Fine-Tuning of Alkyl Side Chains in Ultra-Low Bandgap Polymers To Effectively Suppress the Dark Current for Short-Wavelength Infrared Organic Photodetectors

Short-wavelength infrared organic photodetectors (SWIR OPDs) have great potential for applications in health monitoring, night vision, optical communication, and image sensing. However, the development of SWIR OPDs is limited by challenges in achieving high responsivity (<i>R</i>) and detectivity (<i>D</i>*) due to the exponentially increased nonradiative recombination rate when decreasing the bandgap of conjugated polymers or molecules. In this study, we designed and synthesized a series of donor-acceptor (D-A) type ultralow bandgap (≤0.85 eV) polymers containing [1,2,5]thiadiazolo[3,4-g]quinoxaline (TQ) units as the A unit and selenophene units as the D unit, respectively. The solubility, molecular stacking, charge transport properties, and film morphology of the polymers were finely tuned by varying the side chain lengths on the substituted phenyl groups of TQ units. It was found that the polymer PTQOD with 2-octyldodecyl alkyl chains has lower nonradiative recombination losses and lower trap state density. After device optimization, the PTQOD-based device achieved higher <i>R</i> and lower dark current density (<i>J</i>_d), resulting in a <i>D</i>* of 1.06 × 10¹⁰ Jones at 1300 nm under 0 V bias, representing the highest value for OPDs using TQ-based polymers. This work highlights the importance of optimizing the alkyl chains of ultralow-band gap polymer donor materials and provides a promising approach for developing highly sensitive SWIR OPDs.