**1. Introduction**

Nowadays the development of the society and economy raises up the dependence upon the energy sources. However, the energy crisis and the environmental problems induced by using the fossil energy sources have attached much attention. A consensus about developing new energy as well as the reduction of the fossil energy consumption has been reached all over the world. The solar energy as one of the new energy sources and a regenerated energy is abundant and pollution-free. Photovoltaics (PV) is a method of generating electrical power through transforming solar energy into direct current electricity. The transformation is achieved by using semiconductors that exhibit the photovoltaic effect. The photovoltaic effect refers to photons of light exciting electrons into a higher state of energy, allowing them to act as charge carriers for an electric current. Most photovoltaic devices (solar cells) sold in the market today are based on

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

silicon wafers. There is an argument that the silicon solar cells are the so-called "first generation" technology. The market at present is on the verge of switching to a "second generation" of thin film solar cell technology which offers prospects for a large reduction in material costs by eliminating the costs of the silicon wafers. However, whether the thin film solar cell can be attributed to a "second generation" technology is still a controversial issue. Despite the debate, the silicon photovoltaic industry has reached its industrial maturity. The approach to progress further is to increase the efficiency as well as decrease the cost of the solar cells. The thin film photovoltaic technology is one of the potential alternative approaches.


a ap = aperture area; da = designated illumination area (defined in [1]); t = total area.

b a-Si/nc-Si/nc-Si triple junction solar cell.

c a-Si/nc-Si tandem solar cell.

**Table 1.** The efficiency table of the thin-film solar cells.

**Figure 1.** The cell structure of the Cu(In,Ga)Se2 solar cell.

Currently three types of the thin film solar cells have realized industrialization. They are cadmium telluride (CdTe) solar cells, amorphous silicon (a-Si) solar cells and copper indium gallium diselenide (CIGS) solar cells. The other thin film technologies such as perovskite solar cells, dye-sensitized solar cells (DSSCs), organic solar cells and quantum-dot solar cells (QDSCs) remain in the stage of lab research or pilot line. The a-Si solar cells in the PV industry are fading away because of its relative low efficiency and instability. The CdTe and CIGS solar cells show the rapid development trend in recent years. **Table 1** presents the efficiencies of different thin film solar cells. Among them, the CIGS solar cells show the highest efficiency both in cells and modules.

silicon wafers. There is an argument that the silicon solar cells are the so-called "first generation" technology. The market at present is on the verge of switching to a "second generation" of thin film solar cell technology which offers prospects for a large reduction in material costs by eliminating the costs of the silicon wafers. However, whether the thin film solar cell can be attributed to a "second generation" technology is still a controversial issue. Despite the debate, the silicon photovoltaic industry has reached its industrial maturity. The approach to progress further is to increase the efficiency as well as decrease the cost of the solar cells. The thin film

**)**

**<sup>a</sup> Efficiency (%) References**

photovoltaic technology is one of the potential alternative approaches.

ap = aperture area; da = designated illumination area (defined in [1]); t = total area.

Cu(In,Ga)Se2 (cell) 0.5 (unknown) 22.3 [2] Cu(In,Ga)Se2 (module) 808 (da) 17.5 [3] CdTe (cell) Unknown 22.1 [4] CdTe (module) Unknown 18.6 [5] a-Si (cell)b 1.043 (da) 13.6 [6] a-Si (module)c 14322 (t) 12.3 [7] Dye sensitized (cell) 1.005 (da) 11.9 [8] Dye sensitized (minimodule) 26.55 (da) 10.7 [8] Organic (cell) 0.0429 (ap) 11.5 [9] Organic (module) 802 (da) 8.7 [10] Perovskite (cell) Unknown 22.0 [11]

**Classification Area (cm2**

a

b

c

a-Si/nc-Si/nc-Si triple junction solar cell.

**Table 1.** The efficiency table of the thin-film solar cells.

**Figure 1.** The cell structure of the Cu(In,Ga)Se2 solar cell.

a-Si/nc-Si tandem solar cell.

184 Nanostructured Solar Cells

**Figure 2.** The image of the layers in the CIGS solar cell by scanning electron microscopy.

The structure of the CIGS solar cells is shown in **Figure 1**. The CIGS solar cells consist of a number of films which are deposited onto a rigid or flexible substrate. The first film, typically molybdenum (Mo), serves as a nontransparent back-contact. It is covered by the actual Cu(In,Ga)Se2 film. Most of the light is absorbed by the p-type thin film (absorber) and the photocurrent is generated. The heterojunction is formed by depositing a very thin n-type buffer layer (typically CdS) and an n-type wide gap transparent window layer (usually heavily doped ZnO). **Figure 2** presents the actual image of the layers in the CIGS solar cell, which is measured by a scanning electron microscopy (SEM). From the bottom to the top, they are Mo back contact layer, Cu(In,Ga)Se2 absorber layer, a very thin CdS buffer layer, the ZnO window layer and ZnO nanostructure antireflective coating layer.
