Fine-control of electron trapping for photomultiplication in organic photodetectors by modulating the insulating properties of nonfullerene acceptors

Enhancing photomultiplication (PM) in organic photodetectors (OPDs) requires effective control of photogenerated trapped electrons. Conventional approaches typically adjust the conduction band of the acceptor. In this study, we present a novel strategy for precisely controlling electron trapping and trapped carrier lifetime in PM-OPDs. We synthesized a series of nonfullerene acceptors (NFAs) by modifying the ratio of conducting and insulating components in the conjugated molecular structure. By enhancing the insulating properties of the NFAs, we slowed both electron trapping and de-trapping processes. This resulted in fine-tuned trapped carrier lifetime (280-581 ms), trapped carrier density (2.04-13.4 × 10¹⁷ cm^-3), and Schottky barrier height (0.334-0.229 eV) under illumination. Higher concentrations of trapped electrons accumulated at the P3HT/Al interface, leading to a thinner Schottky barrier, reduced depletion region, and enhanced band bending, improving hole injection from the cathode. Among the NFAs, A-BTP-DT, with the highest insulating properties, exhibited superior PM effects, achieving a maximum external quantum efficiency of 7520% at -10 V (compared to 564% for the Y6-based device). This work represents the first successful demonstration of fine-tuning of electron trapping and PM effects, not by adjusting energy levels, but by modifying the insulating properties.