PROCESS FOR PRODUCING AMORPHOUS SILICON SOLAR CELLS AND PRODUCT PRODUCED THEREBY

Abstract: An amorphous silicon solar cell is provided by forming a p-i-n structure on a substrate by successive deposition of p, i and n layers of amorphous silicon, with the p layer lying bottom­ most and the n layer lying atop the i layer. The i layer is doped with an n-type dopant. The p layer can be formed using boron as a dopant, while the i layer is doped in the range of 1 x 10¹⁶ to 5 x 10¹⁷ phosphorous atoms per cm², with the p layer and the i layer being formed in the same reaction chamber. Alterna­ tively, the i layer is doped in the range of 1 x 10¹⁵ to 5 x 10¹⁷ phosphorous atoms per cm², with the p layer and the i layer being formed in a separate reaction chamber. The p layer can, alterna­ tively, be doped with gallium and the i layer with arsenic. The substrate can be either glass or stainless steel. A stacked amorphous silicon solar cell can be also formed by a plurality of p-i-n structures on a substrate.

Description: RELATED APPLICATIONS Related applications, filed on even date herewith, are Serial Nos. , and , respectively identified by Attorney Docket Nos. WAK 23817, WAK 23818 and WAK 23819. FIELD OF THE INVENTION The present invention relates to a product and process for producing an amorphous silicon (a-Si) solar cell having a p- i-n structure of a-Si that is formed by a film deposition tech­ nique using the glow discharge decomposition process. BACKGROUND OF THE INVENTION Amorphous silicon solar cells having large effective areas can be easily produced at low cost by using the technique of glow discharge decomposition, and active efforts are being made with a view to incorporating such a-Si solar cells as one of the core elements of the large-scale power generating systems projected for future energy needs. Solar cells of this type generally use a glass or stainless steel substrate, and produce a p-i-n structure on this substrate, by successive deposition of p, i and n layers of an a-Si film. The junction structure of a typical a-Si solar cell using a glass substrate is shown in Fig­ ure 3. As shown, this cell has a glass substrate 1 that has formed thereon a transparent electrode 2 of tin oxide (SnO₂), indium tin oxide (ITO), which is successively deposited with a p layer 31 in a thickness of 50-500 Å, an i layer 32 in a thickness of 0.25-1 µ m and an n layer 33 in a thickness of 100-2000 Å, all being made of an a-Si film, with a metal electrode 4 being fin­ ally formed on the n layer 33. The p layer 31 is usually made of a boron-doped a-Si film formed from a mixture of B₂H₆ and SiH₄ gases. Alternatively, in order to provide a wider inhibition band, the layer 31 is made of a boron-doped a-SiC:H film, formed from a mixture of B₂H₆/SiH₄ gas, with CH₄ or C₂H₂ gas. The n layer 32 is usually made of a phosphorus-doped a-Si film. Con­ version efficiencies of 9-11% are attainable from currently- available solar cells at AM1 (air mass = 1) and 100 W/cm², and active research efforts are being made in order to attain even higher efficiencies. However, the conventional a-Si solar cells have one serious disadvantage, i.e., a drop in output character­ istics, as a result of continuous exposure to light. An example of such deterioration of the output charac­ teristics of a conventional a-Si solar cells is depicted in Fig­ ure 4. The tested cell had an initial conversion efficiency of 9% and a structure of the type shown in Figure 3, which comprises the combination of a glass substrate/ITO/B-doped p type a-SiC:H layer (200 Å thick)/i layer (0.5 µ m thick)/P-doped n type a-Si layer (500 Å thick)/A1 electrode. The graph in Figure 4 shows the time-dependent changes in the short-circuit current (J sc ), fill factor (FF) and conversion efficiency (η) relative to their respective initial values when the cell was exposed to light for four continuous hours at AM1 and 100 mW/cm². The values of J sc and FF dropped by about 5%, causing a 10% drop in η. Another solar cell having entirely the same structure as that used in this four-hour test was subjected to an outdoor exposure test. As a result, it was found that the four-hour continuous exposure to light at AM1 mW/cm² corresponds to an outdoor exposure for three days. On the basis of these data, the conversion effi­ ciency of a solar cell subjected to long-term outdoor exposure is estimated to drop by approximately 50% in fifteen years. For most practical applications, solar cells must have a conversion efficiency drop of ≦ 10% in fifteen years, which corresponds to a drop of ≦ 1% for four-hour continuous exposure to light. Actual cell