Invited Paper NS-TuM9
Imaging Performance Variations in Organic Solar Cells with Time-Resolved Electrostatic Force Microscopy and Photocurrent-sensitive Atomic Force Microscopy

Tuesday, October 16, 2007, 10:40 am, Room 616

We study organic/organic and organic/inorganic interfaces in polymer solar cells using time-resolved Electrostatic Force Microscopy (trEFM), and photoconductive Atomic Force Microscopy (pcAFM). These techniques allow us to measure charge generation, collection, and trapping with sub-100 nm resolution so we can correlate variations in performance directly with variations in local film morphology. This talk will describe our trEFM and pcAFM studies of polymer/polymer and polymer/fullerene solar cells. In model blends of polyfluorenes we present surprising evidence that the majority of the collected photocurrent arises away from the visible domain interfaces. In polymer/fullerene cells we show that even the best devices exhibit a distribution of local quantum efficiencies and fill factors that vary on two characteristic length scales. First, we observe performance variations over tens of nanometers that we associate with vertical connectivity of the fullerene domains. Second, in these same devices we observe performance variations over several hundred nanometer length scales (much larger than the characteristic polymer or fullerene domain sizes). We explore the possibility that these larger-scale performance variations are associated with the interface between the polymer blend and the underlying indium tin oxide contact.