Blend films of an amorphous conjugated polymer and a thermal precursor fullerene: effects of annealing temperatures on film structures and photovoltaic properties

A thermal precursor approach can serve as a solution-based process to form high-quality organic thin-films from small molecules with low solubility such as pristine fullerenes. In this study, highly-soluble C-60-9-methylanthracene (9MA) adducts, of which the appended 9MA groups can be easily detached and evaporated by heating, were used as thermal precursors to C-60. In a composite film of C-60-9MA adducts with an amorphous polymer, poly(2,7-carbazole-alt-dithienylbenzothiadiazole) (PCDTBT) for bulk heterojunction (BHJ) solar cells, the C-60 aggregation was suppressed even after the thermal detachment of the 9MA groups at high temperature up to 160 degrees C. This result is contrasted with a composite film of the C-60-9MA adducts and a crystalline conjugated polymer, poly(3-hexylthiophene) (P3HT), yielding undesirable large aggregates by thermal annealing. The organic photovoltaic (OPV) device based on the composite film of the C-60-9MA adducts and PCDTBT showed a low power conversion efficiency (PCE) of 0.69%, which was remarkably improved up to 3.51% by a factor of ca. 5 after the thermal annealing treatment. Importantly, the maximum PCE was achieved at 140 degrees C, where an increase of the short-circuit current density (J(SC)) and a decrease of the open-circuit voltage (V-OC) are well-balanced with increasing the annealing temperature. The results obtained here demonstrate the potential utility of the unique thermal precursor approach to form high-quality BHJ films from amorphous conjugated polymers and highly-insoluble fullerenes, expecting higher LUMO levels and V-OC for applications in organic thin-film OPV devices made by cost-effective solution-based processes.