Unveiling the Operation Mechanism of “Two-dimensional” Perovskite Solar Cells

Two dimensional (2D) perovskites have been shown to improve the stability of perovskite solar cells while its operation mechanism remains unclear. Here we investigate the process for the conversion of light to electrical current in high performance 2D perovskite solar cells by examining the real morphology at multiple scales and anisotropic carrier transport properties. A phase separation at micrometer scale and nanometer scale are identified in both vertical and lateral directions for the 2D perovskite films prepared by the hot-casting method. High resolution transmission electron microscopy revealed that the 2D layer structures with the size much smaller than the film thickness are surrounded by three dimensional (3D) perovskites. In the addition, no preferred orientation of the 2D layer structures was observed in our samples. Based on the morphology scenario, a model of energy transfer from 2D perovskites to 3D perovskite followed by exciton dissociation and free- charge transport via 3D perovskite is proposed to explain the exciton dissociation and charge collection in 2D perovskite solar cells. The impact of the 2D perovskite on efficiency and stability of perovskite solar cells are also discussed to provide guidelines for the further improvement of the “2D perovskite” thin film solar cells.