Abstract:
A resin composition for forming an insulating film suitable for fabricating an amorphous silicon solar cell utilizing a flexible substrate, obtained by mixing a first component comprising a polyfunctional isocyanate compound and a polyol based second component comprising polymers or oligomers having reactive hydroxyl groups which react with isocyanate groups to principally form urethane bonds.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an insulating resin composition and/or a transparent insulating resin composition, and more particularly, it relates to a resin composition suitable for screen printing to use in a thin-film type electronic device, as well as to a resin composition for constituting an insulating film for use in a thin-film type electronic device. 
     The present invention also relates to a solar cell comprising the resin composition above. 
     2. Description of the Prior Art 
     In a thin-film type electronic device (i.e., an electronic device constructed by principally stacking thin films) such as a thin-film solar cell, various types of resin compositions have been studied for forming an insulating film for use as an interlayer dielectric film in a multilayered structure or an insulating film in a multi-level crossed interconnection. 
     The resin composition above should possess insulating properties for use as an interlayer insulating film, adhesion to the upper and the lower layers, weathering resistance, moisture resistance, heat resistance, wear resistance, scratch resistance, flexibility, surface hardness, applicability of an ink by means of screen printing and the like, and hardenability. 
     Studies have heretofore been made on thermosetting, thermoplastic, and ultraviolet (UV) curable resin compositions such as styrene resins, saturated polyester resins, unsaturated polyester resins, epoxy resins, alkyd resins, silicone resins, acrylic resins, and fluororesins. Details of the studies are described in, for example, JP-A-61-218625 (the term &#34;JP-A-&#34; as referred to herein signifies an &#34;unexamined published Japanese patent application&#34;). 
     However, in spite of the above proposed resin compositions above, a resin composition which satisfies the requirements with good balance between the requirements is yet to be developed. 
     Particularly, in a multilayered structure comprising stacked thin films such as that in a solar cell or similar thin-film type electronic device with an interlayer insulating film or an insulating film for multi-level crossed interconnections, none of the known resin compositions was found to satisfy the required characteristics enumerated above as well as printability of an ink, hardenability of the printed film, and productivity. Accordingly, no resin composition developed heretofore is suitable for use as an interlayer insulating film or an insulating film for multi-level crossed interconnections. 
     Furthermore, an extensive study has been made of resin encapsulates for thin-film electronic devices, especially resin compositions that form a transparent protective film on the surface of a solar cell of thin-film structure. 
     The resin compositions for such use need not only transparency but also many characteristic properties required of a protective film (such as weather resistance, moisture resistance, heat resistance, wear resistance, scratch resistance, flex resistance, surface hardness, and adhesion to the substrate and the thin film). It also needs good printability when used in the form of an ink for screen printing. 
     Conventional resin compositions for transparent protective films are formed from a variety of thermoplastic and thermosetting resins and UV-curing resins, such as styrene resin, saturated polyester resin, unsaturated polyester resin, epoxy resin, silicone resin, acrylic resin, and fluoroplastics. Their detailed descriptions will be found in Japanese Patent Publication No. 17342/1988 and Japanese Patent Laid-Open Nos. 69874/1981 and 218625/1986. 
     However, none of the conventional resin compositions meet the requirements for the above-mentioned characteristic properties. Especially when used as a transparent protective film on solar cells of thin film structure, they are not satisfactory in transparency, properties required of a protective film, printability as an ink, curability, and productivity. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a resin composition for forming an insulating film or a transparent protective film suitable for a thin-film type electronic device structure, and particularly, to provide a resin composition for fabricating an insulating film suitable for an amorphous silicon solar cell utilizing a flexible resin substrate, yet, with high productivity and utilizing an inexpensive and lightweight base material. 
     An object of the present invention is, more specifically, to accomplish the improvements described below by overcoming the problems. 
     (1) Improving moisture resistance and water resistance of the insulating film 
     An object of the present invention is to improve the moisture resistance and water resistance of the resin composition constituting an insulating film, thereby improving weathering resistance of the resin composition. 
     The transparent resin composition should have good resistance to deterioration by moisture, adequate ability to prevent the penetration of moisture into the interior of the device covered by the transparent protective film comprising the transparent resin composition, and high weathering resistance. 
     Conventional resin compositions suffered changes in composition due to reaction with the moisture or water of the surroundings. As a result, due to the deterioration of the amorphous silicon layer or to the deterioration of the electrode material, a solar cell using the resin was found to lose the photoelectric conversion effect and to suffer deterioration of the electric properties with passage of time. 
     (2) Improving heat resistance and surface hardness of the insulating film 
     Another object of the present invention is to increase the heat resistance and the hardness of an insulating film comprising the resin composition. More specifically, in the fabrication of a thin-film electronic device such as a solar cell, a flexible printed circuit (FPC) is provided or a lead wire is adhered to the electrode portion that is provided to output the electric power by applying heat under pressure at a temperature of 100° C. or higher for soldering or heat sealing. At the same time, the interlayer insulating film and the multi-level crossed insulating films or an insulator around electrodes are also subjected to heat shock or thermal welding. 
     The thermal deformation of the insulating films due to heat shock or thermal welding can be prevented from occurring by increasing the hardness and the heat resistance of the insulating films or transparent protective films. Furthermore, insulation failure and defective appearance can be avoided. 
     Moreover, the heat resistance of the interlayer insulating film and the multi-level insulation film can be further improved in a device in which heat accumulation occurs during the operation of the device, or in a device which is exposed to a at high temperatures, such as a solar cells and the like used in vehicles. 
     Furthermore, in the case of forming an ITO (indium tin oxide) transparent electrode, the insulating properties can be maintained by preventing physical or chemical damage from occurring on the printed film and thereby increasing heat resistance of the film. 
     Vehicle-mounted solar cells, which are subject to high temperatures, are protected from failure and surface scratches, which reduce light transmission and hence conversion efficiency, if they are covered with a protective film having improved hardness and heat resistance. 
     (3) Improving wear resistance 
     A still an other object of the present invention is to improve the wear resistance of an insulating film comprising the resin composition. 
     In the case of laminating thin films by means of, particularly, a roll-to-roll process (a fabrication process which comprises continuously forming the device by performing each of the unit operations such as film deposition, printing, and laser processing while taking up a rolled flexible substrate on another roll), there are problems such as of scratches and the like which occur on the interlayer insulating film or the transparent surface protective film when the surfaces of the upper layer and the lower layer of the flexible substrates are rubbed against each other, or when the surfaces of the flexible substrate and the guide roll are rubbed against each other. 
