WO2010022530A1 - Method for manufacturing transparent conductive oxide (tco) films; properties and applications of such films - Google Patents

Method for manufacturing transparent conductive oxide (tco) films; properties and applications of such films Download PDF

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WO2010022530A1
WO2010022530A1 PCT/CH2009/000291 CH2009000291W WO2010022530A1 WO 2010022530 A1 WO2010022530 A1 WO 2010022530A1 CH 2009000291 W CH2009000291 W CH 2009000291W WO 2010022530 A1 WO2010022530 A1 WO 2010022530A1
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layer
lpcvd
zno
tco
substrate
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PCT/CH2009/000291
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French (fr)
Inventor
Zimin Dimitry
Arno Zindel
Andrew Teren
Owan Charles Watkins
Laurent Despont
Joachim Wider
Luca Ramoino
Mario Gossla
Benjamin Vogler
Joerg Kerschbaumer
Stefan Schneider
Andreas Mark
Markus Poppeller
Hansjoerg Kuhn
Daniel Borrello
Stefano Benagli
Johannes Meier
Ulrich Kroll
Evelyne Vallat-Sauvain
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Oerlikon Solar Ip Ag, Trübbach
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Publication of WO2010022530A1 publication Critical patent/WO2010022530A1/en

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    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C16/00—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes
    • C23C16/22—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the deposition of inorganic material, other than metallic material
    • C23C16/30—Deposition of compounds, mixtures or solid solutions, e.g. borides, carbides, nitrides
    • C23C16/40—Oxides
    • C23C16/407—Oxides of zinc, germanium, cadmium, indium, tin, thallium or bismuth
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C16/00—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes
    • C23C16/44—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the method of coating
    • 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
    • H01L31/022466—Electrodes made of transparent conductive layers, e.g. TCO, ITO layers
    • H01L31/022483—Electrodes made of transparent conductive layers, e.g. TCO, ITO layers composed of zinc oxide [ZnO]
    • 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 potential barriers
    • H01L31/075—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 potential barriers the potential barriers being only of the PIN type, e.g. amorphous silicon PIN solar cells
    • 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
    • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
    • H01L31/1884—Manufacture of transparent electrodes, e.g. TCO, ITO
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02367—Substrates
    • H01L21/0237—Materials
    • H01L21/02422—Non-crystalline insulating materials, e.g. glass, polymers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02518—Deposited layers
    • H01L21/02521—Materials
    • H01L21/02551—Group 12/16 materials
    • H01L21/02554—Oxides
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02518—Deposited layers
    • H01L21/0257—Doping during depositing
    • H01L21/02573—Conductivity type
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02612—Formation types
    • H01L21/02617—Deposition types
    • H01L21/0262—Reduction or decomposition of gaseous compounds, e.g. CVD
    • 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
    • Y02E10/548—Amorphous silicon PV cells

Definitions

  • This invention addresses transparent conductive / conducting oxide films, especially in their application as material for electrodes in thin film silicon photovoltaic solar cells, solar modules or similar photovoltaic devices.
  • Solar cells also known as photovoltaic cells, are semiconductors that convert electromagnetic energy, such as light or solar radiation, directly to electricity. These semiconductors are characterized by energy bands gaps between their valence electron bands and their conduction electron bands, so that free electrons cannot ordi- narily exist or remain in these band gaps.
  • energy bands gaps between their valence electron bands and their conduction electron bands, so that free electrons cannot ordi- narily exist or remain in these band gaps.
  • light is absorbed by the materials that characterize the photovoltaic cells, electrons that occupy low-energy states are excited and jump the band gap to unoccupied higher energy states.
  • electrons in the valence band of a semiconductor absorb sufficient energy from photons of solar radiation, they jump the band gap to the higher energy conduction band.
  • Electrons excited to higher energy states leave behind them unoccupied low-energy positions which are referred to as holes. These holes may shift from atom to atom in the crystal lattice and the holes act as charge carriers, in the valence band, as do free electrons in the conduction band, to contribute to the crystal's conductivity. Most of the photons that are absorbed in the semiconductor produce such electron-hole pairs . These electron-hole pairs generate photocurrent and, in the presence of a built-in field, the photo- voltage of solar cells.
  • Electron hole pairs produced by the light would eventually recombine, and convert to heat or a photon, unless prevented from doing so.
  • a local electric field is created in the semiconductor by doping or interfacing dissimilar materials to produce a space charge layer.
  • the space charge layer separates the holes and electrons for use as charge carriers. Once separated, these collected hole and electron charge carriers produce a space charge that results in a voltage across the junction, which is the photovoltage . If these separated hole and charge carriers are al- lowed to flow through an external load, they will constitute a photocurrent .
  • a thin- film solar cell generally includes a first electrode, one or more semiconductor thin- film p-i-n or n-i-p junctions, and a second electrode. These layers are successively stacked on a substrate.
  • the i-type layer which is a substantially intrinsic semiconductor layer, occupies the most part of the thickness of the thin-film p-i- n junction. Photoelectric conversion occurs primarily in this i-type layer .
  • Prior Art Fig. 1 shows a simple photovoltaic cell 40 comprising a transparent substrate 41, e. g. glass with a layer of a transparent conductive oxide (TCO) 42 deposited thereon.
  • TCO transparent conductive oxide
  • This layer is also called front contact F/C and acts as first electrode for the photovoltaic element.
  • the next layer 43 acts as the active photovoltaic layer and comprises three sub-layers forming a p-i-n junction.
  • Said layer 43 comprises hydrogenated microcrystalline, nanocrystalline or amorphous silicon or a combination thereof.
  • Sub-layer 44 adjacent to TCO front contact 42 is positively doped, the adjacent sub-layer 45 is intrinsic, and the final sub-layer 46 is negatively doped.
  • the layer sequence p-i-n as described can be inverted to n-i-p, then layer 44 is identified as n- layer, layer 45 again as intrinsic, layer 46 as p- layer.
  • the cell includes a rear contact layer 47 (also called back contact, B/C) which may e. g. be made of zinc oxide, tin oxide or ITO and a reflective layer 48. Alternatively a metallic back contact may be realized, which can combine the physical properties of back reflector 48 and back contact 47. For illustrative purposes, arrows indicate impinging light.
