Morphological Study of Nanoparticle-Polymer Solar Cells Using High-Angle Annular Dark-Field Electron Tomography

Electron tomography in high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) mode is used to image networks of nanoparticles within polymer-nanoparticle blend photovoltaic devices. Blends of CdSe nanoparticles (similar to 6.5 nm diameter spheres and similar to 33 nm long nanorods) with poly[2-methoxy-5-(3'7'-dimethyloctyloxy)-1,4-phenylene vinylene] (OC1C10-PPV) are investigated. Blends using spherical particles are found to contain aggregated regions in which well-connected networks of nanoparticles exist, surrounded by lower-density regions with poorer connectivity. As the ratio of nanoparticles to polymer is increased the aggregated regions increase in size, and the connectivity of the particles in the lower-density regions also increases. This has the effect of increasing the overall percentage of the charge-generating interface connected to the electron-collecting electrode, consistent with the trends observed in device performance. Using nanorods instead of spherical particles results in a highly connected network of particles distributed homogeneously through the film. The nanorods are oriented close to parallel to the plane of the film.