     It is therefore necessary to maintain wear resistance to prevent, for instance, inhomogeneity from occurring on an inorganic thin film such as an amorphous silicon film, a loss in performance, and a drop in production yield due to a defective appearance. 
     Improvement of durability of transparency: 
     Conventional curable resin compositions developed so far to meet the above-mentioned requirements (1) to (3) are liable to discoloration (which decreases transparency) due to attack by heat, moisture, UV light, ozone, etc. when used for outdoor solar cells. The transparent resin composition should form a protective film which retains its transparency for a long period of time. 
     (5) Improving adhesion between the upper and the lower layers 
     A yet other object of the present invention is to improve the adhesiveness between an insulating film comprising the resin composition and the upper and the lower layers adjacent thereto. 
     In a multilayered thin film device, for instance, an amorphous silicon solar cell using a low-cost flexible substrate such as a resin substrate or a metallic substrate as the base material, a flexible insulating resin composition which assures excellent adhesion between layers (adhesion between the insulating film and the adjacent upper and lower layers) is required to achieve insulation between an upper and a lower stacked thin film (layer) under various environments which accompany bending of the thin film device. 
     It is also required to relax the internal stress to prevent deformation or change in size, such as curling, of the flexible solar cell comprising a stacked structure of a thin film having a large internal stress such as an amorphous silicon layer and an ITO transparent electrode layer. 
     (6) Improving laser processability 
     Another object of the present invention is to improve laser processability of an insulating film comprising the resin composition. 
     To implement a multilayered thin film device comprising a multi-level insulation and a multi-level interconnection established in high precision processing such as laser scribing or laser bonding using a YAG laser and the like is indispensable. Thus, the resin composition constituting the insulating film must be designed to be successfully cut at high precision. 
     (7) Improving hardenability 
     A still an other object of the present invention is to lower the thermosetting temperature of a resin composition or a transparent resin composition as possible without impairing the degree of cross linking, yet elongating the pot life in its ink state. 
     Concerning productivity in forming a resin composition for printing to obtain an insulating film or a transparent film, it is advantageous if the thermosetting reaction proceeds efficiently at a temperature as low as possible, and if the pot life of the ink is long. 
     Particularly, in the case of a flexible solar cell and the like in which a plastic film having low thermal resistance is used, an insulating resin composition or a transparent resin which undergoes thermosetting at a lower temperature is important in preventing thermal deformation of the solar cell from occurring. 
     The present invention achieves the objects of the present invention described in (1) to (7) above. 
     According to an aspect of the present invention, there is provided a resin composition obtained by mixing a first component comprising a polyfunctional isocyanate compound; and a polyol based second component comprising polymers or oligomers having reactive hydroxyl groups which react with isocyanate groups to principally form urethane bonds. 
     In accordance with another aspect of the present invention, there is provided a resin composition as above, characterized in that the first component comprises a blocked polyfunctional isocyanate compound which releases an isocyanide group upon heating. 
     In accordance with a still aspect of the present invention, there is provided a resin composition as above, characterized in that the second component comprises a single system or a mixed system of a phenoxy resin or a bisphenol-type epoxy resin having a number average molecular weight in the range of 1,200 to 50,000. 
     In accordance with a yet other aspect of the present invention, there is provided a resin composition as above, characterized in that the second component comprises a single system or a mixed system of a phenoxy resin or a bisphenol-type epoxy resin having a number average molecular weight in the range of 2,500 to 20,000 or less. 
     In accordance with a further other aspect of the present invention, there is provided a resin composition as above, characterized in that an additive comprising a silicone-based or an acrylic-based polymer, or a polymer mixture thereof is added at a quantity of from 0.001 to 5% by weight with respect to the resin composition comprising the first and the second components. 
     In accordance with another aspect of the present invention, there is provided a resin composition as above, characterized in that a composite is formed by adding fine particles of at least one selected from the group consisting of SiO 2 , Al 2  O 3 , TiO 2 , CaCO 3 , and carbon black as fine pigment particles. 
     In accordance with a yet other aspect of the present invention, there is provided an encapsulated solar cell comprising a resin composition above. 
     The present invention provides a highly reliable flexible resin composition particularly useful as an interlayer insulating film. The resin composition according to the present invention is obtained by forming, as represented by phenoxy resin, a polyol of an insulating polymer or oligomer which is resistant against hydrolysis and characterized in that it comprises a bisphenol skeleton with a rigid aromatic ring and an ether bond for rendering the structure flexible as the principal chain, said principal chain provided with reactive hydroxyl groups at high concentration; preparing a resin lacquer containing a polyurethane bond obtained by reacting the resulting polyol with a polyfunctional isocyanate or a block compound thereof in a stoichiometric quantity or in a quantity slightly in excess of stoichiometry; and obtaining a resin composition by dispersing together with a dispersing them agent into a resulting lacquer, a thixotropy imparting agent such as a fine-grained SiO 2  (aerosil), a dye, and a laser radiation absorbing agent such as carbon black having high electric resistivity. 
     The surface tension of the resin composition in the form of a paste before hardening is controlled to a value of 40 μN/cm (at 20° C., as measured according to ASTM D971 by means of platinum ring method using a dynometer manufactured by Bicchemie Co., Ltd.) or lower by adding a defoaming agent, a leveling agent, etc. 
     In accordance with the present invention, an adduct, a biuret, an isocyanurate (a trimer), etc., of an isocyanate monomer and trimethylol propane (TMP) is effective as a polyfunctional isocyanate compound for use as a first component. 
     As isocyanate monomers most generally known as aromatic compounds, there can be mentioned toluene diisocyanate (TDI), 4,4&#39;-diphenylmethane diisocyanate (MDI), or xylene diisocyanate (XDI). An adduct, a biuret, an isocyanurate (a trimer), etc., of the monomers above can be used in the present invention as an aromatic polyfunctional isocyanate. 
     However, an aromatic polyfunctional isocyanate is higher in reactivity as compared with an aliphatic (non-aromatic) compound. It can be expected to completely harden at a lower temperature, however, and the pot life thereof is very short when used as an ink, a paste as a solder resist, or a two-liquid type product by mixing it with a second component polyol. It therefore has difficulties in handling. 
