Better Solar May Lurk In ‘Shadow State’

Theoretically, the maximum efficiency of today’s silicon solar cell is approximately 31 percent, because much of the solar energy that hits the cell is lost as heat. But new research from the University of Texas at Austin indicates that it is possible to double the number of electrons harvested from one photon of sunlight using an organic plastic semiconductor material. The team has demonstrated that capturing these “hot” electrons could increase the efficiency of solar-to-electric power conversion to 44 percent; and theoretically, the rate could go as high as 66 percent.

Austin chemist Xiaoyang Zhu and his team previously demonstrated that capturing hot electrons using semiconductor nanocrystals could lead to efficiency as high as 66 percent. They published that research in the journal Science in 2010. But Zhu says that developing a viable technology based on that research requires using sunlight that is highly focused. This creates problems from an engineering perspective.

image via Shutterstock

Instead, Zhu focused on research that could improve a solar cell’s ability to harvest electricity from the solar energy that typically hits a solar panel. Zhu’s new research shows that absorbing a photon in a pentacene semiconductor creates an “excited electron-hole pair” called an “exciton.” Using quantum mechanics, the exciton is coupled to a dark “shadow state” called a “multiexciton.” This state can be the most efficient method of transferring two electrons  to an electron acceptor material, such as fullerene, which was used in the study. In the journal Science, Zhu and his team demonstrated that exploiting this dark shadow state to produce double the electrons could increase solar cell efficiency to 44 percent.

The research was conducted 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 project was supported by the National Science Foundation and the U.S. Department of Energy.

Lauren Craig is a writer and consultant living in Seattle, WA. She holds an M.S. in International Development from Tulane University, and is co-founder of Sustainable Systems Integrators, LLC., an employee-owned solar energy design and installation firm in New Orleans, LA. She is also certified in PV design and installation by the North American Board of Certified Energy Practitioners (NABCEP).

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