THIN FILM PHOTOVOLTAIC DEVICE

Abstract: A thin film photovoltaic device comprising a first layer of copper indium diselenide p-type semiconductor and a second layer of n-type zinc oxide semiconductor. In a preferred form, the first portion of the zinc oxide film at the junction with the CIS is undoped to have relatively high resistivity while the remaining portion thereof is doped to achieve low resistivity. The zinc oxide is preferably deposited by a low temperature chemical vapor deposition process.

Description: The present invention relates to thin film heterojunction photovoltaic cells formed from compound semiconductors, especially cells having copper indium diselenide (CIS) as a first semiconductor layer, and more particularly to such cells in which the semiconductor forming the light receiving face comprises primarily zinc oxide (ZnO). A known photovoltaic cell structure which is believed relevant to the present invention comprises a first layer of copper indium diselenide forming a junction with one or more layers of cadmium sulfide. The background of such cells is discussed in considerable detail in U.S. Patent 4,335,266 issued to Mickelsen et al on June 15, 1982. As generally discussed in that patent, this type of solar cell is believed by many to be a viable alternative solar cell. The efficiency of such compound semiconductor cells have been gradually increased as various improvements have been made. In addition they offer the opportunity of reduced manufacturing cost. The basic improvement taught by Mickelsen involves a new method for forming the copper indium diselenide layer so as to not have an adverse effect upon the cadmium sulfide layer when deposited. Mickelsen deposits this CIS layer in two distinct regions with the first having an excess of copper and the second having a deficiency thereof. Diffusion between the two layers is said to achieve a uniform CIS structure while avoiding defects such as pure copper nodules at or near the surface upon which the cadmium sulfide is to be deposited. Such copper nodules, and possibly other defects, can cause short circuiting of the later deposited cadmium sulfide layer to the back face electrode of the finished device. Despite the improvements in the CIS layer claimed by Mickelsen, it has still been found necessary to deposit the cadmium sulfide portion of the cell in two distinct regions. The first is a 0.5 to 1.5 micron layer of essentially pure cadmium sulfide which is of n-type conductivity and high resistivity. The high resistivity character of this region is believed necessary to limit the effect of defects such as pure copper or metallic copper selenide nodules in the CIS layer. The cadmium sulfide layer is then completed by deposition of a 2 to 4 micron thick layer of cadmium sulfide which has been doped appropriately to provide low resistivity. While various improvements have been made in such CIS-CdS cells several basic problems remain. One problem is that the relatively thick cadmium sulfide layer absorbs a considerable amount of incoming light before it can reach the active junction area. This necessarily reduces device efficiency. In addition it is well-known that cadmium is a highly toxic heavy metal. While there is no evidence that cadmium in the completed devices presents any danger to the environment or to the user, the material does cause certain hazards in the manufacturing process. Thus, manufacturing costs are increased by the extra time and equipment which must be employed to avoid any danger to employees manufacturing devices incorporating cadmium and to avoid any environmental damage. Some of the problems inherent in use of CdS have been avoided in experimental devices in which a heterojunction has been formed between CIS and ZnO. See for example: "A ZnO/p-CuInSe₂ Thin Film Solar Cell Prepared Entirely by Spray Pyrolysis", M. S. Tomar and F. J. Garcia, Thin Film Solids, 90(1982) pp 419-423; "Chemical Vapor Deposited Copper Indium Diselenide Thin Film Materials Research" Final Report, March 1984, SERI/STR-211-2247 and "Preparation of ZnO Thin Films by Plasma-Enhanced Organometallic Chemical Vapour Deposition", M. Shimizu et al, Thin Solid Films, Vol 96 (1982) Oct No. 2 pp 149-154. In the Tomar publication ZnO was deposited on a SnO₂ coated glass substrate by spray pyrolysis at a temperature of from 350 o C to 550 o C. After similar deposition of a CIS layer, photovoltaic response with about 2% efficiency