  • B/C back contact layer
  • a metallic back contact may be realized, which can combine the physical properties of back reflector 48 and back contact 47.
  • arrows indicate impinging light.
  • Oerlikon Solar TCO 1200 mass production system which is a horizontal inline deposition equipment. It can be used for front and back contact mass production.
  • the system exhibits different modules responsible for transporting, pre-heating and processing of substrates.
  • a loading station, a load lock, four process modules (4PM) , an unload lock, an unload station and a return track are being established.
  • 4PM process modules
  • a throughput of more than 150' 000 substrates/year with a yield of more than 95% is possible.
  • the system can be equipped with six process modules increasing then the system throughput .
  • the load lock is the first module which has an impact on the deposi- tion process and therefore on the layer properties. It first evacuates the chamber from atmosphere to process pressure. Its second task is to preheat the substrate within an adjustable time during the pump down. The heating is realized by independently controllable sets of radiation heaters. After reaching the desired substrate tem- perature the hot substrate moves into the first process module.
  • a low pressure chemical vapor deposition LPCVD process takes place.
  • Each process module incorporates a hot plate with different controllable heating areas and a dedicated process gas distribution system which ensures a suitable gas supply.
  • the glass is laid on the hot plate in order to control the substrate temperature during the deposition.
  • Every process module is capable to deposit ZnO with a deposition rate of 3.5nm/s. All involved process chemicals are delivered into the process module in gaseous phase using an external gas box in which they are evaporated.
  • the appro- priate pumping is provided by symmetrical pump manifolds.
  • Transparent conductive oxides (TCOs) are an essential part of thin film solar cells.
  • the TCO layer acts e. g.
  • the TCO layer has to possess the ability to scatter the incoming light at the TCO-cell interface. This scattering contributes in elongating the path of the light in the (photo) active layers and thus increases the absorption and module efficiency. Besides that a good control of the roughness of the TCO- cell interface allows obtaining multiple reflection of the light be- tween the cell interface to front and back contact thus further increasing the path of light through the absorber.
  • This Light Trapping mechanism is especially important for microcrystalline silicon thin film solar cells due of their poor optical absorption coefficient.
  • LPCVD Boron-doped zinc oxide layers deposited by LPCVD technique have shown to be a good candidate for thin film solar technology. Its raw material constituents are widely available, are low in cost and are harmless. In addition the LPCVD technology is well suited for large- scale device fabrication. Moreover, LPCVD ZnO layers can be used in solar cell production "as-grown", without additional treatment like post processing or etching.
  • ZnO is widely used as back and front contact in thin film solar cells.
  • Doped ZnO films have a high conductivity and a high op- tical transmittance.
  • the electrical and optical properties of TCO layers used in thin film solar cells significantly influence the module efficiency. It is therefore an object of the invention to define a process window for a preferred embodiment of a TCO layer configuration suitable for photovoltaic thin film cells.
  • Fig. 1 Prior Art, layer stack of a thin film silicon PV cell.
  • Fig. 2 Stability of sheet resistance and its uniformity as a function of the number of substrates.
  • Fig. 3 Light transmittance of different TCO layers on glass.
  • Fig. 4 Surface plots of thickness and sheet resistance.
  • Fig. 5 Mobility and Haze at a wavelength at 600 nm (bottom) as a function of the B 2 H S /DEZ ratio for two layers with different thickness.
  • Fig. 6 Haze and Total Transmission for different Haze values (top) . Haze values are considerably higher for LPCVD ZnO compared to commercially available SnO 2 coated glass. The angular resolved scattering measurements indicate that LPCVD ZnO scatters light more efficiently than SnO 2 coated glass.
  • Fig. 7 a) b) Scanning Electron Microscope (SEM) micrographs of the surface of Asahi-U (a) and a typical LPCVD-ZnO (b) layer.
  • SEM Scanning Electron Microscope
  • Fig. 8 a) b) Total transmission (measured with index matching liquid) and diffuse transmission for Asahi-U and LPCVD-ZnO, as used in this study.
  • Fig. 9 V oc , J sc , FF and efficiency as a function of the i-layer thickness (from 200 to 300 nm, at 3.35 A/s) in the initial and light-soaked state. Asahi-U is used as front TCO for the p-i-n cells. For improved statistics, 5 to 10 cells per i-layer thickness were taken into account .
  • Fig. 10 V oc , J sc ⁇ FF and efficiency as a function of the i-layer thickness in the initial and light-soaked state.
  • LPCVD-ZnO is used as front TCO.
  • the i-layer thickness is varied form 180 to 400 nm (at 3.35 A/s) .
  • 4 to 7 cells were considered.
  • Fig. 11 Relative efficiency change due to light-soaking degradation of p-i-n a-Si:H cells deposited LPCVD-ZnO and Asahi-U.
  • Fig. 12 I (V) curves of the best a-Si:H cells obtained so far on LPCVD-ZnO and Asahi-U.
  • Fig. 13 I (V) curves of the best p-i-n a-Si:H (light -soaked) 10x10 cm 2 mini-module on LPCVD-ZnO. The aperture area is certified by the Swiss Federal Office of Metrology.
  • Fig. 14 I (V) curves of the best lcm 2 micromorph tandem cells obtained so far on Asahi SnO 2 and LPCVD-ZnO front TCOs.
  • Pl and P2 Several processes (here: Pl and P2) have been developed for the deposition of the ⁇ c-Si:H i-layer on LPCVD-ZnO front TCO.
  • FIG. 15 AMI.5 I -V characteristics of a stabilized (1000 h, 1 sun, 50 0 C) Micromorph 10x10 cm 2 mini-module of 9.94% efficiency. As front TCO, Asahi SnO 2 has been applied.
