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Fluorine gas etches the surface of silicon into a series of angular peaks that, when viewed with a powerful microscope, look much like the pyramid pattern in the sound-proofing foam shown above. Researchers at PPPL have now modeled how these peaks form in silicon, creating a material that is highly light absorbent. Credit: Pixabay/CC0 Public Domain

Topics: Energy, Environment, Materials Science, Nanomaterials, Solar Power

Researchers at the U.S. Department of Energy's Princeton Plasma Physics Laboratory (PPPL) have developed a new theoretical model explaining one way to make black silicon, an important material used in solar cells, light sensors, antibacterial surfaces, and many other applications.

Black silicon is made when the surface of regular silicon is etched to produce tiny nanoscale pits on the surface. These pits change the color of the silicon from gray to black and, critically, trap more light, an essential feature of efficient solar cells.

While there are many ways to make black silicon, including some that use the charged, fourth state of matter known as plasma, the new model focuses on a process that uses only fluorine gas. PPPL Postdoctoral Research Associate Yuri Barsukov said the choice to focus on fluorine was intentional: The team at PPPL wanted to fill a gap in publicly available research. While some papers have been published about the role of charged particles called ions in the production of black silicon, not much has been published about the role of neutral substances, such as fluorine gas.

"We now know—with great specificity—the mechanisms that cause these pits to form when fluorine gas is used," said Barsukov, one of the authors of a new paper about the work, appearing in the Journal of Vacuum Science & Technology A.

"This kind of information, published publicly and openly available, benefits us all, whether we pursue further knowledge into the basic knowledge that underlines such processes or we seek to improve manufacturing processes," Barsukov added.

How black silicon, a prized material used in solar cells, gets its dark, rough edge, Rachel Kremen, Princeton Plasma Physics Laboratory

Prof. Li Gang invented a novel technique to achieve breakthrough efficiency with organic solar cells. Credit: Hong Kong Polytechnic University

Topics: Chemistry, Green Tech, Materials Science, Photonics, Research, Solar Power

Researchers from The Hong Kong Polytechnic University (PolyU) have achieved a breakthrough power-conversion efficiency (PCE) of 19.31% with organic solar cells (OSCs), also known as polymer solar cells. This remarkable binary OSC efficiency will help enhance these advanced solar energy device applications.

The PCE, a measure of the power generated from a given solar irradiation, is considered a significant benchmark for the performance of photovoltaics (PVs), or solar panels, in power generation. The improved efficiency of more than 19% that was achieved by the PolyU researchers constitutes a record for binary OSCs, which have one donor and one acceptor in the photoactive layer.

Led by Prof. Li Gang, Chair Professor of Energy Conversion Technology, and Sir Sze-Yen Chung, Endowed Professor in Renewable Energy at PolyU, the research team invented a novel OSC morphology-regulating technique by using 1,3,5-trichlorobenzene as a crystallization regulator. This new technique boosts OSC efficiency and stability.

The team developed a non-monotonic intermediated state manipulation (ISM) strategy to manipulate the bulk-heterojunction (BHJ) OSC morphology and simultaneously optimize the crystallization dynamics and energy loss of non-fullerene OSCs. Unlike the strategy of using traditional solvent additives, which is based on excessive molecular aggregation in films, the ISM strategy promotes the formation of more ordered molecular stacking and favorable molecular aggregation. As a result, the PCE was considerably increased, and the undesirable non-radiative recombination loss was reduced. Notably, non-radiative recombination lowers the light generation efficiency and increases heat loss.

Researchers achieve a record 19.31% efficiency with organic solar cells. Hong Kong Polytechnic University. Tech Explore

The LRESE parabolic dish: the solar reactor converts solar energy to hydrogen with an efficiency of more than 20%, producing around 0.5 kg of "green" hydrogen per day. (Courtesy: LRESE EPFL)

Topics: Applied Physics, Energy, Environment, Research, Solar Power

A new solar-radiation-concentrating device produces “green” hydrogen at a rate of more than 2 kilowatts while maintaining efficiencies above 20%. The pilot-scale device, which is already operational under real sunlight conditions, also produces usable heat and oxygen, and its developers at the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland say it could be commercialized in the near future.

