Tag Archives: solar cells

Wash Your Solar Cells

To augment the energy supply, many are installing solar energy systems or residential solar panels. In general, these are flat units, placed at an angle on the rooftop. That naturally leads to the question of keeping them clean, which people equate to cleaning the roof itself. As this cleaning is usually left to the rainwaters, the next question comes as whether we should depend on the rains for cleaning the surface of the installed solar cells as well. Moreover, some also worry about whether water is good for the cells and will not damage them.

For these skeptics, scientists have a new type of waterproof solar cell that generates electricity even when compressed, stretched, or soaked in water. This is good news for those in the wearable solar cell industry. Wearable solar cells provide power to devices for monitoring health, usually as sensors incorporated into clothing, recording heartbeats, body temperature, and other parameters, for providing early warning of medical problems.

These extremely thin and flexible organic solar cells, or photovoltaic cells as scientists call them, are a result of research in the University of Tokyo. A material, by name PNTz4T, coats both sides of the cells with a stretchable and waterproof film. The researchers then deposit the cells within an inverse architecture of a one-micrometer-thick parylene film. After this process, the researchers applied an acrylic-based elastomer coating to both sides of the cell, which prevents water infiltration.

The elastomer is transparent and allows light to enter the cell, but does not allow air and water from leaking into it. This makes the solar cells longer lasting compared to conventional photovoltaic cells. The researchers decided to test the effectiveness of the coating by immersing the coated cells in water for two hours. They found the cells’ resistance to water to be high, as its efficiency to convert from light to electricity dropped by only 5.4 percent.

Next, the researchers tested the durability of the coated cell by subjecting it to compression. They compressed the cell by half for twenty cycles while placing drops of water on it. Even after surviving this brutal test, the researchers found the cell still had more than 80% of its original efficiency still intact. The above tests confirmed the cells’ mechanical robustness, high efficiency, and great environmental stability.

Not only as wearable sensors, these new washable, stretchable, and lightweight organic photovoltaic cells will also be suitable as long-term power sources as rooftop solar panels. Most experts do not recommend washing solar cells regularly for keeping the dust and debris from collecting on the surface. Since these new solar panels have the additional feature of being waterproof, there is no danger from giving them a frequent wash.

Experts feel it is best to let the rain take care of washing the solar panel. By monitoring the system functionality such as checking the energy bills and usage on monthly basis, the user can detect changes in the electricity bill. Another check can be made by visually inspecting the surface of the panels. If cleaning is necessary, washing it with a hose of water will do the job.

Five New Advancements in Solar Cells

The earth receives a huge amount of sunlight every hour. Converted to electricity, this would amount to 52 PW/hr. This is more than ten times the entire amount of electricity produced per hour by China in 2013. In the same year, top countries of the world together produced only 16 PW/hr. of electricity. As this is much less than the actual potential of generation of electricity from the solar energy falling on the planet earth, several countries are actively engaged on research and development on photovoltaic cells.

There have been several breakthroughs in photovoltaic cell technology. For instance, early cells were very expensive and inefficient—almost $1800/watt and 4% respectively. Costs have now come down to $0.75/watt, while the efficiency has increased to 40%. Since, then, there have been several other breakthroughs in the solar cell domain.

Printable Solar Cells

At the New Jersey Institute of Technology (NJIT), researchers have developed a printable solar cell, and they can print or paint this on a surface. According to the lead researcher Dr. Mitra, they are aiming for printable sheets of solar cells that any home-based inkjet printer will be able to print and place on the wall, roof, or billboard to generate power. The printable cells are made of carbon nanotubes 50,000 times smaller than a human hair.

All-Carbon Flexible Solar Cells

Scientists at the Stanford University have made these flexible solar cells from a special form of carbon called graphene. According to Zhenan Bao, one of the team and a professor of chemical engineering at Stanford, the flexible carbon solar cells can be coated on to the surface of cars, windows, or buildings for generating electricity.
By replacing expensive materials when manufacturing conventional solar cells, the all-carbon solar cell is expected to make the cells much cheaper.

Transparent Solar Cells

At the Michigan State University, a team of researchers has made solar cells that appear transparent to the visible spectrum of sunlight. Rather, these non-intrusive solar cells convert light beyond the visible spectrum to electricity. Therefore, these can be used on smartphones, on windowpanes of buildings, or in windshields of vehicles without impeding their performance.

