Judicious Fluorination of Perovskite Quantum Wells Enables Over 25% Efficiency in Inverted Solar Cells

Abstract

The photovoltaic performance of inverted perovskite solar cells (PSCs) is often hindered by trap-induced non-radiative recombination and photochemical degradation occurring at the upper interfaces and the grain boundaries of perovskite films. Herein, ortho-, meta-, and para-isomers of fluorophenylethylammonium iodine (F-PEAI) organic spacer molecules are evaluated for the construction of perovskite quantum wells (2D or quasi-2D, PQWs) to encapsulate 3D perovskites. Among the three variants, p-F-PEAI leads to the most symmetric charge distribution and the weakest steric hindrance which resulting in reinforced interactions with PbI2 and perovskite, the enhanced out-of-plane orientation is confirmed by Grazing incidence wide-angle X-ray scattering (GIWAXS) results. Density functional theory and crystal orbital Hamilton population (COHP) calculations further confirm that p-F-PEAI engages most strongly with the perovskite structure. Moreover, transmission electron microscopy (TEM) characterization is used to illustrate that p-F-PEAI-assisted 2D PQWs effectively passivate both the grain boundaries and surfaces of perovskites. This configuration facilitates effective surface passivation, improves charge carrier transport, and significantly suppresses non-radiative recombination. The resultant inverted PSCs achieve an excellent power conversion efficiency (PCE) of 25.03% with a fill factor (FF) of 85.11%. The unencapsulated devices exhibit enhanced long-term stability under ambient environments and continuous light illumination.