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Home > Nanotechnology Columns > Bo Varga > Nanotechnology Enables Solar Power Advances in 2010 & 2011

Bo Varga
Managing Director
Silicon Valley Nano Ventures

This July, 2011 nano + solar column focuses on emerging & advanced solar technologies including the emerging global R&D role of KAUST in Saudi Arabia, as well as several recent announcements in advanced quantum dot & quantum well technologies & the printing of CIGS type solar cells. My focus is on the business value of technology including manufacturing and application breakthroughs & not on deep technology.

July 6th, 2011

Nanotechnology Enables Solar Power Advances in 2010 & 2011

Recent announcements from Japan indicate a desire to replace nuclear power plants with solar energy, but at a price 1/6 of today's installed solar system pricing by 2020-2030. Achieving that goal will require a reduction from $3.60/WP to 60 cents/WP (Watt Peak = theoretical best case solar power from a panel). Taking solar panels as 50% of system cost that will require a sales price of 30 cents/WP. The lowest cost solar PV manufacturer in the world today, First Solar, expects to achieve 50 - 60 cents/WP cost in the next 3 to 5 years. Achieving 30 cents/WP sales price for solar panels will require major breakthroughs in device stack & manufacturing technologies and also for the Balance of System (BOS) components.

More people with a "higher" standard of living require more energy for agriculture, cooling, electronic devices, heating, and transportation. Oil and its derivatives, aviation fuel, diesel fuel, & gasoline, has the highest energy density by weight & volume, however the world supply of oil has peaked while the demand in oil producing countries has surged. Saudi Arabia is the only producer with major surplus capacity, however based on current trends the kingdom will have zero oil available for export within 20 years. Population growth, the need to desalinate water supplies, and rising standards of living could result in zero export earnings & significant social unrest if current trends continue. No surprise that the kingdom has started a massive program to exploit the abundant solar power resources available in the Middle East, with companies such as IBM (CSP) & ShowaShell (thin film CIS) participating in this effort.

Saudi Oil Export Trends

The King Abdullah University of Science and Technology (KAUST) on the Red Sea and North of Jeddah, has launched an aggressive, well funded, and long term program to both develop Saudi technologies and to partner with leading international research groups. See The Solar and Alternative Energy Engineering Research Center will become a leading institute in renewable energy science and engineering and will provide the foundation for innovation in efficient and low-cost disruptive photovoltaic (PV) foundational technologies. The KAUST Global Solar Research Partnership includes Stanford University, Cornell University, National Taiwan University, & the University of Toronto.

KAUST, with an initial endowment of $10 billion, has been able to provide significant funding for these research collaborations.

The director of KAUST solar research center, Dr. Ghassen E. Jabbour, is a founder of Solterra is developing a Thin Film Quantum Dot PV Solar Cell, a unique technology that can result in lower manufacturing cost, higher efficiency, and broader spectral performance. Solterra manufactures quantum dots using a patent pending, revolutionary process that results in the production of extremely desirable, high quality tetrapod quantum dots at a cost savings in excess of 95%. These third generation quantum dot solar cells do not require custom made, expensive, or complex, processing equipment, or costly silicon or rare earth elements such as indium. Solterra instead will rely on low-cost screen printing and/or inkjet techniques applied to inexpensive substrates. Quantum Dot Solar Cells have extremely high potential efficiency, having demonstrated the production of multiple excitons from a single electron. This phenomenon is the key to increases in conversion efficiency. Quantum dot solar cells can also harvest light energy in the infrared and ultraviolet spectra leading to better low light collection efficiency and the potential to continue harvesting energy even when little or no visible light is present.

I reviewed another quantum dot/nanocrystal company in a previous column. Solexant closed a $41.5 million C round funding in June, 2010, based in part on the proof of concept 2 MW roll-to-roll manufacturing line and the potential to deliver the lowest cost per Watt Peak solar PV panels.
Developed at Lawrence Berkeley National Lab (LBNL) by Dr. Paul Alivisatos and his team, Solexant's printable nanocrystal technology platform can produce flexible thin films using a variety of materials through a fast and simple deposition process. Solexant's first commercial products will be based on printed CdTe nanocrystals. The company plans to commercialize solar cells based on other higher efficiency printed nanocrystal materials over the next few years.

