The effect of illumination intensity and temperature on open circuit voltage (Voc) in organic photovoltaic devices has been investigated. Voc is observed to saturate at high illumination intensities. The illuminated J-V characteristics at different intensities intersect the dark characteristic at a single point. This intersection point is shown to be equal to the built-in voltage (Vbi) in the sample. A reduction in temperature shows increment in saturated Voc. This increment in saturated Voc is attributed to the variation of Vbi with temperature. A model has been presented that explains the observed behavior of Vbi at different temperatures.