Effect of FABr Over-Stoichiometry on the Morphology and Optoelectronic Properties of Wide-Bandgap FAPbBr_3 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 (FAPbBr_3). 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 the passivation of bromine vacancies and to the reduction of defect-assisted recombination. Power-dependent PL confirms this interpretation: the stoichiometric film exhibits a PL band due to donor-acceptor pair (DAP) recombination as identified by the typical excitation-dependent blueshift, whereas FABr-enriched samples show no evidence of DAP emission, indicating effective defect passivation. 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 through controlled FABr addition represents a simple yet effective strategy to enhance the optoelectronic quality and performance of semitransparent perovskite solar cells.

💡 Research Summary

This work investigates how the intentional over‑stoichiometric addition of formamidinium bromide (FABr) to the precursor solution influences the optoelectronic performance and microstructure of wide‑bandgap formamidinium lead bromide (FAPbBr₃) thin films, which are of particular interest for semitransparent and tandem solar cells. Three precursor formulations were prepared: a stoichiometric control (0 % excess), a 5 % FABr excess, and a 10 % FABr excess. All films were deposited by spin‑coating on ITO substrates, followed by antisolvent quenching with ethyl acetate and annealing at 80 °C for 10 min, yielding ~150 nm thick polycrystalline layers.

Steady‑state absorption spectra were fitted with an Elliott model to separate excitonic and continuum contributions. The electronic bandgap (E_g) systematically widened from 2.368 eV (control) to 2.385 eV (+5 % FABr) and 2.406 eV (+10 % FABr), corresponding to a ~40 meV blue shift. Exciton binding energy remained essentially constant at ~40 ± 5 meV, indicating that dielectric screening and effective masses are not strongly altered by the excess. However, Gaussian broadening parameters (σ_X and σ_C) increased markedly for the 10 % sample, reflecting heightened compositional disorder. The non‑parabolicity coefficient b also rose from 0.69 eV⁻¹ to 0.91 eV⁻¹, consistent with band‑structure modifications induced by FABr incorporation.

Low‑temperature (12 K) photoluminescence (PL) revealed a main emission peak that blue‑shifts with increasing FABr, mirroring the absorption trend. The control film displayed an additional low‑energy shoulder, characteristic of donor‑acceptor pair (DAP) recombination associated with bromine vacancies. Power‑dependent PL measurements were analyzed using I_PL ∝ I^n. The extracted exponent n changed from 0.70 ± 0.18 (control) to 1.00 ± 0.09 (+5 % FABr) and 1.14 ± 0.02 (+10 % FABr). Values near unity indicate that excitonic recombination dominates in the FABr‑rich films, whereas n < 1 for the control points to defect‑mediated DAP processes. Thus, FABr excess effectively passivates bromine vacancies, suppressing non‑radiative pathways and enhancing radiative efficiency.

Morphological analysis by field‑emission scanning electron microscopy showed a clear grain‑size reduction as FABr excess increased. Average grain diameters decreased from the micrometer scale in the control to sub‑hundred‑nanometer dimensions in the 10 % sample. While smaller grains can shorten carrier diffusion paths, the increased grain‑boundary density introduces structural disorder, as corroborated by broader X‑ray diffraction peaks.

Solar‑cell devices incorporating these films were fabricated with identical charge‑transport layers and top electrodes. The 5 % FABr‑excess device delivered the highest power conversion efficiency (PCE) of 6.26 %, together with an average visible transmittance (AVT) of 61.6 % and a light‑utilization efficiency (LUE) of 3.85 %. The 10 % excess device exhibited a slightly lower PCE, attributed to excessive band‑gap widening and grain‑boundary‑related losses.

In summary, modest FABr over‑stoichiometry (≈5 %) provides a simple yet powerful lever to passivate bromine vacancies, reduce defect‑assisted recombination, and fine‑tune the bandgap of FAPbBr₃ while maintaining excitonic properties. The trade‑off between electronic quality and structural disorder becomes pronounced at higher excess levels, underscoring the need for careful optimization. These findings offer a practical pathway for improving the performance of semitransparent perovskite solar cells and for integrating wide‑bandgap perovskites as top cells in tandem architectures.