Solar-Cell Layer Comes Into View

Combining analytical transmission electron microscopy with a data analysis technique provides a novel way to image nanoscale features in the photoactive layer of organic photovoltaic cells, according to Martin Pfannmöller of Germany’s Heidelberg University and coworkers, who developed the procedure (Nano Lett., DOI: 10.1021/nl201078t). The imaging technique makes it possible to relate the nanoscale appearance and morphology of the critical solar-cell component to the cell’s photovoltaic performance. Flexible polymer solar cells typically contain a photoactive region consisting of a mostly amorphous blend of a conducting polymer and a fullerene derivative. Those materials, which serve as electron donor and acceptor, respectively, exhibit little visual contrast difference and are tough to distinguish via microscopy. By combining an energy-filtering method, which analyzes the energies of electrons passing through a transmission electron microscope specimen, with a computational procedure, Pfannmöller and coworkers developed a way to impart sharp image contrast to the chemically distinct materials. The method reveals the location and shape of nanoscale domains of each of the pure materials, as well as the interconnecting mixed phases.