, the highest FF of 81.2% could be achieved by reducing not only the metal resistance but also the contact resistance.
  • micro-grid electrodes therefore clearly represent a very promising structure for the high-efficiency MWs solar cells.
  • micro-grid material [mV] [mA/cm 2 ] [%] [%]
  • RTE reactive ion etching
  • An emitter layer was formed by phosphorus diffusion via the spin-on-dopant (SOD) method.
  • SOD spin-on-dopant
  • the Si MWs were covered with photoresist (AZ4330, AZ electronic materials, thickness of - 10 ⁇ ) before the metal deposition using lithography process.
  • photoresist AZ4330, AZ electronic materials, thickness of - 10 ⁇
  • 200-nm-thick Al films were deposited on the top and bottom of samples using a thermal evaporator.
  • the photoresist was removed by dipping samples in acetone solution. Fabrication of micro-grid electrode with a thick Nifilm
  • electroplating method were attempted to deposit a thick Ni film.
  • Ti/Ni as a seed layer for electroplating, was first deposited onto micro-grid pattern of photoresist. After deposition of the seed layer, the residual of photoresist was removed by dipping in acetone solution. Electroplating deposition was then performed in a Ni solution composed of SO 4 .6H 2 O (1 M), CI 2 .6H 2 O (0.5 M), and H 3 BO 3 (1 M) at 40 °C under the constant current density of 5 mA/cm 2 .
  • Optical reflection measurements were performed over wavelengths of 400-1100 nm using a UV-Vis/NIR spectrophotometer (Cary 5000, Agilent) equipped with a 110 mm integrating sphere to account for total light (diffuse and specular) reflected from the samples.

Abstract

Applicants demonstrate novel micro-grid top electrode for highly efficient radial junction Si microwire solar cells. The micro-grid electrode allows the shallow (sheet resistance of -100 Ω/sq) junction emitter to function properly without optical and electrical losses and thus power conversion efficiency of Si microwire solar cell can be enhanced by collecting photocarriers effectively from the shallow emitter through the electrode without serious Auger/surface recombination. By optimizing the micro-grid structure, our microwire solar cells with areas of 1 cm2 show a conversion efficiency of up to 16.5 %, with an open-circuit voltage of 565.2 mV and a short-circuit current density of 35.9 mA/cm2. In particular, over 80 % fill factor (max 81.2 %) is obtained reproducibly due to significant decreases in metal and contact resistances when Applicants applied an electroplating method to form thick Ni electrode of about 1 μπι thickness. The use of the novel micro-grid to build an ideal metal/emitter interface therefore presents a unique opportunity to develop highly efficient microwire solar cells.

Description

METAL MICRO-GRID ELECTRODE FOR HIGHLY EFFICIENT SI MICROWIRE SOLAR CELLS WITH OVER 80% FILL FACTOR
CROSS REFERENCE RELATED APPLICATION
[0001] This application claims priority to U.S. Provisional Application No. 62/161,485, entitled, "Metal Micro-grid Electrode for Highly Efficient Si Microwire Solar Cells with over 80% Fill Factor," and filed on May 14, 2015, the disclosure of which is hereby incorporated by reference in their entirety. This application is related to the disclosures of U.S. Patent Application Nos. 12/204,686, filed September 4, 2008 (now U.S. Patent No. 7,646,943, issued January 12, 2010), 12/648,942, filed December 29, 2009 (now U.S. Patent No. 8,229,255, issued July 24, 2012), 13/556,041, filed July 23, 2012, 15/057,153, filed March 1, 2016, 12/270,233, filed November 13, 2008 (now U.S. Patent No. 8,274,039, issued September 25, 2012), 13/925,429, filed June 24, 2013 (now U.S. Patent No. 9,304,035, issued April 5, 2016), 15/090,155, filed April 4, 2016, 13/570,027, filed August 8, 2012 (now U.S. Patent No. 8,471,190, issued June 25, 2013), 12/472,264, filed May 26, 2009 (now U.S. Patent No. 8,269,985, issued September 18, 2012), 13/621,607, filed September 17, 2012 (now U.S. Patent No. 8,514,411, issued August 20, 2013), 13/971,523, filed August 20, 2013 (now U.S. Patent No. 8,810,808, issued August 19, 2014), 14/459,398 filed August 14, 2014, 12/472,271, filed May 26, 2009 (now abandoned),
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15/093,928, filed April 8, 2016, 13/693,207, filed December 4, 2012, 61/869,727, filed August 25, 2013, 14/322,503, filed July 2, 2014, 14/311,954, filed June 23, 2014, 14/563,781, filed December 8, 2014, 61/968,816, filed March 21, 2014, 14/516,402, filed October 16, 2014, 14/516,162, filed October 16, 2014 and 62/307,018, filed March 11, 2016 are each hereby incorporated by reference in their entirety.