     Other isocyanate monomers free of aromatic rings include hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), hydrogenated MDI (H 12  MDI), and hydrogenated XDI. The aliphatic (non-aromatic) polyfunctional isocyanates of the adduct, biuret, isocyanurate, etc., of TMP with the monomers above are low in reactivity as compared with an aromatic one, however, they are advantageous in increasing device reliability against environment, such as resistance to light, weathering, etc. Accordingly, they are suitable for uses where resistance against yellowing is required, or where transparency or translucency is necessary. 
     Although the aliphatic isocyanide above reacts more sluggishly as compared with an aromatic one, a longer pot life can be obtained when used as an ink (an overcoating agent) or a paste as a solder resist and the like. This ease in handling in production steps is another merit of the aliphatic product. 
     In accordance with another aspect of the present invention, a transparent resin composition which comprises two components mixed therein, the first one being a transparent polyfunctional isocyanate compound having no aromatic rings and the second one being a transparent polyol, in the form of polymer or oligomer, having reactive hydroxyl groups to form urethane linkages upon reaction with isocyanate groups. 
     Also, in the above transparent resin composition, the first component is a blocked polyfunctional isocyanate compound having no aromatic rings, which liberates isocyanate groups upon heating. 
     Also, in the above transparent resin composition, the second component is a phenoxy resin or a bisphenol-type epoxy resin or a mixture thereof which has a number-average molecular weight in the range of 1200 to 50000. 
     Also, in the above transparent resin composition, the second component is a phenoxy resin or a bisphenol-type epoxy resin or a mixture thereof which has a number-average molecular weight in the range of 2500 to 20000. 
     Also, in the above transparent resin composition, it further comprises an additive in an amount of 0.001-5 wt. % which is a silicone polymer or an acrylic polymer or a mixture thereof. 
     Also, the transparent resin composition in accordance with the present invention is particularly advantageous for solar cells. 
     The present invention is designed to form a transparent protective film from a transparent resin composition which is composed of a polyfunctional isocyanate compound and a polyol. The isocyanate compound is one which has no conjugated bonds and is or is not blocked. The polyol is either a polymer or an oligomer typified by a high-molecular weight phenoxy resin having reactive hydroxyl groups. It is transparent and resistant to hydrolysis and has rigid main chains. Reaction between the isocyanate compound and the polyol (with hydroxyl groups in a stoichiometric amount or in an excess amount) gives rise to a product having polyurethane linkages which serves as the protective film. 
     Further, the surface tension of the paste used during the film application is controlled to be 40 μN/cm or less (20° C.) (platinum ring method using a dynometer produced by BYK-Chemie GmbH company under the conditions of ASTM D971) by an additive such as an antifoaming agent or a leveling agent. 
     According to the present invention, the first component of the transparent resin composition may be a polyfunctional isocyanate compound having no aromatic rings. It includes diisocyanates, such as hexamethylenediisocyanate (HDI), and triisocyanates. Examples of the latter include an adduct of trimethylolpropane with three HDI molecules, a trimer of HDI joined by urea linkages (or biuret linkages), and a trimer of HDI forming an isocyanurate. (The last one yields a polyurethane resin superior in weather resistance and heat resistance.) Additional examples of the isocyanate compound having no aromatic rings include isophoronediisocyanate (IPDI). This isocyanate has weather-resistance and heat-resistance as in HDI, and is effective in obtaining a hard polyurethane resin, and has a long pot life when mixed with a polyol. 
     This IPDI forms an adduct with TMP as in HDI, and forms an isocyanurate as a trimer, and is effective in obtaining an excellent polyurethane resin as a trifunctional isocyanate. 
     Hydrogenated diphenylmethandiisocyanate (H 12  MDI) is effective as an isocyanate having no aromatic ring, and a polyfunctioned isocyanate can be used. 
     Preferred representative examples for the block polyfunctional isocyanate compound (block isocyanate) for use as the first component of the above insulating resin component or transparent resin component contain an active hydrogen. A principal example is a compound (which releases free isocyanate groups by heating at a temperature of from 160° to 180° C. for a duration of 30 minutes) comprising isocyanate groups blocked with hydroxyl groups originated from a block agent such as phenol, cresol, and isononyl phenol. By mixing it with a polyol in such a manner that the free isocyanate be in an equivalent quantity with the hydroxyl groups in polyol, a one-liquid type product can be obtained without of taking the pot life into consideration. 
     A curing catalyst such as dibutyltin dilaurate or a dissociation catalyst such as triethylenediamine (DABCO) may be added in a small quantity in preparing the resin. 
     An oxime such as methyl ethyl ketoxime can be mentioned as another example of a blocking agent (which dissociates by heating at about 140° C. for a duration of 30 minutes). 
     A blocking agent comprising a lactam group such as ε-caprolactam, or such comprising an amino group, an amide group, or an imide group can be used as well (these thermally dissociate by heating at about 160° C. for a duration of 30 minutes). 
     Furthermore, a dicarbonyl compound such as diethyl malonate or ethyl acetoacetate is also effective as a blocking agent. Because a blocking agent of this type dissociates the isocyanate groups at a lower temperature (i.e., by heating at about 100° C. for 30 minutes), it can be used advantageously as a one-liquid product having an intensified thermosetting property. 
     The polyfunctional isocyanide compounds described above functions effectively as the isocyanate compounds to be combined with the blocking agents above. An aromatic isocyanate can be used, because no urethane reaction occurs at an ordinary temperature in case it is mixed with a polyol of the second component. Accordingly, the resulting product can be used as a one-liquid type pot life-free ink (overcoat agent), a solder resist, etc. 
     Furthermore, among the isocyanate compounds to be combined with these blocking agents, the above-described polyfunctional isocyanate compound having no aromatic ring is practically preferable to maintain transparency. 
     Polyols for use as the second component comprise a single system or a mixed system of a phenoxy resin or a bispbenol-type epoxy resin having a number average molecular weight in the range 1,200 to 50,000, and preferably, 2,500 to 20,000. 
     In the second component, when a material of a low degree of polymerization such those having a number average molecular weight of 1200 or less, the proportion of the reactive hydroxyl group decreases and the crosslinking density of the urethane bonding between the first and second components decreases. 
     As a result, the obtained resin component cannot sufficiently exhibit the strong adhesion with an underlying surface or the strong wear resistance or flexibility which are caused by urethane bond radicals. Also, there is a tendency that the printability of the resin component as an ink for screen printing decreases since the viscosity and elastic modulus decreases. 
