Abstract
State-of-the-art perovskite solar cells with record efficiencies were achieved by replacing methylammonium (MA) with formamidinium (FA) in perovskite polycrystalline films. However, these films suffer from severe structural disorder and high density of traps; thus, the intrinsic properties of FA-based perovskites remain obscured. Here we report the detailed optical and electrical properties of FAPbX(3) (where X = Br- and I-) single crystals. FAPbX(3) crystals exhibited markedly enhanced transport compared not just to FAPbX(3) polycrystalline films but also, surprisingly, to MAPbX(3) single crystals. Particularly, FAPbBr(3) crystals displayed a 5-fold longer carrier lifetime and 10-fold lower dark carrier concentration than those of MAPbBr(3) single crystals. We report long carrier diffusion lengths-much longer than previously thought-of 6.6 mu m for FAPbI(3) and 19.0 mu m for FAPbBr(3) crystals, the latter being one of the longest reported values in perovskite materials. These findings are of great importance for future integrated applications of these perovskites.