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Their innovation involves a special type of nanocrystal called a quantum dot. Varying in size from about 2 to 50 nanometers, quantum dots are made of semiconducting materials that can accept photons and convert them into electricity at substantial rates.2 Moreover, the size of each quantum dot determines the wavelength it emits, giving these nanocrystals a tuneability that scientists can use to produce an array of colors for a range of applications.
In terms of solar research, the dots' quantum mechanical properties also allow a single photon to excite two or more electrons, a phenomenon known as multiple exciton generation that results in higher yields. Theoretically, photovoltaic (PV) cells could then store this amplified charge, and the whole device could serve as a replenishable energy source.
With this potential, quantum dots could be huge for solar technology, but scaled-up designs have suffered from reabsorption problems that have limited their effectiveness. The issue dealt with the dots' absorption bands, which enter an excited state when receiving photons that's only relieved when nearby emission bands release photons in turn. In previous quantum dot prototypes, there was an overlap between the dimensions of the emission and absorption bands, such that the photons from the emission bands — that were meant to be captured by PV cells — were being soaked up again by the absorption bands instead.
The Los Alamos and UNIMIB labs worked around this constraint by engineering the absorption and emission bands from slightly different materials. They thought this structure would stagger the respective bandgaps, ensuring that the photons released by the emission bands had less energy than the photons accepted by the absorption bands, a difference called a Stokes shift. The goal was to create a big enough Stokes shift so that the absorption bands wouldn't reabsorb the photons released by the emissions bands, letting PV cells grab these packets of energy.
The Los Alamos scientists first created a "giant quantum dot" from a wide outer layer of cadmium sulfide (CdS) and a narrow core of cadmium selenide (CdSe). With this structure, the CdS casing would operate as the energy-absorber and the CdSe core as the energy-emitter, with giant in this context meaning ~10 nanometers across. The UNIMIB lab then integrated these CdSe/CdS quantum dots into slabs of almost transparent polymethylmethacrylate (PMMA) measured in the tens of centimeters.
The outcome was a luminescent solar concentrator that exhibited nearly zero losses from photon reabsorption. Additionally, in experiments with simulated solar radiation, the prototype was able to capture >10% of the total photons possible — a rate that had one researcher likening it to a "light-harvesting antennae."3
Later versions could have even larger surface areas and remain nearly transparent, making CdSe/CdS-quantum dots an ideal candidate for the development of solar windowpanes and other dynamic technologies. These advances could substantially reduce the impact of commercial and residential buildings by transitioning them off the power grid, a timely move in light of the Intergovernmental Panel on Climate Change's (IPCC) latest analysis.
According to the IPCC's most recent report,4 the building sector is a relatively small contributor of greenhouse gas (GHG) emissions, releasing only 6.4% of all GHG direct emissions in 2010, but it was the largest end-consumer of total energy that year, being responsible for 32% of final energy use.5 Relatedly, the energy supply sector was the single biggest contributor of GHG direct emissions, releasing a quarter of all GHG direct emissions that year.
In other words, while energy production was recognized as the sector most urgently needing decarbonization, buildings were the top end users of the electricity and heat being generated by GHG-gushing suppliers.
Quantum dot devices could allow buildings to be more self-sustaining and affect climate change through lowering their energy demands. From this perspective, quantum dot windowpanes could present a clearer view of what to expect from tomorrow's construction materials, as well.
Image credit: Photo of pippettes containing solutions of an array of quantum dots from the Flickr creative commons of Argonne National Laboratory.
1. Meinardi, F., Colombo, A., Velizhanin, K., Simonutti, R., Lorenzon, M., Beverina, L., Viswanatha, R., Klimov, V., & Brovelli, S. (2014). Large-area luminescent solar concentrators based on ‘Stokes-shift-engineered' nanocrystals in a mass-polymerized PMMA matrix. Nature Photonics DOI: 10.1038/nphoton.2014.54
2. Semonin, O., Luther, J., Choi, S., Chen, H., Gao, J., Nozik, A., & Beard, M. (2011). Peak External Photocurrent Quantum Efficiency Exceeding 100% via MEG in a Quantum Dot Solar Cell. Science, 334, 1530-1533 DOI: 10.1126/science.1209845
3. Press Release, Los Alamos National Laboratory. "Shiny Quantum Dots Brighten Future of Solar Cells." April 14, 2014.
4. IPCC, 2014: Summary for Policymakers. In: Climate Change 2014 Mitigation Of Climate Change. Contribution Of Working Group III To The Fifth Assessment Report Of The Intergovernmental Panel On Climate Change. Pages 7, 24 - 29. April 12, 2014.
5. The sectors of industry and transport were a close second and third, though, with the industry sector accounting for 28% of final energy use in 2010, and transport for 27%. [IPCC, 2014, Ibid].
