Tag Archives: #solarcell

A Transparent Phosphate Crystal Immobilizes Lead in Perovskite Solar Cells (Material Science)

Although a very promising solution for capturing solar energy, perovskite solar cells contain lead, which is toxic to the environment and a serious health hazard. EPFL scientists have now found a very elegant and efficient solution by adding a transparent phosphate salt that doesn’t interfere with light-conversion efficiency while preventing lead from seeping into the soil in cases of solar panel failure.

“The solar energy-to-electricity conversion of perovskite solar cells is unbelievably high, around 25%, which is now approaching the performance of the best silicon solar cells,” says Professor László Forró at EPFL’s School of Basic Sciences. “But their central element is lead, which is a poison; if the solar panel fails, it can wash out into the soil, get into the food chain, and cause serious diseases.”

The problem is that in most of the halide perovskites lead can dissolve in water. This water solubility and solubility in other solvents is actually a great advantage, as it makes building perovskite solar panels simpler and inexpensive – another perk along with their performance. But the water solubility of lead can become a real environmental and health hazard when the panel breaks or gets wet, e.g. when it rains.

So the lead must be captured before it gets to the soil, and it must be possible to recycle it. This issue has drawn much and intensive research because it is the main obstacle for regulatory authorities approving the production of perovskite solar cells on a large, commercial scale. However, attempts to synthesize non-water-soluble and lead-free perovskites have yielded poor performance.

Now, Forró’s group has come up with an elegant and efficient solution, which involves using a transparent phosphate salt, which does not block solar light, so it doesn’t affect performance. And if the solar panel fails, the phosphate salt immediately reacts with lead to produce a water-insoluble compound that cannot leach out to the soil, and which can be recycled. The work is published in ACS Applied Materials & Interfaces.

“A few years ago, we discovered that cheap and transparent phosphate salt crystals, like those in soil fertilizers, can be incorporated into various parts of the sandwich-like lead halide perovskite devices, like photodetectors, LEDs or solar cells,” says Endre Horváth, the study’s first author. “These salts instantaneously react with lead ions in the presence of water, and precipitate them into extremely non-water-soluble lead phosphates.”

“The ‘fail-safe’ chemistry keeps lead ions from leaching out and can render perovskite devices safer to use in the environment or close to humans,” says Márton Kollár, the chemist behind the growth of perovskite crystals.

“We show that this approach can be used to build functional photodetectors, and we suggest that the broad community of researchers and R&D centers working on various devices like solar cells and light-emitting diodes implements it in their respective prototypes,” adds Pavao Andričević, who characterized the sensitive photodetectors.

Forró concludes: “This is an extremely important study – I would say, a central one – for large-scale commercialization of perovskite-based solar cells.”

Funding

ERC Advanced Grant “Picoprop”

References

Endre Horváth, Márton Kollár, Pavao Andričević, Lidia Rossi, Xavier Mettan, László Forró. Fighting Health Hazards in Lead Halide Perovskite Optoelectronic Devices with Transparent Phosphate Salts.  ACS Applied Materials & Interfaces 15 July 2021. DOI: 10.1021/acsami.0c21137

Image and video: A transparent phosphate crystal, which incorporated into solar cells, can instantaneously immobilize the lead in case of failure and block its leaching out from the device. © EPFL


Provided by EPFL

Outstanding Organic Solar Cells’ Performance Achieved By Using New Technology (Chemistry)

Organic solar elements with the self-assembling molecular-thin layer (SAM) of hole-transporting material, the technology, which was used in producing a record-breaking tandem solar cell, achieved 18.4 power conversion efficiency. The invention of Lithuanian chemists working at Kaunas University of Technology (KTU), commercialized by several global companies proved versatile and applicable to different solar technologies.

Organic solar cells are made of common organic elements such as carbon, hydrogen, nitrogen, fluorine, oxygen, and sulphur. Their raw materials are cheap, abundant, and can be easily recycled. Although the organic photovoltaic (OPV) elements are lighter, more flexible and cheaper to produce, their efficiency still falls behind that of other photovoltaic technologies, including silicone, perovskite and tandem solar cells. And yet, this aspect may soon change.

