Donor-acceptor copolymers based on dithienopyrrolobenzothiadiazole: Synthesis, characterization, and photovoltaic applications

In this work, we have developed a nitrogen-bridged pentacyclic dithienopyrrolobenzothia diazole (TPBT) arene where the two outer electron-rich thiophene moieties are covalently fastened with the central electron-deficient benzothiadiazole core by two nitrogen bridges. The rigid and coplanar TPBT was first copolymerized with alkylthienyl-substituted benzodithiophene (BDT) unit to obtain a new donor-acceptor (D-A) copolymer (PBDT-T) via Stille coupling polymerization. Then, to tune optoelectronic properties of PBDT-T, two random ternary D-A copolymers (PBDT-D30-T70 and PBDT-D70-T30) were synthesized from an electron-rich BDT unit and two electron-deficient units of diketopyrrolopyrrole (DPP) and TPBT that have complementary light absorption behavior. The optical and electrochemical properties of these random terpolymers can be easily tailored by varying the feed ratio of DPP and TPBT. An increase in TPBT induced increased absorption between 300 and 600 nm, while higher DPP contents resulted in stronger absorption between 600 and 800 nm. Bulk-heterojunction polymer solar cells (PSCs) based on the binary D-A copolymer PBDT-T exhibited a moderate power conversion efficiency (PCE) of 1.86% with a low short-circuit current density (JO of 5.92 mA cm(-2) and fill factor (FF) of 0.42 due to the narrower absorption band and lower hole mobility, whereas the ternary D-A copolymers displayed improved PCEs benefitting from the increased J. Most importantly, the PSCs based on PBDT-D70-T30 achieved a promising PCE of 4.31% with the highest Lc of 11.91 mA cm(-2), indicating that complementary light-absorption random polymer structures have great potential for increasing the photocurrent in bulk heterojunction PSCs. (C) 2015 Elsevier Ltd. All rights reserved.