Physical Science and Engineering | Physical Science & Engineering

Tuesday, September 07, 2021

Helping semiconductors find a cooler way to relax

Extending the lifetime of hot electrons could help make solar cells more efficient.

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Bandgap engineering can improve the performance of optoelectronic devices that aim to harness the energy of “hot” electrons, research from KAUST shows.

Semiconductors have a signature property known as the bandgap that essentially is a range of forbidden electron energies. Electrons with an energy above the top of the bandgap are free to move through the material, whereas those below the bottom of the bandgap are not, and no electrons with an energy in between can exist in the material.

This simple idea defines a semiconductor’s optical properties. The energy from any absorbed light is transferred to the material’s electrons. Most of the free electrons in a semiconductor have an energy near the top of the bandgap. But the absorption of photons with an energy much larger than the bandgap creates, in turn, much higher energy electrons that are also called “hot electrons.”

Understanding the process by which these so-called hot electrons relax to an energy nearer the top of the bandgap is essential for understanding the operation of light-harvesting devices. For example, the efficiency of a solar cell is reduced if this huge excess energy is lost as heat. “However, it was extremely difficult, if not impossible, to sufficiently utilize these hot electrons in real light-conversion applications due to their very short lifetimes,” says material scientist Omar Mohammed.

Mohammed and his colleagues used interface and bandgap engineering to delay hot carriers (electrons and holes) relaxation and to significantly increase their lifetimes.

The team studied a semiconductor known as a lead halide perovskite. They designed and fabricated architectures made up of multiple quantum wells: a thin layer of semiconductor sandwiched between light-absorber layers of larger bandgap material. They compared the optical properties of structures in which the wells were all the same thickness and asymmetric structures in which the well widths varied. They used a technique called femtosecond (1 femtosecond = 10-15 seconds) transient absorption spectroscopy combined with theoretical calculations to determine the timescale of the hot electron relaxation.

They found the cooling rate had a strong dependence on the quantum well thickness in the asymmetric sample and that the relaxation of the hot carriers in the asymmetric multiple quantum wells was 12.5-fold slower compared to that of the symmetric multiple quantum wells.

“This new finding provides a unique strategy on how to significantly slow down the hot carriers cooling in semiconducting materials for their better utilization in solar cells applications,” says postdoc Partha Maity.

- KAUST Discovery

Wednesday, September 01, 2021

Sieving ions with a polymer membrane

Polymer membranes with precisely controlled, uniform and very small pore structure could bring ion-sieving capacity to many applications.

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Ion-sieving polymer membranes can perform with exquisite precision by gaining unprecedented control over pore size and uniformity within the membranes, KAUST researchers have shown.

The biological nervous system works by selective transport of electrically charged particles called ions through cell membranes. If manufactured membranes were able to achieve a similar ion selectivity, it could transform many technologies, including water purification, mineral extraction and energy storage.

“Achieving precise ion separation at the sub-nanometer level by polymer membranes is very challenging” says chemical engineer Zhiping Lai.

Ions are formed when atoms or molecules lose or gain electrons, therefore gaining a positive or negative electrical charge. Those derived from single atoms, such as sodium (Na+), lithium (Li+) or chloride (Cl-) ions, are smaller than 1 nanometer (10-9 meters) across. The researchers used the known sizes of ions to conduct simulation studies, which helped identify suitable monomers that could act as the molecular units needed to link into a conjugated microporous polymer (CMP) membrane structure.

They then used a process called electropolymerization to make their polymer membranes. This process uses a cyclical electric current to control the precise structure that forms when the 1,3,5-tris(N-carbazolyl) benzene monomer molecules link together.

“It was challenging to determine the resulting pore size and level of porosity because of the asymmetric membrane structure,” says Lai, adding, “to overcome this issue we had to make hundreds of samples.”

The tiny size and nature of the pores prevented their analysis from using many common structural determination methods, but a solution was found in gas physisorption, which studies the interaction of a material with gases.

In tests using solutions containing a range of ions, the membranes proved to have a selective ion-sieving performance superior to almost all other reported membranes.

