Layered methylhydrazinium lead halide perovskites: new crystal polymorphs with a tailored band gap and photoluminescence colour via halide substitution

Abstract

An unrelenting focus on improving stability and optoelectronic performance of lead halide three-dimensional hybrid organic–inorganic perovskites (3D HOIPs, ABX₃ formula) leads to the usage of many chemical engineering techniques, including X-site alloying. Formerly we have shown the usefulness of this approach in 3D HOIPs comprising methylhydrazinium (MHy⁺); this time we prove it again in the layered (A₂BX₄) perovskites. Herein we report the synthesis, crystal structure and physicochemical characterization of MHy₂PbBr_4−xI_x in a wide concentration range (x = 0.25, 0.3, 0.4, 1.1, 2.35, 2.5, 3.1). Reported compounds crystallise in the Ruddlesden–Popper phase with corner-sharing [PbX₆]⁴⁻ (X = Br, I) octahedra, forming (010) layers, separated by MHy⁺. An abundant temperature-driven polymorphism is observed, starting from the Br-rich compounds with a sequence of Pnmm, Pnma and P2₁/c phases on cooling, through the region with the P low-temperature phase, and ending with the I-rich compounds with an incommensurately modulated Pnma(00γ)0s0 superspace group. We also report, for the first time in the MHy-based perovskites, the 2D → 3D transition to Pmm cubic symmetry and a 3D perovskite phase with a dominant presence of iodine. With the use of Raman spectroscopy, the dynamics of MHy⁺ cations in particular phases is thoroughly analysed, revealing the positional disorder in room-temperature and high-temperature zones and its freezing at low temperatures. Linear optical studies demonstrate the decrease of the energy band gap (2.91 eV to 2.31 eV) and a strong change in photoluminescence colour (bluish-green to yellow-green) along with increasing iodine contribution.