Enabling Fast Photoresponse in Near-Infrared Organic Phototransistors by Manipulating Minority Charge Trapping and Recombination

The organic phototransistors (OPTs) featuring built-in amplification afford ultrahigh photoresponse. However, the response speed is usually slow in disordered OPTs, limited by the charge trapping/recombination process caused by inherent trap sites. This work studies the relationship between the photoinduced charge carrier dynamics and photoresponse characteristics in a near-infrared (NIR) light-sensitive OPT. It is found that the introduction of a small amount of electron acceptor into a p-type polymer, forming a donor:acceptor bulk-heterojunction (BHJ), in the OPT channel can dramatically improve the overall response speed of the detector. This is because the exciton dissociation and charge separation at the donor/acceptor interface not only improves the photoresponse but also reduces the minority charge trapping. Less charge trapping/recombination is essential to achieve a fast response in OPTs. As a result, the rise and fall time are improved from 1.3 and 3.5 s for the polymer-only-based NIR OPTs to 16 and 119 ms for the organic BHJ-based NIR OPTs, making it suitable for use in medically relevant photoplethysmography applications.