Tag Archives: solar cells

Super Conductive Graphene Will Boost Solar Technology

In 2010, the Nobel Prize in Physics went to the discoverers of graphene. A single layer of carbon atoms, graphene possesses properties that are ideal for a host of applications. Among researchers, graphene has been the hottest material for a decade. In 2017 alone, more than 30,000 research papers on graphene were published worldwide.

Now, two researchers from the University of Kansas (KU), Professor Hui Zhao and graduate student Samuel Lane, both of the Department of Physics & Astronomy, have connected a graphene layer with two other atomic layers (molybdenum diselenide and tungsten disulfide) thereby extending the lifetime of excited electrons in graphene by several hundred times. The finding will be published on Nano Futures, a newly launched and highly selective journal.

The work at KU may speed development of ultrathin and flexible solar cells with high efficiency.

For electronic and optoelectronic applications, graphene has excellent charge transport property. According to the researchers, electrons move in graphene at a speed of 1/30 of the speed of light — much faster than other materials. This might suggest that graphene can be used for solar cells, which convert energy from sunlight to electricity. But graphene has a major drawback that hinders such applications – its ultrashort lifetime of excited electrons (that is, the time an electron stays mobile) of only about one picosecond (one-millionth of one-millionth of a second, or 10-12 second).

These excited electrons are like students who stand up from their seats — after an energy drink, for example, which activates students like sunlight activates electrons,” Zhao said. “The energized students move freely in the classroom — like human electric current.

The KU researcher said one of the biggest challenges to achieving high efficiency in solar cells with graphene as the working material is that liberated electrons — or, the standing students — have a strong tendency to losing their energy and become immobile, like students sitting back down.

The number of electrons, or students from our example, who can contribute to the current is determined by the average time they can stay mobile after they are liberated by light,” explains Zhao. “In graphene, an electron stays free for only one picosecond. This is too short for accumulating a large number of mobile electrons. This is an intrinsic property of graphene and has been a big limiting factor for applying this material in photovoltaic or photo-sensing devices. In other words, although electrons in graphene can become mobile by light excitation and can move quickly, they only stay mobile too short a time to contribute to electricity.”

In their new paper, Zhao and Lane report this issue could be solved by using the so-called van der Waals materials. The principle of their approach is rather simple to understand. “We basically took the chairs away from the standing students so that they have nowhere to sit,” Zhao said. “This forces the electrons to stay mobile for a time that is several hundred times longer than before.”

To achieve this goal, working in KU’s Ultrafast Laser Lab, they designed a tri-layer material by putting single layers of MoSe2, WS2 and graphene on top of each other.

Source: https://news.ku.edu/

Nanoparticles Fom Tea Leaves Destroy 80% Of Lung Cancer Cells

Nanoparticles derived from tea leaves inhibit the growth of lung cancer cells, destroying up to 80% of them, new research by a joint Swansea University (UK) and Indian team has shown. The team made the discovery while they were testing out a new method of producing a type of nanoparticle called quantum dots.  These are tiny particles which measure less than 10 nanometres.  A human hair is 40,000 nanometres thick.

Although nanoparticles are already used in healthcare, quantum dots have only recently attracted researchers’ attention.  Already they are showing promise for use in different applications, from computers and solar cells to tumour imaging and treating cancerQuantum dots can be made chemically, but this is complicated and expensive and has toxic side effects.  The Swansea-led research team were therefore exploring a non-toxic plant-based alternative method of producing the dots, using tea leaf extract.

Tea leaves contain a wide variety of compounds, including polyphenols, amino acids, vitamins and antioxidants.   The researchers mixed tea leaf extract with cadmium sulphate (CdSO4) and sodium sulphide (Na2S) and allowed the solution to incubate, a process which causes quantum dots to form.   They then applied the dots to lung cancer cells. Tea leaves are a simpler, cheaper and less toxic method of producing quantum dots, compared with using chemicals, confirming the results of other research in the field. Quantum dots produced from tea leaves inhibit the growth of lung cancer cells.  They penetrated into the nanopores of the cancer cells and destroyed up to 80% of them.  This was a brand new finding, and came as a surprise to the team.

The research, published in “Applied Nano Materials”, is a collaborative venture between Swansea University experts and colleagues from two Indian universities.

Source: http://www.swansea.ac.uk/

Harvesting Clean Hydrogen Fuel Through Artificial Photosynthesis

A new, stable artificial photosynthesis device doubles the efficiency of harnessing sunlight to break apart both fresh and salt water, generating hydrogen that can then be used in fuel cells.

The device could also be reconfigured to turn carbon dioxide back into fuel.

Hydrogen is the cleanest-burning fuel, with water as its only emission. But hydrogen production is not always environmentally friendly. Conventional methods require natural gas or electrical power. The method advanced by the new device, called direct solar water splitting, only uses water and light from the sun.

