Nav: Home

A new way to image solar cells in 3-D

November 15, 2016

Next-generation solar cells made of super-thin films of semiconducting material hold promise because they're relatively inexpensive and flexible enough to be applied just about anywhere.

Researchers are working to dramatically increase the efficiency at which thin-film solar cells convert sunlight to electricity. But it's a tough challenge, partly because a solar cell's subsurface realm--where much of the energy-conversion action happens--is inaccessible to real-time, nondestructive imaging. It's difficult to improve processes you can't see.

Now, scientists from the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a way to use optical microscopy to map thin-film solar cells in 3-D as they absorb photons.

The method, reported Nov. 15 in the journal Advanced Materials, was developed at the Molecular Foundry, a DOE Office of Science user facility located at Berkeley Lab. It images optoelectronic dynamics in materials at the micron scale, or much thinner than the diameter of a human hair. This is small enough to see individual grain boundaries, substrate interfaces, and other internal obstacles that can trap excited electrons and prevent them from reaching an electrode, which saps a solar cell's efficiency.

So far, scientists have used the technique to better understand why adding a specific chemical to solar cells made of cadmium telluride (CdTe)--the most common thin-film material--improves the solar cells' performance.

"To make big gains in photovoltaic efficiency, we need to see what's happening throughout a working photovoltaic material at the micron scale, both on the surface and below, and our new approach allows us to do that," says Edward Barnard, a principal scientific engineering associate at the Molecular Foundry. He led the effort with James Schuck, the director of the Imaging and Manipulation of Nanostructures facility at the Molecular Foundry.

The imaging method is born out of a collaboration between Molecular Foundry scientists and Foundry users from PLANT PV Inc., an Alameda, California-based company. While fabricating new solar cell materials at the Molecular Foundry, the team found that standard optical techniques couldn't image the inner-workings of the materials, so they developed the new technique to obtain this view. Next, scientists from the National Renewable Energy Laboratory came to the Molecular Foundry and used the new method to study CdTe solar cells.

To develop the approach, the scientists modified a technique called two-photon microscopy (which is used by biologists to see inside thick samples such as living tissue) so that it can be applied to bulk semiconductor materials.

The method uses a highly focused laser beam of infrared photons that penetrate inside the photovoltaic material. When two low-energy photons converge at the same pinpoint, there's enough energy to excite electrons. These electrons can be tracked to see how long they last in their excited state, with long-lifetime electrons appearing as bright spots in microscopy images. In a solar cell, long-lifetime electrons are more likely to reach an electrode.

In addition, the laser beam can be systematically repositioned throughout a test-sized solar cell, creating a 3-D map of a solar cell's entire optoelectronic dynamics.

The method has already shed light on the benefits of treating CdTe solar cells with cadmium chloride, which is often added during the fabrication process.

Scientists know cadmium chloride improves the efficiency of CdTe solar cells, but its effect on excited electrons at the micron scale is not well understood. Studies have shown that the chlorine ions tend to pile up at grain boundaries, but how this changes the lifetime of excited electrons is unclear.

Thanks to the new imaging technique, the researchers discovered the cadmium chloride treatment increases the lifetime of excited electrons at the grain boundaries, as well as within the grains themselves. This is easily seen in 3-D images of CdTe solar cells with and without the treatment. The treated solar cell "lights up" much more uniformly throughout the material, both in the grains and the spaces in between.

"Scientists have known that cadmium chloride passivation improves the lifetime of electrons in CdTe cells, but now we've mapped at the micron scale where this improvement occurs," says Barnard.

The new imaging technique could help scientists make more informed decisions about improving a host of thin-film solar cell materials in addition to CdTe, such as perovskite and organic compounds.

"Researchers trying to push photovoltaic efficiency could use our technique to see if their strategies are working at the microscale, which will help them design better test-scale solar cells--and eventually full-sized solar cells for rooftops and other real-world applications," he says.
-end-
The research was supported by the Department of Energy's Office of Science and by a SunShot Initiative award from the Office of Energy Efficiency and Renewable Energy.

