**2.3. Buffer layer**

opinion, a thin film is defined as a material that is created ab initio by the random nucleation and growth processes of individually condensing/reacting atomic/ionic/molecular species on a substrate. Thin films may encompass a considerable thickness range, varying from a few nanometers to tens of micrometers and thus are best defined in terms of the production processes rather than by thickness. The layers for constructing the CIGS solar cells are back

The back contact layer as the back contact electrode of the solar cell is usually deposited on the substrate such as glass or flexible foils. One of the key requirements for the back contact layer is that the contact between the back contact and the absorber layer should be ohmic contact. The back contact layer should have a good conductivity and can be adhered to substrate firmly in order to make the solar cell stable. Molybdenum (Mo) is used in the majority of the CIGS solar cells. The structural and morphological properties of the Mo thin films will greatly affect

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

contact layer, absorber layer, buffer layer and window layer.

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

**2.1. Back contact layer**

186 Nanostructured Solar Cells

**2.2. Absorber layer**

performance.

the sequent growth of the absorber layer.

The buffer layer is deposited on the absorber layer. CdS is the most widely used buffer layer in the CIGS solar cells. CdS is an n-type semiconductor with a band gap of ~2.4 eV. The buffer layer improves the CIGS solar cells' performance by forming the optimized band alignment between the absorber layer and the window layer. In addition, the buffer layer possesses more advantages to the CIGS solar cells such as the damage prevention of the absorber layer in the sequent sputtering process, the passivation of the absorber surface, the relief of the lattice misfit and so on. Besides CdS, more new buffer layers are in the development process. For example, the new buffer layers include Zn(S,O), ZnMgO, ZnS and In2S3. Since the research on the new buffer layers is a hotspot in the development of the CIGS solar cell in recent years, one can search a lot of published papers regarding the new generation Cd-free buffer layers and some of them showed great progress.
