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. Incorporation of a proper n‐type molecule into a narrow bandgap polymer channel facilitates exciton dissociation and shortens minority charge trapping/recombination process upon near‐infrared light illumination. Organic phototransistors utilizing this bulk‐heterojunction channel have simultaneously afforded a clearly improved photoresponse and a much‐accelerated response speed, which meets the requirement of portable and real‐time health monitoring applications.