π–π Electronic Coupling and Gap‐Plasmonic Enhancement via Fe@C _x Nanoparticles: Synergistic Hole‐Transport Engineering for High‐Responsivity Perovskite Photodetectors

We report a multifunctional interface‐engineering strategy in which carbon‐encapsulated iron nanoparticles (Fe@C x NPs) are blended into poly(3,4‐ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) to co‐optimize energetics, charge transport, and optical field confinement. Organic‐inorganic hybrid optoelectronic interfaces suffer from energetic misalignment and charge transport limitations. This study addresses these challenges through carbon‐encapsulated iron nanoparticles (Fe@Cx NPs) derived from carbon nanotube synthesis byproducts, which modulate the work function of poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) from 5.03 to 5.24 eV via π–π electronic coupling, representing a 0.21 eV improvement, with optimized energy level alignment of the highest occupied molecular orbital. When the concentration of incorporated iron nanoparticles is optimal at 2.5 v%, the dark current of the devices reduces by 87% from 2.12 × 10 − 8 to 2.71 × 10 − 9 A/cm 2 , and a 53% decrease in defect‐state energy from 2.31 to 1.08 meV. Under self‐powered conditions (0 V), the responsivity increases by 13% from 0.38 to 0.43 A/W, whereas shot‐noise‐limited detectivity improves 3.2‐fold from 4.65 × 10 1 2 to 1.49 × 10 1 3 Jones. Frequency response analysis demonstrates a 6.6‐dB signal‐to‐noise ratio enhancement from 59.3 to 65.9 dB with stable operation beyond 300 kHz. This synergistic electronic‐plus‐plasmonic approach provides a scalable route to the development of high‐performance optoelectronic devices.