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Tuning electrode surfaces to optimize solar fuel production

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Tuning electrode surfaces to optimize solar fuel production

Scientists have demonstrated that modifying the topmost layer of atoms on the surface of electrodes can have a remarkable impact on the activity of solar water splitting. As they reported in Nature Energy on Feb. 18, bismuth vanadate electrodes with more bismuth on the surface (relative to vanadium) generate higher amounts of electrical current when they absorb energy from sunlight.

This photocurrent drives the chemical reactions that split water into oxygen and hydrogen. The hydrogen can be stored for later use as a clean fuel. Producing only water when it recombines with oxygen to generate electricity in fuel cells, hydrogen could help us achieve a clean and sustainable energy future.

“The surface termination modifies the system’s interfacial energetics, or how the top layer interacts with the bulk,” said co-corresponding author Mingzhao Liu, a staff scientist in the Interface Science and Catalysis Group of the Center for Functional Nanomaterials (CFN), a U.S. Department of Energy (DOE) Office of Science User Facility at Brookhaven National Laboratory. “A bismuth-terminated surface exhibits a photocurrent that is 50-percent higher than a vanadium-terminated one.”

“”Studying the effects of surface modification with an atomic-level understanding of their origins is extremely challenging, and it requires tightly integrated experimental and theoretical investigations,” said co-corresponding author Giulia Galli from the University of Chicago and DOE’s Argonne National Laboratory.

“It also requires the preparation of high-quality samples with well-defined surfaces and methods to probe the surfaces independently from the bulk,” added co-corresponding author Kyoung-Shin Choi from the University of Wisconsin-Madison.

Choi and Galli, experimental and theoretical leaders in the field of solar fuels, respectively, have been collaborating for several years to design and optimize photoelectrodes for producing solar fuels. Recently, they set out to design strategies to illuminate the effects of electrode surface composition, and, as CFN users, they teamed up with Liu.

“The combination of expertise from the Choi Group in photoelectrochemistry, the Galli Group in theory and computation, and the CFN in material synthesis and characterization was vital to the study’s success,” commented Liu.

Bismuth vanadate is a promising electrode material for solar water splitting because it strongly absorbs sunlight across a range of wavelengths and remains relatively stable in water. Over the past few years, Liu has perfected a method for precisely growing single-crystalline thin films of this material. High-energy laser pulses strike the surface of polycrystalline bismuth vanadate inside a vacuum chamber. The heat from the laser causes the atoms to evaporate and land on the surface of a base material (substrate) to form a thin film.

“To see how different surface terminations affect photoelectrochemical activity, you need to be able to prepare crystalline electrodes with the same orientation and bulk composition,” explained co-author Chenyu Zhou, a graduate researcher from Stony Brook University working with Liu. “You want to compare apples to apples.”

As grown, bismuth vanadate has an almost one-to-one ratio of bismuth to vanadium on the surface, with slightly more vanadium. To create a bismuth-rich surface, the scientists placed one sample in a solution of sodium hydroxide, a strong base.

“Vanadium atoms have a high tendency to be stripped from the surface by this basic solution,” said first author Dongho Lee, a graduate researcher working with Choi. “We optimized the base concentration and sample immersion time to remove only the surface vanadium atoms.”

To confirm that this chemical treatment changed the composition of the top surface layer, the scientists turned to low-energy ion scattering spectroscopy (LEIS) and scanning tunneling microscopy (STM) at the CFN.

In LEIS, electrically charged atoms with low energy – in this case, helium – are directed at the sample. When the helium ions hit the sample surface, they become scattered in a characteristic pattern depending on which atoms are present at the very top. According to the team’s LEIS analysis, the treated surface contained almost entirely bismuth, with an 80-to-20 ratio of bismuth to vanadium.

“Other techniques such as x-ray photoelectron spectroscopy can also tell you what atoms are on the surface, but the signals come from several layers of the surface,” explained Liu. “That’s why LEIS was so critical in this study – it allowed us to probe only the first layer of surface atoms.”

In STM, an electrically conductive tip is scanned very close to the sample surface while the tunneling current flowing between the tip and sample is measured. By combining these measurements, scientists can map the electron density – how electrons are arranged in space – of surface atoms. Comparing the STM images before and after treatment, the team found a clear difference in the patterns of atomic arrangements corresponding to vanadium- and bismuth-rich surfaces, respectively.

“Combining STM and LEIS allowed us to identify the atomic structure and chemical elements on the topmost surface layer of this photoelectrode material,” said co-author Xiao Tong, a staff scientist in the CFN Interface Science and Catalysis Group and manager of the multiprobe surface analysis system used in the experiments. “These experiments demonstrate the power of this system for exploring surface-dominated structure-property relationships in fundamental research applications.”

