Top: A normal perovskite crystal (left) bends and twists after being squeezed between two diamonds. Bottom: The orange perovskite crystal changes color as pressure is applied, indicating that it is absorbing different wavelengths of light. Image: Adam Jaffe and Yu Lin/Stanford & SLAC.Solar cells made of artificial metallic crystalline structures called perovskites have shown great promise in recent years. Now scientists at Stanford University have found that applying pressure can change the properties of these inexpensive materials and how they respond to light.
"Our results suggest that we can increase the voltages of perovskite solar cells by applying external pressure," said Hemamala Karunadasa, an assistant professor of chemistry at Stanford. "We also observed a dramatic increase in the electronic conductivity of these promising materials at high pressures." Karunadasa and Stanford colleague Wendy Mao report their findings in a paper in ACS Central Science.
Perovskites come in several crystalline structures, including hybrid perovskites made of lead, iodine or bromine with organic compounds. These inexpensive materials have potential applications in advanced LEDs and lasers, but one of the hottest areas of research involves solar cells. Recent studies have shown that hybrid perovskites can efficiently absorb sunlight and convert it to electricity; several labs have achieved light-to-electric power efficiencies above 20%, rivaling commercially-available silicon solar cells.
In this new study, Karunadasa and Mao sought to assess how pressure affects the way hybrid perovskites respond to light. To find out, the researchers loaded perovskite samples in a diamond-anvil cell, a high-pressure device consisting of two opposing diamonds. Each tiny sample was placed between the diamonds and then squeezed at very high pressures.
The results were visible. One sample, which is normally orange, turned lighter in color under compression, providing an indication that the perovskite was absorbing higher-energy light waves. But as the pressure increased, the sample darkened, indicating that lower-energy light was also being absorbed.
"Our findings suggest that compression can allow us to tailor the wavelength of absorbed light," explained Mao, an associate professor of geological sciences at Stanford and of photon science at the SLAC National Accelerator Laboratory. "This compression may be attained through either mechanical or chemical means."
Several research groups have been developing low-cost tandem solar cells made of perovskite stacked on top of silicon. But obtaining the high voltages required for high-efficiency tandem cells has proven to be difficult. The results of the new Stanford study suggest that applying pressure could offer a simple way to increase the voltages of perovskite solar cells and should be investigated further.
"By tracking the positions of atoms upon compression using X-ray diffraction, we can explain exactly how the materials' structure responds to pressure," Karunadasa said. "Overall, this work shows that pressure is a tuning knob for improving the properties of perovskite absorbers in a predictable way."
This story is adapted from material from Stanford University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.