Development of New Photovoltaic Conjugated Polymers Based on Di(1-benzothieno)[3,2‑<i>b</i>:2′,3′‑<i>d</i>]pyrrole: Benzene Ring Extension Strategy for Improving Open-Circuit Voltage

We have synthesized a series of novel di­(1-benzothieno)­[3,2-<i>b</i>:2′,3′-<i>d</i>]­pyrrole (DBTP)-based conjugated polymers for organic photovoltaic devices. We first introduced DBTP moiety as an electron donating part in the conjugated polymer system to improve the open-circuit voltage (<i>V</i>_oc) in photovoltaic properties. The DBTP backbone has a fused ring structure with extended benzene rings at the end of the 4<i>H</i>-dithieno­[3,2-<i>b</i>:2′,3′-<i>d</i>]­pyrrole (DTP) moiety. The synthesized DBTP-based polymers effectively enhanced <i>V</i>_oc compared to DTP-based polymers by decreasing the highest occupied molecular orbital (HOMO) energy levels of polymers without damaging other photovoltaic parameters. The alternation of DBTP and 3,6-dithien-2-yl-2,5-dialkyl­pyrrolo­[3,4-<i>c</i>]­pyrrole-1,4-dione (DTDPP) formed a promising donor polymer, PDBTP4, exhibiting an excellent molar absorption coefficient of 17 600 L cm^–1 mol^–1 at the absorption maximum of 719 nm and a broad absorption covering 550–850 nm. PDBTP4:[6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) devices exhibited more than twice improved PCE of 6.80% with 0.28 V higher <i>V</i>_oc than DTP–DTDPP polymer:PC₇₁BM devices at the similar fabrication process. The benzene ring extension strategy is successfully working on the polymers containing strong electron donating DTP moiety. We believe that this approach can be used in other polymer systems showing a poor <i>V</i>_oc value