Comparing C60 and C70 as acceptor in organic solar cells: Influence of the electronic structure and aggregation size on the photovoltaic characteristics

Abstract The difference in aggregation size of the C60 and C70 fullerenes affect the photovoltaic performance of devices assembled in the so-called bilayer architecture with poly[2,7-(9,9- dioctyl- dibenzosilole)- alt-4,7- bis(thiophen-2-yl)benzo- 2,1,3- thiadiazole] (PSiF-DBT) as the electron donor material. Despite the better performance of the C70 devices, which is related to the high absorption coefficient in the visible range and the superior charge transport properties, the short-circuit current variation upon annealing treatment at 100 °C is approximately twice bigger when the C60 is the acceptor. We attribute this effect to the tendency of C60 in form smaller aggregate domains relatively to the C70. The increased roughness on the polymeric surface after annealing results in an enhanced donor/acceptor contact area and assists the fullerene diffusion deeper inside the polymeric layer. This effect leads to a better mixing between donor and acceptor species and create a interpenetrating layer close to the so-called bulk heterojunction. Since C60 forms smaller aggregates, this mechanism is more pronounced for this molecule. Therefore, a significant variation in the performance of the C60 devices is observed after this kind of treatment. Density Functional Theory calculations of the potential energy of interaction between two fullerene molecules and X-Ray measurements gives evidences to support this idea. In addition, combining spectrally resolved external quantum efficiency measurements with optical modeling our results also indicate the occurrence of the bilayer interfacial mixing for PSiF-DBT/C60.