including an amorphous semiconductor film 88 compensated or altered in the manner previously described the film has n type conductivity and it is provided with a highly doped n portion 86 where it contacts a metal substrate 85 so as to provide an ohmic contact 87 for the amorphous film 88 the device also includes a semiconductor film 89 joining the film 88 at 90 the film 89 may be of a different material than the film 88 and has a high band gap so as to be transparent to solar radiation impinging the same the differences in band gap between the films 88 and 89 provide a large potential barrier in the vicinity of the junction 90 which forms a heterojunction for the generation of a large open circuit voltage the differences in the band gap produce band bending at the heterojunction 90 the semiconductor material 89 is heavily doped so as to provide substantially ohmic contact with electrodes 91 because the semiconductor film 89 is highly doped the electrical field generated by the potential barrier at the heterojunction 90 will not penetrate substantially into the film 89 but will penetrate substantially into the film 88 accordingly most of the space charge region i e the depletion region will be generated in the semiconductor film 88 solar radiation which is absorbed within the depletion region in the semiconductor film 88 will generate electron hole carriers which will be swept by the.