Two Donor–One Acceptor Random Terpolymer Comprised of Diketopyrrolopyrrole Quaterthiophene with Various Donor π‑Linkers for Organic Photovoltaic Application

A series of terpolymers based on diketopyrrolopyrrole (DPP) and quaterthiophene (QT) connected randomly using various electron-donating π-linkers such as fluorene (P4TFDPP), thiophene (P5TDPP), phenyl (P4TPDPP), and vinyl (P4TVDPP) have been designed and synthesized via Suzuki and Stille polymerization. In this work we studied the influence of the π-linker and the effect on their photophysical properties, and energy levels, band gap, packing nature, surface morphology, carrier mobility, and photovoltaic behavior were analyzed using various techniques and correlated with quantum chemical calculation. Bulk heterojunction (BHJ) solar cells were fabricated using these polymers as donor materials and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) as an acceptor. A high molecular weight with larger absorption coefficient and high hole mobility were achieved for copolymer P4TFDPP compared to other copolymers. The best photovoltaic performances were achieved for the polymer P4TFDPP with a power conversion efficiency (PCE) of 4.9%, with V_oc = 0.88 V, J_sc = 9.4 mA/cm², and FF = 59%, which is the highest PCE, J_sc, and FF for the fluorene–DPP copolymeric system compared to that previously reported in the literature, while P5TDPP, P4TVDPP, and P4TPDPP show a PCE of 4.1%, 2.9%, and 3.9%, respectively. This work exhibits the importance of selecting a proper π-linker in the random terpolymer to provide an efficient approach for enhancing the PCE in polymer solar cells.