As the demand for solar energy grows worldwide, scientists are working to improve the performance of solar devices – which is important if the technology is to compete with traditional fuels. But researchers face theoretical limits to how efficiently they can make solar cells.
One way to push efficiency beyond the limits is to add nanoparticles to the regenerative materials used in solar devices. Up-conversion materials allow solar cells to harvest energy from a wider spectrum of light than is normally possible. A group of scientists who tested this method found that the nanoparticles increased efficiency, but not for the reason they expected. Their research may suggest a new way forward for developing more efficient solar devices.
“Some researchers in the literature have hypothesized and shown results that up conversion nanoparticles provide performance enhancement,” said Shashank Priya, vice president for research and professor of materials science and engineering at Penn State. “But this research shows that it doesn’t matter whether you add nanoparticles or other nanoparticles – they show improved efficiency due to improved light scattering.”
Adding nanoparticles is like adding millions of tiny mirrors to a solar cell, the researchers said. Light passing through the device hits the nanoparticles and scatters, potentially hitting other nanoparticles and reflecting multiple times in the device, providing a noticeable improvement in photo current.
The researchers reported that this process of light scattering, rather than re-conversion, led to an increase in the efficiency of the solar devices they created.
“It doesn’t matter what nanoparticles you use, as long as they are nanoparticles with specific scattering properties, it always leads to an increase in efficiency by a few percentage points,” said Kai Wang, a research assistant in the Department of Materials Science. and engineering and co-authored the study. “I think our research provides a nice explanation for why this type of light-absorbing composite structure is of interest to the solar community.”
Higher conversion nanoparticles work by absorbing infrared light and emitting visible light, which the solar cell can absorb and convert into more power. About half of the sun’s energy reaches Earth as infrared light, but most solar cells cannot harvest it. The researchers suggest that harnessing this could push the solar cell’s efficiency above its theoretical ceiling, the Shockley-Queisser (SQ) limit, which is around 30% for single-junction solar cells powered by sunlight.
Previous studies have shown efficiency improvements of 1% to 2% using higher conversion nanoparticles. But the team found that these materials provided only modest improvements to the perovskite solar devices they created, the researchers said.
“We initially focused on converting infrared light into the visible spectrum for absorption and energy conversion by the perovskite, but data from our colleagues at Penn State show that this is not a significant process,” said Jim Piper, co-author and professor emeritus at Macquarie. University, Australia. “Then we provide undoped nanocrystals that do not provide optical up-conversion and are just as effective in improving energy conversion efficiency.”
The team performed theoretical calculations and found an improvement in efficiency rather than the ability of the nanoparticles to improve light scattering.
“Basically, we started playing around with the distribution of the nanoparticles in the model, and we started to see that if you spread the particles further apart, you start to see some improved dispersion,” said Thomas Brown, an associate professor at the University of Rome. “Then we had this breakthrough.
The addition of nanoparticles increased the efficiency of the perovskite solar cells in the study by 1%, the researchers reported in the journal ACS Energy Letters. The researchers say that changing the shape, size and distribution of nanoparticles in these devices can make them more efficient.
“That some optimal shape, layout or size can lead to more attractive photography,” says Priya. “That could be a future direction of research based on ideas from this research.”
