CdTe solar cells
With a theoretical efficiency of 29.7 % and the possibility to deposit cadmium telluride (CdTe) with a number of cheap and simple deposition techniques, polycrystalline CdTe thin film solar cells are a promising alternative to established single- and polycrystalline silicon solar cell technologies. Several companies have started to successfully produce CdTe solar modules. The high theoretical efficiency is not reached yet. The best laboratory cell had an efficiency of 16.7 %, commercial modules (≈1 m2) show approximately 10 %.
The aim of our project is to develop high performance CdTe thin film solar cells with controlled morphology, low growth temperature and reduced absorber thickness. By optimising the nucleation of the CdS and CdTe films on the substrate, the thickness and orientation of the growing films are to controlled in a way that the absorber thickness can be kept below 2 µm without suffering losses in efficiency. The gained knowledge can be transferred short-term into industrial base technologies via industry-related research. The reduced film thickness and deposition temperature offer the advantage of economising material and energy costs, increasing station times and thus reducing the production costs of CdTe solar modules.
CdTe solar cells are usually built up in superstrate configuration: the single films are applied one by one onto a glass substrate which also provides mechanical stability.
The setup starts with the front contact, consisting of a transparent conducting oxide (TCO) with a thickness of 100 – 500 nm. Mostly, tin-doped indium oxide (ITO) or fluorine-doped tin oxide (FTO) are applied, sometimes followed by a non-doped tin oxide buffer layer. These materials have wide band gaps (example: SnO2: EG = 3.7 eV), making them transparent for visible light. At the same time, the high doping level gives them metal-like conductivity.
In the next step, n-conducting CdS is applied (thickness 80 – 200 nm). For this, we use the so-called Close Space Sublimation (CS) technique. In the CSS process, CdS is evapourated from a crucible onto the hot substrate surface under vacuum conditions. The substrate is heated up to 500 – 600 °C, where re-evapouration of the species to be deposited is expected to hinder the successful deposition. To avoid this, the substrate is brought closer to the crucible and almost closes it like a lid, allowing for comparably high deposition rates making the technique attractive for the application in commercial products. It is important to deposit thin and yet dense CdS layers due to the absorption of a part of the visual light by CdS. The charge carriers generated by this part do not contribute to the photo current.
The CdTe layer is also deposited by CSS, allowing for the deposition of up to 8 µm in less than 2 min.
The back contact consists of a primary and a secondary contact. The primary contact consists of of a tellurium film created by selective etching of the CdTe absorber using a mixture of nitric and phosphoric acid (NP etching). The secondary contact consists of a metallic conductor; on the laboratory scale, gold is usually applied. On a commercial base, other metals like molybdenum and nickel/vanadium are applied.
Before the deposition of the back contact, the solar cells are exposed to a high temperature treatment in a chlorine containing atmosphere. This so-called activation step leads to a significant improvement of all characteristics of the solar cell (VOC, JSC, FF, efficiency).
The solar cells processed in superstrate configuration are illuminated through the glass side.