was measured. In the SERI report efficiency in the range of 2% to 3% was achieved by ion-beam sputtering of ZnO films onto CIS films deposited by a close space chemical vapor transport technique. Shimizu et al. describes the preparation of zinc oxide thin films by plasma-enhanced organometallic chemical vapour deposition. The preparation of n-ZnO/p-Si and n-ZnO/p-CdTe heterojunction solar cells is also described; however, these cells comprise only a single ZnO layer and the plasma-deposited films are reported to have low resistivities on the order of 10⁻² to 10 ohms. Thus, none of these references indicates that commercially practical efficiency can be achieved when ZnO is used in place of CdS in a thin film CIS heterojunction cell. Accordingly, an object of the present invention is to provide an improved thin film photovoltaic device comprising a first layer of copper indium diselenide semiconductor and a second layer comprising primarily zinc oxide. Zinc oxide containing layer comprises a first high resistivity layer comprising a zinc oxide semiconductor and a second low resistivity layer comprising a zinc oxide semiconductor. In another aspect, the present invention relates to a method for preparing a thin film photovoltaic device including the steps of providing a layer of copper indium diselenide on a supporting substrate and applying to this layer at least one layer of zinc oxide semiconductor, the invention being characterised in that two zinc oxide layers are deposited on the copper indium diselenide layer, the layers comprising a second low resistivity layer and a first high resistivity layer intermediate the copper indium diselenide layer and the second low resistivity layer. Preferably at least the second low resistivity layer is deposited by a low temperature chemical vapor deposition process. The present invention may be better understood by reading the following detailed description of the preferred embodiments with reference to the accompanying drawings wherein: Figure 1 is a cross-sectional illustration of a thin film photovoltaic device having a single zinc oxide layer deposited on a copper indium diselenide layer; and Figure 2 is a cross-sectional illustration of an embodiment of the present invention in which the zinc oxide layer has been deposited as two layers having different resistivities. With reference now to Figure 1, there is provided a cross-sectional illustration of a portion of a photovoltaic cell 10. Cell 10 is structurally supported on a glass substrate 12 which is about 1 to 3 millimeters thick. A back contact comprises a metal layer 14 deposited upon substrate 12. Layer 14 preferably is molybdenum which has been deposited by sputtering to a thickness of about 0.2 to 2 microns. The first active region of the device 10 comprises a semiconductor layer 16 which is primarily p-type copper indium diselenide having a thickness of about 0.2 to 2 microns. The second active region of the device 10 comprises a semiconductor layer 18 having a thickness of from about 0.5 to 2 microns. Layer 18 comprises primarily n-type zinc oxide which has been appropriately doped to provide relatively low resistivity, for example, better than about 2.0 x 10⁻³ ohm-cm and preferably better than 1.0 x 10⁻³ ohm-cm. The device is completed by a series of front face contacts 20 in the form of narrow metal strips which may be, for example, aluminium deposited by sputtering, evaporation or plating. While a device fabricated as illustrated in Figure 1 has proved to be functional, its photovoltaic efficiency is rather low. This is believed to be due to defects in the CIS layer 16 which have an adverse affect on the device just as they have had on the CIS-CdS devices of Mickelsen et al. At this point it has not been shown that "perfect" CIS layers can be deposited or that post-treatments of such layers can remove all defects. As a result, we believe it is preferred to incorporate a high resistivity zinc oxide layer in our device to overcome the defects in the CIS layer. With reference to FIGURE 2, there is illustrated an embodiment of the present invention in which the zinc oxide layer is deposited as two distinct layers. In FIGURE 2 the device 22 is constructed on a glass substrate 24 which may be identical to the substrate 12 of FIGURE 1. Likewise, a molybdenum layer 26 and a CIS 1 yer 28 may be identical to the layers 14 and 16 of FIGURE 1. However, instead of using the single zinc