Interface Engineering via Self-Assembled Monolayers Enables Highly Sensitive Photomultiplication-Type Organic Photodetectors for Photoplethysmography

Photomultiplication-type organic photodetectors (PM-OPDs) were fabricated by employing a blend of P3HT and the narrow-band gap small-molecule acceptor FM4. The blend ratio between P3HT and FM4 was optimized to 100:4 (wt/wt). Under –10 V bias, the PM-OPDs based on P3HT:FM4 (100:4, wt/wt) exhibit a spectral response range from 300 to 900 nm with EQE values of 1600% at 625 nm and 850% at 850 nm. This behavior is attributed to hole tunneling injection triggered by trapped electrons. To further improve the sensitivity of PM-OPDs, PEDOT:PSS is replaced by the self-assembled monolayer (SAM) Ph-4PACz as the interfacial layer. The dark current density (J_D) of PM-OPDs drops from 2.11 × 10⁻⁶ to 4.84 × 10⁻⁷ A cm⁻², while the light current density (J_L) slightly increases by employing Ph-4PACz, resulting in an apparently improved signal-to-noise ratio from 560 to 2700. At –10 V bias, the optimized PM-OPDs deliver EQEs of 1800% (625 nm) and 950% (850 nm), along with specific detectivities (D^*_shot) of 1.36 × 10¹³ Jones at 625 nm and 6.31 × 10¹² Jones at 850 nm. The optimized PM-OPD-based photoplethysmography PPG sensor enables reliable detection of blood oxygen saturation and heart rate, with measurement discrepancies remaining within 2% when benchmarked against a commercial smartwatch.