Three-year project aims to increase solar-cell efficiency on the atomic scale

A supercomputer at ORNL is helping scientists simulate a process leaves do naturally–capturing sunlight and turning it into energy. Silicon-based solar technology is capable of 20 percent efficiency, but its production is expensive and requires massive amounts of energy. Today’s nanostructured solar panels are only 3 to 4 percent efficient. But if nanotechnology can improve, it may be the path to cheaper solar energy.

A simulation of an exciton in a cadmium selenide and cadmium telluride joint nanorod (upper panel). The blue surface shows the shape of the holes; the red surface, the shape of the electrons; and the grey, yellow, and green spheres, cadmium, selenium, and tellurium atoms, respectively. The color contour plots (lower panel) represent the existence of hole quantum states (left) and electron quantum states (right) at different energies. Image courtesy of Lin-Wang Wang, Lawrence Berkeley National Laboratory

“At 10 percent efficiency, these devices could at least enter the market,” explained Lin-Wang Wang, computational scientist at Lawrence Berkeley National Laboratory (LBNL). Wang and his fellow LBNL collaborator, Michael Banda, are using the OLCFs Cray supercomputer known as Jaguar, Americas fastest, to improve the efficiency of photovoltaics by learning more about electrically conductive material on an atomic scale.

The team received a 3-year allocation (2010-2012) on Jaguar through the Innovative and Novel Computational Impact on Theory and Experiment (or INCITE, for short) program and for 2011 was awarded 10 million processor hours. The work is funded by the Department of Energy’s Office of Science.

The research duo’s primary motivation is making solar energy a practical source of power for the masses. “The current solar cells are expensive because they use crystal or polycrystal silicon,” Wang said. “It usually takes 2 to 3 years for solar cells to make the energy back that it took to create [them].” Solar cells are beginning to progressively shorten the payback period, however, by using a hybrid nanostructured design that employs two dissimilar semiconducting materials (for example, copper sulfide and cadmium sulfide) that are cheap and abundant, making them commercially competitive. But complexity in hybrid nanostructures creates a need for materials science researchers to study the minute interactions between electrons and atoms more closely.

Shedding light on photovoltaics

Researchers understand much of the physics for the bulk of silicon-based photovoltaics, or photon-atom interactions. But the electronics, or electron-atom interactions, in the nanostructures of solar cells–highly organized materials ordered on scales as small as atoms–are far more mysterious. The team uses a variety of scientific application codes in its simulations, with the Linear Scaling Three-Dimensional Fragment (LS3DF) code as the centerpiece. LBNL researchers led by Wang developed LS3DF during a 2007-2009 INCITE project, and the group won the 2008 Gordon Bell Prize for algorithm development.

Wang describes LS3DF as a divide-and-conquer code. Simulating millions of atoms interacting with one another across an entire solar panel is a difficult task. When light hits semiconducting solar material, a portion of it is absorbed, exciting the atomic structures and knocking electrons loose. What exactly happens in the nanomaterial from there has been difficult to gauge. Researchers use LS3DF to slice simulations of semiconducting material into smaller portions. By dividing the simulation, the researchers can make observations more efficiently and then patch the individual results together for a more complete picture of the interactions.

With a small group of computer cores to calculate each fragment and a straightforward strategy for parallelization, we can scale the computation very well to the larger number of cores on supercomputers, said Wang. The team typically runs LS3DF on between 20,000 and 60,000 of Jaguar’s 224,000 processing cores.

In addition to getting a more accurate picture of atoms interacting with one another, researchers must also get a clear image of surface electronic structures. The group is simulating zinc oxide, an often-used material for nanosystems. So far its research has uncovered a large dipole moment (when positive and negative charge occurs at opposite ends of a molecule) at the surface of the chemically synthesized zinc oxide nanorod. Such a moment can create an electric field large enough to dramatically change the internal electronic behavior of the system.

Another mystery for materials scientists lies in exciton particles. Excitons are complex electronic phenomena that couple an electron and its theoretical opposite, called a hole, forming an overall quasiparticle with no charge. To generate electricity the electron must be separated from its hole so it can be collected by the electrodes at the opposite side of the solar cell. This turns out to be a particularly challenging task for nanosystems. Because both the electron and hole are confined in the small nanostructure, they interact and strongly influence each other. The team has calculated the exciton binding energy in a nanorod comprised of cadmium selenide and cadmium telluride to begin studying how excitons dissociate in response to the application of an electric field.

The researchers also plan to simulate how electrons move after they are excited by sunlight. The group will run time-domain simulationssimulations moving in small chunks of timefor verification and further insights into the dynamics of electron transport. These simulations occur in thousandths-of-a-second increments, and the massive amount of data requires leadership-class computing resources. “To study electron transport in nanostructured solar materials, time-domain simulations are important,” said Wang. “We often run with 50,000 or more processors for tens of hours. So, one such run is equivalent to running your personal computer for 100 years. Obviously, without the leadership-class computers, such calculations would be impossible.”

After the scientists have a more complete picture of electron-hole interactions in solar-energy nanomaterials, they plan to observe electrons as they move through the materials. From this, Wang and Banda hope they will see ways to improve solar-cell efficiency. The final step of the simulation will be to get all of these interactions working simultaneously. Said Wang, “Through all these simulations, we can have a direct picture for what is going on inside the nanostructures when they are excited by the sunlight. Such knowledge is critical for the design of next-generation solar cells using nanostructures.” —by Eric Gedenk

Related Publications:

N. Vukmirovic, L.W. Wang, “Carrier hopping in disordered semiconducting polymers: How accurate is the Miller-Abrahams model?”, App. Phys. Lett. 97, 043305 (2010).

S. Dag, S.Z. Wang, L.W. Wang, “Large surface dipole moment in ZnO nanorods”, Nano Lett. 11, 2348 (2011).

S.Z. Wang, L.W. Wang, “Exciton dissociation in CdSe/CdTe heterostructure nanorods”, J. Phys. Chem. Lett. 2, 1 (2011).