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Perovskite New Materials 20 plus id42356EPFL scientists have developed a solar-panel material that can cut down on photovoltaic costs while achieving competitive power-conversion efficiency of 20.2%.
Some of the most promising solar cells today use light-harvesting films made from perovskites – a group of materials that share a characteristic molecular structure. However, perovskite-based solar cells use expensive “hole-transporting” materials, whose function is to move the positive charges that are generated when light hits the perovskite film. Publishing in Nature Energy (“A molecularly engineered hole-transporting material for e cient perovskite solar cells”), EPFL scientists have now engineered a considerably cheaper hole-transporting material that costs only a fifth of existing ones while keeping the efficiency of the solar cell above 20%.
FDT on a Perovskite Surface
This is a 3-D illustration of FDT molecules on a surface of perovskite crystals. (Image: Sven M. Hein / EPFL)
As the quality of perovskite films increases, researchers are seeking other ways of improving the overall performance of solar cells. Inadvertently, this search targets the other key element of a solar panel, the hole-transporting layer, and specifically, the materials that make them up. There are currently only two hole-transporting materials available for perovskite-based solar cells. Both types are quite costly to synthesize, adding to the overall expense of the solar cell.
To address this problem, a team of researchers led by Mohammad Nazeeruddin at EPFL developed a molecularly engineered hole-transporting material, called FDT, that can bring costs down while keeping efficiency up to competitive levels. Tests showed that the efficiency of FDT rose to 20.2% – higher than the other two, more expensive alternatives. And because FDT can be easily modified, it acts as a blueprint for an entire generation of new low-cost hole-transporting materials.
“The best performing perovskite solar cells use hole transporting materials, which are difficult to make and purify, and are prohibitively expensive, costing over €300 per gram preventing market penetration,” says Nazeeruddin. “By comparison, FDT is easy to synthesize and purify, and its cost is estimated to be a fifth of that for existing materials – while matching, and even surpassing their performance.”
Source: Ecole Polytechnique Fédérale de Lausanne
SA Solar 5 191b940e-6e05-402a-bfbb-3e7be5f8a46f_16x9_600x338Perovskites, substances that perfectly absorb light, are the future of solar energy. The opportunity for their rapid dissemination has just increased thanks to a cheap and environmentally safe method of production of these materials, developed by chemists from Warsaw, Poland. Rather than in solutions at a high temperature, perovskites can now be synthesized by solid-state mechanochemical processes: by grinding powders.
We associate the milling of chemicals less often with progress than with old-fashioned pharmacies and their inherent attributes: the pestle and mortar. It’s time to change this! Recent research findings show that by the use of mechanical force, effective chemical transformations take place in solid state. Mechanochemical reactions have been under investigation for many years by the teams of Prof. Janusz Lewinski from the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS) and the Faculty of Chemistry of Warsaw University of Technology.
In their latest publication (“Mechanosynthesis of the hybrid perovskite CH3NH3PbI3: characterization and the corresponding solar cell efficiency”), the Warsaw researchers describe a surprisingly simple and effective method of obtaining perovskites – futuristic photovoltaic materials with a spatially complex crystal structure.
perovskite powders
A simple, fast and safe method of obtaining perovskites has been discovered by scientists from IPC PAS in Warsaw, Poland. The perovskite (a black powder) is milled from two powders: a white one, methylammonium iodide, and a yellow one, lead iodide.
“With the aid of mechanochemistry we are able to synthesize a variety of hybrid inorganic-organic functional materials with a potentially great significance for the energy sector. Our youngest ‘offspring’ are high quality perovskites. These compounds can be used to produce thin light-sensitive layers for high efficiency solar cells,” says Prof. Lewinski.
Perovskites are a large group of materials, characterized by a defined spatial crystalline structure. In nature, the perovskite naturally occurring as a mineral is calcium titanium(IV) oxide CaTiO3. Here the calcium atoms are arranged in the corners of the cube, in the middle of each wall there is an oxygen atom and at the centre of the cube lies a titanium atom. In other types of perovskite the same crystalline structure can be constructed of various organic and inorganic compounds, which means titanium can be replaced by, for example, lead, tin or germanium. As a result, the properties of the perovskite can be adjusted so as to best fit the specific application, for example, in photovoltaics or catalysis, but also in the construction of superconducting electromagnets, high voltage transformers, magnetic refrigerators, magnetic field sensors, or RAM memories.
At first glance, the method of production of perovskites using mechanical force, developed at the IPC PAS, looks a little like magic.
“Two powders are poured into the ball mill: a white one, methylammonium iodide CH3NH3I, and a yellow one, lead iodide PbI2. After several minutes of milling no trace is left of the substrates. Inside the mill there is only a homogeneous black powder: the perovskite CH3NH3PbI3,” explains doctoral student Anna Maria Cieslak (IPC PAS).
“Hour after hour of waiting for the reaction product? Solvents? High temperatures? In our method, all this turns out to be unnecessary! We produce chemical compounds by reactions occurring only in solids at room temperature,” stresses Dr. Daniel Prochowicz (IPC PAS).
The mechanochemically manufactured perovskites were sent to the team of Prof. Michael Graetzel from the Ecole Polytechnique de Lausanne in Switzerland, where they were used to build a new laboratory solar cell. The performance of the cell containing the perovskite with a mechanochemical pedigree proved to be more than 10% greater than a cell’s performance with the same construction, but containing an analogous perovskite obtained by the traditional method, involving solvents.
“The mechanochemical method of synthesis of perovskites is the most environmentally friendly method of producing this class of materials. Simple, efficient and fast, it is ideal for industrial applications. With full responsibility we can state: perovskites are the materials of the future, and mechanochemistry is the future of perovskites,” concludes Prof. Lewinski.
The described research will be developed within GOTSolar collaborative project funded by the European Commission under the Horizon 2020 Future and Emerging Technologies action.
Perovskites are not the only group of three-dimensional materials that has been produced mechanochemically by Prof. Lewinski’s team. In a recent publication the Warsaw researchers showed that by using the milling technique they can also synthesize inorganic-organic microporous MOF (Metal-Organic Framework) materials. The free space inside these materials is the perfect place to store different chemicals, including hydrogen.
Source: Institute of Physical Chemistry of the Polish Academy of Sciences