Solar cells developed in Lithuania and Saudi Arabia

At the end of 2018, a group of Lithuanian chemists from Kaunas University of Technology synthesised a material, which self-assembles into a molecule-thick layer, aka monolayer, can cover a variety of surfaces and function as a hole-transporting layer in a solar element. Until recently, the self-assembling monolayers (SAMs) have been used to produce record-breaking perovskite/silicon and CIGS/perovskite tandem solar cells. However, the technology also proved very efficient – reaching nearly record-breaking 18.4 power conversion – when used in an organic solar cell, produced by the group of researchers headed by Professor Thomas Anthopoulos at the KAUST University in Saudi Arabia.

Dr Artiom Magomedov, c0-author of the invention © KTU

“We made some modifications in the material used in SAM formation to tailor it for organic solar elements. However, our technology, offering a breakthrough approach towards photovoltaic elements’ production remains the same: the surface is dipped into a solution and a molecule-thick semiconductor layer is formed. The technology is cheap, efficient and versatile”, says Dr Artiom Magomedov of KTU Faculty of Chemical Technology, the co-author of the invention.

Organic solar cell achieved 18.4 efficiency

As the materials synthesised by KTU chemists are now commercialised and freely available in the market for research groups and companies all over the world, the discovery continues to advance the development of photovoltaic technologies.

“Last year, we noticed an article published by the researchers from KAUST, where they described the achieved very high efficiency of an organic solar cell while using our SAMs. We contacted the scientists and offered to collaborate in enhancing the capacities of the material. Due to the pandemic restrictions, all cooperation was remote – we sent the synthesised materials by post and our colleagues in Saudi Arabia built the organic solar cells and measured their properties”, explains Dr Magomedov.

The organic solar cell using Br-2PACz molecule-thin coating as a hole-transporting layer achieved a power conversion efficiency of 18.4 per cent, which is currently among the highest in OPV technologies. Moreover, the electrode constructed was chemically stable, and after removal of the SAM, it could be recycled and reused to construct fresh highly-performing OPV cells.

All solar technologies will find their niches

The researchers emphasise that the use of similar SAMs could be potentially extended in other applications including light-emitting diodes, photodetectors or organic transistors. According to Dr Magomedov, all different solar technologies, which are currently being developed, will find their niches in the market – as OPV cells are lighter, can be made transparent and flexible, they can be used for charging drones, household electronics, for indoor photovoltaics. Currently, no OPV elements are mass-produced.

Professor Vytautas Getautis © KTU

“The semiconducting properties of organic elements are lower than those of non-organic materials. Therefore, the achieved efficiency results are very impressive for everyone working in the field. After the publication, a Swedish company “Dyenamo” has already obtained the licence to produce our materials tailored for the organic solar elements, as they see the potential of this technology”, says Professor Vytautas Getautis, the Head of the KTU research group behind the invention.

Featured image: Chemists from Kaunas University of Technology synthesised a material, which self-assembles into a molecule-thick layer, aka monolayer, can cover a variety of surfaces and function as a hole-transporting layer in a solar element. © KTU


Reference: Lin, Y., Magomedov, A., Firdaus, Y., Kaltsas, D., El-Labban, A., Faber, H., Naphade, D.R., Yengel, E., Zheng, X., Yarali, E., Chaturvedi, N., Loganathan, K., Gkeka, D., AlShammari, S.H., Bakr, O.M., Laquai, F., Tsetseris, L., Getautis, V. & Anthopoulos, T.D. 18.4% Organic solar cells using a high ionization energy self‐assembled monolayer as hole extraction interlayer, ChemSusChem 2021, 14, 1– 1. Article accessible here.


Provided by KTU

Scientists Develop Transparent Electrode That Boosts Solar Cell Efficiency (Material Science)

Developing new ultrathin metal electrodes has allowed researchers to create semitransparent perovskite solar cells that are highly efficient and can be coupled with traditional silicon cells to greatly boost the performance of both devices, said an international team of scientists. The research represents a step toward developing completely transparent solar cells.

