Prospects of wide-gap chalcopyrites for thin film photovoltaic modules

The development of a standard process for the fabrication of Cu(In,Ga)Se, thin film solar cells by coevaporation leads to highly efficient devices with band-gaps in the range of 1-1.2 eV. The comparison of recent module efficiencies and the corresponding performance of small test cells demonstrates the high uniformity of the cell performance over large areas. In view of their higher open circuit voltage the use of absorbers with higher band-gaps is advantageous for the production of solar modules. However, to date the efficiencies of cells employing chalcopyrite absorbers with higher band-gaps lag behind. A consistent picture of the problems encountered in the case of wide-gap absorbers is drawn on the basis of results from photoelectron spectroscopy in conjunction with electrical measurements on the devices. This picture appoints the high efficiency of the cells with lower band gaps to the successful suppression of recombination at the heterointerface, resulting from a stabilized type inversion at the surface of the absorber. The formation of such a type-inverted surface appears to be a prerequisite for the achievement of high efficiency with wide band-gap absorbers.