PHOTOVOLTAIC DEVICE INCLUDING SHUNT PREVENTING LAYER AND METHOD FOR THE DEPOSITION THEREOF

Abstract: A photovoltaic device (20) which is resistant to shunt and short defects developing between the opposed electrode thereof. The photovoltaic device comprises an electrically conductive substrate (22), a semiconductor (24) including a photogenerative region, a layer of transparent conductive material (26), and means (30) for collecting photogenerated current. In particular, a layer of low-conductivity material (28) is operatively positioned between the layer of transparent conductive material and the collecting means (30), thereby resistively restricting the flow of electric current through short circuit portions (36). A method of fabricating such a shunt-resistant photovoltaic device is also disclosed.

Description: PHOTOVOLTAIC DEVICE INCLUDING SHUNTPREVENTING LAYERAND METHOD FORTHEDEPOSITIONTHEREOF Field of the Invention The present invention relates generally to photovoltaic devices formed on a conductive substrate and particularly to solar cell structures in which the short circuit current path generated by any defect is minimized and to a method of fabricating same. Background of the Invention Of the various types of solar cells, thin film solar cells have demonstrated great potential for widespread future use because they can be manufactured by mass production techniques in large areas and at low cost. However, any large area solar cell will inherently include a large number of electrically shunted or short-circuited portions of the active region thereof, which short circuit portions could result in significantly reduced yields and low photovoltaic conversion efficiencies. These shunts or short circuits (the terms being used synonymously) , arise from defects in the substrate, the semiconductor material or the electrode. The foregoing problem can best be seen from a perusal of the drawings. Figure 1 is a schematic cross- sectional view showing a short circuit portion of a thin film photovoltaic device 1 which has been formed on an electrically conductive substrate. In the drawing, the reference numeral 2 indicates the electrically conductive substrate, 4 indicates a layer of back-reflection material, 6 indicates a multilayered body of semiconductor material which includes a photogenerative region, 6 indicates a layer of transparent conductive material, 10 indicates an upper electrode for collecting photogenerated change carriers, 12 indicates an irregularity which shunts or short circuits said photogenerated change carriers, and 14 indicates an irregularity extending from the upper electrode into and through the semiconductor body 6 so as to form a shunt or short circuit for photogenerated change carriers. The conductive substrate 2 also serves as the lower electrode of the solar cell. Generally, the shunt or short circuit portions 12 and 14 can be of two types: the type in which the lower electrode 2 and the upper transparent electrode 10 are short-circuited; and the type in which the short circuit path is caused when the collecting electrode 10 is formed on a subjacent pinhole section, thereby exposing a portion of the lower electrode 2, i.e., the lower electrode is not covered with a layer of the thin film body of semiconductor material. Accordingly, the electric current photogenerated in the semiconductor body 6 by the light incident on the surface of the transparent conductive layer 8 is at least partially diverted by the current shunting path formed in the photovoltaic device 1, with the result that the photovoltaic-conversion efficiency deteriorates. Brief Summary of the Invention In accordance with the principles of this invention, there is described a thin film photovoltaic device which includes an electrically conductive substrate (the lower electrode layer) , a body of semiconductor material serving as the photogenerative member, a layer of transparent conductive material, and an upper electrode adapted to collect photogenerated charge carriers. An important feature of the present invention is the provision, directly below and in contact with the collecting electrode, a layer of low electrical conductivity material. The layer of low conductivity material is sized and shaped to correspond to the size and shape of the collecting electrode. In this manner, the current carrying capability of the short circuit current path is limited. In the structure of the instant invention, the layer characterized by low electrical conductivity is deposited after the semiconductor body (which includes a photogeneration member) and the layer of transparent conductive material have been successively deposited atop the electrically conductive substrate. This sequence of device formation makes it possible to obtain a semiconductor body and a layer of transparent conductive material of satisfactory quality which is not deleteriously influenced by the conditions for preparing the layer of high-resistance material. Further, since said layer of high-resistance material can be sequentially deposited after performing the electrolytic process disclosed in U.S. Patent No. 4,729,970, there is no fear that the semiconductor body of the photovoltaic device will delaminate from subjacent layers. Furthermore, said electrolytic process helps to reduce the shunt or short circuit current path generated between portions of the layer of transparent conductive material and defective portions of the electrically conductive substrate which serve as the lower electrode of the photovoltaic device. The introduction of the layer of low-conductivity material of the instant invention between the layer of transparent conductive