Fill factor in organic solar cells

Abstract

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 openc-ircuit voltage (V_OC) and short-circuit current density (J_SC), 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 V_sat<V<V_OC) can vary from convex (FF>50%) to concave (FF<12.5%) depending upon these parameters. A general trend of convex shaped JV characteristics (d²J/dV²>0) 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 μ t product) and the thickness of the active layer was favourable for high FF.