SOLAR CELL

Abstract: An electric insulation thin layer comprising a non-­ monocrystalline substance, being deposited on a substrate by means of a plasma CVD method or a spatter method, and having a carbon content of not less than 10 % by atom. The electric insulation layer is thinner and has an excellent heat resistance and a high dielectric breakdown voltage.

Description: ELECTRIC INSULATION THIN LAYER The present invention relates to an electric insulation thin layer comprising a non-monocrystalline substance, which is deposited on a substrate such as a substrate for a solar cell by means of a plasma CVD method or a spatter method. Hitherto, for the production of a solar cell, there has been used a substrate in the form of a light transmitting board coated with a transparent electrically conductive film such as ITO/glass or SnO2/glass; a metal board made of stainless steel, iron, aluminum, nickel, brass, copper, zinc or an alloy thereof; a metal board treated with other metal; and the like. When an electrically conductive board such as a metal board, is used as a substrate for a solar cell, since a plurality of cells are formed in a pattern on a single substrate and are connected in series, it is required that the substrate is electrically insulated from the solar cell electrode on the board side. For this reason, an insulation layer is formed on the substrate. The insulation material used for formation of said insulation layer must be heat resistant since the substrate must be subjected to a heat treatment at a temperature of 200 to 350 0C in the preparation process of a solar cell. Therefore, a heat-resistant organic polymer such as polyimide resin is usually applied to the substrate by such a method as spin coat or dipping and subjected to a heat treatment for curing and degassing. For preparing a solar cell, in general, the higher the semiconductor layer forming temperature, the higher the quality of the resulting solar cell. However, when a solar cell is produced by using a substrate having the insulation layer made of polyimide resin, the semiconductsr layer forming temperature can raise only up to 250 C, such a temperature being insufficient for obtaining a solar cell of high quality. Moreover, condensation occurs in the insulation layer during heating to release H2O or to discharge impurities, which lowers the characteristics of the solar cell. There has been proposed a process for forming an electric insulation layer by using amorphous silicon. In such a case, however, the dielectric breakdown voltage is low i.e. only 20 to 30 V/wm, and thus the deposit of the amorphous silicon must be considerably thick. An object of the present invention is to provide an electric insulation thin layer deposited on a substrate which has an improved heat resistance and can eliminate the complex heat treatment required for the formation of the insulation layer made of polyimide resin which degrades characteristics of the product. According to the present invention, there is provided an electric insulation thin layer comprising a non-monocrystalline substance, being deposited on a substrate by means of a plasma CVD method or a spatter method, and having a carbon content of not less than 10 % (% by atom). The insulation layer has an excellent heat resistance, and does not require a complex heat treatment, and can be made thinner. Fig. 1 is a schematic perspective view of an embodiment according to the present invention; and Fig. 2 is a schematic setional view of an apparatus to be used in the present invention. Nonrestrictive examples of the substrates used in the present invention are, for instance, an electrically conductive board such as a metal board of 0.03 to 1 mm in thickness made of stainless steel, iron, aluminum, nickel, brass, copper, zinc, or the like. The insulation layer of the present invention is formed on the substrate by the usual plasma CVD method or spatter method (spatter vapor deposition method). Ii is preferable that the insulation layer formed by such method contains carbon atoms of not less than 10 %. If the carbon content is less than 10 %, the dielectric breakdown voltage decreases to 50 V/#m or less, which means insufficiency in point of insulating performance. If the carbon content is 30 % or more, the dielectric breakdown voltage of the insulation layer further increases to 100 v/i#m or more, which is preferable for a semiconductor device produced by glow discharge. From the standpoints of relieving stress in the film and imparting structural flexibility thereto, it is preferable that the insulation layer of the present invention is made of an amouphous substance