Influence of Cu-atomic ratio in the 3-stage deposition technique on the efficiency of CuIn1-xGaxSe2 solar cells

Abstract

The 3-stage co-evaporation technique is one of the deposition processes used to fabricate photon absorber layer in high efficiency CuIn1-xGaxSe2 (CIGS) solar cells. For this technique, the [Cu]/[III] ratio (y), where [III] refers to group-III elements, evolves from Cu-poor (y < 1) to Cu-rich (y > 1) in the 2nd stage and finally ends with slightly Cu-poor (y ~ 0.9) in the 3rd stage of the 3-stage process. Here, the highest values of [Cu]/[III] in the 2nd stage are intentionally varied from 1.0 to 1.5 by setting the deposition time of the pre-calibrated Cu and Se fluxes in the 2nd stage. The [Ga]/[III] ratio (x) is set at 0.37 during the 1st and 3rd stages in all devices. The influences of the Cu-atomic ratio are examined for the crystal grain growth, elemental depth profiles of the CIGS absorbers as well as the photovoltaic parameters and external quantum efficiency (EQE) of the CIGS solar cells. The optimal value of y = 1.3 is found to provide the highest efficiency CIGS device. The double-grading depth profile in the [Ga]/[III] ratio has also been observed despite the constant fluxes of group-III elements set during the whole process. The performances of the CIGS solar cells are investigated under AM1.5 condition and found to have open-circuit voltage (V OC) of 670 mV, short-circuit current density (J SC) of 33.2 mA/cm2, fill factor (FF) of 75.5% and the power conversion efficiency of 16.8% for the best CIGS device. The J SC of the device with y = 1.3 is relatively higher than other devices due to the increase of photo-generated currents in the short wavelength region as seen in the EQE spectrum.