Performance enhancement of CIGS solar cells using ITO as buffer layer

Abstract

The purpose of this research is the reduction of parasitic absorption to improve the CIGS solar cell performance. Decreasing the traditional CdS buffer layer thickness is a reduction method for parasitic absorption which yields to a higher photocurrent. However, degradation of the open-circuit voltage (Voc) and fill factor (FF) are observed in J-V measurements as a thin (<30 nm) CdS buffer layer is applied. Moreover, the toxic CdS has a relatively low bandgap which tends to a significant parasitic absorption in the blue wavelength region. Here, Indium Tin Oxide (ITO) with a larger bandgap is suggested as an alternative buffer layer, which unlike the CdS, demonstrates a significant efficiency enhancement without adversely affecting Voc and FF. ITO is an environmentally friendly substance which its ultra-thin thickness leads to a significant short-circuit current (Jsc) enhancement. Hence, an improvement of efficiency about 6% is achieved by successfully replacing the traditional CdS layer with ITO layer in conventional CIGS solar cell. Furthermore, ZnMgO (ZMO) with more transparency and larger bandgap, was applied to replace conventional ZnO window layer of the CIGS solar cell for further increment in Jsc. In addition, ZMO as a proper window layer in the proposed cell structure tends to the modification of J–V curve distortion at low temperatures. Ultimately, the final proposed structure with the alternative window layer (Mo/CIGS/ITO/ZMO/AZO) exhibited 7.5% efficiency improvement in comparison with the conventional solar cells.