Effect of diphenyliodonium ionic additives on crystallization control and interface stabilization in high-efficiency two-step fabricated perovskite solar cells

Abstract

Additive engineering is increasingly being recognized as a critical strategy in the two-step fabrication of perovskite solar cells (PSCs), where the morphology and crystallinity of the initial PbI<inf>2</inf> layer strongly influence the subsequent perovskite conversion and interfacial quality. In this study, we addressed the limited understanding of anionic effects in additive engineering by incorporating diphenyliodonium (DPI)-based ionic additives with various counter anions into a PbI<inf>2</inf> precursor to control early stage crystallization. The nitrate-based additive (DPIN) promoted the formation of a porous PbI<inf>2</inf> framework, enhancing the diffusion of organic halides and enabling complete perovskite conversion. Furthermore, DPIN promoted preferential crystal growth along the (111) facets, particularly in deeper regions of the film, as revealed by grazing-incidence X-ray diffraction (GIXRD). Backside characterization confirmed an improved buried interface morphology with reduced residual PbI<inf>2</inf> and fewer pinholes. Density functional theory (DFT) calculations revealed that nitrate ions effectively passivate iodine vacancies at the SnO<inf>2</inf>/perovskite interface while preserving lattice integrity. These combined effects result in enhanced film quality and device stability. The DPIN-treated device achieved a power conversion efficiency of 25.65 % and retained 95 % of its initial efficiency after 1050 h of ambient storage, along with over 90 % retention under continuous illumination for 500 h, highlighting the dual benefits of nitrate-assisted crystallization and interface engineering.