"Baumkuchen material" makes solar cells more effective

Ultra thin oxide layers increase the current efficiency and make thin cells thinner

Conventional solar cells © DOE / NREL
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A new class of materials could help to build more efficient solar cells: the ultra-thin, layered stacks of so-called oxygen heterostructures ensure that the electrons displaced by sunlight from their places actually flow as electricity rather than back to their old place fall. Although the production of such solar cells from oxide layers is more complex than that of conventional silicon-based, but they provide more power everywhere, where particularly thin, effective panels are in demand, the researchers report in the journal "Physical Review Letters".

The basic principle of the solar cell is the photoelectric effect, the simplest variant of which was explained by Albert Einstein as early as 1905: When a light particle is absorbed by a material, it can cause electrons to leave their whereabouts and electrical current begins to flow. When an electron is removed from its place, a positively charged site remains, a so-called "hole". Both the negatively charged electrons and the positively charged holes can contribute to the flow of current. "However, if in a solar cell electron and hole are not transported away as electricity, but reunite, then everything is as before - the energy can not be used, " explains first author Elias Assmann from the Vienna University of Technology.

Two insulators become a metal

And this is precisely where the new material investigated by the researchers comes in: "The decisive advantage of the new material is that microscopic orders of magnitude create a strong electric field that drives electrons and holes in opposite directions away from each other." This increases the efficiency of the solar cell, In principle, the so-called oxygen heterostructure consists of different thin layers of different oxygen compounds. These oxides are actually non-conductive insulators and would therefore normally be unsuitable for solar cells.

Electrons and holes form in alternating ultrathin layers as a result of light irradiation; conductive cables are attached at the top and bottom, which are used to close a circuit. © TU Vienna

However, when layers of two suitable insulators are packed together, the material at the interfaces at the top and bottom surprisingly develops metallic properties and conducts electrical current. This is of great importance to us: it allows you to very easily derive the top and bottom of the electrical charge carriers and let them flow, "says co-author Karsten Held. In conventional solar cells made of silicon, one has to attach conductive metal wires in order to dissipate the current, but this blocks the way into the interior of the solar cell for part of the sunlight.

Custom-made for sunlight

Another advantage of the new material: In conventional solar cells, not all photons are converted equally efficiently into electricity. For different light colors, different materials are particularly well suited. In the case of oxide heterostructures, suitable properties can be achieved by selecting suitable chemical elements, explains Peter Blaha from the Institute of Material Chemistry at the Vienna University of Technology. In the simulation calculations, the team analyzed oxide layers with lanthanum and vanadium, because the resulting materials are particularly well suited to the radiation of the sun. It is even possible to combine different types of coatings so that different light colors can be optimally transformed into electricity in different material layers, "says Assmann. display

The new solar cells are now to be built and tested at the University of Würzburg. The production of solar cells from oxide layers is more complex than with conventional silicon solar cells. But at least where particularly high energy efficiency or minimum thickness is required, the new structures should be able to replace the previous silicon cells., Held is confident. (Physical Review Letters, 2013; doi: 10.1103 / PhysRevLett.110.078701)

(TU Vienna, 13.02.2013 - NPO)