1.  Field of the Invention
The present invention relates to a solar cell which is formed on a substrate made from an insulator such as glass.  In particular, the present invention provides an amorphous solar cell suitable for use under circumstances of low illuminance, for example, where a white fluorescent light is used. 
2.  Prior Art
Referring to FIG.  1, an amorphous solar cell generally comprises an insulative substrate 11, having on the principal surface thereof a layered structure consisting of, in this order from the substrate side, a clear electrode 12, a photovoltaic conversion layer 13 comprising a PIN junction and comprising amorphous silicon, and a metallic electrode 14. 
The photovoltaic conversion layer 13 is fabricated by a glow discharge process, i.e., by decomposing silane and depositing the resulting product on the substrate having thereon the clear electrode.  A photovoltaic conversion layer having at least a thickness of about 1 .mu.m is required for an amorphous solar cell.  However, because poor adhesion of the amorphous silicon to the clear electrode 12 occurs due to, for example, the surface conditions and stains of the glass substrate and the clear electrode, fine pinholes 16 which penetrate the photovoltaic conversion layer 13 are often formed in the photovoltaic conversion layer. 
If a metal electrode 14 is formed on the photovoltaic conversion layer 13 comprising a pinhole 16, direct contact would be established between the metal electrode 14 and the clear electrode 12, because the pinhole would be filled with the material of the metal electrode.  The resulting portion of direct contact can no longer contribute to the photovoltaic conversion; more disadvantageously, it would provide a short circuit and allow a leakage current to flow through the pinholes 16.  This results in solar cells of poor quality. 
Amorphous solar cells are used not only in the outdoors under the sunlight, but also in the indoors under a light of low illuminance, i.e., under a fluorescent lamp having an illuminance of about 1,000 lux or lower.  In cases where the solar cells are used indoors, the influence of the leakage current which flows through the pinholes 16 is not negligible because the electric current which is generated in the solar cell decreases with lower illuminance.  This results in solar cells with extremely impaired output characteristics. 
Accordingly, the substrates are cleaned with great care before forming the photovoltaic conversion layer 13, such that no pinholes should be generated in the photovoltaic conversion layer However, it is still impossible to completely prevent the pinholes from being formed in the photovoltaic conversion layer 13.  It has been found that such pinholes are the cause for lowering of the product yield.