Exploring Polytype Formation in Double-Cation DMAxFA1–xPbI3 Perovskites: Compositional Engineering and Phase Transitions

Halide perovskites of the general formula ABX 3 are highly promising semiconductor materials for photovoltaic applications due to their unique and tunable band gap, high absorption coefficient, and relatively simple synthesis methods. However, one of the most demanding challenges faced currently is their instability under environmental conditions. One of the popular strategies to overcome this significant drawback is the incorporation of oversized cations into the perovskite lattice. In this study, we focus on the compositional engineering of double-cation A-site DMA x FA 1– x PbI 3 perovskite materials using solvent-free mechanochemical synthesis along with the examination of their temperature-dependent polytypism through in situ variable-temperature powder X-ray diffraction (pXRD) and thermogravimetric analysis coupled with differential scanning calorimetry. pXRD analysis of as-ground materials confirms complete miscibility across the entire substitution range, leading to the formation of one-dimensional δ-perovskitoid materials. Moreover, we revealed that the processing conditions have a huge impact on the temperature-induced phase transitions and the formation of 3D polytypes within the temperature range of 298–573 K. Specifically, subjecting the samples to rapid heating (5 °C/min) supports the coexistence of new different structural phases being formed, while a low heating rate (0.5 °C/min) favors the formation of single-phase materials. Interestingly, slow heating results in the formation of a single black α-perovskite phase for material compositions up to 10% DMA, while in the fast-heating route, black α-perovskite coexists as the secondary phase with the 4H polytype in compositions up to 30% DMA. Thus, this work offers a comprehensive understanding of the effect of composition engineering and processing conditions on the phase transition to a given polytype in double A-cation DMA x FA 1– x PbI 3 materials.