Researchers from the Vienna University of Technology, together with colleagues from the U.S. and Germany, have used computer simulations to show how the unique electrical properties of a new class of materials known as layered oxide heterostructures can potentially be used to create a new type of efficient, ultra-thin solar cell.
The new materials are created by combining single atomic-scale layers of different oxides. When combined, or stacked, these heterostructures display significantly different electrical properties than the single oxides do on their own. After studying the structures in large-scale computer simulations, the research team believe that by designing materials with exactly the right arrangement of physical properties, the layered oxide heterostructures hold great potential for solar cell design.
In general, solar cells are built on a long-established idea known as the photoelectric effect. When a single photon is absorbed, it can allow an electron to leave its place and an electric current begins to flow. The place the electron leaves becomes a positively charged region, or “hole”. Both the negatively charged electrons as well as the holes contribute to the production of an electrical current.
“If these electrons and holes in the solar cell recombine instead of being transported away, nothing happens and the energy cannot be used,” explains Elias Assmann, a member of the Institute for Solid State Physics research team from TU Vienna. “The crucial advantage of the new material is that on a microscopic scale, there is an electric field inside the material, which separates electrons and holes.”
Conventional solar cells made of silicon have traditionally faced a design problem that requires the use of metal wires on their surface to collect charge carriers. These wires, while essential to a solar cell’s operation, also block part of the light from entering the cell, thereby reducing efficiency. In contrast, the oxides used to create the new material are isolators. If the right types of isolators are stacked, the surfaces of the material can become metallic and conduct electrical current, enabling an electrical circuit to be created.
Additionally, the range of photons gathered by solar cells are converted into electrical current with varying rates of efficiency. For different parts of the light spectrum, different materials work better than others – which is why multi-junction solar cells can achieve such high conversion rates. The TU Vienna researchers say that by choosing the right chemicals, the oxide heterostructures can be tuned to operate especially well within the natural light range of the sun, with different parts of the spectrum being simultaneously absorbed through different layers. Some of the promising oxides studied contained the elements Lanthanum and Vanadium.
The results of the study will now be used to design and build new solar cells for testing. While no efficiency figures or production cost estimates have yet been suggested, the scientists hope that cell designs based on layered oxide heterostructures could provide a boost to solar energy production.
“The production of these solar cells made of oxide layers is more complicated than making standard silicon solar cells,” says Professor Karsten Held from the Institute for Solid State Physics at TU Vienna “But wherever extremely high efficiency or minimum thickness is required, the new structures should be able to replace silicon cells.”
The team from TU Vienna was assisted by Satoshi Okamoto from the Oak Ridge National Laboratory in Tennessee, USA, and Professor Giorgio Sangiovanni, a former employee of TU Vienna who is now working at Würzburg University in Germany.