Carrier Recombination and Open-Circuit Voltage Loss in Na-Engineered Cu₂ZnSn(S,Se)₄ Flexible Solar Cells

Kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe), promising earth-abundant materials for thin film solar cells, has been widely studied and active research efforts raise the efficiency of flexible kesterite solar cells to over 11% [1]. To fabricate the robust and highly efficient flexible thin film solar cells, understanding the changes in device properties under mechanical stress is crucial. In this study, transport of photogenerated carriers with mechanical stress was characterized. CZTSSe samples deposited on flexible Mo foil with and without Na element were prepared and solar cell parameters after the mechanical bending were measured. Degradation of open-circuit voltage (V_OC) was varied by the direction of mechanical bending and the existence of Na elements. Photo-assisted Kelvin probe force microscopy was utilized to characterize the transport of photogenerated carriers, and it was found that the magnitude of surface photovoltage (SPV) degraded under mechanical bending state. It refers the degradation of local V_OC and transport of the photogenerated carriers was limited with mechanical stress resulting in decrease of V_OCof devices [2]. In addition, CZTSSe sample with Na doping showed the larger SPV under bending than sample without Na which suggest the passivation of defects and prevention of damage from bending. To quantify the disorder in energy band structure under mechanical bending, its optical properties were obtained through photoluminescence. To clarify the robust carrier transport under mechanical stress, attribution of Na in transport of photogenerated carriers will be investigated further.