Recycling of selenium from CIGS solar cell materials
Today the importance of solar cells as a power source is growing due to the environmental advantages compared to fossil fuels. One of the most promising new solar cell materials, resulting in high efficiency solar cells, is copper indium gallium selenide (CIGS). CIGS is used in thin film solar cells, which require less semiconductor material than conventional silicon solar cells and as a result are much more cost effective. Lately the issue of recycling material from spent solar cells and taking care of production waste has received increasing attention, especially since solar cell technologies utilize expensive, rare, or toxic elements. The production of solar cells requires high purity materials, putting high demands on the recycling processes if the recycled material is to be used directly in new solar cells. One way to obtain high purity materials is to separate the element of interest in the gas phase while other elements remain in the liquid or solid state. To facilitate the separation more volatile compounds of the element can be formed through chemical reactions. As the first step in the development of a recycling process for CIGS materials this study aimed to use such a method for separation of selenium from CIGS.
The feasibility to recycle selenium from CIGS through separation of selenium as either hydrogen selenide or selenium oxide was examined. Several different methods for separation of selenium as hydrogen selenide (H2Se) were tested, but none of them was successful. However, separation of selenium via oxidation of CIGS at elevated temperatures proved to be feasible. The oxidation resulted in the formation of selenium dioxide (SeO2), which after cooling of the gas could be collected as crystals. After oxidation at 1000°C only a few percent by weight of the selenium remained in the CIGS material. To recover the selenium the selenium dioxide was reduced. Two different reduction methods were tested. First the selenium dioxide was reduced by an organic compound, in a so called Riley reaction. Next sulphur dioxide gas was used as a reducing agent. Both methods resulted in high yield and high purity of the recycled selenium. Thus, the aim of the study was achieved and the method will be further developed in the future. The next step in the development of a recycling process for CIGS materials will be the separation of the remaining elements.