Australian solar power experts making up the Victorian Organic Solar Cell Consortium have developed and begun to market solar cells that are created with a 3D printer.

 

The group,  consisting of scientists from the CSIRO, the University of Melbourne and Monash University have been working on the technology for over seven years and have figured out a way to cheaply print the panels onto plastic, including smart-phones and laptops, enabling self charging electronics.  They are also able to print directly on to walls and windows using an opaque solar film and claim that they can line a skyscraper with panels, making it totally electrically self sufficient.

“We print them onto plastic in more or less the same way we print our plastic banknotes,” said Fiona Scholes, senior research scientist at CSIRO. “Connecting our solar panels is as simple as connecting a battery. It’s very cheap. The way in which it looks and works is quite different to conventional silicon rooftop solar.”

The next step is to create a solar spray coating to enhance the power of the panel.  “We would like to improve the efficiency of solar panels – we need to develop solar inks to generate more energy from sunlight,” said Scholes. “We are confident we can push the technology further in the years to come.”

To Read More: Science Alerts

1366 Solar untitled1366 Technologies today announced plans to build a state-of-the-art, commercial solar wafer manufacturing facility in Genesee County New York, strategically located between Buffalo and Rochester, that will eventually scale to 3 GW, house 400 Direct Wafer™ furnaces, and produce more than 600 million high-performance silicon wafers per year – enough to power 360,000 American homes.

1366 Technologies will become the anchor tenant at the high-tech Science and Technology Advanced Manufacturing Park (STAMP) where the company will eventually create more than 1,000 new, full-time jobs in New York’s Finger Lakes Region.

“Today is an exciting day, the culmination of a lot of hard work by a talented group of people. From day one, we have taken a deliberate, highly-measured path to scaling. The facility in Bedford, Massachusetts was our proving ground. New York brings us to commercial scale. The technology is ready and 1366 is squarely positioned to lead in an industry undergoing rapid global growth,” said Frank van Mierlo, CEO, 1366 Technologies. “We are extremely proud to become part of the Upstate New York community and are committed to the region’s vibrant future.”

