Generalized current-voltage analysis and efficiency limitations in non-ideal solar cells: Case of Cu2ZnSn(SxSe1−x)4 and Cu2Zn(SnyGe1−y)(SxSe1−x)4

Detailed electrical characterization of nanoparticle based Cu2ZnSn(SxSe1−x)4 (CZTSSe) and Cu2Zn(SnyGe1−y)(SxSe1−x)4 (CZTGeSSe) solar cells has been conducted to understand the origin of device limitations in this material system. Specifically, temperature dependent current-voltage analysis has been considered, with particular application to the characterization of solar cells with non-ideal device behavior. Due to the presence of such non-ideal device behavior, typical analysis techniques—commonly applied to kesterite-type solar cells—are found to be insufficient to understand performance limitations, and an analysis methodology is presented to account for the non-idealities. Here, the origin of non-ideal device behavior is chiefly considered in terms of electrostatic and band gap potential fluctuations, low minority carrier lifetimes, temperature dependent band edges, high surface/bulk recombination rates, and tunneling enhanced recombination. For CZTSSe and CZTGeSSe, the main limitations to improved device performance (voltage limitations) are found to be associated with significant EA deficits (EA–EG) at 300 K, large ideality factors, and voltage-dependent carrier collection, which we associate with the bulk material properties of the absorbers. The material origin of these non-ideal electrical properties is considered. Additionally, for CZTGeSSe, the effect of Ge-incorporation on the electrical properties of the solar cells is discussed, with improvements in the electrical properties characterized for the Ge-alloyed devices.