Optimising the open-circuit voltage of Cu(In,Ga)S2 solar cells—design and analysis

Exploiting the potential of wide-gap chalcopyrites requires a heterojunction design where the open-circuit voltage is determined by the band gap of the absorber, i.e. a design that avoids barrier reduction by tunneling and/or interface recombination. Furthermore, any variation of the band gap within the absorber should be such that it assists in minority carrier generation and collection while maintaining a high barrier for majority carrier recombination. On the basis of the analysis of films prepared by different methods and of solar cells made from them, we show to which extent these requirements are already fulfilled. In Cu(In,Ga)S2 wide-gap absorber films the band gap is mainly determined by the Ga/(In+Ga) ratio. The Ga/(Ga+In) depth profile depends on the phase formation sequence during preparation. Up to now, barrier reduction could not be totally avoided. Our measurements identify the charge density at internal surfaces as an important parameter governing the recombination mechanism.