Enhancement of open circuit voltage via light soaking in amorphous silicon solar cells

The increase in open-circuit voltage (V-oc) due to light soaking was investigated in amorphous silicon solar cells. The light-induced increase in V-oc appears when i-layers are prepared in H-2-diluted plasma (H-2/SiH4 > 10). It is necessary that more than two-thirds of the i-layer thickness be formed under such hydrogen dilution. This phenomenon is not related to p/i interface effects but reflects the bulk properties of the dilution layer. The increase in V-oc is caused by the voltage shift of dark I-V characteristics. The change of V-oc is determined by the balance between the shift of dark current and the reduction of photocurrent due to the light-induced defects. The voltage shift suggests an increase in the diode barrier height of the pin junction; however, the increase in built-in potential in the active layers (i-layers) is not observed in our experiments. The advantage of this effect is seen with sufficiently thin i-layers, because conventional light-induced degradation is negligible. The increase in V-oc improves conversion efficiency by 2% after light soaking in the case of 500 Angstrom i-layers.