Ruddlesden-Popper-type (RP) quasi-two-dimensional (2D) perovskite solar cells (PSCs) have attracted extensive attention due to their unique photovoltaic properties and excellent device stability. Due to disordered crystallization, the film quality of RP perovskite remains unsatisfactory. To achieve better photovoltaic performance, various strategies focused on crystallization regulation have been successfully applied. At present, the related researches focus on the photovoltaic properties of perovskite films after crystallization regulation, while the understanding of the crystallization process is lacking.

Recently, Prof. ZHOU Huiqiong's research group from National Center for Nanoscience and Technology (NCSNT) of the Chinese Academy of Sciences (CAS) has developed a sulfonium cations-assisted intermediate engineering strategy to study the evolution of intermediates and the film properties of quasi-2D perovskites. The related research results were published online in Advanced Materials (Adv. Mater. 2022, DOI: 10.1002/adma.202207345).

The research group developed a facile strategy for intermediate engineering by employing sulfonium cations toregulate the transformation of intermediates during the crystallization process and improve the film quality of quasi-2D perovskites. The intermediates were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) to reveal the composition and transformation process of the intermediates. The introduction of sulfonium cations inhibits the formation of unfavorable solvated lead iodide and promotes the formation of favorable perovskite intermediates with fiber-like morphology, which is conducive to the formation of high-quality perovskite crystals. The above effects have been confirmed in quasi-2D perovskite with different n values and 3D perovskites.

In-situ photoluminescence spectroscopy (PL) was carried out to study the crystallization during the film formation process. It was found that the introduction of sulfonium cations could not only accelerate but also optimize the crystallization process, leading to high-quality perovskite films. Based on the above results, regulating the properties of intermediates is an effective way to obtain high-quality perovskite films. Induced by the improved film quality, the champion device achieved a power conversion efficiency of 19.08% at room temperature and 20.52% at low temperature (180 K). In addition, the corresponding devices also show improved operational stability, with 84% of the initial efficiency maintained after 1000 h under maximum power point (MPP) tracking at 40 ℃.