“This demonstrates that our innovative polymer membranes have great potential in many energy- and environment-related ion separation processes,” says Zongyao Zhou. Removing ions from seawater to produce drinkable water is one obvious possibility. Rechargeable batteries and other energy storage systems also depend on controlling the movement of ions.

Zhou says that the next target for the team is to explore the potential of the membranes to be used in a variety of innovative chemical sensors. Many chemicals of environmental or medical interest consist of ions. Membranes that selectively allow ions of interest to pass through the membranes could be used in a new generation of more precise and flexible sensor technology.

- KAUST Discovery

Sunday, August 29, 2021

Electric gains in battery performance

Batteries made from earth-abundant metals show great potential for grid-scale energy storage.

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A high-performance version of the zinc-ion battery will enable station energy storage that promises to be cheaper, safer and more environmentally friendly than lithium-ion batteries.

Until now, zinc-ion batteries have been severely hampered by their rapid degradation during use. Now, a KAUST team has developed a new electrolyte and electrode combination that improved several aspects of zinc-ion battery performance, particularly the stability over multiple charge and discharge cycles.

Stationary banks of batteries connected to renewable energy sources, such as solar installations or wind farms, could be key to the transition from the current fossil-fuel powered electricity grid. Unlike batteries for mobile applications, such as laptops or electric cars where battery size and weight are key, stationary batteries can be relatively large and heavy, raising the possibility that alternative rechargeable battery technologies to lithium-ion could be deployed.

Batteries centered on a water-based solution of zinc ions have shown great potential for stationary storage in terms of their high capacity, low cost and lack of toxicity. “But issues including low cycling stability and fast self-discharge have prohibited practical applications of aqueous zinc-ion batteries,” says Yunpei Zhu, a research scientist in Husam Alshareef's group, who led the work. “Both of these issues are related to the design of electrolytes and electrode materials,” he adds. The water-based electrolyte caused problems at both electrodes of the battery, causing damaging side reactions at the anode and rapid dissolution of the cathode.

To combat these issues, the team developed a water electrolyte with a very high salt concentration. The more salt ions present in the solution to bind surrounding water molecules, the fewer free water molecules are available to damage the electrodes.

As zinc salts typically show limited solubility in water, the team added sodium to produce a highly concentrated electrolyte of zinc perchlorate and sodium perchlorate. “We found this combination delivers very high solubility to suppress water activity, without lowering the key attributes of zinc-ion batteries, including their high ionic conductivity, safety or environmental friendliness,” Zhu says.

In addition to the novel electrolyte, the team developed a new nanofiber-based cathode material for batteries. “The nanofiber morphology enhances ion diffusion, which ensures faster charge and discharge rates of the aqueous Zn-ion batteries,” Alshareef says. In testing, the team saw almost no capacity decay over 2,000 charge cycles. “This combination of electrode and electrolyte potentially solves the shortcomings of conventional aqueous Zn-ion batteries,” Alshareef says.

- KAUST Discovery

Thursday, August 26, 2021

Keeping it random

A nano-scale memristor is shown to have superior stability for random number generation as an integral part of secure data transmission.

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Using atomically thin two-dimensional films, researchers have developed a nano-scale random number generator with enhanced long-term stability and reduced power consumption.

Random number generation is a critical underlying function of secure data transmission, used to encrypt and secure data and generate one-time passwords. To generate a random number, a circuit detects the random noise produced by a physical component, such as the thermal noise from diodes or resistors. However, the power consumption of these elements can be significant in highly integrated low-power applications, and the degree of randomness can degrade over time due to a change in the device’s physical and electronic characteristics.

Mario Lanza, who joined KAUST in 2020, saw an opportunity for nano-scale devices called memristors to be used as more reliable true random number generators (TRNGs).

“Memristors are meta/insulator/metal nanocells based on two-dimensional materials that have fast operation speed, low energy consumption and very long endurance and data retention time, as well as being very easy and cheap to fabricate,” says Lanza. “For this reason, memristors are being intensively explored for applications such as high-density electronic memories. They are also particularly useful for encryption systems because they can produce fluctuating electronic signals with an extraordinarily high degree of randomness.”

