Regulating Strain and Suppressing Defect States Through Polymer Ionization and Chemical Chelation for Efficient Inverted Flexible Perovskite Solar Cells

Flexible perovskite solar cells (FPSCs) have great promise for applications in wearable technology and space photovoltaics. However, the unpredictable crystallization of perovskite on flexible substrates results in significantly lower efficiency and mechanical durability than industry standards. A strategy is investigated employing the polymer electrolyte poly(allylamine hydrochloride) (PAH) to regulate crystallization and passivate defect states in perovskite films on flexible substrates. The weakly acidic precursor allows PAH to undergo partial ionization, leading to the protonation of some ─NH₃ ⁺ groups into ─NH₃ ⁺. Simulations and experimental results indicate that multifunctional PAH forms strong chemical interactions with precursors of perovskite materials including Formamidine Hydroiodide (FAI) and Lead Iodide (PbI₂), facilitating homogenous nucleation and growth of crystals of perovskite films. High-resolution transmission electron microscopy (HR-TEM) reveals that PAH strongly anchors to grain boundaries (GBs), consistent with findings from photo-induced force microscopy-based infrared spectroscopy (PiFM-IR). PAH improves the uniformity distribution of Young's modulus between the grains and GBs, facilitating stress relief in the perovskite films. Therefore, the champion efficiency of PAH-modified FPSCs reaches 24.19% and exhibits strong bending durability, retaining 87.9% of their initial efficiency after 2500 bending cycles (r = 5 mm), demonstrating their practical application potential in outdoor wearable electronic products.