BACKGROUND OF THE INVENTION
[0002] A photovoltaic device, also called a solar cell is a solid state device that converts the energy of sunlight directly into electricity by the photovoltaic effect. Assemblies of cells are used to make solar modules, also known as solar panels. The energy generated from these solar modules, referred to as solar power, is an example of solar energy.
[0003] The photovoltaic effect is the creation of a voltage (or a corresponding electric current) in a material upon exposure to light. Though the photovoltaic effect is directly related to the photoelectric effect, the two processes are different and should be distinguished. In the photoelectric effect, electrons are ejected from a material's surface upon exposure to radiation of sufficient energy. The photovoltaic effect is different in that the generated electrons are transferred between different bands (i.e. from the valence to conduction bands) within the material, resulting in the buildup of a voltage between two electrodes.
[0004] Photovoltaics is a method for generating electric power by using solar cells to convert energy from the sun into electricity. The photovoltaic effect refers to photons of light- packets of solar energy-knocking electrons into a higher state of energy to create electricity. At higher state of energy, the electron is able to escape from its normal position associated with a single atom in the semiconductor to become part of the current in an electrical circuit. These photons contain different amounts of energy that correspond to the different wavelengths of the solar spectrum. When photons strike a PV cell, they may be reflected or absorbed, or they may pass right through. The absorbed photons can generate electricity. The term photovoltaic denotes the unbiased operating mode of a photodiode in which current through the device is entirely due to the light energy. Virtually all photovoltaic devices are some type of photodiode.
[0005] Silicon microwires (MWs) have been extensively investigated as a potential means for developing highly efficient and low-cost solar cells thanks to their superior optical absorption and effective carrier separation. Vertically-aligned Si MW arrays have shown drastically enhanced light trapping with ultimately reduced surface reflection of less than 4 %. In addition, wire-based solar cells employing a radial p-n junction are beneficial because light absorption and carrier collection are decoupled through their orthogonal orientation. In other words, photo carriers can be separated along the relatively short radial direction (up to wire radius, <10 μπι) compared to conventional solar cells that require the diffusion length of - 100 μπι. However, power conversion efficiencies (PCEs) of Si MW solar cells are still relatively low compared to that of conventional crystalline Si solar cells. One of possible reasons of the PCE degradation would be non-optimized emitter layer and top electrode on Si MW arrays. For reliable Ohmic contacts without a shading loss by the metal electrode, various transparent conducting electrodes (TCEs), such as indium tin oxide (ITO) and silver nanowires network, have been attempted to form a contact between Si wires and TCEs, but some reports have shown relatively low cell efficiencies caused by low values of experimental FF; e.g., 28% for integration of ITO and metal grids as bus bars and 55% for Ag nanowire networks with Ni nanoparticles. The serious degradation of FF is mainly due to high contact resistances at the interface between Si and TCEs.
[0006] On the other hand, highest FF value of 81% has been demonstrated based on direct metal contact on a single-MW solar cell. For direct metal contact without using TCEs, a highly conductive Si surface is required for Ohmic contact with low contact resistance. The top electrodes have designed as the metal film onto planar junction area surrounded the square area of MW array, so far. High FFs were obtained from very small sized cells (< 8.5 mm2) integrated in either Tungsten probes (65%) or Indium dots (57%) without bus/finger bars. The charge carriers can be collected from Si MWs despite the point-contact with only micro-scale area, as a degenerately-doped emitter layer is relatively conductive for the carrier diffusion in the small cells requiring shorter diffusion length. In previous work, robust Ohmic contact was
demonstrated by selectively-patterned Al electrode onto planar area (called "edge electrode"), resulting in highest / /7 of 75.2 % from MW array device with a cell size of 1 cm2. Although highest FF value was obtained without bus/finger bars, the PCE of MWs solar cell is still not comparable with that of the conventional cell due to a serious Auger/surface recombination in the radial junction MWs with larger junction and surface areas, resulting from a highly- conductive emitter with deep junction (sheet resistance, RSh:~ 30 Ω/sq). Moreover, an increase in length of MWs also intensifies the serious degradation of all photovoltaic parameters (open- circuit voltage (Voc), Jsc, FF) due to an increase in areal density of degenerately-doped region on MW surfaces (supporting information Figure SI). Therefore, a photovoltaic device that does not have this drawback is desired. BRIEF SUMMARY OF THE INVENTION
[0007] A device comprising Si microwire solar cells with over 80% fill factor. The device could further comprise a micro-grid electrode on a grid pattern having a grid spacing of 1 to 1000 microns, wherein the micro-grid electrode is attached to the Si microwire solar cells via a junction. In one embodiment, the grid spacing is 10 to 500 microns.
