Side-Chain Length Modulation in Thieno[3,2- c ]isochromene Small-Molecule Donors for High-Performance Ternary Fullerene Organic Solar Cells

Abstract Side-chain engineering is an effective strategy for precisely tuning the crystallinity and active-layer morphology of high-performance bulk-heterojunction (BHJ) organic solar cells (OSCs). To systematically elucidate the side-chain length–property relationship across a complete homologous series, we herein designed and employed a small-molecule donor TiC6, which, together with the previously reported TiC8 and TiC12 analogues, completes the thieno[3,2-c]isochromene-based donor family with alkyl chains varying from hexyl to dodecyl. Taking the PTB7-Th:PC71BM system as a platform, we employed these three donors as the third component in ternary OSCs and systematically compared their photovoltaic performance and stability. Our comparative study reveals a non-monotonic chain-length dependence: the TiC8-based device delivers the highest PCE of 11.12%, superior to TiC6 (9.80%) and TiC12 (10.42%), which is attributed to optimized π–π stacking. Notably, the stability trend exhibits a reverse order; the TiC12-based device shows dramatically superior thermal stability, retaining 92% of its initial PCE after 48 h at 100 °C, far exceeding TiC8 (86%) and TiC6 (77%) under identical conditions. All ternary devices substantially outperform the binary PTB7-Th:PC71BM control (9.16%) and exhibit markedly enhanced long-term storage stability (e.g., TiC6 retains 90% after 360 h at RT, whereas the control drops to 63% within 10 min at 100 °C). This work not only expands the donor library but also provides unprecedented insights into the side-chain length–property relationship, demonstrating that optimizing efficiency and thermal stability requires distinct chain-length choices.