WO1998047702A1 - Photovoltaic device and its method of preparation - Google Patents
Photovoltaic device and its method of preparation Download PDFInfo
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- WO1998047702A1 WO1998047702A1 PCT/US1997/021090 US9721090W WO9847702A1 WO 1998047702 A1 WO1998047702 A1 WO 1998047702A1 US 9721090 W US9721090 W US 9721090W WO 9847702 A1 WO9847702 A1 WO 9847702A1
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Classifications
-
- 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
- C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
- C23C14/58—After-treatment
- C23C14/5806—Thermal treatment
-
- C—CHEMISTRY; METALLURGY
- C03—GLASS; MINERAL OR SLAG WOOL
- C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
- C03C17/00—Surface treatment of glass, not in the form of fibres or filaments, by coating
-
- 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
- C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
- C23C14/06—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material characterised by the coating material
- C23C14/0623—Sulfides, selenides or tellurides
- C23C14/0629—Sulfides, selenides or tellurides of zinc, cadmium or mercury
-
- 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
- C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
- C23C14/06—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material characterised by the coating material
- C23C14/08—Oxides
- C23C14/086—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
- C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
- C23C14/58—After-treatment
- C23C14/5846—Reactive treatment
-
- 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
-
- 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
- H01L31/072—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 heterojunction type
- H01L31/073—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 heterojunction type comprising only AIIBVI compound semiconductors, e.g. CdS/CdTe 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
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- 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
- Y02E10/543—Solar cells from Group II-VI materials
Definitions
- the present invention relates generally to photovoltaic devices and more particularly to thin film solar cells comprising a thin transparent conducting film of cadmium stannate.
- Background Art Photovoltaic devices, used extensively in a myriad of applications, have generated considerable academic and commercial interest in recent years. Photovoltaic devices (solar cells) utilize the specific conductivity properties of semiconductors to convert the visible and near visible light energy of the sun into usable electrical energy. This conversion results from the absorption of radiant energy in the semiconductor materials which frees some valence electrons, thereby generating electron-hole pairs.
- the energy required to generate electron-hole pairs in a semiconductor material is referred to as the band gap energy, which in general is the minimum energy needed to excite an electron from the valence band to the conduction band.
- Cadmium telluride has long been recognized as a promising semiconductor material for thin-film solar cells due to its near-optimum band gap of
- CdTe is typically coupled with a second semiconductor material of different conductivity type such as cadmium sulfide (CdS) to produce a high efficiency heterojunction photovoltaic cell.
- CdS cadmium sulfide
- Small-area CdS/CdTe heterojunction solar cells with solar energy to electrical energy conversion efficiencies of more than 16% and commercial-scale modules with efficiencies of about 9% have been produced using various deposition techniques, including close-space sublimation or "CSS" (U.S. Patent No. 5,304,499, issued April 19, 1994, to Bonnet et al.), spray deposition (e.g., IF. Jordan, Solar Cells, 23 (1988) pp. 107-113), and electrolytic deposition (e.g., B.M. Basol, Solar Cells, 23 (1988), pp. 69-88).
- CCSS close-space sublimation
- spray deposition e.g., IF. Jordan, Solar Cells, 23 (1988
- Thin film solar cells typically comprise an optically transparent substrate through which radiant energy enters the device, the intermediate layers of dissimilar semiconductor materials (e.g., CdS and CdTe), and a conductive film back contact.
- dissimilar semiconductor materials e.g., CdS and CdTe
- TCO transparent conductive oxide
- conventional TCOs such as tin oxide, indium oxide, and zinc oxide
- sheet resistivities typically about 10 ohms per square
- conductivity at thicknesses necessary for good optical transmission.
- conventional TCOs are not efficient current collectors in solar cells of any appreciable size (i.e., greater than about one square centimeter), particularly in commercial-scale modules.
- One way around the current collection limitation described above is to incorporate a more efficient current collection means, such as a front contact current collector grid, into the TCO layer.
