The
efficiency of conventional solar cells could be significantly
increased, according to new research on the mechanisms of solar energy
conversion led by chemist Xiaoyang Zhu at The University of Texas at
Austin.
Zhu
and his team have discovered that it’s possible to double the number of
electrons harvested from one photon of sunlight using an organic
plastic semiconductor material.
“Plastic
semiconductor solar cell production has great advantages, one of which
is low cost,” said Zhu, a professor of chemistry. “Combined with the
vast capabilities for molecular design and synthesis, our discovery
opens the door to an exciting new approach for solar energy conversion,
leading to much higher efficiencies.”
Zhu and his team published their groundbreaking discovery Dec. 16 in Science.
The
maximum theoretical efficiency of the silicon solar cell in use today
is approximately 31%, because much of the sun’s energy hitting the cell
is too high to be turned into usable electricity. That energy, in the
form of “hot electrons,” is instead lost as heat. Capturing hot
electrons could potentially increase the efficiency of solar-to-electric
power conversion to as high as 66%.
Zhu and his team previously demonstrated
that those hot electrons could be captured using semiconductor
nanocrystals. They published that research in Science in 2010, but Zhu
says the actual implementation of a viable technology based on that
research is very challenging.
“For
one thing,” said Zhu, “that 66% efficiency can only be achieved when
highly focused sunlight is used, not just the raw sunlight that
typically hits a solar panel. This creates problems when considering
engineering a new material or device.”
To
circumvent that problem, Zhu and his team have found an alternative.
They discovered that a photon produces a dark quantum “shadow state”
from which two electrons can then be efficiently captured to generate
more energy in the semiconductor pentacene.
Zhu
said that exploiting that mechanism could increase solar cell
efficiency to 44% without the need for focusing a solar beam, which
would encourage more widespread use of solar technology.
The
research team was spearheaded by Wai-lun Chan, a postdoctoral fellow in
Zhu’s group, with the help of postdoctoral fellows Manuel Ligges, Askat
Jailaubekov, Loren Kaake and Luis Miaja-Avila. The research was
supported by the National Science Foundation and the Department of
Energy.
Science Behind the Discovery:
- Absorption of a photon in a pentacene semiconductor creates an excited electron-hole pair called an exciton.
- The exciton is coupled quantum mechanically to a dark “shadow state” called a multiexciton.
- This
dark shadow state can be the most efficient source of two electrons via
transfer to an electron acceptor material, such as fullerene, which was
used in the study. - Exploiting the dark shadow state to produce double the electrons could increase solar cell efficiency to 44%.
Science feature: Solar Cells Capture Lost Energy
Observing the Multiexciton State in Singlet Fission and Ensuing Ultrafast Multielectron Transfer
