THIN-FILM SOLAR-CELLS

The R & D status of cells and modules based on hydrogenated amorphous silicon (a-Si:H) and those based on CdTe and CuInSe2 is reviewed. The stability of a-Si:H solar cells is still a major concern, Improvements have been achieved on air empirical basis by application of multijunction structures, optimization of interfaces, etc, Stabilized efficiencies of close to 10 % have been reported. In parallel, the introduction of the 'defect-pool model' led to remarkable progress in understanding; it follows that a-SiGe:H instead of a-Si:H should be used for the i-layer (absorber). Improved cell engineering concepts, however, such as enhancement of the built-in electric field via reduction of the i-layer thickness and/or folded structures, are believed to be more promising. Polycrystalline thin-film cells based on CdTe and CuInSe2 ave not affected by inherent degradation mechanisms, The specific properties of these materials demand heterojunctions, and particular problems arise due to the polycrystallinity of the films and to the lattice mismatch and mismatch of the electronic band structures of the materials involved. These are discussed in conjunction with measures currently applied for optimizing solar cell performance. Both cell types exhibit efficiencies in the range 16-17 %. Estimations of, production costs and energy payback times of thin-film photovoltaic modules are reviewed (even below 1 US$ W-p(-1) and as low as 4 months, respectively) and environmental concerns, especially for Cd-containing cells, are summarized.