Simulated STM images based on surface structural models derived from first-principle calculations (those based on the fundamental laws of physics) closely matched the experimental results.

“Our first-principle calculations provided a wealth of information, including the electronic properties of the surface and the exact positions of the atoms,” said co-author and Galli Group postdoctoral fellow Wennie Wang. “This information was critical to interpreting the experimental results.”

After proving that the chemical treatment successfully altered the first layer of atoms, the team compared the light-induced electrochemical behavior of the treated and nontreated samples.

“Our experimental and computational results both indicated that the bismuth-rich surfaces lead to more favorable surface energetics and improved photoelectrochemical properties for water splitting,” said Choi. “Moreover, these surfaces pushed the photovoltage to a higher value.”

Many times, particles of light (photons) do not provide enough energy for water splitting, so an external voltage is needed to help perform the chemistry. From an energy-efficiency perspective, you want to apply as little additional electricity as possible.

“When bismuth vanadate absorbs light, it generates electrons and electron vacancies called holes,” said Liu. “Both of these charge carriers need to have enough energy to do the necessary chemistry for the water-splitting reaction: holes to oxidize water into oxygen gas, and electrons to reduce water into hydrogen gas. While the holes have more than enough energy, the electrons don’t. What we found is that the bismuth-terminated surface lifts the electrons to higher energy, making the reaction easier.”

Because holes can easily recombine with electrons instead of being transferred to water, the team did additional experiments to understand the direct effect of surface terminations on photoelectrochemical properties. They measured the photocurrent of both samples for sulfite oxidation. Sulfite, a compound of sulfur and oxygen, is a “hole scavenger,” meaning it quickly accepts holes before they have a chance to recombine with electrons. In these experiments, the bismuth-terminated surfaces also increased the amount of generated photocurrent.

“It’s important that electrode surfaces perform this chemistry as quickly as possible,” said Liu. “Next, we’ll be exploring how co-catalysts applied on top of the bismuth-rich surfaces can help expedite the delivery of holes to water.”

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Innovative approach to perovskite solar cells achieves 24.5% efficiency

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Innovative approach to perovskite solar cells achieves 24.5% efficiency


Innovative approach to perovskite solar cells achieves 24.5% efficiency

by Simon Mansfield

Sydney, Australia (SPX) Mar 28, 2024






In groundbreaking research published in Nano Energy, a team led by Prof. CHEN Chong at the Hefei Institutes of Physical Science, part of the Chinese Academy of Sciences, has significantly improved the performance of perovskite solar cells (PSCs). By integrating inorganic nano-material tin sulfoxide (SnSO) as a dopant, they have boosted the photoelectric conversion efficiency (PCE) of PSCs to an impressive 24.5%.

Traditional methods of enhancing the charge transport in the critical hole transport layer (HTL) of PSCs involve the use of lithium trifluoromethanesulfonyl imide (Li-TFSI) to facilitate the oxidation of the HTL material spiro-OMeTAD. However, this method suffers from low doping efficiency and can leave excess Li-TFSI in the spiro-OMeTAD film, reducing its compactness and long-term conductivity. Additionally, the oxidation process typically requires 10-24 hours to achieve the desired electrical conductivity and work function.



The HFIPS team’s innovation lies in their development of a rapid and replicable method to control the oxidation of nanomaterials, using SnSO nanomaterial to pre-oxidize spiro-OMeTAD in precursor solutions. This novel approach not only enhances conductivity but also optimizes the energy level position of the HTL, culminating in a high PCE of 24.5%.



One of the key advantages of the SnSO-regulated spiro-OMeTAD HTL is its pinhole-free, uniform, and smooth morphology, which maintains its performance and physical integrity even under challenging conditions of high temperature and humidity. Additionally, the oxidation process facilitated by this method is significantly faster, taking only a few hours- a crucial factor in improving the commercial production efficiency of PSCs.



Prof. CHEN Chong highlighted the importance of this breakthrough, stating, “Also, the oxidation process only takes a few hours, which is good for improving the commercial preparation efficiency of PSCs.” This advancement not only marks a significant leap in the efficiency and stability of PSCs but also holds substantial implications for their commercial viability.



