Dependence of the Electronic and Optical Properties of Methylammonium Lead Triiodide on Ferroelectric Polarization Directions and Domains: A First Principles Computational Study

Organic inorganic perovskites, and in particular methyl ammonium (MA) lead triiodide, recently emerged as promising for thin film solar cell applications and, a a result, have attracted much attention. However, some important phenomena have been less examined in these systems, e.g., effects of ferroelectric domains on the optoelectronic properties. In this work, we investigate the effects of the polarization direction in single domains, and of uncharged and charged ferroelectric domains, on the electronic and optical properties of MA lead triiodide by first principles calculations. Highly accurate quasiparticle band gap calculations enabled characterization of the electronic structure of charged and uncharged domains in comparison to single domains. Additionally, analysis of the effects of a potential on the Born effective charges and respective density of states provided an understanding of changes in the band gap, as dependent on the type of domain, and on the MA moiety direction. Agreement between experimental and calculated optical spectra was achieved by inclusion of electron hole interactions, also discerning specific transitions. However, due to the flexibility in the MA moiety's orientation that causes spectral broadening, consideration of a statistical ensemble of configurations is required, which is not taken into account in a single computation. Indeed, our analysis in considering a number of MA directions leads to better agreement, with experiment. The calculations predict that the optical response is rather sensitive to the type and size of ferroelectric domains, which implies that such a response could be used for their characterization, thus calling for further experimental exploration.