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A New Paradigm for Solar Energy Conversion: Non-Radiative Energy Transfer |
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Sunday, 16 November 2008 00:19 |
Siyuan Lu, Anupam Madhukar, "Nonradiative Resonant Excitation Transfer from Nanocrystal Quantum Dots to Adjacent Quantum Channels", Nano Letters, 7, 3443-3451 (2007)
Historically, the earliest photovoltaic solar cells were investigated in silicon p-n junction devices [1] where the absorption and the created electron-hole pair separation and transport are processes that occur in the same medium. With the development of semiconductor epitaxial deposition techniques, the silicon pn junction solar cells have been augmented with III-V compound semiconductor multiple quantum well based solar cells [2] which exhibit the highest efficiencies although remain also the most expensive to produce.
Low cost solar cell with organic absorbers have been and are continuing to be exploited but the high exciton binding energies typical of such materials requires their mixing with an appropriately energy-aligned different material, inorganic or organic, to create internal heterojunctions at which the photo-generated exciton can be split into its electron and hole components for subsequent collection. These have come to be known as excitonic solar cells [3-6] and in the current implementations of these the separated electrons and holes end up in two different media for transport and collection. These media have so far been ones with very low charge carrier motilities, which is a major contributing factor to their hitherto low overall energy conversion efficiency.
Overcoming or bypassing the bottlenecks of charge carrier extraction and transport to collecting electrodes will constitute a major step forward in the quest for the realization of efficient and cost effective solar energy converters. We have thus introduced an new paradigm for solar energy conversion. To learn about this new paradigm see below (or read our full paper: Siyuan Lu and Anupam Madhukar, "Nonradiative Resonant Excitation Transfer from Nanocrystal Quantum Dots to Adjacent Quantum Channels", Nano Letters, 7, 3443-3451 (2007)
Nonradiative Coupling
Figure 1. Schematic showing a new solar cell architecture utilizing nonradiative couplingbetween dipoles for direct transfer of energy from the excitons created in the light absorber to (a) quantum well (b) nanowire high mobility charge carrier transport channels.
For efficient nonradiative transfer of energy, appropriate matching of the absorption and emission spectra of the donor and acceptor species and their separation is a critical consideration [13]. For our studies, PbS NCQDs are employed as the absorbers (donors) and an appropriately designed adjacent InGaAs quantum well buried in GaAs matrix provides the high mobility charge transport channel for accepting the resonantly transferred exciton energy in the form of electron and hole. To shed light on the time scale and efficiency of excitation transfer from the PbS NCQD into the InGaAs near surface quantum well, we have examined the time resolved behavior of the luminescence decay from the NCQDs adsorbed on quantum well containing substrates and compared it to the behavior on control substrates without the buried quantum well.
Figure 2 shows the room temperature time decay behavior of the luminescence peak of the NCQDs at 965nm. Note the considerably fast decay time of ~207ns in the presence of the quantum well as compared to the ~300ns decay time on the GaAs control substrate. The reduction of the NCQD PL decay time from ~300ns to ~207ns is the manifestation of the opening of an excitation transfer channel provided by the one-dimensionally confined states of the quantum well (Fig.2). From the difference between these two measured decay times, we calculate the nonradiative transfer rate to be ~ 1/(690ns), ~1.4 times faster than their radiative decay rate ~ 1/(960ns) measured for the NCQDs dispersed on a glass substrate. This means for a nanocrystal of 100% quantum yield, the efficiency of the nonradiative transfer from the NCQD to the quantum well is ~60% for the 8.2 nm center-to-center separation of the PbS NCQDs adn the InGaAs QW in these experiments. The transfer efficiency is expected to be further improved by reducing the distance between the NCQDs and the NSQW.
Fig. 2. Time resolved PL of PbS NCs on passivated GaAs (blue) and on passivated NSQW (Red). Excited at 900nm (below GaAs bandgap) and detected at 965nm (PbS PL peak). TRPL curves are fitted using stretched exponential function.
References:
[1] Shockley, W.; and Queisser, H. J.; Jour. App. Phys. 1963, 32, 510-519.
