As climate change garners more attention around the world, scientists at the University of Virginia and Cornell University have made critical advances in understanding the physical properties of an emerging class of solar cells that have the potential to dramatically lower the cost of solar energy. Solar cells remain a focal point of scientific investigation because the sun offers the most abundant source of energy on earth.
The concern, however, with conventional solar cells made from silicon is their cost. Even with recent improvements, they still require a significant amount of electricity and industrial processing to be manufactured.
In 2009, energy researchers turned their attention to a class of materials called "metal halide perovskites," or MHPs. They are sprayed on like paint onto solid objects, says Joshua Choi, an assistant professor of chemical engineering at the University of Virginia. As the solution dries, the MHPs crystallise into a thin film that can be used to capture energy in a solar cell.
Within just a few years, MHP solar cells have been crafted whose performance rivals conventional silicon solar cells. This is the fastest recorded improvement in history for any photovoltaic material and it has been verified by the National Renewable Energy Laboratory in Golden, Colorado.
The challenge is that these existing MHP solar cells are no larger than a human fingernail.
"To be really technologically relevant," Choi said, "we need to be able to scale up this process while maintaining or even improving the efficiency of the solar cell. To do that, we need to understand how this material crystallises and grows from solution into a thin film."
Collaborating with scientists from Cornell University's High Energy Synchrotron Source, which receives funding from the National Science Foundation, Choi and his team monitored in real time the growth of MHP crystals at the atomic level by exposing them to high intensity X-rays.
The scientists will present their findings at the 66th meeting of the American Crystallographic Association, held July 22-26 in Denver, Colorado.
By adding different chemicals to the solution, they were able to control how fast the MHP crystals formed and what direction they grew on a surface. The specific orientation of the MHP crystals on a surface affected how well a solar cell performed, Choi said.
Moreover, this research provides this nascent field with the kinds of insights about MHP crystal formation that scientists will need as they determine how to manufacture the larger MHP solar cells that could reduce the price of solar energy.
But there's more to MHP solar cells than just their potential to cut costs.
"MHP solar cells can be used in flexible, lightweight materials," Choi said. The ultimate goal would be to make manufacturing MHP solar cells as easy as printing newspapers, generating rolls of thin solar cell material that could be easily applied to houses, cars, or anywhere else they were needed.
One significant drawback with many current MHP solar cells is that they contain lead. Researchers are working on identifying viable alternative compositions that are not toxic.
Choi and his research are funded by NASA, which is examining the potential for MHPs to be used in high temperature solar cells that could be installed in solar probes deployed in space. Elsewhere, MHPs have already been used in lasers, photo detectors, transistors and LEDs.
Still, it is the solar cell that arguably offers MHPs the best chance to address pressing global problems.