The new system sits on a concrete foundation on the EPFL campus and consists of a parabolic dish seven meters in diameter. This dish collects sunlight over a total area of 38.5 m2, concentrates it by a factor of about 1000, and directs it onto a reactor that comprises both photovoltaic and electrolysis components. Energy from the concentrated sunlight generates electron-hole pairs in the photovoltaic material, which the system then separates and transports to the integrated electrolysis system. Here, the energy is used to “split” water pumped through the system at an optimal rate, producing oxygen and hydrogen.

Putting it together at scale

Each of these processes has, of course, been demonstrated before. Indeed, the new EPFL system, which is described in Nature Energy, builds on previous research from 2019, when the EPFL team demonstrated the same concept at a laboratory scale using a high-flux solar simulator. However, the new reactor’s solar-to-hydrogen efficiency and hydrogen production rate of around 0.5 kg per day is unprecedented in large-scale devices. The reactor also produces usable heat at a temperature of 70°C.

The versatility of the new system forms a big part of its commercial appeal, says Sophia Haussener, who leads the EPFL’s Laboratory of Renewable Energy Science and Engineering (LRESE). “This co-generation system could be used in industrial applications such as metal processing and fertilizer manufacturing,” Haussener tells Physics World. “It could also be used to produce oxygen for use in hospitals and hydrogen for fuels cells in electric vehicles, as well as heat in residential settings for heating water. The hydrogen produced could also be converted to electricity after being stored between days or even inter-seasonally.”

Concentrated solar reactor generates unprecedented amounts of hydrogen, Isabelle Dumé, Physics World.

Divine light The Dean of Gloucester Cathedral, Stephen Lake, blesses the cathedral’s solar panels after the solar-energy firm MyPower installed them in November 2016. The array of PV panels generates just over 25% of the building’s electricity. (Courtesy: MyPower)

Topics: Alternate Energy, Applied Physics, Battery, Chemistry, Economics, Solar Power

With energy bills on the rise, plenty of people are interested in ditching the fossil fuels currently used to heat most UK homes. The question is how to make it happen, as Margaret Harris explains.

Deep beneath the flagstones of the medieval Bath Abbey church, a modern marvel with an ancient twist is silently making its presence felt. Completed in March 2021, the abbey’s heating system combines underfloor pipes with heat exchangers located seven meters below the surface. There, a drain built nearly 2000 years ago carries 1.1 million liters of 40 °C water every day from a natural hot spring into a complex of ancient Roman baths.

By tapping into this flow of warm water, the system provides enough energy to heat not only the abbey but also an adjacent row of Georgian cottages used for offices. No wonder the abbey’s rector praised it as “a sustainable solution for heating our beautiful historic church.”

But that wasn’t all. Once efforts to decarbonize the abbey’s heating were underway, officials in the £19.4m Bath Abbey Footprint project turned their attention to the building’s electricity. Like most churches, the abbey runs from east to west, giving its roof an extensive south-facing aspect. At the UK’s northerly latitudes, such roofs are bathed in sunlight for much of the day, making them ideal for solar photovoltaic (PV) panels. Gloucester Cathedral – an hour’s drive north of Bath – has already taken advantage of this favorable orientation, becoming – in 2016 – the UK’s first major ancient cathedral to have solar panels installed on its roof.

To find out if a similar set-up might be suitable at Bath Abbey, the Footprint project worked with Ph.D. students in the University of Bath-led Centre for Doctoral Training (CDT) in New and Sustainable Photovoltaics. In a feasibility study published in Energy Science & Engineering (2022 10 892), the students calculated that a well-designed array of PV panels could supply 35.7% of the abbey’s electricity, plus 4.6% that could be sold back to the grid on days when a surplus was generated. The array would pay for itself within about 13 years and generate a total profit of £139,000 ± £12,000 over its 25-year lifetime.

