Rational Design and Synthesis of a DTBDT-Based Polymeric Guest Donor for Efficient Ternary Organic Solar Cells

In this study, a structurally engineered polymeric guest donor, DTBDT-SEH-C8, was rationally designed and synthesized for high-performance ternary organic solar cells. The molecular structure was derived from the PM6 framework by incorporating an extended DTBDT fused-ring core, replacing fluorine substituents with alkyl chains, and introducing sulfur-containing side-chain units between the alkyl chain and thiophene moiety. These synthetic modifications were intended to extend π-conjugation, strengthen intermolecular interactions, and tune the electronic properties of the donor polymer.

The successful preparation of DTBDT-SEH-C8 was confirmed by structural characterization of the intermediates, monomer, and final polymer using NMR spectroscopy and mass spectrometry. Gel permeation chromatography revealed a high molecular weight polymer with Mn and Mw values of 98 and 212 kDa, respectively, and a PDI of 2.17. DTBDT-SEH-C8 also exhibited good solubility in common chlorinated solvents and excellent thermal stability, with a decomposition temperature of 365 °C, indicating its suitability for solution-processed photovoltaic applications.

Owing to the designed structural modification, DTBDT-SEH-C8 showed a deeper HOMO energy level and a higher absorption coefficient than PM6. When incorporated into the PM6 binary blend as a guest donor, it promoted cascade energy alignment and interfacial connection between donor and acceptor domains. As a result, the optimized ternary device containing 1% DTBDT-SEH-C8 achieved a power conversion efficiency of 17.12%, surpassing that of the binary device. These results demonstrate that precise synthetic modification of PM6-derived polymer donors is an effective strategy for improving molecular packing, charge transport, and photovoltaic performance in ternary organic solar cells.