A Low Band Gap, Solution Processable Oligothiophene with a Diketopyrrolopyrrole Core for Use in Organic Solar Cells

We synthesized a low band gap, solution processable oligothiophene derivative functionalized with a diketopyrrolopyrrole core, which is a highly absorbing chromophoric group. This design leads to optical absorption that extends to 720 nm in solution and to 820 nm in the film. Cyclic voltammetry shows quasireversible oxidation and reduction processes. Bulk heterojunction solar cells using [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as the electron acceptor were fabricated. The power conversion efficiencies (PCEs) depend on the donor−acceptor ratio. A 70:30 donor/acceptor by weight mixture provided a PCE of 2.3% under simulated AM 1.5 solar irradiation of 100 mW/cm 2. To date, this is the highest efficiency reported for small molecule-based solution processed bulk heterojunction solar cells.