Understanding the morphology of PTB7:PCBM blends in organic photovoltaics

The structure-property relationships of PTB7-phenyl-C₆₁-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 ≈30 nm, which is critical for producing large currents in devices; a second length scale of ≈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. The morphological features of organic photovoltaic blends based on the low band gap polymer PTB7 are characterized using high power techniques. The additive in the processing solvent fundamentally changes the film-forming process and leads to a multi-length-scale morphology. In bilayer diffusion experiments, it is observed that weak crystallites of PTB7 can be dissolved by phenyl-C₆₁-butyric acid methyl ester (PCBM) diffusion.