Stable 2D Alternating Cation Perovskite Solar Cells with Power Conversion Efficiency >19% via Solvent Engineering

Two‐dimensional alternating cation (ACI)‐type perovskites self‐assemble in solution to form highly ordered periodic stacks with unique physical properties and improved optoelectronic devices. Tailoring composition and distribution of quantum wells (QWs) is of vital importance for the optoelectronic properties and stability, which, however, have been less reported in contrast to their Ruddlesden–Popper (RP) and Dion–Jacobson (DJ) counterparts. Herein, crystallization control via solvent engineering for ACI perovskite (GA)MA n Pb n I 3 n +1 (< n > = 5, GA = guanidinium, MA = methylammonium) that enables preferential QW distribution within the film and augments the crystallinity and smoothness of the films is proposed. It is found that such morphological improvements are further reflected in the optoelectronic properties, including enhanced charge carrier transport/extraction and suppressed nonradiative charge recombination. Thus, efficient and stable ACI perovskite solar cells with a power conversion efficiency (PCE) of 19.18%, standing the highest among all reported RP, DJ, and ACI (< n > ≤ 5) solar cells, are realized. Meanwhile, the device exhibits superior reproducibility and environmental stability. These findings highlight the importance of crystallization control and pave the way for the realization of high‐performance 2D perovskite solar cells.