Scrap metal solar cell

T. Mongstad, S. Zh. Karazhanov, D. M. O. Heggø

A quest for new materials that can be suitable for solar cells has been going on over the last 50 years. Some good candidates have been found and even successfully commercialized, but the rarity of elements that are essential for these technologies is eventually expected to be their Achilles heel. In order to make a serious contribution to the transition from fossil fuels to renewables, a solar cell technology has to be based on abundant elements. IFE is now investigating a new class of materials, which could result in a highly efficient solar cells made of scrap metal!

Limitations for thin film photovoltaics

Currently the most successful alternatives to crystalline silicon (Si) are copper indium gallium selenide (CIGS) and cadmium telluride (CdTe). The last 10-15 years has brought these so-called thin film solar cells from the research labs to the factories. You can now buy both CIGS and CdTe solar panels with efficiencies that are approaching that of crystalline silicon (Si) solar cells, at a lower cost than for Si cells. The efficiencies are still increasing, which is promising. However, there is a show-stopper. An important constituent in the CIGS cells is indium, which is a rare and expensive element. For CdTe cells, which at present is the cheapest solar technology measured in cost per watt, the tellurium (Te) gives a similar problem. The limited abundance of these elements puts a roof on the production, and these technologies may run into difficulties in as little as 10 years [1].

Expensive 3^rd generation cells

Crystalline and multicrystalline silicon solar cells are still dominating the market. The efficiency-to-cost ratio is continuously getting better, but we are bound to reach a limit. The theoretical limit for silicon solar cell efficiency is around 30%, and it is highly unlikely that it goes beyond 20% for reasonably priced silicon solar cells. In spite of this limitation, silicon cells will still be around for a long time. But at some point we will move into more complex technologies, and one solution is tandem solar cells. Tandem solar cells selectively absorb light in different materials to get the most out of every photon. This technology has actually proved to give more that 40% efficiency in laboratory cells, but unfortunately this has only been achieved with the use of extremely expensive materials.

Metal + hydrogen = metal hydride

Metal hydrides can be the solution. By adding hydrogen to different metals we can get materials that are semiconducting, which means that they can be capable of generating electricity in a solar cell device [2]. Semiconducting materials based on abundant metals in combination with hydrogen might be the solution to the problem with rare and expensive elements. And by choosing different metals and alloys, we also can easily make materials that absorb different parts of the sunlight.

Using metal hydrides we might be able to make a highly efficient tandem solar cell out of “scrap” metal. IFE has ongoing experimental work on making thin films of semiconducting metal hydride films for photovoltaics, and we have so far showed results with magnesium [3] and yttrium [4] hydrides.

Figure 1: The author with a transparent semiconducting yttrium hydride sample (left) and a metallic yttrium sample (right). The only difference between these samples is the content of hydrogen!

Further reading:

[1]       B.A. Andersson, “Materials Availability for Large-scale Thin-film Photovoltaics,” Progress in Photovoltaics: Research and Applications, vol. 8, 2000, pp. 61-76.

[2]       S.Z. Karazhanov et al., “Hydrides as materials for semiconductor electronics,” Philosophical Magazine, vol. 88, 2008, pp. 2461-2476.

[3]       C. Platzer-Björkman et al., “Reactive sputtering of magnesium hydride thin films for photovoltaic applications,” Materials Research Society Fall Meeting, Boston: 2009.

[4]       T. Mongstad et al., “Transparent yttrium hydride thin films prepared by reactive sputtering”, Journal of Alloys and Compounds, (in preparation)