represented by the formula: Si C X H (l-x-y) x y (wherein X is at least one member selected from the group consisting of N, O, P, C1, Br and Ge; 1 = > x Z 0.1; 0.9 2 yl 0.1; and 1 > x+y > 0.1). However, the non-monocrystalline substance of the insulation layer of the present invention is not limited thereto. Typical examples of the amorphous substance represented by the general formula are, for instance, C: H, Si (l-x) Cx: H (wherein x is as defined above), si(l-x-y)CxNy: H (wherein x and y are as defined above), Si C F : H (wherein x and y are as defined above), (l-x-y) x y and the like. The insulation layer of the present invention may or may not contain hydrogen atom. Further, the insulation layer is preferably not doped, but it may be doped with a small amount of boron, phosphorus, or the like. The insulation layer can be formed from a gas mixture by the plasma CVD method or spatter method. The gas mixture preferably comprises a gas of a silicon-containing compound and a gas of a carbon-containing compound, and, if necessary, a diluent gas such as hydrogen gas and an inert gas. Examples of the silicon-containing compounds are, for instance, silan, polysilane such as disilane or trisilane, organosilicon derivative such as trimethylsilane or halogenated derivative thereof, a mixture thereof, and the like. Examples of the carbon-containing compounds are, for instance, a saturated or unsaturated hydrocarbon such as methane, ethylene, acetylene or propane, a halogenated derivative thereof, an admixture thereof, and the like. The carbon-containing compound can be admixed so that the insulation layer formed contains carbon atoms of not less than 10 %. Though not specifically limited, the thickness of the insulation layer is preferably about 0.5 to 100 um, more preferably about 1 to 10 um. If the thickness of the insulation layer is about 0.5 to 100 pm, there is obtained a dielectric breakdown voltage of 50 V/m or above. If the thickness is not more than 0.5 pm, pinholes tend to increase, while if it exceeds 100 pm, peel-off tends to occur owing to internal stress. If the thickness is about 1 to 10 pm, there is obtained a dielectric breakdown voltage of about 100 to 1000 V/#m, pinholes decrease, and less peel-off takes place. The insulation layer of the present invention is now described and explained with reference to the drawings. Referring to Fig. 1, an electric insulation layer 2 of the present invention is formed on a substrate 1, and then a metal film electrode 3 is formed thereon. Fig. 2 is a schematic sectional view showing an embodiment of a vacuum apparatus for depositing the insulation layer on a substrate by the plasma CVD method or spatter method. A process of deposition of the insulation layer of the present invention by the plasma CVD method with the apparatus shown in Fig. 2 is carried out, for example, by placing the substrate 1 on an electrode 6 and evacuating a vacuum container 4 once to 1 X 10-6 to 10 X Torr. Thereafter, a valve 7 is opened to introduce a gas mixture. While maintain a degree of vacuum of about 1 X 10 to 5 Torr, and a dc voltage or a high frequency voltage with 1 kHz to tens of MHz is applied between the electrodes 5 and 6 to establish a plasma state in the vacuum container 4. This state is maintained for minutes to hours, whereby there is formed on the substrate the insulation layer having a thickness of about 0.5 to 100 vm, a carbon content of not less than 10 8, and a dielectric breakdown voltage of not less than 50 V/pm. A process of deposition of the insulation layer of the present invention by the spatter method is carried out, for example, by placing the substrate 1 on the electrode 6 of the apparatus shown in Fig. 2 and evacuating the vacuum container 4 once to 1 X 10 6 to 10 X 1016 Torr. Thereafter, a gas mixture of an inert gas such as argon gas or helium gas and hydrogen gas, a hydrocarbon gas or a silicon compound gas is fed through the valve 7 into the vacuum container 4. While maintaining a degree of vacuum of about 5 X 10 to 1 X 10 1 Torr, a dc voltage or a high frequency voltage with 1 kHz to tens of MHz is applied between the electrode 5 having a target of SiC, graphite or silicon, and the electrode 6 on which the substrate 1 is placed so as to establish a plasma state in the vacuum container 4. This state is maintained for minutes or hours, whereby there is formed on the substrate 1 the insulation layer 2 having a film thickness of 0.5 to 100 pm, a carbon content of not less than 10 %, and a dielectric breakdown voltage of not less than 50 V/pm. It is preferable that the applied voltage for establishing the plasma state is about 1 to 