End-Capping Effect of Quinoxalino[2,3‑<i>b</i>′]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-<i>b</i>′]­porphyrin (QP), and quinoxalino­[2,3-<i>b</i>′]­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 <i>ca</i>. 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