oxide layer 18 of FIGURE 1, the device 22 incorporates a first high resistivity zinc oxide layer 30 and a second low resistivity zinc oxide layer 32. Layer 30 may have a thickness ranging from 0.01 to 0.2 microns (100 to 2000 angstroms) whilst layer 32 has a thickness of about 1 micron (10,000 angstroms). The high resistivity layer 30 is achieved by use of essentially pure zinc oxide material while layer 32 is doped to achieve low resistivity. Layer 30 preferrably has a resistivity about two hundred times that of layer 32. For example where layer 32 has the preferred resistivity of about 1x10⁻³ ohm-cm, layer 30 has a resistivity of about 0.2 ohm-cm. As in the FIGURE 1 embodiment, a device 22 is completed by a series of front face contacts 34. As indicated above, layers 12, 14 and 16 of FIGURE 1 and layers 24, 26 and 28 respectively of FIGURE 2 are essentially identical and may therefore be fabricated in essentially the same manner. For example, as indicated above, layer 26 of FIGURE 2 is typically molybdenum which may be applied to the glass substrate 24 by sputtering to a thickness of from 0.2 to 2 microns. The copper indium diselenide layer 28 may be deposited according to the process of the above-referenced Mickelsen et al patent, if desired. However, specific examples of the present invention have been fabricated using a copper indium diselenide layer 28 deposited by different processes. In these processes, layer 28 is formed by first depositing copper to a film thickness of about 0.2 microns (2000 angstroms) on metal layer 26. Thereafter indium is deposited onto the copper film to a thickness of about 0.4 microns (4000 angstroms). These layer thicknesses provide a desired atomic ratio of about 1.0. The copper and indium layers have been deposited both by electrodeposition from aqueous solutions and by sputtering from solid targets. After thus depositing copper and indium by either method, a copper indium diselenide semiconductor layer 28 is formed by "selenization". This process involves heating the substrate to approximately 400 o C in the presence of gas comprising about 3% H₂Se diluted in argon for approximately one hour and thereafter annealing the material for an additional two hours at the same temperature. In this process, the copper and indium interdiffuse while selenium diffuses into and reacts with both the copper and indium. The result is a film of copper indium diselenide having a thickness of about 1 micron. With reference now to FIGURE 2, the preferred method for depositing zinc oxide film on the copper indium diselenide layer will be described. This process is referred to as chemical vapor deposition (CVD) or as low temperature chemical vapor deposition (LTCVD). In this process, the substrate 24 with films 26 and 28 is placed in a vacuum chamber on a heated support. The chamber is then evacuated and substrate temperature is adjusted to about 150 o C and allowed to stabilize for about 20 minutes. To form the high resistivity zinc oxide layer 30 on a substrate having an area of about 0.1M² (1 square foot), reactive gases comprising 8 sccm (standard cubic centimeters per minute) of diethyl zinc, 10 sccm H₂O and 186 sccm of argon were flowed across the substrate at a pressure of 115 Pa (0.86 torr) while the initial temperature was maintained. Flow rates of diethyl zinc and H₂O were estimated from the flow rates of inert carrier gas streams assuming saturation. For deposition times of from two to six minutes, high resistivity zinc oxide films may be deposited to thicknesses ranging between 0.07 and 0.2 microns (700 and 200 angstroms). While the resulting ZnO film is referred to herein as pure, or essentially pure, ZnO, it will be appreciated that the H₂O contributes some hydrogen to the material and this probably has some effect on resistivity. When the desired thickness of high resistivity layer 30 has been deposited, deposition of the low resistivity layer of zinc oxide 32 is commenced by simply changing the reactive gas mixture. Doped layer 32 may be formed from a reactive gas comprising 8 sccm diethyl zinc, 10 sccm H₂O, 0.075 sccm B₂H₆, and 200 sccm argon. A high conductivity layer 32 having a thickness of about 1 micron may be deposited under these conditions in about 30 minutes process time. While specific parameters are given above for the LTCVD process used to produce examples, good ZnO films can be achieved over a range of conditions. The substrate temperature can