A study of bandgap-graded CZTGSe kesterite thin films for solar cell applications

Cu2ZnSn(S,Se)4 kesterite materials are a sustainable and harmless alternative to conventional Cu(In,Ga)Se2 (CIGS) and CdTe absorbers for thin-film photovoltaics but are still lacking efficiency. This study presents the realization of bandgap grading in Cu2Zn(Sn1xGex)Se4 (CZTGSe) kesterite thin films via the incorporation of Ge to partly substitute Sn, and their mutual segregation along the absorber profile. Bandgap values at the front and rear interfaces are, respectively, estimated to be around 1.10 eV and 1.38 eV, based on the Ge–Sn ratio. The kesterite deposition relies on a sequential process involving evaporating metallic precursors then annealing in Se vapour with an optimized temperature profile to favour crystallization and growth of micron-size grains. Further work is needed to solve the problem of the limited open-circuit voltage (Voc) and fill factor of in-house devices. However, the Ge-alloying method proposed in this work to obtain a continuous increase of the kesterite absorber bandgap towards its bottom surface could be applied to possibly enhance the performance of other kesterite solar cells.