**2.4. Window layer**

As the window layer, ZnO is deposited on the buffer layer in the CIGS solar cells. ZnO is not really a newly discovered material. Research on ZnO has continued for many decades with interest. In terms of its characterization, reports go back to 1935 or even earlier. For example, lattice parameters of ZnO were investigated for many decades [14–18].

Most of the group II–VI binary compound semiconductors crystallize in either cubic zinc blende or hexagonal wurtzite structure where each anion is surrounded by four cations at the corners of a tetrahedron, and vice versa. **Figure 4** depicts three kinds of crystal structures shared by ZnO which are rocksalt, zinc blende and wurtzite [19]. The zinc-blende ZnO structure can be stabilized only by growth on cubic substrates, and the rocksalt structure may be obtained at relatively high pressures [18].

The wurtzite structure has a hexagonal unit cell with two lattice parameters, a and c, in the ratio of <sup>c</sup> a = <sup>8</sup> 3 = 1.633and belongs to the spacegroup of C6v 4 or P63mc. In a real ZnO crystal, the lattice parameters deviate from the ideal value due to the existence of the Zn interstitials, O vacancies and dislocations in the crystal structure. The properties vary with different material types (bulk material, thin films, powders and nanostructures) and different synthesis processes.

The window layers in the CIGS solar cells usually are consisted of two layers. The first layer deposited on the buffer layer is the intrinsic ZnO thin films. In the following an aluminumdoped ZnO (AZO) layer is deposited on the undoped ZnO layer. The AZO layer processing high conductivity will be used as the front contact for a CIGS module.

**Figure 4.** Stick and ball representation of ZnO crystal structures: (a) cubic rocksalt, (b) cubic zinc blende, (c) hexagonal wurtzite. The shaded gray and black spheres denote Zn and O atoms, respectively [19]. Copyright 2005 by AIP Publishing LLC.
