## 2 Materials and Methods  
The FTO/glasssubstrate (Hartford Glass, TECA7-RL, IN, USA) was cleaned using acetone (Sigma-Aldrich, MO, USA), ethanol (Sigma-Aldrich, MO, USA), and de-ionizedwater, each for 10 min, and then dried in an oven. Mo film has good conductive properties and was deposited onthe substrate by a radio-frequency (RF) magnetron sputtering system. It was extremely salient to ascertain optimal sputtering conditions for Mo film because good adherence and low resistivity were necessary to ensure a high efficiency for solar cells. In order to ameliorate conventional architecture to develop a novel solar cell device structure, a thinner Mo material was the second back metal-electrode contact layer. The FTO material was the first layer of the back contactelectrode. Thus, the FTO also worked as an adhesion layer in the multilayer-structure of solar cells. Using a transparent conductive oxide film, such as FTO, leads to recombination to substantially lower the absorption at the Mo back metal-electrode contact layer, enhancing the efficiency for multi-junction solar cells. In this study, an RF sputtering system with a different power in conjunction with a different working Ar flow pressure was explored to prepare bi-layer Mo films. The bi-layer Mo films obtained at a low sputtering pressure experienced compressive stress, which displayed low electrical resistivity but adhered poorly to the substrate. On the other hand, the bi-layer Mo films using high-pressure deposition could experience tensile stress and would adhere well to the substrate but displayed high electrical resistivity. The deposition was thus modified to include a porous bottom layer and a dense top layer. The combination of a higher working pressure and a lower working pressure has been attempted. In this experiment, the bottom layer was deposited at a higher Ar flow pressure and the Ar flow rate and RF power were maintained at 80 sccm and 120 W. Additionally, the top layer was deposited at a lower Ar flow pressure condition and the Ar flow rate and RF power were maintained at 40 sccm and 60 W for better adhesion and to ensure a low resistivity. Each Mo film with an approximate thickness of 100 nm was deposited. The Mo appeared as an excellent choice for the back metal-contact material as a result of its relative stability at the annealing temperature, better ohmic-contact for alloying with copper and indium, and its low contact resistance to the CuIn\({}_{1-\mathrm{X}}\)Ga\({}_{\mathrm{X}}\)Se\({}_{2}\) solar cells. The Mo film also acted as a reflective layer in the CuIn\({}_{1-\mathrm{X}}\)Ga\({}_{\mathrm{X}}\)Se\({}_{2}\) solar cells. The CuIn\({}_{1-\mathrm{X}}\)Ga\({}_{\mathrm{X}}\)Se\({}_{2}\) HTM films adjacent to the back-contact layer can also enhance the role of the back metal-electrode contact material as an optical reflector. The CuIn\({}_{1-\mathrm{X}}\)Ga\({}_{\mathrm{X}}\)Se\({}_{2}\) film was further deposited by an RF magnetron system using a CuIn\({}_{1-\mathrm{X}}\)Ga\({}_{\mathrm{X}}\)Se\({}_{2}\) target on the bi-layer Mo/FTO glass substrates. The argon flow rate and RF power were maintained at 60 sccm and 80 W, respectively. The CuIn\({}_{1-\mathrm{X}}\)Ga\({}_{\mathrm{X}}\)Se\({}_{2}\) film was sputtered on bi-layer Mo metal-electrode films as a hole-transport material layer. For ultra-thin CuIn\({}_{1-\mathrm{X}}\)Ga\({}_{\mathrm{X}}\)Se\({}_{2}\) HTM (<400 nm), a surface roughness effect transforms the optical absorbing thickness, which provides shunt paths between the back-electrode and the front-electrode through the optical-conductive buffer layer. At the CIGSe-Mo interface, the diffusion of Se could escalate the structural quality of the CuIn\({}_{1-\mathrm{X}}\)Ga\({}_{\mathrm{X}}\)Se\({}_{2}\) solar cells, the adherence of CuIn\({}_{1-\mathrm{X}}\)Ga\({}_{\mathrm{X}}\)Se\({}_{2}\)/Mo metal-contact and the electrical contact at CuIn\({}_{1-\mathrm{X}}\)Ga\({}_{\mathrm{X}}\)Se\({}_{2}\)/Mo metal-contact interface.