In this paper, we report the dependence of fill factor (FF) on different parameters in organic solar cells. FF is a more sensitive parameter compared to open-circuit voltage (VOC) and short-circuit current density (JSC), and depends on the mobility (μ)-lifetime (τ) product of the bulk material, thickness of the active-polymer layer and on the morphology of the cathode-polymer interface. The curvature of the JV characteristics in the fourth quadrant (which can be adjudged by the sign of d2J/dV2 in the range Vsat<V50%) to concave (FF0) for illuminated devices with good cathode-polymer interfaces and linear or concave JV response (d2J/dV2<0) for inefficient cathode-polymer interfaces has been observed, where the bulk material properties (the μτ product) and the thickness of the active layer was favourable for high FF. © 2008 Elsevier B.V. All rights reserved.