NOVELTY - A multi-bandgap solar cell device is fabricated by depositing layers of polycrystalline or amorphous semiconductor absorber material with bandgap over one of two opposing sides of a transparent substrate (10); and depositing window layers of polycrystalline or amorphous semiconductor material in low temperature conditions on the layer of absorber material.    USE - The method is used for fabricating multi-bandgap solar cell device for converting solar energy to electricity.    ADVANTAGE - The method allows the deposition of absorber layers under high temperature conditions without degrading or destroying previously deposited p/n junctions in the device.    DETAILED DESCRIPTION - Fabrication of a multi-bandgap solar cell device comprises: depositing a first layer of polycrystalline or amorphous semiconductor absorber material with a first bandgap over one of two opposing sides of a transparent substrate in high temperature conditions and depositing a second layer of polycrystalline or amorphous semiconductor absorber material with a second bandgap over another of the two opposing sides of the transparent substrate in high temperature conditions; and depositing a first window layer of polycrystalline or amorphous semiconductor material in low temperature conditions on the first layer of absorber material to form a first p/n junction and depositing a second window layer of polycrystalline or amorphous semiconductor material in low temperature conditions on the second layer of absorber material to form a second p/n junction.    An INDEPENDENT CLAIM is also included for a multi-bandgap solar cell comprising: a transparent substrate having first and second opposing surfaces; a first transparent conductor formed onto the first surface of the substrate and a second transparent conductor formed onto the second surface of the substrate; a first absorber layer of polycrystalline or amorphous semiconductor material with a first bandgap formed onto the first transparent conductor and a second absorber layer of polycrystalline or amorphous semiconductor material with a second bandgap formed onto the second transparent conductor; a first window layer formed onto the wide bandgap absorber to form a first p/n junction, and a second window layer formed onto the narrow bandgap absorber to form a second p/n junction; and a third transparent conductor layer applied to the first window to form a first solar cell, and a fourth transparent conductor layer applied to the second window to form a second solar cell.    DESCRIPTION OF DRAWING(S) - The figure shows a cross-section of a bifacial tandem solar cell device with polycrystalline or amorphous semiconductor heterojunctions.    Substrate (10)    Solar cells (20, 40)    Grid (30)    Lead (31)