Structural Templating of an Organic Solar Cell Absorber by Ellagic Acid To Tune Its Aggregation, Molecular Orientation, and Optical Properties

Structural templating with homogeneous template layers is one of the strategies for controlling the orientation of small molecular absorbers in the photoactive layer of an organic solar cell to increase its power conversion efficiency. A main challenge thereby is the energetic alignment of the template molecules to the photoactive and charge-transporting materials. In the present study, the effects of a cluster-like template layer of ellagic acid (EA) on the morphology and optical properties of side-chain-substituted dicyanovinyl quaterthiophene (DCV4T-Et2) thin films are discussed. In the monolayer regime, J-aggregation of DCV4T-Et2 is confirmed. Insertion of the EA template layer leads to an improved aggregation behavior and formation of J-aggregates in DCV4T-Et2 films near the EA interface. The orientation of DCV4T-Et2 molecules in 30 nm thick films changes from “edge-on” to “face-on” due to a π–π interaction between the flat-lying EA molecules and the DCV4T-Et2 molecules. The face-on orientation by templating is preserved in blend layers with C60, and a considerable increase in the crystallinity of the DCV4T-Et2 phase in the blend is induced. Organic solar cells based on templated DCV4T-Et2:C60 active layers exhibit more than a 50% increase in the efficiency compared to nontemplated active layers. The short-circuit current density and the fill factor are significantly improved. Although the energetic alignment of EA is not ideal, no additional open-circuit voltage losses were observed with templating, due to the cluster-like morphology of the EA layer. Our results demonstrate a cluster-like templating approach with the novel template molecule EA to tailor the molecular orientation, crystallinity, and consequently optical properties of organic semiconducting molecules without significant energetic losses favorable for use in organic electronics.