material and the current collecting electrode of the photovoltaic device, makes it possible to prevent a direct short circuit therebetween in a pinhole section where no semiconductor body exists to limit the flow of shunt current or short circuit current. This novel arrangement of active layers of the device makes it possible to substantially reduce the flow of short circuit current as compared to that flow of short circuit current in prior art devices; and, at the same time, said arrangement prevents the photovoltaic conversion efficiency from deteriorating, thereby optimizing the operational output of the photovoltaic device. Thus, by adopting the principles set forth in the present invention, a photovoltaic device which is resistant to shunt and short circuit defects between the counter- electrodes thereof can be provided. These and other objects and advantages of the instant invention will become apparent from a detailed perusal of the Detailed Description of the Drawings which follows hereinafter. Brief Description of the Drawings FIGURE 1 is a schematic enlarged cross-sectional view, partially cut-away, illustrating defective irregularities which can cause current shunting or short- circuiting paths in photovoltaic device structures of the prior art; FIGURE 2 is an enlarged, schematic cross- sectional view, partially cut-away, illustrating the preferred embodiment of the photovoltaic-device structure of the present invention; FIGURE 3 is a top plan view illustrating the pattern of the upper current collecting electrode of the preferred embodiment of the photovoltaic device of Figure 2 of the present invention, said current collecting electrode being formed of a complex network of bus bars and grid fingers; FIGURE 4 is an enlarged schematic cross-sectional view, partially cut-away, illustrating the preferred embodiment of the photovoltaic-device of Figure 2 of the present invention with a current shunting path operatively disposed directly below the layer of low-conductivity material; FIGURE 5 is an enlarged schematic, cross- sectional view, partially cut-away, illustrating a photovoltaic device of the prior art without the presence of the layer of low-conductivity material of the present invention and, with a current shunting path operatively disposed below the collecting electrode; and FIGURE 6 is an enlarged, schematic cross- sectional view, partially cut-away; illustrating the preferred embodiment of a photovoltaic device of the present invention in which successive PIN-type layers of amorphous silicon alloy material form the semiconductor body. Detailed Description of the Preferred Embodiment The present invention will now be described in detail with specific reference to the accompanying drawings. Generally, the present invention relates to an improved photovoltaic device in which a semiconductor body, including a layer of photogenerative material, is operatively disposed between a lower electrode formed by an electrically conductive substrate and an upper electrode including current collecting means. The photovoltaic device is characterized by a layer of low-conductivity material operatively disposed below the upper electrode. In the fabrication of this photovoltaic device, the semiconductor body, which includes the photogenerative layer of semiconductor material, is first deposited on the electrically conductive substrate and then the layer of transparent conductive material is formed on semiconductor body. Subsequently, a layer of low-conductivity material is formed atop the layer of transparent conductive material, and then the current-collecting layer is formed atop the layer of low-conductivity material. Turning now to Figure 2, the basic structure of the preferred embodiment of the photovoltaic device 20 shown therein includes an electrically conductive substrate 22, a semiconductor body 24 including a layer of photogenerative material, a layer of transparent conductive material 26, a layer of low conductivity material 28 and a complex current collection matrix or network 30 which includes grid fingers and bus bars. Examples of the materials from which the aforementioned electrically-conductive substrate 22 is fabricated include stainless steel, aluminum, copper, titanium, carbon sheet, galvanized-steel plate, and a synthetic plastic film upon which an electrically conductive layer (not shown) is formed. Examples of the materials from which the layer of electrically conductive material may be fabricated include Ti, Cr, Mo, , Al, Ag and Ni. To effectively utilize incident light, a layer of back-reflection material may be operatively disposed between the electrically conductive substrate 22 and the semiconductor body 24. The back-reflection layer may consist of an electrically conductive layer covered or uncovered with a metal oxide layer. Examples of the metal layer and the metal oxide layer can be deposited by resistance-heating, electron beam, sputtering, or any other process well known to those ordinarily skilled in the art. Examples of the semiconductor materials used in the fabrication of the layer of photogenerative material of the photovoltaic device of the instant invention include pin-junction amorphous silicon alloy material, pn-junction polycrystalline silicon, and a compound semiconductor material such as CuInSe 2 /CdS. In the preferred embodiment, wherein amorphous silicon alloy material is used, the aforementioned layer of silicon alloy material is fabricated by plasma enhanced CVD using hydrogen diluted silane gas or the like as a precursor. In the alternatively preferred embodiment wherein polycrystalline silicon is used, the layer of semiconductor material is