Weakened charge trapping at the electrode/active layer interface in a bulk heterojunction-based organic phototransistor for quick photomultiplication†

Abstract

Charge trapping and release have a significant impact on the performance of organic phototransistors (OPTs), especially for the balance of response time and photomultiplication. These processes are usually present near the interface with different energy levels or mobilities. In this paper, charge trapping and release properties at the electrode/active layer interface in PDPPBTT:PC₆₁BM based organic phototransistors have been discussed in detail. We compared the effect of interfacial charge trapping at a Ag or Au electrode on the photodetection performance of OPTs in the absence and presence of acceptors. When acceptors were added to form a bulk heterojunction as the active layer, it was fascinating to observe that not only did the BHJ with a smaller energy level difference become the main charge trap sites, but also the charge trap near the electrode was evidentially weakened and easy to release, which is supported by the turn-on voltage drift (ΔV_on), contact resistance (R) change and accelerated response time. This originated from acceptor-assisted carrier recombination near the metal electrode. The results show that BHJ based phototransistors are good candidates for quick photomultiplication photodetectors.