Surface modification with S-benzylisothiourea hydrochloride for enhancing the efficiency and stability of carbon-based CsPbI₃ perovskite solar cells

Carbon-based CsPbI₃ perovskite solar cells (C-PSCs) have exhibited great application potential because of their suitable optical bandgap and high chemical stability. However, the efficiency of CsPbI₃ C-PSCs still lags behind the theoretical limits, mainly because of issues such as high defect density, non-ideal energy level alignment, and susceptibility to humidity or thermal stress. Herein, a multifunctional molecule, S-benzylisothiourea hydrochloride (BSH), is introduced to modify CsPbI₃ perovskite films. It is demonstrated that the BSH treatment can enhance moisture resistance and enable BSH+ to react with residual PbI₂, forming a low-dimensional (BSH)₂PbX₄ structure that passivates surface defects and improves energy-level alignment at the CsPbI₃/carbon interface. Concurrently, Cl− distributes throughout the surface and buried interface, facilitating film reconstruction and passivating defects at both the surface and the TiO₂/perovskite interface. The optimized devices achieve a PCE of 18.03% with a fill factor of 80.94%, along with enhanced stability, retaining 76.65% of initial PCE after 320 h at 85 °C in 20-30% RH and 88.73% after 1350 h at 25 °C in 20-30% RH.