A fullerene-based dyad for organic photovoltaic cells

We describe the synthesis and the photophysical properties of a fullerene–azothiophene dyad, as well as the photovoltaic performance of cells incorporating the dyad and the surface morphology of the device active layer. The results have been compared with those obtained on the blend, in equimolar ratio, between the azothiophene dye and a fulleropyrrolidine. This revealed a pivotal role played by the morphology on both the photophysical behaviour and the device performance. While scanning force microscopy studies for the dyad exhibited fairly smooth surfaces, in the case of the blend they displayed a micrometre-scale phase segregation between the two components. We suggest that, in the latter case, the lack of photo-induced electron transfer evidenced by the photophysical study, and the relevant reduction of the cell performance (up to more than one order of magnitude with respect to the dyad), could be ascribed to the different morphology. Because of the strong optical absorption in the visible region, the dyad-based solar cells gave notable results if compared with those reported in the literature for other donor–acceptor linked systems. A power-conversion efficiency of 0.37% under 80-mWcm-2 white-light illumination has been achieved by tuning the thickness of the dyad film, though the fullerene–azothiophene is not yet optimised in terms of photo-induced electron transfer.