“Transparent solar cells could someday find a place on windows in homes and office buildings, generating electricity from sunlight that would otherwise be wasted,” said Kai Wang, assistant research professor of materials science and engineering at Penn State and co-author on the study. “This is a big step — we finally succeeded in making efficient, semitransparent solar cells.”

Traditional solar cells are made from silicon, but scientists believe they are approaching the limits of the technology in the march to create ever more efficient solar cells. Perovskite cells offer a promising alternative and stacking them on top of the traditional cells can create more efficient tandem devices, the scientists said.

“We’ve shown we can make electrodes from a very thin, almost few atomic layers of gold,” said Shashank Priya, associate vice president for research and professor of materials science and engineering at Penn State. “The thin gold layer has high electrical conductivity and at the same time it doesn’t interfere with the cell’s ability to absorb sunlight.”

The perovskite solar cell that the team developed achieved 19.8% efficiency, a record for a semitransparent cell. And when combined with a traditional silicon solar cell, the tandem device achieved 28.3% efficiency, up from 23.3% from the silicon cell alone. The scientists reported their findings in the journal Nano Energy.

“A 5% improvement in efficiency is giant,” Priya said. “This basically means you are converting about 50 watts more sunlight for every square meter of solar cell material. Solar farms can consist of thousands of modules, so that adds up to a lot of electricity, and that’s a big breakthrough.”

In previous research, ultrathin gold film showed promise as a transparent electrode in perovskite solar cells, but issues in creating a uniform layer resulted in poor conductivity, the scientists said.

The team found that chromium used as a seed layer allowed the gold to form on top in a continuous ultrathin layer with good conductive properties.

“Normally, if you grow a thin layer of something like gold, the nanoparticles will couple together and gather like small islands,” said Dong Yang, assistant research professor of materials science and engineering at Penn State. “Chromium has a large surface energy that provides a good place for the gold to grow on top of, and it actually allows the gold to form a continuous thin film.”

Perovskite solar cells are composed of five layers and other materials tested as transparent electrodes damaged or degraded layers of the cells. The scientists said solar cells made with the gold electrodes are stable and maintain high efficiencies over time in laboratory tests.

“This breakthrough in the design of tandem cell architecture based on a transparent electrode offers an efficient route toward the transition to perovskite and tandem solar cells,” said Yang.

Also contributing to this research from Penn State were Tao Ye and Jungjin Yoon, postdoctoral scholars; and Yuchen Hou, a doctoral student.

Xiaorong Zhang, Shaanxi Normal University, China; Shengzhong Liu, Chinese Academy of Sciences; Congcong Wu, Hubei University, China; and Mohan Sanghadasa, U.S. Army Combat Capabilities Development Command, also contributed to the research.

The Office of Naval Research, the Army Rapid Innovation Fund, and the Air Force Office of Scientific Research provided funding for this research.

Featured image: Scientists found using a chromium seed layer allowed them to grow ultrathin gold film that serves as a transparent electrode with good conductivity for perovskite solar cells. Image: Penn State


Reference: Dong Yang, Shashank Priya et al., “28.3%-efficiency perovskite/silicon tandem solar cell by optimal transparent electrode for high efficient semitransparent top cell”, Nano energy. Vol. 84, June 2021. https://doi.org/10.1016/j.nanoen.2021.105934


Provided by Penn State University

Researchers Identify the Defect That Limits Solar-cell Performance (Engineering)

Hydrogen in hybrid perovskites: Researchers identify the defect that limits solar-cell performance

Researchers in the materials department in UC Santa Barbara’s College of Engineering have uncovered a major cause of limitations to efficiency in a new generation of solar cells.

Various possible defects in the lattice of what are known as hybrid perovskites had previously been considered as the potential cause of such limitations, but it was assumed that the organic molecules (the components responsible for the “hybrid” moniker) would remain intact. Cutting-edge computations have now revealed that missing hydrogen atoms in these molecules can cause massive efficiency losses. The findings are published in a paper titled “Minimizing hydrogen vacancies to enable highly efficient hybrid perovskites,” in the April 29 issue of the journal Nature Materials.

