Development of high-performance organic photodetectors by understanding origin of dark current density with synthesis of photoconductive polymers

Abstract

Photoconductive polymers (PCPs) are key materials that determine the static and dynamic performance of p-n bulk heterojunction organic photodetectors (OPDs). Herein, we developed benzo[1,2-d:4,5-d']bis(oxazole)-based novel PCPs with π-spacer molecular engineering to promote charge-transport while maintaining low dark-current densities (Jd) in OPDs. The origin of Jd was elucidated by measuring hole charge-transfer (CT) state energy. We found that the degree of hole accumulation at the CT state rather than the CT state energy was crucial to maintaining a low Jd and promoting a fast signal response in the devices. The optimized OPDs showed highly promising responsivity (R) of 0.43 A/W and specific detectivity (D*) of 6.85 × 1012 Jones at −1 V under 532 nm LED (5.0 mW cm−2) illumination, which are superior to those of the control device (R = 0.29 A/W and D* = 5.46 × 1011 Jones). In addition, the dynamic properties were significantly improved by the introduction of π-spacers on the PCPs. −3 dB cut-off frequency was 200.4 kHz, and the rising and falling response times was ultrafast as 2.3 and 2.8 μs, respectively. Understanding the origin of Jd through the molecular engineering of PCPs is expected to contribute greatly toward expanding the knowledge required to develop high-performance OPDs.