Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) remains the most widely used hole-transport layer (HTL) in inverted perovskite optoelectronics, yet its work function is misaligned with the perovskite valence band, and its limited conductivity and transparency cause interfacial losses that limit photodetector sensitivity and interfacial transport [3–7]. Here, we present an HTL-engineering strategy that simultaneously addresses energetics, charge transport, and optical field confinement by blending carbon-encapsulated iron nanoparticles (Fe@Cₓ NPs) into PEDOT:PSS [1]. The Fe@Cₓ NPs are recovered from carbon nanotube synthesis byproducts by dispersion and size-selective filtration, offering an upcycled and scalable additive that was previously shown to provide gap-plasmon enhancement in perovskite solar cells [2].

Two complementary mechanisms operate. First, π–π electronic coupling between the 1–2 nm graphitic shells and the thiophene rings of PEDOT raises the work function from 5.03 to 5.24 eV, as confirmed by ultraviolet photoelectron spectroscopy, cyclic voltammetry, and FTIR changes in the 1300–1600 cm⁻¹ region, improving HOMO alignment with the FA₀.₉Cs₀.₁PbI₃ absorber. Second, controlled shell–shell interactions induce nanoscale clustering of the particles, generating gap-plasmonic field enhancement that compensates for the parasitic losses of pristine PEDOT:PSS. The carbon shell isolates the metal core from the perovskite, suppressing the metal-ion migration typically associated with bare plasmonic additives.

The combined effect is a marked gain in photoresponse. By optimizing the dispersion time to maximize the gap-plasmon effect, the devices exhibit voltage-dependent photoamplification with responsivities reaching 10³ A W⁻¹ under low-light illumination, and even under self-powered operation (0 V) the responsivity increases from 0.38 to 0.43 A W⁻¹. At the optimal loading of 2.5 vol%, this sensitivity gain is accompanied by an 87% reduction in dark current density (from 2.12 × 10⁻⁸ to 2.71 × 10⁻⁹ A cm⁻²), a decrease in defect-state energy from 2.31 to 1.08 meV, a 3.2-fold higher specific detectivity (1.49 × 10¹³ Jones), and a 6.6 dB higher signal-to-noise ratio with stable response beyond 300 kHz.

In this talk, I will discuss how the electronic and plasmonic contributions can be separated and tuned through nanoparticle concentration and dispersion time, and how carbon-shelled metal nanoparticles can serve as a general, low-cost HTL modifier for perovskite photodetectors and solar cells.