## 3 Results and Discussion
The absorption edge of the CIGS:Te thin films tended to be similar or rather slightly shifted toward longer wavelengths (red-shift) due to the Burstein-Moss (B-M) effect once Te was co-sputtered up to condition (5), as shown in Figure 5a, indicating that the band gap became narrower. Under condition (6), a blue-shift of the absorption edge occurred, and the bandgap increased. This highlights the disadvantages of using CIGS:Te thin films as the absorber layer in thin-film photovoltaic devices, namely, an increase in the spectral transmission range for the minimum in the solar emission spectrum under these conditions [40; 41; 42]. The absorption coefficient (\(\alpha\)) of the CIGS:Te thin films are shown in Figure 5b. The absorption coefficient is a measure of how far  
Figure 4: Lattice characteristics of the CIGS:Te thin films with different Te co-sputtering times of (1) 0, (2) 270, (3) 510, (4) 750, (5) 945, and (6) 1140 s. (**a**) Tetragonal distortion parameter (\(\eta\)) and \(d\)-spacing along the (112) plane; (**b**) lattice constants, \(a\) and \(c\), and volume of unit cell; (**c**) microstrain (\(\varepsilon\)) and dislocation density (\(\delta\)). below the surface of the CIS thin film an incident photon of a particular wavelength is absorbed, that is, the ability of a semiconductor to absorb photons [43; 44]. This can be expressed as \(\alpha=(1\ /d)\mathrm{ln}[(1-R)/T]\), where \(d\), \(R\), and \(T\) are the thickness, reflectance, and optical transmittance of the thin film, respectively [45]. Using the Beer-Lambert law in the high-absorption region, the absorption coefficient was calculated as \(\alpha(v)=(2.303A/d)\), where \(A\) is the absorbance and \(d\) is the thickness of the thin film. The absorption coefficient of the CIGS:Te thin film was high (approximately \(10^{4}\) cm\({}^{-1}\)) and stable in the visible to NIR region (380-2000 nm). The mean values of the absorbance over the 380-780 nm (visible) and 380-2000 nm (visible to NIR) spectral ranges are shown in the inset of Figure 5b. The absorbance, which is the amount of light absorbed by the thin film, was calculated as \(A=-\mathrm{log}T=\mathrm{log}(I_{0}/I)\), where \(T\) is the optical transmittance; \(I_{0}\) is the intensity of the incident radiation; and \(I\) is the intensity of the transmitted radiation. In the visible spectral region, the mean absorbance of the CIGS thin film without Te improved to 0.65-0.66 under conditions (3)-(5) in the CIGS:Te thin films. The Planck relation given by \(E_{g}=hc/\lambda\), where \(h\) is Planck's constant (\(4.135667\times 10^{-15}\) eVs); \(c\) is the velocity of light (\(3\times 10^{8}\) m/s); and \(\lambda\) is the wavelength (nm) of the absorption onset (\(1/\mathrm{e}=37\%\)), was used to estimate the band gap (\(E_{g}\)) of the CIGS:Te thin films. The band gap of the CIGS:Te thin films was relatively constant under conditions (1)-(5), albeit larger than the known values; however, it increased rapidly under condition (6), which can cause a corresponding photocurrent loss, although the \(V_{oc}\) can be enhanced. The band gaps of the CIGS:Te thin films were also estimated using Tauc's equation (\(ahv\))\({}^{n}\) = \(B(hv-E_{g})\), where \(\alpha\) is the absorption coefficient; \(h\) is Planck's constant; \(v\) is the photon frequency; \(B\) is a proportional constant; and \(n\) is the transition probability using two for direct allowed transitions. The Tauc plot illustrates the extrapolation of the straight-line portion of the curve to \((\Delta iv)^{2}\) = 0. The band gap from the Tauc plot was also in a similar range and the trend to the values calculated from the optical transmittance. The composition of the thin films resulted in the surplus In + Ga and Se-deficiency composition under all conditions regardless of the Te co-sputtering (Figure 2b), which leads to the relatively large lattice parameter due to the Se-deficiency with the relatively small atomic radius of Se and, hence, the increase in the band gap.