Impact of bulk properties and local secondary phases on the Cu2(Zn,Sn)Se4 solar cells open-circuit voltage

As the development of kesterite solar cells accelerates, the bottlenecks in device performance need to be identified and ways for their circumvention defined and developed. In this work, we use 2-dimensional (2D) numerical simulations to explore possible reasons for low open-circuit voltage (Voc) in Cu2(Zn,Sn)Se4 (CZTSe) solar cells. High defect density in the CZTSe absorber and at the CZTSe/CdS interface can be significant reasons for Voc deficit, but they do not explain all of the losses observed experimentally. Local deviation from stoichiometry could create secondary phases with a lower band gap compared to the absorber. These secondary phases can be severely harmful to Voc if located in the vicinity of the heterointerface and along the grain boundaries.