Polydopamine‐Polyoxometalate Composites in Neutral pH Enable High‐Efficiency and Ultra‐Stable Organic Solar Cells via Mutual Doping Mechanism

Heavy doping critically minimizes depletion region widths for efficient charge transport in organic solar cells (OSCs), yet systematic studies elucidating its underlying mechanisms remain scarce. To address this, two polydopamine-polyoxometalate composites (PDA-PMA and PDA-PMA(N)) are designed via innovative mutual doping pathways. PDA-PMA achieved ultrahigh doping density (1.17 × 10²³ cm^-3) through H₃[P(Mo₃O₁₀)₄] (PMA) initiated oxidative polymerization of dopamine, where electron transfer simultaneously induced p-doped PDA and n-doped PMA. Remarkably, neutralization with ammonia yielded PDA-PMA(N), which retained even higher doping density (3.74 × 10²³ cm^-3) via structural rearrangement-driven organic doping. XPS/ESR studies revealed distinct pathways: dual organic/inorganic doping in PDA-PMA versus organic-dominated doping in PDA-PMA(N). This deep doping compressed depletion region widths from 44.42 nm in undoped controls (the blend of PDA and PMA) to 0.052 nm (PDA-PMA(N)), surpassing PEDOT:PSS (0.238 nm) and enabling barrier-free hole transport. Consequently, PBDB-TF:BTP-eC9-based OSCs with PDA-PMA and PDA-PMA(N) achieved exceptional power conversion efficiencies (PCEs) of 20.02% and 20.29%, respectively. Furthermore, the neutralized PDA-PMA(N) demonstrated superior stability (86.2% PCE retention after 1800 h illumination) by suppressing interfacial corrosion. This work elucidates structure-dependent doping mechanisms and provides a universal strategy for developing high-performance hole transport layers through tailored doping, advancing OSC commercialization.