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The microstructure of MDMO-PPV:PCBM blends as used in bulk hetero-junction organic solar cells was studied by Atomic Force Microscopy (AFM) and Kelvin Force Microscopy (KFM) to image the surface morphology and by means of Transmission Electron Microscopy (TEM) to reveal images of the film's interior.By introducing KFM, it was possible to demonstrate that phase separated domains have different local electrical properties than the surrounding matrix. Since blend morphology clearly influences global electrical properties and photovoltaic performance, an attempt to control the morphology by means of casting conditions was undertaken. By using AFM, it has been proven that not only the choice of solvent, but also drying conditions dramatically influence the blend structure. Therefore, the possibility of discovering the blend morphology by AFM, KFM and TEM makes them powerful tools for understanding today's organic photovoltaic performances and for screening new sets of materials.

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In this paper, a clear view on the bulk microstructure of MDMO-PPV:PCBM blends as used in bulk hetero-junction organic solar cells is obtained by means of TEM (Transmission Electron Microscopy). Using TEM, 3-dimensional information is acquired on phase separated regions, formed during casting. Particle statistics illustrate quantitatively that a.o. drying conditions and choice of solvent dramatically influence the blend structure. More information about the lateral blend structure and distribution is obtained in cross-sectional view. Since blend morphology is strongly related to photovoltaic performance, TEM can be a powerful tool for understanding today's photovoltaic performances and screening new sets of materials.