Influence of Cu off-stoichiometry on wide band gap CIGSe solar cells

Abstract

The electric properties of solar cells based on co-evaporated Cu(In,Ga)Se 2 (CIGSe) thin film show a good tolerance regarding the absorber Cu content ( y = [Cu]/([In] + [Ga])) for standard Ga concentration, i.e. x = [Ga] / ([In] + [Ga]) ~ 0.3. In the present contribution, we show that this tolerance is lost when the gallium content is increased. Wide bandgap CIGSe samples ( x ~ 0.55) with a variation in y from 0.97 to 0.84 have been grown. The efficiency of the cells decreases from 12.6% to 6.5% for y = 0.97 and 0.84 respectively. For the lowest y, the efficiency is harmed because of a low short-circuit current density ( J sc ), an increased voltage dependency in the current collection, which affects the fill factor ( FF), and a decrease of the open-circuit voltage ( V oc ). For y = 0.97 and 0.84 respectively, the decrease of the activation energy ( E a ) from 1.36 to 1.24 eV indicates a shift of the area of the dominant recombination from the space charge region towards the interface. There seems to be evidence that reducing the Cu-content in the CIGSe thin film will cause a decrease in the width of the space charge region. Solar cells based on Cu-rich CIGSe (1.03 < y < 1.09) have also been fabricated and characterized. A strong deterioration of their electrical properties is observed despite the KCN etch of the segregated Cu 2 − x Se binary phases at the surface, suggesting the presence of residual Cu 2 − x Se precipitates within the layer.