Current-voltage characteristics of polymer-fullerene solar cells

We have studied the influence of temperature and light intensity on the current-voltage characteristics of polymer-fullerene bulk-heterojunction solar cells. The open-circuit voltage varies linearly with temperature in the range 200K-300K and approaches a value of 930mV at T=80K. Strictly positive temperature coefficients were found for both, the short-circuit current density and the fill factor. These cause the device efficiency to increase steadily with temperature up to T=330K. The short-circuit current density increases almost linearly with light intensity. The fill factor is not significantly influenced by the incident light intensity in the temperature range from 260K to 330K. At lower temperatures, a negative slope of the fill factor is observed. Since the maximum power point varies sublinearly with light intensity, a decrease of the power efficiency is obtained at light intensities higher than 3mW/cm/sup 2/.