Structural and transport investigation of two different types of organic−inorganic halide perovskite structures

The exciting intrinsic properties of single crystals metal halide perovskites intimately correlate with the metal and cation species and the structural arrangement reached during the crystallization. The mechanisms driving the... [ view full abstract ]

The exciting intrinsic properties of single crystals metal halide perovskites intimately correlate with the metal and cation species and the structural arrangement reached during the crystallization. The mechanisms driving the solidification, the actual deficiencies that prevent long-term stability, and the humidity-related effects that impact the unique behavior of these material, are still poorly understood resulting to be the current bottlenecks towards single crystal-based optoelectronics and stable photovoltaic material. Here we compare experimentally two new different types of thin film perovskite single crystals, to evidence their structural and optoelectronic common and peculiar properties. The first, MAPbBr₃, was obtained exploiting the role of surface tension by rapid synthesis of single perovskite crystals with the innovative inverse temperature crystallization method, the other, a new kind of PbI₄-based material, was obtained by slow crystallization in solution, allowing the generation of a 2D layered structure. We used micro Raman spectroscopy as a useful technique for selectively studying the vibrational configuration and structural dynamics allowing the characterization of the samples quality and crystal structure. Both materials display substantial homogeneity, as can be evaluated for example considering the ratio of MA+ rocking mode (1248 cm-1) and the twisting mode (1589 cm-1) of the NH3+ ions, highly sensitive to the environment. Negligible amount of defects or crystal structure impurities characterize the first, which reaches cm size scale quickly in the triclinic crystal structure, while the second solidifying in a compact 2D-layered well-ordered phase structure freeze the thermal orientation disorder associated to the organic cation, even at ambient temperature. Sharp and well defined low-frequency Stokes and AS peaks allow the complete characterization of the crystal that displays superior photo exposure resistance. Such results have been cross-correlated with AFM topography and electric force microscopy characterization at the nanoscale.