Synergistic binding between an engineered interface and functionalized ferrocene offers remarkable charge extraction efficiency in lead halide perovskites

Abstract

Ferrocene derivatives have been recognized as a new hole-transporting layer (HTL), imparting stability, enhanced charge separation, and transport properties at the perovskite/HTL interface. Perovskite solar cells with functionalized ferrocene have attained an efficiency greater than 25% with enhanced stability. However, the fundamental challenge of the excess energy loss of above-bandgap “hot” charge carriers, which occurs through hot carrier cooling, contributes to the Shockley–Queisser limit of perovskite solar cells. Therefore, the ultrafast transfer of hot carriers to electron/hole-capturing materials can further boost the power conversion efficiency to a great extent. Herein, we demonstrate the importance of interfacial interactions and strong binding between the charge carrier donor and the acceptor to carry out hole transfer in the sub-picosecond time scale. We modified the donor, cesium lead bromide (CPB) nanocrystals, with a cationic NK-12 ligand to enhance the coulombic attraction between the NH₃⁺ group of NK-12 and the COO⁻ group of the acceptor, FcS4, a functionalized ferrocene with two ferrocene units. Double reciprocal plot analysis quantified the interaction between modified CPB and FcS4, with the highest reported association constant, K_app, of 2.5 × 10⁸ M⁻¹. Transient absorption analysis inferred a hot hole transfer within 550 fs with a hole transfer rate, K_ht of 1.45 × 10¹² s⁻¹. This ultrafast transfer of holes is attributed to the strong coulombic attraction between CPB and FcS4 via NK-12 because, with oleylamine and oleic acid-stabilized CPB, we did not observe such ultrafast transfer. These results show that interfacial interactions between the charge carrier donor and acceptor dictate the hot carrier dynamics of perovskites and can play a significant role in overcoming the Shockley–Queisser limit.