Wide-bandgap bromide perovskites, exemplified by FAPbBr₃, have emerged as indispensable components in tandem photovoltaics and semi-transparent solar cells, owing to their large bandgap, elevated theoretical open-circuit voltage, and enhanced environmental robustness relative to iodide-based analogues. Nevertheless, the practical efficiency of FAPbBr₃ devices remains markedly below their theoretical ceiling, largely constrained by poorly regulated crystallization dynamics during film formation. The intrinsically low solubility of PbBr₂, coupled with the rapid conversion kinetics of bromide systems, frequently induces incomplete phase transformation and microstructural heterogeneity, thereby generating pervasive bulk and interfacial defects. These defects act as dominant nonradiative recombination centers, undermining carrier lifetime and operational stability, particularly in carbon-electrode architectures.
To address these limitations, a molecularly engineered interfacial strategy is developed by introducing formamidinium disulfide dihydrochloride (FDD) as a functional interlayer between the FAPbBr₃ absorber and the carbon electrode. The FAPbBr₃ films are constructed via a two-step spin-coating protocol, followed by the deposition of an ultrathin FDD modification layer. Notably, the molecular configuration of FDD enables a cooperative dual-site passivation mechanism: sulfur-containing groups exhibit strong coordination with undercoordinated Pb²⁺ species, while the formamidinium moieties contribute to surface defect passivation through intermolecular interactions.This synergistic interaction effectively suppresses interfacial defect formation and modulates crystallization behavior, leading to concurrent improvements in structural order and electronic quality.
A suite of structural, optical, and electrical characterizations systematically elucidates the role of FDD modification. Morphological analyses (SEM, AFM) reveal enlarged grain domains, enhanced film compactness, and reduced macroscopic roughness, indicative of suppressed grain-boundary defects and improved crystallinity. XRD patterns confirm increased phase purity with diminished residual PbBr₂ signatures. Optical measurements, including UV-Vis absorption and Tauc analysis, demonstrate enhanced absorption coefficients alongside reduced Urbach energy, reflecting decreased sub-bandgap states and minimized energetic disorder.
Photophysical investigations further corroborate the suppression of nonradiative pathways. Steady-state and time-resolved PL measurements exhibit intensified emission and prolonged carrier lifetimes. Space-charge-limited current analysis indicates a reduced trap-state density, while electrochemical impedance spectroscopy reveals elevated recombination resistance and decreased transport resistance, evidencing facilitated charge extraction. Transient photocurrent and photovoltage responses further confirm accelerated carrier dynamics and extended recombination lifetimes, underscoring improved interfacial charge-transfer kinetics.
Benefiting from these collective enhancements, the optimized FDD-modified device delivers a champion PCE of 11.25%, with an open-circuit voltage of 1.64 V, a short-circuit current density of 7.98 mA cm^-2, and a fill factor of 85.96%, substantially surpassing previously reported benchmarks for FAPbBr₃ systems. Device statistics indicate improved reproducibility with reduced performance dispersion. Moreover, unencapsulated devices retain 96.88% of their initial efficiency after 30 days under ambient conditions (~20% RH, 25 °C), highlighting markedly enhanced environmental stability.
In essence, this work establishes a refined molecular-level interfacial engineering paradigm for wide-bandgap FAPbBr₃ photovoltaics. By constructing a synergistic dual-site passivation framework that concurrently targets Pb²⁺-related surface defects and other surface defect sites, the FDD interlayer effectively tailors crystallization behavior, suppresses nonradiative recombination, and promotes efficient charge transport. This strategy not only advances the efficiency ceiling of carbon-based wide-bandgap perovskite solar cells but also provides deeper mechanistic insights into defect governance and interface design in bromide perovskite systems.

Key words: Formamidinium disulfide dihydrochloride, Interfacial engineering, Two-step spin coating, Defect passivation, FAPbBr₃ perovskite solar cells