fabricated by forming molten silicon into a sheet or by heat-treating amorphous silicon. Finally in the alternately preferred embodiment wherein the cell is formed of a compound semiconductor material, CdInSe 2 /CdS is formed by electron beam deposition, sputtering, or electro- precipitation (precipitation through electrolysis in an electrolyte) . Further, the aforementioned semiconductor body of the solar cell may be formed as a spectrum splitting tandem device in which a plurality of photovoltaic cells are stacked in series relationship, each cell dedicated to photogenerating current from a specific portion of the solar spectrum. Examples of the materials from which the layer of transparent electrically conductive material in the photovoltaic device of this invention is fabricated, includes ln 2 0 3 , Snθ 2 , In 2 0 3 -Sn0 2 (ITO) , ZnO, Ti0 2 , Cd 2 Snθ 4 , and a crystalline semiconductor material doped with high concentration impurities. The layer of transparent conductive material may be formed by resistive heating, electron-beam, sputtering, spray pyrolysis, plasma CVD, impurity-diffusion, or any other technique known to ordinarily skilled artisans. The layer of low-conductivity material of the preferred embodiment of the photovoltaic device 20 of the instant invention is preferably at least one of the following materials: polymeric material, semiconductor material, carbon, metal oxide, cermets, and metal. Preferably, said layer of low electrical conductivity material may be formed by dispersing, in a polymeric-resin solution, fine powder selected from the group consisting of at least one of the following: a semiconductor material such as silicon or germanium, carbon, a metallic oxide such as tin oxide, indium oxide, zinc oxide, and titanium oxide, a cermet, a metallic material such as copper, nickel, palladium, and solder. This solution is then dried into a paste-like substance in such a manner that the sheet resistance thereof is characterized by a resistivity which falls substantially in the range from 0.1Ω/D to 1000Ω/D , and more preferably, from about 1Ω/D to 300Ω/D , and most preferably, from about 5Ω/D to 200Ω/D. The current collecting network 30 of the photovoltaic device 20 of the present invention is placed in electrical communication with the layer of transparent conductive material 26 through the intermediation of the aforementioned layer of low-conductivity material 28. It is important that the aforementioned layer of low- conductivity material 28 be of about the same size and shape as, or larger than, the aforementioned current collecting network by, preferably about 5% to 100%, and more preferably by 10% to 50%. The current collecting network 30 is defined by a complex matrix of grid-like electrode fingers and bus bars. The bus bars carry the photogenerated electric current collected by the matrix of grid-like electrode fingers. More specifically, Figure 3 is a top plan view of the current carrying network of the photovoltaic device 20 of the present invention as seen from the light-incident surface thereof. The device 20 shown in Figure 3 includes the layer of transparent conductive material 30, the layer of low-conductivity material 28, grid electrode fingers 32 and a bus bar 34. When irradiated with incident light, the photogenerative layer of the photovoltaic device 20 generates an electric current, the charge carriers of which are collected by the grid electrodes 28 through the layer of low-conductivity material 28 and are then carried by bus bar 34 to the downstream load. The material from which the collecting network 30, which serves as the collecting means, is fabricated can be selected of a conductive ink or paste including therein Ti, Cr, Mo, W, Al, Ag, Ni, Cu, Sn, silver, and combinations thereof. The current collecting electrode may be formed by, for example, sputtering using a mask pattern, by resistive-heating, by plasma CVD, by a method in which patterning is effected through etching after depositing a layer of metallic material over the deposition surface, by directly forming an electrode pattern by photo CVD, by plating after forming a negative-pattern mask of the electrode pattern, or by printing an electrically conductive ink or paste. The conductive ink or paste used may be prepared by dispersing fine powder of a material selected from the group consisting of silver, gold, copper, nickel, indium, tin, and combinations thereof in a binder polymer. The binder polymer may be a synthetic plastic resin selected from the group consisting of polyester resin, epoxy resin, acrylic resin, alkyd resin, polyvinyl acetate, rubber, urethane, phenol, and combinations thereof. The grid electrodes 28 and the bus bar 34 may be formed integrally or separately. When forming the bus bars 34 separately from the grid electrodes 28, tin-plated foils or wires of a metallic material such as copper are preferably used as the bus bars, which are attached to the grid electrodes 28 by means of an electrically conductive adhesive or solder. Figure 4 is a cross-sectional view of the preferred embodiment of the photovoltaic device 20, in which, in accordance with the present invention, an electrically collecting electrode 30 is formed where a shunt or short circuit portion 31 exists between the electrically conductive substrate 22 and the layer of transparent low-conductivity material 28. In Figure 4, the reference numeral 24 continues to refer to the semiconductor body of the photovoltaic device 20. Figure 5 is a cross-sectional view showing the prior art embodiment of the photovoltaic device 20 in which the collecting electrode network 30 is placed in direct contact over the current shunt