Molecular Engineering of Benzobisoxazole-Based Conjugated Polymers for High-Performance Organic Photodetectors and Fingerprint Image Sensors

Various novel conjugated polymers (CPs) have been developed for organic photodetectors (OPDs), but their application to practical image sensors such as X-ray, R/G/B, and fingerprint sensors is rare. In this article, we report the entire process from the synthesis and molecular engineering of novel CPs to the development of OPDs and fingerprint image sensors. We synthesized six benzo[1,2-d:4,5-d’]bis(oxazole) (BBO)-based CPs by modifying the alkyl side chains of the CPs. Several relationships between the molecular structure and the OPD performance were revealed, and increasing the number of linear octyl side chains on the conjugated backbone was the best way to improve J_ph and reduce J_d in the OPDs. The optimized CP demonstrated promising OPD performance with a responsivity (R) of 0.22 A/W, specific detectivity (D*) of 1.05 × 10¹³ Jones at a bias of −1 V, rising/falling response time of 2.9/6.9 μs, and cut-off frequency (f_-3dB) of 134 kHz under collimated 530 nm LED irradiation. Finally, a fingerprint image sensor was fabricated by stacking the POTB1-based OPD layer on the organic thin-film transistors (318 ppi). The image contrast caused by the valleys and ridges in the fingerprints was obtained as a digital signal.