New bulky side chain substituted benzodithiophene based 2D-conjugated polymers for optoelectronic applications

In order to explore the effect of bulky side chain substituted benzodithiophene (BDT) based polymers on optoelectronic properties, here we have designed and synthesized two new 2D conjugated donor–acceptor (D–A) copolymers P1 and P2 via Stille coupling of 2,3-bis(4-(2-ethylhexyloxy)phenyl)thiophene (BAPT) substituted BDT as (D) unit and 1,3-di(2-bromothien-5-yl)-5-(2-ethylhexyl)thieno[3,4-c]pyrrole-4,6-dione (TPD) or 2,5-ethylhexyl-3,6-bis(5-bromothiophen-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-dione (DPP) as (A) units. The new polymers P1 and P2 showed broad absorption windows with vibronic shoulders indicate π–π stacking of polymer backbones. In addition P1 and P2 showed deep highest occupied molecular orbital (HOMO) energy levels of −5.50, −5.35 eV, respectively which allow delivering high open-circuit voltages (V _oc) in bulk heterojunction polymer solar cells (BHJ PSCs). The donor photon energy loss (E _g–eV _oc) of P1 and P2 are 0.87 and 0.57 which is comparable to the previous reports. BHJ PSCs were fabricated with P1 and P2, and they displayed high V _oc of 0.99 and 0.78 V, respectively, with maximum power conversion efficiency of 2.05 and 0.96 % in additive free BHJ PSCs. The polymer field effect transistor mobilities of P1 and P2 are 8.0 × 10⁻³, 9.2 × 10⁻⁵ cm²/V s, respectively.