The lattice symmetrization worked, but with a plot twist: effects of methylhydrazinium doping of MAPbI3 on phase transitions, cation dynamics and photoluminescence

Abstract

Lattice symmetrization is a term coined for the intentional doping of methylammonium lead iodide (MAPbI₃) with larger organic cations to lower the transition temperature to the cubic phase, whose stability is much preferred over the tetragonal one to avoid undesired lattice strain harmful for device operation, which could appear due to the cubic-tetragonal phase transition (PT) present near 330 K in undoped MAPbI₃. The following case study of three-dimensional (3D) organic cation-alloyed perovskites of formula MA_1−xMHy_xPbI₃ (MHy⁺ = methylhydrazinium; x < 0.25) unveils the complex impact of MHy⁺ doping on the stability of crystal phases. For low doping of x ≤ 0.115 the cubic-tetragonal PT temperature strongly decreases on doping, as expected. The MHy⁺-induced lattice symmetrization worked to the point that at room-temperature (RT) the cubic phase could be observed at a doping of x = 0.057, the lowest among so far used organic dopants. By contrast, the temperature of the tetragonal-orthorhombic PT increases on doping, opposite to what was observed for analogous doped MAPbI₃ systems. Unexpectedly, however, beyond x > 0.2 the tendency reverses, as the temperatures of tetragonal-to-cubic PTs shift strongly to higher values. Significant changes in PT mechanism at high doping conditions are inferred from large thermal hystereses, Raman scattering data showing the presence of two unique MA⁺ cations in the orthorhombic phase, and dielectric spectroscopy demonstrating dipolar relaxation for low-doped systems, and its suppression for highly-doped ones. The substitution of MA⁺ with MHy⁺ leads to a weak widening of the band gap while retaining efficient emission and extended absorption, suitable for optoelectronic applications.