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 Stile 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 optimization strategies such as using additives and thermal annealing. Furthermore, thermal aging experiments showed increased stability of the optimal morphology of P(BT-DPP)-QPZn:PC71BM blend film after the use of Bipy. This study provides a promising strategy to design donor acceptor copolymers with multifunctional end-caps and to use ligand additives for achieving PSCs with both high-efficiency and long-term stability, which are critical for the devices to be commercially useful.