Flexible Kesterite Solar Cells—Optoelectronic Characteristics Implied by Mechanical Deformation

We will present recent progress of photovoltaic devices using chalcogen-based compounds: kesterites and chalcopyrites. 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 close to 12%. 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 with and without Na deposited on flexible Mo foil 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 changed 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. 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, Urbach energy was obtained from absorption coefficient function. Attribution of Na in transport of photogenerated carriers will be investigated to clarify the robust carrier transport under mechanical stress compared to the CZTSSe without Na.