Polymer-Assisted Crystallization and Defect Passivation in Planar Wide-Bandgap FAPbBr₃ Perovskite Solar Cells

Wide-bandgap lead bromide perovskites such as FAPbBr₃ are promising candidates for tandem solar cells and high-voltage optoelectronic applications, yet their performance is limited by surface and bulk defects that induce severe nonradiative recombination and limit stability. In this work, we present a defect passivation and crystallization control strategy by incorporating poly-(methyl methacrylate) (PMMA) into the antisolvent during FAPbBr₃ film fabrication. PMMA treatment leads to improved film morphology with larger grains, reduced surface roughness, and enhanced crystallinity. FTIR analysis reveals that the carbonyl groups in PMMA coordinate with undercoordinated Pb²⁺ ions, effectively passivating electronic trap states. Photothermal deflection spectroscopy (PDS) shows reduced sub-bandgap absorption and lower Urbach energy, indicating suppressed deep-level defects and reduced energetic disorder. Enhanced photoluminescence intensity, prolonged carrier lifetimes, and decreased trap densities further confirm suppressed nonradiative recombination. As a result, PMMA treatment increases Voc by over 100 mV and improves power conversion efficiency by more than 1%, achieving a Voc of up to 1.510 V with reduced hysteresis and improved ambient stability. These findings demonstrate the effectiveness of polymer-assisted strategies for improving both efficiency and stability of wide-bandgap perovskite solar cells, offering a pathway toward high-voltage and tandem photovoltaic applications.