Enhanced Photovoltaic Performance of 9,10-Difluorodithieno[3,2-a:2′,3′‑c]phenazine-Based Polymer Donors by the Synergistic Effect of Alkyl-Thiophene π‑Bridges and Halogen Atom Modification

The fused electron-accepting (A) unit 9,10-difluorodithieno[3,2-a:2′,3′-c]phenazine (FTP) was synthesized in a simple way with high yield by only one-step reaction. Then, four D–A copolymer donors, PD8-F, PD8-Cl, PD4-F, and PD4-Cl, were designed and synthesized based on FTP as the A-unit, benzodithiophene with halogen-substituted thiophene-conjugated side chains (BDTT) as the D-unit, and alkyl-thiophene as the π-bridge. It was found that the shorter alkyl chain length on the thiophene π-bridge could effectively improve the crystallinity of the polymers, and different halogen substitutions on the thiophene side chains of BDTT could finely regulate the energy levels of the polymers. The photovoltaic properties of the polymer donors were studied by fabricating the polymer solar cells (PSCs) with Y6 as an acceptor. It was found that the PSC with the F-substituted PD8-F as the donor had higher J_sc and FF than the Cl-substituted PD8-Cl-based device. Furthermore, the crystallinity of the polymers PD4-F and PD4-Cl with short alkyl side chains on their thiophene π-bridge is enhanced. Finally, the power conversion efficiency (PCE) of the PD4-F-based PSCs reached 15.71% with a high J_sc of 26.71 mA cm^–2 and an FF of 68.41%, which was higher than that of the PCEs of 13.26, 13.59, and 13.61% for the devices based on PD4-Cl, PD8-F, and PD8-Cl, respectively. The results indicate that the FTP unit has great potential as an A-unit in designing high-performance D–A copolymer donors for PSCs. Moreover, the synergistic optimization of changing the alkyl chain length of the thiophene π-bridge and adjusting halogen substitution on the thiophene-conjugated side chain of BDTT D-unit is important for the high-performance D–A copolymer donors of the PSCs.