p- and n-Channel Photothermoelectric Conversion Based on Ultralong Near-Infrared Wavelengths Absorbing Polymers

Organic materials absorbing near-infrared (NIR) light are very attractive for the fabrication of optoelectronic devices. In this study, we developed an ultralow energy gap copolymer TzQI-TDPP composed of thiadiazoloquinoxalinimide (TzQI) and thiophene-flanked diketopyrrolopyrrole (TDPP) repeat units. TzQI-TDPP has a nearly identical narrow energy gap (0.60 eV) to that of the p-channel thienoisoindigo-based homopolymer PTII. Both polymers exhibit broad and intense optical absorption in the NIR-II light window (1000–1700 nm). Examination of charge polarity using field-effect transistors indicates p-channel conduction for PTII and n-channel-dominant ambipolar conduction for TzQI-TDPP with moderate mobilities, in which a thin film of TzQI-TDPP displayed air-stable n-channel performance with a persistent mobility of over 0.003 cm2 V–1 s–1 after 30 days. In addition, we explored the photothermal (PT) and thermoelectric (TE) effects in the NIR-II light window by fabricating a photothermoelectric device. Both polymers exhibit PT conversion efficiencies of ∼30%, and the TE effect is observed in p-channel PTII and n-channel TzQI-TDPP. Notably, the PTII and TzQI-TDPP films display excellent photostability during on–off irradiating light cycles, indicating prominent NIR light detection. Our work not only provides a set of p-channel and n-channel-dominant ambipolar polymers with ultralow energy gaps but also demonstrates their underlying structure–property correlations based on electronic structures and their promising potential in applications utilizing NIR-II light.