Flow of Tiny Bubbles Mimics Computer Circuitry
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Figure 1. Schematic of a laser focused through a transparent electrode onto a photovoltaic blend film. Current is collected with a metal-coated atomic force microscope tip.
active devices with an area of 1.5 mm2 (see Figure 1). In between the Al electrodes of a device, the researchers aligned a conductive AFM probe at the center of a diffraction-limited laser (532 nm) spot, which generated a photocurrent signal sufficient for imaging at current levels in the range of ~1–10 pA at low intensity or at ~1 nA at high intensity, corresponding to observation of the photocurrent intensity dependence over nearly six orders of magnitude in illumination intensity. The research team collected photocurrent images under short circuit conditions (zero applied bias) as well as with various voltages applied between the ITO layer and the conductive, Pt-coated AFM probe tip. Contact mode was used to simultaneously measure the current and film height. The researchers observed variations in photocurrent by as much as a factor of three between invididual PCBM domains with similar topography. They attributed these variations to differences in vertical film structure. The researchers speculate that “MDMO-PPV-rich regions generate low photocurrents because of poor charge mobility and/or energy level alignment” whereas “PCBM-rich regions generate more photocurrent, but the vertical position of a PCBM-rich domain and domain-stacking can limit electron extraction or enhance electron extraction depending on the thickness of the MDMO-PPV:PCBM overlayer.” In addition, the researchers observed smaller photocurrent fluctuations on a 384
length scale in the range of ~200–600 nm. Photocurrents averaged from hundreds of measurements made with the Pt tip agree well with the photocurrents measured for bulk devices with Al contacts, demonstrating that the pcAFM measurements provide information relevant to device operation, despite the differences in geometry and intensity. The researchers concluded that local heterogeneity diminishes solar cell performance; gaining greater control and regularity of film morphology in both lateral and vertical directions will improve performance. In addition, the researchers said that their technique’s “simplicity should allow for the characterization of many other organic blends over a wide range of illumination intensity. Further, [because] this technique is based on standard current-voltage measurements, albeit on solar cells the size of the AFM tip, it is interesting to speculate that other current-voltage based characterizations, including spacecharge limited photocurrent and time-offlight mobility measurements, might be accessible with high resolution.” STEVEN TROHALAKI
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