We systemically investigate the size-dependent behavior of organic photovoltaics (OPVs) tested under partial illumination. An illumination mask was hypothesized to limit uncertainty associated with device size and illumination conditions when measurements are conducted on laboratory scales. Introducing the illumination mask into the measurement improves the accuracy of J(SC); however partial illumination will result in a decrease in V(OC) and an increase in FF. This last observation is explained by diodes contributing both light and dark currents in the measurement. In addition to loss due to the energy required to separate bound excitons, these results suggest that non-optimized photo-dark current ratio could be another reason why measured V(OC) is less than ideal V(OC). (C) 2011 Elsevier B.V. All rights reserved.