  • Fig. 16 Thickness as a function of the DEZ/H 2 0 ratio and the total transmittance for several B 2 H 6 /DEZ ratios
  • a process window for a TCO layer configuration suitable for photovoltaic thin film cells employs manufacturing, on a substrate, a transparent conductive zinc oxide layer from at least diethylzinc, water and diborane by low pressure chemical vapour deposition (LPCVD) in a process chamber of a deposition system comprising the steps of:
  • a respective transparent conductive zinc oxide layer deposited by a low pressure chemical vapour deposition (LPCVD) process chamber as mentioned above will exhibit an intrinsic layer transmittance in the range between 400 to 800nm of more than 93% and between 400 and llOOnm of more than 92%.
  • the sheet resistance will be smaller than 10 ⁇ /D over a whole substrate of 1.4 m 2 size.
  • a thin film photovoltaic cell can be equipped with such a zinc oxide layer acting as first and/or second electrode in a stacked arrangement as follows: substrate - first electrode - one or more semicon- ductor thin- film p-i-n or n-i-p junctions - second electrode.
  • the TCO layer properties are determined by many parameters such as substrate temperature, process gas mixture and pressure.
  • Figure 2 shows a scanning electron microscope (SEM) picture of a typical ZnO layer surface. The specific surface morphology is one of the key factors to get an optimal Light Trapping. The ZnO light scattering efficiency is quantified with the haze which can be easily tuned between 10 and 25% at a wavelength of 600nm.
  • the TCO 1200 mass production system produces ZnO layers with thickness uniformity below 20% and a sheet resistance smaller than 10 ⁇ /D over the whole substrate of 1.4 m 2 size.
  • the layer quality remains constant as a function of the number of substrates as seen in Figure 2 for sheet resistance.
  • the layer transmittance plays a very important role.
  • the intrinsic layer transmittance in the range of 400 to 800nm is higher than 93% and between 400 and llOOnm higher than 92%.
  • Figure 3 shows a comparison of transmittance measurements between different TCO layers on different glass substrates. It shows that ZnO deposited with a TCO 1200 production system has the highest transmittance over a very wide range of wavelengths .
  • Fig. 4 shows thickness and sheet resistance measurements for a 1100 x 1300 mm 2 V5 glass. Thickness and sheet resistance measurements have been performed on 143 points distributed uniformly over the glass.
  • the ZnO layers are doped with boron (source of boron is diborane B 2 H 6 , which is commercially available as 2%dilution in hydrogen) in order to increase layer conductivity.
  • the doping level can be changed easily by variation of the process inputs without hardware changes. This is contrary to physical vapor deposition (PVD) , where the doping level is mainly given by the chemical composition of the sputtering target.
  • PVD physical vapor deposition
  • doping negatively affects haze as well as transmission.
  • the wide process window of the LPCVD ZnO deposition process allows being less strict to the stability of process parameters without influence on layer properties. For example, properties of TCO layer remaining the same by pressure variation ( ⁇ 15%) or by slight changes in gas flows (absolute and relative) .
  • This stability of LPCVD processes allows the tool owners to receive stable output of coated glasses in spite of minor process changes.
  • flow ratio of two main precursors H 2 O and diethyl zinc (DEZ) can be changed in a wide range without major changes of ZnO layers (see Fig. 16) .
  • EXPERIMENTAL RESULTS II As indicated, for thin film solar cell devices light-trapping is a key feature in order to increase the cell efficiency and to significantly decrease the costs of photovoltaic cells. Therefore properties of the front TCO have been compared between a high performance laboratory TCO (Asahi U) and the in-house fabricated LPCVD ZnO. A single-junction, 1 cm 2 a-Si:H cell' s properties as a function of the i-layer thickness were studied in details, for both TCOs. Maximal stable efficiency of 8.6% was obtained on Asahi-U. In particular, the difference between the two studied substrate is in the device J " sc value, and this observation is in good agreement with the measured optical parameters of the TCOs.
  • the reduction of the a-Si:H absorber layer thickness is beneficial for both production throughput and device stability (after light- soaking) .
  • rough TCOs are used for enhanced light- trapping within the device. Efficient light-trapping leads to sev- eral-fold enhanced optical path and allow a reduction of the absorber layer thickness.
  • the enhanced light-trapping properties of the TCO are even more important due to the lower optical absorption of ⁇ c-Si:H compared to a-Si:H.
  • Asahi-U SnO 2 has good optical and electrical properties, that lead to excellent performances of a-Si:H p-i-n cells.
  • Zinc oxide (ZnO) fabricated by Low Pressure Chemical Vapor Deposi- tion (LPCVD) is considered to have the potential for the fabrication of excellent thin- film silicon solar cells (due to its outstanding light -trapping capability) .
  • LPCVD Low Pressure Chemical Vapor Deposi- tion
  • Zinc oxide (ZnO) fabricated by Low Pressure Chemical Vapor Deposi- tion (LPCVD) is considered to have the potential for the fabrication of excellent thin- film silicon solar cells (due to its outstanding light -trapping capability) .
  • LPCVD-ZnO front contacts are disclosed. They illustrate the poten- tial of LPCVD-ZnO to obtain excellent initial and stable a-Si:H cells performances.
  • the comparison has been extended to mi- cromorph tandem cells and mini-modules.
  • the presented p-i-n a-Si:H solar cells are deposited in a R&D sin- gle-chamber KAI-M system (52 x 41 cm 2 substrate size) manufactured by Oerlikon Solar.
  • the PECVD process was adapted to an excitation frequency of 40.68 MHz.
  • Cleaning of the KAI reactor was based on an in- situ plasma process and is executed after each cell run.
  • the deposition rate of the intrinsic device-quality a-Si:H is 3.35 A/s, whereas ⁇ c-Si:H i-layers (in mi- cromorph Tandem) have been deposited at rates up to 5 A/s.
  • LPCVD-ZnO p-i-n a- Si :H cells
  • Asahi-U SnO 2 The best p-i-n a-Si:H cells of each substrate type have been used as top cells for the micromorph tandem fabrication; finally their performances are compared, as well .
  • In-house prepared LPCVD- ZnO is also applied as back contact in combination with a white reflector (WR) on all cells and mini-modules.
  • the deposition parameters of the ZnO layers are optimized to obtain efficient light- trapping, high transparency and conductivity on both front and back contacts of the device.
