Dynamic Disorder and Potential Fluctuation in Two-Dimensional Perovskite.

The structural and electronic properties of 2D perovskite (C4H9NH3)2PbBr4 are investigated from first-principles calculations. It is found that despite the existence of carbon chain, the organic molecule C4H9NH3 in 2D perovskite is able to rotate at room temperature, showing a highly dynamic behavior. The dynamic disorder of C4H9NH3 introduces dynamic potential fluctuation, which is sufficient to localize the wave function and to separate the valence band maximum and conduction band minimum states. We further showed that, rather than a pure dipole rotation model, the disorder effect of the molecules can be better described by the motion of the net charge centers of the molecules, which contributes to the potential fluctuation. Hence, polar molecule is not the necessary condition to create potential fluctuation in perovskite, which is further demonstrated by the calculations of 2D inorganic perovskite Cs2PbBr4.