- These current collector grids generally comprise a network of very low resistivity material that collects electrical current from the transparent conductive layer and channels the current to a central current collector.
- U.S. Patent Nos. 4,647,711, 4,595,790 and 4,595,791 to Basol et al. each disclose a photovoltaic device having a metallic conductive grid integrated into the TCO layer to decrease the series resistance of the device.
- a metallic grid may theoretically enhance the current collecting capacity of the solar cell, because the grid material is not optically transparent, the presence of the grid can actually reduce the overall conversion efficiency of the photovoltaic device.
- Other disadvantages and potential problems commonly associated with the use of current collector grids include diffusion of the grid material into the semiconductor layers, short circuiting of the device, and incomplete or uneven deposition of the semiconductor layers due to the geometry of the grid.
- U.S. Patent No. 4,808,242 to Murata et al. discloses a photovoltaic device having a substrate on which a plurality of transparent electrodes for each photoelectric conversion cell are arranged. Each transparent electrode has a coupling conductor and a plurality of collecting electrodes connected to the coupling conductor.
- the Murata et al. device includes a transparent current collecting network, and thus avoids the problems associated with a non-optically transparent system, it is difficult and expensive to produce due to the additional materials and processing steps required to integrate the intricate arrangement of electrodes and coupling conductors.
- Low sheet resistance is a primary requirement of any contact on a semiconductor device to reduce the barrier to carrier flow between the semiconductor device and the external electronic circuit.
- High optical transmission is also very important to increase the amount of electromagnetic radiation that is absorbed by the semiconductor material, thereby optimizing the operation of the photovoltaic device by maximizing the number of photogenerated electrons available for collection.
- conventional TCOs have high inherent resistivity. High sheet resistance causes ohmic losses in the transparent conducting film, which decreases the overall conversion efficiency of the device.
- the TCO To reduce the sheet resistance of these conventional TCO films, and thus potentially improve device performance, the TCO must be deposited as a relatively thick layer. However, the thicker the transparent conducting film, the lower the transmission and thus the less electromagnetic radiation that reaches the semiconductor material, thereby reducing the conversion efficiency of the solar cell.
- Another disadvantage associated with conventional TCO layers in thin film solar cell devices is their generally rough surface morphology. For example, one of the most popular TCOs currently in use, tin oxide (SnO 2 ), when deposited as a thin film by chemical vapor deposition (CND) typically produces an average surface roughness of between about 100 and 250 A. Such high surface roughness has several significant disadvantages.
- This improved device should include a transparent conducting film (TCO) which features a variety of desirable optical, electronic and mechanical properties.
- TCO transparent conducting film
- the transparent conducting layer should exhibit high electrical conductivity, high optical transmission, relatively smooth surface morphology, good chemical and environmental stability, be easy and inexpensive to produce, and be easily patternable for module production.
- this high efficiency device should have a front contact with a sheet resistivity as low as 2-5 ohms per square and an optical transmittance greater than 85 percent. Until this invention, no such device existed. Disclosure of the Invention
- the articles of manufacture of this invention may comprise a substrate, a layer of cadmium stannate (Cd 2 SnO 4 or "CTO") disposed on said substrate as a front contact, a thin film comprising two or more layers of semiconductor materials disposed on said layer of Cd 2 SnO 4 , and an electrically conductive film disposed on said thin film of semiconductor materials to form a rear electrical contact to said thin film.
- Cd 2 SnO 4 or "CTO”
- one embodiment of this invention comprises a process for preparing a photovoltaic device having a layer of Cd 2 SnO 4 as a front contact.
- the method of this invention includes depositing a Cd 2 SnO 4 layer onto a substrate, depositing a thin film of semiconductor materials onto the layer of Cd 2 SnO 4 , and depositing an electrically conductive film onto the thin film of semiconductor materials.
- Figure 1 is a cross-sectional view (not in actual scale or proportion) of a photovoltaic device, such as a solar cell, in accordance with the present invention.
