## 3 Results and Discussion
Acceptable electrical properties, including \(V_{oc}\) and the short-circuit current density (\(J_{sc}\)), can be obtained by reducing both carrier recombination and the leakage current originating from the structural defects in the thin films by using densely packed (112)-, (220/224)-, and (312)-oriented grains [10; 27]. The full width at half maximum (FWHM) of the (112) diffraction peaks, as a function of Te co-sputtering time, is shown in Figure 3b. The FWHM of the CIGS thin film decreased from 1.1082\({}^{\circ}\) to 0.5635\({}^{\circ}\) under condition (3) of the CIGS:Te thin film after Te co-sputtering because of the closest stoichiometric condition, indicating fewer defects overall, including both Se vacancy and In + Ga interstitial. The crystallite sizes of the CIGS/CIGS:Te thin films were calculated from the (112) diffraction peak using the Debye-Scherrer formula, \(D=0.94\lambda/\omega\cdot\)cos\(\theta\), where \(\lambda\) is the K\(\alpha\) radiation wavelength of Cu (\(\lambda\) = 0.15406 nm); \(\omega\) is the FWHM of the (112) diffraction peak in radians; and \(\theta\) is the Bragg angle corresponding to the (112) diffraction peak. The crystallite size of CIGS thin film without Te was 7.12 nm, which increased to 15.88 nm under condition (3), as shown in Figure 3b, whereas it reached a minimum value of 6.79 nm under condition (6). The average grain size is shown in Figure 3c, which was estimated using the FESEM images in Figure 1 and the ImageJ software [30]. Both, the crystallite size from XRD and average grain size from FESEM, showed a similar tendency with respect to \(\Delta\)s from EDX. Figure 4a shows the interplanar distances corresponding to the \(d\)-spacings for tetragonal CIGS:Te orientation along the (112) plane, \(d_{112}\), at different co-sputtering times for the Te target. \(d_{112}\) was calculated using the equation \(d_{112}=\lambda/2\)sin\(\theta\), where \(\lambda\) is the K\(\alpha\) radiation wavelength of Cu (\(\lambda\) = 0.15406 nm) and \(\theta\) is the Bragg angle corresponding to the diffraction peak obtained from the XRD data [31]. The \(d_{112}\) value of the CIGS:Te thin films was in the range of 0.333-0.334 nm, which agrees with a previous study [32]. The tetragonal distortion parameter \(\eta=c/a\) was in the range of 2.005-2.026. The diffraction data analysis with lattice constants \(a\) and \(c\) in a tetragonal chalcopyrite structure was performed using the equation \(1/d^{2}=(h^{2}+k^{2})/a^{2}+l^{2}/c^{2}\), combined with Bragg's law, \(d=\lambda/2\)sin\(\theta\), where \(d\) is the spacing between the planes in the atomic lattice (interplanar spacing); \(hkl\) are Miller indices; \(\lambda\) is the wavelength of CuK\(\alpha\) radiation; and \(\theta\) is the angle between the incident ray and the scattering planes [33]. For a particular incident X-ray wavelength \(\lambda\) and angle \(\theta\) from XRD, the \(d\) spacing can be determined from Bragg's law. Lattice constants \(a\) and \(c\) can also be calculated using the (112), (220/204), and (312) peaks shown in Figure 4b. The lattice constants of the tetragonal CIGS:Te thin films are generally \(a\) = 0.56-0.58 nm and \(c\) = 1.10-1.15 nm [34]. For the CIGS thin films without Te, \(a\) and \(c\) were 0.575 and 1.164 nm, respectively. Lattice constant \(a\) gradually increased and then decreased, attaining a maximum value of 0.578 nm under condition (3) as the Te co-sputtering time increased. Lattice constant \(c\) instantly decreased as Te co-sputtering was performed, and it gradually decreased as a function of Te co-sputtering time in the CIGS:Te thin films, reaching a minimum value of 1.156 nm under condition (6). The volume of the unit cell was estimated from the expression for the tetragonal system: \(V=abc\).