| The current-voltage characteristics of many illuminated heterostructure diodes are shifted to lower voltages than those expected from the superposition principle. The practical result is often a low open circuit voltage. The voltage shift for CdS/CuInSe(,2) solar cells is as much as 100 mV at room temperature. These cells are studied in detail and a heterojunction model proposed for both dark and light conditions. 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. The states trap charge under illumination and modify the effective barrier to carrier transport. Unipolar or dipolar electric charge densities near the interface the order of 10('12) cm('-2) are sufficient to explain the observed shift of 100 mV. The shift increases with light intensity until near 100 mW/cm('2) where the charging of states approach saturation. The charging effect is due to a mixture of states in the CuInSe(,2) bandgap which dominate the shift and states in the CdS 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. |
