In-depth carrier recombination mechanism at grain boundaries in flexible kesterite thin film solar cells with 12.2% efficiency

Kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe), an earth-abundant and non-toxic material, is widely regarded as a promising light absorber for photovoltaic devices. Extensive research has focused on understanding recombination mechanism at the surface grain boundaries (GBs) which detrimentally impacts device performance, while their behavior within the material's bulk remains relatively unexplored. In this study, in-depth carrier recombination mechanism was investigated using atomic force microscopy-based characterizatio. A flexible CZTSSe thin film solar cell with an efficiency of 12.2% was prepared using a sputtering method on a Mo foil substrate. In particular, the sample was subjected to mechanical etching using a focused-ion beam to expose a plane at the specific depth. After that, electrostatic potential at the revealed surface was obtained by Kelvin probe force microscopy. At the surface and subsurface, formation of both conduction (E_C) and valence (E_V) upward bending was observed due to the widen band gap and increased charged defects at the GBs than in intra grains. This type of band bending is efficient for carrier separation as it repels electrons from the GBs while collects holes. [1] Additionally, large upward band bending contributed to a formation of conduction path along the intra grains. In contrast, center of absorber presented E_C downward and E_V upward band bending besides the E_C and E_V upward band bending, that is unfavorable as it can be the recombination sites. Hence, in-depth carrier recombination mechanism exhibited non-uniform behavior between the materials’ surface and the bulk. In conclusion, the uniform E_C and E_V upward band bending at the GBs across the entire absorber layer is desirable to minimize the recombination. [2] Additionally, our research suggested the in-depth characterization methods that can be applied other materials beyond the kesterite thin film solar cells.<br/> <br/>[1] H. K. Park et al., npj Flexible Electronics, 6 (2022) 91.<br/>[2] D.-H. Son, H. K. Park, and K.-J. Yang et al., Carbon Energy, (2023) Accepted.