Mitigation of Dark Current in Photomultiplication Organic Photodetectors

Organic photodetectors (OPDs) offer a promising alternative to conventional inorganic counterparts due to the customizable optical and electrical properties of the organic semiconductor materials. Photodiode-type OPDs are limited to an external quantum efficiency (EQE) of 100%. The photomultiplication (PM) effect is demonstrated in OPDs achieving an EQE of > 100%. Different PM OPDs having dispersed electron or hole traps in a bulk heterojunction (BHJ) have been reported. Under illumination, lightinduced current can be produced, due to the trap-assisted charge injection at the BHJ/electrode interface in a PM OPD under an operation voltage. However, the ratios of donors to acceptors that produce either electron or hole traps in a BHJ must be very low. An excess fraction of the donor molecules creating hole traps, or an extra fraction of acceptor molecules producing electron traps in a BHJ, will result in a high dark current in a PM OPD. This occurs because a high density of charge traps forms a percolation
pathway, causing hopping conduction by the charges in the BHJ. The performance of the PM OPD deteriorates as the donor-to-acceptor ratio shifts away from the respective optimal values. In addition, a low donor-to-acceptor ratio across a BHJ also requires a high operation voltage, impacting the practicality of the PM OPD as a viable option for
different applications.

This project proposes to develop a novel approach to mitigate the dark current in PM OPD by employing a charge trap gradient absorber. The donor-to-acceptor ratio is not uniformly across the BHJ, instead it has a vertical gradually changing ratio of donor to acceptor. The use of a charge trap gradient BHJ has two advantages: (1) it effectively eliminates the charge percolation pathway due to the existence of a very low charge trap density region, which reduces the dark current, (2) it creates a high charge trap density region to enhance the band bending in the BHJ near the electrode vicinity, and thereby facilitating an efficient tunneling charge injection due to an accumulation of the trapped photo-generated charges. This design helps to reduce the dark current and enhances trap-induced tunneling charge injection simultaneously at a low operation voltage. The resulting technology will enable high-performance PM OPDs for a plethora of applications in areas such as high-resolution image sensing, weak light detection, security monitoring, artificial intelligence, Internet of Things, and optical communications.