Home, green home: scientific solutions for cutting carbon and (maybe) saving money, Margaret Harris, Physics World

V. ALTOUNIAN/SCIENCE

Topics: Alternate Energy, Applied Physics, Chemistry, Materials Science, Solar Power

As ultrathin organic solar cells hit new efficiency records, researchers see green energy potential in surprising places.

In November 2021, while the municipal utility in Marburg, Germany, was performing scheduled maintenance on a hot water storage facility, engineers glued 18 solar panels to the outside of the main 10-meter-high cylindrical tank. It’s not the typical home for solar panels, most of which are flat, rigid silicon and glass rectangles arrayed on rooftops or in solar parks. The Marburg facility’s panels, by contrast, are ultrathin organic films made by Heliatek, a German solar company. In the past few years, Heliatek has mounted its flexible panels on the sides of office towers, the curved roofs of bus stops, and even the cylindrical shaft of an 80-meter-tall windmill. The goal: expanding solar power’s reach beyond flat land. “There is a huge market where classical photovoltaics do not work,” says Jan Birnstock, Heliatek’s chief technical officer.

Organic photovoltaics (OPVs) such as Heliatek’s are more than 10 times lighter than silicon panels and in some cases cost just half as much to produce. Some are even transparent, which has architects envisioning solar panels, not just on rooftops, but incorporated into building facades, windows, and even indoor spaces. “We want to change every building into an electricity-generating building,” Birnstock says.

Heliatek’s panels are among the few OPVs in practical use, and they convert about 9% of the energy in sunlight to electricity. But in recent years, researchers around the globe have come up with new materials and designs that, in small, lab-made prototypes, have reached efficiencies of nearly 20%, approaching silicon and alternative inorganic thin-film solar cells, such as those made from a mix of copper, indium, gallium, and selenium (CIGS). Unlike silicon crystals and CIGS, where researchers are mostly limited to the few chemical options nature gives them, OPVs allow them to tweak bonds, rearrange atoms, and mix in elements from across the periodic table. Those changes represent knobs chemists can adjust to improve their materials’ ability to absorb sunlight, conduct charges, and resist degradation. OPVs still fall short of those measures. But, “There is an enormous white space for exploration,” says Stephen Forrest, an OPV chemist at the University of Michigan, Ann Arbor.

Solar Energy Gets Flexible, Robert F. Service, Science Magazine

A floating artificial leaf – which generates clean fuel from sunlight and water – on the River Cam near King's College Chapel in Cambridge, UK. (Courtesy: Virgil Andrei)

Topics: Climate Change, Energy, Environment, Materials Science, Solar Power

Leaf-like devices that are light enough to float on water could be used to generate fuel from solar farms located on open water sources. This avenue hasn’t been explored before, according to researchers from the University of Cambridge in the UK who developed them. The new devices are made from thin, flexible substrates and perovskite-based light-absorbing layers. Tests showed that they can produce either hydrogen or syngas (a mixture of hydrogen and carbon monoxide) while floating on the River Cam.

Artificial leaves like these are a type of photoelectrochemical cell (PEC) that transforms sunlight into electrical energy or fuel by mimicking some aspects of photosynthesis, such as splitting water into its constituent oxygen and hydrogen. This differs from conventional photovoltaic cells, which convert light directly into electricity.

Because PEC artificial leaves contain both light harvesting and catalysis components in one compact device, they could, in principle, be used to produce fuel from sunlight cheaply and simply. The problem is that current techniques for making them can’t be scaled up. What is more, they are often composed of fragile and heavy bulk materials, which limits their use.

In 2019 a team of researchers led by Erwin Reisner developed an artificial leaf that produced syngas from sunlight, carbon dioxide, and water. This device contained two light absorbers and catalysts, but it also incorporated a thick glass substrate and coatings to protect against moisture, which made it cumbersome.

