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Electronic Structure Methodology Reveals New Materials for Solar Cells

Final Report Summary - SOLAR CELL MATERIALS (Electronic Structure Methodology Reveals New Materials for Solar Cells)

Summary of “Solar Cell Materials” project
Conducted during the outgoing phase at Princeton University and the incoming phase at the Technion Institute
Funded by the Research Executive Agency, Marie Curie Actions – International Fellowships

Current solar energy conversion technologies are either too expensive or inefficient to compete with our most commonly used energy source, fossil fuels. Therefore, the goal of this project is to use quantum mechanics theory to find better materials that utilize solar energy to generate electricity and fuel. In a solar energy conversion device several processes must take place. In the first step, light is absorbed. The light excites an electron from the valence band to the conduction band. In this way, an electron-hole pair is formed. In the second step, the electron is collected away from the hole to produce electricity. If the material is able to catalyze, the electron and/or hole participates in a reaction that forms fuel. Finding a material that can perform well in all of these steps is challenging.

The project started with selecting materials that have potential. Metal oxides and transition metal oxides were selected as a parent materials due to several advantages these material have, including relative abundance, low cost, stability under exposure to ultraviolet light, and resistance to corrosion. After selecting parent materials to work on, the materials were characterized. The first characteristic is the band gap, which determines a material’s ability to efficiently absorb solar energy. A
many-body method (non-self consistent GW) was used to calculate the (quasiparticle) band gap. With this theoretical method, we found that several oxides, such as iron (II) oxide and iron (III) oxide, have proper band gaps for efficient solar energy absorption. This work was published in the Physical Chemistry Chemical Physics journal and has been cited in more than 20 peer reviewed publications.

Hence, iron oxides are strong candidates. Other oxides, such as nickel (II) oxide and manganese (II) oxide, have a band gap that is too large for efficient solar energy absorption. For nickel (II) oxide, we calculated the band gap with several computational approaches. These approaches gave consistent qualitative results and a band gap that is within the experimental range. We found that the band gap can be reduced by alloying with iron (II) oxide and discovered new materials that can absorb light better. We also found a general strategy to reduce band gaps of semiconductors based on their relative band alignments. We published this work in Journal of Materials Chemistry A and also filed a U.S. patent application on new alloyed materials.

The second property is conductivity. Transition metal oxides suffer from low conductivity that limits solar energy conversion efficiency. We researched charge transport in iron oxides and found several potential dopants that should improve conductivity. We concluded that dopants with low ionic potential and a covalent-type bonding do not trap charge carriers. These dopants include silicon and hydrogen. The work on iron oxides was published in Nano Letters and Journal of Physical Chemistry C.

The third property is band edge positions. Knowledge of the band edge positions provides the first test that a material must pass to be a potentially effective photoelectrode or photocatalyst. We have proposed a new scheme for calculating band edge positions of semiconductors. This scheme has an advantage over the conventional approaches in that it is much less computationally expensive. The position of the band edges of iron (II) oxide show that this material may be useful for reducing carbon dioxide to generate fuels such as hydrogen, and the band edge positions of iron (III) oxide show that it can oxidize water. This work was published in the Physical Chemistry Chemical Physics journal.

This work contributes to the trend of seeking novel materials acting in solar energy conversion devices. We now have a better understanding of how to engineer solar energy conversion materials and discovered several materials that show promise. We anticipate that our insights will impact not only the academy, but will be beneficial for companies that develop and manufacture solar cells. We hope this will impact on the socio-economic energy needs of Europe (and the world).

1. P. Liao, M. Caspary Toroker, and E. A. Carter, "Electron transport in pure and doped hematite",
Nano Lett. 11, 1775 (2011).
2. M. Caspary Toroker, D. K. Kanan, N. Alidoust, L. Y. Isseroff, P. Liao, and E. A. Carter, "First
principles scheme to evaluate band edge positions in potential transition metal oxide photocatalysts
and photoelectrodes", Phys. Chem. Chem. Phys. 13, 16644 (2011).
3. M. Caspary Toroker and E. A. Carter, "Hole transport in non-stoichiometric and doped wustite", J. Phys. Chem. C 116, 17403 (2012).
4. M. Caspary Toroker and E. A. Carter, "Transition metal oxide alloys as potential solar energy conversion materials", J. Mater. Chem. A 1, 2474 (2013).

Contact details:
Dr. Maytal Caspary Toroker
Assistant Professor
Department of Materials Science and Engineering
Technion - Israel Institute of Technology
Haifa 32000, Israel

Skype: maytal.toroker