     In particular, the viscosity and the elastic modulus decreases in the case of a transparent resin component which is not added with a pigment, making it more difficult to be printed. 
     Accordingly, it is preferable that the second component has a number average molecular weight not lower than 1200, more preferably, not lower than 2500. 
     For example, it is effective to use a material having a bisphenol A-type epoxy structure unit of which the degree of polymerization is not lower than 4 and having four or more reactive hydroxyl groups in each of which one principal molecular chain is arranged in a pendant form. In this case, the molecular weight is about 1350. 
     Also, if the number average molecular weight of the second exceeds 50000, the kinds of solvents that can be used are limited. Even if it can be dissolved, the viscosity of the solution tends to be high and the flowability is low. As a result, it difficult to make an ink for screen printing. 
     For example, when a screen printing is used with an ink of which the main component is a material having a bisphenol A-type epoxy structure unit having a high degree of polymerization and having a molecular weight exceeding 50000, it is difficult for the ink to pass through the mesh of a screen having a desired fine pattern. 
     Also, because of the high viscosity, there is no defoaming effect with respect to enclosed bubbles when using a silicon containing defoaming agent as an additive and because it exhibits a strong consistency, it is almost impossible to uniformly mix such a second component and a polyfunctional isocyanate having no aromatic ring as a first component, resulting in that it is extremely difficult to form a urethane resin component having urethane bonds uniformly arranged. 
     Accordingly, in the case of using the second component having a high degree of polymerization alone, it is effective if the number average molecular weight is not higher than 50000, preferably not higher than 20000 in order to effectively utilize the characteristics of the urethane resins in view of the printability as a screen printing ink and the ease of handling. 
     Also, in the case where mixed resins having different molecular weights are used as a second component, it is desirable for the same reasons that the number average molecular weight is in the range of 1200 to 50000, more preferably, not lower than 2500 and not higher than 20000. 
     For example, phenoxy resins produced by Union Carbide Corp., such as PKHC (having a number average molecular weight of about 14,000), PKHH (having a number average molecular weight of about 15,400), or PKHJ (having a number average molecular weight of about 18,700), and Phenoade YP=50, a phenoxy resin produced by Tohto Kasei, may be used. 
     To control the suitability for printing an ink or forming a film with an ink, a bisphenol-type epoxy resin produced by Shell Kagaku K. K., such as epikote 1007 (having a number average molecular weight of about 2,900) or epikote 1009 (having a number average molecular weight of about 3,750), can be effectively used either singly or as a mixture with a phenoxy resin. 
     In the case of utilizing the resin composition having a polyurethane bond as above as, for example, an interlayer insulating film or a transparent protective film in a flexible solar cell by uniformly forming the desired pattern by screen printing, the defoaming properties and the leveling properties of the ink (paste), as well as the wetting properties with respect to the underlying amorphous silicon film, ITO electrically conductive film, transparent insulating film, etc., must be improved. Preventing pinholes from generating and avoiding repelling of ink, as well as improving the suitability to re-coating are also important factors for realizing a higher uniformity and decreasing reject rate of the solar cells. 
     As an effective means for satisfying the aforementioned requirements for a resin composition, from 0.001 to 5% by weight with respect to the resin components (the first and the second components) of a silicone based, acrylic based, or vinyl ether based additive is added if necessary to more favorably maintain screen printability of the resin ink. 
     Concerning a silicone based additive, for instance, it is effective to mix a trace quantity of a methylalkyl polysiloxane type silicon compound which is partially polyether-modified, an alkyl-modified compound, or a polyester-modified dimethyl polysiloxane. 
     Particularly, in the case of a silicone based defoaming agent or a leveling agent, an alkyl-modified compound of methylalkyl polysiloxane as shown in chemical formula (I) below was found favorable. ##STR1## 
     Not only silicone compounds, but also acrylic or polyvinyl ether based oligomers or polymers (having a number average molecular weight of about 20,000 or less) added in a small quantity were found effective. 
     In the case of uniformly coating an insulating ink (e.g., a solder resist, overcoat agent, etc.) at high precision according to the desired pattern by means of screen printing, it is especially important to control the fluid properties of the ink, and particularly the thixotropy. Favorable results can be obtained by uniformly dispersing particularly fine particles of a pigment in the resin composition. 
     Effective fine-grained pigments include SiO 2  (for instance, Aerosil produced by Degussa Corp.), Al 2  O 3 , TiO 2 , and high resistivity carbon black. Particularly favorable are the fine particles having a primary particle diameter in a range of from about 5 to 30 nm. 
     Among the fine-grained pigments above, carbon black absorbs laser radiation. Accordingly, carbon black is also effective in improving suitability of the resin to laser scribing or laser cutting. 
     The point in preparing an ink (paste) is to sufficiently disperse the fine-grained pigment in the resin component provided for preparing an ink (particularly, the second component). The addition of a polymer based dispersant which transferless to the printed film surface was found effective. 
     Examples of the dispersant include polymer dispersants soluble to the second and the first components of the resin composition, and having, on the terminal of the polymer (i.e., an acrylic resin or a polyester resin), a proton-donating groups such as --COOH, --SO 3 , --P(O), (OH) 2 , or --OH; a proton-accepting group such as the salts of the proton-donating groups enumerated above, --NH 2 , --NHR, --NR, or R 2  ; and furthermore a polar group having an ion pair bond such as a quaternary ammonium group. 
     The fine-grained pigment is preferably added at a concentration of from 3 to 20% by weight with respect to the total of the resin composition components (the total of first component, second component, fine-grained pigment, and solvent). The dispersant functions effectively by adding it at a concentration of from 1 to 5% by weight with respect to the total of the resin composition components. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1(A) to 1(D) are a diagram showing process steps for fabricating a flexible amorphous silicon solar cell according to an embodiment of the present invention; 
     FIG. 2 shows the characteristics of amorphous silicon solar cells obtained in Example 1 according to the present invention and in a Comparative Example; and 
     FIG. 3 shows a series connected example of an amorphous silicon solar cell. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is described in further detail below by making reference to the preferred embodiments according to the present invention. It should be understood, however, that the present invention is not to be construed as being limited to the examples below. 
     In the Examples 1-5, solar cells of a thin film type module structure are fabricated using polyethylene naphthalate for the flexible substrate. By means of screen printing, pastes of the resin compositions are each applied according to their individual printing patterns as an upper layer of an amorphous silicon film or an ITO transparent electrically conductive film. The resulting multilayered structures are each overcoated to obtain flexible solar cells, and are subjected to device evaluation. In the Examples below, each of the thin films is formed in-line by a roll-to-roll process to obtain a layered structure. 
