Dual-band and binary response of high-performance photodetectors with bithiophene-difluoroquinoxaline based copolymers via side chain engineering towards secure optical communication

Abstract

In present work, three bithiophene-difluoroquinoxaline (TQ) random copolymers (PTQ1-PTQ3) with the similar conjugated skeleton and different substituted side chains such as alkyl, phenyl, and ester groups are designed and synthesized using the direct C–H/C–H coupling polymerization method. All polymers exhibit intense UV absorbance and shallow highest occupied molecular orbital (HOMO) levels, and their photophysical properties could be effectively manipulated by engineering the TQ side chains. The PTQ3, which contains two phenyl groups on 2- and 3-position of 6,7-difluoroquinoxaline units and two dioctyl groups on 4- and 4′- position of 2,2′-bithiophene units, displays intramolecular charge transfer (ICT) character, optimized energy levels, and high hole mobility of 6.4 × 10−4 cm2 V−1 s−1. Furthermore, these polymers blending with small molecular acceptor 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetralcis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d']-s-indanone[1,2-b:5,6-b']-dithiophene (ITIC) are evaluated as photosensitive layers in polymer photodetectors (PPDs) for light detection. All devices show the dual-band photoresponse and self-powered effect. Especially, PTQ3-based device exhibits an extremely low dark current density of 8.1 × 10−10 A cm−2, a superior external quantum efficiency (EQE) of 33.9% and a high photo-detectivity exceeding 6.0 × 1012 Jones under 365 nm at 0 V, outperforming the reported thiophene-quinoxaline based PPDs. Moreover, an interesting binary response of optimal PPD has been demonstrated by imposing small external bias voltages, which is conducive to the transmission of multi-encoding information. Then, we also propose a feasible encryption scheme for optical communication by modulating the UV and red dual-band response of the PPD. The results illustrate a viable design strategy for side chain engineered bithiophene-difluoroquinoxaline based polymers and provide some design guidelines for high-performance PPDs applied in secure optical communication.