**2.2. Absorber layer**

The absorber layer is deposited on the back contact layer. CuInSe2 (CIS) was the first absorber layer developed in the 1970s. In the next two decades gallium was introduced into the absorber layer, resulting in the deposition of the Cu(In,Ga)Se2 thin films. The invention of the new generation absorber layer boosted the solar cells' efficiency by increasing the absorber's band gap. In addition, the possibility of fabricating graded band gap Cu(In,Ga)Se2 absorber layer further increased the device efficiency. Several other improvements were developed such as the thinner CdS buffer layer (less than 50 nm) and the use of the soda lime glass during this period. These technologies resulted in the progress of the Cu(In,Ga)Se2 absorber layer's performance.

**Figure 3.** The Cu(In,Ga)Se2 crystal structure. Red = Cu, yellow = Se, blue = In/Ga [13].

The Cu(In,Ga)Se2 material possesses a chalcopyrite crystal structure which is shown in **Figure 3**. Each anion (Se) is coordinated by two cations of each type (Cu, In/Ga). The Cu(In,Ga)Se2 is obtained by partially replacing indium in a CIS structure with Ga. Since the atomic radii of the Ga are smaller than In, the lattice constants will be decreased with increasing Ga content.

The band gap of the Cu(In,Ga)Se2 material depends on the ratio of Cu/(In + Ga). The band gaps of CuInSe2 and CuGaSe2 are 1.02 and 1.67 eV, respectively. The band gap of the Cu(In,Ga)Se2 varies between 1.02 and 1.67 eV by the change in Ga/(In + Ga) ratio.