performance, however, falls far short of this re­ quirement and improved stability is absolutely indispensable to future efforts toward the development of commercial solar cells. SUMMARY OF THE INVENTION The principal object, therefore, of the present inven­ tion is to provide an a-Si solar cell that achieves an improved conversion efficiency by preventing any drop in the output char­ acteristics under continuous exposure to light, thus eliminating the major problem of conventional solar cell fabricated by deposition of a-Si layers with the aid of glow discharge decompo­ sition techniques. In accordance with the present invention, the stated object is achieved by incorporating an n-type dopant in the i layer, and this is based on the recognition that the drop in the characteristics of an a-Si solar cell, as a result of continuous exposure to light, is due to a p-type dopant that is auto-doped into the growing i layer. If boron is used as a dopant for the formation of a p layer, phosphorous may be used an an n-type dopant to cancel the autodoping effect of the boron. If the i layer is formed in the reaction chamber where the p layer is formed, the effective amount of phosphorous ranges from 1 x 10¹⁶ to 5 x 10¹⁷ atoms per cm². If the i layer is formed in a reac­ tion chamber separate from the p layer forming chamber, the ef­ fective amount ranges from 1 x 10¹⁵ to 5 x 10¹⁷ atoms per cm². BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, advantages and features of the present invention will be more fully understood when consid­ ered in conjunction with the following drawings, of which: Figure 1 is a graph showing the initial conversion efficiency of an a-Si solar cell on a glass substrate and the change in conversion efficiency resulting from continuous expo­ sure to light as a function of the doping level of phosphorous incorporated in the i layer in accordance with the present inven­ tion; Figure 2 is a graph showing the time-dependent change in conversion efficiency of an a-Si solar cell on a glass sub­ strate in accordance with one embodiment of the invention; Figure 3 is a cross-sectional view of a solar cell with a p-i-n structure on a glass substrate incorporating the concept of the present invention; Figure 4 is a graph showing the time-dependent change in the output characteristics of a conventional solar cell, of the type shown in Figure 1, that occurs as a result of continuous exposure to light; Figure 5 is a graph showing, like Figure 1, the initial conversion efficiency of a solar cell having three a-Si layers formed on a glass in separate reaction chambers, as well as the change in conversion efficiency resulting from continuous light exposure, as a function of phosporous doping in the i layer in accordance with the present invention; Figure 6 is a cross-section of an a-Si solar cell with a p-i-n structure on a stainless steel substrate incorporating the concept of the present invention; Figure 7 is a cross-section of a solar cell having two layers of p-i-n structure stacked on a glass substrate incorpor­ ating the concept of the present invention; and Figure 8 is a cross-section of a solar cell having two layers of p-i-n structure stacked on a stainless steel substrate incorporating the concept of the present invention. The following reference numerals correspond to the following elements throughout the drawings and the discussion which follows: 1: glass substrate; 2: transparent electrode; 31, 34: p-layer of a-Si film; 32, 35: i-layer of a-Si film; 33, 36: n-layer of a-Si film; 4: metal electrode; 5: stainless steel substrate. DETAILED DESCRIPTION OF THE INVENTION In accordance with the first embodiment of the present invention, there is provided an a-Si solar cell composed of an assembly of a glass substrate/ITO/SnO₂ film/p-i-n a-Si layer/ metal electrode. If, in the prior art, p, i and n layers of a-Si film are successively formed within a single reaction chamber, the i layer is autodoped with a p-type dopant, for example, boron, originating from the wall of the reaction chamber or from the substrate support. Given the p-i-n structure shown above, wherein the p layer is made of a boron-doped a-SiC:H layer with a thickness of 150 Å, the i layer of an a-Si layer with a thickness of 0.5 µ m and the n layer of a phosphorous-doped a-Si layer with a thickness of 500 Å, a depth analysis of the i layer reveals a uniform distribution of about 1 x 10¹⁷ boron atoms per cm². A solar cell having improved stability against continuous exposure to light can be obtained by incorporating a trace level of phos­ phorous in the i layer being formed. Figure 1 shows the initial