Effect of light intensity on current collection in thin-film solar cells

We have measured the current-voltage curves of thin-film solar cells using focused laser spots (30-500 μm) using DC and modulated (AC) photocurrent techniques. The AC short-circuit current response (I _SC) and the AC fill factors (FF) decrease for small spot sizes corresponding to several 100 sun light intensities. Laser line scans across the devices produced significant but reproducible spatial fluctuations in AC I_SC. These spatial variations depend on spot size and are reduced by scanning with lower light intensity. The reduction of AC FF and AC I_SC was largest in a-Si:H, intermediate in CdTe and CuInSe₂ (CIS), barely noticeable in some Cu(Ga,In)Se₂ (CIGS) cells and absent in a silicon cells. The observations on CIGS and some CIS cells can be explained by internal series resistance, but field dependent collection and recombination effects must be invoked to explain results on most thin-film solar cell materials. Such field modification is not accounted for in standard exponential diode equation models