Diketopyrrolopyrrole-Coumarin Conjugates for Efficient Binary Organic Solar Cells

Two coumarin (Cou)-functionalized DPP derivatives, DPP-Cou and Cou-DPP-Cou, have been designed, synthesized, and characterized. The extension of the molecular backbone by an additional coumarin derivative in symmetrical Cou-DPP-Cou extends the absorption to ∼750 nm ( E g opt = 1.67 eV) compared to ∼700 nm ( E g opt = 1.78 eV) for asymmetric DPP-Cou. Thermogravimetric analysis showed higher thermal stability for symmetrical conjugates compared to the asymmetric one. Both materials showed complementary absorption spectra and well-matched Frontier molecular orbital energy levels with the low band gap fullerene-free acceptor Y7 and used as a donor in organic solar cells. After optimization using solvent vapor annealing treatment, the organic solar cells based on DPP-Cou:Y7 and Cou-DPP-Cou:Y7 attained overall power conversion efficiencies (PCE) of 9.63 and 13.48%, respectively. The improved PCE for the Cou-DPP-Cou derivative is mainly due to an appropriate morphology for faster charge extraction and lower charge recombination.