- Figure 2 is a graph showing the relative intensities of x-ray diffraction peaks as a function of 2 theta for a Cd 2 SnO 4 layer prior to heat treatment and the cadmium stannate film after treatment.
- Figure 3 a is a graph showing the transmittance for the cadmium stannate film shown in Figure 2 and a commercially available SnO 2 thin film.
- Figure 3b is a graph showing the absorbance for the cadmium stannate film shown in Figure 2 and a commercially available SnO 2 thin film.
- Figure 4a is an atomic force micrograph showing the surface morphology of a conventional SnO 2 film.
- Figure 4b is an atomic force micrograph showing the surface morphology of a cadmium stannate film.
- Figure 5 is an optical micrograph of a photolithographically patterned cadmium stannate film which was etched in dilute HC1 for 2 minutes.
- Figure 6 is a graph of resistivity as a function of temperature for the cadmium stannate and SnO 2 films shown in Figures 3 and 4.
- Figure 7 is a graph of current and voltage for a photovoltaic device of the present invention.
- Figure 8 is a graph of open circuit voltage as a function of
- FIG. 1 shows a cross-sectional view of a photovoltaic device of the present invention.
- the photovoltaic device generally referred to by reference number 20 comprises a transparent substrate 22 through which radiant energy enters the device.
- a thin transparent conducting oxide (TCO) film of cadmium stannate (Cd 2 SnO 4 ) 24 is deposited onto the transparent substrate 22 and is contiguous thereto.
- the cadmium stannate film 24 is deposited between the transparent substrate 22 and the first semiconductor layer 26 (described below) to function as a front contact current collector.
- the cadmium stannate film 24 thus replaces the conventional TCO films commonly used in photovoltaic devices.
- a first semiconductor layer 26 is disposed on the cadmium stannate film 24 and a second semiconductor layer 28 is disposed on the first semiconductor layer 26.
- the conductivity types of semiconductor layers 26 and 28 are not the same.
- a back electrical contact 30 is disposed over the second semiconductor layer 28 and in ohmic contact therewith.
- the present invention also provides a method of making a photovoltaic device 20, which method includes depositing a film of cadmium stannate 24 onto a transparent substrate 22.
- the cadmium stannate film 24 is formed by RF sputtering a layer of substantially amorphous Cd 2 SnO 4 onto a suitable transparent substrate 22.
- a layer of CdS is formed on a second substrate (not shown), such as soda lime glass, by a suitable technique, such as RF sputtering or chemical bath deposition.
- the coated CdS layer is placed in contact with the Cd 2 SnO 4 layer and heated to a treatment temperature sufficient to induce crystallization of the Cd 2 SnO 4 layer.
- the resulting uniform, crystalline cadmium stannate film 24 has significantly improved electrical and optical properties when compared to the properties of previously available TCO films.
- the above-described method for depositing a film of cadmium stannate 24 onto a transparent substrate 22 is described in a related copending U.S. patent application entitled "Thin Transparent Conducting Films of Cadmium Stannate,” which is incorporated by reference herein.
- at least two semiconductor materials of differing conductivity types are deposited on the cadmium stannate film 24 to function as a semiconductor for the device 20.
- this description refers to a first semiconductor layer 26 of CdS and a second semiconductor layer 28 of CdTe.
- the present invention can be practiced using any suitable combination of semiconductor materials of differing conductivity types including, but not limited to, CdS/CdTe, CdS/HgCdTe, CdS/CdZnTe, and CdS/ZnTe.
- the substrate 22 for the cadmium stannate film 24 must be optically transparent over the range of light wavelengths for which transmission through the substrate is desired. Suitable transparent substrates 22 allowing transmission of visible light include silica and glass. Also, the transparent substrate 22 must be of a material capable of withstanding heat treatment at temperatures of 550° C or more, as described below, and the cadmium stannate film 24 must adhere to the transparent substrate 22 material. The thermal expansion coefficient of the transparent substrate 22 must be close enough to the thermal expansion coefficient of the cadmium stannate film 24 to prevent cracking or buckling of the cadmium stannate film 24 during heat treatment.
