CdTe and CuInGaSe2 Thin-Film Solar Cells

Thin film solar cells are based on materials, which show an extraordinary high absorption coefficient so that there is no need to build thick solar cells to absorb all the light. Using this kind of materials allows one to fabricate devices with an overall thickness of less than 10 micrometers and a clear advantage in terms of material supply and fabrication energy. Thin-film solar cells offer a wide variety of choices in terms of device design, fabrication methods and substrates (flexible or rigid, metal or insulator). The deposition of different layers (contact, buffer, absorber, reflector, etc.) can be done using several techniques, which will be described later. Indeed, such versatility allows for tailoring and engineering of the layers, in order to match the solar spectrum and to improve device performance. Typically, the thin films used in these devices are polycrystalline materials, where the layer is a pattern of small crystals, whose width can range between 0.1 and 5 m. As shown in Figure 8.1, the layer is a patch of differently sized grains with different orientations. This configuration looks very disordered and irregular considering that it has to allow carriers to move through the material; however the films, when properly prepared, have the required conductivity, and devices can reach very high efficiencies up to 25%. On the other hand, the advantage of such a disordered structure is that it does not need a very precise control of crystal growth; neither does it need high energy for crystallization: this is an advantage compared to other technologies such as crystalline silicon.