conversion efficiency of a solar cell as a function of phosphorous levels in the i layer. Figure 1 also shows the relationship between P levels in the i layer and the change in the solar cell efficiency during a four- hour continuous exposure to light at AM1 and 100 mW/cm². It is apparent from Figure 1 that the stability of the cell against continuous exposure to light is improved as the doping level of phosphorous is increased, and by adding at least 1 x 10¹⁶ phos­ phorous atoms per cm² of the i layer, the drop in the conversion efficiency of the cell is reduced to 1% or below. On the other hand, the initial characteristics of the cell remain constant so long as the number of phosphorous atoms present in the i layer is not more than 5 x 10¹⁷/cm, but if this value is exceeded, there occurs a drop in the conversion efficiency of the cell. With the a-Si solar cell having a p-i-n structure on a glass substrate that is fabricated within a single reaction chamber, as is usu­ ally the case, about 10¹⁷ boron atoms are present per cm² of the i layer. If such boron atoms are compensated by adding 1 x 10¹⁶ to 5 x 10¹⁷ phosphorous atoms per cm² of the i layer being formed, a solar cell can be obtained that exhibits high initial characteristics, but which will not experience a greater than 1% change in the conversion efficiency during a continuous four-hour light exposure. Figure 2 shows the time versus conversion efficiency profile of a solar cell that contained 1 x 10¹⁷ phosphorous atoms per cm² of the i layer and which was exposed to light for four continuous hours. It is apparent from Figure 2 that the time­ dependent change in efficiency was negligible. In accordance with the second embodiment of the present invention, there is provided an a-Si solar cell having a p-i-n structure on a glass substrate that is formed by a glow discharge decomposing apparatus, which allows the respective p, i and n layers to be formed in separate reaction chambers. The apparatus consists of five chambers, i.e., a substrate mounting chamber, a p-layer forming chamber, an i-layer forming chamber, an n-layer forming chamber and a delivery chamber. A support carrying the substrate successively passes through each of the chambers for deposition of the respective a-Si layers. The boron level that is autodoped in the i layer being formed is lower than when the three a-Si layers are formed within a single reaction chamber, and the approximate value is 1 x 10¹⁶ boron atoms per cm². Fig­ ure 5 shows both the initial conversion efficiency and the sta­ bility against continuous light exposure of the solar cell of interest as a function of phosphorous level in the i layer. Compared with the case where the p, i and n layers are formed within the same reaction chamber, the optimum level of phosphor­ ous doping in the i layer shifts to the lower side, i.e., the range of 1 x 10¹⁵ to 5 x 10¹⁷ atoms/cm². By incorporating phos­ phorous atoms in the i layer so that their doping level is within these limits, a solar cell can be obtained that exhibits high initial characteristics, but which will not experience a greater than 1% change in conversion efficiency during a continuous four- hour exposure to light. In accordance with the third embodiment of the present invention, there is provided an a-Si solar cell having a p-i-n structure on a stainless steel substrate. As shown in Figure 6, this cell consists of a stainless steel substrate 5 on which are deposited successively p layer 31, an i layer 32 and an n layer 33, which is overlaid with a transparent electrode 2 and a top­ most metal electrode 4. As in the case of the cell arrangements shown above, using a glass substrate, if the p, i and n layers are formed within a single reaction chamber, about 10¹⁷ boron atoms are autodoped per cm² of the i layer, and this value is decreased to about 1 x 10¹⁶/cm² if the three a-Si layers are formed in separate reaction chambers. As a result, the conver­ sion efficiency at the end of continuous four-hour exposure to light is decreased by about 10% in the former case and by about 7% in the latter case. Experiments were also conducted to inves­ tigate the effects of phosphorous doping of the i layer on the stability of the solar cell, having the construction shown in Figure 6, and the change in its initial conversion efficiency. As a result, it was confirmed that if the three a-Si layers are formed within a single reaction chamber, a solar cell that exhi­ bits high initial characteristics, but which will not experience a more than 1% drop in conversion efficiency during a continuous four-hour light exposure can be obtained by