be varied from 60 o C to 350 o C and preferably from 100 o C to 200 o C. Even at the 350 o C level this is a low temperature process as compared to other methods of depositing transparent conductors and therefore well suited for use with semiconductors. Vacuum system pressure can be varied from 13.3 to 266 Pa (0.1 torr to 2 torr), which range is not considered high vacuum. The chamber should however be capable of developing high vacuum levels to allow removal of all ambient gases from the chamber before deposition conditions are established. Other organozinc compounds having the formula R₂Zn, where R is a lower alkyl radical, for example dimethyl zinc (CH₃)₂Zn, may be used in place of the diethyl zinc (C₂H₅)₂Zn specified above. Other oxygen sources, such as O₂ gas, may be useful in place of the H₂O used in the preferred embodiment. Other inert gases, such as helium or possibly even nitrogen, could be used in place of argon as a carrier for the organozinc compound and water. The organozinc compound vapor and water vapor are preferably introduced into the vacuum chamber by bubbling the inert gas through a reservoir of the compound or deionized water. A mole ratio of water to organozinc compound greater than 1.0 is preferred to provide the proper reaction conditions. With reference to FIGURE 1, it can be seen that the zinc oxide layer 18 corresponds to the layer 32 of FIGURE 2 and is deposited in the same manner. That is, the device of FIGURE 1 is fabricated by the above-described process except that the deposition of a high resistivity layer corresponding to layer 30 of FIGURE 2 is omitted. The devices of both FIGURES 1 and 2 are completed by deposition of a metal grid 20, 34 to the upper surface of layers 18 or 32 respectively. This grid may be formed from aluminium or silver metal deposited by any well-known method. Several test devices having four square centimeters of area were fabricated as described above with reference to FIGURE 2. Layer 30 had a estimated thickness of about 0.07 micron (700 angstroms) while layer 32 had a thickness of about 1 micron. At an insolation level of 100mW/cm², devices in which the copper and indium were electrodeposited had efficiencies of between six and seven percent and those in which the copper and indium were sputtered had efficiencies exceeding seven percent. Similar devices with high resistivity layers 30 in the range of 100 to 200 angstroms also demonstrated reasonable photovoltaic response. Likewise, devices according to FIGURE 1 in which no high resistivity zinc oxide layer is employed have shown photovoltaic response however with very low efficiency. Such low efficiency is believed to be due to defects in the copper indium diselenide layer. While the layers 16 and 28 of FIGURES 1 and 2 respectively are specified as copper indium diselenide, it will be appreciated that various other materials may be alloyed with this layer for various purposes. For example, it is known that aluminium, gallium, tellurium or sulfur may be included in copper indium diselenide films to adjust bandgaps and such alloyed materials would be considered equivalent to copper indium diselenide for the purposes of this invention. Likewise, the precise ratios of copper, indium and selenium may be adjusted to improve the qualities of the final layer, for example, in attempts to eliminate pure copper nodules. Likewise, while layer 18 of FIGURE 1 and layers 30 and 32 of FIGURE 2 are indicated as being zinc oxide, it will be appreciated that various materials may be added for various purposes. For example, as indicated above, both layers 18 of FIGURE 1 and 32 of FIGURE 2 include boron. In similar fashion, the particular reactive gases used to deposit zinc oxide in the preferred embodiment are believed to produce a certain amount of hydrogen doping in these layers. Hydrogen is provided both by the H₂O and the B₂H₆ gases. While boron is normally considered to increase the conductivity of semiconductors, we are not sure if this is true with ZnO and believe that the extra hydrogen contributed by the B₂H₆ gas may be the primary source of lower resistivity. Resistivity of ZnO may also be reduced by doping with aluminium, either alone or in combination with hydrogen. In addition, it is believed that the presence of sulfur in at least layer 30 of FIGURE 2, and possibly layers 18 of FIGURE 1 and 32 of FIGURE 2, may improve open circuit voltage of the device and thereby improve overall