  • test cells are structured by laser-patterning to a well-defined area of 1.0029 cm 2 , as certified by the Swiss Federal Office of Metrology (METAS) .
  • 10x10 cm 2 dimension are realized by applying laser scribing for the monolithic series connection.
  • Mini-module aperture area has been also certified by METAS.
  • the I (V) characteristics of the a-Si:H cells were measured under AM 1.5 illumination (Wacom WXS-155S-L2 double-source simulator) at 25°C. Light-soaking experiments were carried out under an illumination close to AMI.5 at 50 0 C during 1000 h. Best light-soaked a-Si:H mini-modules were sent for independent characterization to ESTI laboratories of JRC in Ispra, Italy.
  • the in-house zinc oxide is fabricated by a LPCVD process at a temperature below 200 0 C.
  • the sheet resistance of the front TCO layers is below 10 ⁇ /D.
  • the polycrystalline films are constituted of large grains, whose extremities appear at the growing surface as large pyramids (see Fig. 7b) . This as-grown rough surface texture provides for an efficient diffuse light scattering into the silicon device, as shown in Fig. 7.
  • LPCVD-ZnO possesses excellent optical properties for its application as front TCO in thin film silicon solar cells.
  • the total transmission of the layer in the visible range of light is well above 80% and Asahi-U.
  • its light scattering abilities, as evaluated from the diffuse transmission data of Fig. 8a) and b) are even better than those of Asahi-U.
  • V oc -values are essentially independent of the i-layer thickness, whereas in the light-soaked state, V oc decreases slightly with increasing thickness, for both TCOs.
  • initial FF-values on ZnO are slightly lower compared to Asahi-U.
  • after light-soaking FF s are comparable on both TCOs.
  • Fig. 11 the relative efficiency degradation of cells obtained in the study of Fig. 3 and 4 is shown. According to these results, the relative degradation of the device efficiency depends on the i- layer thickness in a similar way for both TCOs, even if the absolute values of the initial and stabilized efficiencies are different.
  • the optimal i- layer thickness lies between 240 and 300nm. Based on that result the device processes were further improved and four deposition regimes were found (see table below; regimes 1 to 4) where it is possible to boost the stabilized effi- ciencies even up to 9%. A confirmation of the robustness of the process is that stabilized efficiencies of around 9% using different process windows could be achieved.
  • Fig. 14 shows the I (V) curves of our best results on both TCOs.
  • Efficiencies of the micromorph devices can be further optimized by tuning the i-layer of the bottom cell and the recombination junc- tion. Moreover, no reflector between the top and bottom cell is incorporated in these devices, and it is expected that this could further improve the efficiency.
  • a-Si:H single-junction p-i-n devices were investigated on Asahi-U and LPCVD-ZnO front TCOs.
  • i-layer thickness series of p-i-n cells have been deposited in our R&D KAI M reactor.
  • This study shows that reducing the i-layer thickness on Asahi-U yields to an increased stabilized efficiencies (up to 8.6% for an i-layer thickness of 200nm) .
  • the situation is different for LPCVD-ZnO front TCO, where this study indicates that the maximum stabilized efficiency is obtained for i- layers in the range of 240-300nm.
  • the best stabilized efficiency reaches 9.1%. This result was obtained without applying an antireflection coating (ARC) onto the front glass substrate.
  • ARC antireflection coating

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Abstract

A method for manufacturing a boron doped, transparent, conductive zinc oxide layer from on a substrate is disclosed. The layer is being deposited from at least diethylzinc, water and diborane by low pressure chemical vapour deposition (LPCVD) in a process chamber of a deposition system comprising wherein the gas flow ratio of diethylzinc and water is kept between 0.87 and 1.3 and the gas flow ratio of diborane and diethylzinc is being kept between 0.05 and 0.4. The haze of such manufactured layer, measured as ratio of diffuse transmittance to total transmittance at 600nm, is between 10 and 25%.

Description

METHOD FOR MANUFACTURING TRANSPARENT CONDUCTIVE OXIDE (TCO) FILMS; PROPERTIES AND APPLICATIONS OF SUCH FILMS
This invention addresses transparent conductive / conducting oxide films, especially in their application as material for electrodes in thin film silicon photovoltaic solar cells, solar modules or similar photovoltaic devices.
BACKGROUND OF THE INVENTION Solar cells, also known as photovoltaic cells, are semiconductors that convert electromagnetic energy, such as light or solar radiation, directly to electricity. These semiconductors are characterized by energy bands gaps between their valence electron bands and their conduction electron bands, so that free electrons cannot ordi- narily exist or remain in these band gaps. However, when light is absorbed by the materials that characterize the photovoltaic cells, electrons that occupy low-energy states are excited and jump the band gap to unoccupied higher energy states. Thus, when electrons in the valence band of a semiconductor absorb sufficient energy from photons of solar radiation, they jump the band gap to the higher energy conduction band.
Electrons excited to higher energy states leave behind them unoccupied low-energy positions which are referred to as holes. These holes may shift from atom to atom in the crystal lattice and the holes act as charge carriers, in the valence band, as do free electrons in the conduction band, to contribute to the crystal's conductivity. Most of the photons that are absorbed in the semiconductor produce such electron-hole pairs . These electron-hole pairs generate photocurrent and, in the presence of a built-in field, the photo- voltage of solar cells.
Electron hole pairs produced by the light would eventually recombine, and convert to heat or a photon, unless prevented from doing so. To prevent this phenomenon, a local electric field is created in the semiconductor by doping or interfacing dissimilar materials to produce a space charge layer. The space charge layer separates the holes and electrons for use as charge carriers. Once separated, these collected hole and electron charge carriers produce a space charge that results in a voltage across the junction, which is the photovoltage . If these separated hole and charge carriers are al- lowed to flow through an external load, they will constitute a photocurrent .
A thin- film solar cell generally includes a first electrode, one or more semiconductor thin- film p-i-n or n-i-p junctions, and a second electrode. These layers are successively stacked on a substrate.
