Surface Modification in CsPb_0.5Sn_0.5I₂Br Inorganic Perovskite Solar Cells: Effects of Bifunctional Dipolar Molecules on Photovoltaic Performance

Inorganic tin–lead binary perovskites have piqued the interest of researchers as effective absorbers for thermally stable solar cells. However, the nonradiative recombination originating from the surface undercoordinated Sn 2+ cations and the energetic offsets between different layers cause an excessive energy loss and deteriorate the perovskite device’s performance. In this study, we investigated two thioamide derivatives that differ only in the polar part connected to their common benzene ring, namely, benzenecarbothioamide and 4-fluorophenylcarbothioamide (F-TBA). These two molecules were implemented as modifiers onto the inorganic tin–lead perovskite (CsPb 0.5 Sn 0.5 I 2 Br) surface in the perovskite solar cells. Modifiers that carry C═S and NH 2 functional groups, equipped with lone electron pairs, can autonomously associate with surface Sn 2+ through coordination and electrostatic attraction mechanisms. This interaction serves effectively to passivate the surface. In addition, due to the permanent dipole moment of the intermediate layer, an interfacial dipole field appears at the PCBM/CsPb 0.5 Sn 0.5 I 2 Br interface, reducing the electron extraction potential barrier. Consequently, the planar solar cell with an ITO/PEDOT:PSS/CsPb 0.5 Sn 0.5 I 2 Br/PCBM/BCP/Ag layered structure featuring an F-TBA surface post-treatment demonstrated a noteworthy power conversion efficiency of 14.01%. Simultaneously, after being stored for 1000 h in an inert atmosphere glovebox, the non-encapsulated CsPb 0.5 Sn 0.5 I 2 Br solar cells managed to preserve 94% of their original efficiency.