High open-circuit voltage in wide-bandgap bromide perovskite solar cells: the role of hole transport materials

We report the fabrication and characterization of mesoporous TiO₂-based wide-bandgap bromide perovskite (FAPbBr₃) solar cells employing both fluorene-dithiophene and spiro-OMeTAD as hole transport materials (HTMs). The devices were fabricated using the same protocol as those investigated for spectroscopy, ensuring consistent material deposition and interface quality. Current-voltage (I-V) measurements under one sun illumination revealed promising photovoltaic performance, with power conversion efficiencies (PCE) of 6.7 % (Voc = 1.40 V, Jsc = 6.80 mA/cm², FF = 70 %) for spiro-OMeTAD and 6.3 % (Voc = 1.39 V, Jsc = 6.60 mA/cm², FF = 68 %) for fluorene-dithiophene-based devices. The exceptionally high open-circuit voltage (∼1.40 V) achieved by both HTMs highlights excellent interface quality and reduced non-radiative recombination losses. Both, X-ray Photoelectron Spectroscopy (XPS) and Secondary Ion Mass Spectrometry (SIMS) were employed to investigate the chemical composition, elemental distribution, and depth profiling of the fabricated FAPbBr₃-based solar cells with different hole transport materials (HTMs). XPS analysis confirmed the presence of characteristic Pb 4f, Br 3d, and N 1s peaks, verifying the composition of the perovskite layer. The SIMS results revealed a uniform distribution of bromide (Br⁻) within the perovskite layer, confirming the stability of the material and the absence of significant halide migration. Depth profiling further demonstrated well-defined interfaces between the perovskite, mesoporous TiO₂, and the respective HTMs, with minimal interdiffusion, which aligns with the high open-circuit voltage (∼1.40 V) observed in the I-V measurements. We have also studied the charge extraction behavior and recombination dynamics using steady-state and transient optoelectronic characterization tools. FDT-based devices confirm better charge injections and better Voc compared to Spiro-OMeTAD devices. These results underscore the potential of bromide-based perovskites for high-voltage photovoltaic applications and emphasize the critical role of HTM selection in optimizing device performance.