The remarkable photovoltaic performance of hybrid perovskites has created a great deal of excitement, given their potential to advance solar-cell technology. “Hybrid” refers to the embedding of organic molecules in an inorganic perovskite lattice, which has a crystal structure similar to that of the perovskite mineral (calcium titanium oxide). The materials exhibit power-conversion efficiencies rivaling that of silicon, but are much cheaper to produce. Defects in the perovskite crystalline lattice, however, are known to create unwanted energy dissipation in the form of heat, which limits efficiency.

Chris Van de Walle © UC Santa Barbara

A number of research teams have been studying such defects, among them the group of UCSB materials professor Chris Van de Walle, which recently achieved a breakthrough by discovering a detrimental defect in a place no one had looked before: on the organic molecule.

“Methylammonium lead iodide is the prototypical hybrid perovskite,” explained Xie Zhang, lead researcher on the project. “We found that it is surprisingly easy to break one of the bonds and remove a hydrogen atom on the methylammonium molecule. The resulting ‘hydrogen vacancy’ then acts as a sink for the electric charges that move through the crystal after being generated by light falling on the solar cell. When these charges get caught at the vacancy, they can no longer do useful work, such as charging a battery or powering a motor, hence the loss in efficiency.”

The research was enabled by advanced computational techniques developed by the Van de Walle group. Such state-of-the-art calculations provide detailed information about the quantum-mechanical behavior of electrons in the material. Mark Turiansky, a senior graduate student in Van de Walle’s group who was involved in the research, helped build sophisticated approaches for turning this information into quantitative values for rates of charge carrier trapping.

“Our group has created powerful methods for determining which processes cause efficiency loss,” Turiansky said, “and it is gratifying to see the approach provide such valuable insights for an important class of materials.”

“The computations act as a theoretical microscope that allows us to peer into the material with much higher resolution than can be achieved experimentally,” Van de Walle explained. “They also form a basis for rational materials design. Through trial and error, it has been found that perovskites in which the methylammonium molecule is replaced by formamidinium exhibit better performance. We are now able to attribute this improvement to the fact that hydrogen defects form less readily in the formamidinium compound.

“This insight provides a clear rationale for the empirically established wisdom that formamidinium is essential for realizing high-efficiency solar cells,” he added. “Based on these fundamental insights, the scientists who fabricate the materials can develop strategies to suppress the harmful defects, boosting additional efficiency enhancements in solar cells.”

Funding for this research was provided by the Department of Energy’s Office of Science and Office of Basic Energy Sciences. The computations were performed at the National Energy Research Scientific Computing Center.

Featured image: A hydrogen vacancy (the black spot left of center) created by removing hydrogen from a methylammonium molecule, traps carriers in the prototypical hybrid perovskite, mehtylammonium lead iodide CH3NH3Pbl3 © ILLUSTRATION BY XIE ZHANG


Reference: Zhang, X., Shen, JX., Turiansky, M.E. et al. Minimizing hydrogen vacancies to enable highly efficient hybrid perovskites. Nat. Mater. (2021). https://doi.org/10.1038/s41563-021-00986-5


Provided by UC Santa Barbara

The Perfect Recipe For Efficient Perovskite Solar Cells (Chemistry)

A long-cherished dream of materials researchers is a solar cell that converts sunlight into electrical energy as efficiently as silicon, but that can be easily and inexpensively fabricated from abundant materials. Scientists at the Helmholtz-Zentrum Berlin have now come a step closer to achieving this. They have improved a process for vertically depositing a solution made from an inexpensive perovskite solute onto a moving substrate below. Not only have they discovered the crucial role played by one of the solvents used, but they have also taken a closer look at the aging and storage properties of the solution.

Solar cells made of crystalline silicon still account for the lion’s share of roof installations and solar farms. But other technologies have long since become established as well – such as those that convert sunlight into electrical energy through use of extremely thin layers of solar-cell material deposited upon a substrate. The perovskite solar cells that Prof. Eva Unger and her team at the Helmholtz-Zentrum Berlin (HZB) are researching belong to this group. “These are the best solar cells to date that can be made using a 2D ink”, the researcher explains. “And now their efficiencies are approaching those for cells made of crystalline silicon.”

