ИСТИНА

In recent years, the excellent optoelectronic properties and facile solution processing of hybrid halide perovskite-based materials have called great attention of researchers due to high perspectives for applications in thin film solar cells and LED devices. A widespread strategy to improve the luminescence properties of hybrid perovskites such as MAPbI3, FAPbI3 (where MA+ = methylammonium, FA+ = formamidinium) is addition of excess organic halides [1, 2]. However, there is a lack of understanding of the key dependencies between the non-stoichiometry, structure and optical properties of the hybrid halide perovskites. Under specific conditions, a formation of perovskite-like phases with partial replacement of [PbI]+ moieties with organic cations of appropriate size ([PbI]+ ↔ A+) can take place. In this work, we have shown this effect for the MAI-FAI-PbI2 system and studied the structure and optical properties of new phases. Powders of the new phases were synthesized from corresponding iodides mixed in desired ratio and reacted at 50-70 °C in toluene with a small additive of iodine acting as a mass transfer agent (I- + I2 ↔ [I3]-). Such a mild reaction conditions without usage of coordinating solvents and annealing provides the formation of equilibrium phase composition. The XRD analysis of obtained powders showed 3 new phases: “hollow” - (FAyMA1-y)1.5PbI3.5, (0.75≥y≥0.6), “2D” - (FAyMA1 y)2PbI4 (1≥y≥0.9), and “1D” - FA3PbI5. For each phase full profile refinement was carried out (Table 1) and model structures were proposed. “Hollow”-phase has a perovskite-like structure with an ordered set of parallel empty channels without [PbI6] octahedra. “2D” phase is a layered perovskite with LSF of [0,0]. Finally, the “1D”-phase structure is linear unbranched chains of [PbI6] octahedra bounded by vertices and tilted by 45° each other. For the “hollow” phase a decrease of cell parameters with increasing methylammonium content, which is directly related to cations size. But there is an inverse relation for the “2D” phase presumably because higher packing density of flat FA+ cations compared with more isotropic MA+ cations. A wide composition area ((FAyMA1-y)xPbI2+x 3≥x≥1, 1≥0) is being investigated, and a corresponding phase diagram is in development. There discovered a couple of other phases, but their structure is still unknown. Optical properties were studied by diffuse reflectance spectroscopy and photoluminescence (PL) spectroscopy on thin films. The optical band gap (2.16, 2.3, and 2.6 eV for “hollow”, “2D” and “1D” phases respectively) and PL maximums were defined for new phases. During the transition to lower-dimensional phases, the optical band gap increases significantly, and PL peaks are shifted to the higher energies. The MA/FA ratio has only a small influence on optical properties.