Floating artificial leaves could produce solar-generated fuel, Isabelle Dumé, Physics World

Surfing excitons: Cambridge’s Alexander Sneyd with the transient-absorption microscopy set-up. (Courtesy: Alexander Sneyd)

Topics: Alternate Energy, Applied Physics, Materials Science, Nanotechnology, Solar Power

Organic solar cells (OSCs) are fascinating devices where layers of organic molecules or polymers carry out light absorption and subsequent transport of energy – the tasks that make a solar cell work. Until now, the efficiency of OSCs has been thought to be constrained by the speed at which energy carriers called excitons to move between localized sites in the organic material layer of the device. Now, an international team of scientists led by Akshay Rao at the UK’s University of Cambridge has shown that this is not the case. What is more, they have discovered a new quantum mechanical transport mechanism called transient delocalization, which allows OSCs to reach much higher efficiencies.

When light is absorbed by a solar cell, it creates electron-hole pairs called excitons and the motion of these excitons plays a crucial role in the operation of the device. An example of an organic material layer where light absorption and transport of excitons takes place is in a film of well-ordered poly(3-hexylthiophene) nanofibers. To study exciton transport, the team shone laser pulses at such a nanofiber film and observed its response.

Exciton wave functions were thought to be localized due to strong couplings with lattice vibrations (phonons) and electron-hole interactions. This means the excitons would move slowly from one localized site to the next. However, the team observed that the excitons were diffusing at speeds 1000 times greater than what had been shown for similar samples in previous research. These speeds correspond to a ground-breaking diffusion length of about 300 nm for such crystalline films. This means energy can be transported much faster and more efficiently than previously thought.

Exciton ‘surfing’ could boost the efficiency of organic solar cells, Rikke Plougmann, Physics World

FIG. 1. (a) Schematic of La Mer and Dinegar's model for the synthesis of monodispersed CQDs. (b) Representation of the apparatus employed for CQD synthesis. Reproduced with permission from Murray et al., Annu. Rev. Mater Res. 30(1), 545–610 (2000). Copyright 2000 Annual Reviews.

Topics: Energy, Materials Science, Nanotechnology, Quantum Mechanics, Solar Power

ABSTRACT
Solution-processed colloidal quantum dot (CQD) solar cells are lightweight, flexible, inexpensive, and can be spray-coated on various substrates. However, their power conversion efficiency is still insufficient for commercial applications. To further boost CQD solar cell efficiency, researchers need to better understand and control how charge carriers and excitons transport in CQD thin films, i.e., the CQD solar cell electrical parameters including carrier lifetime, diffusion length, diffusivity, mobility, drift length, trap state density, and doping density. These parameters play key roles in determining CQD thin film thickness and surface passivation ligands in CQD solar cell fabrication processes. To characterize these CQD solar cell parameters, researchers have mostly used transient techniques, such as short-circuit current/open-circuit voltage decay, photoconductance decay, and time-resolved photoluminescence. These transient techniques based on the time-dependent excess carrier density decay generally exhibit an exponential profile, but they differ in the signal collection physics and can only be used in some particular scenarios. Furthermore, photovoltaic characterization techniques are moving from contact to non-contact, from steady-state to dynamic, and from small-spot testing to large-area imaging; what are the challenges, limitations, and prospects? To answer these questions, this Tutorial, in the context of CQD thin film and solar cell characterization, looks at trends in characterization technique development by comparing various conventional techniques in meeting research and/or industrial demands. For a good physical understanding of material properties, the basic physics of CQD materials and devices are reviewed first, followed by a detailed discussion of various characterization techniques and their suitability for CQD photovoltaic devices.

Advanced characterization methods of carrier transport in quantum dot photovoltaic solar cells, Lilei Hu, Andreas Mandelis, Journal of Applied Physics