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.
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

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

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

DOE/Lawrence Berkeley National Laboratory

Related Solar Cells Articles:

Solar cells more efficient thanks to new material standing on edge
Researchers from Lund University in Sweden and from Fudan University in China have successfully designed a new structural organization using the promising solar cell material perovskite.
Printable solar cells just got a little closer
A University of Toronto Engineering innovation could make printing solar cells as easy and inexpensive as printing a newspaper.
A big nano boost for solar cells
Solar cells convert light into electricity. While the sun is one source of light, the burning of natural resources like oil and natural gas can also be harnessed.
Game changer for organic solar cells
Researchers develop a simple processing technique that could cut the cost of organic photovoltaics and wearable electronics.
Physics, photosynthesis and solar cells
A University of California, Riverside assistant professor has combined photosynthesis and physics to make a key discovery that could help make solar cells more efficient.
Throwing new light on printed organic solar cells
Researchers at the University of Surrey have achieved record power conversion efficiencies for large area organic solar cells.
A new way to image solar cells in 3-D
Berkeley Lab scientists have developed a way to use optical microscopy to map thin-film solar cells in 3-D as they absorb photons.
Toward 'greener,' inexpensive solar cells
Solar panels are proliferating across the globe to help reduce the world's dependency on fossil fuels.
A new technique opens up advanced solar cells
Using a novel spectroscopic technique, EPFL scientists have made a much-needed breakthrough in cutting-edge photovoltaics.
OU physicists developing new systems for next generation solar cells
University of Oklahoma physicists are developing novel technologies with the potential to impact utility-scale energy generation, increase global energy capacity and reduce dependence on fossil fuels by producing a new generation of high efficiency solar cells.

Related Solar Cells Reading:

PHYSICS OF SOLAR CELLS, THE (Properties of Semiconductor Materials)
by Jenny Nelson (Author)

The Physics of Solar Cells: Perovskites, Organics, and Photovoltaic Fundamentals
by Juan Bisquert (Author)

Physics of Solar Cells: From Basic Principles to Advanced Concepts (No Longer Used)
by Peter Würfel (Author), Uli Würfel (Author)

Principles of Solar Cells, LEDs and Related Devices: The Role of the PN Junction
by Adrian Kitai (Author)

Solar Cells: Operating Principles, Technology, and System Applications (Prentice-Hall series in solid state physical electronics)
by Martin A. Green (Author)

Build A Solar Hydrogen Fuel Cell System
by Phillip Hurley (Author)

Materials Concepts For Solar Cells (Energy Futures)
by Thomas Dittrich (Author)

The Complete Guide About Solar Energy: A Practical Beginners Guide To Solar Panels, Cells and Electricity
by Russel Hobbs (Author)

Perovskite Solar Cells Principle, Materials and Devices (Series on Chemistry, Energy and the Environment)
by Eric Wei-Guang Diau (Author), Eric Wei-Guang Diau (Editor), Peter Chao-Yu Chen (Editor)

Solar Cell Array Design Handbook: The Principles and Technology of Photovoltaic Energy Conversion
by Hans S. Rauschenbach (Author)

Best Science Podcasts 2018

We have hand picked the best science podcasts for 2018. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

We're told if the economy is growing, and if we keep producing, that's a good thing. But at what cost? This hour, TED speakers explore circular systems that regenerate and re-use what we already have. Guests include economist Kate Raworth, environmental activist Tristram Stuart, landscape architect Kate Orff, entrepreneur David Katz, and graphic designer Jessi Arrington.
Now Playing: Science for the People

#504 The Art of Logic
How can mathematics help us have better arguments? This week we spend the hour with "The Art of Logic in an Illogical World" author, mathematician Eugenia Cheng, as she makes her case that the logic of mathematics can combine with emotional resonance to allow us to have better debates and arguments. Along the way we learn a lot about rigorous logic using arguments you're probably having every day, while also learning a lot about our own underlying beliefs and assumptions.