## 3 Results and Discussion
When the volume of the unit cell in the CIGS thin film without Te was 384.12 \(\times\) 10\({}^{-3}\) nm\({}^{3}\), the volumes of the  
unit cell of all CIGS:Te thin films were in the range of 384.56-387.92 \(\times\) 10\({}^{-3}\) nm\({}^{3}\), which is higher than that of the CIGS thin films, as shown in Figure 4b. The diffraction peak corresponding to the (112) orientation also shifted to higher 2\(\theta\) values, from 26.58\({}^{\circ}\), for the CIGS thin film, to 26.71\({}^{\circ}\), for the CIGS:Te thin films, when Te was co-sputtered, and then shifted to a lower 2\(\theta\) value of 26.63\({}^{\circ}\) under condition (6). According to Bragg's law, the shift in the Bragg angle was caused by a change in the spacing of the crystallographic planes, whereas strain increased the \(d\)-spacing, causing a shift in the Bragg angle of the diffraction peak towards lower 2\(\theta\) values in the XRD pattern [35]. Figure 4c shows the microstrain (\(\epsilon\)) due to distortions and crystal imperfections of the CIGS:Te thin films as calculated using the equation \(\epsilon=\omega\cdot\)cos\(\theta/4\), where \(\omega\) is the FWHM of the preferred diffraction peak in radians and \(\theta\) is the Bragg angle corresponding to the predominant diffraction peak obtained from the XRD data [36; 37; 38; 39]. The microstrain along the (112) orientation decreases from a minimum of 2.391 \(\times\) 10\({}^{-3}\) under condition (3) to a maximum of 5.210 \(\times\) 10\({}^{-3}\) under condition (6). The dislocation density (\(\delta\)) was calculated using the equation \(\Delta=1/D^{2}\), where \(D\) is the mean crystallite size of the CIGS:Te thin films. The dislocation density exhibited a similar trend in the microstrain along the (112) orientation. The lowest value of 4.3629 \(\times\) 10\({}^{15}\) line/m\({}^{2}\) was obtained under condition (3), when the dislocation density decreased from the highest value of 20.7105 \(\times\) 10\({}^{15}\) line/m\({}^{2}\) under condition (6). This indicates that the largest crystallite size was obtained under condition (3) because of the released microstrain and dislocation density, as shown in Figure 1c. With an increase in the Te sputtering time for the strained CIGS thin film, the increased Te\({}^{2+}\) ions occupied the Se vacancies, expanding the volume of the unit cell up to condition (3) as the strain was relieved rapidly by the compensated strain due to relaxation in the horizontal direction, whereas volumetric contraction occurred as the additional Te sputtering time increased up to the condition (f). As shown in Figure 2e, the normalized stoichiometric deviation of Se + Te decreased up to condition (6), that is, the number of Se + Te vacancies increased, causing the dislocation to increase again and resulting in a decrease in the volume of the unit cell, as shown in Figure 4c. Figure 3: (**a**) X-ray diffraction (XRD) patterns, (**b**) crystallite size along the (112) plane and full widths at half maximum (FWHM), and (**c**) average grain sizes 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.  
Figure 5 illustrates the optical properties of the CIGS:Te thin films at different co-sputtering times for the Te target over the spectral region of 200-2000 nm. Under all conditions, the optical transmittance of the CIGS:Te thin films converges to '0' below approximately 500 nm, exhibits absorption characteristics in which an absorption edge is formed in the wavelength range of 500-600 nm, and exhibits low transmittance characteristics even in the near-infrared (NIR) spectral region. Direct band gap semiconductors usually have a sharp transmittance onset, so all thin films have a direct band gap nature. As shown in Figure 5a, the mean transmittance of the CIGS:Te thin films was 27-28% under conditions (1)-(5) in the visible spectral region 380-780 nm; however, it increased to 30-31% under condition (6).