During the past few years, metal-halide perovskites have become one of the most promising semiconducting materials for the fabrication of high-efficiency solar cells. One of the greatest advantages of perovskite semiconductors lies in the possibility to easily tune the bandgap by simply modifying their stoichiometry. A commonly used strategy to widen the bandgap consists in partially or completely substituting the iodide ions with smaller-sized bromide ions. Notably, bromide-based perovskites exhibit elevated bandgaps around 2.3 eV, which can also translate into extraordinarily high open-circuit voltages (V_OC) up to 1.9 V according to the Schockley-Queisser limit. For this reason, full-bromide perovskites are highly regarded as potential candidates for the use in photoelectrochemical water splitting systems and as the topmost cell in multijunction solar cells.

However, it is well known that wide-bandgap solar cells still suffer from considerable energy losses compared to their narrower bandgap counterparts. This can be mainly ascribed to the presence of defects or to a poor energetic alignment between the perovskite and the charge-transport layers (CTLs). In this work, efficient formamidinium lead bromide (FAPbBr₃) perovskite solar cells were obtained by engineering the interfaces between the absorber layer and the adjacent CTLs. FAPbBr₃ thin-films were obtained following a two-step deposition method and employed for the fabrication of planar solar cells in a p-i-n configuration. First, high-quality FAPbBr₃ layers with appropriate morphology and thickness were obtained by finely tuning the precursors stoichiometry, concentration, and the solvents ratio. Then, the use of a pseudo-halide into the precursor solution, namely formamidinium thiocyanate (FASCN), lead to a considerable increase in grain size and a to large gain in V_OC, possibly due to a reduction of trap states. Using propane-1,3-diammonium iodide (PDAI₂) for surface passivation, it was possible to reduce interface recombination and further suppress non-radiative losses. Numerous charge-transport layers characterized by different HOMO and LUMO levels were tested to finely match the ones of our perovskite. Thanks to its good energy alignment respect to our perovskite, together with its excellent hole-extraction selectivity and low interface trap density, the use of Me-4PACz as hole-transport layer allowed to obtain a global increase in photovoltaic performances. To mitigate its strong hydrophobic nature, which often results in an incomplete coverage of the overlying perovskite layer, small quantities of 1,6-hexylene diphosphonic acid (HDPA) were introduced into the Me-4PACz layers. For the electron-transport layer, an innovative strategy which consists in mixing different fullerene derivatives, such as PCBM, CMC, and ICBA in equal ratios was employed. Mixing these materials provides a higher V_OC, short-circuit current density (J_SC), and fill factor (FF) combination than with CTLs composed of one or two fullerenes. As the last step, devices were completed with LiF as a buffer layer and Al as top contact. Following the application of an antireflective (AR) coating, a champion device with a V_OC of 1.63 V, J_SC of 7.45 mA cm^-2, FF of 0.75, and power conversion efficiency (PCE) of 9.1% was obtained, showing the effectiveness of interfacial engineering for the fabrication of high-efficiency wide-bandgap solar cells.