Study on Self-Assembly of Thiophene-Based All-Conjugated Amphiphilic Diblock Copolymers and Their Application to Organic Solar Cells

Abstract

A various block ratio of amphiphilic poly(3-hexylthiophene-b-3-(2-(2-{2-[2-(2-methoxy-ethoxy)-ethoxy]-ethoxy}-ethyl))thiophene) (P(3HT-b-3EGT)) polymers was synthesized. Size exclusion chromatograms (SEC) revealed that the molecular weight distributions of the P3HT blocks in the block copolymers have narrow distribution. The correlations among the block ratio, the amphiphilicity, and the self-assembled nanostructures of the block copolymers in thin films and in solution were examined. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) studies revealed that the crystallinity of the BP93 was higher than the crystallinity of the P3HT alone due to the packing effects caused by repulsion among the hydrophobic hexyl and hydrophilic ethylene glycol side chains. A long relaxation time was required to observe the ordering among P3HT blocks in the BP26, suggesting that self-assembly could occur if induced on the molecular level. We demonstrated that the molecular-level self-assembly of BP26 (at dilute concentrations) via a slow dialysis method produced highly ordered polymer vesicles with the size of 200–250 nm scales under thermodynamic control. The size could be controlled by competitive hydrophobic interactions using polystyrene. In contrast, the fast process of precipitation method yielded 5–20 nm micelles. Additionally we incorporated the BP93 into active layer of organic solar cells, resulting enhanced device performances.