Determining Material-Specific Morphology of Bulk-Heterojunction Organic Solar Cells Using AFM Phase Imaging

A technique for determining the material-specific morphology of a polymer–fullerene blend is presented. This technique is applied to solution processed bulk-heterojunction organic solar cells with different weight ratios of polymer–fullerene blend using the PTB7:PCBM material system. Optical and electrical characterizations show that the light absorption increases for larger polymer (PTB7) content, while the fill factor of the fabricated solar cells is improved for larger fullerene (PCBM) content. The material-specific morphologies of polymer–fullerene bulk-heterojunctions are measured by employing AFM phase imaging. The measured AFM phase images reveal that fullerene material forms flake-like clusters which are embedded in the polymer network. The size of the flakes is increasing with larger content of fullerene material. By correlating the optical and electrical properties with the measured bulk-heterojunction morphologies, the relation between bulk-heterojunction structure and solar cell performances i...