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


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

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Solution coating the easy way


201306047919620Researchers in the US and China have developed the first solution-coating technique capable of producing high-quality, large-area single-crystalline organic semiconductor thin films suitable for high-performance, low power and inexpensive printed electronic circuits. The technique, dubbed FLUENCE (fluid-enhanced crystal engineering) can be used to make thin film organic semiconductors with record charge carrier mobilities.

Fluid flow around micropillars

Solution coating of organic semiconductors is an excellent method for making large-area and flexible electronic materials. However, it is not at all good for making aligned single-crystalline thin films – the ideal form for organic semiconductors and that have the best electronic properties. Aligned crystals are preferred in these materials because charge carrier transport through these structures depends on the crystal orientation.

A team led by Zhenan Bao at Stanford University and Stefan Mannsfeld of the SLAC National Accelerator Laboratory is now reporting on a new solution-coating method that can produce high-quality, millimetre-wide and centimetre-long highly aligned single-crystalline organic semiconductor thin films. The essence of FLUENCE is that we are able to control the flow of liquid in which the organic semiconductor is dissolved, explains team member Ying Diao. During fast printing, this “ink” often distributes itself unevenly – something that leads to defects and other structural imperfections quickly appearing in the semiconducting crystals.

FLUENCE tackles this problem from two angles, she says. First, it works using a microstructured printing blade containing tiny pillars that mixes the ink uniformly. Second, specially designed chemical patterns on the substrate prevent the crystals from aligning randomly or “stochastically” in a direction that would be the opposite to that in which printing is taking place. These two methods combined lead to large-area highly aligned single crystalline films that are much more structurally perfect.

To prove that their technique works, the researchers fabricated an organic semiconductor made from TIPS-pentacene, a routinely used and much studied organic semiconductor material, and found a charge carrier mobility of as high as 11 cm2 V−1 s−1. This is the first time a mobility of greater than 10 cm2 V−1 s−1 has been reported for TIPS-pentacene.

“The concepts we have developed in FLUENCE could easily be scaled up and applied to commercial printing methods,” said Diao. “The significant improvement in structural quality and electrical performance of the thin films printed with our method could allow to make higher performance, lower power, small and inexpensive organic circuitry,” she told nanotechweb.org. “We hope that our work will help advance such a morphology-by-design approach to make organic semiconductors for high-performance, large-area printed electronics.”

The team, which includes researchers from Nanjing University in China, says that it will now look at pattering aligned crystals at length scales suitable for making sub-micron devices.

The present work is reported in Nature Materials.

About the author

Belle Dumé is contributing editor at nanotechweb.org.

Self-assembling Solar-harvesting Films Reveals New Low-Cost Tool for 3D Circuit Printing


4 March 2013 (created 4 March 2013)

QDOTS imagesCAKXSY1K 8Scientists from Imperial College London, working at the Institut Laue-Langevin, have presented a new way of positioning nanoparticles in plastics, with important applications in the production of coatings and photovoltaic material that harvest energy from the sun.  The study used neutrons to understand the role that light – even ambient light – plays in the stabilisation of these notoriously unstable thin films. As a proof of concept the team have shown how the combination of heat and low intensity visible and UV light could in future be used as a precise, low-cost tool for 3D printing of self-assembling, thin-film circuits on these films.
Thin films made up of long organic molecule chains called polymers and fullerenes (large football-shaped molecules composed entirely of carbon) are used mainly in polymer solar cells where they emit electrons when exposed to visible or ultraviolet sun rays. These so-called photovoltaic materials can generate electrical power by converting solar radiation into direct electrical current.
Polymer solar cells are of significant interest for low-power electronics, such as autonomous wireless sensor networks used to monitor everything from ocean temperature to stress inside a car engine. These fullerene-polymer mixtures are particularly appealing because they are lightweight, inexpensive to make, flexible, customisable on the molecular level, and relatively environmentally-friendly.
However current polymer solar cells only offer about one third of the efficiency of other energy harvesting materials, and are very unstable.
In order to improve science’s understanding of the dynamics of these systems and therefore their operational performance, the team carried out neutron reflectometry experiments at the ILL, the world’s flagship centre for neutron science, on a simple model film made up of pure fullerenes with a flexible polymer. Neutron reflectometry is a non-destructive technique that allows you to ‘shave’ layers off these thin films to look at what happens to the fullerenes and the polymers separately, at atomic scale resolution, throughout their depth.
Whilst previous theories suggested that thin film stabilisation was linked to the formation of an expelled fullerene nanoparticle layer at the substrate interface, neutron reflectometry experiments showed that the carbon “footballs” remain evenly distributed throughout the layer. Instead, the team revealed that the stabilisation of the films was caused by a form of photo-crosslinking of the fullerenes. The process imparts greater structural integrity to films, which means that ultrathin films, (down to 10000 times smaller than a human hair) readily become stable with trace amounts of fullerene.
The implications of this finding are significant, particularly in the potential to create much thinner plastic devices which remain stable, with increased efficiency and lifetime (whilst the smaller amount of material required minimises their environmental impact).

