Science: Solar-powered hydrogen fuels a step closer

perovskite

They tested the waterproofing by submerging the coated perovskite cells in water and using the harvested solar energy to split water into hydrogen and oxygen.

A cheaper, cleaner and more sustainable way of making hydrogen fuel from water using sunlight is step closer thanks to new research from the University of Bath’s Centre for Sustainable Chemical Technologies.

With the pressure on global leaders to reduce carbon emissions significantly to solve a climate change emergency, there is an urgent need to develop cleaner energy alternatives to burning fossil fuels. Hydrogen is a zero carbon emission fuel alternative that can be used to power cars, producing only water as a waste product.

It can be made by splitting water into hydrogen and oxygen, however the process requires large amounts of electricity. Most electricity is made by burning methane so researchers at the University of Bath are developing new solar cells that use light energy directly to split water.

The approach

Most solar cells currently on the market are made of silicon, however they are expensive to make and require a lot of very pure silicon to manufacture. They are also quite thick and heavy, which limits their applications.

Perovskite solar cells, using materials with the same 3D structure as calcium titanium oxide, are cheaper to make, thinner and can be easily printed onto surfaces. They also work in low light conditions and can produce a higher voltage than silicon cells, meaning they could be used indoors to power devices without the need to plug into the mains.

perovskite solar cell

The downside is they are unstable in water which presents a huge obstacle in their development and also limits their use for the direct generation of clean hydrogen fuels.

The team of scientists and chemical engineers, from the University of Bath’s Centre for Sustainable Chemical Technologies, has solved this problem by using a waterproof coating from graphite, the material used in pencil leads.

They tested the waterproofing by submerging the coated perovskite cells in water and using the harvested solar energy to split water into hydrogen and oxygen. The coated cells worked underwater for 30 hours – ten hours longer than the previous record.

After this period, the glue sandwiching the coat to the cells failed; the scientists anticipate that using a stronger glue could stabilize the cells for even longer.

The next breakthrough

Previously, alloys containing indium were used to protect the solar cells for water splitting, however indium is a rare metal and is therefore expensive and the mining process to obtain it is not sustainable.

The Bath team instead used commercially available graphite, which is very cheap and much more sustainable than indium.

Dr Petra Cameron, Senior Lecturer in Chemistry, said: “Perovskite solar cell technology could make solar energy much more affordable for people and allow solar cells to be printed onto roof tiles. However at the moment they are really unstable in water – solar cells are not much use if they dissolve in the rain!’

“We’ve developed a coating that could effectively waterproof the cells for a range of applications. The most exciting thing about this is that we used commercially available graphite, which is much cheaper and more sustainable than the materials previously tried.”

Perovskite solar cells produce a higher voltage than silicon based cells, but still not enough needed to split water using solar cells alone. To solve this challenge, the team is adding catalysts to reduce the energy requirement needed to drive the reaction.

Isabella Poli, Marie Curie FIRE Fellow and PhD student from the Centre for Sustainable Chemical Technologies, said: “Currently hydrogen fuel is made by burning methane, which is neither clean nor sustainable.

“But we hope that in the future we can create clean hydrogen and oxygen fuels from solar energy using perovskite cells.”

The research was done in collaboration with the SPECIFIC team at Swansea University. The study is published in the open access journal Nature Communications: Poli, Hintermair, Regue, Kumar, Sackville, Baker, Watson, Eslava & Cameron (2019)

— Solar Builder magazine

Science: Caffeine improved the performance of perovskite solar cells

California nano science

Scientists at the California NanoSystems Institute at UCLA have found that caffeine improves the stability of materials under heat – a property known as thermal stability — of perovskite solar cells, which could someday replace traditional silicon-based solar cells.

The research, published in the journal Joule, was led by Yang Yang, UCLA’s Carol and Lawrence E. Tannas Jr. Professor of Engineering.

For the past few years, perovskite solar cells have been thought to be the future of solar power because they could eventually cost less to produce than today’s silicon solar cells and they have the potential to be more energy-efficient. Research on perovskite solar cells dates back only to the early 2010s, but they are already nearly as efficient as silicon solar cells, which have been researched for more than 40 years.

But perovskite solar cells are not yet commercially viable, in part because of their inability to withstand sustained heat from sunlight.