     EXAMPLE 1 
     A polyurethane based thermosetting resin according to the present invention is used in this Example. The resin comprises the following components: 
     •Second Component: 
     Phenoxy resin PKHH 20 pts. by weight (a product of Union Carbide Corp., having a number average molecular weight of about 15,400 and containing 6% by weight of hydroxyl groups) 
     •Solvent: 
     Cyclohexanone 40 pts. by weight 
     Isophorone 20 pts. by weight 
     •Pigment: 
     High resistivity Carbon Black: 4 pts. by weight (a product of Degussa Corp., 25 nm in average particle diameter) 
     •Fine Particles: 
     Aerosil 10 pts. by weight (a product of Degussa Corp., 15 nm in average diameter) 
     •Dispersant: 
     Oleic acid 3 pts. by weight 
     •Additives: 
     Defoaming agent KS-69, 0.5 pts. by weight (a product of Shin-Etsu Silicone Co., Ltd.) 
     Leveling agent KS-66, 1 pt. by weight (a product of Shin-Etsu Silicone Co., Ltd.) 
     After completely dissolving the phenoxy resin in a mixed solvent (cyclohexanone and isophorone), carbon black, Aerosil, and the dispersant are added and dispersed for a duration of 48 hours using a zirconia ball mill. Then, the defoaming agent and the leveling agent are added to the resulting dispersion and mixed further for 2 hours. 
     By adding 17 pts. by weight of an hexamethylene diisocyanate (HDI trimer) as a first component to the resulting mixture above in such a manner that the isocyanate groups are present in stoichiometric quantity with hydroxyl groups in the phenoxy resin and further mixing the resulting product for 20 minutes, a resin composition is obtained in the form of a paste. 
     A flexible amorphous silicon solar cell having an electrode structure as shown in FIGS. 1(A) to 1(D) is fabricated thereafter by using the resulting paste resin composition. 
     Referring to FIG. 1(A), an electrode 2 comprising aluminum or a layered structure of aluminum and stainless steel is formed on a polyethylene naphthalate flexible substrate 1. 
     Then, as shown in FIG. 1(B), an amorphous silicon layer 3 having a PIN junction or the like and which exhibits a photovoltaic effect is formed by means of plasma CVD. The paste of the resin composition obtained above is printed on the upper layer portion of the amorphous silicon film by using a 150-mesh polyester screen, and is thermally set in an oven by heating at 160° C. for a duration of 10 minutes. Thus there is obtained a first interlayer insulating film 4. 
     By means of Ar gas sputtering, an ITO (indium tin oxide) film is deposited on the first interlayer insulating film 4 to provide a transparent electrode layer 5. 
     Even after effecting the sputtering above, no physical or chemical damage is observed on the resin composition provided as a first interlayer insulating film 4. 
     Then, by using a YAG laser, trenches and holes are formed for insulation or for electric connection. The processing of the first interlayer insulating film 4 is found to be accomplished with high dimensional precision. 
     Then, the paste resin composition is applied again to the surface of the transparent electrode layer 5 by using a 150-mesh polyester screen, and is subjected to thermal setting in an oven at 160° C. for a duration of 10 minutes. 
     Referring to FIG. 1(C), a second interlayer insulating film 6 is obtained in this manner. The resin composition is formed in such a manner to fill the trenches and holes formed by laser processing. 
     No flaws and the like are observed to be generated on the interlayer dielectric films 4 and 6 during the process steps above conducted according to a roll-to-roll process. 
     Referring to FIG. 1(D), an interconnection electrode 7 is printed by using an electrically conductive paste, and a flexible amorphous silicon solar cell is thus obtained by printing and forming a translucent insulating resin film 8. 
     The performance of the flexible amorphous silicon solar cell thus obtained is evaluated. 
     •Moisture Resistance 
     The change in current-voltage characteristics of the solar cell fabricated in the present Example is observed for a duration of 2,000 hours under conditions of 80° C. and 90% RH. The results are illustrated in FIG. 2. Referring to FIG. 2, the ordinate in each of the graphs is expressed by a normalized value. It can be seen from FIG. 2 that the solar cell according to the present invention is substantially free of degradation of short circuit current (Isc), open circuit voltage (Voc), and fill factor (F.F.) even after passage of 2,000 hours. 
     •Adhesion Strength 
     Adhesion strength of the translucent electrode layer is evaluated by means of chess-board tape peeling off test. A value of 100/100 obtained in the initial evaluation is found unchanged to yield 100/100 after exposing the solar cell to a temperature of 80° C. at 90% RH for a duration of 2,000 hours. 
     •Glass Transition Temperature (Tg) 
     Tg is measured by using DSC. Tg of 110° C. and 103° C. are obtained after and before thermal setting, respectively. The drop in elasticity at temperatures of Tg or higher is observed to be very sluggish. 
     •Surface Tension 
     Surface tension of 37 μN/cm is obtained by means of platinum ring method using a Dynometer at 20° C. 
     •Pot Life 
     No change in printing ability is observed even after passage of 24 hours at an ink temperature of 20° C. 
     COMPARATIVE EXAMPLE 
     A known thermosetting alkyd resin composition containing the components below is used to fabricate a solar cell similar to that described in Example 1. The performance of the resulting cell is evaluated in the same manner as in Example 1. Composition of Alkyd Resin 
     
         ______________________________________Aminoalkyd resin     12 pts. by weightMelamine resin        8 pts. by weightButyl cellosolve     25 pts. by weightTrimethylbenzene     25 pts. by weightHigh resistivity carbon black                 4 pts. by weight(a product of Degussa Corp., 25 nm in average particlediameter)Aerosil              10 pts. by weight(a product of Degussa Corp., 15 nm in average diameter)Dispersant (Oleic acid)                 8 pts. by weightDefoaming agent TSA-720                 1 pt. by weight(a product of Toshiba Silicone Co., Ltd.)Leveling agent KS-66  1 pt. by weight(a product of Shin-Etsu Silicone Co., Ltd.)______________________________________ 
    
     After completely dissolving aminoalkyd resin and melamine resin in a mixed solvent (butyl cellosolve and benzene), carbon black, Aerosil, and the dispersant are added and dispersed for a duration of 48 hours using a zirconia ball mill. Then, the defoaming agent and the leveling agent are added to the resulting dispersion and mixed further for 2 hours. 
