Stable Open-Shell Conjugated Terpolymer with Extended NIR Absorption for Organic Photodetectors Detecting Beyond 1000 nm

Near-infrared (NIR) photodetectors play crucial roles in many scientific, industrial, and medicinal fields. However, conventional organic photodetectors (OPDs) often do not utilize the NIR region due to poor absorption beyond 1000 nm. In this study, an open-shell conjugated terpolymer is synthesized for NIR detection. This polymer contains diketopyrrolopyrrole (DPP), thiophene, and benzo[1,2-c;4,5-c′]bis[1,2,5]thiadiazole (BBT); these components form the novel random terpolymer poly{2,5-bis(2-decyltetradecyl)-3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione]-co-thiophene-co-benzo[1,2-c;4,5-c’]bis[1,2,5]thiadiazole} (PDPPTBBT) via Stille coupling polymerization. The diradicals generated by the open-shell characteristics of PDPPTBBT become stronger as molecular packing is enhanced. This enhancement enables absorption at wavelengths beyond 1000 nm. PDPPTBBT exhibits temperature-independent Pauli paramagnetic properties. Additionally, electron paramagnetic resonance measurements reveal that compared with the singlet ground state, the polymer exhibits a higher stability in the triplet ground state and a high spin (S = 1). PDPPTBBT can act as an acceptor or a donor in films in which the material is blended with either poly(3-hexylthiophene-2,5-diyl) or Y6. OPDs prepared using the blended films display detection wavelengths exceeding 1000 nm with a maximum external quantum efficiency of 126% at 1050 nm and a specific detectivity (D*) of 7.5 × 10¹¹ Jones.