“Solar cells need high thermal stability since they are constantly exposed to sunlight, which warms up the devices,” said Yang, who is also a professor of materials science and engineering at the UCLA Samueli School of Engineering. “While perovskites are an attractive option for solar cells, the materials degrade and become less stable over time. We need them to last 20 to 30 years like traditional solar cells.”

Perovskite solar cells got their name not because they contain the mineral perovskite but because their crystalline structure mimics perovskite’s molecular structure. They contain an ultra-thin film made of inexpensive materials like methylammonium, lead and iodine, the combination of which produce that crystalline structure.

And it’s that structure that makes the solar cells highly effective at converting photons — the basic unit of light — into electricity.

The idea to test caffeine as a possible solution for perovskite cells’ thermal instability arose in March 2018 when Rui Wang, a UCLA graduate student, was drinking coffee with some colleagues. He considered caffeine’s chemical structure and wondered whether it could interact with the materials used in perovskite solar cells.

“The boiling point of caffeine is 300 degrees Celsius, which is higher than the operational temperature of solar cells, so it seemed like a possible candidate,” said Wang, a co-first author of the study.

To test whether caffeine would improve the device’s thermal stability, the team made a custom perovskite film by mixing dimethylformamide, methylammonium iodide and lead iodide to create a liquid solution, adding caffeine, and then pouring the solution onto indium tin oxide glass to form a black layer of perovskite.

They incorporated the new film into a solar cell and tested its ability to withstand high temperatures by placing it on a plate heated to 85 degrees Celsius (about 185 degrees Fahrenheit). Measuring its energy output every four days for two months, the researchers found that the device retained its thermal stability for more than 1,300 hours, or about 55 days, while preserving 86 percent of the energy it took in — a measure called power conversion efficiency.

For comparison, the team also tested a perovskite solar cell made without caffeine; it retained only 60 percent of its power conversion efficiency after 175 hours, or about seven days.

To understand why the caffeine worked, the team used a transmittance electron microscope to analyze the way that the new film’s crystalline structure evolved. They determined that there was a strong interaction — a “molecular lock” — between the caffeine and the lead ions.

“Parts of caffeine’s chemical structure were forming very strong binding with the lead ions and stabilizing the crystals,” said Jingjing Xue, a UCLA graduate student and another co-first author of the study. “The molecular lock between caffeine and lead also slowed down the growth of perovskite crystals, allowing them to align into an orientation that is beneficial for electric charge transfer.”

With an understanding of the effect of the molecular lock created by caffeine molecules, researchers now can explore whether chemicals other than caffeine could produce similar effects that further improve perovskite cells’ thermal stability.

“The molecular lock may help push perovskite solar cells to commercialization in the future,” Yang said. “Caffeine is the first compound we identified, but there may be others that can work even more efficiently.”

Yang’s research group has been investigating perovskite and other types of solar cells for several years. Its recent accomplishments include the development of a dual-layer solar cell that generates more energy from sunlight than typical solar panels.

The research was supported by the Air Force Office of Scientific Research, the Office of Naval Research, the University of California Advanced Solar Technologies Institute, the Natural Science Foundation of China, Suzhou Nano Science and Technology’s Collaborative Innovation Center, Jiangsu Higher Education Institutions’ Priority Academic Program Development and Jinzhou Solargiga Energy.

— Solar Builder magazine

NREL argues for value of ‘watts per kilogram’ in emerging thin-film, flexible solar technology

NREL lightweight CIGS

This lightweight CIGS photovoltaic cell, on flexible stainless steel, was made by Matthew Reese and his team at NREL. Photo by Dennis Schroeder / NREL

Rigid silicon solar panels dominate the utility and residential markets, but opportunity exists for thin-film photovoltaic and emerging technologies notable for being lightweight and flexible, according to scientists at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL).

Thin films such as cadmium telluride (CdTe) and copper indium gallium selenide (CIGS), along with perovskites and other new technologies, could be ideal for generating the electricity needed for unmanned drones, portable chargers, and building facades. The opportunities and challenges inherent in widespread adoption of these ideas appear in the new Nature Energy paper, “Increasing Markets and Decreasing Package Weight for High Specific Power Photovoltaics.

“We explore the limits behind power-to-weight ratios and how this can generate value for emerging players in photovoltaics to enable them to reach gigawatt scale without having to directly compete with silicon solar panels,” said Matthew Reese, an NREL researcher and lead author of the paper. The paper was co-authored by Stephen Glynn, Michael Kempe, Deborah McGott, Matthew Dabney, Teresa Barnes, Samuel Booth, David Feldman, and Nancy Haegel, all from NREL.

