Cooperative Assembly of Pyrene-Functionalized Donor/Acceptor Blend for Ordered Nanomorphology by Intermolecular Noncovalent π–π Interactions

A facile approach to develop the stable and well-defined bulk heterojunction (BHJ) nanomorphology has been demonstrated. Novel pyrene (Py)-functionalized diblock copolymers poly(3-hexylthiophene)-block-poly[3-(10-(pyren-1-yloxy)decyloxy)thiophene] (P3HT-b-P3TPy), and pyrene-functionalized fullerene [6,6]-phenyl-C61-butyric acid 1-pyrene butyl ester (PCBPy), were successfully synthesized. The π-π interactions of Py mesogens interdigitated between the functionalized fullerene and P3TPy segment can allow for the cooperative assembly of P3HT-b-P3TPy and PCBPy. The orientation of the Py mesogens also can further enhance the molecular arrangement. Compared with the as-cast and thermal annealing, solvent annealing can promote cooperative assembly of P3HT-b-P3TPy:PCBPy undergoing the slow film growth. Note that the assembly microstructure strongly depends on the molar ratio of P3HT and P3TPy with Py mesogens. Low loading of P3TPy block in the copolymers blends keeps the same behavior to the P3HT, whereas relatively high loading of Py mesogens favors the better intermolecular π-π stacking interactions between P3HT-b-P3TPy and PCBPy. As a result, the P3HT-b-P3TPy(3/1) forms the orientated nanowires with PCBPy in bulk heterojunction, and the average domain size is estimated to be 10-20 nm, which is desirable for enlarge surface area for donor/acceptor interfaces and give a bicontinuous pathway for efficient electron transfer. Furthermore, the cooperative assembly between P3HT-b-P3TPy and PCBPy is found to effectively suppress the PCBPy macrophase separation, and stabilize the blend morphology.