Seed crystal free growth of high-quality double cation - double halide perovskite single crystals for optoelectronic applications

Organo-lead trihalide perovskites (OLTPs) form a highly interesting class of semiconductors, which might play an important role in future photovoltaics and optoelectronics. Particularly, formamidinium lead triiodide - methylammonium lead tribromide mixed perovskite (FAPbI(3))(0.9)(MAPbBr(3))(0.1)is one of the most important representatives of this material class. In order to estimate the full optoelectronic potential of this perovskite system and thus to foster its future technological use, it is essential to investigate high-quality single crystals with the lowest structural as well as chemical defect density and with a stoichiometry relevant for their thin-film counterparts. However, the liquid growth of perovskite crystals without seed crystals is usually challenging and becomes even more demanding in the case of mixed cation-mixed halide single crystals, making it difficult to access their inherent properties. Here, we introduce a new efficient seed crystal free re-fill crystallization method (RFCM) based on inverse temperature crystallization (ITC) to grow large-sized single crystals. We performed qualitative and quantitative analyses, which confirmed the targeted elemental composition and the exact stoichiometry of the grown crystals. By means of polychromatic and monochromatic X-ray diffraction (XRD), we have demonstrated the high single crystal quality of the RFCM crystals, superior to crystals obtained by the seed crystal method. Steady-state photoluminescence (PL), absorption and temperature-dependent electrical measurements completed the investigation and enabled the determination of the optical band gap, relative permittivity and electrical conductivity of the grown single crystals. The conductivity clearly exhibits an ionic contribution and is therefore relevant for photovoltaic and optoelectronic implementation of this perovskite system.