ZnO nanoparticles enhanced near-infrared photodetection in organic phototransistors based on narrow bandgap polymer

Abstract

Highly sensitive near-infrared (NIR) photodetectors have attracted extensive attention due to their wide applications in thermal imaging, night vision and human health monitoring. Organic phototransistors (OPTs) are an important type of photodetector with three-terminal contact, which provides high sensitivity and tunable gain with an external quantum efficiency (EQE)>100% by controlling unbalanced charge transport through an optically controlled gate terminal. Thus, NIR OPTs are considered as a promising alternative NIR light detecting technology to conventional inorganic NIR phototransistors. The design and fabrication flexibility provided by the organic semiconductors and solution processes also have significant cost benefit, creating next-generation large-area, low-cost and flexible NIR photodetectors. However, now the performance of NIR-OPT is not high enough, which is still far from practical applications. Low field-effect mobility, limited exciton dissociation, and poor NIR light absorption are the main reasons to hamper the performance of NIR OPTs. A key technical development to circumvent this hurdle is to create an organic channel layer with ordered microstructure to provide high field-effect mobility and a smart device design to enhance exciton dissociation. In this work, an air stable conjugated semiconducting polymer of diketopyrrolopyrrole-dithienylthieno[3,2- b ]thiophene (DPP-DTT) with narrow band gap was selected as the NIR light response medium. The DPP-DTT nanowire network was obtained by a solution-processed and scalable approach called polymer-matrix-mediated molecular self-assembly. The matrix polymer of polystyrene (PS) was selectively removed from the resulted DPP-DTT:PS blend film, leaving the pristine DPP-DTT nanowire network thin film as the active channel layer for the NIR OPTs. The electrical properties and NIR response of the OPTs based on DPP-DTT nanowire network were studied. It is found that the pristine DPP-DTT nanowire network layer exhibited a typical p-type field-effect transport, showing the hole mobility of >1 cm2/(V s) and on/off ratio >106. These devices also showed sensitive NIR response with photoresponsivity ( R ) of 336 A/W and specific detectivity ( D *) of 5.6×1012 Jones under NIR light intensity of 8 μW/cm2. To further improve the NIR photodetection of the DPP-DTT nanowire network OPTs, zinc oxide (ZnO) nanoparticles were introduced into the devices as electron acceptor material. A small amount of ZnO nanoparticles (1.0 mg/mL) were spin coated onto the DPP-DTT nanowire network to increase the separation efficiency of photodenerated excitons and then capture the photogenerated electrons in trap sites, thereby minimizing the negative impact of photogenerated electrons on hole transport in the NIR OPTs. As a result, the NIR photodetection of the ZnO-modified OPTs was significantly improved. The photosensitivity of ZnO-modified devices reaches as high as 107, which is increased by about 4 to 5 orders of magnitude compared with the pristine DPP-DTT nanowire network NIR OPTs. Correspondingly, the R and D * of ZnO-modified NIR OPTs have been also enhanced. The maximum R value was increased to 721 A/W, which is more than twice the R value of the pristine DPP-DTT nanowire network NIR OPTs. It is noted that such a large R value has achieved the performance level of conventional inorganic NIR phototransistors. Moreover, the maximum D * of the ZnO-modified NIR OPTs reaches 1.3×1013 Jones under the NIR intensity of 8 μW/cm2. Therefore, this work provides a good idea for the structural design and device performance optimization of high-performance NIR OPTs.