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Spring 2017

The Ultrafast Interplay between Molecules and Materials

Computer calculations shed light on nebulous interactions in semiconductors relevant to dye-sensitized solar cells

Shannon McCullough

Researchers used sophisticated calculations to see how the electrons (see purple and green shapes) move around certain atoms in cyanidin (top), a red pigment naturally occurring in grapes and cherries, and interact with silicon. The results are critical to understanding how — and how quickly — electrons navigate through complex systems required for fuel production from sunlight and air. Image courtesy of Nathan Johnson, Pacific Northwest National Laboratory

In pursuit of a renewable fuel source, a long-run objective is to create a device that synthesizes fuels from sunlight and air. This goal is the focus of the Center for Solar Fuels, an Energy Frontier Research Center. At UNC, a diverse group including chemists and physicists are collaborating to achieve this objective.

Of particular interest to the team is the exact way electrons move around between semiconductors and molecules attached at the surface. Semiconductors are materials that can both conduct electricity and insulate against it. Semiconductors, such as silicon, are essential to separate charges. Without high-yield charge separation, producing electricity or turning sunlight into fuels would not be effective or efficient. Recent research by team members Lesheng Li and Yosuke Kanai shows how calculations can simulate the electron pathways and timescales at semiconductor-molecule interfaces. The specific system studied silicon as the semiconductor surface and cyanidin, a red pigment naturally occurring in many red fruits such as grapes and cherries, as the molecule.

The calculations used are based exclusively on what scientists call first principles. First-principles calculations use only the laws of physics and known physical measurements such as the charge of an electron to solve equations that model real-world systems. Using first-principles calculations, the team investigated physical and chemical systems independent of any empirical or fitting parameters.

Kanai and his team have leveraged these calculations to better understand solar devices. The inner workings of microscopic systems such as at the molecular scale and nanoscale can prove to be very difficult to figure out precisely. With advanced high-performance computers now available, scientists can use simulations to model microscopic systems of interest and obtain highly specific descriptions.

The simulations allowed them to view and model processes that happen very quickly — as fast as on the femtosecond (one quadrillionth of a second) timescale. These ultrafast dynamics are critical for the success of optical and electronic devices, and first-principles calculations provide a unique vantage point to view how quickly electrons move within semiconductor-molecular interfaces.

With the first-principles computational modeling, the team precisely defined every atom in the lattice. The entire geometry is exactly defined within the model. The control afforded by using these explicit models let the team ask more precise questions and, consequently, yield more informative results about complex systems.

They used the ability to define a precise atomic model to introduce a singular defect by misplacing or omitting an individual atom within the lattice. Defects in semiconductors significantly impact the electronic properties making the exact characterization of critical importance.

The team’s results indicate that the electron transfer in the forward direction (from the silicon to the cyanidin) is mostly independent from the backward direction (from the cyanidin to the silicon). The simulations also showed that electron transfer occurs within a picosecond (one trillionth of a second) in a defect-free material. Future calculations will explore how the introduction of material defects slows the dynamics of electron transfer.

For solar cells and other systems, electron dynamics at semiconductor-molecule interface determine the solar-to-electric efficiency and sources of failure in the system. Thus, better understanding these dynamics has clear consequences for improving optical-electronic system performance. The research presented here lays the foundation for future calculations, and the first-principles simulations employed in this study can be extended to other systems by varying the semiconductor surface and/or varying the molecules that are modeled. By describing a more exact picture of how electrons move through systems relevant to solar cells, the team at the Center for Solar Fuels is one step closer to creating a fuel from sunlight and air.

Acknowledgments

The work was supported by the Center for Solar Fuels, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. The National Energy Research Scientific Computing Center, a U.S. Department of Energy Office of Science user facility, provided computational resources.

More Information

Li L and Y Kanai. 2016. “Excited Electron Dynamics at Semiconductor-Molecule Type-II Heterojunction Interface: First-Principles Dynamics Simulation.” Journal of Physical Chemistry Letters 7(8):1495-1500. DOI: 10.1021/acs.jpclett.6b00436

About the author(s):

  • Shannon McCullough is a graduate student at the University of North Carolina at Chapel Hill. A member of the Center for Solar Fuels, she is studying photocathode materials for implementation in a dye-sensitized photoelectrosynthesis cell. Shannon earned her undergraduate degree at the College of William and Mary.

An Electron’s Journey

Scientists use complex calculations to see how electrons move across materials

How electrons move matters in solar panels, supercomputers, and more. Researchers determined that electrons move amazingly fast from cyanidin (top) to silicon (bottom). Understanding electrons (key areas noted with purple and green balloons) could help create materials that change the energy efficiency of countless devices. Image courtesy of Nathan Johnson, Pacific Northwest National Laboratory

Solar energy is often a use-or-lose proposition. The electricity produced isn’t stored. But what if it could be packed into fuels? Scientists are working to create such fuels. A key challenge is knowing how electrons, generated by sunlight, move through a solar cell. To study these changes, scientists employed calculations to see how electrons transit from one material to another. Driving across materials is vital if an electron is to eventually produce a fuel molecule. The team found the move is amazingly fast, occurring in a trillionth of a second. The team’s calculations can be used for other materials, such as those in energy-efficient lights. The team is from the Center for Solar Fuels, a University of North Carolina Energy Frontier Research Center.

More Information

Li L and Y Kanai. 2016. “Excited Electron Dynamics at Semiconductor-Molecule Type-II Heterojunction Interface: First-Principles Dynamics Simulation.” Journal of Physical Chemistry Letters 7(8):1495-1500. DOI: 10.1021/acs.jpclett.6b00436

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.