[0008] In one embodiment, the grid spacing is 50 to 400 microns.
[0009] In one embodiment, the micro-grid electrode allows the junction to function properly without optical and electrical losses.
[0010] In one embodiment, the junction has a sheet resistance of 10-500 Ω/sq.
[0011] In one embodiment, the junction has a sheet resistance of approximately 100 Ω/sq.
[0012] In one embodiment, power conversion efficiency of the Si microwire solar cell is enhanced by collecting photocarriers through the micro-grid electrode without substantial Auger/surface recombination.
[0013] In one embodiment, the Si microwire solar cells with areas of 1 cm2 show a conversion efficiency of up to 16.5 %, with an open-circuit voltage of 565.2 mV and a short-circuit current density of 35.9 mA/cm2.
[0014] In one embodiment, the fill factor is 81.2 %.
[0015] In one embodiment, the micro-grid electrode comprises a Ni electrode of about 1 μπι thickness. BRIEF DESCRIPTION OF THE DRAWINGS
[0016] Figure 1. (a) Experimental (solid symbols and lines) and simulated current
density-voltage (J-V) curves (open symbols) for the edge electrode cells with deep (RSh of -30 Ω/sq) and shallow (RSh of -100 Ω/sq) junctions, (b) PC1D simulation results of FF values as a function of sheet resistance of emitter layer in the various device size ranges.
[0017] Figure 2. (a-d) Optical microscope images of metal micro-grids on glass substrates with a line width of 2μιη and different grid spacings of 50, 100, 200, and 400 μιη. (e) Optical transmission (black solid symbol), averaged over the main spectral range from 400 to 1000 nm, and sheet resistance (blue solid symbol) of micro-grid as a function of grid spacing. The red solid line represents the calculated sheet resistance of the micro-grid electrode. Also shown is the sheet resistance of deep junction emitter layer (blue dashed line). The inset shows the optical image of the micro-grid on glass substrate, showing the symbol mark (UNIST) in the
background.
[0018] Figure 3. (a) J-V curves of planar junction Si solar cells with different spacing of metal micro-grid; 50 (black), 100 (red), 200 (green), and 400 (blue solid line and symbol) μιη. (b) Open-circuit voltage (black) and short-circuit current density (blue solid line and symbol) extracted from the J-V curves plotted in panel a. (c) Fill factor (red) and efficiency (green solid line and symbol) are plotted as a function of grid spacing. The efficiency was improved by increasing the spacing of micro-grid due to higher transmittance of wider micro-grid.
[0019] Figure 4. (a) Schematic illustration, (b) optical, and (c-d) SEM images of Si MWs solar cells with micro-grid electrode. The vertically-aligned Si MW arrays were formed between grid lines. [0020] Figure 5. (a) J-V curves of Ni film electrode measured by a solar simulator (red) and a Suns- Voc (blue solid line). The inset shows a thick Ni film of micro-grid electrode deposited by the electroplating method, (b) Two J-V curves (red: solar simulator, blue solid line: Suns- Voc) and power output curve (red dashed line) of Al film electrode. AFwas recorded at Jmmp by comparing two different J-V curves, (c) Experimental (red solid symbol) and calculated sheet resistance (red solid line) of Ni film electrode as a function of the film thickness. Experimental sheet resistance of Al film electrode at the film thickness of 200 nm is marked with black solid symbol, (d) Experimental (solid symbols) and calculated contact resistance (solid lines) of Ni and Al film electrodes as a function of the grid width. The inset shows plot of resistances measured by varying the pitches of contact pads; Al (black) and Ni (red line and symbol) film electrodes.
[0021] Figure SI. (a) Optical and (b) SEM images of Si microwires solar cells with the edge electrode, (c) J-V curves of planar and microwires solar cells with various lengths of Si microwires. (d) Secondary ion mass spectrometry profile of the deep emitter with sheet resistance of - 30 Ω/sq. Increase in length of microwires intensifies the serious degradation of the power conversion efficiency due to increase in areal density of degenerately-doped region on microwire surfaces.
[0022] Figure S2. Secondary ion mass spectrometry profile of the shallow emitter with sheet resistance of -100 Ω/sq in which junction depth was estimated to be -220 nm and surface concentration was measured to be 6.2 x 1019 cm-3. [0023] Figure S3. Transmittance spectra of the micro-grids onto the glass substrates. An overall increase in transmittance is observed for larger spacing, which is attributed to the decrease in the surface area that is covered by Al film as the spacing increases.
