Effects of Ga contents on properties of CIGS thin films and solar cells fabricated by co-evaporation technique

Abstract

This study examined the effects of Ga content in the CIGS absorber layer on the properties of the corresponding thin films and solar cells fabricated using a co-evaporation technique. The grain size of CIGS films decreased with increasing Ga content presumably because Ga diffusion during the 2nd stage of the co-evaporation process is more difficult than In diffusion. The main XRD peaks showed a noticeable shift to higher diffraction angles with increasing Ga content, which was attributed to Ga atoms substituting for In atoms in the chalcopyrite structure. Band gap energy and the net carrier concentration of CIGS films increased with Ga/(In + Ga) ratios. Regarding the solar cell parameters, the short circuit current density ($J_{SC}$) decreased linearly with Ga/(In + Ga) ratios due to the lack of absorption in the long-wavelength portion of the spectrum, while the open circuit voltage ($V_{OC}$) increase with those. However, $V_{OC}$ values at high Ga/(In + Ga) regions (>0.35) was far below than those extrapolated from the low Ga contents regions, finally resulting in an optimum Ga/(In + Ga) ratio of 0.28 where the solar cell showed the highest efficiency of 15.56% with $V_{OC}$, $J_{SC}$ and FF of 0.625 V, 35.03 mA $cm^{-2}$ and 0.71, respectively.

Keywords

• Solar cell;
• Thin film;
• $CuInGaSe_2$;
• Co-evaporation;
• Ga ratio