Singlet fission in pentacene-fullerene solar cells

In recent years there is a considerable interest for singlet fission as a strategy to further increase the efficiency of organic solar cells. The prototypical example of singlet fission in organic molecules is pentacene which produces two triplet states after photoexcitation with a single photon with high quantum yield. Pentacene-fullerene solar cells in which the external quantum efficiency (EQE) exceeds 100% have been reported. One remarkable issue of these pentacene-fullerene solar cells is that their open-circuit voltage (V_oc = 0.35 V) is relative high compared to the energy of the triplet state of pentacene (E_T = 0.85 eV) from which the charges are created. Because the energy of the interfacial charge transfer state in organic solar cells can be estimated from E_CT ≈ qV_oc + 0.5 eV = 0.85 eV, the triplet and charge-transfer state are almost degenerate.

Here we study the relation between E_CT, E_T and the external quantum efficiency (EQE) for a series of inverted pentacene-fullerene bilayer solar cells employing five fullerene derivatives with different reduction potentials. By using fullerenes with more negative reduction potentials than C60, the open-circuit voltage of these cells can be increased to 0.55 V. In this case E_CT ≈ 1.05 eV, which is much higher than E_T. We find that when increasing E_CT above E_T there is a significant loss in EQE at wavelengths were pentacene absorbs light. This provides indirect, but compelling, evidence for singlet fission being the main mechanism for charge generation in pentacene-fullerene bilayers because charge transfer from the triplet state is now endergonic and will not occur efficiently. In contrast, if charge generation would occur from the singlet state of pentacene (E_S = 1.83 eV), charge transfer would still be exergonic and the drop in EQE would not be expected.