‘Shadow state’ ups solar cell efficiency
Posted:28 Dec 2011
The discovery of a quantum “shadow state” could double the efficiency of solar cells.
The quantum “shadow state” allows not only one but two high-energy electrons to be produced by optoelectronic energy conversion. Chemistry professor Xiaoyang Zhu of the University of Texas at Austin led the latest research on the mechanisms of solar energy conversion.
Zhu and his team have discovered that it’s possible to double the number of electrons harvested from one photon of sunlight using pentacene, an organic plastic semiconductor material.
“Plastic semiconductor solar cell production has great advantages, one of which is low cost,” said Zhu, in a statement. “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.”
The maximum theoretical efficiency of the silicon solar cell in use today is approximately 31 percent. This is because much of the photonic energy hitting the cell is not that at wavelengths that can be turned into usable electricity. That energy is instead lost as heat. Capturing the thermally excited hot electronic energy could potentially increase the efficiency of solar-to-electric power conversion to as high as 66 percent, according to the research team.
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. The primary one being that it requires focused sunlight rather than ambient light that typically hits a solar panel.
The team have found an alternative. They discovered that in the semiconductor pentacene a photon produces a dark quantum “shadow state” from which two electrons can then be efficiently captured to generate more energy.
The absorption of a photon creates an excited electron-hole pair, called an exciton.
The exciton is coupled quantum mechanically to a dark “shadow-state” called a multiexciton.
It so happens that the multiexciton can be an efficient source of two electrons via transfer to an electron acceptor material such as fullerene, the ball-shaped 60-atom allotrope of carbon, which was used in the study.
Exploiting this could raise pentacene solar cell efficiency to 44 percent
without the need for a focused solar beam, according to the researchers.
The research team was led 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.
– Peter Clarke