Neat C-60:C-70 buckminsterfullerene mixtures enhance polymer solar cell performance

We demonstrate that bulk-heterojunction blends based on neat, unsubstituted buckminsterfullerenes (C-60, C-70) and a thiophene-quinoxaline copolymer (TQ1) can be readily processed from solution. Atomic force and transmission electron microscopy as well as photoluminescence spectroscopy reveal that thin films with a fine-grained nanostructure can be spin-coated, which display a good photovoltaic performance. Replacement of substituted fullerenes with C-60 or C-70 only results in a small drop in open-circuit voltage from 0.9 V to about 0.8 V. Thus, a power conversion efficiency of up to 2.9% can be maintained if C-70 is used as the acceptor material. Further improvement in photovoltaic performance to 3.6% is achieved, accompanied by a high internal quantum efficiency of 75%, if a 1:1 C-60:C-70 mixture is used as the acceptor material, due to its improved solubility in ortho-dichlorobenzene.