Measurement shows potential for building better solar cells by imaging fundamental properties of the material
Solar cells made with films mimicking the structure of the mineral perovskite are the focus of worldwide research. But only now have researchers at Case Western Reserve University directly shown the films bear a key property allowing them to efficiently convert sunlight into electricity.
Identifying that attribute could lead to more efficient solar panels.
Electrons generated when light strikes the film are unrestricted by grain boundaries -- the edges of crystalline subunits within the film -- and travel long distances without deteriorating, the researchers showed. That means electric charge carriers that become trapped and decay in other materials are instead available to be drawn off as current.
The scientists directly measured the distance traveled--called diffusion length -- for the first time by using the technique called "spatially scanned photocurrent imaging microscopy." Diffusion length within a well-oriented perovskite film measured up to 20 micrometers.
The findings, published in the journal Nano Letters, indicate that solar cells could be made thicker without harming their efficiency, said Xuan Gao, associate professor of physics and author of the paper.
"A thicker cell can absorb more light," he said, "potentially yielding a better solar cell."
Efficiency built in
Solar power researchers believe perovskite films hold great promise. In less than five years, films made with the crystalline structure have surpassed 20 percent efficiency in converting sunlight to electricity, a mark that took decades to reach with silicon-based solar cells used today.
In this research, Gao's lab performed spatially scanned photocurrent image measurements on films made in the lab of Case Western Reserve chemistry professor Clemens Burda.
Perovskite minerals found in nature are oxides of certain metals, but Burda's lab made organo-metallic films with the same crystalline structure using methyl ammonium lead tri-iodide (CH3NH3PBI3), a three-dimensional lead halide surrounded by small organic methyl ammonium molecules that hold the lattice structure together.
"The question has been, 'How are these solar cells so efficient? If we would know, we could further improve perovskite solar cells" Burda said. "People thought it could be due to unusually long electron transport, and we directly measured it."
Diffusion length is the distance an electron or its opposite, called a hole, travels from generation until it recombines or is extracted as electric current. The distance is the same as transport length when no electric field (which usually increases the distance traveled) is applied.
The labs made repeated measurements by focusing a tiny laser spot on films 8 millimeters square by 300 nanometers thick. The films were made stable by coating the perovskite with a layer of the polymer parylene.
The light generates electrons and holes and the photocurrent, or stream of electrons, is recorded between the electrodes positioned about 120 microns away from each other while the film is scanned along two perpendicular directions. The scanning yields a two-dimensional spatial map of carrier diffusion and transport characteristics.
The measurements showed diffusion length averaged about 10 microns. In some cases, the length reached 20 microns, showing the functional area of the film is at least 20 microns long, the researchers said.
In some materials, grain boundaries decrease conductivity, but imaging showed that these interfaces between grains in the film exerted no influence on electron travel. Gao and Burda say this may be because grains in the film are well aligned, causing no impedance or other detrimental effects on electrons or holes.
Burda and Gao are now seeking federal funds to use the microscopy technique to determine whether different grain sizes, orientations, halide perovskite compositions, film thicknesses and more change the film's properties, to further accelerate research in the field.
Kevin Mayhood | EurekAlert!
Borophene shines alone as 2-D plasmonic material
21.11.2017 | Rice University
Quantum dots amplify light with electrical pumping
21.11.2017 | DOE/Los Alamos National Laboratory
The WHO reports an estimated 429,000 malaria deaths each year. The disease mostly affects tropical and subtropical regions and in particular the African continent. The Fraunhofer Institute for Silicate Research ISC teamed up with the Fraunhofer Institute for Molecular Biology and Applied Ecology IME and the Institute of Tropical Medicine at the University of Tübingen for a new test method to detect malaria parasites in blood. The idea of the research project “NanoFRET” is to develop a highly sensitive and reliable rapid diagnostic test so that patient treatment can begin as early as possible.
Malaria is caused by parasites transmitted by mosquito bite. The most dangerous form of malaria is malaria tropica. Left untreated, it is fatal in most cases....
The formation of stars in distant galaxies is still largely unexplored. For the first time, astron-omers at the University of Geneva have now been able to closely observe a star system six billion light-years away. In doing so, they are confirming earlier simulations made by the University of Zurich. One special effect is made possible by the multiple reflections of images that run through the cosmos like a snake.
Today, astronomers have a pretty accurate idea of how stars were formed in the recent cosmic past. But do these laws also apply to older galaxies? For around a...
Just because someone is smart and well-motivated doesn't mean he or she can learn the visual skills needed to excel at tasks like matching fingerprints, interpreting medical X-rays, keeping track of aircraft on radar displays or forensic face matching.
That is the implication of a new study which shows for the first time that there is a broad range of differences in people's visual ability and that these...
Computer Tomography (CT) is a standard procedure in hospitals, but so far, the technology has not been suitable for imaging extremely small objects. In PNAS, a team from the Technical University of Munich (TUM) describes a Nano-CT device that creates three-dimensional x-ray images at resolutions up to 100 nanometers. The first test application: Together with colleagues from the University of Kassel and Helmholtz-Zentrum Geesthacht the researchers analyzed the locomotory system of a velvet worm.
During a CT analysis, the object under investigation is x-rayed and a detector measures the respective amount of radiation absorbed from various angles....
The quantum world is fragile; error correction codes are needed to protect the information stored in a quantum object from the deteriorating effects of noise. Quantum physicists in Innsbruck have developed a protocol to pass quantum information between differently encoded building blocks of a future quantum computer, such as processors and memories. Scientists may use this protocol in the future to build a data bus for quantum computers. The researchers have published their work in the journal Nature Communications.
Future quantum computers will be able to solve problems where conventional computers fail today. We are still far away from any large-scale implementation,...
15.11.2017 | Event News
15.11.2017 | Event News
30.10.2017 | Event News
21.11.2017 | Physics and Astronomy
21.11.2017 | Physics and Astronomy
21.11.2017 | Life Sciences