Mapping Binding Sites for Efficient Hole Extraction in Lead Halide Perovskites through Sulfur-Based Ligand Engineering

Abstract

Lead halide perovskite nanocrystals (NCs) rapidly emerge as promising materials for photovoltaics. However, to fully harness their potential, efficient charge extraction is crucial. Despite rapid advancements, the specific active sites where acceptor molecules interact remain inadequately understood. Surface chemistry and interfacial properties are pivotal, as they directly impact charge transfer efficiency and overall device performance. This study identifies and maps binding sites for hole transporters, examining their influence on charge transfer dynamics through ligand engineering with 2,3-dimercaptopropanol (DMP), a compound with a strong affinity for lead (Pb). DMP effectively passivates Pb sites in CsPbBr₃ (CPB) NCs, enhancing photoluminescence (PL) by forming stable chelating bonds. DMP-modified CPB nearly completely suppresses hole transfer to ─COOH-functionalized ferrocene (FcA) and partially suppresses transfer to ─NMe₂-functionalized ferrocene (FcAm), suggesting an alternative hole extraction pathway for FcAm. This is further supported by enhanced hole transfer in bromine-excess CPB (CPB-Br(XS)) synthesized via SOBr₂ treatment. The distinct binding interactions and charge transfer dynamics are validated through steady-state and time-resolved PL, along with transient absorption spectroscopy. These findings underscore the role of strategic ligand engineering in enhancing perovskite NC-charge acceptor interactions, enabling better charge extraction, higher solar cell efficiency, and reduced lead toxicity through strong Pb binding.