Diketopyrrolopyrroles with a Distinct Energy Level Cascade for Efficient Charge Carrier Generation in Organic Solar Cells

Three structurally different low molecular weight diketopyrrolopyrroles (DPPs) are synthesized in order to provide donors with a precise offset in their energy levels. The DPPs are characterized for optical, electrochemical, and thermal properties. By changing the terminal aryl groups attached to the DPP core from phenyl over m-pyridine to p-pyridine, different solid state packing is observed in thin film studies using UV/VIS absorption spectra and X-ray diffraction. Most importantly it is shown that both, reduction as well as oxidation potentials can be precisely tuned with a gradual stepping of about 100 meV by changing the terminal groups attached to the DPP core. Exploiting this energy level modification, these materials are tested in planar cascade organic photovoltaic devices using C-60 as acceptor. A sub nm thick interlayer of a suitable DPP derivative is introduced to obtain a distinct energy level cascade at the donor/acceptor interface. Power conversion efficiency as well as short-circuit current density is doubled with respect to the reference bilayer devices lacking the interface cascade. Spectrally resolved analysis of external quantum efficiency reveals that this enhancement can mainly be attributed to destabilization of bound charge transfer states formed in the C-60 layer at the interlayer interface, thus reducing geminate recombination losses.