If we can directly store solar energy as a chemical fuel, like what nature does with photosynthesis, we could solve a fundamental challenge of renewable energy,” said Zetian Mi, a professor of electrical and computer engineering at the University of Michigan who led the research while at McGill University in Montreal.

Faqrul Alam Chowdhury, a doctoral student in electrical and computer engineering at McGill, said the problem with solar cells is that they cannot store electricity without batteries, which have a high overall cost and limited life.

The device is made from the same widely used materials as solar cells and other electronics, including silicon and gallium nitride (often found in LEDs). With an industry-ready design that operates with just sunlight and seawater, the device paves the way for large-scale production of clean hydrogen fuel.

Previous direct solar water splitters have achieved a little more than 1 percent stable solar-to-hydrogen efficiency in fresh or saltwater. Other approaches suffer from the use of costly, inefficient or unstable materials, such as titanium dioxide, that also might involve adding highly acidic solutions to reach higher efficiencies. Mi and his team, however, achieved more than 3 percent solar-to-hydrogen efficiency.

Source: https://news.umich.edu/

Squeeze And Get More Power Out Of Solar Cells

Physicists at the University of Warwick have published new research in the Journal Science  that could literally squeeze more power out of solar cells by physically deforming each of the crystals in the semiconductors used by photovoltaic cells. The paper entitled the “Flexo-Photovoltaic Effect” was written by Professor Marin Alexe, Ming-Min Yang, and Dong Jik Kim who are all based in the University of Warwick’s Department of Physics.

The Warwick researchers looked at the physical constraints on the current design of most commercial solar cells which place an absolute limit on their efficiency. Most commercial solar cells are formed of two layers creating at their boundary a junction between two kinds of semiconductors, p-type with positive charge carriers (holes which can be filled by electrons) and n-type with negative charge carriers (electrons). When light is absorbed, the junction of the two semiconductors sustains an internal field splitting the photo-excited carriers in opposite directions, generating a current and voltage across the junction. Without such junctions the energy cannot be harvested and the photo-exited carriers will simply quickly recombine eliminating any electrical charge. That junction between the two semiconductors is fundamental to getting power out of such a solar cell but it comes with an efficiency limit. This Shockley-Queisser Limit means that of all the power contained in sunlight falling on an ideal solar cell in ideal conditions only a maximum of 33.7% can ever be turned into electricity.

There is however another way that some materials can collect charges produced by the photons of the sun or from elsewhere. The bulk photovoltaic effect occurs in certain semiconductors and insulators where their lack of perfect symmetry around their central point (their non-centrosymmetric structure) allows generation of voltage that can be actually larger than the band gap of that material. Unfortunately the materials that are known to exhibit the anomalous photovoltaic effect have very low power generation efficiencies, and are never used in practical power-generation systems. The Warwick team wondered if it was possible to take the semiconductors that are effective in commercial solar cells and manipulate or push them in some way so that they too could be forced into a non-centrosymmetric structure and possibly therefore also benefit from the bulk photovoltaic effect.

Extending the range of materials that can benefit from the bulk photovoltaic effect has several advantages: it is not necessary to form any kind of junction; any semiconductor with better light absorption can be selected for solar cells, and finally, the ultimate thermodynamic limit of the power conversion efficiency, so-called Shockley-Queisser Limit, can be overcome“,  explains Professor Marin Alexe  (University of Warwick).

Source: https://warwick.ac.uk/

How Solar Cells Absorb 20 % More Sunlight

Trapping light with an optical version of a whispering gallery, researchers at the National Institute of Standards and Technology (NIST) have developed a nanoscale coating for solar cells that enables them to absorb about 20 percent more sunlight than uncoated devices. The coating, applied with a technique that could be incorporated into manufacturing, opens a new path for developing low-cost, high-efficiency solar cells with abundant, renewable and environmentally friendly materials.

Illustration shows the nanoresonator coating, consisting of thousands of tiny glass beads, deposited on solar cells. The coating enhances both the absorption of sunlight and the amount of current produced by the solar cells

The coating consists of thousands of tiny glass beads, only about one-hundredth the width of a human hair. When sunlight hits the coating, the light waves are steered around the nanoscale bead, similar to the way sound waves travel around a curved wall such as the dome in St. Paul’s Cathedral in London. At such curved structures, known as acoustic whispering galleries, a person standing near one part of the wall easily hears a faint sound originating at any other part of the wall.

Using a laser as a light source to excite individual nanoresonators in the coating, the team found that the coated solar cells absorbed, on average, 20 percent more visible light than bare cells. The measurements also revealed that the coated cells produced about 20 percent more current.

Source: https://www.nist.gov/