Formamidinium lead bromide (FAPbBr₃) perovskite solar cells (PSCs) have attracted increasing attention for applications in tandem photovoltaics, semi-transparent devices, and photocatalysis owing to their wide bandgap, excellent thermal stability, and high open-circuit voltage. Compared with iodide-based perovskites, FAPbBr₃ exhibits superior resistance to moisture, heat, and phase instability. However, its relatively wide bandgap limits the short-circuit current density in single-junction devices, and severe interfacial defect-induced nonradiative recombination at the electron transport layer (ETL)/perovskite interface further constrains device performance, especially in hole-transport-layer-free (HTL-free) architectures.
In this work, high-quality FAPbBr₃ perovskite films were fabricated via a two-step solution spin-coating method, in which PbBr₂ films were first deposited followed by the spin-coating of a formamidinium bromide (FABr) methanol solution. To regulate interfacial charge transport and suppress defect-assisted recombination, tetraethylammonium hydroxide (TEAOH) was introduced as an interfacial modifier at the TiO₂/FAPbBr₃ interface. By systematically tuning the concentration of the TEAOH aqueous solution, the influence of interfacial modification on perovskite crystallization, defect passivation, and carrier dynamics was comprehensively investigated.
Morphological characterization by scanning electron microscopy revealed that an optimal TEAOH concentration of 4 mg·mL^-1 significantly improved the film quality, featuring enlarged grain size, reduced grain boundary density, and a more compact and uniform morphology. X-ray diffraction analysis confirmed enhanced crystallinity and improved phase purity of the FAPbBr₃ films after TEAOH modification. Meanwhile, ultraviolet-visible absorption spectra demonstrated strengthened light-harvesting capability, which can be attributed to the improved microstructure and reduced optical scattering losses.
Electrical and spectroscopic analyses, including steady-state and time-resolved photoluminescence, space-charge-limited current measurements, electrochemical impedance spectroscopy, and transient optoelectronic characterization, consistently revealed that TEAOH modification effectively reduced interfacial defect density, suppressed nonradiative recombination, and promoted faster electron extraction at the TiO₂/FAPbBr₃ interface. As a result, the optimized HTL-free FAPbBr₃ PSC delivered a high open-circuit voltage of 1.63 V, a short-circuit current density of 7.99 mA·cm^-2, and a fill factor of 83.74%, achieving a power conversion efficiency of 10.91%, which represents the highest reported efficiency for HTL-free FAPbBr₃-based PSCs to date. In addition, the modified devices exhibited stable current output under continuous illumination.
This study demonstrates that TEAOH interfacial engineering is a simple yet highly effective strategy to simultaneously improve crystallization quality, interfacial charge transport, and photovoltaic performance in wide-bandgap perovskite solar cells, offering valuable insights for the development of efficient and stable perovskite optoelectronic devices.

Key words: FAPbBr₃ perovskite solar cells, TiO₂/FAPbBr₃ interface, tetraethylammonium hydroxide, two-step spin-coating, interfacial modification