According to MSU assistant professor Richard Lunt, their aim is to produce solar harvesting surfaces that are invisible. However, the present efficiency of these cells is a mere 1%, as they are in their initial stages.

Wearable Ultra-Thin Solar Cells

In South Korea, at the Gwangju Institute of Science and Technology, scientists have used gallium arsenide to develop solar cells with a thickness of just one micrometer, more than 100 times thinner than human hair. According to Jongho Lee, an engineer at the institute, such thin cells can be integrated into fabric or glass frames to power the next wave of wearable electronics.

To create such thin cells, the scientists removed extra adhesives from the traditional cells, and cold-welded them on flexible substrates at 170°C.

Solar Cells with 100% Efficiency

By extracting all the energy from excitons, researchers at the University of Cambridge have found methods of making solar cells that are more efficient. Such a hybrid cell combines organic material and inorganic material into high conversion efficiency.

Rectannas : Will They Make Solar Cells Obsolete?

Professor Baratunda Cola and colleagues at the Georgia Institute of Technology, Atlanta, claims to have improved on the solar cells available. They have reported their findings in Nature Nanotechnology. The new type of solar cell is actually a rectenna – half antenna and half rectifier that can be tuned to any frequency as a detector, while generating electricity from solar and infrared light falling on it.

The team claims they can achieve a broad-spectrum efficiency of 40 percent with their new cell, although the efficiency they have achieved so far is only one percent. Comparatively, conventional solar cells such as the silicon and multi-junction gallium arsenide types have a maximum efficiency of 20 percent. The team also claims their rectenna can achieve an upper limit of 90 percent efficiency for single wavelength conversion at only a one-tenth the cost of conventional solar cells.

The theory of rectennas is not new, but was discovered more than 50 years ago. However, so far, technology was not advanced enough to fabricate them. According to Professor Baratunda Cola, with currently available technology, it is now possible to make cheap solar-to-electricity converters from carbon nanotubes with ends turned into a special tunnel diode. Cola says the concept is well suited for mass production.

Rectennas are made by growing fields of vertical carbon nanotubes. Their length roughly matches the wavelength of the energy source – for solar radiation, it is one micron. An insulating dielectric such as aluminum oxide caps the carbon nanotubes on the tethered end of the bundles. On the dielectric grows a low-work function of metal – calcium/aluminum. This arrangement makes each nanotube a rectenna with a two electron-volt potential when collecting sunlight and converting it to direct current.

According to Cola, the process uses three steps. In the first step, they grow a large array of vertical nanotube bundles. Then one end of the tubes is coated with a dielectric, while a layer of metal is deposited. One end of the nanotubes changes to a super-fast metal-insulator-metal type of tunnel diode by this process. This method is eminently suitable for mass production, and up to ten times cheaper than making crystalline silicon cells.

With its metal-insulator-metal form, the structure resembles a capacitor with a rating of a few attofarads (1aF = 10-18F). Each nanotube bundle is only 10-20 microns in diameter and consequently, the area of the capacitor plates is so small that the electrical field concentration at the end of the nanotube is very high. With the low work function of the metal, the device behaves just as a tunnel diode does in the peta-hertz (1015 Hertz) region when excited by solar energy and emits electrons in bursts of femtoseconds (10-15 seconds).

Commercialization will require several trillions of nanotube bundles growing side-by-side. Once optimized for higher efficiency, this bunch of nanotube bundles could ramp the power output well into the megawatt range. According to Cola, increasing the efficiency can be achieved by lowering the contact resistance between the antenna and diode. The team expects to improve the efficiency up to 40 percent in only a few years.

New Combination of Materials for Efficient Solar Cells

An international team of scientists have come together to build solar cells with high efficiency using a new blend of materials. The materials used allow the cells to convert sunlight into electrical energy without the addition of dopants. The design, termed DASH, uses molybdenum oxide and lithium fluoride.

Doing away with doping

Most of the solar cells use silicon wafers in the crystalline form. The wafer, along with the layers of materials deposited on it are doped or injected with special impurity atoms that either introduce free electrons or create electron deficiencies called holes. The presence of these extra electrons or holes increases the electrical conductivity of the material. However, the impurities introduce certain complexities within the crystalline structure, which bring down the performance.