incorporating 1 x 10¹⁶ to 5 x 10¹⁷ phosphorous atoms per cm² of the i layer. Finally, in accordance with the fourth embodiment of the present invention, a stacked a-Si solar cell is provided wherein a plurality of p-i-n structures are made of an a-Si film in stacked relation on a substrate. Figures 7 and 8 show junc­ tion structures having two layers of p-i-n structure. Figure 7 refers to a cell using a glass substrate, whereas Figure 8 shows a cell using a stainless steel substrate. The cell shown in Figure 7 is fabricated by first providing a glass substrate 1 having a vapor-deposited layer of transparent electrode 2, then forming the first stack of p layer 31, i layer 32 and n layer 33, and a second stack of such layers (34, 35 and 36), which is overlaid with a metal electrode 4. In a similar manner, the cell shown in Figure 8 is fabricated by first providing a stainless steel substrate 5, then forming the first stack of p layer 31, i layer 32 and n layer 33, and the second stack of such layers (34, 35 and 36), which is overlaid with metal electrodes 2 and 4. By properly designing the thicknesses of the individual layers, the stacked-type solar cells shown in Figures 7 and 8 are given the ability to achieve conversion efficiencies comparable to those of the non-stacked type cells having only one layer of p-i-n structure and, at the same time, the open-circuit voltage V OC of the stacked solar cell is twice as high as that of the non-stacked cell. Therefore, the concept of stacking two layers of p-i-n structure is currently used in the solar cells for con­ sumer use, that require high operational voltages, such as cells in hand-held electronic calculators intended for use under fluor­ escent lamps. Also available today is a tandem a-Si solar cell, wherein three layers of p-i-n structures are stacked with a view to provide a larger photocurrent through efficient use of inci­ dent light. In a typical layer arrangement, the i layer in the topmost cell facing the light source is made of a nitrogen-doped a-Si film having the greatest energy gap, and the bottommost cell is made of a germanium-doped a-Si film having the smallest energy gap. The present inventors investigated not only the stabi­ lizing effects of adding phosphorous to the i layer in stacked a- Si solar cells, but also the change in their initial conversion efficiency, as a function of the phosphorous doping of the i layer. As in the case of the cell using a single layer of p-i-n structure, stacked cells that exhibit high initial characteris­ tics, and yet do not experience a more than 1% change in the conversion efficiency during four-hour continuous exposure to light, can be obtained by incorporating 1 x 10¹⁶ to 5 x 10¹⁷ phosphorous atoms per cm² of the i layer in each of the stacked p-i-n structures if they are formed within a single reaction chamber, and 1 x 10¹⁵ to 5 x 10¹⁷ phosphorous atoms per cm² of the i layer if the stacked p-i-n structures are formed in separ­ ate reaction chambers. Therefore, by incorporating the specified amount of phosphorous in the i layer of the p-i-n structure on a substrate, solar cells can be obtained which have practical stability in performance and which exhibit high initial characteristics, with­ out experiencing a more than 10% drop in conversion efficiency during a fifteen-year use in outdoors. The applicability of the concept of the present inven­ tion is not limited to using boron as a p-type dopant. The in­ vention can also be implemented effectively if the autodoping of other p-type dopants, such as gallium, is compensated by incor­ porating phosphorous of other n-type dopants, such as arsenic, in the i layer. In accordance with the present invention, an a-Si solar cell, with the p-i-n structure that exhibits high initial conver­ sion efficiency and whose performance remains highly stable against continuous exposure to light, can be fabricated by incor­ porating in the i layer a sufficient concentration level of an n- type dopant to compensate for the p-type dopant that is autodoped during the formation of the i layer at one stage of the succes­ sive deposition of p, i and n layers of a-Si film on a substrate. The above-described description and the accompanying drawings are merely illustrative of the application of the prin­ ciples of the present invention and are not limiting. Numerous other arrangements which embody the principles of the invention and which fall within its spirit and scope may be readily devised by those skilled in the art. Accordingly, the invention is not limited by the foregoing description, but is only limited by the scope of the appended claims.