[2] King, R. R.; Law, D. C.; Edmondson, K. M.; Fetzer, C. M.; Kinsey, G. S.; Yoon, H.; Sherif, R. A.; Karam, N. H.; Appl. Phys. Lett., 2007, 90, 183516.
[3] Oregan, B.; Gratzel, M.; Nature, 1991, 353, 737-740.
[4] Hoppe, H.; Sariciftci, N. S.; J. Mater. Res. 2004, 19, 1924-1945.
[5] Kim, J. Y. ; Lee, K. ; Coates, N. E. ; Moses, D. ; Nguyen, T. Q. ; Dante, M. ; Heeger, A. J. ; Science 2007, 317, 222-225.
[6] Nozik, A. J. Physica E, 2002, 14, 115; Gur, I.; Fromer, N. A.; Alivisatos, A. P.; J. Phys. Chem. B 2006, 110, 25543-25546.
[7] Weisbuch, C. Fundamental properties of III-V semiconductor two-dimensional quantized structures: The basis for optical and electronic device applications, Ch.1 in Semiconductors and Semimetals: Application of Multiquantum Wells, Selective Doping, and Superlattices, Ed. R. Dingle, Academic Press, New York, (1987); Interfaces, Quantum Wells, and Superlattices, Eds, C. R. Leavens and R. Taylor Plenum, New York, (1988).
[8] Madhukar, A.; Thin Solid Films, 1993, 231, 8-42.
[9] Konkar, A.; Madhukar, A.; Chen, P.; Appl. Phys. Lett., 1998, 72, 220-222.
[10] Kiravittaya, S.; Heidemeyer, H.; Schmidt, O. G.; Physica E, 2004, 23, 253-259.
[11] Westwater, J.; Gosain, D. P.; Tomiya, S.; Usui, S.; Ruda, H. J.; Vac. Sci. Technol. B, 1997, 15, 554-557.
[12] Dick, K. A.; Deppert, K.; Karlsson, L. S.; Wallenberg, L. R.; Samuelson, L.; Seifert, W.; Advanced Functional Materials, 2005, 15, 1603-1610; Zhong, Z. H.; Qian, F.; Wang, D. L.; Lieber, C. M.; Nano Lett., 2003, 3, 343-346; Kim, Y.; Joyce, H. J.; Gao, Q.; Tan, H. H.; Jagadish, C.; Paladugu, M.; Zou, J.; Suvorova, A.; Nano Lett., 2006, 6,599 - 604.
[13] Förster, Th.; Annu. Rev. Phys., 1948, 2, 55-75; Förster, Th.; Discuss. Faraday Soc., 1959, 27, 7-17.
[14] Lu, S.; Madhukar, A.; Nano Lett. 2007, 7, 3443-3451. |
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What is Clean Coal Technology? |
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Friday, 07 November 2008 09:09 |
 Clean coal is an umbrella term used in the promotion of the use of coal as an energy source by emphasizing methods being developed to reduce its environmental impact. These efforts include chemically washing minerals and impurities from the coal, gasification (see also IGCC), treating the flue gases with steam to remove sulfur dioxide, and carbon capture and storage technologies to capture the carbon dioxide from the flue gas. These methods and the technology used are described as clean coal technology. Major politicians and the coal industry use the term "clean coal" to describe technologies designed to enhance both the efficiency and the environmental acceptability of coal extraction, preparation and use,[1] with no specific quantitative limits on any emissions, particularly carbon dioxide.
It has been estimated that commercial-scale clean-coal power stations (coal-burning power stations with carbon capture and sequestration) cannot be commercially viable and widely adopted before 2020 or 2025.[2] This time frame is of concern to environmentalists because, according to the Stern report, there is an urgent need to mitigate greenhouse gas emissions and climate change.
The concept of clean coal is said to be a solution to climate change and global warming by coal industry groups, while environmental groups maintain that it is greenwash, a public relations tactic that presents coal as having the potential to be an environmentally acceptable option. Greenpeace[3] is a major opponent of the concept because emissions and wastes are not avoided, but are transferred from one waste stream to another
Byproducts
The byproducts of coal combustion are very hazardous to the environment if not properly contained. This is Clean Coal's largest challenge, both from the practical and public relations perspectives.
