Ge-Alloyed Kesterite Thin-Film Solar Cells: Previous Investigations and Current Status - a Comprehensive Review

Abstract

The incorporation of Ge into kesterite thin-film absorbers for photovoltaics (PV) applications is thoroughly reviewed. Kesterite materials constitute a promising and critical raw material-free alternative to other inorganic thin-film PV compounds such as Cu(In,Ga)Se2 and CdTe. The interest of Ge alloying is to solve the critical open-circuit voltage (Voc) deficit for kesterite solar cells, still unresolved and under strong debate. First, the substitution of Sn by Ge from 0 to 100 % in the composition of Cu2Zn(Sn1−x,Gex)(Sy,Se1−y)4 absorbers is largely discussed, with a complete overview on the existing literature. It is concluded that fine composition tuning is essential to ensure the Ge-induced enhancement of morphology and single-phase growth through modified reaction pathway. In this regard, choosing between vacuum- and non vacuum-based deposition methods also plays a crucial role, since the former allows further up-scaling whereas the latter leads to higher thin-film quality. Second, at the solar cell level, Se-rich devices with less than 40 % Ge currently exhibit the lowest Voc and fill factor deficits with the highest efficiencies beyond 13 %. This is mainly due to defect and band tail compensation as well as graded bandgap and enlarged crystalline grains. This study unveils encouraging prospects for Ge-boosted kesterite PV devices.