The light sensitivity also suggests a unique and simple tool for imparting patterns and designs onto these notoriously unstable films. To prove the concept the team used a photomask to spatially control the distribution of light and added heat. The combination causes the fullerenes to self-assemble into well-defined connected and disconnected patterns, on demand, simply by heating the film until it starts to soften. This results in spontaneous topography and may form the basis of a low-cost tool for 3D printing of thin film circuits.

Other potential applications could include patterning of sensors or biomedical scaffolds.
In the future, the team is looking to apply its findings to conjugated polymers and fullerene derivatives, more common in commercial films, and industrial thin film coatings.

Source: From A neutron investigation into self-assembling solar-harvesting films reveals new low-cost tool for 3D circuit printing. This work is detailed in the paper “Patterning Polymer–Fullerene Nanocomposite Thin Films with Light” by Him Cheng Wong, Anthony M. Higgins, Andrew R. Wildes, Jack F. Douglas, João T. Cabral.

Third Generation Solar Technology And DSC Technology – An Interview With Gordon Thompson


 

 

Date Added: Nov 7, 2012 | Updated: Nov 7, 2012

 

 
In the first part of this ‘insight from industry’ interview, Gordon Thompson, CEO & Executive Director at Dyesol Ltd, talks to AZoM about recent innovations in solar technology. Interview conducted by Gary Thomas.
What is meant by 3rd generation solar technology – how has solar technology progressed in the last decade?

Solar technology has progressed significantly over the last 10 years although unlike some industries where older technologies become obsolete quicjkly, consumers and developers still work across all ‘generations’ of solar technology. These generations include:

1st Generation – Crystalline Silicon

By far the most prevalent bulk material in solar cells is silicon. It is separated into multiple categories: monocrystalline, polycrystalline and ribbon silicon. Crystalline silicon cells account for around 90 per cent of the market. The annual growth rate is expected to be 30 per cent. CSi uses higher cost, high energy, super “clean-room” manufacturing environment.

2nd Generation – Thin Film Semiconductor

Categorized by the cell materials: amorphous or nano-crystalline, e.g. CdTe. The thin film share, in terms of actual production, was 13.5 per cent in 2010. Thin Film technologies use more rare materials in manufacture.

3rd Generation – Artificial Photosynthesis, Nanotechnology

Third generation PV includes multiple technologies, including DSC, that seek to improve upon first two generations through a combination of cost reduction, increased energy efficiency, improved aesthetics, and opportunity for product integration. Dye Solar Cell technology uses less energy in manufacture – making it a much more environmentally friendly choice, is cheaper to manufacture, and one of the key benefits is that it works well in low-light real-world solar conditions, such as on cloudy days which are common in the heavily populated northern hemisphere.

Could you briefly explain the term ‘DSC technology’ and how this is manufactured?

