Organic thin-film photovoltaic cells based on oligothiophenes with reduced bandgap

The best polymeric solar cells reported so far are based on a so-called bulk heterojunction of a polythiophene as donor and a soluble fullerene derivative as acceptor. However, these cells still suffer from an unsatisfying photovoltage, typically below 0.7 V. Here, we show that we can achieve higher photovoltages using a new terthiophene end-capped with electron withdrawing dicyanovinyl groups (DCV3T) that increase both the ionization energy and even more strongly the electron affinity of the compound. The new material is tested in cells using a photoactive heterojunction to separate the excitones generated in the oligomer and a p-doped wide-gap transport layer. The solar cells show an open circuit voltage of up to 1.04 V and a broad spectral sensitivity band ranging from 420 nm to 620 nm. Solar cells based on such oligothiophenes are promising candidates for stacked organic solar cells tailored to the sun-spectrum. Moreover, we present first examples of a new concept for organic solar cells: By blending DCV3T with fullerene C-60, an enhanced generation of triplet excitons on the oligomer can be achieved via a back and forth transfer of excitons (ping-pong-efffect).