I have been invited to give a lecture at the Yerevan Physics Institute in Armenia at the end of this month, on "Solar Energy and Space Applications", in which I plan to stress technology for keeping satellites going, and hence maintaining the global information function of the world, even if other aspects of its connectivity begin to fade. During the process, I came across "Quantum Dots" which strike me as rather interesting materials in this respect, particularly in terms of increasing the energy conversion efficiency of solar radiation to electricity (photovoltaic capacity), radiation resistance and lightweight payload for launching. Given that energy efficiency is probably the key feature to exploit in our riding-down Hubbert's peak, I thought I would share this with you.
The term “quantum dot” (QD) was coined by Mark Reed at Yale University. A QD is a semiconductor whose excitons are confined in all three spatial dimensions. Accordingly, they have properties that are between those of bulk semiconductors and those of discrete molecules. They were discovered by Louis E. Brus, who was then at Bell Labs. QDs are nanocrystalline materials (or materials that contain nanocrystals) in which the dimension of the crystal is smaller (in all directions) than the Bohr exciton radius of the exciton pair (M+ ... e-).
This causes the energy levels to become quantised (quantum confinement), as in individual molecules, rather than coalescing into the “band structure” of bulk semiconductors Traditional (bulk) semiconductors lack versatility, since their band-gap and hence optical and electronic properties cannot be easily changed, if at all. By tuning the size of the QD particle, the band-gap can be tailored for specific applications. The gap enlarges as the crystalline dimension decreases, so that the fluorescence wavelength shortens; and conversely, as the crystal becomes bigger, the wavelength increases, so the fluorescence shifts toward the red end of the visible spectrum.
QDs range in size from 2 - 10 nanometers (10 - 50 atoms) in diameter and contain as few as 100 to 100,000 atoms. Nearly 3 million quantum dots could be lined up end to end and fit within the width of a human thumb. There are several ways to confine excitons in semiconductors, resulting in different methods to produce quantum dots. In general, quantum wires, wells and dots are grown by advanced epitaxial techniques in nanocrystals produced by chemical methods or by ion implantation, or in nanodevices made by state-of-the-art lithographic techniques.
There are also colloidal methods to produce many different QD semiconductors, including cadmium selenide, cadmium sulfide, indium arsenide, and indium phosphide. Large quantities of quantum dots may be synthesized via colloidal synthesis., which can be done under benchtop conditions, i.e. you just mix chemicals in a flask, rather than complex molecular beam epitaxy techniques. QDs are less rapidly damaged by radiation because ejected electrons and positive holes can recombine harmlessly (i.e. without molecular structure changes, e.g. atomic displacements, bond breaking, cascade ionisation and creating further damage centres etc.)
There is a dimensional restriction on the normal reactivity of the bulk material, since the QD is smaller than the radiation spur (track) distance, which limits the extent of chemical reactions normally induced in the bulk semiconductor, and in the absence of alternative routes, the holes and electrons are more likely to simply recombine. Thin-films too are relatively radiation-resistant, and one can invoke a simple geometric argument, in that the total concentration of active material is comparatively small, hence kinetically the relative rate of damage is lower.
Quantum dots offer the potential to improve the efficiency of solar cells in two respects: (1) by extending the band gap of solar cells so they can harvest more of the solar spectrum, and (2) by generating more excitons from a single photon.
Extending the solar cell band gap into the IR region.
Almost half the intensity of sunlight ranges within the IR region of the electromagnetic spectrum. Thus Photovoltaic cells that respond to IR – ‘thermovoltaics’ - can even capture radiation from a fuel-fire emitter; and co-generation of electricity and heat are said to be quiet, reliable, clean and efficient. A 1 cm2 silicon cell in direct sunlight will generate about 0.01W, but an efficient infrared photovoltaic cell of equal size can produce theoretically 1W in a fuel-fired system.
It was discovered in the 1970s that chemical doping of conjugated organic polymers increased electronic conductivity by several orders of magnitude, leading to the application of electronically conducting materials as sensors, light-emitting diodes, and solar cells. Conjugated polymers provide ease of processing, low cost, physical flexibility and large area coverage. They now work reasonably well within the visible spectrum.
In order to make conjugated polymers work in the infrared range, researchers at the University of Toronto wrapped the polymers around lead sulphide quantum dots tuned (by size) to respond to infrared . The polymer poly(2-methoxy-5-(2’-ethylhexyloxy-p-phenylenevinylene)] (MEH-PPV) absorbs between ~400 and ~600 nm. QDs of lead sulphide (PbS) have absorption peaks that can be tuned from ~800 to ~2000 nm.
By wrapping MEH-PPV around the QDs shifted the absorption spectrum of the polymer was shifted into the infrared. Commercial implementation is predicted to come about within 3-5 years.
Multiple excitons from one photon..
Researchers led by Arthur Nozik at the National Renewable Energy Laboratory Golden, Colorado in the United States showed recently that the absorption of a single photon by their QDs yielded - not one exciton as is usual for bulk semiconductors - but three excitons!
The formation of multiple excitons per absorbed photon requires that the energy of the photon absorbed is far greater than the semiconductor band gap. This phenomenon does not readily occur in bulk semiconductors where the excess energy simply dissipates away as heat before it can cause other electron-hole pairs to form. In QDs, the rate of energy dissipation is significantly reduced, and the charge carriers are confined within a minute volume, thereby increasing their interactions and enhancing the probability for multiple excitons to form.
A quantum yield of 300 percent was recently demonstrated for 2.9nm diameter PbSe (lead selenide) QDs when the energy of the photon absorbed is four times that of the band gap. However, multiple excitons start to form as soon as the photon energy reaches twice the band gap. Quantum dots made of lead sulphide (PbS) also showed this phenomenon.
“We have shown that solar cells based on quantum dots theoretically could convert more than 65 percent of the sun’s energy into electricity, approximately doubling the efficiency of solar cells”, said Nozik.
QDs do seem to offer remarkable potential in photovoltaic applications generally, but in space-applications particularly, in terms of radiation resistance, low payload weight, and light to electricity conversion efficiency.