Unveiling the Role Reversal of Guest and Host in OverTolerant Hybrid Perovskites.

The structural and electronic changes are investigated in a 3D hybrid perovskite, methylhydrazinium lead chloride (MHyPbCl) from a host/guest perspective as it transitions from a highly polar to less polar phase upon cooling, using first-principles calculations. The two phases vary structurally in the guest (MHy) orientation and the two differently distorted host (lead halide) layers. These findings highlight the critical role of guest reorientation in reducing host distortion at high temperatures, making the former the primary order parameter for the transition, a notable contrast to the case of other hybrid perovskites. This is also confirmed by the dominating contribution of guest reorientation along the transition pathway. Analysis using maximally localized Wannier functions reveals that polarization enhancement upon heating is primarily due to host atoms, particularly of the more distorted octahedral layer. Despite its pivotal role in the transition, the contribution of the guest to polarization is relatively weaker, in contrast to previous suggestions. Furthermore, host distortion induces a significant (∼9%) feedback polarization on the guest. This distortion is also responsible for significantly altering the density of states occupied by the guest closer to band-edges, suggesting a non-trivial contribution of the guest in impacting the optoelectronic properties and exciton binding energies.