Sputtering Laboratory – TCO and Reflector Development
Motivation and Objectives
On the sun-facing side of solar cells, in addition to high conductivity, high transparency is also required for electrical contacting. Transparent conductive oxides (TCOs) as front contacts and, in combination with metals, as back reflectors, decisively influence the efficiency of silicon thin-film solar cells. This is why IPV is developing and optimizing various TCO materials for use in solar cells. The films are produced by magnetic sputtering of ceramic or metallic targets.
A sputtering system for substrates up to 10×10 cm² has been operated for many years. It consists of a load chamber and a large deposition chamber with four heatable substrate retainers. Since it is equipped with four different 6" sputter sources, versatile film systems can be developed. Each substrate can be transferred to any of the coating stations by rotating the substrate retainer. The system's flexible design allows a multitude of variations and is thus ideal for our research work.
Within the scope of the scale-up to the large-area technology, an in-line sputtering system for substrates of up to 30×30 cm² was put into operation at the end of 2001. The system has a sputtering station for RF and DC processes, a station for purely DC excitation and a double cathode system for bipolar pulsed excitation. These cathode systems are equipped with 10×75 cm² targets. A new process chamber was installed in May 2007. It has a double tubular cathode system and a linear ion source.
These systems provide standard processes for solar cell development and simultaneously drive further developments with industry-oriented processes and innovative concepts.
- development of TCO films (ZnO, TiO2) with a focus on aluminium-doped zinc oxide films (ZnO:Al)
- production of surface-textured ZnO:Al films by means of wet chemical etching after deposition
- development of back reflectors
- TCO for other optoelectronic components
- development of relevant processes for TCO production in commercial solar panel manufacturing
Films and Film Systems
- ZnO:Al: el. resistivity: 2.0×10−4 Ω cm, highly transparent T > 80 %
- textured ZnO:Al: el. resistivity: 3 - 4×10−4 Ω cm, highly transparent, adjustable light dispersing ability
- other materials: ZnO, TiO2, SnO2, ITO, Ag, Al...
- reflectors: ZnO/Ag, surface texture due to textured zinc oxide, textured silver or combinations