Small-molecule thiophene-C₆₀ dyads as compatibilizers in inverted polymer solar cells

This study details the synthesis and characterization of thiophene-C ₆₀ derivatives, nT-C₆₀, as compatibilizers for inverted bulk-heterojunction organic solar cells comprising poly(3-hexylthiophene), P3HT, and [6,6]-phenyl-C₆₁-butyric acid methyl ester, PCBM. We find that the extent with which these compatibilizers can reduce interfacial energy and prevent domain coarsening and macrophase separation in active layers depends strongly on their molecular architecture. 4T-C₆₀ is the most effective among the derivatives tested; its incorporation at 5 wt % to P3HT and PCBM effectively suppresses macrophase separation, even upon extended thermal annealing of the active layers. In sharp contrast to 4T-C₆₀, 8T-C₆₀ is miscible with P3HT. The addition of 8T-C₆₀ to active layers of inverted solar cells therefore does not arrest macrophase separation on extended annealing. 2T-C₆₀ is also ineffective as a compatibilizing agent, presumably because its thiophene segment is too short so its presence at the interface between P3HT and PCBM does not increase interfacial adhesion.