3 kV, the current is 100 to 300 mA, and the power is about 100 to 300 W. By forming a metal layer on the insulation layer by means of the vacuum evaporation method or the spatter method, the substrate with the insulation layer and the metal electrode shown in Fig. 1 can be prepared. The metal layer has an appropriate electric conductivity and a thickness of about 500 to 1 pm. Materials of the metal layer are, for instance, aluminum, molybdenum, stainless steel, antimony, chromium, nickrome, and the like. The insulation layer of the present invention does not need to undergo a heat treatment during production as it was needed in the case of use of polyimide resin, and its heat resistance is higher than when polyimide resin is used. Further, since there is no influence from degassing by heating during the production of a solar cell, semiconductor layer formation can be effected at a high temperature and the performance of a solar cell obtained can be improved. Furthermore, as compared with the case where polyimide resin is formed by the spin coat method or dipping method, the formation of the insulation layer by the plasma CVD method or spatter method ensures uniform formation of the layer, so that the yield is improved. The insulation layer of the present invention has a dielectric breakdown voltage of 50 V/;m or above, preferably a dielectric breakdown voltage of 100 v/m or above. The present invention is more particularly described and explained by means of the following Example. It is to be understood that the present invention is not limited to the Example and various changes and modifications may be made in the invention without departing from the spirit and scope thereof. EXAMPLE This example was carried out by the plasma CVD method by using an apparatus similar to the one shown in Fig. 2. A stainless 304 sheet having a thickness of 0.2 mm was employed as the substrate 1 and was placed on the electrode 6. The distance between the electrodes 5 and 6 was 20 mm, and the substrate was heated so that its surface temperature was 250 C. After the vacuum container 4 was evacuated to 1 X 10 Torr, a gas mixture of 2 seem (standard cubic centimeter) of silane and 180 seem of methane deluted with hydrogen to 10 % by volume was introduced through the valve 7, and the degree of vacuum was maintained at 1.5 Torr. In this state, a high frequency voltage (30 W, 13.56 MHz) was applied between the electrodes 5 and 6 to establish a plasma state in the vacuum container, and the state was maintained for 3 hours, whereby an insulation layer 2 was formed. The insulation layer obtained had a film thickness of 2 vm and an optical inhibition band width of Eg (Opt) 2.6 eV. The carbon content of the insulation layer measured by the Auger electron spectroanalysis was 45 %. Thereafter, chromium was deposited as the metal electrode 3 on the surface of the insulation layer 2 by the electron beam vapor deposition method so that its thickness was 1000 #. Voltage was applied between the substrate 1 and the metal electrode 3. The dielectric breakdown voltage was found to be about 300 V/i'm. In addition to the ingredients and the method used in the Example, other ingredients and method can be used in the Example as set forth in the specification to obtain substantially the same results.

Claims: WHAT WE CLAIM IS: 1. An electric insulation thin layer comprising a non-monocrystalline substance being deposited on a substrate by means of a plasma CVD method or a spatter method, and having a carbon content of not less than 10 % by atom. 2. The electric insulation thin layer of Claim 1, wherein the carbon content of the layer is not less than 30 % by atom. 3. The electric insulation thin layer of Claim 1, wherein the layer has a dielectric breakdown voltage of not less than 50 V/m. 4. The electric insulation thin layer of Claim 1, wherein the layer has a dielectric breakdown voltage of not less than 100 V/i'm. 5. The electric insulation thin layer of Claim 1, wherein the layer has a thickness of 0.5 to 100 pm. 6. The electric insulation thin layer of Claim 1, wherein the layer has a thickness of 1 to 10 pm. 7. The electric insulation thin layer of Claim 1, wherein the non-monocrystalline substance is an amorphous substance of the formula: Si(l-x-y)CsXy: H wherein X is at least one member selected from the group consisting of N, o, F, C1, Br and Ge, l x 0.1, 0.9 # y # 0; and 1 # ¯ x+y # 0.1.

Inventors: NAKAYAMA, TAKEHISA, NISHIMURA, KUNIO, TAWADA, YOSHIHISA, TSUGE, KAZUNORI
Applicants: KANEGAFUCHI KAGAKU KOGYO KABUSHIKI KAISHA
Application year: 1984
Main classification: H01B3
IPC codes: H01L31, C23C16, C23C14, H01B3, H01B17