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Interfacial trap-assisted non-radiative recombination and residual stress impede the further increase of power conversion efficiency (PCE) and stability of the methylammonium-free (MA-free) perovskite solar cells (PSCs). Here, we report an interfacial defect passivation and stress release strategy through employing the multi-active-site Lewis base ligand (i.e., (5-mercapto-1,3,4-thiadiazol-2-ylthio)­acetic acid (MTDAA)) to modify the surface and grain boundaries (GBs) of MA-free perovskite films. Both experimental and theoretical results confirm strong chemical interactions between multiple active sites in the MTDAA molecule and undercoordinated Pb2+ at the surface or GBs of perovskite films. It is demonstrated theoretically that multi-active-site adsorption is more favorable thermodynamically as compared to single-active-site adsorption, regardless of PbI2 termination and formamidinium iodide (FAI) termination types. MTDAA modification results in much reduced defect density, increased carrier lifetime, and almost thoroughly released interfacial residual stress. Upon MTDAA passivation, the PCE is boosted from 20.26% to 21.92%. The unencapsulated device modified by MTDAA maintains 99% of its initial PCE after aging under the relative humidity range of 10–20% for 1776 h, and 91% after aging at 60 °C for 1032 h.

Bulk and interfacial nonradiative recombination hinders the further enhancement of power conversion efficiency (PCE) and stability of SnO2-based planar perovskite solar cells (PSCs). To date, it is still a huge...

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We modified perovskite/Spiro‐OMeTAD interface by using two novel phosphonium salts containing PF6− counter anion (i.e., ClTPPPF6 and BrTPPPF6). The cation and anion in phosphonium salts possess not only ionic bonds but also coordination bonds with perovskites. The anion and cation vacancies at the surface and GBs of perovskite films can be filled by phosphonium cations and PF6− anions, respectively, resulting in reduced defect density and prolonged carrier lifetimes. The stronger chemical interaction and accordingly better defect passivation were certified for BrTPPPF6 than ClTPPPF6. As a result, the devices modified by ClTPPPF6 and BrTPPPF6 deliver a PCE of 21.73% and 22.15%, respectively, which far exceed 20.6% of the control device. The unsealed BrTPPPF6 modified device maintains 98.2% of its initial efficiency value after thermal aging of 1320 h whereas merely 84.7% for the control device. 96.4% of its original efficiency was retained for BrTPPPF6‐modified device after ambient exposure of 2016 h.