KAUST has provided funding for the Stanford Center for Advanced Molecular Photovoltiacs (CAMP) where fundamental research is in progress to dramatically decrease the cost of solar PV. CAMP's technological mission is to revolutionize the global energy landscape by developing the science & technology for stable, efficient molecular photovoltaic (solar) cells.

Molecular photovoltaic cells can be fabricated at low cost using roll-to-roll coating processes similar to those used to make newspapers. They can be much cheaper than conventional cells because, in addition to low materials costs, the cells can be printed and connected to each other in a high-throughput, integrated architecture. Today's best organic solar cells have an efficiency of 6.5 % & last approximately 1 year under sunlight. The Center has plans for taking the efficiency to at least 15% & making the cells stable for 10 years or more.

One of CAMP's primary focus topics will be designing & synthesizing molecules with optimally tailored energy levels & controlled packing to maximize wave function overlap, both of which are important for charge carrier & exciton transport. Achieving the highest possible efficiencies with molecular solar cells will also require fabricating the right nanostructures. If the semiconductor domains are too large, the number of excitons reaching the interface is limited; if the donor and/or acceptor domains are isolated, charge carriers can be trapped. Thus, a variety of approaches will be utilized to produce ideal nanostructures. Recent advances in plasmonics will be exploited to increase absorption at donor-acceptor interfaces & enable the use of thinner films, thereby enhancing both the current & fill factor. Methods to print transparent electrodes based on meshes of carbon nanotubes, graphene sheets, as well as silver & ZnO nanowires, will be developed, decreasing the cost of the substrate & collecting electrode. Multijunction approaches will be exploited to obtain ultrahigh efficiency."

Among other projects, Cornell is developing a low cost alternative to c-Si:

Silicon solar cells are versatile and reliable, but their high cost has limited the technology's adoption. Led by Professors Sandip Tiwari (ECE), Jiwoong Park (CHEM), and Christopher Ober (ENG), this project promises a new low-cost, low-energy technique for creating silicon solar cells. The group's innovative process developed at Cornell replaces expensive single-crystal silicon wafers with grass-like silicon nanowires grown on metal. In the course of the study, researchers will refine the mechanism for single-crystal nanowire growth on a metal substrate, demonstrate the technique's characteristics, and develop appropriate production technology and prototypes.

If proven comparable to current crystalline solar cells, nanowire solar cells will provide an inexpensive alternative to solar cells now on the market, a major step toward the goal of making clean, sustainable energy widely available.

KAUST also funded the National Taiwan University Solar Energy Research Center (SERC), which focuses on the applications of solar energy to buildings.
«About 80% of the energy in the world is consumed within various buildings. Air conditioning & heating make up about 30-50% of the total building energy consumption, & indoor & outdoor lighting make up about 15-25%. Reducing energy consumption in cooling/heating systems & lighting can thus contribute a great deal to reducing global warming. Water supply is another equally important issue for buildings. The utilization of solar energy in the cooling, heating, lighting, & water supply systems for buildings has become a very important subject in addressing future oil shortages. Advancing solar building technology for the future is the main theme of NTU's SERC, which intends to pursue a unique & leading edge solar building technology with complex engineering systems & technologies that have the potential to incubate or spawn a new industry.

SERC will be modeled on the New Energy Center of NTU, with emphasis on academic-supported development, system integration, innovation, & working together with industry. Companies are invited to join for various product development projects. The research topics include ejector cooling/heating technology, advanced solar collectors for cooling & desalination, solar-assisted membrane desalination, dye-sensitized solar cells, solar-powered LED lighting, & indoor LED lighting.

SERC will work with teams from KAUST & Taiwan industries in the demonstration & field testing of solar-powered LED highway & indoor lighting, & solar cooling/heating & water supply systems in buildings in Saudi Arabia. Because the applications in Saudi Arabia are in an entirely different environment (hot & dry), SERC will face a big challenge in modifying all the related technologies."

