Abstract
A method was developed for the electrodeposition of Cu-In-Ga precursor layers to elaborate Cu(In,Ga)(S,Se)2 (CIGS) thin films on silicon substrates for future application as silicon/wide-gap CIGS tandem solar cells. An underlayer of Ag was first deposited on silicon substrates to ensure a good adhesion of the electrodeposited stack and to serve as cathode during the deposition process. Cu, In and Ga layers were then sequentially electrodeposited. Ag-Cu-In-Ga precursor layers were finally subjected to elemental sulfur annealing at 600 °C. Formation of compact and adherent AgCIGS is observed. X ray diffraction and photoluminescence analyses confirm the formation of wide-gap CIGS of about 1.6 eV, with a spontaneous gallium grading over the depth of the sample leading to the formation of a bi-layer structure with a gallium rich layer at the interface with silicon.
Highlights
Cu(In,Ga)(S,Se)2 (CIGS) is a good candidate for tandem solar cell applications thanks to the tunability of its bandgap, modified by changing the ratios In/Ga and Se/S
The aim of this study is to develop an efficient route to synthesize CIGS on silicon substrates
A different approach was developed, consisting of depositing a 50 nm layer of silver by evaporation on the silicon’s surface. This intermediate layer offers two direct advantages, (i) the adhesion of silver on silicon is greatly improved compared to electrodeposited copper on silicon, and (ii) its sheet resistance is estimated at 0.32 V/□, which is sufficiently low to avoid passing the electrodeposition current through the silicon solar cell and to prevent voltage drops across the substrate’s surface
Summary
Cu(In,Ga)(S,Se) (CIGS) is a good candidate for tandem solar cell applications thanks to the tunability of its bandgap, modified by changing the ratios In/Ga and Se/S. Wide-gap CIGS can be efficiently combined with silicon in tandem solar cells, the CIGS acting as the top semi-transparent solar cell. Barreau et al recently reported a 14.2% pure sulfide solar cell deposited by coevaporation for a bandgap of about 1.65 eV [3], together with an extensive study of the pure sulfide CIGS system [4]. Bandgap engineering and efficiency improvement can be achieved by partial substitution of copper by silver [5]. Recent investigations on (Ag,Cu) (In,Ga)Se2 led to a 15.1% efficiency solar cell with a wide bandgap of 1.45 eV [6] and to a 20.6% solar cell [7]
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