Phase Control of Organometal Halide Perovskites for Development of Highly Efficient Solar Cells.

To develop a highly efficient solar cell using organometal halide perovskites, its microscale structure control is one of the most important factors because the microstructural defects inside the organometal halide perovskite are harmful to charge carrier flow and, thus, degrade device performance. In this study, we confirmed the existence of large physical gaps at the grain boundary in a methylammonium iodide (MAPbI, MA = CHNH) perovskite with transmission electron microscopy (TEM) analysis and revealed that the physical gap prevents charge carrier flow in the MAPbI perovskite. To minimize the physical gap and its negative influences, the grain size of the MAPbI perovskite was optimized by increasing the portion of the cubic phase via microstructural phase control using liquid nitrogen (LN). Through microstructural phase control of the MAPbI perovskite, its grain boundaries and physical gap were significantly decreased, and 20.23% power conversion efficiency (PCE) was achieved with a single cation MAPbI perovskite solar cell.