Improving redox reactions of Spiro-OMeTAD via p-type molecular scaffold to reduce energy loss at Ag-electrode in perovskite solar cells

2,2′,7,7′-Tetrakis(N,N-di(4-methoxyphenyl)amino)-9,9′-spirobifluorene (Spiro) is an essential hole-transport material used in perovskite solar cells (PSCs). However, the redox reaction of Spiro and its impact at the interface with the metal electrode are not yet fully understood. In this study, we introduced a crystalline additive (CA) to regulate the redox process of Spiro and its interface with an Ag electrode. Our findings indicate that CA functions as a molecular scaffold, improving the crystallinity and stability of radicals in Spiro throughout the entire redox reaction. This enhancement increases the hole mobility of Spiro and strengthens the internal electric field, thereby improving hole extraction and transport efficiency at both interfaces. Moreover, the optimized redox reaction of Spiro reduces energy loss at the Ag electrode, significantly boosting the power conversion efficiency to 25.21%. Furthermore, CA mitigates the aggregation of lithium salt and enhances the stability of the device. Our findings contribute to a deeper understanding of hole-transport mechanisms of Spiro and emphasize the importance of reducing energy loss at the Spiro/Ag electrode interface in PSCs.