The site selection marks the start of a phased program to methodically scale 1366 Technologies Direct Wafer™ technology – a transformative manufacturing process that produces a uniformly better silicon solar wafer at half the cost – from 250 MW to 3 GW. 1366 Technologies will first construct a 250 MW facility that will produce more than 50 million standard silicon wafers per year. The facility will quickly ramp to 1 GW of production capacity and employ 300 people.

“Our goal has always been two-fold: deliver solar at the cost of coal and manufacture – at scale – in the United States,” continued van Mierlo. “Today’s announcement signifies that we’re on our way to achieving both.”

To encourage 1366 Technologies to invest and establish operations in New York, Governor Cuomo’s administration offered a competitive and attractive incentive package through various state and local resources including Empire State Development, New York’s chief economic development agency; New York State Energy Research and Development Authority (NYSERDA); New York State Homes and Community Renewal (HCR); New York Power Authority (NYPA); and Genesee County Industrial Development Agency. In September 2011, 1366 was also issued a $150 million loan guarantee from the U.S. Department of Energy (DOE) to build a commercial-scale manufacturing facility.

Construction of the 130,000 square-foot facility is slated to begin no later than Q2 of 2016 and is expected to be completed in 2017.

“Today’s announcement is an example of how we are combining this region’s natural strengths with our vision to develop New York’s entrepreneurial future and make the Empire State a true leader in developing the clean energy technologies of tomorrow. I am proud to continue building on Upstate’s economic resurgence and I am pleased to have 1366 helping us lead the way forward,” said Governor Cuomo.

“STAMP, the site of this expansion, is strategically located between Buffalo and Rochester, which enables 1366 Technologies to draw on the highly-skilled and talented workforce available in our region,” said Mark S. Peterson, president and CEO of Greater Rochester Enterprise. “1366 Technologies’ decision to expand its operations here not only marks the largest business attraction success story in our organization’s history, but it also brings two great cities even closer together, strengthening our efforts to make Upstate New York a hot-bed for high-tech development.”

“The strategy Governor Cuomo has developed to create a statewide high tech and advanced manufacturing corridor from Albany to Buffalo will change the economic fortunes for Upstate New York for generations to come,” said Steve Hyde, president and CEO, Genesee County Economic Development Center (GCEDC). “We are very excited to welcome 1366 Technologies to Genesee County and stand ready to assist the company in any way we can as the first phase of the development of the STAMP site begins.”

“I want to congratulate 1366 Technologies and thank them for bringing this exciting project to upstate New York,” said Buffalo Niagara Enterprise President & CEO Thomas Kucharski. “1366 Technologies is bringing a revolutionary process to an industry that is transforming our regional economy. The very assets and partnerships that attracted 1366 to the STAMP site remain in place, and are well positioned to ensure the success of this company and industry well into the future.”

Source: http://www.1366tech.com/

IMAGEIMAGE: A surfactant template guides the self-assembly of functional polymer structures in an aqueous solution. view more

Credit: Image credit: Oak Ridge National Laboratory, U.S. Dept. of Energy; image by Youngkyu Han and Renee Manning.

OAK RIDGE, Tenn., Oct. 5, 2015–The efficiency of solar cells depends on precise engineering of polymers that assemble into films 1,000 times thinner than a human hair.

Today, formation of that polymer assembly requires solvents that can harm the environment, but scientists at the Department of Energy’s Oak Ridge National Laboratory have found a “greener” way to control the assembly of photovoltaic polymers in water using a surfactant– a detergent-like molecule–as a template. Their findings are reported in Nanoscale, a journal of the Royal Society of Chemistry.

“Self-assembly of polymers using surfactants provides huge potential in fabricating nanostructures with molecular-level controllability,” said senior author Changwoo Do, a researcher at ORNL’s Spallation Neutron Source (SNS).

The researchers used three DOE Office of Science User Facilities–the Center for Nanophase Materials Sciences (CNMS) and SNS at ORNL and the Advanced Photon Source (APS) at Argonne National Laboratory–to synthesize and characterize the polymers.

“Scattering of neutrons and X-rays is a perfect method to investigate these structures,” said Do.

The study demonstrates the value of tracking molecular dynamics with both neutrons and optical probes.

