Electrochemical Synthesis and Characterization of Copper Indium Gallium Selenide (CIGS) Absorber Layer for Solar Cells

Abstract

The quest for searching alternative energy resource is of current interest due to the scarcity of the conventional energy sources, which are major pollutants of the atmosphere. Research on renewable energy has gained attention due to its advantageous features sides like no noise, impollutant, static working parts and long life time. The much matured technology in solar cells is silicon (Si). Thought it have gained a dominant place in commercial aspects, it recedes in terms of cost effective purification process, defect tolerance, Indirect band gap nature (less absorption coefficient), which made researchers to thin for a better alternative direct band gap semiconducting materials like CIGS and CdTe. CIGS have gained much attention in last two decades due to their high absorption coefficient (direct band gap nature). Photovoltaics utilizing CIGS (Copper indium gallium Selenide) material is considered to be the most efficient solar energy converter of any thin film device. They exhibit the potential to reduce the device fabrication cost when compared to Si-based solar devices. The high efficiency CIGS thin film solar cells 20.5% were reported by NREL (National Renewable energy laboratory,USA) for an aperture area of 1 cm2. Very recently, featuring news release of Center for Solar Energy and Hydrogen Research Baden-Württemberg) of Stuttgart, Germany have reported 21.7% (0.5 Cm2) as confirmed by Fraunhofer Institute for Solar Energy Systems ISE. Impressive efficiencies were also reported for mini-modules of CIGS devices of efficiency 18.7% (15cm2).
The efficiency percentage motivates to give an impressive scope to explore about the material.
All the reported high efficiency devices were fabricated by co-deposition technique (Physical Vapor Techniques). Inspite of high efficiency, the cost of production and mass production are the hurdles for the PVD technique. Electrodeposition is one of the promising deposition techniques, have achieved an efficiency of 12.25% and 10% for an area of 102 cm2 and 1.07 m2.
The work pertains to the electrodeposition of CIS/CIGS absorber layer using two and three electrode systems. From the linear sweep voltammetry (LSV), the bath composition and deposition potential were optimized for ternary system (CIS). Similar approach was extended to quaternary systems using cyclic voltammetry (CV). Using sodium dodecyl sulphate (SDS) as surfactant , it was found to be beneficial in obtaining crystalline and dense CIS/CIGS thin films which is confirmed by XRD and FE-SEM studies. From the preliminary experiments for CIS electrodeposition in a neutral electrolyte without any complexing agents, it was found that CIS can be conveniently co-deposited between the potential of -1 to -1.3 V. This proposed simplified bath scheme reduces the complexity in bath chemistry. The compositional analysis by EDS for CIS/CIGS absorbers deposited using two electrode system revealed that a potential higher than -2 V is required to obtain near stoichiometric CIS/CIGS films.