Effect of heat-bias soaking on cesium fluoride-treated CIGS thin film solar cells

The combined effect of heating and forward bias voltage, that is, heat-bias soaking (HBS) on cesium fluoride (CsF)-free and CsF-treated Cu(In1-xGax)Se-2 (CIGS) solar cells using chemical bath deposition-cadmium sulfide (CBD-CdS) buffer layer was investigated with varying the heating temperature, bias voltage, and biasing time. Heat bias soaking followed by heat soaking (HS) (ie, HBS/HS) treatments improved the open-circuit voltage (V-OC) and conversion efficiency for the CsF-treated CIGS solar cell, whereas such a beneficial effect was not observed for CsF-free CIGS solar cells. Capacitance-voltage measurement confirmed the significantly increased carrier concentration (N-CV) after HBS for the CsF-CIGS solar cell, which is one of the possible reasons for the increased V-OC. Because of the extremely high N-CV, the short-circuit current density (J(SC)) decreased owing to the narrower depletion width. However, the high N-CV could be tuned to an appropriate value via a subsequent moderate heating procedure without biasing. As a result, the J(SC) loss was reduced, thereby improving the cell efficiency. These results open a new route to improve the efficiency of alkali-treated CIGS solar cells.