The base material to manufacture photovoltaic cells are monocrystalline or multicrystalline silicon blocks. These blocks (ingots) are sawed to silicon discs or wafers of defined size. Then surface damages and impurities are removed by wet chemical etching. Special etching and texturing agents are used to optimize the wafer surface, reduce crystal damages and increase the wafer life span. Additionally, the etching and texturing process has an advantage regarding energy input. The roughened, optimized structure is able to absorb more light and increase the efficiency of the photovoltaic cell.
Various texturing chemicals can be used depending on the wafer type and desired outcome of the process. Typical alkaline etching agents are caustic soda and caustic potash solutions. Acidic etching of wafers and silicon is done with sulphuric acid, nitric acid and hydrofluoric acid. The completion steps are neutralization and the removal of residues.
Alkaline or acidic etching agents remove the surface damage induced by the sawing process and texture the surface of the wafer. Acidic etching agents, like HF and HNO3 are used for isotropic etching. Typical alkaline etching agents are caustic soda (NaOH), caustic potash (KOH) and tetramethylammonium hydroxide (TMAH) solutions. Wafer surface texturing is important to improve the light trapping into the cell and the energy absorption capacity. The major factors of influence in this process are the current bath concentration and residence time.
Because of the consumption and discharge of etching chemicals during the texturing process, it is necessary to measure the concentration and temperature continuously. This insures consistant product quality and, if needed, more etching agents can be added. LiquiSonic® inline analyzers monitor the current texturing and etching bath concentration. This enables an automatic additional dosing through the incorporation in to existing process control systems.
The LiquiSonic® immersion sensor is easily installed into the bath circulation pipeline or directly in the texturing or etching bath. By using the LiquiSonic® controller 30, up to four sensors can be connected, allowing the simultaneous monitoring of several measuring points.
Typical measuring range:
concentration range: 0 to 55 wt%
temperature range: 80 to 120 °C (170°F to 250°F)
The robust sensor construction and the optional special materials, like Halar oder PFA, promote long process life. LiquiSonic® enables a reduction of labor cost through the elimination of manual process steps:
- time saving: 1 h per day
The avoidance of insufficient dosages saves material costs and secures constant process quality.