Enhanced interface regulation via π-conjugated heterojunctions for high-efficiency inverted perovskite solar cells

Abstract

Self-assembled monolayers (SAMs) are increasingly utilized as effective hole-collecting materials to boost the efficiency of inverted perovskite solar cells (PSCs). However, issues such as incomplete surface coverage and suboptimal interfacial bonding persist, leading to non-radiative recombination and compromising long-term stability. To address these challenges, we developed an innovative strategy by integrating 1-benzyl-3-methylimidazolium tetrafluoroborate (BzMIMBF₄) into the SAM, optimizing the buried interface and enhancing perovskite crystallization. BzMIMBF₄ enhances SAM surface coverage through BzMIM⁺ interactions, forming a robust π-conjugated heterojunction with the [4-(3,6-dimethyl-9H-carbazol-9-yl)butyl]phosphonic acid (Me-4PACz) SAM that optimizes interfacial bonding, inhibits detrimental Pb²⁺/I⁻ ion migration, and safeguards the bottom electrode. BzMIMBF₄ stabilizes crystal nucleation, minimizing defect-related non-radiative recombination, promotes rapid α-phase formation, and enhances (100) plane alignment and charge carrier transfer to the hole-transport layer (HTL). Besides, time-of-flight secondary ion mass spectrometry (ToF-SIMS) depth profiling confirms the distribution of BF₄⁻ anions throughout the perovskite film. Simultaneously, BF₄⁻ anions effectively passivate the perovskite surface and bulk defects, such as uncoordinated Pb²⁺ ions and iodine vacancies, thereby suppressing non-radiative recombination centers. The resulting perovskite films exhibit a pinhole-free structure, increased grain sizes, smoother surfaces, and significantly reduced residual strain. Consequently, BzMIMBF₄-treated devices achieve remarkable power conversion efficiencies of up to 26.45% (certified 26.37%) and retain 90.8% of their initial efficiency after 700 hours of operation under one-sun illumination, demonstrating excellent stability. This approach paves the way for high-performance, durable PSCs and their potential use in advanced photovoltaic applications.