Mitigating Dark Current in Photomultiplication Organic Photodetectors via the Charge Trap Gradient Bulk Heterojunction

Abstract

Photomultiplication-type organic photodetectors (PM-OPDs) with dispersed electron or hole traps in a bulk heterojunction (BHJ) have external quantum efficiency far exceeding unity. However, it typically requires a very low donor-to-acceptor ratio, as excess donor or acceptor molecules in the BHJ lead to a high dark current by forming dense charge trap pathways, resulting in hopping conduction. The BHJ layer with a low donor-to-acceptor ratio often associates with a high operating voltage, limiting the use of the PM-OPDs. In this study, we report the results of a new approach to reducing dark current by employing a charge trap gradient design in PM-OPD. This gradient provides two key benefits: (1) it reduces dark current by eliminating charge percolation pathways through regions with low charge trap concentration and (2) it enhances band bending near the electrode by creating regions with high charge trap concentration, facilitating efficient tunneling charge injection. The PM-OPD with a gradient charge trap enables the dark current to be 1 order of magnitude lower than that of an optimal BHJ-based conventional PM-OPD, achieving a high responsivity of 25.40 A/W at 890 nm, operated under 0.3 V, which is nearly 40 times higher than the commercial Si photodiode. These results offer promising opportunities for diverse applications.