Dual Functionality of Bidentate Ligands for Optimized Charge Extraction in Lead Halide Perovskites: Implications for Perovskite Solar Cells

Perovskite nanocrystals (NCs) have emerged as key materials in photovoltaics. Yet, a fundamental understanding of their interfacial interactions with charge acceptor molecules is essential for optimizing charge transport pathways and enhancing device performance. In this study, we systematically investigate the binding sites of hole acceptors on CsPbBr₃ (CPB) NCs by employing targeted ligand engineering with pyridine-based capping agents. Using 1,10-Phenanthroline (Phen), 2,2′-bipyridine (2,2′-BPY), and 4,4′-dipyridine (4,4′-DPY), we demonstrate that bidentate coordination significantly influences anchoring to the perovskite surface. Among these, Phen exhibits the strongest binding affinity to Pb sites, effectively suppressing hole transfer to ferrocene-based acceptors, FcA and FcAm. However, FcAm retains its hole extraction ability through alternative interactions with Br sites, underscoring the presence of multiple charge transfer pathways. These binding interactions and charge transfer dynamics are comprehensively validated through steady-state and time-resolved photoluminescence (PL) spectroscopy and transient absorption measurements. Additionally, our designed dipyrido[3,2-a:2′,3′-c] phenazin-11-amine phenazine (PhZ), a hole acceptor with extended π-conjugation, enhances charge separation by stabilizing long-lived charge-separated states via the Stark effect. Even in the presence of Phen, which blocks Pb²⁺ sites, PhZ efficiently extracts holes, demonstrating strong binding affinity and favorable electronic properties. This study provides a direct methodology for identifying charge acceptor binding sites and underscores the critical role of surface chemistry in guiding charge transfer. By offering molecular-level insights into perovskite-acceptor interactions, these findings inform the rational design of optimized charge transport pathways, advancing high-performance perovskite solar cells.