Developing scalable methods

Many methods have been developed and used to fabricate small test cells in the laboratory, where they can be studied and improved. But industrial-scale fabrication is still a long way off. Unger knows from her own experience: “Unfortunately, processes that are optimised for fabricating small surface areas cannot always be scaled up.” In other words: Not everything that works perfectly in the lab also necessarily works economically on the factory floor. “That’s why we are taking the next step and developing scalable methods. This means our team is focussing on processes for coating larger surfaces.” At the Hybrid Silicon Perovskite Research, Integration & Novel Technologies (HySPRINT) Innovation Lab, an infrastructure for collaboration between HZB and industry, the team is concentrating on processes that have already proven their importance in industry to start with.

“We have experimented here with slot-die coating”, she explains. In this process, the “ink”, as the thin liquid solution of perovskite precursor, solvent, and additive is known in the trade, flows from a slit-shaped nozzle and falls like a curtain onto the glass substrate being conveyed below that will later become a solar cell. After application, crystallisation begins. An ultra-thin layer of a semiconducting perovskite structure grows that gives the material group its name and the solar cell its capabilities. Unger, together with her team members doctoral student Jinzhao Li and Dr. Janardan Dagar, have now discovered that the exact amount of an organic solvent called dimethyl sulfoxide (DMSO) in the material ink is critical for this process. Unger uses it as an additive because it has an amazing effect on the ink. “DMSO induces crystallisation nuclei for the perovskite”, says the researcher. Crystallisation nuclei usually are tiny grains that help jump-start a crystal and promote its growth. “During X-ray diffraction experiments at BESSY II, we saw quite a big difference between inks with and without DMSO added”, the physical chemist explains.

It’s the amount that counts

However, as her team has found out in many experiments, the amount added plays a decisive role here. More DMSO favours crystal growth – up to a certain point. If this is exceeded, other processes come into play and the resulting microstructure reduces the performance of the solar cells. “It’s like seasoning a soup”, says Unger. “If you add too little, it remains bland. If you add too much, it won’t taste good either. So you need to add just the right amount to make it best.” In addition to the optimal composition, the HZB team has also thoroughly investigated the ageing processes and thus the storage life of the inks. “This is an aspect that has received little attention so far”, Unger explains. “The age of a perovskite precursor ink can influence device performance. This is an important factor that must be considered when developing inks and processes.”

Featured image: The liquid solution of perovskite precursor, solvent, and additive flows from a slit-shaped nozzle onto the glass substrate being conveyed below. © Jinzhao Li / HZB


Reference: Jinzhao Li, Janardan Dagar, Oleksandra Shargaieva, Marion A. Flatken, Hans Köbler, Markus Fenske, Christof Schultz, Bert Stegemann, Justus Just, Daniel M. Többens, Antonio Abate, Rahim Munir, and Eva Unger, “20.8% Slot-Die Coated MAPbIPerovskite Solar Cells by Optimal DMSO-Content and Age of 2-ME Based Precursor Inks”, Adv. Energy Materials (2021). DOI: 10.1002/aenm.202003460


Provided by Helmholtz Zentrum Berlin

Tiny 3D Structures Enhance Solar Cell Efficiency (Nanotechnology / Engineering)

A new method for constructing special solar cells could significantly increase their efficiency. Not only are the cells made up of thin layers, they also consist of specifically arranged nanoblocks. This has been shown in a new study by an international research team led by the Martin Luther University Halle-Wittenberg (MLU), which was published in the scientific journal Nano Letters.

