Design, synthesis, and self-assembly of diketopyrrolopyrrolo derivatives for fabrication of reproducible bulk heterojunction solar cells

Organic solar cells potentially offer a low cost, large area, flexible, light-weight, clean, and quiet alternative energy sources for indoor and outdoor applications. Recently, there are a few reports on combining the solution processing method of conjugated polymers with high purity, ease of synthesis, and order of small molecules. To date, small molecule based solar cells have PCEs up to 1.3 %. These efficiencies remain low when compared to either thermally deposited small molecule or polymer-based solar cells. Most importantly, conjugated small molecules do not suffer from batch to batch variations, broad molecular weight distributions, end group contamination, and difficult purification methods, as is the situation for their polymeric counterparts. Our research in this area focuses on the design and synthesis of materials having a broad absorption spectrum and high charge carrier mobility. The molecular design incorporates the high mobility and ordered molecular packing of oligothiophenes together with the large optical density of the diketopyrrolopyrrole (DPP) core. The resulting molecules exhibit strong absorption at long wavelengths, up to 800 nm when in the solid state. Functional groupsattached to the DPP core can be used to tune the energy level, the bandgap, the solubility, and hence, the film morphology and charge mobility. Furthermore, this material in combination with the widely-used acceptor C71-PCBM can be used to form the active layer in organic solar cells with high power conversion efficiencies (PCEs)up to 4.5% under simulated AM 1.5 solar irradiation. This is the highest PCE reported to date for BHJ solar cells using solution processable small molecules.