While it is possible to remove most of the sulfur dioxide (SO2), nitrogen oxidesparticulate (PM) emissions from the coal-burning process, carbon dioxide (CO2) emissions and radionuclides [10] will be more difficult to address. Technologies do exist to capture and store CO2, but they have not yet been utilized on a large-scale commercial basis due to the high economic costs.[11] (NOx) and
In terms of mercury, coal-fired power plants are the largest aggregate source: 50 tons/year come from coal power plants out of 150 tons emitted nationally in the USA and 5000 tons globally.[12] In the USA, neither the combustion products of oil[13], nor their associated solid or liquid waste streams[14], are considered to be major contributors to mercury pollution.[15]
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Support and Criticism
In the United States, Clean Coal has been mentioned by United States President George W. Bush on several occasions, including his 2007 State of the Union Address. Bush's position is that clean coal technologies should be encouraged as one means to reduce the country's dependence on foreign oil. Senator Hillary Clinton has also recently said that "we should strive to have new electricity generation come from other sources, such as clean coal and renewables."[20] The US Department of Energy is working with private industry in developing clean coal technologies.[21] One of the clean coal technologies being developed is carbon sequestration, capturing carbon dioxide and eliminating or slowing its release back into the atmosphere. Another technology under development is Integrated Gasification Combined Cycle or IGCC. [22] During the 2008 US Presidential campaign, both candidates John McCain and Barack Obama expressed interest in the development of clean coal technologies as part of an overall comprehensive energy plan.[23] The development of clean coal also creates the possibility of international business for the United States and other world markets.[24]
In Australia, clean coal is often referred to by Prime Minister Kevin Rudd as a possible way to reduce greenhouse gas emissions.[25] (The previous Prime Minister John Howard has stated that nuclear power is a better alternative, as clean coal technology may not prove to be economically favorable.[26])
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Criticism
Greenpeace has released a report titled False Hope, Why carbon capture and storage won’t save the climate. The report lists five key reasons that carbon capture is not a viable solution. [27]
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CCS cannot deliver in time to avoid dangerous climate change. The earliest possibility for deployment of CCS at utility scale is not expected before 2030. To avoid the worst impacts of climate change, global greenhouse gas emissions have to start falling after 2015, just seven years away.
CCS wastes energy. The technology uses between 10 and 40% of the energy produced by a power station. Wide scale adoption of CCS is expected to erase the efficiency gains of the last 50 years, and increase resource consumption by one third.
Storing carbon underground is risky. Safe and permanent storage of CO2 cannot be guaranteed. Even very low leakage rates could undermine any climate mitigation efforts.
CCS is expensive. It could lead to a doubling of plant costs, and an electricity price increase of 21-91%. Money spent on CCS will divert investments away from sustainable solutions to climate change.
CCS carries significant liability risks. It poses a threat to health, ecosystems and the climate. It is unclear how severe these risks will be.
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—Greenpeac
See Wikipedia, Clean coal technology, http://en.wikipedia.org/wiki/Clean_coal_technology (as of Nov. 7, 2008, 09:19 GMT).
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New Industrial Revolution |
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Saturday, 21 June 2008 00:00 |
The New Industrial Revolution has more to do with rectifying or even undoing some of the damage that resulted from the last Industrial Revolution. Rather than a call for industrialization, expansion of mechanization, or a broadening of global markets, the New Industrial Revolution is characterized by merging traditionally contraditory disciplines: environmental sustainability and economic competitiveness.
Ushered in by world-renowned industrial designer William McDonough, the New Industrial Revolution is viewed as a necessity in order to change the direction of the current industrial modality. The New Industrial Revolution will produce a world of abundance and good design - a delightful, safe world that our children can play in.
At the heart of the New Industrial Revolution is a quantum leap in the way that humans think of the products that we purchase and consume. The traditional “cradle to grave” product lifecycle must be changed to a system of “cradle to cradle” product flow. This alternative product flow can be characterized as “reuse": returning consumer products to the environment as biological nutrients, or to industry as technical nutrients that can be infinitely recycled. |
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