Chemical polishing and sub-surface passivation of perovskite film towards high efficiency solar cells

Eliminating surface defects and impurities on metal halide perovskite (MHP) films through chemical reactions represents a novel strategy to improve the performance of perovskite solar cells (PSCs), which can be referred to as "chemical polishing". This approach is anticipated to be more facile, precise, and distinct from the extensively documented surface passivation methods in terms of its underlying mechanism. However, to date, the underlying selective chemical reaction mechanism still requires in-depth study. In this context, we present a novel two-step chemical polishing method that eliminates surface impurities while simultaneously passivating the sub-surface. The core principle of this method involves two primary steps: (1) the creation of two-dimensional (2D) perovskite via selective reactions between polishing agents (n-octylammonium bromide, OABr) and undesired metastable amorphous species, as well as residual PbI2 nanocrystals present on the surface of MHP films. (2) Subsequently, the 2D perovskite and excess polishing agents are efficiently eliminated using a mixed solvent. Following the polishing process, the sub-surface, which is passivated by residual OABr, contains fewer defects and can establish improved electrical contact with the hole transport layer (HTL). As a result, the power conversion efficiency (PCE) of PSCs is enhanced from 21.7% to 23.6%. Moreover, the PSCs processed with chemical polishing exhibit enhanced long-term operational stability, with a capability to retain 80.2% of their original PCE value after 1500 h of illumination.