Progress Toward Efficient Wide‐Gap Cu (In,Ga)(S,Se)$_2$ Thin‐Film Solar Cells

In this paper, wide-gap Cu (In,Ga)(S,Se)$_2$ thin-film solar cells are studied in view of their performance, limitations, and opportunities for further optimization. To this end, a wide variety of properties is investigated. This includes the role of gallium gradients, grain size effects, electronic properties, doping metastabilities, and minority carrier lifetime. Particular emphasis is placed on the impact of alkali atoms. A comparison of surface, interface, and grain boundary chemistry shows systematic atomic accumulation and depletion effects. This leads to electronic modifications in the grain boundary regions of the absorber. Heavy alkali treatments also influence the device properties, giving a clear boost of open-circuit voltage. By the combination of different experimental results, this positive open-circuit voltage effect has been explained in terms of reduction of interface recombination. The latter effects are discussed in view of a possible alkali–indium–selenium bond formation at the interface between the absorber and the buffer layer. The properties of a 14.2%-efficient Cu (In,Ga)Se$_2$-based device with [Ga]/([Ga] + [In]) = 0.8 and a wide optical band gap of 1.48 eV are investigated, also in view of further opportunities for improvement.