Each p-i-n junction or thin- film photoelectric conversion unit includes an i-type layer sandwiched between a p-type layer and an n- type layer (p-type = positively doped, n-type = negatively doped) . The i-type layer, which is a substantially intrinsic semiconductor layer, occupies the most part of the thickness of the thin-film p-i- n junction. Photoelectric conversion occurs primarily in this i-type layer .
Prior Art Fig. 1 shows a simple photovoltaic cell 40 comprising a transparent substrate 41, e. g. glass with a layer of a transparent conductive oxide (TCO) 42 deposited thereon. This layer is also called front contact F/C and acts as first electrode for the photovoltaic element. The next layer 43 acts as the active photovoltaic layer and comprises three sub-layers forming a p-i-n junction. Said layer 43 comprises hydrogenated microcrystalline, nanocrystalline or amorphous silicon or a combination thereof. Sub-layer 44 adjacent to TCO front contact 42 is positively doped, the adjacent sub-layer 45 is intrinsic, and the final sub-layer 46 is negatively doped. In an alternative embodiment the layer sequence p-i-n as described can be inverted to n-i-p, then layer 44 is identified as n- layer, layer 45 again as intrinsic, layer 46 as p- layer.
Finally, the cell includes a rear contact layer 47 (also called back contact, B/C) which may e. g. be made of zinc oxide, tin oxide or ITO and a reflective layer 48. Alternatively a metallic back contact may be realized, which can combine the physical properties of back reflector 48 and back contact 47. For illustrative purposes, arrows indicate impinging light. Recently, efforts have focused on the TCO layers employed with PV thin film silicon solar cells due to their important contributions to the overall effectiveness of PV cells. A very promising material for TCO layers to be used as front- and back contacts or electrodes in PV solar cell systems is zinc oxide ZnO. Layers of ZnO suitable for PC applications can e. g. be produced by an Oerlikon Solar TCO 1200 mass production system, which is a horizontal inline deposition equipment. It can be used for front and back contact mass production. The system exhibits different modules responsible for transporting, pre-heating and processing of substrates. In a standard configuration a loading station, a load lock, four process modules (4PM) , an unload lock, an unload station and a return track are being established. With such a standard 4PM system a throughput of more than 150' 000 substrates/year with a yield of more than 95% is possible. The system can be equipped with six process modules increasing then the system throughput .
The load lock is the first module which has an impact on the deposi- tion process and therefore on the layer properties. It first evacuates the chamber from atmosphere to process pressure. Its second task is to preheat the substrate within an adjustable time during the pump down. The heating is realized by independently controllable sets of radiation heaters. After reaching the desired substrate tem- perature the hot substrate moves into the first process module.
In the process module PM a low pressure chemical vapor deposition LPCVD process takes place. Each process module incorporates a hot plate with different controllable heating areas and a dedicated process gas distribution system which ensures a suitable gas supply. The glass is laid on the hot plate in order to control the substrate temperature during the deposition. Every process module is capable to deposit ZnO with a deposition rate of 3.5nm/s. All involved process chemicals are delivered into the process module in gaseous phase using an external gas box in which they are evaporated. The appro- priate pumping is provided by symmetrical pump manifolds. Transparent conductive oxides (TCOs) are an essential part of thin film solar cells. The TCO layer acts e. g. as a transparent front window and needs to have a high conductivity and a high optical transmittance . Further the TCO layer has to possess the ability to scatter the incoming light at the TCO-cell interface. This scattering contributes in elongating the path of the light in the (photo) active layers and thus increases the absorption and module efficiency. Besides that a good control of the roughness of the TCO- cell interface allows obtaining multiple reflection of the light be- tween the cell interface to front and back contact thus further increasing the path of light through the absorber. This Light Trapping mechanism is especially important for microcrystalline silicon thin film solar cells due of their poor optical absorption coefficient.
Boron-doped zinc oxide layers deposited by LPCVD technique have shown to be a good candidate for thin film solar technology. Its raw material constituents are widely available, are low in cost and are harmless. In addition the LPCVD technology is well suited for large- scale device fabrication. Moreover, LPCVD ZnO layers can be used in solar cell production "as-grown", without additional treatment like post processing or etching.
Therefore, ZnO is widely used as back and front contact in thin film solar cells. Doped ZnO films have a high conductivity and a high op- tical transmittance. The electrical and optical properties of TCO layers used in thin film solar cells significantly influence the module efficiency. It is therefore an object of the invention to define a process window for a preferred embodiment of a TCO layer configuration suitable for photovoltaic thin film cells.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1: Prior Art, layer stack of a thin film silicon PV cell. Fig. 2: Stability of sheet resistance and its uniformity as a function of the number of substrates. Fig. 3: Light transmittance of different TCO layers on glass. Fig. 4: Surface plots of thickness and sheet resistance. Fig. 5: Mobility and Haze at a wavelength at 600 nm (bottom) as a function of the B2HS/DEZ ratio for two layers with different thickness.
Fig. 6: Haze and Total Transmission for different Haze values (top) . Haze values are considerably higher for LPCVD ZnO compared to comercially available SnO2 coated glass. The angular resolved scattering measurements indicate that LPCVD ZnO scatters light more efficiently than SnO2 coated glass. Fig. 7 a) b) : Scanning Electron Microscope (SEM) micrographs of the surface of Asahi-U (a) and a typical LPCVD-ZnO (b) layer.
Fig. 8 a) b) : Total transmission (measured with index matching liquid) and diffuse transmission for Asahi-U and LPCVD-ZnO, as used in this study. Fig. 9: Voc, Jsc, FF and efficiency as a function of the i-layer thickness (from 200 to 300 nm, at 3.35 A/s) in the initial and light-soaked state. Asahi-U is used as front TCO for the p-i-n cells. For improved statistics, 5 to 10 cells per i-layer thickness were taken into account . Fig. 10: Voc, Jscι FF and efficiency as a function of the i-layer thickness in the initial and light-soaked state. LPCVD-ZnO is used as front TCO. The i-layer thickness is varied form 180 to 400 nm (at 3.35 A/s) . For an improved statistics, 4 to 7 cells were considered. Fig. 11: Relative efficiency change due to light-soaking degradation of p-i-n a-Si:H cells deposited LPCVD-ZnO and Asahi-U. Fig. 12: I (V) curves of the best a-Si:H cells obtained so far on LPCVD-ZnO and Asahi-U.
