Time dependent voltage in CuInSe2 and CdTe solar cells

The open circuit voltage of CuInSe₂ and CdTe solar cells continues to increase during the time range of 10^-3 to 10⁴ s after the cell is illuminated or otherwise placed in forward bias. The effect is qualitatively similar in all samples measured to date, however, the magnitude of the voltage shift varies from 3 to 40 mV. The shift increases with increasing light intensity. The voltage shift depends on the voltage history of the sample and not the illumination history. Application of an appropriate pre-measurement bias in the dark can significantly reduce the voltage shift. When the cell remains in the dark with a constant forward current, a nearly identical voltage shift is seen. The time required for the cell to relax back to the dark zero-bias steady-state condition is similar to the time the cell has been in forward bias. A likely cause for the shift is electron-trapping centers near the diode junction which change occupation with changes in voltage. The voltage shift has important consequences for pulse-simulator measurements, which, because of the shift, tend to underestimate the steady-state voltage