efficiency. Sulfur can be incorporated into the layers simply by inclusion of hydrogen sulfide (H₂S) gas in the reactive gas used in the CVD process by which the zinc oxide films are deposited. In similar fashion, addition of selenium or tellurium to the ZnO layers may improve performance and can be accomplished by including H₂Se or H₂Te in the reactive gas mixture. The resulting films are still considered to be substantially zinc oxide according to the present invention. In the above description the CIS layer is referred to as being of p-type conductivity and is considered to form a junction at its physical interface with ZnO which is of n-type conductivity. However there is evidence that the junction in CIS devices is actually a buried homojunction between the bulk of the CIS layer and a thin n-type CIS layer at the surface which contacts the transparent layer. Such a n-type CIS layer may be formed by loss of selenium from the surface of the CIS layer caused by later processing steps, e.g. heating. This is one of the reasons that the low temperature ZnO deposition process is preferred and believed to provide better results. While the present invention has been illustrated and described with reference to particular structures and methods of fabrication, it will be apparent that other changes and modifications can be made therein within the scope of the present invention as defined by the appended claims.

Claims: A thin film photovoltaic device comprising a layer of copper indium diselenide semiconductor (28) and a layer comprising primarily zinc oxide, characterized in that the zinc oxide containing layer comprises a first high resistivity layer (30) comprising a zinc oxide semiconductor deposited on said layer of copper indium diselenide and a second low resistivity layer (32) comprising a zinc oxide semiconductor deposited on said first high resistivity layer. A thin film photovoltaic device according to Claim 1, characterized in that the second layer is doped to provide a low resistivity of less than about 2.0 x 10⁻³ ohm-cm. A thin film photovoltaic device according to Claim 2, characterized in that the second layer has a resistivity of less than about 1.0 x 10⁻³ ohm-cm. A thin film photovoltaic device according to any one of Claims 1 to 3, characterized in that the first layer comprises essentially pure zinc oxide material. A thin film photovoltaic device according to any one of Claims 1 to 4, characterized in that the first layer has a resistivity about two hundred times that of the second layer. A thin film photovoltaic device according to any one of Claims 1 to 5, characterized in that the first layer has a thickness in the range of about 0.01 to about 0.2 micrometer. A method for preparing a thin film photovoltaic device according to claim 1 including the steps of providing a layer comprising a copper indium diselenide semiconductor (28) on a supporting substrate (24, 26), depositing on said layer of copper indium diselenide a first layer of zinc oxide semiconductor (30) of high resistivity, and depositing a second low resistivity layer (32) of zinc oxide semiconductor on said first high resistivity layer. A method according to Claim 7, characterized in that at least the second low resistivity layer is deposited by a low temperature chemical vapor deposition process. A method according to Claim 7 or 8, characterized in that the first high resistivity layer is deposited by a low temperature chemical vapor deposition process. A method according to any one of Claims 7 to 9, characterized in that low resistivity of the low resistivity layer is achieved by inclusion of B₂H₆ in a reactive gas mixture used to deposit the low resistivity layer. A method according to any one of Claims 7 to 10, characterized in that the substrate is maintained at a temperature of from 60 o C to 350 o C while flowing reactant gases including water vapor and an organozinc compound at a pressure of from 13.3 to 266 Pa across the layer of copper indium diselenide. A method according to Claim 11, characterized in that the substrate temperature is maintained at about 150 o C and the reactant gas pressure is maintained at about 115 Pa. A method according to Claim 11 or 12, characterized in that the organozinc compound is diethyl zinc.

Inventors: POTTER, RICHARD R., WIETING, ROBERT D.
Applicants: SIEMENS SOLAR INDUSTRIES L.P.
Application year: 1986
Main classification: H01L31
IPC codes: H01L31, C23C28, H01L51