     A flexible amorphous silicon solar cell is obtained in a similar process as described in Example 1 above, and the performance thereof is evaluated. 
     •Moisture Resistance 
     The short circuit current (Isc) and the curve factor (F.F.) were each observed to deteriorate by 30% with respect to the initial values after passage of 2,000 hours (FIG. 2). 
     •Adhesion Strength 
     Adhesion strength is evaluated on the ITO layer. A value of 100/100 obtained in the initial evaluation is found to drop to 80/100 after exposing the solar cell to a temperature of 80° C. at 90% RH for a duration of 2,000 hours. 
     •Glass Transition Temperature (Tg) 
     Tg of 98° C. is obtained after thermal setting. 
     •Surface Tension 
     Surface tension of 42 μN/cm is obtained by means of platinum ring method using a Dynometer at 20° C. 
     •Pot Life 
     No problem in printing suitability is observed even after passage of 24 hours at an ink temperature of 20° C. 
     However, in printing the transparent insulating resin of the uppermost layer, ink is found to be frequently repelled. It can be seen therefore that the resin is inferior concerning recoating properties. As compared with the productivity in Example 1, only 60% of the products are found acceptable. 
     EXAMPLE 2 
     A block isocyanate obtained by active-hydrogen blocking three isocyanate groups in HDI isocyanate using MEK (methyl ethyl ketone) oxime is utilized for the first component, and is added at such a quantity that it may be chemically equivalent to the hydroxyl group content (OH content) accounting for 6% by weight of PKHH (i.e., the second component in Example 1). The resulting mixture is mixed for a duration of 30 minutes in a mixer to obtain a resin composition for use as an overcoat. The other materials are mixed in the same quantities as in Example 1. 
     The resulting resin composition is used in the trial fabrication of a solar cell in the same manner as in Example 1, and the properties of the resulting cell are evaluated. 
     •Moisture Resistance 
     The short circuit current (Isc) and the curve factor (F.F.) were each observed to be substantially free of deterioration even after passage of 2,000 hours under conditions of 80° C. and 90% RH. 
     •Adhesion Strength 
     Adhesion strength is evaluated by means of a chess-board tape peeling-off method on the translucent electrode layer. A value of 100/100 obtained in the initial evaluation is found to remain unchanged to yield a value of 100/100 after exposing the solar cell to a temperature of 80° C. at 90% RH for a duration of 2,000 hours. 
     •Glass Transition Temperature (Tg) 
     Tg is measured by using DSC. Tg of 110° C. and 103° C. are obtained after and before thermal setting, respectively. The drop in elasticity at temperatures of Tg or higher is observed to be very sluggish. 
     •Pot Life 
     No problem in printing ability is observed even after passage of 24 hours at an ink temperature of 20° C. Thus, the resin composition can be used pot-life free as a one-liquid type urethane ink. 
     EXAMPLE 3 
     Thirty parts by weight of PKHH and 70 parts by weight of a bisphenol A epoxy resin (Epikote 1007 produced by Shell Kagaku K. K., having a number average molecular weight of about 2,900 and an OH content of 6% by weight) are mixed to give 100 parts by weight of a mixed resin. 
     Then, an isocyanate compound of toluene diisocyanate (TDI) is added as the first component in a quantity chemically equivalent to the hydroxyl group content of the resin of the second component. A resin composition for an overcoat is obtained in the same manner as in Example 1 except for the points described above, and the properties of the resulting cell are evaluated. 
     •Moisture Resistance 
     Almost no change in the short circuit current (Isc) and the fill factor (F.F.) is observed even after passage of 2,000 hours under conditions of 80° C. and 90% RH. 
     •Adhesion Strength 
     Adhesion strength is evaluated by means of a chess-board tape peeling-off method on the translucent electrode layer. A value of 100/100 obtained in the initial evaluation is found to remain almost unchanged to yield a value of 98/100 after exposing the solar cell to a temperature of 80° C. at 90% RH for a duration of 2,000 hours. 
     •Glass Transition Temperature (Tg) 
     Tg is measured by using DSC. Tg of 103° C. is obtained after thermal setting. 
     •Pot Life 
     Deterioration in printing ability is observed to initiate after passage of 24 hours at a temperature of 20° C., and a loss of pattern uniformity and a drop in pattern precision are found. 
     EXAMPLE 4 
     One hundred parts by weight of a polycarbonate ester having a straight chain aliphatic carbonate structure with hydroxyl groups on the both terminals thereof, Nipolane 980 (produced by Nippon Polyurethane industry Co., Ltd., having a number average molecular weight of about 2,000 and an OH value of 56.1) is used as a second component, and is dissolved in 40 parts by weight of isophorone solvent. A resin composition is prepared in the same manner as in Example 1. 
     An ink is prepared thereafter by adding IPDI isocyanurate as a hardening agent at a quantity chemically equivalent to the resin. Thus, a solar cell is fabricated in the same manner as in Example 1, and the properties of the resulting cell are evaluated. 
     •Moisture Resistance 
     Almost no change in the short circuit current (Isc) and the curve factor (F.F.) is observed even after passage of 2,000 hours under conditions of 80° C. and 90% RH. 
     •Adhesion Strength 
     Adhesion strength is evaluated by means of a chess-board tape peeling-off method on the translucent electrode layer. A value of 100/100 obtained in the initial evaluation is found to remain almost unchanged to yield a value of 95/100 after exposing the solar cell to a temperature of 80° C. at 90% RH for a duration of 2,000 hours. 
     •Glass Transition Temperature (Tg) 
     Tg is measured by using DSC. Tg of 115° C. is obtained after thermal setting. 
     •Pot Life 
     Deterioration in printing suitability is observed to initiate after passage of 24 hours at a temperature of 20° C. The pot life of the resin composition of the present Example is found to be longer than that of the resin composition described in Example 1. 
     EXAMPLE 5 
     In the present Example, the compounds below are used as additives. 
