Synthesis and Optoelectronic Properties of Benzo[1,2-b:4,5-b′]dithiophene-Based Copolymers with Conjugated 2-(2-Ethylhexyl)-3,4-dimethoxythiophene Side Chains

In order to regulate the electronic ability of benzo[1,2-b:4,5-b']dithiophene (BDT), the new electron-donating unit (BDTOT) is designed and synthesized which consists of a BDT backbone with conjugated 2-(2-ethylhexyl)-3,4-dimethoxythiophene side chains. By alternating copolymerization of BDTOT with electron-accepting units of fluorinated benzothiadiazole (FBT), benzothiadiazole (BT), and pyrrolo[3,4-c]pyrrole-1,4-dione (DPP), three donor-acceptor (D-A) copolymers (PBDTOT-FBT, PBDTOT-BT, and PBDTOT-DPP) have been developed for PSC applications. The impact of dimethoxythiophene substituent and the electron-accepting strength of the acceptor units on the absorption, HOMO/LUMO energy levels, and photovoltaic properties of the resultant polymers is investigated in detail. PBDTOT-BT and PBDTOT-DPP exhibit relatively narrower bandgaps and PBDTOT-FBT possesses a down-shifted HOMO energy level as compared to their corresponding analogs without methoxy groups onto the conjugated thiophene side chains. The screening of the different blend ratio, the processing additive, and polar solvent post-treatment is conducted to optimize the polymer solar cell (PSC) devices. PSCs with PBDTOT-FBT as donor deliver a power conversion efficiency (PCE) of 2.55%. By treatment of the active layer with methanol to tailor the morphology, the solar cell based on PBDTOT-FBT exhibits the remarkably improved PCE of 4.84% with a V-oc of 0.92 V, a J(sc) of 8.71 mA cm(-2), and an FF of 60.3%.