“We would like to create very specific polymer stacking in solution and translate that into thin films where flawless, defect-free polymer assemblies would enable fast transport of electric charges for photovoltaic applications,” said Ilia Ivanov, a researcher at CNMS and a corresponding author with Do. “We demonstrated that this can be accomplished through understanding of kinetic and thermodynamic mechanisms controlling the polymer aggregation.”

The accomplishment creates molecular building blocks for the design of optoelectronic and sensory materials. It entailed design of a semiconducting polymer with a hydrophobic (“water-fearing”) backbone and hydrophilic (“water-loving”) side chains. The water-soluble side-chains could allow “green” processing if the effort produced a polymer that could self-assemble into an organic photovoltaic material. The researchers added the polymer to an aqueous solution containing a surfactant molecule that also has hydrophobic and hydrophilic ends. Depending on temperature and concentration, the surfactant self-assembles into different templates that guide the polymer to pack into different nanoscale shapes–hexagons, spherical micelles and sheets.

In the semiconducting polymer, atoms are organized to share electrons easily. The work provides insight into the different structural phases of the polymer system and the growth of assemblies of repeating shapes to form functional crystals. These crystals form the basis of the photovoltaic thin films that provide power in environments as demanding as deserts and outer space.

“Rationally encoding molecular interactions to rule the molecular geometry and inter-molecular packing order in a solution of conjugated polymers is long desired in optoelectronics and nanotechnology,” said the paper’s first author, postdoctoral fellow Jiahua Zhu. “The development is essentially hindered by the difficulty of in situ characterization.”

In situ, or “on site,” measurements are taken while a phenomenon (such as a change in molecular morphology) is occurring. They contrast with measurements taken after isolating the material from the system where the phenomenon was seen or changing the test conditions under which the phenomenon was first observed. The team developed a test chamber that allows them to use optical probes while changes occur.

Neutrons can probe structures in solutions

Expertise and equipment at SNS, which provides the most intense pulsed neutron beams in the world, made it possible to discover that a functional photovoltaic polymer could self-assemble in an environmentally benign solvent. The efficacy of the neutron scattering was enhanced, in turn, by a technique called selective deuteration, in which specific hydrogen atoms in the polymers are replaced by heavier atoms of deuterium–which has the effect of heightening contrasts in the structure. CNMS has a specialty in the latter technique.

“We needed to be able to see what’s happening to these molecules as they evolve in time from some solution state to some solid state,” author Bobby Sumpter of CNMS said. “This is very difficult to do, but for molecules like polymers and biomolecules, neutrons are some of the best probes you can imagine.” The information they provide guides design of advanced materials.

By combining expertise in topics including neutron scattering, high-throughput data analysis, theory, modeling and simulation, the scientists developed a test chamber for monitoring phase transitions as they happened. It tracks molecules under conditions of changing temperature, pressure, humidity, light, solvent composition and the like, allowing researchers to assess how working materials change over time and aiding efforts to improve their performance.

Scientists place a sample in the chamber and transport it to different instruments for measurements. The chamber has a transparent face to allow entry of laser beams to probe materials. Probing modes–including photons, electrical charge, magnetic spin and calculations aided by high-performance computing–can operate simultaneously to characterize matter under a broad range of conditions. The chamber is designed to make it possible, in the future, to use neutrons and X-rays as additional and complementary probes.

“Incorporation of in situ techniques brings information on kinetic and thermodynamic aspects of materials transformations in solutions and thin films in which structure is measured simultaneously with their changing optoelectronic functionality,” Ivanov said. “It also opens an opportunity to study fully assembled photovoltaic cells as well as metastable structures, which may lead to unique features of future functional materials.”

Whereas the current study examined phase transitions (i.e., metastable states and chemical reactions) at increasing temperatures, the next in situ diagnostics will characterize them at high pressure. Moreover, the researchers will implement neural networks to analyze complex nonlinear processes with multiple feedbacks.

The title of the Nanoscale paper is “Controlling molecular ordering in solution-state conjugated polymers.”

