Wide band-gap CuIn _1-XGa _XSe ₂ based chalcopyrite absorbers for tandem cell applications

Device efficiencies of 20% have been achieved in CIGS-based chalcogenide absorbers, however progress has slowed in recent years. There has been a substantial interest in tandem cell structures to achieve a discrete jump in device efficiencies for these materials. Top cell absorbers with band-gap of 1.6 eV to 1.7 eV are the optimum choice for the wide band-gap top cell. CGS, CIGSS, CIAS and similar materials have all been investigated. However, for all of these materials the grain size decreases and the trap density rises sharply as the material approaches the required band-gap. This poses significant problems of interface recombination and increased bulk trap states which impede transport in the absorber. In this paper we propose a CIGS-based material developed by doping CIGS with moderate amount of aluminum (CIAGS) thus increasing the band-gap. A single step process with constant Cu, In, Ga, Al, Se fluxes has been used. The films are grown in the copper deficient regime throughout the deposition. Band-gap measurements were done by calculating the absorption coefficients using transmission spectroscopy. A substantial increase in the band-gap was observed for moderate amounts of Al and Ga in CIAGS films compared to CIGS. We investigated the grain structure of the films and find that moderate to large grains were observed, even for bandgaps as large as 1.5 eV.