Orientation Behavior of Bulk Heterojunction Solar Cells Based on Liquid-Crystalline Polyfluorene and Fullerene

Novel p-type materials, liquid-crystalline polyfluorene and its copolymer containing a terphenyl mesogen pendant, namely, poly{9,9-bis[6-(4′-hexyloxy-terphenyloxy)-hexyl]-fluorene} (PFBHeT) and poly{9,9-bis[6-(4′-hexyloxy-terphenyloxy)-hexyl]-fluorene-co-3-hexyl-thiophene} (PFBHeT-3HT), were designed and synthesized, respectively. The effects of the structural variation on their properties, especially the influences of the thermal treatment on the blend morphology of polymers and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), have been characterized and investigated. The spontaneous orientation of terphenyl mesogen endows the polymer and blend films with a good ordered morphology. The annealed and quenched films exhibit better liquid-crystalline properties, a lower LUMO, and better ordered domains compared with the untreated one, and the annealed films are favorable. It indicates that the spontaneous assembly of the liquid-crystalline molecules pushes PCBM clusters to form an oriented nanodispersing structure with highly oriented channel layers upon heating at liquid-crystalline states, and the annealing process offers enough stress relaxation time for the molecules to pack into a well-ordered stacking. Furthermore, the bulk heterojunction devices based on the PFBHeT-3HT:PCBM (1:2) active layer have been constructed. Without extensive optimization, annealing the devices yields a V oc of 0.63 V and a power conversion efficiency of 0.25%, showing a significantly increased J sc and FF with respect to its untreated counterpart.