or short circuit portion 36 which is formed between the electrically conductive substrate 22 and the layer of transparent conductive material 26 without the intermediation of the layer of low- conductivity material of the instant invention. In Figure 5, the reference numeral 24 indicates the photogenerative layer of the semiconductor body. When electrically connected to a load and irradiated with light, the photovoltaic device 20 of Figure 5 generates an electric current, the charge carriers of which are extracted by the load. Since, however, the photogenerated current is shunted through the short circuit defect 36, the amount of electric current actually delivered to said load is less than that photogenerated by the aforementioned photogenerative layer. In the case of the photovoltaic device 20 illustrated in Figure 4, in contrast to Figure 5, the presence of the layer of low-conductivity material 28 operatively disposed between the short circuit defect 36 and the current collecting electrode 30 provides a high shunting resistance. The result of that layer of low conductivity material 26 is a reduction of the amount of current which is shunted. The amount of electrical current that can be extracted at the load is greater than that in the case of the photovoltaic device 20 of Figure 5; i.e., the photoconversion efficiency of the photovoltaic device 20 has been improved. Figure 6 is a cross-sectional view of an amorphous-silicon solar cell which constitutes the preferred embodiment of the instant invention, which invention will now be described with specific reference to that preferred embodiment. THE FIGURE 6 EMBODIMENT A 3000A thick Al film and a 700A thick ZnO film were successively deposited, as by sputtering, on a cleansed stainless-steel substrate 42, thus forming a dual layered of back-reflection material 44. Then, a layer of n-type a-Si alloy material was formed from a precursor gaseous mixture of SiH 4 , BF 3 and H 2 , at a substrate temperature of about 250 to 300 ° C by r. f. or microwave plasma CVD. Using these layers of a-Si alloy material, the following layers were successively deposited: an n-layer 46 (75 to 150A thick)/ an i-layer 48 (2000 to 4000A thick)/ a p-layer 50 (75 to lOOA)/ an n-layer 52 (75 to lOOA thick)/ an i-layer 54 (750 to lOOOA thick) / a p-layer 56 (75 to lOOA thick) , thereby forming a multi-layered semiconductor body 58. Subsequently, a 500-lOOθA thick ln 2 0 3 film 60 was formed by the resistive-heating deposition of In- in an 0 2 atmosphere while keeping the substrate temperature at about 250 to 300 ° C. After deposition of the ln 2 0 3 layer was complete, the cell may optionally be subjected to a passivation process which involved disposing it in etchant bath of a mild acid such as FeCl 3 or dilute hydrochloric acid and a reverse bias current was passed therethrough to passivate any defect regions. These techniques are more fully detailed in Patent No. 4,729,970 discussed hereinabove. Then, a 1-liquid carbon paste of epoxy type manufactured by Emerson Cummings Corp. (also by Taiyo Ink . ) printed at predetermined positions by a screen printer and dried for three minutes in an extreme- infrared-radiation furnace at about 150°C, thereby forming the layer of low-conductivity material 62 characterized by a sheet resistance of about 20Ω/D. Further, silver paste #5007 manufactured by DuPont was then screen-printed on the layer of low-conductivity material 64 and dried for three minutes in an extreme-infrared-radiation furnace at 130°C, thereby forming the grid electrode network 66 whose size is about 60% of the layer of low-conductivity material 64. In this way, a tandem photovoltaic device 40 was fabricated. Sixteen photovoltaic devices 40 were fabricated by the aforedescribed method. Each of said devices 40 had the grid electrode 66 thereof electrically connected in parallel to a tin-plated-copper-foil bus bar 68 by means of an electrically conductive adhesive agent containing silver so that a predetermined level of photogenerated electric current can be controlled. Thus, a unit photovoltaic cell was produced. One hundred unit photovoltaic cells prepared in this way were tested for the electrical characteristics thereof under an incident irradiation of AM 1.5, i.e., 100 mW/cm . The number of photovoltaic cells characterized by a fill factor of 65% or greater was two times greater as compared to the percentage of photovoltaic cells in the case in which no layer of the low-conductivity material of the present invention was employed. It should be apparent from this result that the introduction of the layer of low- conductivity material of the present invention markedly improves solar cell characteristics. In practical use, the bus bars 66 and the stainless-steel substrate 42 of a plurality of adjacent unit photovoltaic cells 40 as described hereinabove are electrically interconnected in series so as to form a photovoltaic cell system which provides a desired level of voltage. As described above, the layer of low-conductivity material of the present invention prevents, or at least substantially reduces, performance degradation attributable to any current shunt or short circuit pathway formed in a thin film photovoltaic device and, at the same time improves product yield. It is to be understood that the instant invention is not limited to the precise structure of the illustrated embodiments. It is intended that the foregoing description of the presently preferred embodiments be regarded as illustrative rather than as a limitation of the present invention. It is the claims which follow, including all equivalents, which are intended to define the scope of this invention.

