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)Se2(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 (VOC) 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 (NCV) after HBS for the CsF‐CIGS solar cell, which is one of the possible reasons for the increasedVOC. Because of the extremely highNCV, the short‐circuit current density (JSC) decreased owing to the narrower depletion width. However, the highNCVcould be tuned to an appropriate value via a subsequent moderate heating procedure without biasing. As a result, theJSCloss was reduced, thereby improving the cell efficiency. These results open a new route to improve the efficiency of alkali‐treated CIGS solar cells.