(Poster Award - 1st Place) Emerging Effect of Vertical-Oriented WSe2 Interfacial Layers in the CIGSe Thin Film Solar Cells

Abstract

Copper indium gallium selenide (CIGSe) thin film solar cells become popular due to their potential as a solution for green energy and space technology recently. These solar cells have a similar electrical performance to traditional silicon solar cells, making them ideal for light, flexible, and high-performance applications. Molybdenum is a popular material for the rear electrode due to its conductivity and low cost. During the selenizing process of the absorber layer, MoSe2 will be formed between Mo and CIGSe layer, which improves adhesion between Mo and CIGSe and facilitates carrier transport from the absorber layer to the rear electrode. Therefore, previous studies show that oxides or nitride-based materials, such as Titanium Nitride and Alumina, are used as the passivation layer at the back contact to address the trade-off of the critical thickness of the MoSe2 layer. That is, designing a nanostructure and creating a contact hole can attract carriers and avoid current blocking. Similarly, Oxide-based material interfacial layers, such as Molybdenum oxide and tungsten oxide, can also play a role in the obstacle of selenium diffusion and modify the band structure. In this work, a few layers of WSe2 interfacial layer were introduced between the CIGSe absorber layer and Mo electrode. First, 20 nm WOx was deposited on Mo by E-beam evaporation. Second, WOx was selenized by a vertical furnace with ICP coils plasma system to form WSe2. By tuning the substrate temperature of plasma-enhanced selenization process, the full vertically lamellar-oriented WSe2 was formed on Mo. The results show that inserting a few layers of WSe2 improved the efficiency of the CIGS solar cell from 12.5 to 14.3%, with a significant increase in short circuit current density up to 38mA/cm2. This improvement was attributed to the improved carrier transportation through the backside interface, facilitated by the specific vertically lamellar-orientated WSe2 and the induced Gallium gradient measured by TEM, SIMS, and XPS. The remarkable enhancement by WSe2 provides the potential for further applications. Figure 1