[0024] Figure S4. J-V curves of Si microwires solar cells with edge (black) and micro-grid electrode (red symbol and line). The power conversion efficiency increased from 9.6 to 15.4% along with the improved Voc and FF values.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0025] Described herein is a photovoltaic device operable to convert light to electricity, comprising a substrate, a plurality of structures essentially perpendicular to the substrate, and one or more recesses between the structures. The term "photovoltaic device" as used herein means a device that can generate electrical power by converting light such as solar radiation into electricity. The term single crystalline as used herein means that the crystal lattice of the entire structures is continuous and unbroken throughout the entire structures, with no grain boundaries therein. An electrically conductive material can be a material with essentially zero band gap. The electrical conductivity of an electrically conductive material is generally above 103 S/cm. A semiconductor can be a material with a finite band gap upto about 3 eV and general has an electrical conductivity in the range of 103 to 10~8 S/cm. An electrically insulating material can be a material with a band gap greater than about 3 eV and generally has an electrical conductivity below 10~8 S/cm. The term "structures essentially perpendicular to the substrate" as used herein means that angles between the structures and the substrate are from 85° to 90°. The term "recess" as used herein means a hollow space in the substrate and is open to a space outside the substrate. [0026] The single crystalline semiconductor material could be selected from a group consisting of silicon, germanium, group III-V compound materials, group II- VI compound materials, and quaternary materials. A group III-V compound material as used herein means a compound consisting of a group III element and a group V element. A group III element can be B, Al, Ga, In, Tl, Sc, Y, the lanthanide series of elements and the actinide series of elements. A group V element can be V, Nb, Ta, Db, N, P, As, Sb and Bi. A group II- VI compound material as used herein means a compound consisting of a group II element and a group VI element. A group II element can be Be, Mg, Ca, Sr, Ba and Ra. A group VI element can be Cr, Mo, W, Sg, O, S, Se, Te, and Po. A quaternary material is a compound consisting of four elements.
[0027] The structures could be cylinders or prisms with a cross-section selected from a group consisting of elliptical, circular, rectangular, and polygonal cross-sections, strips, or a mesh. The term "mesh" as used herein means a web-like pattern or construction. The structures could be pillars with diameters from 50 nm to 5000 nm, heights from 1000 nm to 20000 nm, a center-to- center distance between two closest pillars of 300 nm to 15000 nm. The term "electrode" as used herein means a conductor used to establish electrical contact with the photovoltaic device.
[0028] The substrate could have a flat surface opposite the structures. The flat surface could have a doped layer and optionally a metal layer metal layer disposed on and forming an Ohmic contact with the doped layer. An Ohmic contact is a region a current-voltage (I-V) curve across which is linear and symmetric. The substrate could have a thickness of at least 50 microns.
[0029] The structures could have pillars arranged in an array; each structure is about 5 microns in height; a pitch of the structures is from 300 nm to 15 microns.
[0030] According to the embodiments herein, the strategies for improving the performance of MWs solar cells should include i) reducing the dopant concentration and junction depth of MW emitter so as to suppress Auger recombination, which leads to higher Voc and Jsc, (ϋ) narrowing the distance between electrode to collect the charge carriers from non-conductive emitter (i.e., shallow junction) without the electrical loss, which increases FF, and (iii) minimizing the area of metal electrode on the cell surface for reducing shading loss, which directly enhances Jsc-
[0031] According to the embodiments herein, Applicants incorporate a novel metal micro-grid to the MWs solar cells as top electrode. Owing to superior optical transmittance and electrical conductivity of micro-grid electrodes, robust and reliable Ohmic contacts would be realized for effective collection of charge carriers through the top electrode while diminishing the loss of light absorption associated with shading by metal electrode. In addition, the electrical conductivity of the micro-grid electrode was significantly improved by electroplating a thick Ni film with thickness of about 1 μπι, leading to a FF of over 81% in the MWs solar cells. The remarkable improvement of FF could be originated from the decreased contact resistance between metal-grid and emitter layer as well as the low resistance of micro-grid electrode.
Applicants' MWs solar cells with metal micro-grid electrode exhibit a conversion efficiency of 16.5% which is 72% improved value compared to that with the edge electrode. This novel electrode structure is therefore considered an important step toward the practical realization of highly efficient microwire solar cells in the future.
