Near-Infrared Organic Photodetectors with Tailored Junction Thickness for Resonance-Enhanced Photoresponse and Suppressed Dark Current

Organic photodetectors (OPDs) based on nonfullerene acceptors (NFAs) offer a promising platform for near-infrared (NIR) detection, yet their performance beyond 1100 nm is often limited by low external quantum efficiency (EQE) due to inefficient exciton dissociation and severe nonradiative recombination in narrow-bandgap systems. In this work, we address these challenges by employing a low-dark-current NFA (QXIC-4F) and tailoring the junction thickness to induce resonance-enhanced absorption. Optical simulations guided the optimization of the active layer to 340 nm, establishing constructive resonance at 1130 nm. As a result, the device achieves an EQE of 23% under −5 V bias, nearly twice that of a nonresonant reference, while maintaining a low dark current density (J_d) of 1.8 × 10^–8 A cm^–2 and a noise current spectral density as low as 10^–16 A Hz^–1/2, yielding specific detectivities (D*) up to 10¹³ Jones at zero bias. The thick active layer also suppresses trap-assisted transport, preserving D* above 10¹¹ Jones under −5 V. All devices exhibit microsecond-scale response times with −3 dB bandwidths of 90 kHz, supporting their potential for high-speed operation. Furthermore, integration onto flexible substrates enables wearable photoplethysmography (PPG) monitoring. This work presents a scalable strategy that leverages optical resonance to simultaneously enhance sensitivity and reduce noise in NIR OPDs, advancing their applicability in biomedical and low-light sensing.