Impact of organic molecule rotation on the optoelectronic properties of hybrid halide perovskites

The importance of organic molecular cation orientation and interaction with surrounding inorganic framework in the optoelectronic hybrid halide perovskites is a topic of considerable interest. To that end, we study the effect of organic molecule rotations on the properties of such hybrid semiconductors using the swarm intelligence-based structure prediction method combined with ab initio density functional calculations. Adopting the cubic phases of ${\mathrm{APbI}}_{3}$ [$\mathrm{A}={\mathrm{CH}}_{3}{\mathrm{NH}}_{3}$(MA), ${\mathrm{CHNH}}_{2}{\mathrm{NH}}_{2}$(FA) and ${\mathrm{CH}}_{3}{\mathrm{CH}}_{2}{\mathrm{NH}}_{3}$(EA)], we determine the energetically stable/metastable configurations of organic molecules inside the quasicubic perovskite cages. Different cation orientations result in up to $\ensuremath{\sim}250\phantom{\rule{0.16em}{0ex}}\mathrm{meV}/\mathrm{formula}$ changes in the material free energy, reflecting the complex energy landscape of these hybrid materials containing dynamically rotating organic components. Notably, the [012] orientation of MA is found to give the most stable structure of ${\mathrm{MAPbI}}_{3}$, while the conventional [001], [011], or [111] directions remain slightly higher in energy. Molecular orientations clearly influence the fundamental electronic bandgap and also modify the magnitude of Rashba-type energy band splitting resulting in indirect gap behavior. However, the dielectric screening remains large for all orientations thus creating weakly bound excitons. Based on the analysis of many organic molecule configurations obtained through the structure search technique, our results provide insightful understanding on the effects of organic molecule rotation on optoelectronic properties and carrier dynamics of hybrid halide perovskites.