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

Abstract

The effects of heat-light soaking (HLS) and heat-bias soaking (HBS) were investigated on NaF-treated Cu (In,Ga)Se 2 (CIGS) solar cells. The HLS improved the cell efficiency with increased open-circuit voltage (V OC ) for NaF-treated CIGS solar cells, whereas the short-circuit current density (J SC ) slightly decreased. The reduced J SC was caused by the significantly high carrier concentration due to the HLS, leading to a fairly narrow space charge region. However, the reduced J SC 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 donorlike 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.

Metastable Behavior on Cesium Fluoride‐Treated Cu(In1–x,Gax)Se2 Solar Cells

“…W190113). HLS treatment increased the V OC and carrier concentration ( N CV ) and reduced J SC, whereas the HS treatment yielded opposite results similar to the previous reports on alkali‐treated samples . The change in the width of the space charge region (SCR) was consistent to the change in N CV .…”

“…Na treatment and growth of CIGS thin films under “Se‐rich” and “Cu‐rich” conditions have been suggested as methods to suppress the formation of the divacancy complex in the absorber . However, in our previous investigation, we observed performance improvements by HLS or HBS treatments in NaF‐treated CIGS solar cells having a CGI ratio of 0.95 . Furthermore, Nishinaga et al.…”

“…A recent report suggested that the carrier collection efficiency in the solar cell after HLS also depends on the presence of alkali metals near the Mo back contact . However, through the controlled HLS followed by HS treatment, the improvement in device performance can be realized . In addition, as shown in Figure , a reversible change in FF occurred due to HLS and HS treatments, which may be related to the defect states at the interface .…”

“…Both the positive and negative effects due to this metastable behavior have been reported . However, in CdS/alkali metal‐treated CIGS solar cells, HLS guarantees an improvement in V OC and FF under the optimized condition . In this study, we analyzed the contributing factors for the metastable effects in CsF‐treated CIGS solar cells.…”

“…This indicates that CsF‐treated CIGS solar cells are applicable to terrestrial and space applications. Furthermore, our previous studies revealed the beneficial effect of heat‐light soaking (HLS) or heat‐bias soaking (HBS) on alkali metal‐treated CIGS solar cells. These post‐treatments increased the efficiency ( η ) of solar cells by enhancing the open‐circuit voltage ( V OC ).…”