Claims: 1. In a process for producing an amorphous silicon solar cell by forming a p-i-n structure on a substrate by succes­ sive deposition of p, i and n layers of amorphous silicon, with the p layer lying bottomost and the n layer lying atop the i layer, the improvement comprising doping the i layer with an n- type dopant. 2. The process according to Claim 1, wherein the p layer is formed using boron as a dopant, while the i layer is doped in a range of 1 x 10¹⁶ to 5 x 10¹⁷ phosphorous atoms per cm², said p layer and said i layer being formed in a same reac­ tion chamber. 3. The process according to Claim 1, wherein the p layer is formed using boron as a dopant, while the i layer is doped in a range of 1 x 10¹⁵ to 5 x 10¹⁷ phosphorous atoms per cm², said p layer and said i layer being formed in separate reac­ tion chambers. 4. The process according to Claim 1, wherein said p layer is formed from a boron-doped a-SiC:H layer with a thickness of approximately 150 Å, said i layer is formed of an a-Si layer with a thickness of approximately 0.5 microns, and said n layer is formed of a phosphorous-doped a-Si layer with a thickness of approximately 500 Å. 5. The process according to Claim 1, wherein the substrate is formed of glass. 6. The process according to Claim 3, wherein said solar cell is formed by a glow discharge decomposing process. 7. The process according to Claim 1, wherein the substrate is formed of stainless steel. 8. The process according to Claim 1, wherein said p layer is doped with gallium and said i layer is doped with arsenic. 9. In a process for producing a stacked amorphous silicon solar cell by forming a plurality of p-i-n structures on a substrate by successive deposition of p, i and n layers of amorphous silicon, with each p-i-n structure comprising a p layer lying bottommost and an n layer lying atop an i layer, the im­ provement comprising doping each i layer with an n-type dopant. 10. The process according to Claim 9, wherein each p layer is formed using boron as a dopant, while each i layer is doped in the range of 1 x 10¹⁶ to 5 x 10¹⁷ phosphorous atoms per cm², each said p layer and each said i layer being formed in a same reaction chamber. 11. The process according to Claim 9, wherein each p layer is formed using boron as a dopant, while each i layer is doped in the range of 1 x 10¹⁵ to 5 x 10¹⁷ phosphorous atoms per cm², each said p layer and each said i layer being formed in separate reaction chambers. 12. An amorphous silicon solar cell comprising a p-i-n structure formed on a substrate, with the p layer lying bottom­ most and the n layer lying atop the i layer, said i layer includ­ ing an n-type dopant. 13. A solar cell as in Claim 12, wherein said p layer is doped with boron and said i layer is doped with phosphorous in a range of 1 x 10¹⁵ to 5 x 10¹⁷ phosphorous atoms per cm². 14. A solar cell as in Claim 12, wherein said p layer is doped with gallium and said i layer is doped with arsenic. 15. A solar cell as in Claim 12, wherein said sub­ strate is formed of glass. 16. A solar cell as in Claim 12, wherein said sub­ strate is formed of stainless steel. 17. A solar cell as in Claim 12, wherein said p layer is formed from a boron-doped a-SiC:H layer with a thickness of approximately 150 Å, said i layer is formed of an a-Si layer with a thickness of approximately 0.5 microns, and said n layer is formed of a phosphorous-doped a-Si layer with a thickness of approximately 500 Å. 18. A stacked amorphous silicon solar cell comprising a plurality of p-i-n structures formed on a substrate, with each p-i-n structure comprising a p layer lying bottommost and an n layer lying atop an i layer, each said i layer including an n- type dopant. 19. A solar cell as in Claim 18, wherein each said p layer is doped with boron and each said i layer is doped with phosphorous in a range of 1 x 10¹⁵ to 5 x 10¹⁷ phosphorous atoms per cm². 20. A solar cell as in Claim 18, wherein each said p layer is doped with gallium and each said i layer is doped with arsenic. 21. A solar cell as in Claim 18, wherein said sub­ strate is formed of glass. 22. A solar cell as in Claim 18, wherein said sub­ strate is formed of stainless steel.

Inventors: KAMIYAMA, MICHINARI FUJI ELECTRIC CORPORATE RES., NABETA, OSAMU FUJI ELECTRIC CORPORATE RESEARCH, UCHIDA, YOSHIYUKI FUJI ELECTRIC CORPORATE RESEARCH
Applicants: FUJI ELECTRIC CORPORATE RESEARCH AND DEVELOPMENT LTD.
Application year: 1985
Main classification: H01L31
IPC codes: H01L31