DSC are made from a few key materials applied in very thin (many times thinner than a human hair) layers to a substrate (such as glass, steel or plastic).  On top is a transparent anode made of fluoride-doped tin dioxide deposited on the back of a glass plate or other substrate. On the back of this conductive plate is a thin layer of titanium dioxide (TiO2), which forms into a highly porous structure with an extremely high surface area. The plate is then immersed in a mixture of a photosensitive ruthenium-polypyridine dye (also called molecular sensitizers) and a solvent. After soaking the film in the dye solution, a thin layer of the dye is left covalently bonded to the surface of the TiO2.

A separate plate is then made with a thin layer of the iodide electrolyte spread over a conductive sheet, typically platinum metal. The two plates are then joined and sealed together to prevent the electrolyte from leaking. Although DSC uses a number of advanced materials, these are inexpensive compared to the silicon needed for traditional silicon solar cells because they require no expensive manufacturing steps.

TiO2, for instance, is already widely used as a paint base.  The materials are applied to a substrate by screen printing, soaking, and baking and the whole process takes place in a much less stringent manufacturing environment – at standards similar to the food manufacturing industry – not the “space-suit”, high energy/high cost, super clean room environments of Crystalline Silicon technology.
When sunlight, indirect light, indoor/artificial light, dappled light, or low light strikes the DSC, the light excites electrons in the dye which are absorbed by the titanium to generate an electric current.

What are the advantages of using DSC technology over Silicon based photovoltaic technology?

DSC technology has been widely recognised as a technology of the future because DSC has a number of outstanding characteristics and benefits, which include:

  • DSC performs well in real world sun conditions – shade, dawn, dusk, dappled light, haze, cloud, and even indoor light
  • Low energy, low cost manufacturing process,
  • No toxic materials,
  • Small layers/quantities of product saves resources,
  • Aesthetically appealing options for building integration, and
  • Widely available raw materials.

These features combine to make DSC a clean, green technology inherently suitable to application in the built environment where the largest part of human activity occurs and where electricity demand is highest.

Good Performance in Real-World Solar Conditions

DSC technology works well – and relatively better than other solar technologies – in real-world solar conditions, including: cloudy days, hazy days, polluted skies, at higher latitudes (i.e. Europe, Asia, North America), at dawn and at dusk, not just at noon on a sunny day.

Low Embodied Energy & Nanotechnology

An important distinction of DSC, which distinguishes this technology from all other photovoltaic systems, is its nanotechnology basis.  One of DSC’s key materials is a nanostructured titanium dioxide (TiO2), which provides the host matrix for the photoactive dye, offers unique electric properties, unique optical properties (such as transparency), and unique mechanical properties.  Nanomaterials can be processed at much lower temperatures.  For example, micrometer-sized TiO2 particles are processed at temperatures around 1,000 ˚C, but nano-TiO2 particles are processed at temperatures around 500 ˚C.  This  saves considerable amounts of energy and means that Dye  Solar Cells have less embodied energy than competitor technologies.  DSC is truly a clean, green technology.

Low Cost Manufacturing Processes

Since traditional PV technologies rely heavily on vacuum processing and require extremely high purity materials and stringent cleanliness for the manufacturing environment, these technologies are generally based on expensive equipment, including the most sophisticated and energy-hungry clean rooms and all factory workers wearing ‘space-suit’-type work gear.  In contrast, DSC manufacture relies mainly on printing, ‘baking’ and packaging processes.  Only relatively moderate control of atmospheric dust and moisture is required for DSC assembly.  Most production steps are similar to high throughput processes used by the coating, printing, lamination and food packaging industries. Therefore, capital expenditure for manufacturing is much lower for DSC, a fact which is certainly appreciated by our commercial partners.