The market opportunity

Silicon panels constitute 95 percent of the global solar market, generating electricity for utilities, residences, and businesses, but the researchers identified applications that must consider value propositions beyond the standard value triad of cost, efficiency, and reliability used for conventional photovoltaic (PV) panels. Flexibility and portability will be important factors, with the performance of the technology quantified in terms of watts per kilogram.

The researchers identified three high-value markets, each with a potential to cumulatively generate a gigawatt (GW) of electricity—at a price above $1 a watt—over the next 10 years:

Aerospace and unmanned aerial vehicles – Powering satellites is driven by extremely high launch costs; whereas, there is an increasing desire to keep drones aloft for very long periods. For both of these applications, limited space makes efficiency and weight critical and cost secondary. A key player in this market is III-V PV, but while highly efficient it’s also too expensive for many applications.

Portable charging – Making it easy for one person to install or move a portable charger is driving the need for PV technology that’s efficient and flexible. Finding the correct balance between those requirements and cost could put millions of units into service by the military, disaster relief workers, and recreational users.

Ground transportation – The integration of PV in electric vehicles will compete with electricity coming from the grid, but the addition could extend the driving range. The PV would have to use smaller panels and be flexible enough to conform the contours of the roof.

The researchers identified these markets as smaller but significant and ones that will pay a premium for the added value of the technology being lightweight to support initial, low-scale production. As production increases, lower costs will follow.

The NREL team determined the lower limit for a lightweight PV device is between 300 and 500 grams per square meter. Below that would reduce reliability, durability, and safety. A lightweight module on the lower side of that range could generate more than a kilowatt of electricity from something that weighs as little as a six pack of soda. Conventional modules, even without the additional weight from the mounting equipment, might require 150–200 pounds to generate this much power.

— Solar Builder magazine

Solar-Tectic patents perovskite, crystalline silicon thin-film tandem solar cell

solar tectic perovskite

Perovskite materials are always on the horizon for the solar industry, holding promise as a future solution to the long-standing problem of solar cell efficiency, which is of primary importance in today’s solar panel market. And while there have been numerous reports of perovskite/silicon (wafer) tandem solar cells, remarkably there has been none on a perovskite/crystalline silicon thin-film tandem solar cell, until now.

The US Patent and Trademark Office (USPTO) awarded Solar-Tectic LLC  two patent applications for perovskite thin film solar cells, one of which covers all kinds of perovskites. The inventors are Ashok Chaudhari, Founding Manager of Solar-Tectic, and the late Dr. Praveen Chaudhari, renowned materials physicist.

Tandem cells explaned

Wafer-sized bottom poly- and monocrystalline silicon layers in PERC, PERL, HIT, HJ, or perovskite/silicon tandem cells are typically 200-280 microns thick, whereas Solar-Tectic’s thin-film crystalline inorganic bottom layers can be as thin as 20-30 microns with the same or similar efficiency; moreover, they can be processed at much lower temperatures thereby lowering costs of production significantly. The top perovskite layer is less than only 1 micron – an ultra-thin film — and a thin film crystalline silicon (CSiTF) bottom layer decouples the need for a silicon wafer. If the price of polysilicon rises less silicon material use will be an additional cost savings.

RELATED: Modules and integration: Four reasons why AC, smart modules are on the rise

Tandem perovskite solar cells are capable in theory of 45 percent efficiency, though Solar-Tectic has set a more realistic 30 percent efficiency goal, higher than the best silicon wafer technologies such as PERC, PERL, HIT, HJ cells with 25-26.6 percent efficiencies. The efficiencies of today’s solar cells on the market in general range from 14 – 25 percent. A cost effective 30 percent efficient solar cell with a simple design would revolutionize the solar energy industry by dramatically reducing the balance of system (BoS) costs, thereby lessening the need for fossil fuel generated electricity significantly. Silicon wafer technology based on polycrystalline or monocrystalline silicon, which is 90 percent of today’s market, would become obsolete.

Non-toxic

Importantly, the entire Solar-Tectic process is environmentally friendly since non-toxic Sn (tin) or Au (gold) is used to deposit the crystalline silicon thin-film material for the bottom layer in the tandem/heterojunction configuration as well as in the top, perovskite, layer. The more commonly used toxic Pb (lead) is not used in the perovskite here. The manufacturing methods used in this technology – sputtering or electron beam evaporation — are conventional and similar to those used in today’s thin-film solar cell industry, and importantly also in the display industry with which there is much overlap and potential for synergy.