[0032] Figure la shows the experimental and simulated current density-voltage (J-F) curves for the edge electrode cells with deep (RSh of -30 Ω/sq) and shallow (RSh of -100 Ω/sq) junctions. Simulation results were obtained by using the one-dimensional solar cell modeling program (PC ID) and are well-matched with experimental data. In the case of the shallow junction (red and green circles in Figure 2a), the J- V curve shows a relatively sudden increase of current density value and lower Voc compared to that of the deep junction. The carriers generated in the shallow junction are ineffectively collected through the edge electrode with the high series resistance due to the insufficient conductivity of the emitter. On the other hand, the value of Jsc was significantly increased from 20.2 to 25 mA/cm2 owing to reduction of Auger recombination because depth of the highly-doped region (> 1020 cm-3) is decreased (supporting information Figure S2). Furthermore, FF value is significantly decreased with increase in the sheet resistances of emitter layer only except the case of extremely small cell size of 0.01 cm2, as shown in Figure lb. The simulation results indicate that the design of the edge electrode would not be effective for the charge collection in the large-scale cells due to the serious degradation of FF. Therefore the top metal electrode should be re-designed to collect the charge carriers effectively while minimizing the optical and electrical losses.
[0033] Figure 2a-d show optical microscope images of novel metal micro-grids on glass substrates with a line width of 2μπι and different grid spacings of 50, 100, 200, and 400 μπι, which are taken under the same magnification. The micro-grids are successfully fabricated using a lithography process followed by thermal evaporation of 200-nm-thick Al film, resulting in the formation of continuous and uniform square pattern. The optical and electrical properties of micro-grid electrodes were measured as shown in Figure 2e. An averaged transmittance value increases with increasing the spacing, which is attributed to the decrease in the surface area covered by Al film as the spacing increases (supporting information Figure S3). Due to the high transparency (transmittance of 97.9%) of the electrode with 400 μπι spacing, the photograph in the inset of Figure 2e clearly shows the symbol mark (UNIST) in the background. For the electrical characteristic of micro-grid electrode, the experimental RSh values (blue solid symbol) with various spacings were plotted with the calculated RSh (red solid line of Figure 2e). The RSh of micro-grid electrode was be calculated by sh = ,— : x - (l) N + 1 wt
where N, p, L, w, and t represent the number of lines in 1 cm width, resistivity of Al, gird spacing, width, and thickness of Al film, respectively, as the micro-grid electrode consisting of N x N lines with the square pattern follows Kirchhoff s rules. In the cell area of 1 cm x lcm (N > 24), the calculated RSh is simply given by pLIwt because N/(l+N is close to unity. The experimental Rshs are 3.1 + 1.7, 7.2 + 2.0, 14.5 + 1.9, and 24.9 + 2.8 Ω/sq for the 50, 100, 200, and 400 μπι spacing, respectively. These values are well-matched with the calculated RSh, showing the good agreement with the model. The RSh of micro-grid electrode linearly scales with the grid spacing. In can be anticipated that RSh will be higher than 30 Ω/sq which is RShof deep junction emitter (horizontally-dashed line at left y-axis in figure 2e) if the grid spacing increases over 450 μπι. That is, the solar cell with larger grid spacing than 450 μπι would show severe FF degradation as explained with the simulation results in Figure lb.
[0034] To evaluate the potential use of the fabricated micro-grid as a top electrode in Si solar cells, Applicants analyzed J-V curves of planar junction Si solar cells with metal micro-grid electrodes under AM 1.5G illumination (Figure 3a). The Jsc values increase significantly with increasing the spacing of micro-grid due to the transmittance change as shown in Figure 3b. Similar to the tendency of Jsc, Voc values also tends to increase with increasing the grid spacing. The improved Voc is mainly due to the increase in Jsc as following relationship:
Figure imgf000014_0001
where q, kB, T, JL, and Jo represent electron charge, Boltzmann's constant, temperature, light- generated current, and reverse saturation current, respectively. On the other hand, the FF is not greatly influenced by the spacing of micro-grid electrode as shown in Figure 3c. According to simulation data, the photocarriers are effectively collected through top electrode without electrical loss if RSh of emitter is less than 30 Ω/sq in 1 cm2 cell size. Table 1 shows the photovoltaic properties of solar cells with edge and micro-grid electrodes. Due to enhancement of Voc and Jsc, the conversion efficiency is improved from 6.7% (edge electrode) to 11.3% by using metal micro-grid electrode with grid spacing of 400 μπι.
[0035] The micro-grid electrode with grid spacing of 400 μπι, shown the highest PCE in the planar solar cells, was applied to the Si microwire solar cells. The detailed device features are illustrated in Figure 4a. Vertically-aligned Si MW arrays were designed between grid lines, so that the photo-generated carriers in wire arrays could be effectively collected through the top electrode of metal micro-grid owing to shorter diffusion length up to half of grid spacing (-200 μπι). The Si MWs and micro-grid were fabricated using photolithography followed by reactive ion etching (RIE), resulting in the formation of MW arrays in the square patterns of micro-grids as shown in SEM images of Figure 4b and 4c.