Memristors, short for “memory resistors,” are tiny passive electronic components that have a resistance that can be switched between two states by applying electrical stress. However, they also produce a type of electrical noise called random telegraphic noise (RTN), sometimes called popcorn noise for the sound it would make if connected to a speaker, which turns out to be perfect for random number generation. The challenge for Lanza’s team was to design and fabricate a memristor device that has stable RTN over time.

“The main challenge was that the atomic structure of the resistive thin film degrades over time, which causes the RTN signal to disappear,” says Lanza. “In our devices, we used two-dimensional multilayer hexagonal boron nitride, which is a two-dimensional material that has a very stable atomic structure and is immune to this effect.”

The team fabricated hundreds of devices using industry-compatible methods and characterized them using a range of techniques including a randomness test involving the generation of one-time passwords.

“A key aspect of our work was the use of fabrication processes that are compatible with industry, which facilitates integration in commercial products,” Lanza says. “We also presented yield and variability information for hundreds of devices; it was a tremendous effort, but it gives more reliability to our study.”

-KAUST Discovery

Sunday, August 22, 2021

Solar gains stack up

Clever contacts enhance perovskite/silicon tandem solar-cell performance.

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Flipping the structure of perovskite/silicon tandem solar cells can lead to dramatic gains in solar-cell performance, KAUST scientists have discovered. To enable the flipped cells’ high power-conversion efficiency, the team developed new materials to extract an electrical charge from the silicon side while maximizing the amount of incoming light reaching the perovskite layer. The development opens multiple new avenues for further gains in tandem cell performance.

Tandem solar cells, which combine two light-harvesting materials in one device, potentially generate significantly more electricity than a conventional single-junction silicon solar cell for a small additional cost. “Silicon-based tandem solar cells have been identified as the ultimate long-term technology to further enhance solar panel power conversion efficiencies,” says electrical engineer Stefaan De Wolf, who led the study. “We, therefore, focus on this technology.”

When semiconductors such as silicon and perovskites absorb light, electrons are collected at “n-type contacts” on one side of the material, and positively charged holes are collected at “p-type contacts” on the opposite side, explains Erkan Aydin, a research scientist in De Wolf’s group. In single-junction thin-film perovskite solar cells, the p-side usually faces uppermost, an arrangement known as the n-i-p structure. These cells recently exceeded efficiencies of 25 percent. “However, perovskite/silicon tandem solar cells are usually in opposite p-i-n polarity,” Aydin says.

Inspired by recent gains in n-i-p single-junction perovskite solar-cell performance, De Wolf, Aydin, and their colleagues revisited n-i-p tandem cells in a bid to overcome the limitations that had previously hampered tandem solar cells with this structure. The key hurdles had been the lack of an efficient n-side contact material for efficient charge extraction and a sufficiently transparent p-side contact material to maximize the incoming sunlight reaching the light-harvesting layers in the device.

The team applied a transparent contact material, called spiro-TTB, and improved the performance of the n-side layers of the device by developing a niobium oxide-based contact material that was chemical bridged to the perovskite layer. “Overall, we allowed more light to be captured in the n-i-p tandem solar cell, and we converted the absorbed sunlight to energy more effectively thanks to our newly developed contacting materials,” Aydin says. The team’s improved device achieved a power-conversion efficiency exceeding 27 percent, a significant improvement over the 21.8 percent efficiency of the previous best n-i-p tandem cell.

“Our agenda-setting results enable a new tandem device platform, from which we expect rapid progress to be achieved in the field of perovskite/silicon tandems in the n-i-p configuration,” De Wolf says.

- KAUST Discovery

Sunday, August 22, 2021

Congratulations Professor Derya!

Associate Professor Derya Baran has been selected C&EN's Talented 12.

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KAUST Associate Professor of Material Science and Engineering Derya Baran has been selected C&EN's Talented 12 by Chemical and Engineering News (C&EN's).

"Each year, C&EN introduces you to a dozen dazzling young scientists with our Talented 12 issue. The researchers we feature are always engaged in world-changing work."

