Decoupling Stress Relief from Electrical Compromise via Carboxylated PMMA Nanospheres for Flexible Perovskite Solar Cells

Flexible perovskite solar cells (f-PSCs) hold great promise for next-generation wearable electronics, portable power sources and even space power systems. However, these applications are fundamentally limited by optoelectronic degradation under mechanical strain. Although incorporating continuous polymeric networks enhances mechanical robustness, these insulating dielectric materials inevitably introduce carrier-transport barriers, leading to severe charge accumulation and a compromised fill factor (FF). Herein, we introduce 50-nm carboxyl-functionalized polymethyl methacrylate (PMMA) nanospheres (CPNs) to fundamentally decouple mechanical stress dissipation from interfacial carrier transport kinetics in inverted f-PSCs. Unlike continuous dielectric buffer layers, the CPNs self-assemble into a discontinuous nano-island network at the buried interface, maintaining unobstructed conductive pathways for efficient cross-interfacial charge transfer. Mechanically, the elastic nano-islands modulate the local strain field to efficiently dissipate mechanical and thermal stresses. Furthermore, the localized electrostatic field induced by the negatively charged carboxyl groups spatially repels electrons and accelerates hole extraction, profoundly suppressing non-radiative interfacial recombination. Consequently, the optimized f-PSCs achieve a power conversion efficiency (PCE) exceeding 26% with a remarkably high FF of 0.845. The devices demonstrate outstanding structural and operational stability, exhibiting negligible degradation after 5,000 bending cycles at a 6-mm radius and retaining 96.3% of their initial efficiency after 200 thermal cycles (−60 to +80 °C). This work provides a robust micromechanical and optoelectronic strategy for highly reliable flexible photovoltaics.