Dual-Ion Synergy Breaks 20% Barrier in Electron Transport Layer-Free Perovskite Solar Modules

Electron transport layer-free (ETL-free) perovskite solar cells (PSCs) represent a promising research direction for the industrial-scale deployment of next-generation technologies. However, achieving efficiency exceeding 20% in ETL-free PSCs requires the incorporation of an intermediate layer between perovskite and electrode, which undermines the fundamental goal of simplified device architecture. Herein, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIMTFSI) is used to passivate perovskite film. This process occurs through distinct enrichment mechanisms involving both the anionic and cationic species of EMIMTFSI, which results in notable enhancements in the efficiency and stability of ETL-free PSCs. Experimental results confirm that EMIM⁺ and TFSI^- interact with uncoordinated iodide and lead ions in the perovskite, respectively. Furthermore, the fluorine in TFSI^- forms a hydrogen bond with the N-H group in perovskite. The synergistic effects of these interactions contribute to enhanced crystallization, improved energy level alignment, and reduced defects in perovskite film. As a result, the device achieves an efficiency of 22.08% with excellent stability under environmental and light exposure. Importantly, the ETL-free perovskite solar module with an efficiency of 20.27% is first reported. This study presents a simplified approach to boost the performanceof ETL-free PSCs, providing an effective pave for the cost-effective commercialization of perovskite photovoltaics.