A study at the University of Rochester, Rochester, N.Y., suggests perovskites — a family of materials nicknamed for their crystalline structure that have shown extraordinary promise in recent years as a far less expensive, equally efficient replacement for silicon in solar cells and detectors — may become far more efficient.
Researchers typically synthesize perovskites in a wet lab, and then apply the material as a film on a glass substrate and explore various applications. Chunlei Guo, professor of optics at the University of Rochester leading the study reported in Nature Photonics, instead proposes a novel, physics-based approach.
By using a substrate of either a layer of metal or alternating layers of metal and dielectric material—rather than glass—he and his coauthors found they could increase the perovskite’s light conversion efficiency by 250 percent.
“No one else has come to this observation in perovskites,” Guo says. “All of a sudden, we can put a metal platform under a perovskite, utterly changing the interaction of the electrons within the perovskite. Thus, we use a physical method to engineer that interaction.”
Metals are probably the simplest materials in nature, but they can be made to acquire complex functions. The Guo Lab has extensive experience in this direction. The lab has pioneered a range of technologies transforming simple metals to pitch black, superhydrophilic (water-attracting), or superhydrophobic (water-repellent). The enhanced metals have been used for solar energy absorption and water purification in their recent studies.
In this new paper, instead of presenting a way to enhance the metal itself, the Guo Lab demonstrates how to use the metal to enhance the efficiency of pervoskites.
“A piece of metal can do just as much work as complex chemical engineering in a wet lab,” says Guo, adding that the new research may be particularly useful for future solar energy harvesting.”
In a solar cell, photons from sunlight need to interact with and excite electrons, causing the electrons to leave their atomic cores and generating an electrical current, Guo explains. Ideally, the solar cell would use materials that are weak to pull the excited electrons back to the atomic cores and stop the electrical current.
Guo’s lab demonstrated that such recombination could be substantially prevented by combining a perovskite material with either a layer of metal or a metamaterial substrate consisting of alternating layers of silver, a noble metal, and aluminum oxide, a dielectric.
The result was a significant reduction of electron recombination through “a lot of surprising physics,” Guo says. In effect, the metal layer serves as a mirror, which creates reversed images of electron-hole pairs, weakening the ability of the electrons to recombine with the holes.
The lab was able to use a simple detector to observe the resulting 250 percent increase in efficiency of light conversion.
Several challenges must be resolved before perovskites become practical for applications, especially their tendency to degrade relatively quickly. Currently, researchers are racing to find new, more stable perovskite materials.
“As new perovskites emerge, we can then use our physics-based method to further enhance their performance,” Guo says.
Image – This illustration from the Guo Lab shows the interaction between a perovskite material (cyan) and a substrate of metal-dielectric material. The red and blue pairings are electron-hole pairs. Mirror images reflected from the substrate reduce the ability of excited electrons in the perovskite to recombine with their atomic cores, increasing the efficiency of the perovskite to harvest solar light. Courtesy of: Chloe Zhang.
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