Campus researchers use semiconductors in solar technology

Nicole Lim/Staff

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Researchers with the Lawrence Berkeley National Laboratory and UC Berkeley have developed a new technology that allows low-cost, high-efficiency solar cells to be developed from almost any type of semiconductor material.

Semiconductors are materials that absorb sunlight and transfer the energy to electrons in their material. The high-energy electrons in the material can then be pulled out and used to provide power.

The research team — led by campus physics professor and Berkeley Lab senior scientist Alex Zettl and campus physics professor Feng Wang, a principal investigator at the Ultrafast Nano-Optics Group — have developed a way to use almost any type of semiconductor for this purpose.

The team released a paper on June 16 detailing their work entitled “Screening-Engineered Field-Effect Solar Cells,” which has been published in the science journal Nano Letters.

“Only a couple of the many semiconductors we know of are being used,” said William Regan, a co-author of the study.

Regan said a p-n junction must be used to extract the high-energy electrons from the semiconductor. Solar cells currently use a process known as chemical doping in order to form the junction.

“Chemical doping is a process that makes semiconductors conduct better,” said Will Gannett, another co-author of the study, in an email. “The dopants add or remove electrons from the semiconductor atoms and thus change the energy of the electrons that remain.”

Gannett explained that if the dopants add electrons to the semiconductor atoms, the semiconductor is “n-type, and if the dopants remove electrons the semiconductor is “p-type.”

“If you dope a piece of silicon p-type on one side and n-type on the other such that the two halves touch, you have created a p-n junction,” Gannett said in the email.

According to Steven Byrnes, a graduate student and member of the Nano-Optics group, the team’s design uses the field effect as a replacement for doping in solar cells.

The field effect uses an electric field to alter charge-carrying particles in the semiconductor and induce a p-n junction.

When the material in a semiconductor absorbs light, electrons within the material are excited to a higher energy state, according to Gannett.

“While it (the electron) is excited (and before it can fall back down) the field in the p-n junction pushes it off to one side where it can be collected by an electrode and then used for whatever we want,” Gannett said in the email.

Using the electric field allows for the creation of p-n junctions in a wider range of materials than just the range that chemical doping works in, such as copper oxide.

“This design can open a path for solar cells made of new, inexpensive materials that otherwise would not work well,” Byrnes said in an email.

Regan echoed a similar sentiment as Byrnes, and said the materials that can now be used as semiconductors are composed of constituent materials that are very abundant in nature.

“As the use of solar increases, we have to find materials that won’t run out,” Regan said.