Effect of FABr over-stoichiometry on the morphology and optoelectronic properties of wide-bandgap FAPbBr3 films

In this study, we investigate the impact of formamidinium bromide (FABr) over-stoichiometry in the precursor solution on the optoelectronic properties and morphology of the resulting films of formamidinium lead bromide. Optical characterization, including steady-state absorption, photoluminescence (PL), and femtosecond transient absorption spectroscopy, reveals a systematic blueshift in emission energy with increasing FABr content, attributed to compositional variations induced by FABr excess. The excess of FABr also induces the reduction of defects associated with radiative recombination. Indeed, the stoichiometric film exhibits a PL band due to donor–acceptor pair (DAP) recombination as identified by the typical blueshift for increasing excitation intensity, whereas FABr-enriched samples show no evidence of DAP emission. Additionally, morphological characterization shows a reduction in grain size with increasing FABr excess, indicating a trade-off between improved electronic quality and enhanced structural disorder. The film synthesized with a 5% excess of FABr provides the optimal balance, yielding the highest power conversion efficiency (6.26%), average visible transmittance (61.6%), and light utilization efficiency (3.85%). These results demonstrate that fine-tuning the precursor stoichiometry represents a simple yet effective strategy to enhance the optoelectronic quality and performance of semitransparent perovskite solar cells.