The University of Toronto is another Kaust Global Research partner under the KAUST Investigator Award program, Nanotechnology for Solar Energy, see
"We will create low-cost paint-on solar cells to convert the sun‘s power efficiently into electrical energy. Our goal is to break the current compromise between high efficiency & low cost in solar cells with colloidal quantum dots - semiconductor particles a few nanometers in diameter. These particles can be sprayed from the solution phase onto large, flexible substrates. Quantum dots also represent a highly tunable materials system: their bandgap is determined not only by the choice of semiconductor material used, but also by the size of the particles.

Any solar architecture that seeks to achieve ultra-high power conversion efficiencies must efficiently harvest the considerable energy of high-energy (blue) photons from the sun & also absorb low-energy (infrared) photons. Our first architecture is based on multijunction devices: layers of different-bandgap photovoltaic cells stacked atop one another. The power from each layer is added together either within the device or through an external circuit.

We will pursue the realization of high-efficiency solar cells based on new classes of colloidal quantum dots. Successful optoelectronic devices based on this class of materials have, until now, included heavy metals such as lead or cadmium as constituent materials. We will optimize the properties of colloidal quantum dots that do not contain heavy metals, showing that these can be transformed into efficient solar energy harvesting devices."

Dr. Sargent recently reported the first efficient tandem solar cell based on colloidal quantum dots (CQD) in a paper published in Nature Photonics, with press release here: Sargent is hopeful that in five years solar cells using the graded recombination layer published in Nature Photonics paper will be integrated into building materials, automobiles and mobile devices. The solar community and the world needs a solar cell that is over 10 per cent efficient, and that dramatically improves on today's photovoltaic module price points, said Sargent. The U of T device is a stack of two light-absorbing layers, one tuned to capture the sun's visible rays, the other engineered to harvest the half of the sun's power that lies in the infrared, said coauthor Dr. Xihua Wang. We needed a breakthrough in architecting the interface between the visible & infrared junction, said Sargent, a Professor of Electrical and Computer Engineering at the University of Toronto, who is also the Canada Research Chair in Nanotechnology. The team engineered a cascade, really a waterfall, of nanometers-thick materials to shuttle electrons between the visible and infrared layers.

The team pioneered solar cells using CQDs, nanoscale materials that can readily be tuned to respond to specific wavelengths of the visible & invisible spectrum. By capturing a broad range of light waves wider than normal solar cells tandem CQD solar cells can in theory reach up to 42% efficiencies. The best single-junction solar cells are constrained to a maximum of 31% efficiency. In reality, solar cells that are on the roofs of houses & in consumer products have 14 - 18% efficiency. The work expands the Toronto team's world-leading 5.6 % efficient colloidal quantum dot solar cells.

Lastly, two breakthroughs in manufacturing were announced in June, one by the Oregon State University and the other by Quantasol, a UK company. Inkjet Printing Could Change the Face of Solar Energy Industry: ScienceDaily (June 28, 2011) Engineers at Oregon State University have discovered a way for the first time to create successful "CIGS" solar devices with inkjet printing, in work that reduces raw material waste by 90 percent and will significantly lower the cost of producing solar energy cells with some very promising compounds. High performing, rapidly produced, ultra-low cost, thin film solar electronics should be possible, however further research is needed to increase the efficiency of the cell. The work could lead to a whole new generation of solar energy technology, researchers say The findings have been published in Solar Energy Materials and Solar Cells.

"This is very promising and could be an important new technology to add to the solar energy field," said Chih-hung Chang, an OSU professor in the School of Chemical, Biological and Environmental Engineering. "Until now no one had been able to create working CIGS solar devices with inkjet technology." Part of the advantage of this approach, Chang said, is a dramatic reduction in wasted material. Instead of depositing chemical compounds on a substrate with a more expensive vapor phase deposition -- wasting most of the material in the process -- inkjet technology could be used to create precise patterning with very low waste.

