Ferrocene-Based Self-Assembled Monolayers Enable Ultra-High Specific Detectivity in Perovskite Photodetectors

Molecular design of self-assembled monolayers (SAMs) for perovskite photodetector (PD) applications has largely been inherited from photovoltaics, where molecules have been designed to maximize charge extraction rather than to minimize dark current density (J_d). Here, we move beyond conventional carbazole-core SAMs and introduce ferrocene (Fc)-based SAMs (Fc-SAMs) as hole transport layers specifically designed for PD operation. These Fc-SAMs enable a unique energy alignment strategy where the work function of ITO is modified to misalign with the valence band maximum of an adjacent metal halide perovskite. In doing so, interfacial band bending is reduced, suppressing thermally activated charge injection. Simultaneously, the Fc moiety passivates interfacial defects, lowering defect density two-fold when compared to a benchmark SAM. These effects enable an ultralow J_d of 2.52 × 10^-10 A cm^-2 and a noise current of 3.80 × 10^-15 A Hz^-1/2, which generates a state-of-the-art specific detectivity (D*) of 2.56 × 10¹³ Jones at 680 nm and -0.5 V. We also establish that Fc-SAM performance is independent of alkyl chain length (0 ≤ n ≤ 6), contrasting established SAM design rules. Moreover, Fc-SAM devices exhibit excellent operational stability, supporting LiFi communication rates exceeding 1 Mbps and highlighting their potential for commercial optoelectronic applications.