Fig. 1. Cross-sectional transmission electron microscope (TEM) photographs of the device structure of the Cu(In1-xGax)Se2 (CIGS) thin-film solar cells. It consists of Pt, ITO, ZnO and CdS as transparent front contact layers, CIGS as absorbing layer and Mo as a back-contact layer. The inset indicates that a lamellar-typed MoSe2 phase appears at the CIGS/Mo interfacial region.

Fig. 2 shows the theoretical X-ray penetration depth of the Cu(In1-xGax)Se2 absorbing layer with x = 0.0, 0.2, 0.5, 0.8 and 1.0 as a function of the incident angle. In GIXRD, the theoretical X-ray penetration depth of the incident X-ray beam, τ(α), is given by the following equation [[30], [31], [32], [33]]: (1)$\tau (\alpha )=\frac{\lambda }{4\pi \times {{2}^{-1/2}}}{{\left( \sqrt{{{({{\alpha }^{2}}-\alpha _{c}^{2})}^{2}}+4{{\beta }^{2}}}+{{\alpha }^{2}}-\alpha _{c}^{2} \right)}^{{}^{1}/{}_{2}}}$

Fig. 2. Theoretical X-ray penetration depth of the Cu(In1-xGax)Se2 (x = 0.0, 0.2, 0.5, 0.8 and 1.0) absorbing layer as a function of the incident angle.

Fig. 3. (a) Glancing incidence X-ray diffraction (GIXRD) patterns of the Cu(In1-xGax)Se2 (CIGS) absorbing layer at incident angles between 0.50° and 5.00°, (b) the shift in the position of 112 and (c) the shift in the position of 204 peak of the CIGS absorbing layer as a function of the incident angle.

Fig. 4. Average chemical composition of constituent atoms (Cu, Ga, Se and In) of the Cu(In1-xGax)Se2 (CIGS) absorbing layer.

Fig. 5. Lattice parameters (a and c) and unit-cell volume of the Cu(In1-xGax)Se2 (CIGS) absorbing layer as functions of the incident angle.

Fig. 6. (a) Cross-sectional transmission electron microscope (TEM) photograph and (b) compositional profile of constituent atoms (Cu, Ga, Se and In) of the Cu(In1-xGax)Se2 (CIGS) absorbing layer as a function of depth from the surface to the bottom of the absorbing layer.

Fig. 7. Unit-cell volume, theoretical penetration, In/(Ga + In) ratio and bandgap of the Cu(In1-xGax)Se2 (CIGS) absorbing layer as functions of the incident angle.