Conjugated random copolymers of benzodithiophene–benzooxadiazole–diketopyrrolopyrrole with full visible light absorption for bulk heterojunction solar cells

Abstract

We have used Stille coupling polymerization to synthesize a series of new donor–acceptor (D–A) conjugated random copolymers—PBDTT-BO-DPP—that comprise electron-rich alkylthienyl-substituted benzodithiophene (BDTT) units in conjugation with electron-deficient 2,1,3-benzooxadiazole (BO) and diketopyrrolopyrrole (DPP) moieties that have complementary light absorption behavior. These polymers exhibited excellent thermal stability with thermal degrading temperatures higher than 340 °C. Each of these copolymers exhibited (i) broad visible light absorption from 400 to 900 nm and (ii) a low optical band gap that is smaller than 1.4 eV and a low-lying highest occupied molecular orbital that is deeper than −5.22 eV. As a result, bulk heterojunction photovoltaic devices derived from these polymers and fullerenes provided a high short-circuit current density that is larger than 12 mA cm⁻². In particular, a photovoltaic device prepared from the PBDTT-BO-DPP (molar ratio, 1 : 0.5 : 0.5)/PC₇₁BM (w/w, 1 : 2) blend system with 1-chloronaphthalene (1 volume%) as an additive exhibited excellent photovoltaic performance, with a value of V_oc of 0.73 V, a high short-circuit current density of 17 mA cm⁻², a fill factor of 0.55, and a promising power conversion efficiency of 6.8%, indicating that complementary light-absorption random polymer structures have great potential for increasing the photocurrent in bulk heterojunction photovoltaic devices.