- a cadmium stannate film 24 is created by RF sputtering from a hot-pressed target containing stoichiometric amounts of SnO 2 and CdO onto the transparent substrate 22.
- the sputtering can be conducted in substantially pure oxygen which is substantially free of impurities which could react with the metal oxides present. Preferably, the oxygen is 99.999% pure. It is also preferred that the sputtering is at room temperature.
- the Cd 2 SnO 4 layer is yellowish in color and substantially amorphous. As is apparent to one skilled in the art, higher transmittance is obtained with a thinner film, and lower sheet resistance is obtained with a thicker film.
- the CdS layer may be formed on a second substrate (not shown) by any method known to those skilled in the art.
- the CdS layer may be formed by chemical bath deposition or sputtering. Although there is no maximum layer thickness, the coating should be thick enough for reuse.
- the CdS layer must also have a smooth surface and be uniform and free of pinholes.
- the second substrate may be any material known to those skilled in the art which is capable of withstanding heat treatment up to 550°C or more. The material must be one to which CdS will adhere. Also, the coated substrate must have a CdS layer surface which is sufficiently flat to make good contact with the Cd 2 SnO 4 layer, as described below. Further, the second substrate must not react chemically with CdS at the treatment temperature.
- a suitable, inexpensive substrate is soda lime glass.
- the Cd 2 SnO 4 and CdS layers are then placed in contact with each other in an environment substantially free of water and oxygen. Water and oxygen cause stains in the cadmium stannate film 24 which inhibit its optical and electrical properties.
- a suitable environment is flowing argon at ambient pressure.
- the substrates and layers are then heated for a period of time sufficient for the Cd 2 SnO 4 layer to form a uniform single-phase layer of Cd 2 SnO 4 with a spinel crystal structure, substantially free of CdO, SnO2 and CdSnO 3 phases.
- This uniform single-phase layer of Cd 2 SnO 4 is referred to herein as the "cadmium stannate film 24," as distinguished from the layer of substantially amorphous Cd 2 SnO 4 prior to treatment, which is generally referred to herein as the “Cd 2 SnO 4 layer.
- the treatment temperature should be low enough to prevent softening and damage to the transparent substrate 22. Although higher temperatures result in superior cadmium stannate film 24 properties, satisfactory films have been obtained at temperatures less than 600 °C. Twenty minutes was found to be a satisfactory period for treatment in laboratory experiments. Cadmium stannate films 24 prepared by the method described herein and having thicknesses of about 0.5 microns were found to have desirable optical and electrical properties.
- the two substrates are cooled at a rate slow enough to avoid stress in the substrates and removed from each other.
- the transparent substrate 22 with its cadmium stannate film 24 is further processed to produce the photovoltaic device 20 of the present invention, as described below.
- CdS layer may be reused until the CdS layer is too thin to perform properly.
- the second substrate may then be re-coated with a new layer of CdS and reused.
- FIG. 2 shows a comparison of the X-ray diffraction patterns for the amorphous Cd 2 SnO 4 layer prior to heat treatment and the cadmium stannate film after treatment.
- the data indicate a single-phase spinel crystal structure having a slightly larger lattice constant than Cd 2 SnO 4 without interstitial cadmium.
- a first semiconductor layer 26 is deposited on the top surface of the cadmium stannate film 24. Contiguous to the first semiconductor layer 26 is a second semiconductor layer 28 having a conductivity type that is different from the conductivity type of first semiconductor layer 26.
- a back electrical contact 30 is disposed over the second semiconductor layer 28 and in ohmic contact therewith.
- the semiconductor layers 26 and 28 and the back contact 30 may be formed by any known process, such as chemical bath deposition, vapor deposition, electro-deposition, and the like.
