Exploring Light Polarization Effects of Photovoltaic Actions in Organic-Inorganic Hybrid Perovskites with Asymmetric and Symmetric Unit Structures.

Hybrid perovskites are known as electrically polarizable semiconductors based on theoretical prediction and experimental information. Here we show the light polarization effects on the photovoltaic actions in perovskite solar cells by optically generating directional and random electronic transition dipoles within asymmetric and symmetric unit structures. In an asymmetric tetragonal unit structure, we observed the stripes with different orientations onto perovskite grains and that the randomly polarized photoexcitation can generate a higher photocurrent (sc) by 6.5 ± 0.5% as compared to the linearly polarized photoexcitation at same intensity in the hysteresis-free perovskite solar cells [ITO/PEDOT:PSS/MAPbI/PCBM/PEI/Ag]. Clearly, switching the photoexcitation between linear and random polarizations leads to a Δsc, which provides an experimental indication that all-directional and one-directional transition dipoles generate higher and lower photocurrents in organic-inorganic hybrid perovskites (MAPbI) with an asymmetric tetragonal unit structure. This implies that all-directional and one-directional transition dipoles develop stronger and weaker dissociative interactions, consequently giving rise to more and less dissociation toward generating photocurrent. This is confirmed by the experimental observation that the Δsc almost disappears when the temperature increases up to 55 °C, where the asymmetric tetragonal structure is changed to a symmetric cubic structure. Furthermore, the Δsc is shown to decrease with increasing light intensity. This indicates that the electronic transition dipoles encounter a polarization relaxation caused by mutual interaction. We show that the polarization relaxation time in MAPbI is comparable to exciton dissociation time (∼ps). This presents the necessary condition to demonstrate light polarization effects of photovoltaic actions in perovskite solar cells.