Time-dependent open-circuit voltage in CuInSe2/CdS solar cells: Theory and experiment

The increase with illumination time of the open-circuit voltage of the CuInSe2/CdS solar cell has been studied as a function of illumination intensity, temperature, and preillumination bias voltage. The results indicate time constants on the order of minutes (40 to 500 s at room temperature), with an activation energy of ∼0.20 eV. The magnitude of the effect (ΔVoc ≤30 mV) and the time constants both decrease with increasing preillumination voltage bias, indicating that the effect is due to voltage rather than illumination. A model involving the tunneling of electrons trapped in deep states in the CdS, near the junction with the CuInSe2 layer to holes in the CuInSe2 valence band, seems to account for the magnitude, as well as the time and voltage dependence of the effect. The increased positive charge near the interface shifts the division of the diffusion voltage between the two sides of the junction, thereby increasing Voc. The fill factor and efficiency of the cell are also increased, while the short-circuit current is unaffected.