Since solar cells are all about increasing the performance of these devices, researchers have been looking at means to eliminate the doping process. The international team has made available a cell prototype with a simple architecture. The journal Nature Energy has published an article on the design of the solar cell. James Bullock, a faculty member of Australian National University and a team member is the principal author of the paper. He has been a visiting researcher at Lawrence Berkeley National Laboratory and UC Berkeley as a part of the project.

Bullock explains that the simple structure of the cell designed by them would cut down the production and operational costs considerably, thereby enhancing the efficiency. The silicon cell, free from doping impurities designed by the team is termed DASH, which is the acronym for dopant-free asymmetric heterocontact. The efficiency of this product is 19%, which exceeds that of other dopant-free cells. The efficiency of previous cells of this category did not exceed 14%.

Special properties of the contact materials

The researchers applied several layers of amorphous silicon over a wafer of crystalline silicon. This was overlaid with very thin films of molybdenum oxide on the sun exposed surface and lithium fluoride at the bottom one. These two external layers are only a few nanometers thick and act as contacts for the holes and electrons. Ali Javey, a team member and a professor of Electrical Engineering and Computer Science at UC, Berkeley, explains that both molybdenum oxide and lithium fluoride have been selected for making up the contacts because of their special properties. The materials form transparent layers at this thickness. Molybdenum oxide has several imperfections in its crystalline structure that allow it to act as an effective hole contact.

Likewise, the defects in the lithium fluoride structure allow it to be a useful electron contact. Stefaan de Wolf, another team member has described in the article how the molybdenum oxide and lithium fluoride layers work as effective contacts when used with a combination of amorphous and crystalline silicon. In addition, these materials have allowed the scientists to come up with remarkable variations in their properties with different thicknesses.

The scientists used the thermal evaporation technique at room temperatures to apply the coatings. Javey said that the researchers intend to use the material combination for other semiconductor applications like transistors to improve their performance.

Can a Solar Cell Store Its Own Power?

Can a Solar Cell Store Its Own Power?

Researchers at Ohio State University have invented a device that looks like a solar cell but has the ability to store the power it generates. The patent-pending device is the world’s first solar battery. On October 3, 2014, the researchers reported in the journal – Nature Communication – that they have succeeded in combining a solar cell and a battery into a single hybrid device.

The innovation is a special solar panel in the form of a mesh that allows entry of air into the battery. Another unique process allows electrons to be transferred between the solar panel and the electrodes of the battery. Light and oxygen entering the device enable chemical reactions to charge the battery.

According to Yiying Wu, Professor of chemistry and biochemistry at the Ohio State University, they will license the new solar battery to industry. Wu expects that the solar battery will tame the costs of renewable energy.

A solar panel is typically used to capture light for converting it to electricity, which is then stored in a cheap battery for later use. By integrating the two functions into a single device, installation becomes simpler and costs go down. The new solar battery may typically bring down the costs by about 25 percent.

The invention also has another advantage. The long interconnections between solar panels and its battery introduce ohmic resistance that reduces the solar energy efficiency because of heat generation when charging. Typically, about 20 percent of the electricity generated by the solar cells is wasted as heat when charging the battery. With the new design, nearly all the electricity generated reaches the battery.

Wu and his students have also developed a high-efficiency battery for use with their solar cells. An earlier designed battery, invented by Wu and his research team, won them the 2014 clean energy prize of $100,000 from the US Department of Energy. The researchers have created a technology spinoff – KAir Energy Systems, LLC – to develop the battery.

The high-efficiency battery is air-powered, meaning it breathes in air when discharging and breathes out when charging. The battery discharges by the chemical reaction of potassium and oxygen. The researchers faced a formidable challenge when trying to combine a solar panel with the KAir type of battery. Typical solar cells are solid panels of semiconductor material and this would prevent air from entering the battery.

Wu and his research team had to redesign the solar panel to make it permeable. They did this by using titanium gauze, a flexible fabric. They grew vertical rods of titanium dioxide on the fabric, similar to blades of grass growing on soil. The rods capture sunlight, while air passes freely through them and the gauze.

Normally, interconnecting a solar cell and a battery requires four electrodes – two on the solar panel and two on the battery. The hybrid design of the researchers has reduced the number of electrodes required to three.

The mesh in the solar panel forms the first electrode. Under this, a thin sheet of porous carbon forms the second electrode, while a lithium plate forms the third. Layers of electrolyte sandwiched between the electrodes forms the battery to store electricity.