Optically Clear Films of Formamidinium Lead Bromide Perovskite for Wide-Band-Gap, Solution-Processed, Semitransparent Solar Cells

Solvent engineering and antisolvent methods have been used extensively to achieve high-quality, homogeneous, and crystalline perovskite thin films. Usually, highly concentrated (>1.1 M) precursor solutions are used to achieve the maximum power conversion efficiency (PCE), and most fabrication studies focus on iodide-based metal halide perovskites (MHPs). However, high concentrations of precursors are not suitable for semitransparent (ST) MHP solar cells (STPSCs), which require thinner films to achieve a high average visible transmittance (AVT). The deposition of high-quality perovskites with variable concentrations in a one-step method is challenging due to the complexity of the antisolvent crystallization process. Here, we have developed an in situ technique based on photoluminescence (PL) measurements to identify the optimum delay time for antisolvent crystallization in formamidinium lead bromide (FAPbBr₃). By monitoring the in situ PL, the nucleation, crystal growth, and early perovskite formation phases are easily identified for a range of concentrations. Subsequently, we fabricated opaque and ST solar cells with optically clear, ST perovskite films formed from precursors with varying concentrations. These all-solution-processed STPSCs achieved AVTs of up to 35.6, 42.5, and 49.2%, with the corresponding PCEs of 5.71, 3.25, and 1.86% in p-i-n type, FAPbBr₃ perovskite solar cells with transparent Ag nanowire electrodes. These devices show good stability over several weeks and an impressive V_oc as high as 1.24 V for STPSCs and 1.38 V for opaque cells produced with a thick Ag electrode. This work demonstrates the potential use of in situ spectroscopy to tailor the film growth of halide perovskites with varying concentrations and the feasibility of using wide-band-gap perovskites for ST solar cells with exceptional clarity and higher V_oc.