## 2 Materials and Methods  
Mo bilayer films were deposited on soda lime glass substrates (20 \(\times\) 20 \(\times\) 1 mm) by RF magnetron sputtering for the bottom layer and DC sputtering for the top layer with a circular Mo target (99.97% purity, \(\Phi\)50 mm \(\times\) 5 mm), respectively. Prior to deposition, the substrates were cleaned using an ultrasonic cleaning machine with acetone, alcohol and deionized water for 15 min in order followed bydrying with N\({}_{2}\). Finally, they were placed on the backplane in the sputtering chamber with a distance of 80 mm between the target and the substrate. The base pressure in the sputtering chamber was 2 \(\times\) 10\({}^{-4}\) Pa before deposition. Pre-sputtering process was performed for 15 min to remove contaminants from the target surface. In the following experiments, the bottom layer RF sputtering power and the top layer DC sputtering power were 120 W and 100 W, respectively. In order to study the effects of the bottom layer sputtering pressures on the structures and properties of Mo bilayer films and the lattice matching between the top and bottom layer, we fixed the top layer sputtering pressure at 0.3 Pa and changed the bottom layer sputtering pressures in the range of 0.4-1.2 Pa. The gas flow rates changed in the range of 17-73 mL/min, the substrates were kept at room temperature and the holder was biased negatively up to 150 V during deposition. Generally, the thickness of back electrodes of CIGS solar cells is about 500-1000 nm. In order to save resources and increase adhesion, the thickness of Mo films were set to be 500 nm. For all depositions, the bottom and top layer thicknesses were controlled to be 100 nm and 400 nm, respectively, by a quartz crystal monitor. In order to study the bonding of bottom and top layer Mo films and the growth of top layer Mo films, we also studied the surface conditions of bottom layer Mo films. The Mo bilayer films prepared at bottom layer sputtering pressure of 0.4 Pa and top sputtering pressure of 0.3 Pa and were annealed at various temperatures in the range of 100-400 \({}^{\circ}\)C for 30 min in a tube furnace. X-ray diffraction (XRD, Panalytical, Almelo, Netherlands), scanning electron microscopy (SEM, ZEISS, Oberkochen, Germany), atomic force microscopy (AFM, SPA-400, Tokyo, Japan), an ultraviolet (UV)-visible spectrophotometer (Hitachi U-4100, Tokyo, Japan) and Hall effect measurement system (HMS ECOPIA 3000) were used to characterize the Mo films. The degree of adhesion was qualitatively tested using a Scotch tape test, if the shedding area exceeds a certain proportion, it is considered that it will not pass the adhesion test [18]. The photo-conversion efficiency of CIGS solar cell was measured and calculated using the current-voltage (I-V) characteristics under standard AM 1.5 and 100 mW/cm\({}^{2}\) illumination at room temperature [19].  
The grain sizes (D) of the films was calculated using the Scherrer formula [20]:  
\[\mathrm{D}=\frac{0.9\lambda}{\beta\mathrm{COS}\delta} \tag{1}\]  
the micro-strain (\(\epsilon\)) and dislocation density (\(\delta\)) developed in the films were calculated from Equations (2) and (3), respectively [21].  
\[\epsilon=\frac{\beta}{4\tan\theta} \tag{2}\]  
\[\delta=\frac{1}{\mathrm{D}^{2}} \tag{3}\]  
here, \(\lambda\) is the X-ray wavelength (0.15406 nm), \(\beta\) is the full width at half maximum (FWHM) of XRD and \(\theta\) is the Bragg angle of diffraction peak of XRD; all variables are SI units.