Commercially available solar cells are mostly made of silicon. “Based on the properties of silicon it’s not feasible to say that their efficiency can be increased indefinitely,” says Dr Akash Bhatnagar, a physicist from the Centre for Innovation Competence (ZIK) “SiLi-nano” at MLU. His research team is therefore studying the so-called anomalous photovoltaic effect which occurs in certain materials. The anomalous photovoltaic effect does not require a p-n junction which otherwise enables the flow of current in silicon solar cells. The direction of the current is determined at the atomic level by the asymmetric crystal structure of the corresponding materials. These materials are usually oxides, which have some crucial advantages: they are easier to manufacture and significantly more durable. However, they often do not absorb much sunlight and have a very high electrical resistance. “In order to utilise these materials and their effect, creative cell architectures are needed that reinforce the advantages and compensate for the disadvantages,” explains Lutz Mühlenbein, lead author of the study.

In their new study, the physicists introduced a novel cell architecture, a so-called nanocomposite. They were supported by teams from the Bergakademie Freiberg, the Leibniz Institute of Surface Modification in Leipzig and Banaras Hindu University in India. In their experiment, the researchers stacked single layers of a typical material only a few nanometres in thickness on top of one another and offset them with nickel oxide strips running perpendicularly. “The strips act as a fast lane for the electrons that are generated when sunlight is converted into electricity and which are meant to reach the electrode in the solar cell,” Bhatnagar explains. This is precisely the transport that would otherwise be impeded by the electrons having to traverse each individual horizontal layer.

The new architecture actually increased the cell’s electrical output by a factor of five. Another advantage of the new method is that it is very easy to implement. “The material forms this desired structure on its own. No extreme external conditions are needed to force it into this state,” says Mühlenbein. The idea, for which the researchers have now provided an initial feasibility study, could also be applied to materials other than nickel oxide. Follow-up studies now need to examine if and how such solar cells can be produced on an industrial scale.

The study was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation), the Federal Ministry of Education and Research (BMBF) and with funds from the European Regional Development Fund (ERDF).

Featured image: This schematic representation shows the new structure: nickel oxide stripes run perpendicular to the actual material, serving as a passing lane for the electrons. © Lutz Mühlenbein


Reference: Mühlenbein L. et al. Nanocomposites with Three-Dimensional Architecture and Impact on Photovoltaic Effect. Nano Letters (2020). Doi: 10.1021/acs.nanolett.0c03654 https://pubs.acs.org/doi/10.1021/acs.nanolett.0c03654


Provided by Martin Luther University Halle-Wittenberg

On the Road to Invisible Solar Panels: How Tomorrow’s Windows Will Generate Electricity (Engineering)

A new study led by scientists from Incheon National University in Korea shows how to make a fully transparent solar cell.

In a new study in Journal of Power Sources, an international team of researchers, led by Prof. Joondong Kim from Korea, demonstrate the first transparent solar cell. Their innovative technique rests on a specific part of the solar cell: the heterojunction, made up of thin films of materials responsible for absorbing light. By combining the unique properties of titanium dioxide and nickel oxide semiconductors, the researchers were able to generate an efficient, transparent solar cell.

Five years after the Paris climate agreement, all eyes are on the world’s progress on the road to a carbon-free future. A crucial part of this goal involves the energy transition from fossil fuels to renewable sources, such as sun, water, wind and wave energy. Among those, solar energy has always held the highest hope in the scientific community, as the most reliable and abundant energy source on Earth. In recent decades, solar cells have become cheaper, more efficient, and environment friendly. However, current solar cells tend to be opaque, which prevents their wider use and integration into everyday materials, constrained to being lined up on roofs and in remote solar farms.

But what if next-generation solar panels could be integrated to windows, buildings, or even mobile phone screens? That is the hope of Professor Joondong Kim from the Department of Electrical Engineering at Incheon National University, Korea. In a recent study published in Journal of Power Sources, he and his colleagues detail their latest invention: a fully transparent solar cell. “The unique features of transparent photovoltaic cells could have various applications in human technology,” says Prof. Kim.

The idea of transparent solar cells is well known, but this novel application where scientists have been able to translate this idea into practice is a crucial new finding. At present, the materials making the solar cell opaque are the semiconductor layers, those responsible for capturing light and translating it into an electrical current. Hence, Prof. Kim and his colleagues looked at two potential semiconductor materials, identified by previous researchers for their desirable properties.