- the first semiconductor layer 26 is deposited by a chemical bath deposition and the second semiconductor layer 28 is deposited by close-space sublimation, as. described in the Examples hereof.
- the photovoltaic device 20 may include an electrical contact or electrode pad (not shown) on the cadmium stannate film 24, the function and construction of which is known in the art and not a part of this invention.
- the photovoltaic device 20 may further include an anti-reflective (AR) coating (not shown) on the front surface of the transparent substrate 22 to enhance the initial transmission of light into the semiconductor material, which is also known in the art and not a part of this invention.
- AR anti-reflective
- a significant advantage associated with the photovoltaic device of the present invention is the improved device performance due to the high conductivity of the cadmium stannate film. As will be appreciated by those of skill in the art, the conductivity of the transparent conducting film can be improved by increasing either the carrier concentration or the electron mobility.
- the transparent conducting films in the photovoltaic devices of the present invention exhibit unusually high electron mobilities, even at high carrier concentrations.
- CdS vapor is sublimated from the CdS layer and diffuses into the
- Table 1 compares the thicknesses and electrical properties of two commercially available SnO 2 films, a SnO 2 film prepared using a tetramethyltin (TMT) process, and cadmium stannate films prepared as described in Example 1 below.
- R_ is the sheet resistivity
- ⁇ is electron mobility
- n is the charge carrier concentration
- p is the resistivity. All values reported in Table 1 were obtained in the same laboratory using the same equipment and analytical techniques. Film thicknesses were determined from the position of neighboring interference maxima in optical transmittance curves, and cross checks were performed using a Dektak thickness profilometer. The values for n, ⁇ , and p were obtained by the Hall effect method, and R j was measured by the four-point probe technique
- the photovoltaic devices of the present invention offer the important advantage of reduced series resistance and increased fill factor, and hence improved efficiency, due to the presence of these low- resistivity cadmium stannate films
- this advantage applies to both small and large-area photovoltaic devices, reducing the series resistance in commercial modules is of particular interest, since the distance between laser scribe lines can be significantly increased without significant resistive power losses Moreover, because the distance between scribe lines is increased, fewer scribe lines are required, thus improving throughput and reducing manufactu ⁇ ng costs
- Another significant advantage associated with the photovoltaic device of the present invention is the improved optical transmission and electrical resistance of the cadmium stannate film, as compared to existing devices comprising conventional TCO films.
- the photovoltaic device of the present invention offers improved short circuit currents, and hence improved performance, over prior art devices.
- a yet further significant advantage of the photovoltaic device of the present invention is the improved surface morphology of the transparent conducting film, which provides improved device performance.
- Figure 4 provides a comparison of the surface morphologies of a conventional SnO 2 film ( Figure 4a) and a cadmium stannate film ( Figure 4b).
- the surface of the cadmium stannate film is significantly smoother than the surface of the SnO 2 film.
- Data obtained from atomic force micrography indicate that the average surface roughness of the cadmium stannate film is an order of magnitude lower than that of the SnO 2 film. It is well known that in a heterojunction solar cell, reducing the window layer absorption increases short circuit current (J sc ).
- CdS/CdTe solar cells this is typically achieved by reducing the CdS thickness (C. Ferekides, et al, 23rd IEEE SPVC Proc. (1993) pp. 389-393).
- CdS thickness C. Ferekides, et al, 23rd IEEE SPVC Proc. (1993) pp. 389-393.
- a CdS/CdTe solar cell with a thin CdS layer has much better spectral response in the blue.
- reducing the thickness of the CdS film to between 600 A and 700 A can reduce the open circuit voltage and fill factor.
- the CdS film is either partially or completely consumed forming a CdS,. x Te x intermixed layer.
- the CdS consumption reportedly increases as the CdS film thickness decreases (B.E. McCandless and S.S. Hegedus, 22nd IEEE SPVC Proc. (1991) pp. 967-972).