Professor Deryawas recognized for her work on developing "organic photovoltaics and other energy conversion technologies that could broaden access to solar power."

Research

Professor Baran's research interests lie in the area of solution processable organic/hybrid soft materials for electronic devices. Such soft semiconductor materials possess a viable platform for printed, large area, stretchable and wearable electronics that can be used as solar cells, smart windows, OFETs, thermoelectrics, sensors and bio-electronics.

Professor Baran is particularly interested in interface engineering for organic/hybrid solar cells, transparent solar cells for building integrated photovoltaics and stability/degradation studies for long lifetime organic solar cells. She has led projects on i) conjugated polymers for electrochromic devices; ii) non-fullerene acceptors for organic solar cells; iii) multi-component and multi-layered solar cell devices; and iv) understanding the correlation between recombination and nano-morphology in solution processed solar cells.

Professor Baran aims to expand the applications of solution processable organic/hybrid semiconductors and to explore their limits in organic/hybrid thermoelectric devices and bio-electronics in the future.

Wednesday, August 18, 2021

Minding the gaps to boost perovskite performance

A simple holistic solution plugs the performance-sapping defects that hamper new alternative solar materials.

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A multifunctional molecule that plugs various atomic-scale defects in perovskite solar materials can significantly boost the longevity and electrical output of this promising solar technology, KAUST researchers have shown.

Perovskites are an alternative solar-cell material to silicon. They typically comprise a combination of negatively charged iodine or bromine ions and positively charged lead and organic ions. Adjusting the ion combination can generate wide bandgap perovskites that capture the solar wavelengths that silicon does not efficiently absorb. This suggests the two materials could be combined in tandem cells to capture even more energy from the Sun.

However, perovskites inevitably feature defects, such as where a particular ion did not slot into place during fabrication, leaving a gap in the structure. These reactive sites can contribute to rapid performance decline — unless they can be plugged. “Defect passivation is very important for improving the long-term stability of perovskite solar cells,” says Furkan Isikgor, a researcher in Stefaan De Wolf’s group.

Defect sites in perovskites can be positively or negatively charged depending on which ion is missing, but chemical additives for defect passivation can typically plug one type of defect or the other. But a molecule called phenformin hydrochloride (PhenHCl) overcomes this problem, Isikgor, De Wolf and their colleagues have shown.

“PhenHCl works very well owing to its multifunctional structure,” Isikgor says. The molecule consists of an electro-positive ammonium head group that can plug negatively charged defects and an electro-negative amine and imine group body to plug positive gaps.

The researchers showed that PhenHCl boosted the power conversion efficiencies (PCEs) of wide bandgap perovskites from 16.7 percent in untreated cells up to 20.5 percent in treated cells. “Moreover, PhenHCl passivation improves the PCE of textured perovskite/silicon tandem solar cells from 25.4 percent to 27.4 percent,” Isikgor says.

PhenHCl passivation also significantly improved the stability of the perovskite solar cell. “Under continuous light soaking, the PhenHCl-passivated device retained 80 percent of its post-burn-in efficiency for around 106 hours of operation,” Isikgor says. The untreated device retained 80 percent of its post-burn-in efficiency for only 5 hours.

“Our simple holistic defect-passivation strategy has drastically improved the semiconductor quality of solution-processed perovskites,” says De Wolf. “Passivating the different types of defects that may be present in perovskites with a single molecule is highly attractive for industry,” he adds. The next step will be to integrate PhenHCl passivation with scaled-up perovskite production. “Simultaneously, we are working on further improvements in device performance and stability through extensive outdoor testing,” De Wolf adds.

-KAUST Discovery

Thursday, August 05, 2021

Slow slips offer insights into earthquakes

Studying slow, gentle and previously undetectable events will help reveal the origins of damaging earthquakes

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Significant earthquakes, from gentle shaking to devastating tremors, are hazards caused by a sudden release of stress that has built up in geological faults. More subtle events called slow slips are attracting increasing attention as nonshaky versions of the dramatic seismic fractures of the largest earthquakes.

