Material and Interface Design of Colorless, Heat-Resistant Fluorinated Polyimides for Flexible and Rigid Perovskite Solar Cells

To achieve efficient solar-to-electricity conversion, third-generation photovoltaics represented by perovskite solar cells (PSCs) have attracted tremendous interest. However, perovskites are vulnerable to humidity, oxygen and heat, and insufficient device stability hinders their commercialization. Polyimides (PIs), polymers with facile preparation and superior comprehensive properties, are widely used in electronics for their great mechanical strength, high-temperature tolerance and biocompatibility. PIs exhibit good moisture and oxidation resistance under harsh conditions, which well offsets perovskites’ intrinsic environmental-sensitivity drawback and makes PIs promising candidates for solving the stability bottleneck of PSCs. Herein, three fluorinated transparent PIs were synthesized through molecular-structure design and adopted as flexible substrates and auxiliary modification layers for self-assembled monolayers (SAMs). The 6FDA-TFMB film shows a maximum visible-light transmittance of 90.7%, a 5% weight-loss temperature above 400 °C and a glass-transition temperature of 339 °C, and flexible PSCs based on it obtain a power conversion efficiency (PCE) of 18.06%. Hydroxyl-containing 6FDA-6FAP was further used to construct PI/SAM composite hole-selective layers, which suppress SAM aggregation, increase SAM anchoring coverage on ITO, optimize perovskite crystallization and passivate buried-interface defects. Rigid 1.68 eV wide-bandgap perovskite devices with this composite interface deliver a PCE improvement from 22.48% to 23.15% with greatly enhanced thermal and environmental stability. This work demonstrates that transparent heat-resistant PIs can act as both high-performance flexible substrates and synergistic buried-interface regulators, providing new material strategies for efficient, stable perovskite photovoltaics.