Redox-Modulated Polyoxometalate-Incorporated Carbazole-Based Interface for High-Performance Perovskite Photodetectors

High-performance self-powered perovskite photodetectors demand hole transport layers (HTLs) concurrently possessing excellent energy level alignment, optimal wettability, and effective defect passivation. Although the widely used [2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid (Me-4PACz) self-assembled monolayer (SAM) satisfies energy-level requirements, its hydrophobicity and limited defect passivation hinder concurrent optimization of film quality and interface defects, limiting device performance. Herein, we address this via a Sn²⁺-mediated interfacial chemical reconstruction of phosphomolybdic acid (PMA). Preforming a PMA:Sn composite and blending it with Me-4PACz yields a ternary HTL, Me-4PACz:(PMA:Sn). Sn²⁺ reduces partial Mo(VI) to Mo(V) in PMA, enabling triple interface optimization: (i) tuning the PMA acidity to preserve the SAM transport characteristics, (ii) enhancing the interfacial polarity for the uniform perovskite film growth, and (iii) moderating Pb²⁺ coordination to suppress detrimental PbI₂ formation. This results in high-quality, vertically oriented methylammonium lead triiodide (MAPbI₃) films. The corresponding photodetector achieves an ultralow dark current of 1.38 × 10^-10 A cm^-2, a high detectivity of 5.76 × 10¹³ Jones, a wide dynamic range of 143 dB, fast response of ∼5 μs, and stable operation. This work demonstrates redox-mediated polyoxometalate chemistry as a versatile strategy for rational SAM interface design in perovskite optoelectronics.