Terahertz spectroscopy helps improve performance of solar cells

https://optics.org/news/11/1/89

Researchers at NIST make the most sensitive measurements to date of silicon conductivity.

A project at the US National Institute of Standards and Technology (NIST) has made the most sensitive measurements so far of how quickly electric charge moves in silicon, a gauge of its performance as a semiconductor.

In particular, NIST has found a way to measure the charge-carrier mobility at extremely low levels of electric charge, in a way that does not require physical contact with the silicon sample. The new results may suggest ways to improve semiconductor materials and their applications, including solar cells and high-speed cellular networks, and were reported in Optics Express.

The findings build on previous NIST research into the use of time-resolved terahertz spectroscopy (TRTS) to assess other semiconductor materials, and represent the first comparison between the spectroscopic technique and more conventional contact-based methods to measure charge-carrier behavior.

The technique involves using a pulse of visible, ultraviolet or infrared light to first excite a resonant transition in the semiconductor and create a controlled density of charge carriers, effectively "photo-doping" the material.

Coupling a probe pulse of THz illumination into the photoexcited region then allows the motion of those charge carriers to be measured, as the probe irradiation is integrated over all the conduction pathways inside the interaction volume of the material, and THz light can penetrate even opaque materials such as silicon semiconductor samples. How much of that light is absorbed by the sample then depends on how many charge carriers are freely moving, allowing the charge mobility to be calculated.

In addition, the low-intensity pulsed light used in this study simulates the kind of low-intensity light a solar cell would receive from the Sun, so a greater understanding of charge-carrier behavior could lead to real-world benefits for power generation.

"The light we use in this experiment is similar to the intensity of light that a solar cell might absorb on a sunny spring day," said Tim Magnanelli of NIST. "So the work could potentially find applications in improving solar-cell efficiency."

New discoveries about silicon

A key breakthrough was the use of two-photon excitation for the photo-doping process. Earlier work using single-photon excitation had been limited by the small penetration depth achieved into the sample, usually around 10 to 100 nanometers. That meant that surface variations were an inevitable complication, negating the advantage of a contactless measurement technique.

But a two-photon technique can penetrate deeper, and more effectively complement the ability of THz light to completely penetrate a sample. The project successfully used the TRTS method to study a number of un-doped, n-, and p-doped Si wafer samples about half a millimeter thick.

"Combining two-photon excitation with a terahertz probe serves as a more accurate method to extract absolute and integrated carrier mobility, by minimizing of the impact of surface defects and providing an explicit depth of photoexcitation," commented the team in its published paper.

Having successfully lowered the threshold for measuring free holes and electrons, the NIST researchers discovered that carrier mobility eventually plateaus once the density of careers drops to a certain level. Although this effect had been noted before, the new project was able to determine that this plateau occurs at a lower carrier density than previously thought.

"An unexpected result like this shows us things we didn’t know about silicon before," said NIST's Ted Heilweil. "And though this is fundamental science, learning more about how silicon works could help device makers use it more effectively, for example if some semiconductors can be made to work better at lower doping levels than currently used."

To test whether these findings only held for silicon, the project tested gallium arsenide (GaAs) as well, and found that there too the carrier mobility continues to increase with lower charge carrier density, to a point about 100 times lower than the conventionally accepted limit.

"This observation for both Si and GaAs suggests that the effect is not uniquely due to silicon's indirect bandgap structure, and may be the product of differing selection rules or effective masses for populated carriers," commented the team.

Future work may now involve applying different photodoping techniques to samples, or experimenting with thicker samples. "Using the two-photon method on thicker samples may produce even lower carrier densities, that we can then probe with the THz pulses," said Heilweil.

Nanoparticle Breakthrough Could Capture Unseen Light for Solar Energy Conversion

Scientists demonstrate how organic dyes work as antennas to help harness, convert light

News Release Glenn Roberts Jr. 

http://newscenter.lbl.gov/2018/04/23/nanoparticle-breakthrough-solar-energy-conversion/

ANIMATION: Energy transfers from a ytterbium atom (blue), which absorbs near-infrared light, to an erbium atom (red). The erbium atom then releases visible, green light. A study led by researchers at Berkeley Lab’s Molecular Foundry found a way to enhance this process, known as “upconversion,” by coating nanoparticles with dyes. Scientists hope to use this process to develop solar cells that capture and convert previously missed sunlight into usable energy. (Credit: Andrew Mueller)

An international team of scientists has demonstrated a breakthrough in the design and function of nanoparticles that could make solar panels more efficient by converting light usually missed by solar cells into usable energy.

The team, led by scientists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), demonstrated how coating tiny particles with organic dyes greatly enhances their ability to capture near-infrared light and to reemit the light in the visible light spectrum, which could also be useful for biological imaging.