- CdTe/TCO specifically CdTe/SnO 2
- the probability of pinhole formation increases, particularly for sputter deposited or CSS deposited CdS, as the SnO 2 surface roughness increases (A. Rohatgi, et al, 22nd IEEE SPVC Proc. (1991) pp. 962-966).
- the photovoltaic device of the present invention provides a significant improvement in device performance.
- Cadmium stannate films are also much easier to pattern than conventional SnO 2 films (by etching in either HC1 or HF), thus facilitating production and significantly expanding commercial applications of the photovoltaic device of the present invention.
- Figure 5 shows a photolithographically patterned cadmium stannate film which was etched in dilute HC1 for 2 minutes. As shown in Figure 5, cadmium stannate films provide excellent edge definition, which is particularly important for certain commercial applications, such as in advanced photovoltaic module and flat panel display device processing.
- the photovoltaic device of the present invention is also more durable and stable (chemically and thermally) than existing devices due to the improved mechanical properties of the cadmium stannate film.
- cadmium stannate films deposited on glass substrates have good adhesion, are reasonably hard and scratch resistant, and exhibit high stability at elevated temperatures and over long periods of time.
- the cadmium stannate films are less affected by post-deposition processing (i.e., CdCl 2 heat-treatment) than conventional TCO films, thus improving process reproducibility and product yield.
- the photovoltaic device of the present invention is also economically advantageous. Highly efficient thin film devices can be produced using low cost substrates, such as soda lime glass substrates, thus reducing production costs.
- the following examples demonstrate the practice and utility of the present invention but are not to be construed as limiting the scope thereof.
- Cd 2 SnO 4 layers are deposited onto substrates using a modified SC-3000 evaporation system, manufactured by CNC Products, Inc.
- Optical measurements are made with a Cary 2300 spectrophotometer, manufactured by Narian Company.
- the sputtering was carried out at room temperature in a modified SC3000 evaporation system, evacuated to a background pressure of ⁇ 5 x 10 "7 Torr and then backfilled with high purity oxygen.
- Corning 7059 glass substrates (sample Cd 2 SnO 4 -l) and soda-lime glass substrates (sample Cd 2 SnO 4 -2) were placed on a water-cooled sample holder parallel to the target surface. The distance between the substrate and the target was varied from 6 to 9 cm.
- the targets 33 mol % SnO 2 and 67 mol % CdO) were reacted using a commercial hot pressed oxide target.
- X-ray diffraction showed that the target was single-phase orthorhombic Cd 2 SnO 4 .
- Deposition was performed at an oxygen partial pressure of 10-20 mTorr with the RF power between 100 and 140 Watts, providing an average deposition rate of about 10 nm min "1 .
- Sample Cd 2 SnO 4 -l was heated at 680 °C and sample Cd 2 SnO 4 -2 was heated at between 580° and 600°C for about 20 minutes in a tube furnace containing argon of 99.999% purity flowing at a rate of 1500 seem.
- the samples were placed in contact with a CdS-coated glass substrate during the heat treatment.
- the CdS was previously deposited by chemical bath deposition as a thin layer on a glass substrate.
- Figure 2 compares X-ray diffraction patterns obtained with a DMAX-A X-ray diffractometer, manufactured by Rigaku Company.
- the pattern of the as-deposited Cd 2 SnO 4 layer shows an amorphous structure.
- the diffraction pattern of the Cd 2 SnO 4 film after the Ar/CdS heat treatment indicates a single-phase, cubic spinel crystal structure.
- secondary phases such as CdO, SnO 2 , and CdSnO 3 form in these films after crystallization.
- Table 1 provides the electrical properties of the cadmium stannate films, two commercially available SnO 2 films, and a film obtained from Dr. Christopher Ferekides at the University of South Florida (Sample USF), which was prepared using a tetramethyltin (TMT) process similar to that described by J. Proscia and R.G. Gordon, Thin Solid Films, 214 (1992), pp. 175-187.