###

Zhu, Do and Ivanov led the study. Zhu, Ivanov and Youngkyu Han conducted synchrotron X-ray scattering and optical measurements. Sumpter, Rajeev Kumar and Sean Smith performed theory calculations. Youjun He and Kunlun Hong synthesized the water-soluble polymer. Peter Bonnesen conducted thermal nuclear magnetic resonance analysis on the water-soluble polymer. Do, Han and Greg Smith performed neutron measurement and analysis of the scattering results. This research was conducted at CNMS and SNS, which are DOE Office of Science User Facilities at ORNL. Moreover, the Advanced Photon Source, a DOE Office of Science User Facility at Argonne National Laboratory, was used to perform synchrotron X-ray scattering on the polymer solution. Laboratory Directed Research and Development funds partially supported the work.

UT-Battelle manages ORNL for DOE’s Office of Science. The single largest supporter of basic research in the physical sciences in the United States, the Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit http://science.energy.gov/.

NASA Discovers Evidence for Liquid Water on Mars

wired.com– For years, scientists have known that Mars has ice locked away within its rusty exterior. More elusive, though, is figuring out how much of that water is actually sloshing around in liquid form. No…

St. Mary’s College Maryland: New research puts us closer to DIY Spray-on Solar Cell Technology

genesisnanotech.wordpress.com– In a 2014 study, published in the journal Physical Chemistry Chemical Physics, St. Mary’s College of Maryland energy expert Professor Troy Townsend introduced the first fully solution-processed all…

GSA and DOE: Gaps in Knowledge About ‘Best Practices’ for Fracking and a Sustainable Energy Future: The Water Impact

genesisnanotech.wordpress.com – Though applied since the 1940s, hydraulic fracturing boomed in the 1990s, according to The Geological Society of America. New technology allowed the practice to be applied to horizontal wells for e…

Case Western University: Using Solar Cells (Energy) to Charge a lithium-ion Batteries for Electric Vehicles

genesisnanotech.wordpress.com– Charging cars by solar cell would appear to be the answer. But most cells fail to meet the power requirements needed to directly charge lithium-ion batteries used in today’s all-electric and plug-i…

Oak Ridge National Laboratory: Super Capacitors for Electric Cars … From Scrap Tires … Really??

genesisnanotech.wordpress.com – Oak Ridge National Laboratory: Super Capacitors for Electric Cars … From Scrap Tires … Really?? Some of the 300 million tires discarded each year in the United States alone could be used in superca…

MIT: Analysis sees many Promising Pathways for Solar Photovoltaic Power

genesisnanotech.wordpress.com – New study identifies the promise and challenges facing large-scale deployment of solar photovoltaics. In a broad new assessment of the status and prospects of solar photovoltaic technology, MIT res…

St Mary Spray on Solar 150928083119_1_540x360A new study out of St. Mary’s College of Maryland puts us closer to do-it-yourself spray-on solar cell technology — promising third-generation solar cells utilizing a nanocrystal ink deposition that could make traditional expensive silicon-based solar panels a thing of the past.

In a 2014 study, published in the journal Physical Chemistry Chemical Physics, St. Mary’s College of Maryland energy expert Professor Troy Townsend introduced the first fully solution-processed all-inorganic photovoltaic technology.

While progress on organic thin-film photovoltaics is rapidly growing, inorganic devices still hold the record for highest efficiencies which is in part due to their broad spectral absorption and excellent electronic properties. Considering the recorded higher efficiencies and lower cost per watt compared to organic devices, combined with the enhanced thermal and photo stability of bulk-scale inorganic materials, Townsend, in his 2014 study, focused on an all-inorganic based structure for fabrication of a top to bottom fully solution-based solar cell.

A major disadvantage compared to organics, however, is that inorganic materials are difficult to deposit from solution. To overcome this, Townsend synthesized materials on the nanoscale. Inorganic nanocrystals encased in an organic ligand shell are soluble in organic solvents and can be deposited from solution (i.e., spin-, dip-, spray-coat) whereas traditional inorganic materials require a high temperature vacuum chamber. The solar devices are fabricated from nanoscale particle inks of the light absorbing layers, cadmium telluride/cadmium selenide, and metallic inks above and below. This way, the entire electronic device can be built on non-conductive glass substrates using equipment you can find in your kitchen.

The outstanding challenge facing the (3-5 nm) inorganic nanocrystals is that they must be annealed or heated to form larger ‘bulk scale’ grains (100 nm to 1 μm) in order to produce working devices. Townsend recently teamed with Navy researchers to explore this process.

