Elucidation of the Formation Mechanism of Highly Oriented Multiphase Ruddlesden–Popper Perovskite Solar Cells

The crystallographic orientation and phase distribution of two-dimensional Ruddlesden–Popper perovskites (2D-RPPs) should be carefully controlled to obtain high-performance 2D-RPP-based optoelectronic devices. However, these characteristics are still unclear. Herein, we systematically examine the formation mechanism of highly oriented multiphase 2D-RPPs. We argue that the 3D-like perovskites containing small organic cations nucleate first with out-of-plane (111) preferential orientation, followed by the further growth of two-dimensional perovskites incorporating bulky organic cations owing to the difference in the solubility between small and bulky cations. This spatial segregation of organic cations across the film depth induces the formation of multiple perovskite phases, which produces n-value-graded 2D-RPP films with continually upshifted band energy alignment. Highly oriented multiphase 2D-RPP films with isobutylammonium (isoBA2(Cs0.02MA0.64FA0.34)4Pb5I16) were successfully employed as a photoabsorbers for perovskite solar cells (PSCs), exhibiting remarkable efficiency of over 16% and significantly enhanced environmental stability compared with their three-dimensional counterparts.