Research Report:A nanomaterial-regulated oxidation of hole transporting layer for highly stable and efficient perovskite solar cells


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Revolutionary technique boosts flexible solar cell efficiency to record high

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Revolutionary technique boosts flexible solar cell efficiency to record high


Revolutionary technique boosts flexible solar cell efficiency to record high

by Simon Mansfield

Sydney, Australia (SPX) Mar 28, 2024






Researchers at Tsinghua University have made a significant breakthrough in the efficiency of flexible solar cells, leveraging a novel fabrication technique to set a new efficiency record. This advancement addresses the longstanding challenge of the lower energy conversion efficiency in flexible solar cells compared to their rigid counterparts, offering promising implications for aerospace and flexible electronics applications.

Flexible perovskite solar cells (FPSCs), despite their potential, have historically lagged in efficiency due to the polyethylene terephthalate (PET)-based flexible substrate’s inherent softness and inhomogeneity. This limitation, coupled with durability issues arising from the substrate’s susceptibility to water and oxygen infiltration, has hindered the practical deployment of FPSCs.



The team from the State Key Laboratory of Power System Operation and Control at Tsinghua University, alongside collaborators from the Center for Excellence in Nanoscience at the National Center for Nanoscience and Technology in Beijing, introduced a chemical bath deposition (CBD) technique. This method facilitates the deposition of tin oxide (SnO2) on flexible substrates without the need for strong acids, which are detrimental to such substrates. Tin oxide is essential for the FPSCs as it acts as an electron transport layer, crucial for the cells’ power conversion efficiency.



Associate Professor Chenyi Yi, a senior author of the study, explained, “Our method utilizes SnSO4 tin sulfate instead of SnCl2 tin chloride, making it suitable for acid-sensitive flexible substrates. This approach not only enhances the efficiency of FPSCs but also their durability, with a new power conversion efficiency benchmark set at 25.09%, certified at 24.90%.”



The novel fabrication technique also contributes to the FPSCs’ stability, as demonstrated by the cells maintaining 90% of their initial efficiency after being bent 10,000 times. The researchers noted an improved high-temperature stability in SnSO4-based FPSCs over those made with SnCl2, pointing towards the dual benefits of efficiency and durability enhancements.



The research signifies a leap towards industrial-scale production of high-efficiency FPSCs, with potential applications ranging from wearable technology and portable electronics to aerospace power sources and large-scale renewable energy solutions. The team’s findings, supported by Ningyu Ren, Liguo Tan, Minghao Li, Junjie Zhou, Yiran Ye, Boxin Jiao, and Liming Ding, mark a pivotal step in transitioning FPSCs from laboratory to commercial use.



Research Report:25% – Efficiency flexible perovskite solar cells via controllable growth of SnO2


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KAUST advances in perovskite-silicon tandem cells

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KAUST advances in perovskite-silicon tandem cells


KAUST advances in perovskite-silicon tandem cells

by Sophie Jenkins

London, UK (SPX) Mar 28, 2024






In 2009, researchers introduced perovskite-based solar cells, highlighting the potential of methylammonium lead bromide and methylammonium lead iodide-known as lead halide perovskites-for photovoltaic research. These materials, notable for their excellent light-absorbing properties, marked the beginning of an innovative direction in solar energy generation. Since then, the efficiency of perovskite solar cells has significantly increased, indicating a future where they are used alongside traditional silicon in solar panels.

Erkan Aydin, Stefaan De Wolf, and their team at King Abdullah University of Science and Technology (KAUST) have explored how this tandem technology could transition from experimental stages to commercial production. Perovskites are lauded for their low-temperature production process and their flexibility in application, offering a lighter, more adaptable, and potentially cost-effective alternative to silicon-based panels.



Combining perovskite with silicon in a single solar cell leverages the strengths of both materials, enhancing sunlight utilization and reducing losses that aren’t converted into electrical energy. “The synergy between perovskite and silicon technologies in tandem cells captures a broader spectrum of sunlight, minimizing energy loss and significantly boosting efficiency,” Aydin notes.



However, Aydin and his colleagues acknowledge challenges in scaling tandem solar-cell fabrication for the marketplace. For instance, the process of depositing perovskite on silicon surfaces is complicated by the silicon’s texture. Traditional laboratory methods like spin coating are not feasible for large-scale production due to their inefficiency and material wastage. Alternatives such as slot-die coating and physical vapor deposition present their own set of advantages and challenges.



Moreover, the durability of perovskite components under environmental stressors such as moisture, heat, and light remains a critical concern. Aydin emphasizes the need for focused research to enhance the reliability and lifespan of perovskite/silicon tandem cells, especially in harsh conditions.



Although tandem modules have already been demonstrated in proof-of-concept stages, the timeline for their market readiness is uncertain. Nonetheless, the successful development of efficient, commercial-grade perovskite/silicon solar cells is essential for meeting global energy demands sustainably.



Research Report:Pathways toward commercial perovskite/silicon tandem photovoltaics


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