Multifunctional Thiophene Cascading SnO₂/Perovskite Interfaces for Efficient and Stable MAPbI₃ Photovoltaics

The power conversion efficiency (PCE) and stability of n-i-p perovskite solar cells (PSCs) are significantly affected by inherent defects of SnO₂ and perovskite layers. In this work, we incorporate 2-bromo-3-thiophenic acid (BrThCOOH) as a multifunctional passivant to simultaneously passivate the defects of SnO₂ surface and perovskite layer. BrThCOOH permeates evenly into the MAPbI₃ and coordinates with Pb²⁺ and iodine vacancies (V_I⁺) to reduce surface defect density and inhibit the decomposition of MAPbI₃. Carboxylic acid effectively passives the oxygen vacancy on the surface of SnO₂ through coordination bonds, reducing the probability of electron capture by SnO₂ surface defects, thus contributing to electron transport in device. The interaction of BrThCOOH with MAPbI₃ and SnO₂ surfaces leads to an upward shift in energy levels, reducing energy loss during charge migration. The optimal MAPbI₃ device with BrThCOOH-modified SnO₂ (T-SnO₂) reveals an improved PCE of 21.12%, much higher than that of the control one (19.12%). The hydrophobicity of BrThCOOH-modified MAPbI₃ is also improved, which is beneficial to the durability of the device. After 100 h of storage in the environment, the generated PSCs maintain their initial PCE of 75%, demonstrating excellent long-term stability without any encapsulation.