"Some of the materials we want to work with for the most advanced solar cells, such as indium, are relatively expensive," Chang said. "If that's what you're using you can't really afford to waste it, and the inkjet approach almost eliminates the waste." One of the most promising compounds and the focus of the current study is called chalcopyrite, or "CIGS" for the copper, indium, gallium and selenium elements of which it's composed. CIGS has extraordinary solar efficiency -- a layer of chalcopyrite one or two microns thick has the ability to capture the energy from photons about as efficiently as a 50-micron-thick layer made with silicon. In the new findings, researchers were able to create an ink that could print chalcopyrite onto substrates with an inkjet approach, with a power conversion efficiency of about 5 percent. The OSU researchers say that with continued research they should be able to achieve an efficiency of about 12 percent, which would make a commercially viable solar cell.

"In summary, a simple, fast, and direct-write, solution-based deposition process is developed for the fabrication of high quality CIGS solar cells," the researchers wrote in their conclusion. "Safe, cheap, and air-stable inks can be prepared easily by controlling the composition of low-cost metal salt precursors at a molecular level."

Lastly, Quantasol, see , announced in June at the IEEE Photovoltaics conference the development of a triple junction multiple quantum well device (MQW) to maximize conversion efficiency. The top cell in a triple junction solar cell is often current poor in real CPV systems, once absorption & reflection losses in the module optics have been taken into account. MQW top cells, however, can improve the performance of multijunction solar cells by enabling the absorption edge of top & middle subcells to be tuned. QuantaSol's simulations have also shown that photon coupling resulting from the radiative dominance of the MQW top cell can make the multijunction cell less sensitive to variations in the incoming spectrum, thus further improving energy harvesting.

Today's triple junction solar cells are the workhorse of a typical high concentration photovoltaic (HCPV) system.  By splitting the solar spectrum into three colour ‘bands', each sub-cell in a triple junction device can convert the band of the spectrum to which it is sensitive at the best efficiency this has led to world record efficiencies of in excess of 40% as compared to around 25% for traditional Silicon based technology.

Although well proven, the materials used in today's leading triple-junction cells (InGaP, InGaAs, and Ge) have been adopted from previous industrial processes & are not the optimum combination of materials to maximise the potential efficiency of a triple-junction solar cell. QuantaSol can further enhance the efficiency of a triple-junction solar cell by using its Quantum Well technology to adjust, or ‘tune', the sub-cells of a triple junction cell.  By doing this, efficiency increases are possible, since the cell can be tuned to absorb and convert more of the available light. Both triple-junction solar cells and optoelectronic devices containing quantum wells (for example, semiconductor laser diodes) are technologies with a long heritage in both research and commercial production. QuantaSol is able to combine these two technologies to enhance the efficiency of today's market leading multijunction solar cells, maintaining reliability and at a competitive cost to the CPV market in manufacture."

Professor Keith Barnham's research team at Imperial College pioneered the application of nanostructures such as quantum wells & quantum dots for solar photovoltaics. The IP generated has resulted in patents for the use of quantum wells in concentrator cells, the use of strain-balanced quantum wells in photovoltaic cells (SB-QWSC), & quantum dot light concentration (QDC).
QuantaSol was formed in 2007 to commercialize the technology, and has quickly developed world record high efficiency single junction quantum well solar cells, & has now developed very high efficiency triple junction solar cells, enabling the market for new, lower-cost per kW solar electricity generating systems.

Bo Varga can be reached via Bo has worked since 1979 on business development, strategy, & the marketing, initially with high power pulse amplifiers, real-time image acquisition & processing systems, advanced materials, reconfigurable computers, & C compilers for firmware. Since 2000 his focus is on nanotechnology-enabled materials, coatings, & products, as well as the economics of manufacturing processes & scale up. He provides consulting services for emerging & large companies, investors, & government programs, especially in solar, energy storage, & water-related domains. He recently completed a landscape study of US advanced battery companies & technologies & is working on a similar study for ultracapacitors.

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