Potential-induced optimization of ultra-thin rear surface passivated CIGS solar cells

Abstract

Ultra-thin Cu(In,Ga)Se2 (CIGS) solar cells with an Al2O3 rear surface passivation layer between the rear contact and absorber layer frequently show a "roll-over" effect in the J-V curve, lowering the open circuit voltage (VOC), short circuit current (JSC) and fill factor (FF), similar to what is observed for Na-deficient devices. Since Al2O3 is a well-known barrier for Na, this behaviour can indeed be interpreted as due to lack of Na in the CIGS absorber layer. In this work, applying an electric field between the backside of the soda lime glass (SLG) substrate and the SLG/rear-contact interface is investi-gated as potential treatment for such Na-deficient rear surface passivated CIGS solar cells. First, an electrical field of +50 V is applied at 85 °C, which increases the Na concentration in the CIGS absorber layer and the CdS buffer layer as measured by glow discharge optical emission spectroscopy (GDOES). Subsequently, the field polarity is reversed and part of the previously added Na is removed. This way, the J -V curve roll-over related to Na deficiency disappears and the VOC (+25 mV), JSC(+2.3 mA/cm2) and FF (+13.5% absolute) of the rear surface passivated CIGS solar cells are optimized. Rear surface passivated CIGS solar cells often are Na-deficient, as seen by a "roll-over" effect in the J-V curve that lowers the open circuit voltage (VOC), short circuit current (JSC) and fill factor (FF). In this work, potential-induced degradation (PID) and subsequent accelerated recovery are used to redistribute Na in these cells and thus optimize VOC, JSC and FF.