St Mary Spray on Solar 150928083119_1_540x360

A spray-on nanocrystal solar cell array.
Credit: Image courtesy of St. Mary’s College of Maryland

“When you spray on these nanocrystals, you have to heat them to make them work,” explained Townsend, “but you can’t just heat the crystals by themselves, you have to add a sintering agent and that, for the last 40 years, has been cadmium chloride, a toxic salt used in commercial thin-film devices. No one has tested non-toxic alternatives for nanoscale ink devices, and we wanted to explore the mechanism of the sintering process to be able to implement safer salts.”

In his latest study, published this year in the Journal of Materials Chemistry A, Townsend, along with Navy researchers, found that ammonium chloride is a non-toxic, inexpensive viable alternative to cadmium chloride for nanocrystal solar cells. This discovery came after testing several different salts. Devices made using ammonium chloride (which is commonly used in bread making) had comparable device characteristics to those made with cadmium chloride, and the move away from cadmium salt treatments alleviates concerns about the environmental health and safety of current processing methods.

The team also discovered that the role of the salt treatment involves crucial ligand removal reactions. This is unique to inorganic nanocrystals and is not observed for bulk-scale vacuum deposition methods. “A lot of exciting work has been done on nanocrystal ligand exchange, but, for the first time, we elucidated the dual role of the salt as a ligand exchange agent and a simultaneous sintering agent. This is an important distinction for these devices, because nanocrystals are typically synthesized with a native organic ligand shell. This shell needs to be removed before heating in order to improve the electronic properties of the film,” said Townsend about the discovery. Because nanomaterials are at the forefront of emerging new properties compared to their bulk counterpart, the study is important to the future of electronic device fabrication.

The research comes in the wake of the Obama Administration’s announcement in July to put more solar panels on low-income housing and expand access to solar power for renters, and recent pledge to get 20 percent of the U.S. total electricity from renewable sources by the year 2030.

“Right now, solar technology is somewhat unattainable for the average person,” said Townsend. “The dream is to make the assembly and installation process so cheap and simple that you can go to your local home improvement store and buy a kit and then spray it on your own roof. That is why we we’re working on spray-on solar cells.” Townsend plans for further research to increase the efficiency of the all-inorganic nanocrystal solar cells (currently reaching five percent), while building them with completely non-toxic components.

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Story Source:

The above post is reprinted from materials provided by St. Mary’s College of Maryland. Note: Materials may be edited for content and length.


Journal References:

  1. Troy K. Townsend, William B. Heuer, Edward E. Foos, Eric Kowalski, Woojun Yoon, Joseph G. Tischler. Safer salts for CdTe nanocrystal solution processed solar cells: the dual roles of ligand exchange and grain growth. J. Mater. Chem. A, 2015; 3 (24): 13057 DOI: 10.1039/C5TA02488A
  2. Troy K. Townsend, Edward E. Foos. Fully solution processed all inorganic nanocrystal solar cells. Physical Chemistry Chemical Physics, 2014; 16 (31): 16458 DOI: 10.1039/C4CP02403F

Hong Kong Poly U 150908103652_1_540x360

PolyU’ invention of semitransparent perovskite solar cells with graphene electrodes, the power conversion efficiencies (PCEs) are around 12% which is much higher than existing semitransparent solar cells.
Credit: Image courtesy of The Hong Kong Polytechnic University

Developing transparent or semitransparent solar cells with high efficiency and low cost to replace the existing opaque and expensive silicon-based solar panels has become increasingly important due to the increasing demands of the building integrated photovoltaics (BIPVs) systems. The Department of Applied Physics of The Hong Kong Polytechnic University (PolyU) has successfully developed efficient and low-cost semitransparent perovskite solar cells with graphene electrodes. The power conversion efficiencies (PCEs) of this novel invention are around 12% when they are illuminated from Fluorine-doped Tin Oxide bottom electrodes (FTO) or the graphene top electrodes, compared with 7% of conventional semitransparent solar cells. Its potential low cost of less than HK$0.5/Watt, more than 50% reduction compared with the existing cost of Silicon solar cells, will enable it to be widely used in the future.

KAUST QD SOLAR img_0519-0

The triplet state lifetime varies with the distance and the strength of binding between the porphyrin and the surface of the quantum dot.