The U.S. Department of Energy SunShot Initiative is a collaborative national effort that aggressively drives innovation to make solar energy fully cost-competitive with traditional energy sources before the end of the decade. Through SunShot, the Energy Department supports efforts by private companies, universities, and national laboratories to drive down the cost of solar electricity to $0.06 per kilowatt-hour. Learn more at energy.gov/sunshot.

Lawrence Berkeley National Laboratory addresses the world's most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab's scientific expertise has been recognized with 13 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy's Office of Science. For more, visit http://www.lbl.gov.

DOE's Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

DOE/Lawrence Berkeley National Laboratory

Related Solar Cells Articles:

Next gen solar cells perform better when there's a camera around
A literal ''trick of the light'' can detect imperfections in next-gen solar cells, boosting their efficiency to match that of existing silicon-based versions, researchers have found.
On the trail of organic solar cells' efficiency
Scientists at TU Dresden and Hasselt University in Belgium investigated the physical causes that limit the efficiency of novel solar cells based on organic molecular materials.
Exciting tweaks for organic solar cells
A molecular tweak has improved organic solar cell performance, bringing us closer to cheaper, efficient, and more easily manufactured photovoltaics.
For cheaper solar cells, thinner really is better
Researchers at MIT and at the National Renewable Energy Laboratory (NREL) have outlined a pathway to slashing costs further, this time by slimming down the silicon cells themselves.
Flexible thinking on silicon solar cells
Combining silicon with a highly elastic polymer backing produces solar cells that have record-breaking stretchability and high efficiency.
Perovskite solar cells get an upgrade
Rice University materials scientists find inorganic compounds quench defects in perovskite-based solar cells and expand their tolerance of light, humidity and heat.
Can solar technology kill cancer cells?
Michigan State University scientists have revealed a new way to detect and attack cancer cells using technology traditionally reserved for solar power.
Solar cells with new interfaces
Scientists from NUST MISIS (Russia) and University of Rome Tor Vergata found out that a microscopic quantity of two-dimensional titanium carbide called MXene significantly improves collection of electrical charges in a perovskite solar cell, increasing the final efficiency above 20%.
Welcome indoors, solar cells
Swedish and Chinese scientists have developed organic solar cells optimised to convert ambient indoor light to electricity.
Mapping the energetic landscape of solar cells
A new spectroscopic method now makes it possible to measure and visualize the energetic landscape inside solar cells based on organic materials.
More Solar Cells News and Solar Cells Current Events

Trending Science News

Current Coronavirus (COVID-19) News

Top Science Podcasts

We have hand picked the top science podcasts of 2020.
Now Playing: TED Radio Hour

Listen Again: Meditations on Loneliness
Original broadcast date: April 24, 2020. We're a social species now living in isolation. But loneliness was a problem well before this era of social distancing. This hour, TED speakers explore how we can live and make peace with loneliness. Guests on the show include author and illustrator Jonny Sun, psychologist Susan Pinker, architect Grace Kim, and writer Suleika Jaouad.
Now Playing: Science for the People

#565 The Great Wide Indoors
We're all spending a bit more time indoors this summer than we probably figured. But did you ever stop to think about why the places we live and work as designed the way they are? And how they could be designed better? We're talking with Emily Anthes about her new book "The Great Indoors: The Surprising Science of how Buildings Shape our Behavior, Health and Happiness".
Now Playing: Radiolab

The Third. A TED Talk.
Jad gives a TED talk about his life as a journalist and how Radiolab has evolved over the years. Here's how TED described it:How do you end a story? Host of Radiolab Jad Abumrad tells how his search for an answer led him home to the mountains of Tennessee, where he met an unexpected teacher: Dolly Parton.Jad Nicholas Abumrad is a Lebanese-American radio host, composer and producer. He is the founder of the syndicated public radio program Radiolab, which is broadcast on over 600 radio stations nationwide and is downloaded more than 120 million times a year as a podcast. He also created More Perfect, a podcast that tells the stories behind the Supreme Court's most famous decisions. And most recently, Dolly Parton's America, a nine-episode podcast exploring the life and times of the iconic country music star. Abumrad has received three Peabody Awards and was named a MacArthur Fellow in 2011.