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          Technology Trends

Quantum Dots: Future Energy Solution

DEC 22 2009

Technology Trends


Quantum Dots: Future Energy Solution

 

If we are going to solve the global warming issue, we will need to start believe quantum dot solar technology is the only viable solution to the worlds future energy needs.

 

By Stephen Squires

 

Photo by Solterra Renewable Technologies

 

I dont believe in quantum dots because that is my business. I am in the quantum dot business because I believe in quantum dots. It is widely believed that if we do not embrace clean renewable energies quickly, massive destruction is inevitable, and the human race will go the way of the dinosaurs. Although there are many interesting and novel ideas and solutions, I see solar as the only solution with no downside. Many people believe wind power can be the answer. However, local environmental effects and a global potential to disrupt weather patterns (think of the Butterfly Effect?one turbine is roughly equivalent to a billion butterflies?combined with the Law of Unintended Consequences) and the real possibility exists to heighten global warming. Further, I believe we are kidding ourselves with the concept of clean coal, which to me is the ultimate oxymoron. Only with solar, do we have such a small carbon footprint in production, while also harnessing the suns energy that in and of itself takes heat directly out of the environment. This is particularly interesting in the case for quantum dot solar, as the potential exists to harvest from UV and IR ranges where much heat energy exists (but I will expand on this later). So, it seems obvious to me that solar is the only viable solution to satisfy both our incredible appetite for energy and our urgent need to curtail and ultimately reverse the catastrophic effects of global warming.

 

Why Quantum Dot Solar

So why quantum dot solar? I notice the barrage of press releases from numerous solar cell manufacturers touting the funding for a new solar project (sometimes referred to as a solar farm or solar project) to produce 20, 50, or even more Megawatts (MW) of electricity, and government announcements across the globe of legislature to mandate 20% clean electricity for energy consumption by 2020. But, there hardly seems to be means to the end. The worlds governments have not detailed how many thousands of gigawatts (not mere megawatts) of energy production per year over the next 20 years will be required to achieve these lofty, yet imperative goals. We have devoted time to solve this equation, and although there are variables which are open for debate, I believe my calculations are in the ball park?to achieve 20% by 2020 world renewable energy production, it is estimated that we will need to generate well over an additional 4,000 Terawatt-hours of electricity per year. Existing, non-solar renewable energy sources contribute the lions share of renewable energy production currently, with hydroelectric being the largest provider. Expansion of these non-solar sources is not trivial, with no major hydroelectric being planned in the United States due to lack of suitable large sites. Wind power, which arguably has had the most significant ramp up in recent production, accounts for about 55 terawatt-hours of U.S. production, but even under rosy expansion goals, will meet less than a quarter of the 20 x 2020 goals. The remainder of renewable technologies, including biomass (which seems to be in contradiction to emissions goals), either do not have much current base of production or have limits to significant expansion. I believe the potential for solar provides the only true solution to meeting these goals. However, if we were to continue to rely only on Silicon-based (Si) photovoltaic cells, we would need to build over six thousand new 100 MW plants in the United States alone! First Solar, the fastest growing solar company, has recently announced many new installation initiatives, depleted with significant scale-up even before 2020. Surely as their end grows near, the laws of supply and demand will undoubtedly result in steep and rapid price increases for both materials negating any cost benefits derived from their lower cost print processing. I have analyzed each and every solar cell technology, and here is why I believe Quantum Dot (qdot or QD) solar tech is the only viable solution to satisfy the worlds energy demands without placing the earth and all its inhabitants in peril in the process.

 

Cost Effectiveness and Conversion Efficiencies

First, for any solar cell technology to be cost effective, the basic raw materials must be cheap and abundant. Usually if theyre cheap, it is because they are abundant. Such is the case with CdSe, our current material of choice, although we have many other materials from which we can produce qdot semiconductors, including silicon. Second, the materials must be able to achieve high conversion efficiencies. This is another topic that is near and dear to me. Currently, the benchmark for rating solar cells is conversion efficiency during the peak of a sunny day where maximum sun exposure is achieved. I dont know about you but unless that solar cell can convert enough of the suns energy during that short peak exposure to operate my household everyday, then Peak Watts is not a very useful metric. I am much more interested in conversion efficiency over a 24-hour period, and in my opinion, you should be too. Quantum dots have the very unique ability to be easily tuned to specific segments of the solar spectrum. In other words, they can be produced to just respond and convert energy from a variety of bands of the energy spectrum. What makes this both interesting and important, is the fact that you can easily produce a very broad selection of quantum dots to cover nearly the entire solar spectrum including non-visible sources in the ultraviolet and infrared ranges. By combining this broad coverage of qdots into a soup, and then printing them on a substrate, you can in effect, produce a solar cell capable of much higher conversion efficiency over a 24-hour period even if the cells peak conversion were substantially less than a competing conventional solar cell. That said, it is also true that qdots can provide the energy solution due to high maximum potential conversion efficiency.

 

Multiple Exciton Generation Effect

Quantum dots have several very unique qualities that are the primary result of their small sizes and the optical and electrical effects at these dimensions. One of the most interesting and most significant is their ability to create Multiple Exciton Generation (MEG). What is MEG and why is it important? Thought you would never ask? The MEG effect is important because it creates the potential to harvest 3 electrons per photon from the sun, and some reports have been published of up to 7 electrons for each photon. Like a farmer harvesting corn, solar cells are nothing more than a way to collect photons sent to earth by the sun and collecting these photon so we can harvest electrons from them which we in turn use to produce electricity. Prior to quantum dots, you could only extract one electron from each photon. Now with quantum dots the potential is there to collect 3 or more opening the window for exponential increases in conversion efficiency and not just during that single peak window of sunlight, but throughout the entire day. Although the technology is not there yet, NREL has substantiated to the theoretical conversion efficiency for qdots is 30% greater than for other materials.

