Lead-free metal halide perovskite inspired materials (PIM) are attracting heightened interest for indoor photovoltaic (IPV) applications, driven by the escalating need for sustainable, low-toxicity energy solutions in low-light environments. Within this class of materials, bismuth-based PIM, including FA₃Bi₂I₉, present intrinsic benefits, such as enhanced chemical stability and environmental suitability. Nevertheless, their practical implementation is impeded by constrained optoelectronic performance, deficient charge transport, and insufficient long-term stability, which primarily stem from uncontrolled crystallization processes and suboptimal precursor chemistries [1,2,3].

This study introduces a systematic A-site cation engineering strategy for the fabrication of FA₃Bi₂I₉ thin films, with the objective of concurrently improving crystallinity, optoelectronic characteristics, precursor solution processability, and operational stability for indoor photovoltaic applications. Through the incorporation of appropriate mono-cationic FA-substituting moieties at the A-site, we successfully modulate precursor solution chemistry and crystallization kinetics while maintaining the structural integrity of the bismuth-iodide framework. This methodology yields substantially enhanced precursor solubility and solution homogeneity, thereby facilitating the formation of uniform, dense, and pinhole-free thin films fabricated under moderate conditions. Detailed structural analysis demonstrates heightened crystallinity, diminished lattice disorder, and enhanced phase purity within the engineered compositions. These structural enhancements consequently yield superior optoelectronic characteristics, as demonstrated by increased photoluminescence intensity, extended carrier lifetimes, and mitigated non-radiative recombination. Employing steady-state and time-resolved photoluminescence, in conjunction with femtosecond transient absorption spectroscopy, we illustrate enhanced carrier relaxation dynamics and more effective exciton dissociation, both of which are essential for optimal energy harvesting under low-intensity interior illumination. To elucidate the fundamental mechanisms, comprehensive X-ray photoelectron spectroscopy and nuclear magnetic resonance investigations were utilized to examine precursor coordination environments and chemical interactions. These analyses demonstrate that A-site substitution modifies precursor complexation and ion-solvent interactions, thereby regulating nucleation and crystal growth. This controlled crystallization behavior is recognized as a critical determinant of the observed improvements in film quality and electronic coupling. Moreover, spectroscopic analyses reveal optimized electron-phonon coupling within the fabricated films, thereby enhancing charge transport and photovoltaic efficiency.

This research elucidates key relationships between precursor solution chemistry, crystallization processes, and resultant properties in lead-free PIMs, thereby demonstrating A-site cation engineering as a viable design approach for achieving high-performance, stable indoor photovoltaic devices.