Probe of field collapse in a-Si:H solar cells

We study the effect of illumination intensity on solar cell performance in a-Si:H solar cells. We find that the fill factor strongly depends on light intensity. As we increase the illumination intensity from low levels to one sun we observe a decrease in fill factor of approximately 15% in as grown cells. We attribute this effect to electric field collapse inside the cell. We propose that photogenerated space charge (free and trapped charge) increases with light intensity and causes field collapse. We describe the origin of space charge and the associated capacitance-photocapacitance. We measure the photocapacitance as a barometer to probe the collapsed field. We obtain a good agreement between photocapacitance experiments and theory. We also explore the light intensity dependence of photocapacitance and explain the decrease of FF with the increasing light intensity.