Optimization of energy levels by molecular design: Evaluation of bis-diketopyrrolopyrrole molecular donor materials for bulk heterojunction solar cells

We report a series of solution-processable, small-molecule, donor materials based on an architecture consisting of two diketopyrrolopyrrole (DPP) cores with different aromatic π-bridges between the DPP units and different end-capping groups. In general, this architecture leads to desirable light absorption and electronic levels for donor materials. Out of the compounds investigated, a material with a hydrolyzed dithieno(3,2-b;2′,3′-d) silole (SDT) core and 2-benzofuran (BFu) end capping groups leads to the most favorable properties for solar cells, capable of generating photocurrent up to 800 nm while producing an open-circuit voltage of over 850 mV, indicating a small loss in electrical potential compared to other bulk heterojunction systems. Device properties can be greatly improved through the use of solvent additives such as 2-chloronaphthalene and initial attempts to optimize device fabrication have resulted in power conversion efficiencies upwards of 4%.