**3. Technology of the CIGS solar cells**

The term "photo-voltaic" has been in use in English since 1849. A photovoltaic cell (also called a solar cell) is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect [20]. The operation of a solar cell requires three basic attributes. At first, the absorption of light generates either electron-hole pairs or excitons. Afterwards, various types of charge carriers are separated. Finally, those carriers are extracted to an external circuit. Conventionally, photovoltaic materials use inorganic semiconductors. Ideally, the absorber material of an efficient solar cell should be a semiconductor with a bandgap of 1–1.5 eV with a high solar optical absorption (104 to 105 cm−1) in the wavelength region of 350–1000 nm, a high quantum yield for the excited carriers, a long diffusion length and low recombination velocity [21].

The magnetron sputtering is extensively used to deposit the Mo back contact layer and the ZnO window layer. The CdS buffer layer is usually fabricated by a chemical bath deposition method. Several more methods such as atomic layer deposition and spray ion layer gas reaction technique have been adopted to obtain the new buffer layers. Several techniques were developed to fabricate Cu(In,Ga)Se2. Among them the coevaporation method and the twostage process are two of the most important techniques.

#### **3.1. Coevaporation process**

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

**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 Pub-

The term "photo-voltaic" has been in use in English since 1849. A photovoltaic cell (also called a solar cell) is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect [20]. The operation of a solar cell requires three basic attributes. At first, the absorption of light generates either electron-hole pairs or excitons. Afterwards, various types of charge carriers are separated. Finally, those carriers are extracted to an external circuit. Conventionally, photovoltaic materials use inorganic semiconductors. Ideally, the absorber material of an efficient solar cell should be a semiconductor with a bandgap of 1–1.5 eV with

high quantum yield for the excited carriers, a long diffusion length and low recombination

The magnetron sputtering is extensively used to deposit the Mo back contact layer and the ZnO window layer. The CdS buffer layer is usually fabricated by a chemical bath deposition method. Several more methods such as atomic layer deposition and spray ion layer gas reaction technique have been adopted to obtain the new buffer layers. Several techniques were developed to fabricate Cu(In,Ga)Se2. Among them the coevaporation method and the two-

cm−1) in the wavelength region of 350–1000 nm, a

to 105

high conductivity will be used as the front contact for a CIGS module.

lishing LLC.

188 Nanostructured Solar Cells

velocity [21].

**3. Technology of the CIGS solar cells**

a high solar optical absorption (104

stage process are two of the most important techniques.

The brief setup of the coevaporation process is shown in **Figure 5**. During the process, Cu, In, Ga and Se sources are heated and evaporated to be grown on a heated substrate. The heating temperature is specific for each source. The temperature for Cu, In and Ga is higher than that for Se. It is very important to control the element flux in the coevaporation process. The in-situ feedback controls based on electron impact emission spectrometry, quadruple mass analysis or atomic absorption spectrometry have been successfully adopted to control the element flux. Although the progress for the coevaporation process is in rapid development, the problems regarding the process control in the absorber deposition remain to be improved.

**Figure 5.** The setup of the coevaporation process.

#### **3.2. Two-stage process**

The two-stage process was invented by Boeing company. For the first stage, a stack of precursor layers including the Cu, In and Ga metals is deposited. In the following stage, the deposited precursor alloy layers are transferred to a furnace or a specific reaction setup for selenization. Either H2Se gas or Se powders are used for selenization process. **Figure 6** shows the brief setup of the selenization furnace.

**Figure 6.** The setup of the selenization furnace.