A database of information on previous slow slips to develop a model of the mechanisms of these geological events has now been compiled by KAUST and University of Geneva researchers, in collaboration with colleagues at ETH Zürich in Switzerland, GFZ in Germany and the University of Bologna in Italy.

Slow slips, also known as silent earthquakes, are fractures of the Earth's crust that propagate very slowly without producing seismic waves. They typically proceed over a few weeks or months, and their duration can range from less than a day to more than a year.

"Slow slips alone pose no threat since they don't generate shaking and therefore cause no damage or disruption to properties, but they cause disturbances that can trigger regular earthquakes nearby," says Luigi Passarelli, formerly of KAUST and now in ETH Zürich.

The potential of triggering regular earthquakes is one reason why researchers are keen to learn more about slow slip events. Another reason is that they come with smaller tremors or sometimes a flurry of small earthquakes, a so-called seismic swarm, in a process not yet fully understood.

Slow slip events occur in many regions where tectonic plates collide and slide but especially in subduction zones around Japan, New Zealand and North and Central America. They also occur near volcanoes such as Mt. Etna in Italy and Kilauea in Hawaii.

"We have compiled the first systematic catalogue of the aseismic and seismic characteristics of slow slip events to better understand the physical mechanisms that generate both," says Passarelli. He adds that this was a big challenge as the data are not all consistently archived or readily available.

The results have allowed the team to assess the correlation between the characteristics of each slow event and the triggered tremor activity, as size and duration, and the likely immediate and possible long-term consequences. This information can now be used to improve a model to predict the changes and hazards associated with specific types of event.

"We have made a step toward an improved understanding of the processes that control slow slips and seismic tremors, although there is more to investigate," says Passarelli. The researchers hope that their database and modeling can be developed further to build a better understanding of those complex processes.

- KAUST Discovery

Wednesday, July 14, 2021

Sweet spot for membrane thickness offers sustainable separations

Perfecting the dimensions of chemical separation membranes is a step toward a sustainable chemical industry.

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Super-thin carbon molecular sieve (CMS) membranes may not be best for separating industrially important chemical mixtures. However, ensuring the CMS film thickness is just right could enable more energy-efficient purification of chemical products, KAUST researchers have shown.

CMS membranes, as their name suggests, can purify mixtures of liquids or gases by permitting only certain molecules to pass through their subnanometer-sized pores. Currently, the chemical industry mainly uses heat-based processes such as distillation to separate product mixtures, but these processes consume about 10 percent of global energy output. "This situation is highly unsustainable," says Wojciech Ogieglo, a research scientist at KAUST. "We believe that a good portion of these energy-intensive separations could be replaced by much more environmentally friendly membrane separations."

CMS membranes are created by depositing a layer of carbon-rich polymers on to a suitable support, then applying heat to convert the polymer into a microporous CMS film. "CMS materials display by far the best performance for a wide variety of highly energy-intensive membrane-based gas separation applications," says group leader Ingo Pinnau.

"These materials are also particularly chemically robust," notes Ogieglo. "They are promising for situations such as plastics production or greenhouse gas capture because they perform reliably even in very harsh chemical environments and at high temperatures," he says.

One aspect of CMS membrane research is to optimize the CMS film thickness to minimize the energy required to separate a chemical mixture. "Intuitively, one could think that the thinner the membrane, the better," Ogieglo says. A thinner CMS layer would be expected to pose the least transport resistance to molecules passing through its pores. However, the team found that when they created sub-50 nanometer CMS films, the CMS layer was very compact with low microporosity. "Such extremely thin films turn out to pose much more transport resistance than expected," Ogieglo says. Thicker 300 nanometer CMS films had significantly higher microporosity, the team showed.

"We believe that there must be a sweet spot in the thickness range — not too thin, not too thick — where the membrane performance is optimal," Ogieglo says. "We are currently trying to find out where this sweet spot lies for different types of membrane materials."

"The results will feed into the team's wider efforts to create scalable, industry-ready CMS separation membranes," Pinnau says. "We are currently scaling up the production of CMS composite membranes to test their performance and long-term stability in membrane modules," he adds.

- KAUST Discovery

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