Cascade energy alignment and absorption enhancement via a guest donor strategy for efficient ternary organic solar cells

Abstract

Ternary blend bulk heterojunction (BHJ) strategies have emerged as an effective route to enhance the performance of organic solar cells (OSCs), yet the development of guest donors remains relatively underexplored. Here, a polymeric guest donor, DTBDT-SEH-C8, is designed and synthesized as a derivative of PM6 with extended It-conjugation, replacement of fluorine atom with alkyl chain, and incorporation of sulfur atom between alkyl chain and thiophene. This structural modification affords favorable cascade energy alignment through a deeper HOMO energy level and stronger light harvesting in the donor spectral region compared to PM6. Incorporation of DTBDT-SEH-C8 into the binary matrix promotes the formation of interstitial connections between the donor and acceptor phases. This structural feature is consistent with the improved charge transfer characteristics and reduced recombination losses observed in the ternary devices. These synergistic effects deliver a notable increase in device performance: the optimized ternary BHJ device achieves a power conversion efficiency (PCE) of 17.12%, representing an enhancement over the binary counterpart (16.26%), with improvements in shelf and thermal stability. This work highlights the functional role of structurally tuned polymeric guest donors and underscores the importance of developing new guest donors for high-performance ternary OSCs.