Flexible Applications & Scalable Production

DSC technology may be applied onto a range of substrates, including glass, metal, and polymeric substrates.  DSC technology can be applied onto rigid and flexible substrates and is bifacial, meaning it can take in light from both sides of a pane of glass for example.  DSC technology is also scalable to high levels and the ability to integrate DSC into roll-to-roll manufacturing lines (such as in DSC enabled steel roofing material) makes high volume product manufacture achievable.

Low Environmental Impact – Non-Toxic Raw Materials

None of the materials used in today’s DSC is known to be toxic according to international standards and regulations.  The main ruthenium-based dye used today has been biologically tested (AMES test) and found not to be mutagenic. As DSC does not utilise toxic raw materials in cell production, there is minimal remediation risk and other additional protective measures in production are minimal. In contrast, some of the potential competitor technologies to DSC rely on very toxic materials such as cadmium and rather toxic materials such as selenium used for CdTe (cadmium telluride) and CIGS photovoltaics.
Small quantities of product saves resources

DSC is the technology with the thinnest-possible photoactive absorbing layer: one single molecular layer of a sensitiser dye spread out over a high surface area and low cost titanium dioxide (TiO2) layer.

Added up, the sensitiser layers on any DSC panel amount to a thickness corresponding to around 1 micrometer, i.e. many times thinner than a human hair.

Thus DSC is the ultimate ‘miser’ when it comes to the usage of natural resources.  In comparison, silicon wafers used for standard solar panels, are more than 100x thicker once in the product, plus there are considerable material losses and waste during processing, at the wafer sawing stage in particular.

Aesthetics

No other photovoltaic technology offers nearly as much flexibility in terms of colouration and transparency as Dye Solar Cell technology due to the very nature of the Dye Solar Cell chemistry.  Many architects are attracted towards DSC for its virtually endless possibilities of colours and transparency for windows, doors, atriums, skylights and internal dividing walls, all whilst producing clean energy.  DSC windows will not only provide electricity, but can also moderate harsh sunlight and provide thermal and noise insulation. While the most efficient dyes used today are red to yellow – orange, green, grey and brown colours offer attractive efficiencies as well.  DSC windows for office buildings can be coloured in a neutral grey, whereas art galleries and music halls may opt for more vibrant and expressive colours.  DSC integrated into glass houses could filter and scatter stark sunlight, whilst converting the part of the solar spectrum – which does not contribute effectively to the growth of plants to electricity – right at the point of use.

Widely Available Raw Materials

The major chemical materials used in DSC are carbon, oxygen, nitrogen, hydrogen, titanium, and silicon (glass) or iron (steel) for substrates, plus very small amounts of platinum and ruthenium.  With the exception of the latter two, these are all very common materials and there is no shortage in sight.  The most critical component in today’s DSC in terms of natural resources is ruthenium.  Annual production of 20 million m2 of DSC panels producing close to 1.5 GW at maximum power would today require about 2 tonnes of ruthenium, which corresponds to 6-7% of the annual worldwide ruthenium production or to only about 0.03% of the estimated mineable world resources.

Material supply availability is even less significant with platinum since this metal is used in even smaller quantities in DSC compared to ruthenium and because natural platinum resources are significantly higher than those of ruthenium. In comparison, second generation solar technology CIGS (which stands for copper, gallium, indium, and diselenide), requires significant amounts of indium.  Assuming again an annual production of 20 million m2 of CIGS PV panels, about 120 tonnes of indium would be required annually.  This corresponds to about 20% of the annual worldwide indium production or about 1% of the estimated mineable worldwide resources, which is a very significant amount.  With all of the other applications for indium, such as LCD displays, it is likely that indium will become harder and harder to source and thus more and more expensive than today.

A ruthenium dye is used in the manufacture of DSC technology. Why is ruthenium used? Could this be substituted by another element? 

In dye sensitised solar cells, the dye is one of the key components for high-power conversion efficiencies.  Ruthenium based dyes are used because of their better performance and stability characteristics.  Yes, other dyes may be used and there is a body of work on organic dyes as alternatives, however, at the moment these alternates do not match the ruthenium based dyes on performance or stability.