Proceedings of Online International Conference on Hybrid and Organic Photovoltaics (OnlineHOPV20)
Publication date: 22nd May 2020
The complete phase diagram of organic-cation solid-solutions of lead iodide perovskites (FAxMA1-xPbI3, where MA stands for methylammonium and FA for formamidinium) with compositions x ranging from 0 to 1 in steps of 0.1 was constructed in the temperature range from 10 to 365 K by combining Raman scattering and photoluminescence (PL) measurements. The occurrence of phase transitions was inferred from both the temperature-induced changes in the optical emission energies and/or the phonon frequencies and linewidths, complementing X-ray and neutron scattering literature data. For MA-rich perovskites (x<0.3) the same structural behavior as for MAPbI3 is observed with decreasing temperature: cubic ==> tetragonal-I ==> orthorhombic. Being the FA molecule larger, more symmetric but less polar than MA, a tetragonal crystal structure is favored at low temperatures and FA compositions x>0.4, in detriment of the orthorhombic phase. As a consequence, with decreasing temperature the phase transition sequence for FA-rich compounds is cubic ==> tetragonal-II ==> tetragonal-III. The symmetry of the latter is according to neutron scattering data. Strikingly, the isostructural (tetragonal-to-tetragonal) transformation, which occurs between 200 and 150 K, exhibits a kind of critical point for x=0.7. For intermediate FA contents the perovskite solid solution transforms close to 250 K directly from the cubic phase to the tetragonal-III phase. The latter is characterized by a non-monotonous dependence of the band gap energy with temperature. We ascribe such behavior to a substantial tilting of the PbI6 octahedrons in the tetragonal-III phase. In this way, we established important links between crystal-phase stability and electronic as well as vibrational properties of mixed organic-cation halide perovskites, which might impact the current search for more stable best-performing optoelectronic materials.
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