Molecular model T6:C60 bulk-heterojunction solar cells

Organic photovoltaic devices offer a large technological potential as a renewable source of electrical energy because of their low cost and ease of processing. For the last few years, the interest in such devices has grown rapidly, allowing multiplication of the solar conversion efficiency by 5 within about 10years. The highest conversion efficiency obtained is now close to 5% with a spin-coated polythiophene-fullerene blend. For a better understanding of those polymer blend systems, we have fabricated a coevaporated solar cell from model molecules. The molecular blend was composed of sexithiophene (α,α′-dihexylsexithiophene, T6) as a donor and fullerene C60 as an acceptor. The influence of the T6:C60 proportion was studied for an active layer of 100nm thickness. The conversion efficiency obtained with the optimal proportion of T6:C60 (40–60) is 0.7%. Reverse bias annealing effect was studied by measuring the current-voltage characteristics after each postprocessing step. The conversion efficiency reached 1% but still remained much less than for the polymer bulk heterojunction. This major difference may come from the spontaneous phase separation which takes place in polymers to form an interpenetrated network