## 1 Introduction
The TCO is deposited on top of a glass substrate, such as in dye-sensitized solar cells and perovskite-based solar cells. The ITO electrical conductivity can be reduced in the presenceof oxygen at a relatively high temperature, such as around 500 \({}^{\circ}\)C. However, FTO is much more stable under a heat annealing environment. In addition, FTO has been distinguished for its use in numerous multi-structure photovoltaics and nano-structured devices [28; 29]. Moreover, few investigators have studied metal-selenide as a carrier-transporting material for perovskite photovoltaic devices [30; 31]. The metal-selenide material has attracted significant attention due to its outstanding optical properties and electric characteristics. Moreover, the energy band gap can be adjusted between 1.0 and 1.7 eV by varying the In:Ga ratio [32]. This also provides the possibility for tandem solar cells. The traditional standard thickness of CuIn\({}_{1-x}\)Ga\({}_{x}\)Se\({}_{2}\) absorber layer has been about 2-3 \(\upmu\)m [33; 34; 35]. It is desirable to thin down the absorber layer's thickness to below 1 \(\upmu\)m. This would improve recombination at the back contact layer [36; 37]. The recombination can be decreased by changing the In:Ga ratio toward the back contact layer, directing to a beneficial electron reaction and reducing the active absorber layer thickness to essentially decrease light absorption. Nevertheless, light trapping in CuIn\({}_{1-x}\)Ga\({}_{x}\)Se\({}_{2}\) ultra-thin solar cellsfaces challenges in crystallization growth and parasitic photon absorption at the Mo back metal-electrode contact layer. The absorption loss at the back contact layer should be considered for CuIn\({}_{1-x}\)Ga\({}_{x}\)Se\({}_{2}\) solar cell before its application in advanced photovoltaic devices [38; 39]. Even now, CuIn\({}_{1-x}\)Ga\({}_{x}\)Se\({}_{2}\) thin-films are one of the most promising materials for photovoltaic devices because of their high solar photon absorption coefficient and direct energy band gap. They also exhibit a high conversion efficiency on various flexible substrates [39; 40]. A small contact resistance between the metallic film and the optical absorber film is inevitable, as the contact resistance addstoa serial resistance. The Schottky barrier has been reported with a height of approximately 0.8 eV for the p-doped CuInSe\({}_{2}\) (011)/Mo metal-contact film [41]. The molybdenum-diselenide (MoSe\({}_{2}\)) film has significant effects on the behavior of the Mo/CuIn\({}_{1-\mathrm{x}}\)Ga\({}_{\mathrm{x}}\)Se\({}_{2}\) layer interface [42; 43]. Mo-O and Mo-O-Se compounds were discovered while being selenized with a Mo film deposited on the substrate with a CuInS film [44]. For a substrate temperature higher than 550 \({}^{\circ}\)C, the MoSe\({}_{2}\) film thickness increased witha higher annealing temperature [45]. The nano-crystal film orientation and growth rate of the MoSe\({}_{2}\) film was dependent upon the selenization temperature. It has been indicated that the construction of a MoSe\({}_{2}\) film at the Mo/CuIn\({}_{1-\mathrm{x}}\)Ga\({}_{\mathrm{x}}\)Se\({}_{2}\) layer interface during a three-stage process using the annealing procedure [46]. However, the MoSe\({}_{2}\) film did not form under copper-rich conditions, and the characteristics of the MoSe\({}_{2}\) film relied on the CuIn\({}_{1-\mathrm{x}}\)Ga\({}_{\mathrm{x}}\)Se\({}_{2}\) deposition technique and the annealing conditions [47; 48]. In the past years, researchers have studied the contribution of nano-materials to the efficiency of ultra-thin CuIn\({}_{1-\mathrm{x}}\)Ga\({}_{\mathrm{x}}\)Se\({}_{2}\) solar cells; that would lead to even thinner and more cost-effective devices [49; 50]. In addition, a highly defective CuIn\({}_{1-\mathrm{x}}\)Ga\({}_{\mathrm{x}}\)Se\({}_{2}\)/TCO interface could affect the connection to the depletion region in a thinned CuIn\({}_{1-\mathrm{x}}\)Ga\({}_{\mathrm{x}}\)Se\({}_{2}\) solar cell, resulting in a high recombination rate between the absorber layer and the back contact layer [51; 52].