 

Raw Material Vs. Quantum Dots

Another piece to our puzzle relates to comparative complexity of processing the raw materials and solar cell manufacturing techniques. The range of materials being considered is vast, and I couldnt possibly cover them all here. So, we will start with the big dog so-to-speak?silicon. It makes up 98% of todays solar market, with the bulk of cells being the monocrystalline variety. As the precursor material for silicon is sand, I think we can all agree it is in relative abundance. Unfortunately, solar cells require high purity silicon and the road from sand to high-grade silicon is long and paved in gold. A typical silicon refining plant costs over a billion dollars to build and the process of preparing the silicon wafers for the solar cell is neither cheap nor trivial. In order to satisfy the 20 by 2020 energy scenario, we would need nearly 20 more silicon refining plants each year over the next 10 years. This estimate even assumes that these new plants would be built to incorporate the latest in micromaching technology to minimize wafer and silicon consumption. If the reliance on thicker wafer silicon persists, this number could easily need to be increased by a factor of 10! Furthermore, the carbon footprint from these silicon foundries is significant and diminishes much of the gains offered by the Si cells when considering the total global warming and emissions mandates that accompany the demands for cleaner direct energy production. It bears repeating the significance of the statistic for solar farms above?for silicon to fill the need, and with the United States historically consuming a quarter of global energy, the world would require approximately ten thousand conventional 100 MW Si cell solar generation projects. And even if we accomplished all of the above, it is still extremely unlikely silicon solar cells will ever be cost competitive with conventional energy sources. In the absence of grid parity, I do not see the rapid adoption of solar energy that is required, and history has demonstrated that in the long run, subsidies place unnatural and often detrimental burdens on economic systems. Quantum dots on the other hand, are produced using a low cost synthesis process. Additionally, the raw material cost are relatively low to begin with, and a single qdot solar cell only requires a few grams or less per square meter of solar cell, so a little goes a long way.

 

Solar Cell Production

Last, but not least, let me discuss the production of the actual solar cell. We have mined the raw materials. These have been processed into final form and we are now ready to apply these to make a solar cell. In the case of mono-Si, you actually mount the individual wafers to a substrate, which is usually glass. With most thin films, you apply the active materials to a flexible substrate. Although the preferred method is roll-to-roll print processing, currently every active material, with the exception of quantum dots, requires some form or another of vapor deposition or vacuum bonding. There are many claims of roll to roll print processing from solar manufactures.


Tetrapod quantum dots (Photo by Solterra Renewable Technologies)

However, typical roll speeds are much less than 100 m per minute, due to processes that are complicated and as a result, this ultimately limits throughput. On the other hand, our qdot cell design is based upon simple roll-to-roll print processing that has been around for many years. Even 20-year-old equipment runs over 300 m per min and todays newer machines easily exceed 600 m per minute. If you had a machine capable of printing money at 600 m per minute would you operate it at 10 m per minute? I think not. Its not being done because up until qdot solar comes online, it simply cannot be done. For quantum dot solar (and for that matter any quantum dot-based technology), we are presented with the classic chicken and the egg scenario. In order to print meter wide qdot solar cells, even at a meager 100 m per minute print speed, you need to produce 100 kg of quantum dots per day. On the other hand, if you begin producing this daily amount of QD material before the solar cells start printing, in a young quantum dot market, you begin to have a stockpile of QDs. Quantum dots have been a novel material with many potential applications, but with nearly all of these technologies waiting for a low cost and ready input material supply. Even though quantum dot science is showing tremendous potential, all the qdot manufacturers combined have only produced less than 100 kg per year!

 

Challenges Remain

So what are the challenges that remain for qdot solar? We will continue to optimize

qdot solar cell designs to achieve a greater portion of the 66% theoretical conversion efficiency and broad solar spectrum  performance from multiple qdot size soups, and we will scale up quantum dot synthesis to achieve 100 kg/day production.

This QD output will become solar cell print line input for operation at the initial target of 100 m per minute. At a nominal work schedule of 5 days per week and 8 hours per day, this will create an expandable output of well over 1 GW (1,000 MW) per year in solar cell production. This will enable the largest, but minimally complex solar lines in the industry, with production technology for both the quantum dot production and the solar cell line that are globally scalable and repeatable.

 

The Impact

Now you are probably asking that, based on the 20 by 2020 mandates, just how significant of an impact is qdot solar really going to have? Well, Ill tell you?I think it will be huge. Huge for the following reasons:

1) For every 1% gained in collection efficiency the delivery cost of electricity drops by approximately one cent per kw-hour to quickly match conventional energy generation methods;

2) With cost equal to or below unsubsidized energy source, adoption will accelerate;

3) With the potential to print at 300 meters or more per minute, a single qdot solar plant will produce more MWs of cell power per day than a mono si plant produces in one month (so you need less than one-30th the number of qdot plants to produce the same energy potential); and, last but not least,

4) Although we are focusing on CdSe as our initial raw material, quantum dot semiconductors can be produced from a wide variety of raw materials, so there is inherent flexibility in this technology.

 

 

As started this article, I made it clear that I dont believe that qdot solar is the solution because Im in the qdot solar business, but rather, Im in the qdot solar business because I believe it is the only viable solution to the worlds future energy needs. If we are going to solve the global warming issue, we will need to make all of you believers, too.

 

Stephen Squires is President & CEO of Solterra Renewable Technologies, Inc. (www.solterrasolarcells.com).

 

For more information, please send your e-mails to pved@infothe.com.

2009 www.exposolar.org. All rights reserved.

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