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

Hole-transport-layer (HTL)-free CsPbI 2 Br 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 CsPbI 2 Br, 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-CF 3 OTf) as an efficient precursor additive to regulate the crystallization and passivate defects in CsPbI 2 Br. The TT-CF 3 OTf modification can effectively enhance the crystallinity and reduce the surface roughness of the CsPbI 2 Br layer. Density functional theory calculations reveal that the presence of OTf – can effectively passivate uncoordinated Pb 2+, thereby suppressing nonradiative recombination. Benefiting from these positive effects of the TT-CF 3 OTf, the demonstration device with a simple configuration of ITO/SnO 2 /CsPbI 2 Br/C delivers an enhanced PCE of 14.86%, significantly improved from 11.80% for the control device without TT-CF 3 OTf. Moreover, the TT-CF 3 OTf-modified CsPbI 2 Br device exhibits improved thermal and light stability.