Terthiophene-C₆₀ dyads as donor/acceptor compatibilizers for developing highly stable P3HT/PCBM bulk heterojunction solar cells

Forming nano-scale interpenetrating networks in electron donor/acceptor blends via phase-separation tuning and maintaining their optimal morphology are of paramount importance for the favorable performance and operational lifetime of bulk heterojunction (BHJ) polymer solar cells. In this work, two fulleropyrrolidine derivatives that bear π-conjugated terthiophene (3T) and two hexyl (H) or ethylhexyl (EH) chains, abbreviated as 3T-H-C₆₀ and 3T-EH-C₆₀, are separately used as a compatibilizer of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM). The effect of the loading of such an agent on the photovoltaic properties of solar devices and the morphological stability of the P3HT/PCBM blend was examined. The substitution of PCBM with a tiny amount of 3T-H-C₆₀ or 3T-EH-C₆₀ apparently enhances the short-circuit current density, reflecting the formation of small PCBM clusters within the P3HT matrix. More importantly, optical microscopy, transmission electron microscopy and photoluminescence measurements verify that the presence of 1 wt% 3T-H-C₆₀ or 3 wt% 3T-EH-C₆₀ effectively hinders the thermal motion of PCBM molecules to form micro-scale clusters, leading to an extremely stable morphology even under long-term exposure to elevated temperatures. Accordingly, the power conversion efficiencies of P3HT/PCBM:3T-H-C₆₀ and P3HT/PCBM:3T-EH-C₆₀ devices remain very steady during prolonged aging at 130 °C. This work provides valuable guidelines for the design of an effective compatibilizer for thermally stable polymer/fullerene BHJ solar cells.