Effects of formamidinium and bromide ion substitution in methylammonium lead triiodide toward high-performance perovskite solar cells

Highlights: • The roles of FA⁺ and Br^- ions in MA_xFA_1-xPb(I_yBr_1-y)₃ perovskite were revealed. • An average PCE of 17.34% was realized in the optimized MA_0.7FA_0.3Pb(I_0.9Br_0.1)₃-based PVSC. • A perovskite with a large bandgap of 1.69 eV yields a high PCE_AVG over 15%. Compositional engineering of organic-inorganic hybrid perovskite has attracted great research interests recently for seeking a better perovskite system to address existed challenges, such as the thermal and moisture instability, anomalous hysteresis, and toxic lead contamination, etc. In this study, we systematically investigated the structural, optophysical, and photovoltaic properties of the compositional MA_xFA_1−xPb(I_yBr_1−y)₃ perovskite by sequentially introducing FA⁺ and Br^- ions into the parental MAPbI₃ to elucidate their respective roles when they were inserted into the perovskite lattice. We unraveled that such dual compositional tuning in perovskite can improve the crystallinity of the resultant film and thus reduce its density of defect states as evidenced by admittance spectroscopy, resulting in a prolonged carrier lifetime over 500 ns. As a result, a promising average PCE (PCE_AVG) of 17.34% was realized in the optimized MA_0.7FA_0.3Pb(I_0.9Br_0.1)₃-based PVSC with little hysteresis and stable photocurrent output. More significantly, another compositional MA_0.7FA_0.3Pb(I_0.8Br_0.2)₃ perovskite with a large bandgap of 1.69 eV can yield an impressively high PCE_AVG over 15%. To the best of our knowledge, this performance is among the state-of-the-art large bandgap (~1.7 eV) PVSCs reported so far, which paves the way for the development of high-performance tandem cells using efficient large bandgap PVSCs as the top subcells. This study not only manifests the pivotal roles of dual compositional tuning in MA_xFA_1−xPb(I_yBr_1−y)₃ perovskites but also highlights the importance of compositional engineering for developing an even more efficient perovskite.