Understanding the Morphology of PTB7: PCBM Blends in Organic Photovoltaics

The structure-property relationships of PTB7-phenyl-C-61-butyric acid methyl ester (PCBM)-based organic photovoltaics are investigated. The morphology is investigated in an active layer setting where a multi-length-scale morphology is observed using a solvent additive-assisted film processing. This multi-length-scale structure consists of a phase separated morphology with a characteristic length scale of approximate to 30 nm, which is critical for producing large currents in devices; a second length scale of approximate to 130 nm, arises from face-on PTB7 crystalline aggregates. This latter morphological feature is also observed in films prepared without the use of an additive. By observing the structure formation in situ during solvent evaporation for blade coated thin films, the additive is found to promote the formation of ordered domains of the PTB7 at an earlier stage during the solvent evaporation, which is critical in the development of the final morphology. In studies on PTB7/PCBM bilayers, PCBM is found to diffuse into the PTB7 layer. However, the performance of devices prepared in this manner is low. This diffusion leads to a swelling of the PTB7 and a reduction in the crystallinity of the PTB7, reflecting the strong miscibility of PCBM with PTB7. The morphology resulting from the interdiffusion is single-length-scale with slightly large phase separation. This leads to devices with poor performance.