- TMT tetramethyltin
- the cadmium stannate films have a lower resistivity (p) and higher mobility ( ⁇ ) and carrier density (n) than Sn0 2 films of comparable thickness.
- Figures 3a and 3b compare the transmittance (T) and absorbance (A), respectively, of a cadmium stannate film (Sample Cd 2 SnO 4 -l) and a commercially available Sn0 2 thin film deposited on Corning 7059 glass (Sample SnO 2 -l), manufactured by Solarex Corp. (Thin Film Division, 826 Newton Yaraley Road, Newton, PA 18940).
- the cadmium stannate film has a higher average transmittance (about 85 percent) over the visible portion of the spectrum as compared to the SnO 2 film, even though the cadmium stannate film has a much lower resistivity.
- the cadmium stannate film has a lower absorbance in the visible range as compared to the SnO 2 film, even at a carrier concentration of about 9 x 10 20 cm- 3 .
- Figure 4 shows atomic force micrographs of the surface of both a cadmium stannate film (Sample Cd 2 SnO 4 -l) and a SnO 2 thin film (Sample SnO 2 -l).
- the SnO 2 thin film ( Figure 4a) deposited by atmospheric pressure chemical vapor deposition using a SnCl 4 chemistry, has an average surface roughness of 212 A.
- the Cd 2 Sn0 4 -l thin film Figure 4b
- FIG. 5 shows a photolithographically patterned cadmium stannate film (Cd 2 SnO 4 -l) which was etched in dilute HC1 for 2 minutes.
- Cadmium stannate films can be easily patterned by etching in either HC1 or HF, with excellent edge definition.
- EXAMPLE 2 A film of cadmium stannate (Cd 2 SnO 4 -l) is prepared as described in Example 1 and heat treated in argon for 20 minutes. The thermal stability of the cadmium stannate film was compared to a similarly treated SnO 2 thin film (Sample SnO 2 -l). As indicated in Figure 6, the resistivity of the SnO 2 film degrades when annealed at temperatures in excess of 500°C. In contrast, the cadmium stannate thin film is extremely stable at temperatures as high as 650°C.
- EXAMPLE 3 A film of cadmium stannate (Cd 2 SnO 4 -l) was prepared as described in Example 1 and subjected to a residual gas analysis to determine the temperature stability of the film with respect to release of cadmium. The sample was heated to 400K at a pressure of 10 "5 Torr in a UTI 100C Precision Gas Analyzer, manufactured by Uthe Technology International. No cadmium-containing gaseous species were detected, up to the detection limit of the technique. Thus, the cadmium stannate film is stable and does not decompose readily at elevated temperatures such as those likely to be encountered in most uses of TCO films.
- EXAMPLE 4 A film of cadmium stannate (Cd 2 SnO 4 -l) was prepared as described in Example 1 and heat treated in contact with a layer of CdS at 680° C. The film was stored at room temperature, and the sheet resistance of the film was measured periodically during the year. No change in sheet resistivity (R-) was observed, indicating that the cadmium stannate film properties are stable over extended periods of time.
- Example SnO 2 -l Films of cadmium stannate (Cd 2 SnO 4 -l) were prepared as described in Example 1; SnO 2 thin films (Sample SnO 2 -l, Table 1) were purchased from Solarex Corp. (Thin Film Division, 826 Newton Yaraley Road, Newton, PA 18940).
