Highly efficient solar cells could result from quantum dot research

Conventional solar cell efficiency could be increased from the current limit of 30 per cent to more than 60 per cent, suggests new research on semiconductor nanocrystals, or quantum dots.

Director of the Centre for Materials Chemistry and Professor of Chemistry, Dr Xiaoyang Zhu and his colleagues at The University of Texas at Austin reported the breakthrough in an issue of Science magazine in June.

Dr Zhu has discovered a method to capture the higher energy sunlight that is lost as heat in conventional solar cells.

The maximum efficiency of the silicon solar cell in use today is about 31 per cent. According to the researchers, this is because energy from sunlight hitting a solar cell is too high to be turned into usable electricity. That energy, in the form of so-called “hot electrons,” is lost as heat.

If the higher energy sunlight, or more specifically the hot electrons, could be captured, solar-to-electric power conversion efficiency could be increased theoretically to as high as 66 per cent.

“There are a few steps needed to create what I call this ‘ultimate solar cell’,” Dr Zhu said.

“First, the cooling rate of hot electrons needs to be slowed down. Second, we need to be able to grab those hot electrons and use them quickly before they lose all of their energy.”

Dr Zhu said that semiconductor nanocrystals, or quantum dots, are promising for these purposes.

As for the first problem, a number of research groups have suggested that cooling of hot electrons can be slowed down in semiconductor nanocrystals. In a 2008 paper in Science, a research group from the University of Chicago showed this to be true unambiguously for colloidal semiconductor nanocrystals.

Dr Zhu’s team has now figured out the next critical step: how to take those electrons out.

They discovered that hot electrons can be transferred from photo-excited lead selenide nanocrystals to an electron conductor made of widely used titanium dioxide.

“If we take the hot electrons out, we can do work with them,” Dr Zhu said. “The demonstration of this hot electron transfer establishes that a highly efficient hot carrier solar cell is not just a theoretical concept, but an experimental possibility.”

The researchers used quantum dots made of lead selenide, but Dr Zhu said that their methods will work for quantum dots made of other materials, too.

He cautions that this is just one scientific step, and that more science and a lot of engineering need to be done before the world sees a 66 per cent efficient solar cell.

In particular, a third piece of the science puzzle Dr Zhu is working on is connecting solar cell to an electrical conducting wire.

“If we take out electrons from the solar cell that are this fast, or hot, we also lose energy in the wire as heat,” Dr Zhu said.

“Our next goal is to adjust the chemistry at the interface to the conducting wire so that we can minimise this additional energy loss. We want to capture most of the energy of sunlight. That’s the ultimate solar cell.

“Fossil fuels come at a great environmental cost,” Dr Zhu said. “There is no reason that we cannot be using solar energy 100 per cent within 50 years.”

Funding for this research was provided by the US Department of Energy.