Artist’s representation of a halide perovskite-based photocathode. Researchers from Rice University and Los Alamos National Laboratory discovered that halide-perovskite semiconductors (silver) coated by a thin layer of Cesium (blue-green) can be modified to release liberated electrons (gray) across both the ultraviolet and visible spectrum (colored the arrows) and that a new layer of Cesium could be used to regenerate damaged photocathodes. Credit: Image courtesy of A. Mohite/Rice UniversityRice University and Los Alamos National Laboratory make inexpensive, scalable photocathodes using Halide Perovskites.
Rice University engineers have discovered techniques that could cut down the price for semiconductor electronic sources, which are essential elements in devices that range from night-vision glasses and low-light cameras to particle accelerators and electron microscopes.
With an access-to-all-universities Nature Communications paper, Rice researchers and colleagues from Los Alamos National Laboratory (LANL) discuss the first step to making electron sources from thin films of perovskite halide that efficiently convert light into free electrons.
A spends billions of dollars annually on photocathode sources made of semiconductors that contain rare elements such as gallium, selenium, cadmium, and even tellurium.
“This should be orders of magnitude lower in cost than what exists today on the market,” said study co-author Aditya Mohite, who is a Rice scientist of materials also a chemical engineer. The perovskites with halide can surpass current electronic sources for semiconductors in many ways.”First is the quantum efficiency and longevity,”
Mohite said. “Even as an initial proof of concept, which was the initial demonstration of the use of halide perovskites for electron sources, Quantum efficiency was approximately four times lower than the gallium arsenide photocathodes that are commercially available. We also found that halide perovskites have a longer life as compared to gallium arsenide.
“Aditya Mohite is an associate professor of biomolecular and chemical engineering and nanotechnology and materials science at Rice University. Credit photo by Jeff Fitlow/Rice UniveristyAn additional benefit is that perovskite photocathodes can be manufactured using spin coating, an inexpensive method that can be easily expanded. According to Mohite, who is an associate professor of biomolecular and chemical engineering and Nanoengineering and materials science.
“We also discovered that degraded perovskite photocathodes could be easily regenerated compared to conventional materials that typically require high-temperature annealing,” he added.
The researchers conducted tests on some perovskite-halide photocathodes, others with quantum efficiency up to 2.2 percent. They proved their method by creating photocathodes using organic and inorganic components. They demonstrated that they could adjust electron emission across each wavelength of both spectrums. Quantum efficiency measures how efficient the photocathode is in changing light into usable electrons.
“If each incoming photon generates an electron and you collected every electron, you’d have 100% quantum efficiency,” the study’s lead researcher Fangze Liu, a postdoctoral researcher at LANL. “The most efficient semiconductor photocathodes in the present have quantum efficiency of 10 to 20 percent and are composed of highly expensive materials made of intricate manufacturing processes. Metals are sometimes also utilized as sources of electrons. However, the quantum efficiency for copper is low, about .01 percent, however, it’s still being used, and it’s a useful technology.”
The savings in cost from perovskite halide photocathodes will be in two forms: ingredients for their production are plentiful and cheap. At the same time, the manufacturing process is less complicated and more affordable than conventional semiconductors.
“There is a tremendous need for something low-cost, and that can be scaled up,” Mohite stated. “Using solution-processed materials, where you can paint a sizable area, is uncommon in making the type of high-quality semiconductors necessary for photocathodes.”
The term “perovskite” refers to a specific mineral discovered in Russia in 1839, as well as any other compound with its crystal form similar to the mineral. Halide perovskites are one of the latter and are created by mixing lead, Tin, and other metals with bromide and iodide salts. Research into the semiconductors halide was a huge success when scientists from the United Kingdom used sheet-like crystals of the substance to create solar cells with high efficiency in 2012. Some labs have since demonstrated that the materials can produce photodetectors, LEDs, photo-splitting photochemical cells, and other gadgets.
Mohite is an expert on perovskites who was a researcher at LANL before joining Rice in 2018, explained that one of the reasons why the halide perovskite photocathode was a success is because his LANL colleagues in the Applied Cathode Enhancement and Robustness Technologies research group are “among the greatest teams on earth for exploring new materials and technologies for photocathodes.”
Photocathodes function following Einstein’s photoelectric effect that releases free electrons when struck by the light of a specific frequency. The quantum efficiency of photocathodes is usually small because even the tiniest imperfections, such as one atom out of position in the crystal’s lattice, would create “potential wells” that trap free electrons.
“When you have defects, all your electrons are likely to get lost,” Mohitestated. “It requires an enormous amount of control. And it required many hours of work to devise the process that would make an excellent perovskite material.”Mohite and Liu utilized spin-coating, a widely used technique where a liquid is poured onto a spinning disk, and centrifugal force distributes the liquid over the surface of the disk. The results of Mohite and Liu’s research the spin-coating process took place under an argon-based atmosphere to eliminate the presence of impurities. After being spun, the disks were heated before being placed in a high vacuum, transforming the liquid into crystals with a clear surface.