- a first semiconductor window layer of CdS was deposited on the top surface of the respective films by a chemical bath deposition (CBD) technique using CdAc 2 , NH 4 Ac, NH 4 OH and thiourea in an aqueous solution
- CBD chemical bath deposition
- the substrates with a SnO 2 layer were cleaned using a 1 % Liquinox in hot DI (deionized) water with thorough rinsing (5 minutes running DI water, 5 sonications in fresh DI, one sonication in hot DI)
- Substrates with a cadmium stannate film were cleaned by rinsing with TCE, then acetone, then IP A, followed by thorough rinsing
- the substrates were loaded into a quartz holder in a jacketed beaker and the chemical deposition bath was prepared as follows (1) 550 mL of water was added to the jacketed beaker and was heated to 86-87°C using a recirculator, (2) 8 mL of a
- CdTe was deposited on the CdS films by close-space sublimation (CSS), as described by C Ferekides, et al., 23rd IEEE SPVC Proc. (1993) pp 389-393)
- CSS close-space sublimation
- the substrate and source temperatures were 600 °C and 660 °C, respectively
- the distance between the substrate and the source was 0 2 cm, and the ambient pressure was 15
- the cadmium stannate film replaces the SnO 2 layer as the front contact
- the thickness of the cadmium stannate film was between 0 5 and 0 6 microns, with a sheet resistivity of about 3 ⁇ /square Flowever, as will be appreciated by those of skill in the art, the thickness of the cadmium stannate film can be varied as appropriate for the particular application A thinner film provides a higher transmittance, and a thicker film provides a lower sheet resistance
- the thickness of the cadmium stannate film can be increased accordingly Alternatively, the film thickness can be minimized, thereby increasing the sheet resistance, when optimal transmission is required
- the cadmium stannate film is between about 0 2 and 2 0 microns thick, more preferably between about 0 3 and 0 7 microns thick, and most preferably
- Figure 8 compares the open circuit voltage (V oc ) of the cadmium stannate- based and SnO 2 -based solar cells as a function of CdCl 2 concentration As shown in this figure, the cadmium stannate-based device has a significantly higher N oc than the SnO 2 -based device for any particular CdCl 2 concentration. This advantage likely results, in part, from the improved surface morphology of the cadmium stannate film, as previously discussed. However, the chemistry at the Cd 2 SnO 4 /CdS interface may also be an important contributing factor. Figure 8 also indicates that the N oc of Cd 2 SnO 4 -based CdS/CdTe devices is less dependent on the CdCl 2 heat-treatment.
- the photovoltaic devices of the present invention provide improved process reproducibility and product yield.
- EXAMPLE 6 To compare the thermal stability of the photovoltaic devices of the present invention with conventional devices, three sets of thin film CdS/CdTe solar cells comprising either cadmium stannate or SnO 2 TCO films were fabricated as described in Example 5. Films of cadmium stannate (Cd 2 SnO 4 -l) were prepared as described in Example 1; SnO 2 thin films (Sample SnO 2 -l) were purchased from Solarex Corp. (Thin Film Division, 826 Newton Yaraley Road, Newton,
- SnO 2 thin films (Sample SnO 2 -3) were prepared by a TMT process similar to that described by J. Proscia and R.G. Gordon, Thin Solid Films, 214 (1992), pp. 175-187.
- Table 2 compares the sheet resistance (R of the three sets of CdS/CdTe cells before and after CSS CdTe deposition, as measured by a Tencor M-gauge. As can be seen in Table 2, only the cadmium stannate films provide a sheet resistivity of less than 3 ⁇ /square following CSS deposition.
- the thermal stability of the photovoltaic devices of the present invention is a particularly significant advantage, since CSS deposition typically involves processing temperatures as high as 600 °C.
Abstract
Description
Claims
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DE19782118T DE19782118T1 (en) | 1996-11-18 | 1997-11-18 | Photovoltaic device and its preparation process |
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US9276142B2 (en) | 2010-12-17 | 2016-03-01 | First Solar, Inc. | Methods for forming a transparent oxide layer for a photovoltaic device |
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Also Published As
Publication number | Publication date |
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US5922142A (en) | 1999-07-13 |
JP2001504281A (en) | 2001-03-27 |
DE19782118T1 (en) | 1999-10-14 |
AU5358298A (en) | 1998-11-13 |
AU712220B2 (en) | 1999-11-04 |
CA2271412A1 (en) | 1998-10-29 |
CA2271412C (en) | 2005-02-08 |
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