Resonance-induced electron delocalization for efficient lead-free perovskite solar cells

Tin perovskites have emerged as promising lead-free alternatives for eco-friendly perovskite solar cells; however, their device performance still lags behind that of lead-based counterparts, mainly due to poor thin-film quality, high defect densities, and the intrinsic instability Sn2+. Here, we report a resonance-induced electron delocalization strategy to fundamentally stabilize tin perovskite by enhancing Sn-centered interactions using trithiocyanuric acid (Cy-SH) and cyanuric acid (Cy-OH) molecules as interfacial layer. We further demonstrate the electron delocalization between the N–C = O and N = C–OH tautomeric configurations of Cy-OH at the perovskite buried interface, which strengthens its interaction with uncoordinated Sn atoms, thereby significantly suppressing non-radiative recombination and enhancing carrier extraction and transport. As a result, the best-performing device achieves a power conversion efficiency (PCE) of 12.1%, maintaining 86% of its initial efficiency after 1000 h aging in N2 containing 50-100 ppm O2. This work demonstrates that tautomeric molecules can be rationally designed and utilized to regulate tin perovskite crystallization and defect states, thereby providing guidelines for the development of high-performance lead-free photovoltaics. Tin perovskites are promising lead-free alternatives for eco-friendly perovskite solar cells but are limited by defects and Sn2+ instability. Here, the authors report a resonance induced strategy to stabilize tin perovskites using trithiocyanuric and cyanuric acid interfacial layers.