2D Monolayers Could Yield Thinnest Solar Cells Ever

Efforts to improve solar cells have historically focused on improving energy conversion efficiencies and lowering manufacturing costs. But new computer simulations have shown how using a different type of material could yield thinner, more lightweight solar panels that provide power densities – watts per kilogram of material – orders of magnitude higher than current technologies.

Using NERSC supercomputers and density functional theory calculations,  researchers demonstrated that an effective solar cell could be made from a stack of two 1-molecule-thick materials: graphene (a one-atom-thick sheet of carbon atoms, shown in blue) and molybdenum disulfide (with molybdenum atoms shown in red and sulfur in yellow). Image: J. Grossman, M. Bernardi

Solar cells are most often made from multi-crystalline silicon semiconductors, which have energy conversion efficiencies in the 15 to 20 percent range. But silicon-based photovoltaics, typically hundreds of microns thick, do not absorb light very efficiently and must be mounted between layers of glass, which can be heavy and expensive. In fact, about half the cost of today’s panels is in support structures, installation, wiring and control systems. And alternative active-layer materials such as polymers, while lighter and less costly to produce, suffer from lower energy conversion efficiencies as well as degradation over time due to exposure to ultraviolet (UV) light and moisture.

Now, using supercomputers at the Department of Energy’s National Energy Research Scientific Computing Center (NERSC) and density functional theory calculations to investigate the energy storage capacity, thermal stability, and degradation resistance of chromophore/template nanostructures, an international team of scientists has demonstrated that two-dimensional materials – specifically, transition metal dichalcogenides (TMDs) and graphene -- could be used to produce 1-nanometer thick solar cells that yield 1 to 2 percent energy conversion efficiency.

While that conversion rate may not seem impressive at first glance, it is achieved using material that is 20 to 50 times thinner than the thinnest solar cell made today, according to Jeffrey Grossman, an engineering professor at MIT and co-author of the study, which was published in Nano Letters. Because the cells would be so thin, several could be stacked to improve the energy efficiency while still being much smaller and lighter than existing solar cells.

“Our calculations show that, theoretically at least, one can stack 2D materials to make something a little thicker and get up to 20 percent efficiency – the same as current cutting-edge photovoltaics,” he said. In addition, the 2D materials -- which in this case were molybdenum disulfide and molybdenum diselenide -- have very good thermal and UV robustness, he noted.

They also have keen light-absorption capabilities, Grossman emphasized. Solar cells using these monolayers as active layers could show significantly higher power density than existing ultra-thin solar cells because TMDs can absorb sunlight effectively despite their extremely thin thickness.

“The question we wanted to answer was how well do these materials absorb light,” he said. “And that is something that surprised us: how with just two layers of material it is possible to absorb 10 percent of the solar spectrum and have a cell that is 2 percent efficient as a result.”

NERSC supercomputers were critical in enabling these findings, which open up new avenues for nanoscale solar energy conversion, Grossman added.

“In order to model accurately enough the optical properties of these two materials, we couldn’t use traditional methods. There were too many electrons and we needed too much accuracy," he said. "So basically the only place we could get these algorithms to run was on NERSC.”

The researchers also used the National Science Foundation's XCEDE computing resources and the CINECA supercomputer in Italy.

The next step is to take what they learned from the simulations and begin building devices using these and other 2D materials, Grossman noted. One of the students in his group is currently working on this, but there are challenges.

“What our calculations show is that when you have two layers, you have extraordinary optical absorption properties,” he said. “How much light each of these single layers can absorb, given how thin they are, is extremely high. Now, the efficiency of a solar cell made with a stack of two layers is around 1 to 2 percent, which may sound pretty low, but if we consider that this is only a few atoms thick, it's really quite remarkable. And if you keep stacking them together, I believe you could reach efficiencies that are comparable to today's solar cells. At this point we have been able to build devices with 10-20 layers each of two different materials, so the optical properties are good, but not extraordinary in terms of how much power they generate per mass. But we're only at the beginning stages of the experimental work.”

The researchers also plan to do more computer simulations to investigate the role of material defects, test other 2D materials, and look at what kinds of energy conversion efficiencies larger stacks may yield.

Demonstrating the potential of atom-thick materials for solar generation and related applications is just the beginning, Grossman noted. He believes that even at the current lower energy efficiencies, these ultrathin films could find application in a number of fields – say, where weight is a crucial factor (such as spacecraft and aviation) or in remote areas of the developing world where transportation costs are significant. They could also find their way into consumer products.

“It is exciting, bringing alternative energy to people in an entirely new way,” Grossman said.

M. Bernardi, M. Palummo, J. Grossman, 2013, Extraordinary sunlight absorption and one nanometer thick photovoltaics using two-dimensional monolayer materials Nano Letters 13(8), 3664–3670.