Diazapentalene-Containing Ultralow-Band-Gap Copolymers for High-Performance Near-Infrared Organic Phototransistors

Because of the limited availability of synthetic strategies and strong acceptor units, constructing new types of low-band-gap donor–acceptor-type copolymers for use in multiple functional applications remains a big challenge. Herein, we report the synthesis, characterization, and optoelectronic applications (i.e., organic field-effect transistors (OFETs) and organic phototransistors (OPTs)) of a novel class of ultralow-band-gap copolymers (PDAP–Fu, PDAP–Th, and PDAP–Se) on the basis of the unique, interesting, yet rarely researched bicyclic 2,5-diazapentalene (DAP) strong acceptor in conjugation with chalcogenophene donors (furan (Fu), thiophene (Th), or selenophene (Se)). All of the copolymers exhibit broad near-infrared (NIR) absorption and optical band gaps as low as ∼1.0 eV. The effects of the actual chalcogen atoms on the geometry, optical properties, energy levels, and film organization are carefully determined for OFET and OPT applications. Regarding the OFET studies, all of the copolymers show unipolar transport behavior in bottom-gate and top-contact OFETs, and PDAP–Se exhibits the highest hole mobility of 4.76 × 10–1 cm2 V–1 s–1. Besides, investigations of the OPTs indicate that a high photoresponse is achieved for all of the copolymers at a wavelength of 1060 nm in the NIR spectral region combined with an excellent external quantum efficiency (η) and photodetectivity (D*). This is particularly true for PDAP–Se (η = 6.56 × 104% and D* = 1.80 × 1012 Jones). Thus, such ultralow-band-gap copolymers are promising candidates for use in integrated circuits and optoelectronic devices.