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  • Space Charge Limited Curren...
    Reid, Obadiah G; Munechika, Keiko; Ginger, David S

    Nano letters, 06/2008, Volume: 8, Issue: 6
    Journal Article

    We describe local (∼150 nm resolution), quantitative measurements of charge carrier mobility in conjugated polymer films that are commonly used in thin-film transistors and nanostructured solar cells. We measure space charge limited currents (SCLC) through these films using conductive atomic force microscopy (c-AFM) and in macroscopic diodes. The current densities we measure with c-AFM are substantially higher than those observed in planar devices at the same bias. This leads to an overestimation of carrier mobility by up to 3 orders of magnitude when using the standard Mott−Gurney law to fit the c-AFM data. We reconcile this apparent discrepancy between c-AFM and planar device measurements by accounting for the proper tip−sample geometry using finite element simulations of tip−sample currents. We show that a semiempirical scaling factor based on the ratio of the tip contact area diameter to the sample thickness can be used to correct c-AFM current−voltage curves and thus extract mobilities that are in good agreement with values measured in the conventional planar device geometry.