Fig. 13: I (V) curves of the best p-i-n a-Si:H (light -soaked) 10x10 cm2 mini-module on LPCVD-ZnO. The aperture area is certified by the Swiss Federal Office of Metrology. Fig. 14: I (V) curves of the best lcm2 micromorph tandem cells obtained so far on Asahi SnO2 and LPCVD-ZnO front TCOs. Several processes (here: Pl and P2) have been developed for the deposition of the μc-Si:H i-layer on LPCVD-ZnO front TCO. Fig. 15: AMI.5 I -V characteristics of a stabilized (1000 h, 1 sun, 500C) Micromorph 10x10 cm2 mini-module of 9.94% efficiency. As front TCO, Asahi SnO2 has been applied. Fig. 16: Thickness as a function of the DEZ/H20 ratio and the total transmittance for several B2H6/DEZ ratios
SUMMARY OF THE INVENTION A process window for a TCO layer configuration suitable for photovoltaic thin film cells employs manufacturing, on a substrate, a transparent conductive zinc oxide layer from at least diethylzinc, water and diborane by low pressure chemical vapour deposition (LPCVD) in a process chamber of a deposition system comprising the steps of:
Providing diethylzinc and water in a gas flow ratio between 0.87 and 1.3
Maintaining a diborane / diethylzinc gas flow ratio between 0.05 and 0.4 - Thus depositing a boron doped, conductive ZnO layer on a substrate, with a haze of the zinc oxide layer between 10 and 25%, measured as ratio of diffuse transmittance to total transmittance at 600nm.
A respective transparent conductive zinc oxide layer deposited by a low pressure chemical vapour deposition (LPCVD) process chamber as mentioned above will exhibit an intrinsic layer transmittance in the range between 400 to 800nm of more than 93% and between 400 and llOOnm of more than 92%. The sheet resistance will be smaller than 10Ω/D over a whole substrate of 1.4 m2 size.
A thin film photovoltaic cell can be equipped with such a zinc oxide layer acting as first and/or second electrode in a stacked arrangement as follows: substrate - first electrode - one or more semicon- ductor thin- film p-i-n or n-i-p junctions - second electrode.
DETAILED DESCRIPTION OF THE INVENTION
The TCO layer properties are determined by many parameters such as substrate temperature, process gas mixture and pressure. Figure 2 shows a scanning electron microscope (SEM) picture of a typical ZnO layer surface. The specific surface morphology is one of the key factors to get an optimal Light Trapping. The ZnO light scattering efficiency is quantified with the haze which can be easily tuned between 10 and 25% at a wavelength of 600nm.
The TCO 1200 mass production system produces ZnO layers with thickness uniformity below 20% and a sheet resistance smaller than 10Ω/D over the whole substrate of 1.4 m2 size. In addition, the layer quality remains constant as a function of the number of substrates as seen in Figure 2 for sheet resistance.
Used as a front contact the layer transmittance plays a very important role. The intrinsic layer transmittance in the range of 400 to 800nm is higher than 93% and between 400 and llOOnm higher than 92%. Figure 3 shows a comparison of transmittance measurements between different TCO layers on different glass substrates. It shows that ZnO deposited with a TCO 1200 production system has the highest transmittance over a very wide range of wavelengths .
EXPERIMENTAL RESULTS An easy and precise tuning of the TCO properties is a very desirable feature for solar cell producers as it allows modifying the layer properties according to the cell design.
By increasing the thickness of the TCO layer the sheet resistance can be improved, but the transparency will decrease. The ideal trade off ensuring the best module performance clearly depends on the absorber type. Thickness can be controlled by deposition time as well as by acting on parameters such as temperature, pressure and process-gas flows during deposition (see Fig. 16, top) . In all cases thickness can be varied ensuring good layer uniformity. Fig. 4 shows thickness and sheet resistance measurements for a 1100 x 1300 mm2 V5 glass. Thickness and sheet resistance measurements have been performed on 143 points distributed uniformly over the glass.
Another relevant parameter is the doping level. The ZnO layers are doped with boron (source of boron is diborane B2H6, which is commercially available as 2%dilution in hydrogen) in order to increase layer conductivity. The doping level can be changed easily by variation of the process inputs without hardware changes. This is contrary to physical vapor deposition (PVD) , where the doping level is mainly given by the chemical composition of the sputtering target. However, doping negatively affects haze as well as transmission. By Hall Mobility measurements it is possible to estimate the doping level which guarantees the optimal trade off. In Fig. 5 the Hall Mobility and the Haze (Haze is defined as a ratio of diffused trans- mittance to total transmittance) is shown as function of the B2HS/DEZ ratio.
The wide process window of the LPCVD ZnO deposition process allows being less strict to the stability of process parameters without influence on layer properties. For example, properties of TCO layer remaining the same by pressure variation (± 15%) or by slight changes in gas flows (absolute and relative) . This stability of LPCVD processes allows the tool owners to receive stable output of coated glasses in spite of minor process changes. Thus, flow ratio of two main precursors H2O and diethyl zinc (DEZ) can be changed in a wide range without major changes of ZnO layers (see Fig. 16) .
Besides transparency and conductivity, one other TCO characteristic affecting the performance of PV modules is the light scattering at the TCO-cell interface. Adsorption coefficients for silicon are relatively low and this is especially true for microcrystalline silicon. Therefore it is desirable to increase the path of light through the absorber. The main parameter controlling light scattering is Haze. Results show that LPCVD ZnO layers grown according to this disclosure have superior haze compared to most commercial SnO TCO glasses (see Fig. 6 top) .
By a more accurate control of the TCO surface structure it is possible to induce a significant portion of the light undergoing multiple total internal reflection at the interface of absorbers with front and back contact. This helps increasing adsorption significantly. In order to measure and control these crucial properties of the ZnO surface a setup for Angular Resolved Scattering (ARS) was developed. Figure 6, bottom, indicates that as grown LPCVD ZnO layers have very- good light scattering properties.
