Mitigating the Trade-Off between Bandgap Narrowing and Trap-Limited Transport for Low-Noise Organic Short-Wave Infrared Photodetectors

Solution-processed organic photodetectors present a promising route to short-wave infrared detection; however, accessing the spectral region beyond 1200 nm requires sub-1.0 eV organic absorbers, typically achieved through strong intramolecular charge transfer, enhanced quinoidal character, and extended π-conjugation; whereas, conformational and energetic disorder in such narrow-bandgap systems can contribute to trap-mediated transport losses and elevated dark current. This study introduces Fc2BS and Fc2BT, narrow-bandgap nonfullerene acceptors differing solely in the π-bridge unit (benzoselenadiazole vs benzothiadiazole), to examine how chalcogen substitution influences bandgap narrowing and trap-related transport in sub-1.0 eV organic absorbers. Fc2BS achieves an optical bandgap of 0.98 eV with absorption extending to 1263 nm, where benzoselenadiazole substitution is associated with reduced structural and energetic disorder relative to benzothiadiazole. Compared to the Fc2BT-based device, the Fc2BS-based device suppresses trap density by 8-fold and extends the critical carrier transport length by an order of magnitude, mitigating the dark current–responsivity trade-off and achieving a dark current density of 7.88 × 10^–9 A cm^–2 with an EQE of 12.5% at 1200 nm even at a low bias of −0.1 V, while enabling stable low-bias operation through suppressed bias-dependent collection bottlenecks and dark current drift. A directly measured noise equivalent power of 2.14 pW and corresponding specific detectivity of 1.47 × 10¹¹ cm·Hz^1/2·W^–1 at 1200 nm highlight the benzoselenadiazole π-bridge as a promising molecular-design motif for extending SWIR response while limiting noise and transport penalties.