Synthesis of the novel 2,2-bithiophene-3,3-dicarboximide-based conjugated copolymers for OPVs

• The linear alkyl chains on the electron-deficient units are preferred to offer higher hole mobility and interchain interaction.
• The substitution of the branched chain with the linear chain on the electron deficient unit provided increased PCEs.
• The device based on the P1:PCBM (1:2) blend had PCE of 1.49%.

A new accepter unit, 4H-dithieno[3,2-c:2′,3′-e]azepine-4,6(5H)-dione, was prepared and utilized for the synthesis of the conjugated polymers containing electron donor–acceptor pair for OPVs. The dithienoazepinedione, bithiophene imide (BTI) group, is an attractive electron deficient unit since it demonstrates strong electron-withdrawing character, a planar architecture, and good solubility. The polymer with linear dodecyl-BTI unit and benzodithiophene unit substituted with bulky 2-octyldodecyl side chain was synthesized using Stille polymerization to generate P1. The new polymer, P2, was also synthesized by Suzuki polymerization of carbazole, thiophene and linear octyl BTI unit to increase conjugation length. The spectra of P1 and P2 solid films show absorption bands with maximum peaks at 395, 538 and 380, 491 nm, and the absorption onsets at 652 and 634 nm, corresponding to band gaps of 1.90 and 1.96 eV, respectively. The best device with P1: PC71BM in ODCB with 2% DIO showed a VOC of 0.97 V, a JSC of 3.06 mA/cm2, and a FF of 0.49, which yielded PCE of 1.49%. The highest PCE of P2:PC71PM (1:2) solar cells reaches 0.76% with a VOC of 0.84 V, a JSC of 2.48 mA/cm2, and a FF of 0.36.