The first is titanium dioxide (TiO2), a well-known semiconductor already widely used to make solar cells. On top of its excellent electrical properties, TiO2 is also an environment-friendly and non-toxic material. This material absorbs UV light (a part of the light spectrum invisible to the naked eye) while letting through most of the visible light range. The second material investigated to make this junction was nickel oxide (NiO), another semiconductor known to have high optical transparency. As nickel is one of the mist abundant elements on Earth, and its oxide can easily be manufactured at low industrial temperatures, NiO is also a great material to make eco-friendly cells.

The solar cell created by the team is transparent, allowing its use in a wide range of applications
Photo courtesy: Joondong Kim from Incheon National University

The solar cell prepared by the researchers was composed of a glass substrate and a metal oxide electrode, on top of which they deposited thin layers of the semiconductors (TiO2 first, then NiO) and a final coating of silver nanowires, acting as the other electrode in the cell. They ran several tests to evaluate the device’s absorbance and transmittance of light, as well as its effectiveness as a solar cell.

Their findings were encouraging; with a power conversion efficiency of 2.1%, the cell’s performance was quite good, given that it targets only a small part of the light spectrum. The cell was also highly responsive and worked in low light conditions. Furthermore, more than 57% of visible light was transmitted through the cell’s layers, giving the cell this transparent aspect. In the final part of their experiment, the researchers demonstrated how their device could be used to power a small motor. “While this innovative solar cell is still very much in its infancy, our results strongly suggest that further improvement is possible for transparent photovoltaics by optimizing the cell’s optical and electrical properties,” suggests Prof. Kim.

Now that they have demonstrated the practicality of a transparent solar cell, they hope to further improve its efficiency in the near future. Only further research can tell whether they will indeed become a reality, but for all intents and purposes, this new technology opens a—quite literal—window into the future of clean energy.

Reference: Thanh Tai Nguyen, Malkeshkumar Patel, Sangho Kim, Rameez Ahmad Mir, Junsin Yi, Vinh-Ai Dao, Joondong Kim, “Transparent photovoltaic cells and self-powered photodetectors by TiO2/NiO Heterojunction”, Journal of Power Sources, 2020. 10.1016/j.jpowsour.2020.228865

Provided by Incheon National University

Powering The Future: New Insights Into How Alkali-metal Doped Flexible Solar Cells Work (Chemistry)

Scientists place another piece of the puzzle on how introducing alkali metals into crystals of thin-film flexible solar cells improves their efficiency.

“When eco-friendly, inexpensive, versatile, and efficient solar cells are developed, all thermal and nuclear power plants will disappear, and solar cells installed over the ocean or in outer space will power our world,” says Professor Dong-Seon Lee of the Gwangju Institute of Science and Technology in Korea. His highly optimistic view of the future mirrors the visions of many researchers involved in the effort to improve solar cells.

Flexible thin-film solar cells constructed via doping with eco-friendly, earth-abundant, and inexpensive alkali metals could be the future of a sustainable energy economy. ©Pixabay on Pexels.

Over time, in this effort, scientists have come to realize that doping–distorting a crystal structure by introducing an impurity–polycrystalline solar cells made by melting together crystals called CZTSSe with earth-abundant and eco-friendly alkali metals, such as sodium and potassium, can improve their light to electricity conversion efficiency while also leading to the creation of inexpensive flexible thin-film solar cells which, needless to say, could find many applications in a society that is increasingly making wearable electronics commonplace. But why doping improves performance is yet unknown.

In a recent paper published in Advanced Science, Prof Lee and team reveal one part of this unknown. Their revelations come from their observations of composition and electric charge transport properties of CZTSSe cells doped with layers of sodium fluoride of varying thicknesses.

Upon analyzing these doped cells, Prof Lee and team saw that the amount of dopant determined the path that charge carriers took between electrodes, making the cell either more or less conductive. At an optimal doped-layer thickness of 25 nanometer, the charges flowed through the crystal via pathways that allowed for maximum conductivity. This in turn, the scientists hypothesized, affected the “fill factor” of the cell, which indicates the light-to-electricity conversion efficiency. At 25 nanometers, a record fill factor of 63% was obtained, a notable improvement over the previous limit of 50%. The overall performance was also competitive with this amount of doping.

