The physics of Vbi-related IV crossover in thin film solar cells: Applications to ink deposited CZTSSe

IV measurements of thin film solar cells often show a crossover between the illuminated and dark curves. Crossover can occur for several different reasons. In this paper, we explore crossover in CZTSSe solar cells fabricated using nanocrystalline ink deposition with selenization and compare it to crossover in ink-based CIGSSe solar cells. Crossover in CIGSSe appears to be related to traps, as is commonly observed, but crossover in CZTSSe appears to be due to a different mechanism. Using numerical simulation, we show that crossover can arise from a simple explanation that is common to solar cells with different structures and closely related to the built-in potential of the device. Using IVT and CV measurements, we show that both simulations and experimental analysis point to a cross-over voltage in our CZTSSe cells that is directly related to the built-in voltage of the device, and which may play a role in limiting the open-circuit voltage.