The breakthrough patents correspond to a “Tandem Series” of solar cell technologies which has been launched by Solar-Tectic, and that includes a variety of different proven semiconductor photovoltaic materials (i.e. III-V, CZTS, a-Si, etc) for the top layer on silicon (or germanium) bottom layer, on various substrates such as cheap soda-lime glass. A paper reporting a successful step in this approach was recently published. Last year, Solar-Tectic announced the first patent ever granted for this perovskite/silicon thin-film tandem approach.

A patent for a copper oxide thin-film tandem solar cell was also granted to ST (US 9,997,661) this month thereby expanding the IP portfolio of the tandem series.

— Solar Builder magazine

NREL update: The puzzle of scaling perovskite solar cells (and possible solutions)

perovskite solar cell

As perovskite solar cells set efficiency records and the nascent technology becomes more stable, another major challenge remains: the issue of scalability, according to researchers at the Department of Energy’s National Renewable Energy Laboratory (NREL).

“It is scalable,” said Kai Zhu, a materials science researcher at NREL. “We just need to demonstrate efficiency and yield at a large-scale to move the technology beyond the laboratory.”

Lead author of a new Nature Reviews Materials paper titled, “Scalable Fabrication of Perovskite Solar Cells,” Zhu and his colleagues at NREL reviewed efforts to move perovskites from the laboratory to the rooftop. Zhen Li, Talysa Klein, Dong Hoe Kim, Mengjin Yang, Joseph Berry, and Maikel van Hest are the co-authors.

Most solar panels on the market today are made of silicon, but perovskite solar cells have the potential to accelerate the growth of photovoltaic (PV) manufacturing in the United States because they’re much cheaper to make and have shown performance potential in the lab. Perovskites have achieved record efficiency levels faster than any other solar cell technology with the current record—certified last summer—now standing at 22.7 percent. But efficiency in a perovskite solar cell declines as the cell and module area increases. A combination of factors is attributed to the decline, including the non-uniform coating of chemicals in the cell. Also, when any type of solar cells are joined together to create modules, inactive zones form between cells where sunlight isn’t converted to electricity, leading to efficiency declines.

RELATED: Module Evolution: What big-time PV improvements will boost panel efficiency?

To make a perovskite solar cell in the laboratory, scientists deposit chemicals onto a substrate. The perovskite material forms as the chemicals crystallize. The most commonly used deposition method in the laboratory, called spin coating, produces devices with the highest efficiency, but the process wastes more than 90 percent of the chemicals used, the so-called perovskite ink. Spin coating also works best on cells smaller than four square inches, but there isn’t an easy way to enable this technology to be used on a larger surface.

The NREL researchers examined potential scalable deposition methods, including:

• Blade coating, which uses a blade to spread the chemical solution on substrates to form wet thin films. The process can be adapted for roll-to-roll manufacturing, with flexible substrates moving on a roller beneath a stationary blade similar to how newspapers are printed. Blade coating wastes less of the ink than spin coating.

• Slot-die coating, which relies on a reservoir to supply the precursor ink in order to apply ink over the substrate. The process hasn’t been as well explored as other methods and so far has demonstrated lower efficiency than blade coating. But the reproducibility of slot-die coating is better than blade coating when the ink is well-developed, so this is more applicable for roll-to-roll manufacturing.

• Ink-jet printing, which uses a small nozzle to disperse the precursor ink. The process has been used to make small-scale solar cells, but whether it is suitable for the high-volume, large-area production will depend on the printing speed and device structure.

Other methods exist, such as electro-deposition, but there haven’t been any reports of that being used to make direct deposition of halide perovskites in perovskite solar cells.

Despite numerous challenges, impressive progress is being made toward scaling up production of these solar cells, the NREL researchers noted in the paper. The new paper outlined research that needs to be addressed to scale-up the technology. One area in particular that needs more attention is the ideal architecture of a perovskite solar module.

Several studies have estimated perovskite solar cells could generate electricity at a lower cost than other photovoltaic technologies, although those figures are based on hypothetical research. But one conclusion that can be drawn from the studies is that the highest input costs for perovskite modules will come from substrates and electrode materials, which points to a range of opportunities for innovation in these areas.

— Solar Builder magazine