[0036] Phosphorus doping was then performed using a spin-on-doping (SOD) technique to form shallow p-n junctions with RSh of -100 Ω/sq in which junction depth was estimated to be -220 nm and surface concentration was measured to be 6.2 x 1019 cm-3 based on the secondary ion mass spectrometry (SIMS) profile (supporting information Figure S2). A thin SiNx layer (60-nm- thick) was deposited by plasma-enhanced chemical vapor deposition (PECVD) as a passivation layer as well as an anti -reflection layer. The emitters of both MWs solar cells with the micro-grid electrode and the edge electrode have shallow junction with RSh of - 100 Ω/sq. As the micro-grid electrode was applied to MWs cells, the PCE increased from 9.6 to 15.4% along with the improved Voc and FF values (supporting information Figure S4). In particular, FF (75.1%) of the micro-grid electrode was notably improved compared to the edge electrode (52.1%) which requires longer carrier diffusion length (up to half of cell size, ~ 5 mm) through relatively non- conductive channel of shallow emitter. This is mainly due to decrease in the series resistance (Rs) of solar cells which is the sum of the resistance of the metal electrode (Rm), the contact (Rc), the emitter, and the base. Applicants can ignore the influence of the emitter and base resistances, since it is assumed that they do not change with the structure of front electrode. Among the resistance parameters, the Rc can be calculated using18
Figure imgf000016_0001
where RSh is sheet resistance, L a grid length, pc contact resistivity, JFgrid width, Lc unit cell length, s grid spacing. This calculation enables the estimation of Rs for different s values of the edge and micro-grid electrodes. Since the edge electrode is considered as millimeter-scale grid structure, the spacing of edge electrode (1 cm) is much longer than that of micro-grid electrode (400 μιη). According to Eq. 3, the Rc of micro-grid electrode can remarkably be reduced by 25 times compared to edge electrode. Hence, the improved FF could be explained by the fact that the sum of Rs and Rc decreased with applying the micro-grid electrode. In addition, Voc was also improved from 501 mV to 564 mV, as the recombination is suppressed by transporting carriers to the top electrode with shorter diffusion length, leading to decrease in the reverse saturation current (Eq. 2). Although Jsc slightly decreased from 37.0 to 36.3 mA/cm2 in the micro-grid electrode due to the light reflection from the metal grid with only 1% coverage of the front surface, the cell efficiency of MWs solar cells is remarkably improved by effectively collecting the carriers with highly conductive micro-grid electrode. To achieve the high FF of over 80%, Applicants attempted to devise a further improved top electrode with reducing Rm and Rc while minimizing the shading loss.
[0037] In order to minimize the Rs without the optical loss, Applicants applied electroplating method to deposit a thick Ni film. Compared to the vacuum process such as the physical vapor deposition (PVD), the electroplating method enables the deposition of metal film with high deposition rate in the solution for cost-effective solar cells. A Ι-μιη-thick Ni film deposited using the electroplating method is shown in the inset of Figure 5a. The highly dense, thick Ni layer was uniformly coated with the micro-grid pattern, resulting in the significant improvement of FF from 75.1 to 81.2% for the MWs solar cells. As a result, the best performance of the MWs solar cells achieved the PCE of 16.5%, along with Voc and Jsc of 565 mV and 35.9 mA/cm2, respectively. To quantitatively estimate the effect of the micro-grid electrode on the improved FF, the Rs values of MWs solar cells with thick Ni film (Ι-μιη-thick, electroplating method) and Al electrode (200-nm-thick, thermal evaporation) were extracted by comparing the J-V curves obtained using a solar simulator (red lines) to those measured using a Suns- Voc (blue lines) as shown in Figure 5a and 5b. The Rs is calculated by dividing the difference between the voltages in two different J-V curves (AV) by the current density at the maximum power point (Jmmp) given in Figure 5b, as the Suns- Voc measurement provides an ideal J- curve in the absence of Rs. The real J-V curve (Rs-included, solar simulator) of the Ι-μιη-thick Ni film is almost overlapped with the ideal J-V curve (Rs-excluded, Suns-Foe) as shown in Figure 5a, while the 200-nm-thick Al film electrode has significant voltage gap between two J-V curves (Figure 5b). Based on the calculation, the extracted Rs of the Ni film electrode (0.645 Ω cm2) is much lower than that of deposited Al film electrode (1.51 Ω cm2). As mentioned above, Applicants can consider only Rm and Rc values due to the same cell structures except the top electrode. The RSh of Ni film decreases with increasing the Ni thickness easily controlled by the deposition time, as shown in Figure 5c. Due to the increased thickness, RSh of the Ni film electrode decreases up to 3.7 Ω/sq compared to 24.9 Ω/sq of 200-nm-thick Al film electrode (blue symbol in Figure 5c).
