Proceedings of MATSUS Spring 2024 Conference (MATSUS24)
Publication date: 18th December 2023
Hybrid perovskites offer more structural and chemical flexibility than traditional metal halide perovskites. In supertolerant 3D perovskites, a strong organic-inorganic sublattice coupling can influence several physical properties significantly. Here, we have carried out a detailed first-principles investigation of the structural and polar properties of 3D hybrid perovskite, methylhydrazinium lead chloride, MHyPbCl3, as it transitions from a highly polar (Phase-I) to less polar (Phase-II) phase upon cooling [1,2]. From a host/guest perspective, the two phases vary structurally in the orientation of the guest (MHy) and the two differently distorted host (lead halide) layers. We show that lowering of host distortion at high temperatures necessarily requires guest reorientation, making the latter the primary order parameter for the transition, in contrast to the case of most other hybrid perovskites. The same is also confirmed by a minimum energy path calculation, which shows that the predominant contribution to the transition barrier is due to a high guest reorientation barrier. Maximally localized Wannier functions (MLWFs) identify the atomistic origin of the polarization enhancement upon heating with changes at the host atoms, especially those in the more distorted octahedral layer. The guest, despite being the primary effect leading to transition, has a relatively weaker contribution to the polarization change, in contrast to previous suggestions [2]. Furthermore, a significant (~9%) feedback polarization is induced on the guest by the host distortion. This directly affects the density of states occupied by the guest, which shifts away from the band edge with the increase in the host distortion (for Phase-II guest orientation). Thus, the organic cations in 3D perovskites can also have a non-trivial contribution to the optoelectronic properties and exciton binding energies.
The authors gratefully acknowledge computational resources provided by IISER Bhopal as well as “PARAM Shivay” at Indian Institute of Technology (BHU), Varanasi, which is implemented by C-DAC and supported by the Ministry of Electronics and Information Technology (MeitY) and the Department of Science and Technology (DST), Government of India, through National Supercomputing Mission (NSM). P.S. acknowledges funding through UGC-JRF (India) for carrying out the Ph.D. program. S.M. acknowledges funding through the Integrated Ph.D. program at IISER Bhopal.
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