EXPERIMENTAL RESULTS II As indicated, for thin film solar cell devices light-trapping is a key feature in order to increase the cell efficiency and to significantly decrease the costs of photovoltaic cells. Therefore properties of the front TCO have been compared between a high performance laboratory TCO (Asahi U) and the in-house fabricated LPCVD ZnO. A single-junction, 1 cm2 a-Si:H cell' s properties as a function of the i-layer thickness were studied in details, for both TCOs. Maximal stable efficiency of 8.6% was obtained on Asahi-U. In particular, the difference between the two studied substrate is in the device J" sc value, and this observation is in good agreement with the measured optical parameters of the TCOs. In further process development investigations the stable efficiency on LPCVD ZnO could be improved to the remarkable value of 9.1%. With the high performance a-Si:H processes on LPCVD-ZnO, several mini-modules (10x10 cm2) were fabricated and light-soaked. ESTI laboratories of JRC in Ispra certified an ap- erture efficiency of 8.32% on the best LPCVD-ZnO mini -module. On the basis of the acquired knowledge on a-Si:H single- junction devices, micromorph tandem cells and mini-modules were realized. Cells show high initial (>11.8%) efficiency, and a mini -module with stabilized efficiencies close to 10% was obtained.
The reduction of the a-Si:H absorber layer thickness is beneficial for both production throughput and device stability (after light- soaking) . For this purpose, rough TCOs are used for enhanced light- trapping within the device. Efficient light-trapping leads to sev- eral-fold enhanced optical path and allow a reduction of the absorber layer thickness. Moreover, in the micromorph tandem device the enhanced light-trapping properties of the TCO are even more important due to the lower optical absorption of μc-Si:H compared to a-Si:H. These light-trapping properties are interesting in the visi- ble range of light and especially in the near- infrared range. Finally, excellent light -trapping capabilities, high transmission and high conductivity are important aspects for TCOs in thin film sili- con solar cells. Asahi-U SnO2 has good optical and electrical properties, that lead to excellent performances of a-Si:H p-i-n cells.
Zinc oxide (ZnO) fabricated by Low Pressure Chemical Vapor Deposi- tion (LPCVD) is considered to have the potential for the fabrication of excellent thin- film silicon solar cells (due to its outstanding light -trapping capability) . In the following the best performances achieved with p-i-n a-Si:H cells and mini-modules on Asahi-U and LPCVD-ZnO front contacts are disclosed. They illustrate the poten- tial of LPCVD-ZnO to obtain excellent initial and stable a-Si:H cells performances. Finally, the comparison has been extended to mi- cromorph tandem cells and mini-modules.
The presented p-i-n a-Si:H solar cells are deposited in a R&D sin- gle-chamber KAI-M system (52 x 41 cm2 substrate size) manufactured by Oerlikon Solar. In order to enhance the deposition rate, the PECVD process was adapted to an excitation frequency of 40.68 MHz. Cleaning of the KAI reactor was based on an in- situ plasma process and is executed after each cell run. The deposition rate of the intrinsic device-quality a-Si:H is 3.35 A/s, whereas μc-Si:H i-layers (in mi- cromorph Tandem) have been deposited at rates up to 5 A/s. For comparison purposes, two i-layer thickness series of p-i-n a- Si :H cells have been deposited on LPCVD-ZnO, respectively Asahi-U SnO2. The best p-i-n a-Si:H cells of each substrate type have been used as top cells for the micromorph tandem fabrication; finally their performances are compared, as well . In-house prepared LPCVD- ZnO is also applied as back contact in combination with a white reflector (WR) on all cells and mini-modules. The deposition parameters of the ZnO layers are optimized to obtain efficient light- trapping, high transparency and conductivity on both front and back contacts of the device.
For careful characterization, all test cells are structured by laser-patterning to a well-defined area of 1.0029 cm2, as certified by the Swiss Federal Office of Metrology (METAS) . Mini-modules of
10x10 cm2 dimension are realized by applying laser scribing for the monolithic series connection. Mini-module aperture area has been also certified by METAS. The I (V) characteristics of the a-Si:H cells were measured under AM 1.5 illumination (Wacom WXS-155S-L2 double-source simulator) at 25°C. Light-soaking experiments were carried out under an illumination close to AMI.5 at 500C during 1000 h. Best light-soaked a-Si:H mini-modules were sent for independent characterization to ESTI laboratories of JRC in Ispra, Italy.
The in-house zinc oxide is fabricated by a LPCVD process at a temperature below 2000C. The sheet resistance of the front TCO layers is below 10 Ω/D. The polycrystalline films are constituted of large grains, whose extremities appear at the growing surface as large pyramids (see Fig. 7b) . This as-grown rough surface texture provides for an efficient diffuse light scattering into the silicon device, as shown in Fig. 7.
LPCVD-ZnO possesses excellent optical properties for its application as front TCO in thin film silicon solar cells. The total transmission of the layer in the visible range of light is well above 80% and Asahi-U. Moreover, its light scattering abilities, as evaluated from the diffuse transmission data of Fig. 8a) and b) , are even better than those of Asahi-U.
The overall behavior of p-i-n cells performances are the following: after p-layer optimization for both TCOs (LPCVD-ZnO and Asahi-U) , similar values and trends are observed for the Voc-values. In the initial state, Voc-values are essentially independent of the i-layer thickness, whereas in the light-soaked state, Voc decreases slightly with increasing thickness, for both TCOs. As already known, initial FF-values on ZnO are slightly lower compared to Asahi-U. However, after light-soaking FF s are comparable on both TCOs.
In agreement with the optical data, for a given i-layer thickness, the »JSC-values are much higher on LPCVD-ZnO than on Asahi-U. For instance, at 300 nm a 1 mA/cm2 higher ι7sc-value (in the stabilized state) is found on LPCVD-ZnO compared to Asahi-U.
