Light-induced junction modification in (Cd, Zn)S/CuInSe2solar cells

The current-voltage characteristics of illuminated (Cd, Zn)S/ CuInSe2solar cells are shifted to lower voltages than those expected from the superposition principle. The practical result is a reduced open-circuit voltage,sometimes by as much as 100 mV at room temperature. Dependence of the shift on light intensity, wavelength, temperature, and time after the light, is blocked is investigated. Thermal heating of the junction by the light is discounted in favor of optical excitation of interfacial states. A heterojunction model is proposed for both dark and light conditions. The states trap charge under illumination and approach saturation near 100 mW/cm2. The result is a modified effective barrier to carrier transport. Unipolar or dipolar electric charge densities near the interface, the order of 1011cm-2are sufficient to explain the observed shift of 100 mV. The charging effect is due to a mixture of states in the CuInSe2bandgap which dominate the shift and states in the (Cd, Zn)S which result in a second-order spectral dependence. The relaxation to the equilibrium dark condition indicates that many states with long time constants are involved.