Scientists at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) have determined that surface recombination limits the performance of polycrystalline perovskite solar cells, and that protective material on the surface of the polycrystalline thin films could improve device performance.
Study of surface recombination in polycrystalline perovskite solar cells: Outcome
“We found that in the polycrystalline perovskite films the surface recombination is lower than in bulk crystals of perovskite—by about six times,” said Matthew Beard from NREL’s Chemistry and Nanoscience Center. “We correlated the lower surface recombination to a methyl-ammonium-rich environment that occurs during the synthesis of the polycrystalline films — without any post-processing.”
The researcher explains that the surface recombination is already very low in the unpassivated perovskite films, compared with traditional unpassivated semiconductors. Yet, the top and bottom surfaces control the total carrier lifetime in the samples. That is, surface recombination dominates over bulk and grain boundary recombination, according to Beard, who concluded: “Thus, improvements in carrier lifetime can be expected from intentional surface passivation schemes.”
Overcoming a challenge in thin films
The NREL researcher explains that in thin films, separating surface recombination from bulk or grain recombination can be challenging. And yet, he can report: “Our experiments were able to do this.”
Transient reflection spectroscopy
Beard and his collaborators have used transient reflection (TR) spectroscopy to examine surface recombination in single-crystal and polycrystalline films. The scientist explains the technique: “TR spectroscopy is a pump-probe spectroscopy. The pump wavelength is tuned to provide different carrier distributions at the surface. A broadband probe light is reflected from the surface and is monitored as a function of pump-probe delay time and for different pump wavelengths. The probe light is sensitive to carriers that reside within about 10–15 nm from the surface. Thus, transient reflection spectroscopy is only sensitive to the surface carrier dynamics.”
Knowing where the recombination is coming from
Beard notes it is important to know where the recombination is coming from: “To obtain the highest power conversion efficiency in a solar cell, one must eliminate all sources of non-radiative recombination,” he said, adding that knowing where the major sources of non-radiative recombination arise allows researchers to specifically target those areas for improvements in device performance.
Impact on the design of next-gen solar cells
The new understanding the NREL scientists gained from their close look at surface recombination in single-crystal and polycrystalline films potentially has significant impact on the design of the next generation of solar cells, perovskite solar cells in particular. “Researchers should target the top and bottom surfaces in order to increase the total carrier lifetimes within the perovskite absorber layer,” Beard urged.
The NREL team, however, has not yet suggested any particular protective materials one could use on the surface of the polycrystalline thin films to help to improve perovskite solar cells.
What’s next at NREL?
“We are investigating various specific perovskite interfaces to better understand interfacial carrier dynamics, and how that governs devices performance,” Beard shared about the next steps ahead at the Laboratory.
Written by Sandra Henderson, Research Editor, Solar Novus Today