Crystec Technology Trading GmbH

Chalcopyrite or chalcopyrite ore (chalcopyrite) are commonly occurring minerals and I–III–VI2 compounds that contain copper, additional metals such as indium and gallium, as well as sulfur and selenium. Many chalcopyrites absorb light very well, exhibit semiconducting properties, and are therefore well suited for manufacturing thin-film solar cells. By varying the chemical composition of the individual elements, the semiconductor properties such as lattice constant and band gap can be adjusted. These are excellent prerequisites for optimizing solar cells.

A chalcopyrite or CIGS thin-film solar cell consists of multiple layers. A glass carrier is commonly used as the substrate. A non-transparent molybdenum layer is applied as the back contact. To form the chalcopyrite layer, copper, indium and gallium are first deposited on the substrate. Subsequently, this layer is selenized or sulfurized in a furnace or an RTP system at temperatures around 550 °C. Hydrogen selenide H₂Se or hydrogen sulfide H₂S serve as selenium or sulfur sources.

On top of this CIGS layer, which is also referred to as the absorber, a thin cadmium sulfide CdS layer is deposited, followed by a transparent conductive oxide (TCO) that serves as the front contact, for example zinc oxide ZnO. At the interface between the CdS and CIGS layers, charge carrier generation occurs when exposed to sunlight.

Constructing the CIGS solar cell in reverse order, i.e. with the glass substrate on the front side, is also fundamentally possible. This configuration is called the superstrate configuration, because the carrier sits above the cell rather than below it as in the previously described substrate configuration.

Selenization and sulfurization can be performed in various furnaces or RTP systems. The image shows a large vertical furnace model VFS-4000 from JTEKT Thermo Systems (formerly Koyo Thermo Systems). This furnace was originally developed for the deposition of poly-silicon layers on glass plates for TFT-LCD production. It can process 25 vertically standing glass plates in solar panel size.

Like all vertical furnaces from the semiconductor field, this furnace provides excellent temperature uniformity and can safely handle toxic gases such as H₂Se and H₂S. The tube and boat of the furnace are made of quartz glass, ensuring a clean process environment, but they are also relatively expensive. Multiple furnaces can be combined into a process cluster and loaded via cassette stations by a robot. This allows for quasi-continuous operation.

Production can also be carried out in a fully continuous inline process. After sputtering the molybdenum contact, the metallic precursors Cu, Ga, In are deposited. A preheating step follows, and finally the selenization and sulfurization using hydrogen selenide and hydrogen sulfide or via a thin layer of selenium or sulfur.

In the example, a continuous furnace from JTEKT (Koyo) is described for the thermal process steps. Individual process chambers are separated by airlocks. The thermal process chambers can be evacuated and purged. The further transport of the glass plates from process to process is performed stepwise via a roller transport system.

Recent developments enable the introduction of selenium by diffusion from an applied or embedded solid selenium layer or phase. This opens the way for the use of simpler roll-to-roll furnaces or other continuous furnaces similar to those used for producing CdTe cells. Highly toxic gases no longer need to be used, which can significantly reduce production costs.

Alternatively, an RTP system can be used. In this case, lamp heating replaces continuous heating with Moldatherm® heating elements. RTP systems are also suitable for single-process handling of smaller glass plates at laboratory scale.