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(S_xSe_1-x)₄ (CZTSSe) and Cu₂Zn(Sn_yGe_1-y)(S_xSe_1-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 E_A deficits (E_A-E_G) 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.