Photocurrent pattern formation in polymer/methanofullerene blends imaged by near-field scanning photocurrent microscopy

The dimensional dependence of micron-sized current inhomogeneities in poly[2'-methoxy-5-(2-ehtylhexyloxy)-1,4-phenylene vinylene]/1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6) C₆₁ (MEH-PPV/PCBM) solar cells on processing conditions has been investigated. These current inhomogeneities typically consist of a central PCBM aggregate surrounded by a larger region of poor photocurrent production. For films spin coated from chlorobenzene, both the diameter of the current inhomogeneities and the size of the corresponding PCBM aggregates systematically increase with the time allowed for the film to dry. These observations indicate that the current inhomogeneities are a consequence of the nucleation and growth of the central PCBM cluster at the expense of PCBM from the surrounding area. In contrast to the results observed with chlorobenzene as the solvent, no micron-sized current inhomogeneities are observed using toluene as the solvent, despite the increased density of PCBM clusters in the film. The solubility of PCBM and the solvent volatility are identified as key parameters determining the formation of micron-sized current patterns. Also discussed is the influence of these current features on overall device performance, along with the influence of the film nanomorphology as processing conditions are changed.