     
         ______________________________________Dispersant         3 pts. by weight______________________________________ 
    
     (Sulfonic acid terminated acrylic copolymer, i.e., a 90:9.5:0.5 (by molar) butyl acrylate/methyl methacrylate/sulfonic acid monomer; having a number average molecular weight of about 9,900 and a solid content of 50% by weight in isophorone solution) 
     
         ______________________________________• Defoaming agent 0.2 pt. by weight (an aralkyl-modified methylalkyl polysiloxane of formula (I),where R.sub.1 represents a structure expressed by formula (II) and nis 5) ##STR2##• Leveling agent 0.8 pt. by weight (an aralkyl-modified methylalkyl polysiloxane of formula (I),where R.sub.1 represents a structure expressed by formula (III) and nis 8) ##STR3##• Alumina 10 pts. by weight (Aluminum Oxide C, a product of Degussa Corp., 13 nm in averagediameter)• High resistivity carbon black 4 pts. by weight (a product of Degussa Corp., 25 nm in average particlediameter)______________________________________ 
    
     In a manner similar to that described in Example 1, the above additives are dispersed for a duration of 48 hours using a zirconia ball mill. Then, the defoaming agent and the leveling agent are added to the resulting dispersion and are mixed and dispersed further for 2 hours. 
     A resin component for an overcoat is prepared in the same process as that described in Example 1 by adding a trimethylol propane (TMP) adduct of isophorone diisocyanate (IPDI) at such a quantity that the isocyanate groups are chemically equivalent to the OH groups contained in PKHH. 
     A flexible amorphous solar cell is fabricated in the same manner as in Example 1 by means of a roll-to-roll process, and the properties of the resulting cell are evaluated. 
     •Moisture Resistance 
     The short circuit current (Isc) and the curve factor (F.F.) were each observed to deteriorate by about 26% with respect to the initial values after passage of 2,000 hours at 80° C. and 90% RH. 
     •Adhesion Strength: 
     Adhesion strength is evaluated by means of a chess-board tape peeling-off method on the translucent electrode layer. A value of 100/100 obtained in the initial evaluation is found to remain unchanged to yield a value of 100/100 after exposing the solar cell to a temperature of 80° C. at 90% RH for a duration of 2,000 hours. 
     •Glass Transition Temperature (Tg): 
     Tg is measured by using DSC. Tg of 113° C. is obtained after thermal setting. 
     •Pot Life: 
     Deterioration in printing ability is observed to initiate after passage of 24 hours at a temperature of 20° C. The degree of deterioration is better than that in Example 1, and is about the same as that in Example 4. 
     In examples 6 to 8 that follow, a transparent resin composition was prepared and it was made into a transparent protective film for amorphous silicon solar cells of thin film structure as shown in FIG. 3. A cross-sectional structure of an amorphous Si solar cell having a thin film type electronic device structure is shown in FIG. 3. The solar cells and the protective film were formed in-line by the roll-to-roll process. 
     The solar cell shown in FIG. 3 is formed on a flexible substrate 11 of polyethylene terephthalate. It is composed of a lower electrode 12 (which is aluminum thin film), a photoelectric conversion layer 13 (which is an amorphous silicon thin film), and a transparent upper electrode 14 (which is an indium-tin oxide film). These layers were patterned by the laser scribe technique. 
     Except for a small area for electrodes for power output, the entire surface (for the transparent upper electrode layer 14) of the solar cell was covered with a transparent protective film 15, which was formed by screen printing from the transparent resin composition 15 of the present invention. The screen printing was followed by leveling for a few minutes and heat curing in an oven at 160° C. for 10 minutes. 
     EXAMPLE 6 
     A transparent resin composition of the present invention was prepared as follows: 
     •The first component: Isocyanurate-linked hexamethylenediisocyanate (HDI trimer, &#34;Coronate HX&#34; containing 21.3 wt. % of NCO, a product of Nippon Polyurethane Co., Ltd.) 
     •The second component: Phenoxy resin (&#34;PKHH&#34; having a number-average molecular weight of about 15400 and containing 6 wt. % of hydroxyl groups, a product of Union Carbide Corporation) This phenoxy resin (100 pbw) was dissolved in a solvent composed of 100 pbw each of cyclohexanone and isophorone. The resulting solution was incorporated with 5 pbw each of anti-foam agent (&#34;TSA-720&#34;, a product of Toshiba Silicone Co., Ltd.) and leveling agent (&#34;KS-66&#34;, a product of Shin-Etsu Silicone Co., Ltd.). 
     The phenoxy resin is completely dissolved in a mixed solvent (cyclohexanon/isophorone) and is sufficiently mixed with antifoaming and leveling agent. 
     The first component (80 pbw), i.e. isocyanurate bonded hexamethylenediisocyanate (HDI trimer: Nihon Polyurethane Co., Ltd. Coronate NX; NCO containing ratio 21.3 wt. %) was thoroughly mixed with the second component (phenoxy resin solution) such that the amount of the hydroxyl groups is stoichiometrically equal to that of the isocyanate groups. 
     The resulting resin composition was applied to the surface of the upper electrode by screen printing through a 150-mesh polyester screen. The screen printing was followed by heat-curing in an oven at 160° C. for 10 minutes. The thus obtained transparent protective film was found to have the following properties. 
     •Moisture resistance: high enough to prevent the degradation of the solar cell conditioned at 80° C. and 90% RH for 2000 hours. 
     •Pencil hardness: 4H 
     •Adhesion: (according to cross-cut test) 
     100/100 immediately after application to the transparent electrode. 
     100/100 after conditioning at 80° C. and 90% RH for 2000 hours. 
     •Glass transition point (Tg) (DSC method): 110° C. for the cured film; 103° C. for the uncured film. Very slow decrease in elastic modulus at temperatures above Tg. 
     •Transparency: The sample remained unchanged in transmission of light having a wavelength of 400 nm after conditioning at 85° C. for 1000 hours. 
     •Yellowing: None 
     •Surface tension: 36 μN/cm at 20° C. (measured by the platinum ring method, using Dynometer) 
     •Pot life: The ink remained unchanged in printability after standing at 20° C. for 24 hours. 
     COMPARATIVE EXAMPLE 
     A transparent resin composition was prepared in order to obtain a protective film. The second component polyol is obtained by sufficiently dissolving and mixing a solvent, antifoaming and leveling agent in the same kind and quantity as in Embodiment 6 to 100 pbw phenoxy resin (PKHH) in the same manner as in Example 1 except that the first and second components were replaced as follows: 
     •The first component: An adduct of toluene diisocyanate (TDI, which has an aromatic ring) and trimethylolpropane (TMP) (which is a trifunctional polyisocyanate, &#34;Coronate L&#34;, a product of Nippon Polyurethane Co., Ltd., containing 13.2 wt. % of NCO) where the amount of the isocyanate groups is equivalent to the amount of hydroxyl groups contained in the phenoxy resin. After this component is printed with mesh polyester screen as in Embodiment 6, it is cured in an oven at 160° C. for 10 minutes. 
