Impact of heat‐light soaking and heat‐bias soaking on NaF‐treated CIGS thin film solar cells

Abstract

AbstractThe effects of heat‐light soaking (HLS) and heat‐bias soaking (HBS) were investigated on NaF‐treated Cu (In,Ga)Se2 (CIGS) solar cells. The HLS improved the cell efficiency with increased open‐circuit voltage (VOC) for NaF‐treated CIGS solar cells, whereas the short‐circuit current density (JSC) slightly decreased. The reduced JSC was caused by the significantly high carrier concentration due to the HLS, leading to a fairly narrow space charge region. However, the reduced JSC could be successfully recovered by the subsequent moderate heat soaking (HS) in the dark, thereby improving cell efficiency. On the other hand, no beneficial effect was observed for NaF‐free solar cells. Secondary ion mass spectrometry (SIMS) measurements revealed that the Na and K ions increased especially near the CIGS surface region after the HLS process. This result suggests that these ions were shifted by the photo‐induced electric field. The thermally and electrically activated alkali ions may eliminate the donor‐like defects in CIGS absorber, resulting in an increased acceptor concentration. Furthermore, we found that an external forward bias without light irradiation (HBS) resulted in the same effect as HLS. Based on these results, a possible mechanism of HLS and HBS effects was proposed on NaF‐treated CIGS solar cells.