Claims: Claims 1. A photovoltaic device comprising: an electrically conductive substrate, a body of semiconductor material, a layer of transparent conductive material, a layer of low-conductivity material, and means for collecting current photogenerated by said photovoltaic device; said semiconductor body having opposed first and second surfaces, the second surface contacting said substrate; said layer of transparent conductive material formed on the first surface of said semiconductor body so as to contact at least a part of said first surface of said semiconductor body and said layer of low-conductivity material; said low-conductivity material operatively disposed below said collecting means and formed of a material having a sheet resistance ranging from about 0.1Ω/ to 1000Ω/D so as to restrict the flow of electric current between at least portions of said collecting means and portions of said layer of transparent conductive material. 2. A device as in claim 1, wherein said layer of low-conductivity material is formed of a material selected from the group consisting essentially of polymeric material, semiconductor material, carbon, ceramics, cermet material, and combinations thereof. 3. A device as in claim 1, wherein said layer of low-conductivity material is 0% to 100% longer and wider than the length and width of said collecting means. 4. A device as in claim 1, wherein said collecting means includes a network of grid electrodes and bus bars. 5. A device as in claim 4, wherein said layer of low-conductivity material is 0% to 100% longer and wider than the length and width of said grid electrodes. 6. A device as in claim 4, wherein said layer of low-conductivity material is 0% to 100% longer and wider than the length and width of said bus bars. 7. A device as in claim 1, wherein said layer of low-conductivity material is deposited after selectively passivating portions of said layer of transparent conductive material which shunt current between said layer of transparent conductive material and said conductive substrate. 8. A method of fabricating a photovoltaic device, comprising the steps of: (1) providing an electrically conductive substrate; (2) forming a semiconductor body having opposed first and second surfaces such that the second surface thereof contacts said conductive substrate; (3) forming a layer of transparent conductive material on the first surface of said semiconductor body; (4) forming means for collecting current photogenerated by said photovoltaic device atop a portion of said layer of transparent conductive material; and (5) operatively disposing a layer of a low- conductivity material having a sheet resistance of 0.1Ω/D to 1000Ω/D for restricting the flow of electric current on at least that portion of said layer of transparent conductive material beneath said collecting means, whereby the flow of electric current between portions of said collecting means and portions of said layer of conductive material is restricted. 9. A method as in claim 8, comprising the step of selecting said layer of low-conductivity material from the group consisting of: polymeric material, semiconductor material, carbon, ceramics, cermets, and combinations thereof. 10. A method as in claim 8, comprising the step of forming said layer of low-conductivity material on said layer of transparent conductive material so as to be 0% to 100% longer and wider than said collecting means. 11. A method as in claim 8, further comprising the step of forming said collecting electrode of a network of grid electrodes and bus bars. 12. A method as in claim 11, comprising the step of forming said layer of low-conductivity material to be 0% to 100% longer and wider than the length and width of said grid electrode. 13. A method as in claim 11, comprising the step of forming said low-conductivity layer, to be 0% to 100% longer and wider than the length and width of said bus bars. 14. A method as in claim 8, comprising the step of forming said layer of low-conductivity material after selectively passivating those portion of said layer of transparent conductive material which shunt current from said conductive substrate. 15. A method as in claim 8, comprising the step of forming said layer of transparent conductive material of a material selected from the group consisting of indium oxide, tin oxide, zinc oxide, titanium oxide, cadmium/tin oxide, cadmium stannate, and mixtures thereof.

Inventors: NATH PREM, VOGELI CRAIG, NATH, PREM, VOGELI, CRAIG
Applicants: UNITED SOLAR SYSTEMS CORP, UNITED SOLAR SYSTEMS CORPORATION
Application year: 1992
Main classification: H01L31
IPC codes: H01L31