Nanotechnology could improve the efficiency of organic photovoltaic technology, researchers at King Abdullah University of Science and Technology (KAUST) have demonstrated1.
In general, solar cells made from organic materials offer a cheap, simple and sustainable approach to harvesting light from the sun. But there is an urgent need to improve the efficiency of these organic cells.
The performance of these devices is limited by the re-emission of light that has been absorbed, thus detracting energy that should be converted to electricity. When an organic material absorbs light, it can create an exciton — an electron paired to a positively charged equivalent called a hole. This exciton exists for a very short period before recombining radiatively or non-radiatively. So, for a useful current to be produced, the electron and hole must separate before they recombine.
Research by Omar Mohammed and his colleagues from the KAUST Solar and Photovoltaics Engineering Research Center show how the lifetime of excitons in an organic material can be extended by using quantum dots.
Quantum dots are nanometer scale particles. Their advantage in solar-cell technologies is their tunability: the optical properties, such as absorption wavelength, can be changed by varying the size of the dot. Additional molecules attached to the surface of the nanostructure can tailor the functionality of the dots even further.
Mohammed’s team investigated a family of organic compounds (commonly used in solar applications) known as porphyrins. The electron-hole pair generated in porphyrin by light absorption forms a high-energy exciton, which then relaxes to one of two different lower-energy excitons know as a singlet and a triplet.
“The photo-generated singlet excitons exhibit very short lifetimes and consequently they have short diffusion lengths, which is one of the greatest challenges for achieving high power-conversion efficiencies in solar-cell devices,” explains Mohammed. “Triplet excitons with their long lifetimes are an alternative way to overcome this problem.”

abu-dhabi-solar
The researchers showed that cadmium telluride quantum dots can improve not only the path from excited exciton to triplet exciton — so-called intersystem crossing, but also the elongation of the triplet exciton lifetime. They were able to tune the intersystem crossing and the triplet state lifetime by changing the size of the quantum dots in the solution.
“We are currently testing other absorber materials and other semiconductor quantum dots,” says Mohammed. “In addition, we are planning to fabricate solar cell devices from these nano-assemblies.”

© 2015 KAUST

Reference
Ahmed, G. H., Aly, S. M., Usman, A., Eita, M. S., Melnikov, V. A. & Mohammed, O. F. Quantum confinement-tunable intersystem crossing and the triplet state lifetime of cationic porphyrin–CdTe quantum dot nano-assemblies. Chemical Communications 51, 8010—8013 (2015). | article

*** For More on Quantum Dots and Solar Energy ‘Search’ Quantum Dots on our Blog. ***

“Great Things from Small Things”

GNT Thumbnail Alt 3 2015-page-001

 

Graphene Perovskite 081115 324x182 EPFL scientists have created the first perovskite nanowire-graphene hybrid phototransistors. Even at room temperature, the devices are highly sensitive to light, making them outstanding photodetectors.

 

The lead-containing perovskite materials can turn light into electricity with high efficiency, which is why they have revolutionized solar cell technologies. On the other hand, graphene is known for its super-strength as well as its excellent electrical conductivity. Combining the two materials, EPFL scientists have created the first ever class of hybrid transistors that turn light into electricity with high sensitivity and at room temperature. The work is published in Small.

The lab of László Forró at EPFL, where the chemical activity is led by Endre Horváth, used its expertise in microengineering to create nanowires of the perovskite methylammonium lead iodide. This highly non-trivial route for the synthesis of nanowires was developed by him in 2014 and called slip-coating method. The advantage of nanowires is their consistency, while their manufacturing can be controlled to modify their architecture and explore different designs.

Making a device by depositing the perovskite nanowires onto graphene has increased the efficiency in converting light to electrical current at room temperature. “Such a device shows almost 750,000 times higher photoresponse compared to detectors made only with perovskite nanowires,” added Massimo Spina who fabricated the miniature photodetectors. Because of this exceptionally high sensitivity, the graphene/perovskite nanowire hybrid device is considered to be a superb candidate for even a single-photon detection.

This work was founded by the Swiss National Science Foundation. The hybrid devices were fabricated in part at EPFL’s Center for Micro/Nanotechnology.

Reference


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