Binary Solvent Engineering for High-Performance Two-Dimensional Perovskite Solar Cells

Two-dimensional (2D) organic–inorganic hybrid perovskite materials have recently attracted tremendous attention for use in perovskite solar cells (PSCs) by virtue of their considerable long-term stability. However, the primary stumbling block for their application in PSC is their relatively lower power conversion efficiency (PCE) compared with the conventional 3D perovskite. In this work, through a binary solvent engineering of dimethylformamide (DMF)/dimethyl sulfoxide (DMSO) in the precursor solution, high-performance 2D perovskite planar solar cells are fabricated with beyond 10% PCE and outstanding environmental stability. In the formation of the 2D perovskite film during the hot-coating process, heterogeneous nucleation primarily occurs at the air–liquid interface, contributing to the formation of the thin, flaky cap layer on the outmost surface of the film. Sequentially, for the unary solvent DMF, its fast volatilization facilitates homogeneous nucleation, resulting in random-orientated perovskite grains in the inner layer. In contrast, with the binary solvent engineering of DMF/DMSO, the formation of intermediate can slow the crystallization process, making the perovskite cap layer serve as the seed that impels the subsequent perovskite crystallization to be vertically orientated in the inner layer of the perovskite film. These findings provide not only a new understanding of the 2D perovskite crystallization process but also a simple and effective method to fabricate high-performance 2D perovskite-based photovoltaics.