The use of doped wide-gap charge transport layers with high conductivity and low absorption in the visible range enables one to achieve high internal quantum efficiencies and to optimize the devices with respect to optical interference effects. Here, it is shown that this architecture is particularly useful for stacking several cells on top of each other. The doping eases the recombination of the majority carriers at the interface between the cells, whereas the recombination centers are hidden for excitons and minority carriers. By stacking two p-i-n cells both with a phthalocyanine-fullerene blend as photoactive layer, a power efficiency of up to 3.8% at simulated AM1.5 illumination as compared to 2.1% for the respective single p-i-n cell has been achieved. Numerical simulations of the optical field distribution based on the transfer-matrix formalism are applied for optimization. The concept paves the way to even higher efficiencies by stacking several p-i-n cells with different photoactive materials that together cover the full visible spectrum.

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We present the material ␣--bis-͑dicyanovinylen͒-sexithiophen ͑DCV6T͒ as donor material in organic solar cells. A systematic study on the potential of DCV6T is given for different active layer concepts. DCV6T is a member of a class of acceptor-substituted oligothiophenes, which showed efficiencies of up to 3.4% and open circuit voltages ͑V oc ͒ of 1.0 V, which were recently reported ͓K. Schulze et al. Adv. Mater. ͑Weinheim, Ger.͒ 18, 2875 ͑2006͔͒. To verify the potential of the material ͑DCV6T͒, organic solar cells with planar heterojunctions, bulk heterojunctions, and a hybrid-planar-mixed heterojunction are investigated. The planar heterojunction solar cells of DCV6T and C60 show the highest V oc of 0.90 V. The mixed heterojunction solar cells have improved currents but a lower V oc of 0.82 V. The solar cell using the hybrid-planar-mixed heterojunction achieves the best combination of parameters. It has a V oc of 0.88 V, a short circuit current ͑j sc ͒ of 5.7Ϯ 0.4 mA cm −2 , a fill factor of 41.6%, and a power conversion efficiency of 2.1Ϯ 0.2%.