Nanostructure determines the intensity-dependence of open-circuit voltage in plastic solar cells

Abstract

We use photoconductive atomic force microscopy to make local measurements of the open-circuit voltage (V-OC) as a function of light intensity in several polymer/fullerene bulk heterojunction blend solar cells. We find significant local variations in the slope of the open-circuit voltage plotted versus the log of the light intensity. By studying a model alkoxy-poly (p)-pheneylene-vinylene/phenyl-C-61-butyric acid methyl ester system with known vertical structure, and by comparing our results with a simple numerical model, we associate these local differences in V-OC versus light intensity with lateral variations in vertical morphology/composition. These results not only provide a qualitative method of mapping lateral variations in vertical structure/composition by making local measurements of V-OC as a function of light intensity but suggest that the unusual light-intensity dependence of V-OC (diode ideality factors in the light) of many organic photovoltaics can be linked with morphological heterogeneity. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3482009]