[0038] In addition, the Rc value of the Ni film electrode (red symbol) significantly decreased compared to Al film electrode (black symbol) as shown in Figure 5d. The Rc and contact resistivity (pc) was measured by a transmission line model (TLM). The values of pc at the interface between various metals and Si were extracted by measuring the resistance of metal/Si excluding the intrinsic resistances of both metal and Si. The resistances were measured by adjusting the number of pitches between two metal pads. At the zero-point (the intersection with the y-axis in the inset of Figure 5d) of the grid pitches, resistance is induced by the two grid contacts without the influence of the emitter. The extracted pc of Ni/Si contact was 1.69 mQ cm2 at the 1.35 x 10~3 cm2 contact area while Al/Si contact achieved the pc of 2.63 mQ cm2. The improvement of pc could be explained by new current paths, which were formed by filling pinholes of SiNx layer with Ni layer during electroplating. Based on the extracted pc, the Rc values of Ni (red line) and Al electrodes (black line) were calculated from Eq. 3 as a function of the grid width (Figure 5d). In order to obtain lower Rc than that of Ni film electrode, the grid width of the Al film electrode is required to be wider than over 10 μπι, leading to the shading loss and then Jsc degradation. In other words, the shading loss which is proportional to the grid width can be reduced by using narrower width of Ni film electrode with the same or even lower Rc, as the decrease in Rc of Ni film electrode is mainly due to the reduced pc at the interface between Ni and Si. Therefore, the highest FF of 81.2% could be achieved by reducing not only the metal resistance but also the contact resistance. [0039] Applicants developed highly efficient (16.5%) MWs solar cells with FF value of 81.2% by using the metal micro-grid electrode as the top contact. Due to superior transmittance and conductivity of micro-grid electrode, FF and Voc values were improved by effectively collecting photocarriers thorough the top electrode in the shallow emitter solar cells. In addition, a thick Ni film was deposited onto the micro-grid electrode using electroplating method; consequently, the Rm and Rc of micro-grid electrode were drastically decreased in comparison with Al film electrode formed by thermal deposition. Our micro-grid electrodes therefore clearly represent a very promising structure for the high-efficiency MWs solar cells.
Table 1. Photovoltaic performance of planar and MW solar cells.
Structure Spacing of Electrode Voc Jsc FF CE
micro-grid material [mV] [mA/cm2] [%] [%]
Planar 50 μηι Al film 527.6. 15.6 75.1 6.2
100 μηι 536.7 20.3 75.5 8.2
200 μηι 547.7 23.8 74.3 9.7
400 μηι 557.8 26.7 75.6 11.3
Microwires 400 μηι Al film 564.0 36.3 75.1 15.4
Ni film 565.2 35.9 81.2 16.5
[0040] Supporting Information. Experimental section; Photovoltaic characteristic of Si microwires solar cells with the deep emitter and edge electrode; Secondary ion mass
spectrometry profiles of the deep and shallow emitters; Transmittance spectra of the micro-grids; Photovoltaic characteristic of Si microwires solar cells with the edge and micro-grid electrodes.
EXPERIMENTAL
Fabrication of vertical Si microwire arrays [0041] Si microwire (MW) arrays with micro-grid lines were fabricated from Float-zone (Fz) p- type Si wafers (resistivity of 1-5 Q.cm, 550-μιη thick). Circular-shaped photoresist dot arrays (2 μπι in diameter, 1 μιη spacing) were periodically patterned using AZ nLOF 2035 photoresist (AZ Electronic Materials) through the photolithography. Then, vertically-aligned Si MWs were fabricated by reactive ion etching (RTE, Tegal 200) with 1500 W source power, 100 W stage power, 45 mTorr gas pressure under a source gas ratio of SF6/C4F8 = 250/150. After RIE process, the resulting polymeric coating and etching damage on the wire sidewalls were removed using a piranha solution (H2S04:H202 = 2: 1) followed by slow Si etching (RSE-100, Transene) for 10 sec.
