Efficient heterojunction organic solar cells with high photovoltage containing a low gap oligothiophene derivative

Abstract

In this work, we report on efficient heterojunction organic solar cells containing a new oligothiophene derivative α,α'-bis-(2,2-dicyanovinyl)-quinquethiophene (DCV5T) as donor (D) and fullerene C₆₀ as acceptor (A). The oligothiophene carries electron withdrawing substituents which increase the ionization energy and even more strongly the electron affinity. In thin films, the absorption is significantly broadened compared to solution and the optical gap is reduced to 1.77 eV. Nevertheless, the material shows strong fluorescence with low Stokes shift (peak at 1.71 eV), i.e. low energy loss upon reorganisation in the excited state. At the heterointerface between the low band-gap oligothiophene and fullerene C₆₀, photogenerated excitons from both materials are efficiently separated into electrons on the LUMO of C₆₀ and holes on the low-lying HOMO of the oligothiophene. This step involves only low energetic losses since both the HOMO and the LUMO offset of the two materials are below 0.6 eV, close to the expected exciton binding energy. We can thus reach high open circuit voltages of up to 1.0 V. The most efficient solar cells with power efficiencies around 4 % are obtained when the photoactive heterojunction is embedded between a p-doped hole transport layer on the anode side and a combination of a thin exciton blocking layer and aluminium on the cathode side. However, due to the high ionization energy of the oligothiophene (approx. (5.6 ± 0.1) eV), hole injection from any anode or hole transport layer is difficult and the IV curves thus show a characteristic S-shape which reduces the fill factor FF. It is found that the actual FF sensitively depends on the work function of the p-doped hole transport layer, that can be influenced by doping.