Due to technological limitations and the cost of producing cells based on crystalline silicon, alternative materials and methods have been sought to produce photovoltaic cells. About \( 50\% \) of the production cost of crystalline silicon solar cells comes from the preparation of the silicon, while the processing of the cells and panels is respectively the remaining \( 20\% \) and \( 30\% \). The search was therefore carried out to find materials other than mono- and polycrystalline silicon for cell construction. In the 1970s, the first thin-film cells based on amorphous silicon were successfully produced, followed by other materials.
The most common types of thin film cells ( Fig. 1 [1]) are cells made of amorphous silicon, CIGS cells and CIS cells (based on copper, indium, gallium, selenium). Their efficiency ranges from \( 14-23\% \). Cadmium sulphide and cadmium telluride (CdS and CdTe) are also popular materials. Silicon cells dominate the PV market ( \( 92\% \)), followed by cells based on CdTe, \( 5\% \) CIS/CIGS/CIGSS- \( 2\% \) and amorphous silicon ( \( 1\% \)) [2].

Figure 1: Thin film cell. Aut. photo by Fieldsken Ken Fields, licensed under CC BY-SA 3.0, source: Wikimedia Commons.

Thin film cells are cheaper to produce due to lower material consumption, automated production and less labour-intensive processes. The problem in the future may be a shortage of key materials - the prices of indium and tellurium are constantly rising. Another advantage of cells, and thus thin-film panels, is their weight - they are lighter than silicon cells [3]. They achieve slightly lower efficiencies than crystalline silicon cells and also have a lower lifetime. However, they have the undoubted advantage of being able to be applied to flexible surfaces (foils).

The first thin film cells built were amorphous silicon cells. An important element in the development of this technology was the properties of amorphous silicon - it has a higher light absorption coefficient than crystalline silicon. A cross-section through the cell structure is shown on Fig. 2. A layer of intrinsic conductor with a thickness of about 0.5-1 \( \mu m \) is sandwiched between additional layers of p-type (layer thickness is about 8 nm) and n-type materials (layer thickness is about 20 nm) [4].

CIS/CIGS/CIGSS cells are cells in which the structural material is indium copper diselenide \( CuInSe_{2} \) (CIS). This material has a high absorption coefficient (its energy gap is about 1 eV). By adding gallium, a semiconductor compound is formed having the chemical formula \( CuIn_{(1-x)}Ga_{x}Se_{2} \).

By controlling the gallium content (x in the formula), the energy gap can be modified in the range 1.02 eV-1.64 eV. The energy gap can be increased by adding sulphur. The compound CIGSS is then formed. CIS/CIGS/CIGSS cells are produced by depositing the active layer on sodium glass covered with a molybdenum layer (base contact). The second contact is often a layer of zinc oxide ZnO doped with aluminium [5]. CIGS cells have a characteristic black color.

Another material for photovoltaic cells is cadmium telluride (CdTe). The energy gap of CdTe is 1.45 eV [6], which results in high absorption of the solar spectrum. The material is temperature stable compared to other semiconductor materials. The elements Cd and Te and their oxides are highly toxic, which was a concern before the application of this technology. CdTe has been shown to be minimally harmful to living organisms. The cells were therefore approved for production. CdTe-based cells produce the smallest carbon footprint, the lowest water consumption and the shortest energy payback time. Due to the good combination of thermal properties, CdTe is combined with cadmium selenide (CdS). The structure of a CdTe-based cell is shown in Fig. 3 (based on [7]).