Fabrication of Si microwires solar cells with micro-grid electrode
[0042] An emitter layer was formed by phosphorus diffusion via the spin-on-dopant (SOD) method. First, diluted phosphorus dopant source (P509, Filmtronics, Inc.) with ethanol
(P509:ethanol = 1 :3) was spin-coated on a dummy Si wafer, and then baked at 200 °C for 10 min. To form the conformal doping on MWs, we positioned Si MWs sample so that it faced the phosphorus-coated dummy wafer. The diffusion doping was carried out in a tube furnace under a mixed ambient of 20 % 02 and 80 % N2 at 850 °C. Phosphorus glass that remained after the SOD diffusion was removed by using a diluted FIF solution. After removing phosphorus glass and SiOx layer, a thin SiNx layer (60-nm-thick) was deposited by PE-CVD (PEH-600,
SORONA). In creating the micro-grid electrode, the Si MWs were covered with photoresist (AZ4330, AZ electronic materials, thickness of - 10 μιη) before the metal deposition using lithography process. For the top and bottom contacts, 200-nm-thick Al films were deposited on the top and bottom of samples using a thermal evaporator. Finally, the photoresist was removed by dipping samples in acetone solution. Fabrication of micro-grid electrode with a thick Nifilm
[0043] In order to further improve the electrical characteristics of the micro-grid electrode, electroplating method were attempted to deposit a thick Ni film. Ti/Ni, as a seed layer for electroplating, was first deposited onto micro-grid pattern of photoresist. After deposition of the seed layer, the residual of photoresist was removed by dipping in acetone solution. Electroplating deposition was then performed in a Ni solution composed of SO4.6H2O (1 M), CI2.6H2O (0.5 M), and H3BO3 (1 M) at 40 °C under the constant current density of 5 mA/cm2.
Characterization of vertical Si microwires solar cells
[0044] Current-voltage (I-V) characteristics of the devices in the dark were investigated using a semiconductor parameter analyzer (4200-CSC, Keithley). The photovoltaic properties of our solar cells were investigated using a solar simulator (Class AAA, Oriel Sol3 A, Newport) under AM 1.5G illumination. Incident flux was measured using a calibrated power meter, and double- checked using a NREL-calibrated solar cell (PV Measurements, Inc.). EQE was measured using a Xe light source and a monochromator in the wavelengths range of 400-1100 nm. Optical reflection measurements were performed over wavelengths of 400-1100 nm using a UV-Vis/NIR spectrophotometer (Cary 5000, Agilent) equipped with a 110 mm integrating sphere to account for total light (diffuse and specular) reflected from the samples.

Claims

CLAIMS What is claimed is:
1. A device comprising Si microwire solar cells having a fill factor of more than 80%.
2. The device of claim 1, further comprising a micro-grid electrode having a grid pattern having a grid spacing of 1 to 1000 microns, wherein the micro-grid electrode is attached to the Si microwire solar cells via a junction.
3. The device of claim 2, wherein the grid spacing is 10 to 500 microns.
4. The device of claim 2, wherein the grid spacing is 50 to 400 microns.
5. The device of claim 2, wherein the micro-grid electrode allows the junction to function properly without optical and electrical losses.
6. The device of claim 2, wherein the junction has a sheet resistance of 10-500 Ω/sq.
7. The device of claim 2, wherein the junction has a sheet resistance of approximately 100 Ω/sq.
8. The device of claim 2, wherein power conversion efficiency of the Si microwire solar cell is enhanced by collecting photocarriers through the micro-grid electrode without substantial Auger/surface recombination.
9. The device of claim 2, wherein the Si microwire solar cells with areas of 1 cm2 show a conversion efficiency of up to 16.5 %, with an open-circuit voltage of 565.2 mV and a short-circuit current density of 35.9 mA/cm2.
10. The device of claim 2, wherein the fill factor is 81.2 %.
11. The device of claim 2, wherein the micro-grid electrode comprises a Ni electrode of about 1 μπι thickness.
12. A method comprising fabricating a vertical Si microwire array, fabricating Si microwires solar cells comprising a micro-grid electrode, depositing a Ni film onto the micro-grid electrode, and forming a photovoltaic device comprising the Si microwire solar cells.
13. The method of claim 12, wherein the Si microwire solar cells have a fill factor of more than 80%.
14. The method of claim 12, wherein the micro-grid electrode comprises a grid pattern
having a grid spacing of 1 to 1000 microns, wherein the micro-grid electrode is attached to the Si microwire solar cells via a junction.
15. The method of claim 14, wherein the grid spacing is 10 to 500 microns.
16. The method of claim 14, wherein the grid spacing is 50 to 400 microns.
17. The method of claim 14, wherein the micro-grid electrode allows the junction to function properly without optical and electrical losses.
18. The method of claim 14, wherein the junction has a sheet resistance of 10-500 Ω/sq.
19. The method of claim 14, wherein the junction has a sheet resistance of approximately 100 Ω/sq.
0. The method of claim 14, wherein the Si microwire solar cells with areas of 1 cm2 show a conversion efficiency of up to 16.5 %, with an open-circuit voltage of 565.2 mV and a short-circuit current density of 35.9 mA/cm2.
PCT/US2016/032774 2015-05-14 2016-05-16 Metal micro-grid electrode for highly efficient si microwire solar cells with over 80% fill factor WO2016183589A1 (en)

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