New thiophene-phenylene-thiophene acceptor random conjugated copolymers for optoelectronic applications

New low band gap thiophene-phenylene-thiophene (TPT)-based donor-acceptor-donor random copolymers were synthesized for optoelectronic device applications by a palladium-catalyzed Stille coupling reaction under microwave heating. The acceptors included 2,3-bis(4-(2-ethylhexyloxy)phenyl)-5,8-bis[5′-bromo-dithien-2-yl-quinoxalines] (DTQ) and 3,6-bis(5-bromothiophen-2-yl)-2,5-bis(2-ethyl-hexyl)-pyrrolo[3,4-c]-pyrrole-1,4-dione (DPP). The prepared random copolymers were named as PTPTDTQ₀ . ₅₅, PTPTDTQ₀ . ₃₄DPP₀ . ₁₄, and PTPTDTQ₀ . ₂₆DPP₀ . ₃₄ depending on the copolymer ratio. The optical band gaps (Eopt g) of PTPTDTQ₀ . ₅₅, PTPTDTQ₀ . ₃₄DPP₀ . ₁₄, and PTPTDTQ₀ . ₂₆DPP₀ . ₃₄ were 1.74, 1.56, and 1.48 eV, respectively. The hole mobility obtained from the field-effect transistor devices prepared from PTPTDTQ₀ . ₅₅, PTPTDTQ₀ . ₃₄DPP₀ . ₁₄, and PTPTDTQ₀ . ₂₆DPP₀ . ₃₄ were 2.2 x 10⁻³, 2.4 x 10⁻³, and 4.7 x 10⁻³ cm² V⁻¹ s⁻¹, respectively, with the on-off ratios of 4.0 x 10⁴, 4.0 x 10⁴, and 5.3 x 10⁴. It suggested that the significant intramolecular charge transfer between the TPT and acceptor led to the band gap reduction and hole mobility enhancement. Polymer solar cells of these TPT-based copolymers blended with 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-C-71 (PC₇₁BM) under illumination of AM 1.5G (100 mW cm⁻²) solar simulator exhibited a power conversion efficiency (PCE) as high as 3.71%. Besides, the near-infrared photodetector device prepared from PTPTDTQ₀ . ₂₆DPP₀ . ₃₄ showed a high external quantum efficiency exceeding 32% at 700 nm (under -3 V bias) and fast-speed response. This study suggests that the prepared TPT-based donor-acceptor random copolymers exhibited promising and versatile applications on optoelectronic devices.