Once they understood the mechanism that enables the dyes on nanoparticles to function as antennas to gather a broad range of light, they successfully reengineered the nanoparticles to further amplify the particles’ light-converting properties. Their study was published online April 23 in Nature Photonics.

“These organic dyes capture broad swaths of near-infrared light,” said Bruce Cohen, a scientist at Berkeley Lab’s Molecular Foundry who helped to lead the study along with Molecular Foundry scientists P. James Schuck (now at Columbia University), and Emory Chan. The Molecular Foundry is a nanoscience research center.

“Since the near-infrared wavelengths of light are often unused in solar technologies that focus on visible light,” Cohen added, “and these dye-sensitized nanoparticles efficiently convert near-infrared light to visible light, they raise the possibility of capturing a good portion of the solar spectrum that otherwise goes to waste, and integrating it into existing solar technologies.”

Researchers found that the dye itself amplifies the brightness of the reemitted light about 33,000-fold, and its interaction with the nanoparticles increases its efficiency in converting light by about 100 times.

An erbium atom (red) in a nanocrystal emits visible, green light via a process known as upconversion that could lead to the development of improved solar cells that capture some previously missed solar energy. Scientists discovered that coating the particles with dyes (blue and purple molecules at right) can greatly enhance this light-converting property. (Credit: Berkeley Lab)

Cohen, Schuck, and Chan had worked for about a decade to design, fabricate, and study the upconverting nanoparticles (UCNPs) used in this study. UCNPs absorb near-infrared light and efficiently convert it to visible light, an unusual property owing to combinations of lanthanide metal ions in the nanocrystals. A 2012 study suggested that dyes on the UCNPs’ surface dramatically enhances the particles’ light-converting properties, but the mechanism remained a mystery.

“There was a lot of excitement and then a lot of confusion,” Cohen said. “It had us scratching our heads.”

Although many researchers had tried to reproduce the study in the following years, “Few people could get the published procedure to work,” added Chan. “The dyes appeared to degrade almost immediately upon exposure to light, and nobody knew exactly how the dyes were interacting with the nanoparticle surface.”

The unique mix of expertise and capabilities at the Molecular Foundry, which included theoretical work and a mix of experiments, chemistry know-how, and well-honed synthetic techniques, made the latest study possible, he noted. “It’s one of those projects that would be difficult to do anywhere else.”

Experiments led by David Garfield, a UC Berkeley Ph.D. student, and Nicholas Borys, a Molecular Foundry project scientist, showed a symbiotic effect between the dye and the lanthanide metals in the nanoparticles.

The proximity of the dyes to the lanthanides in the particles enhances the presence of a dye state known as a “triplet,” which then transfers its energy to the lanthanides more efficiently. The triplet state allowed a more efficient conversion of multiple infrared units of light, known as photons, into single photons of visible light.

The studies showed that a match in the measurements of the dye’s light emission and the particles’ light absorption confirmed the presence of this triplet state, and helped inform the scientists about what was at work.

“The peaks (in dye emission and UCNP absorption) matched almost exactly,” Cohen said.

They then found that by increasing the concentration of lanthanide metals in the nanoparticles, from 22 percent to 52 percent, they could increase this triplet effect to improve the nanoparticles’ light-converting properties.

“The metals are promoting dyes to their triplet states, which helps to explain both the efficiency of energy transfer and the instability of the dyes, since triplets tend to degrade in air,” Cohen said.

The nanoparticles, which measure about 12 nanometers, or billionths of meters, across, could potentially be applied to the surface of solar cells to help them capture more light to convert into electricity, Schuck said.

“The dyes act as molecular-scale solar concentrators, funneling energy from near-infrared photons into the nanoparticles,” Schuck said. Meanwhile, the particles themselves are largely transparent to visible light, so they would allow other usable light to pass through, he noted.

Another potential use is to introduce the nanoparticles into cells to help label cell components for optical microscopy studies. They could be used for deep-tissue imaging, for example, or in optogenetics – a field that uses light to control cell activity.

There are some roadblocks for researchers to overcome to realize these applications, Cohen said, as they are currently unstable and were studied in a nitrogen environment to avoid exposure to air.

More R&D is needed to evaluate possible protective coatings for the particles, such as different polymers that serve to encapsulate the particles. “We have even better designs in mind going forward,” he said.

The Molecular Foundry is a DOE Office of Science User Facility.

Researchers from UC Berkeley, the Korea Research Institute of Chemical Technology, Sungkyunkwan University in South Korea, and the Kavli Energy NanoScience Institute at UC Berkeley also participated in this study. This work was supported by the DOE Office of Science; the National Science Foundation; the China Scholarship Council; and the Ministry of Science, Information and Communication Technology, and Future Planning of South Korea.

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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 www.lbl.gov.

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