These findings provide insight into CZTSSe and other polycrystalline solar cells, paving the way for improving them further and realizing a sustainable society. But the competitive performance of the solar cell that yielded these findings gives it real-world applications more tangible to us common folks, as Prof Lee explains: “We have developed flexible and eco-friendly solar cells that will be useful in many ways in our real lives, from building-integrated photovoltaics and solar panel roofs, to flexible electronic devices”. And given the bold vision that Prof Lee carries, perhaps a green economy is not too far away.

References: Jeong, W.‐L., Kim, K.‐P., Kim, J., Park, H. K., Min, J.‐H., Lee, J.‐S., Mun, S.‐H., Kim, S.‐T., Jang, J.‐H., Jo, W., Lee, D.‐S., Impact of Na Doping on the Carrier Transport Path in Polycrystalline Flexible Cu2ZnSn(S,Se)4 Solar Cells. Adv. Sci. 2020, 1903085. https://doi.org/10.1002/advs.201903085 link: https://onlinelibrary.wiley.com/doi/10.1002/advs.201903085

Provided by Gwangju Institute of Science and Technology (GIST)

Reseachers Developed Colloidal Quantum Dot Photovoltaics Using Ultrathin, Solution-Processed Bilayer In2O3/ZnO Electron Transport Layers with Improved Stability (Material Science)

Colloidal quantum dots are tiny semiconductor particles capable of absorbing light over a broad range of wavelengths. Because these dots are easy to mix into liquid solvents, researchers have used them as ‘solar inks’ that can be printed onto bendable plastic sheets. However, early prototypes revealed that exposure to air and ultraviolet radiation degraded the cell’s ability to transform sunlight into electricity.

A lead sulfide quantum dot with long-chain surface ligands. Solar cells made with quantum dots show great promise as the next generation photovoltaic technology, but need to demonstrate long-term stability. Credit: KAUST, Ahmad Kirmani

The latest quantum dot solar cells sandwich the tiny particles between two films referred to as either electron- or hole-transporting layers. These coatings are designed to quickly extract negative or positive charges generated by photoexcited dots to an external circuit. In addition, the layers provide much-needed protection against external elements.

Schematic showing a control solar cell with a thick zinc oxide electron-transport layer (ETL) (left) and a solar cell employing the ultrathin and stable electron-transport layer developed in this work (right). SEM images are behind each schematic. Credit: KAUST 2020; Ahmad R. Kirmani

Kirmani and his colleagues realized that reducing the size of the electron-transporting layer could boost quantum dot solar cell performance. These films often comprise ultraviolet-sensitive materials, such as zinc oxide, and typically need to be more than 100-nanometers thick to prevent formation of defects that may short out the device. In contrast, thinner films are more desirable because they can extract photogenerated electrons at higher speeds.

The KAUST team developed a two-step technique to produce ultrathin films that are smooth enough for efficient electron collection. First, they deposited an indium oxide coating onto a transparent electrode to promote highly ordered film growth. A second deposition of zinc oxide, only 20-nanometers high, sealed up any porous defects and generated an extremely uniform interface.

They found that this bilayer ETL results in solar cells with significantly improved overall stability without compromising performance, with an 11.1% power conversion efficiency hero device. While, comparisons with a control device demonstrated that the ultrathin electron-transporting layer worked just as efficiently as a thicker zinc oxide film. Surprisingly, the mix of zinc and indium oxides in the new solar cell prolonged its shelf life, operational stability and tolerance to ultraviolet rays—advantages that the team attributes in part to enhanced optical transmittance through the device.

References: Ahmad R. Kirmani et al. Colloidal Quantum Dot Photovoltaics Using Ultrathin, Solution-Processed Bilayer In2O3/ZnO Electron Transport Layers with Improved Stability, ACS Applied Energy Materials, 3(6), (2020). DOI: 10.1021/acsaem.0c00831 link: https://pubs.acs.org/doi/10.1021/acsaem.0c00831