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Contacts for solar cells

Solar cells require contact materials to extract the electrical energy from active absorber layers. Many photovoltaic (PV) technologies utilize thin film contacts, which are coated locally or on large area.
Metals like copper or silver are highly conductive and are often used as contact material. However, the light needs to enter the absorber to be converted to electricity. The contacts on the illuminated side of the device must therefore consist of transparent and conductive materials (TCM). One option are very thin metal layers of only a few nanometers . These are transparent due to their very low thickness. Alternatively, metals are deposited as grids of narrow finger lines, such that light can still penetrate into the absorber. An extreme case of those narrow metal contacts is represented by a network of silver nanowires. The nanowire contacts are coated from liquids and are already well established in the production of touch screen displays.
Some metal oxides belong to another class of materials with the required properties: the so-called Transparent Conductive Oxides (TCO). They are transparent and conductive as a bulk layer. The most common TCO is tin doped indium oxide (ITO). ITO is widely applied in displays and a range of solar cell technologies. Due to the rareness of indium its price is subject to fluctuations and feasibility of terascale deployment is questionable. Fortunately, the oxides of more abundant metals like tin and zinc can also serve as TCOs and are considered as more appropriate candidates for mass production of PV. Typically they are extrinsically doped by halides or group III elements, e.g. resulting in transparent conductive fluorine doped tin oxide (SnO2:F) or aluminum doped zinc oxide (ZnO:Al).
Further properties of the TCOs which are important for application in photovoltaic devices: the interfaces to adjacent semiconductor materials must not provide barriers for the required electronic transport; TCMs should be stable against various environments; and some technologies require light management, which might be governed by the TCMs to a certain extent.
Such contact materials and associated fabrication processes are developed with partners from research and industry. Additionally, characterization of structural, optical and electronic properties of materials and interfaces are important research topics.
In the past transparent front contacts and back reflectors have been developed for silicon thin-film solar cells. Current research topics include selective contacts for organic, hybride and silicon-heterojunction solar cells.

Contact

Dr. Jürgen Hüpkes
Phone: +49 2461 61-2594
email: j.huepkes@fz-juelich.de

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