     The thus obtained transparent protective film was found to have the following properties. 
     •Pencil hardness: 4H 
     •Adhesion: (according to cross-cut test) 
     100/100 immediately after application to the transparent electrode. 
     90/100 after conditioning at 80° C. and 90% RH for 2000 hours. 
     •Glass transition point (Tg): 113° C. for the cured film; 103° C. for the uncured film. 
     •Transparency: The sample decreased by 20% in transmission of light having a wavelength of 400 nm after conditioning at 85° C. for 1000 hours. 
     •Yellowing: Significant 
     •Surface tension: 38 μN/cm at 20° C. (measured by the platinum ring method, using Dynometer) 
     •Pot life: The ink became a gel (and hence incapable of screen printing) after standing at 20° C. for 24 hours. 
     Embodiment 7 
     A transparent resin composition was prepared in the same manner as in Embodiment 6 except that the first and second components were replaced as follows: 
     •The first component: A blocked isocyanate prepared by blocking the three isocyanate groups of &#34;Coronate HX&#34; (used in Embodiment 6) with the reactive hydrogen of methyl ethyl ketone oxime. 
     •The second component: Phenoxy resin &#34;PKHH&#34; (100 pbw) dissolved in solvents and incorporated with an anti-foam agent and leveling agent as in Embodiment 6. 
     The first and second components were thoroughly mixed such that the amount of hydroxyl groups is stoichiometrically equal to that of isocyanate groups. 
     The resulting resin composition (in the form of ink or paste) underwent screen printing through a 150-mesh polyester screen, which was followed by heat-curing in an oven at 160° C. for 10 minutes. 
     The thus obtained transparent protective film was found to have the following properties. 
     •Moisture resistance: high enough to prevent the degradation of the solar cell conditioned at 80° C. and 90% RH for 2000 hours. 
     •Pencil hardness: 4H 
     •Adhesion: (according to cross-cut test) 
     100/100 immediately after application to the transparent electrode; 
     100/100 after conditioning at 80° C. and 90% RH for 2000 hours. 
     •Glass transition point (Tg): 110° C. for the cured film; 103° C. for the uncured film. 
     •Transparency: The sample remained unchanged in transmission of light having a wavelength of 400 nm after conditioning at 85° C. for 1000 hours. 
     •Yellowing: None 
     •Pot life: The ink remained unchanged in printability after standing at 20° C. for 1 week. 
     •Curability: The ink gave no sign of degradation even after storage at 20° C. for 1 week, as indicated by the fact that the coating film of the ink gave an adequate degree of cross-linking when heat-cured in an oven at 160° C. for 10 minutes. 
     Embodiment 8 
     A transparent resin composition of the present invention was prepared as follows: 
     •The first component: Isocyanurate-linked isophorone diisocyanate (IPDI trimer) 
     •The second component: A mixture composed of 30 pbw of phenoxy resin (&#34;PKHH&#34; as used in Embodiment 6) and 70 pbw of bisphenol A-type epoxy resin (M n  =2900, &#34;Epikote 1007&#34;, a product of Shell Chemical Co., Ltd.) This mixture contains 6 wt. % of hydroxyl groups. This mixture (100 pbw) was thoroughly dissolved in a solvent composed of 50 pbw of cyclohexanone and 100 pbw of isophorone. The resulting solution was incorporated with 0.2-0.4 wt. % of anti-foam agent and leveling agent, which are a silicone compound represented by the formula (4) below. ##STR4## 
     The first and second components were thoroughly mixed such that the amount of hydroxyl groups is stoichiometrically equal to that of isocyanate groups. 
     The resulting resin composition was made into a transparent protective film for solar cells in the same manner as in Example 1. 
     The thus obtained transparent protective film was found to have the following properties. 
     •Moisture resistance: high enough to prevent the degradation of the solar cell conditioned at 80° C. and 90% RH for 2000 hours. 
     •Pencil hardness: 4-5H 
     •Adhesion: (according to cross-cut test) 
     100/100 immediately after application to the transparent electrode; 
     100/100 after conditioning at 80° C. and 90% RH for 2000 hours. 
     •Glass transition point (Tg): 115° C. for the cured film; 95° C. for the uncured film. Very slow decrease in elastic modulus at temperatures above Tg. 
     •Transparency: The sample remained unchanged in transmission of light having a wavelength of 400 nm after conditioning at 85° C. for 1000 hours. 
     •Yellowing: None 
     •Pot life: The ink remained unchanged in printability after standing at 20° C. for 24 hours. 
     As described in the foregoing, the resin composition according to the present invention is improved in resistance against moisture, heat, wear, and scratches, and is also improved in surface hardness, adhesiveness, printing suitability, and laser processability. 
     In particular, a resin composition having excellent productivity and processability satisfying the requirements for an interlayer insulating film of a flexible amorphous silicon solar cell and the like is developed. 
     Furthermore, the transparent resin composition of the present invention has the following characteristics: 
     •Moisture resistance: High enough to prevent the solar cell from degradation even after conditioning at 80° C. and 90% RH for 2000 hours. 
     •Heat resistance: High enough to prevent thermal deformation and deterioration in insulation and appearance despite the fact that the protective film near the electrodes is subject to heat compression at the time of bonding to lead wires and flexible printed circuits. This effect is due to the high Tg resulting from curing. 
     •Wear resistance, scratch resistance, and surface hardness: Owing to its high surface hardness and wear resistance, the protective film protects the solar cells from scratches which otherwise occur during the roll-to-roll process. This contributes to good appearance and high yields. 
     •Weather resistance: After an outdoor exposure test, the protective film did not show yellowing, loss of clarity, and adverse effect on solar cells. 
     •Flex resistance: Flexible solar cells with the protective film remain flexible, with thin films firmly sticking to the substrate, and show no clacking. 
     •Printability: The resin composition in the form of ink has a surface tension lower than 40 μN/cm at 20° C. owing to the additive (silicone or acrylic compound) in a small amount (0.001-3 wt. %) which improves the anti-foaming and leveling properties. With a low surface tension, the ink is suitable for screen printing. 
     While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof. For example, although solar cells are described as an example, the resin component of the present invention may be used in other types of electronic devices, for example, liquid crystal displays or image sensors and the like.