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February 10, 2017
CWRU researchers directly measure how perovskite solar films efficiently convert light to power
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.
Schematic of scanning photocurrent imaging microscopy of halide perovskite film (side view).
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.
This article was originally published Jan. 10.
January 09, 2017
Benjamin Monreal is joining the department this spring and will be co-teaching PHYS 302/318. He is an experimental physicist working in particle/astrophysics and was formerly at the University of California, Santa Barbara. You can learn more about Prof. Monreal at http://www.physics.ucsb.edu/people/benjamin-monreal.
Emanuela Dimastrogiovanni is joining the department this spring as a Visiting Assistant Professor, teaching PHYS 310. Prof. Dimastrogiovanni is a theoretician working in particle astrophysics.
January 09, 2017
RESEARCH NEWS – “It’s a medical breakthrough story that begins with a long line.”
This Device Could Revolutionize How Malaria Is Detected Around the World -
December 05, 2016
Physics Reading Day Potluck
Potluck 2016 - click
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December 16, 2016 – Rockefeller, 2nd floor
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August 19, 2016
Thesis Defense – Ms. Johanna Nagy
Optical Development for the SPIDER Balloon-Borne CMB Polarimeter
The generation of a stochastic background of gravitational waves is a key prediction of inflation. At large angular scales, these gravitational waves imprint a B-mode polarization pattern in the Cosmic Microwave Background, providing a new window into the physics of the early universe and helping to constrain and distinguish between inflationary models. SPIDER is a balloon-borne telescope uniquely optimized to search for the inflationary B-mode signature. Over the course of two Antarctic flights, SPIDER will make polarization maps over 10% of the sky in three frequency bands with degree-scale angular resolution. The SPIDER optics are designed to take advantage of the low atmospheric loading and large sky coverage accessible from the ballooning platform through a combination of high optical efficiency, low in-band loading, and strong sidelobe rejection. These goals are applied to the design, fabrication, and testing of many optical components including forebaffles, windows, and half-wave plate polarization modulators. A review of instrument performance is presented as a validation of the optical system, including the polarization angle calibration and preliminary data analysis from the first flight in January 2015. Preparations for a second flight in December 2017 are currently underway.
August 23, 2016
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May 12, 2016
A Life In Physics
Check out the article about Professor Robert Brown and his group and how many of his students feel about the work he is involved in.
April 21, 2016
Andrew Matas – Foundations of Mass Gravity
Thursday, May 19, 2016
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April 21, 2016
Cuyahoga Valley House Concert – May 15, 2016
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For our series, Ed invites different guest
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March 16, 2016
First-Year Grad Students – Qualifying Exam
The dates for the May 2016 Qualifying exam have been set. The exam will be in the week of May 23 – 27. Part One of the exam will be on Tuesday May 24 at 2 p.m. – 5 p.m. Part Two of the exam is on Wednesday May 25 at 2 p.m. to 5 p.m. The venue for the exam will be determined and announced a bit closer to the date. Results should be available by the end of Friday May 27.
Please contact Harsh Mathur (email@example.com) if you have any questions.
March 16, 2016
Churna Bahadur Bhandari – Thesis Defense
First-Principles Study of Electronic and Vibrational Properties of Bulk and Monolayer V2O5
Thursday, March 24, 2016
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