End-Capping Effect of Quinoxalino[2,3-b′]porphyrin on Donor–Acceptor Copolymer and Improved Performance of Polymer Solar Cells

A series of end-capped donor–acceptor copolymers, P(BT-DPP)-T, P(BT-DPP)-QP, and P(BT-DPP)-QPZn, were synthesized by Stille coupling of 2,2′-bithiophene (BT) and diketopyrrolopyrrole (DPP) with end-caps of thiophene (T), quinoxalino[2,3-b′]porphyrin (QP), and quinoxalino[2,3-b′]porphyrinatozinc (QPZn), respectively. Compared with the counterpart, P(BT-DPP), which contains no end-caps, P(BT-DPP)-QP and P(BT-DPP)-QPZn showed remarkably enhancement of light absorption in the range of ca. 400–550 nm resulted from the end-capping effect of porphyrins. Bulk heterojunction polymer solar cells (PSCs) based on these polymers were fabricated and the results showed significant improvement of power conversion efficiencies (PCEs) of the end-capped polymers. Especially, ligand addivie, 4,4′-bipyridine (Bipy), was applied to the PSCs based on polymers end-capped by porphyrins and the PCE of the photovoltaic device based on P(BT-DPP)-QPZn significantly improved from 2.92% to 4.45% with the comprehensive benefits of optim...