American Institute of Physics (AIP)
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.
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 crystallize 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 crystallizes 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.
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 light emitting diodes (LEDs).
Still, it is the solar cell that arguably offers MHPs the best
chance to address pressing global problems.
"To mitigate the impact of climate change and also to
ensure the energy security of the United States and the world, it is very
important to come up with renewable energy sources rather than just be relying
on fossil fuel-based energy," Choi said.
