Enhancing the Photovoltaic Performance of Carbon-Based Hole-Transport-Layer-Free CsPbI₂Br Solar Cells via Trifluoromethyl Thianthrenium Triflate Additive

Hole-transport-layer (HTL)-free CsPbI2Br solar cells based on carbon electrodes have emerged as a promising photovoltaic architecture, owing to their good thermal stability and low cost. However, the power conversion efficiency (PCE) is relatively low, mainly due to the inferior crystalline quality of CsPbI2Br, mismatched energy levels, and poor contact between the perovskite layer and the carbon electrode. Herein, we introduce 5-(trifluoromethyl)-5H-thianthren-5-ium trifluoromethanesulfonate (TT-CF3 OTf) as an efficient precursor additive to regulate the crystallization and passivate defects in CsPbI2Br. The TT-CF3 OTf modification can effectively enhance the crystallinity and reduce the surface roughness of the CsPbI2Br layer. Density functional theory calculations reveal that the presence of OTf– can effectively passivate uncoordinated Pb2+, thereby suppressing nonradiative recombination. Benefiting from these positive effects of the TT-CF3 OTf, the demonstration device with a simple configuration of ITO/SnO2/CsPbI2Br/C delivers an enhanced PCE of 14.86%, significantly improved from 11.80% for the control device without TT-CF3 OTf. Moreover, the TT-CF3 OTf-modified CsPbI2Br device exhibits improved thermal and light stability.