Finally, it is possible to conclude that on Asahi-U the optimal thickness for the i-layer of the device is about 200 nm or below. On the other hand, on LPCVD-ZnO the absorber layer should have a thickness in the range of 240 to 300nm in order to maximize the stabilized efficiency. In this study, on LPCVD ZnO the highest efficiency- is 8.8%, whereas on Asahi-U 8.6% is obtained. According to these new findings we demonstrated the successful replacement of the Asahi-U TCO by our in-house ZnO for obtaining best stabilized efficiency single-junction a-Si:H devices.
In Fig. 11 the relative efficiency degradation of cells obtained in the study of Fig. 3 and 4 is shown. According to these results, the relative degradation of the device efficiency depends on the i- layer thickness in a similar way for both TCOs, even if the absolute values of the initial and stabilized efficiencies are different.
According to the previous findings for p-i-n a-Si:H devices deposited on LPCVD-ZnO, the optimal i- layer thickness lies between 240 and 300nm. Based on that result the device processes were further improved and four deposition regimes were found (see table below; regimes 1 to 4) where it is possible to boost the stabilized effi- ciencies even up to 9%. A confirmation of the robustness of the process is that stabilized efficiencies of around 9% using different process windows could be achieved.
Figure imgf000014_0001
Table: Achieved stable efficiencies on LPCVD ZnO front TCO for 4 differently optimized p-i-n a-Si:H deposition regimes (4-13 individual lcm2 cells measured for each regime) .
Several attempts of process fine tuning were also investigated in order to improve the efficiency on Asahi-U, but without further progress. However, on LPCVD-ZnO we could significantly improve the electrical performance up to the level shown in the table above and Fig. 12. The I (V) curves of the best a-S:H cells obtained so far on different front TCOs are shown in Fig. 12.
Stabilized cell efficiencies (on Asahi-U) of 8.6% could be obtained with an i-layer thickness of 200 nm (J" sc=14.8 mA/cm2, FF=66.6%, and VOC=873 mV, see Fig. 12) . In case of LPCVD-ZnO front TCO, the highest stabilized cell efficiency that could be obtained is 9.1%; using an i-layer thickness of 240nm (Jsc=15.6mA/cm2, FF=66.7%, and Voc=876mV, see Fig. 12) .
This result was obtained without using any antireflective coating (ARC) and is of comparable level as the world champion cell of 9.47% with ARC (confirmed by NREL) , as well obtained with LPCVD-ZnO.
It has to be noted that this 9.1% stable a-Si:H cell was deposited in a commercially available KAI-M reactor.
The findings on lcm2 cells size were used to optimize mini -modules on LPCVD-ZnO. The best light-soaked mini-module was independently characterized by ESTI laboratories of JRC in Ispra (see Fig. 13) . The stabilized efficiency of 8.32% obtained on LPCVD-ZnO is 0.5% higher (absolute efficiency) than the previous record mini-module on Asahi - U.
Optimized a-Si:H top cells on Asahi SnO2 and on LPCVD-ZnO have been combined with μc-Si:H bottom cells to realize micromorph tandem devices. Fig. 14 shows the I (V) curves of our best results on both TCOs.
Various processes for maximizing the micromorph device efficiency on LPCVD-ZnO were under investigation, two of them are plotted in Fig. 12, Pl and P2. On one hand, cells with high J" sc-value can be fabricated (Pl) , on the other hand, devices with high Voc-values are pos- sible as well (P2) . Therefore, the best efficiencies obtained so far on LPCVD-ZnO are still slightly below the one reached on Asahi SnO2. A well documented way to maintain a high Voc and FF without decreas- ing Jsc is to use silane concentration profiling during the i-layer deposition. This route has been followed in process 2 (P2) .
By implementing the best a-Si:H cells on Asahi SnO2 as top cells in micromorph mini-modules a stabilized aperture efficiency skimming the 10% could be realized (Fig. 9) .
Efficiencies of the micromorph devices can be further optimized by tuning the i-layer of the bottom cell and the recombination junc- tion. Moreover, no reflector between the top and bottom cell is incorporated in these devices, and it is expected that this could further improve the efficiency.
In summary, a-Si:H single-junction p-i-n devices were investigated on Asahi-U and LPCVD-ZnO front TCOs. After optimization of the p- layer for each TCO, i-layer thickness series of p-i-n cells have been deposited in our R&D KAI M reactor. This study shows that reducing the i-layer thickness on Asahi-U yields to an increased stabilized efficiencies (up to 8.6% for an i-layer thickness of 200nm) . The situation is different for LPCVD-ZnO front TCO, where this study indicates that the maximum stabilized efficiency is obtained for i- layers in the range of 240-300nm. After further device optimization on this TCO, the best stabilized efficiency reaches 9.1%. This result was obtained without applying an antireflection coating (ARC) onto the front glass substrate.

Claims

CLAIMS :
1. A method for manufacturing, on a substrate, a transparent conductive zinc oxide layer from at least diethylzinc, water and dibo- rane by low pressure chemical vapour deposition (LPCVD) in a proc- ess chamber of a deposition system comprising the steps of :
Providing diethylzinc and water in a gas flow ratio between 0.87 and 1.3
Maintaining a diborane / diethylzinc gas flow ratio between 0.05 and 0.4 - Thus depositing a boron doped, conductive ZnO layer on a substrate, whereas the haze of the zinc oxide layer, measured as ratio of diffuse transmittance to total transmittance at 600nm, is between 10 and 25%.
2. The method according to claim 1, whereas the zinc oxide layer is being fabricated at a temperature below 2000C.
3. The method according to claims 1-2, wherein the substrate is glass.
4. A transparent conductive zinc oxide layer deposited by a low pressure chemical vapour deposition (LPCVD) process according to claims 1-3, characterized by an intrinsic layer transmittance in the range between 400 to 800nm of more than 93% and between 400 and llOOnm of more than 92%.
5. A zinc oxide layer according to claim 4, characterized by a sheet resistance smaller than 10Ω/D over a whole substrate of 1.4 m2 size.
6. Thin film photovoltaic cell comprising, successively stacked on a substrate, a first electrode, one or more semiconductor thin- film p- i-n or n-i-p junctions and a second electrode, characterized by a a TCO layer according to claim 4-5 being used as first and/or second electrode.
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