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Space-Separated Quantum Cutting Anthony Yeh EE C235, Spring 2009
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Introduction Shockley-Queisser limit ~30% for single-junction cells Multi-junction cells Theoretically up to ~68% But more complex/expensive Is there another alternative? Quantum Cutting (QC) Space-Separated QC in Silicon: D. Timmerman, I. Izeddin, P. Stallinga, I. N. Yassievich, and T. Gregorkiewicz Van der Waals-Zeeman Institute, University of Amsterdam “Shockley-Queisser limit,” Wikipedia http://en.wikipedia.org/wiki/File:Solar_Spectrum.png
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Motivation for Quantum Cutting Photon energy smaller than bandgap: not absorbed Quantum cutting cannot help here Photon energy larger than bandgap: waste heat Quantum cutting reclaims some of the excess energy “Slicing and dicing photons,” Nature Photonics, February 2008
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Space-Separated Quantum Cutting One high-energy photon => Multiple low-energy photons “Cutting” the energy quantum of the photon into pieces Multiple low-energy photons can be more efficiently converted to electricity by a cheap, single-junction cell Space-separated The lower-energy excitons are generated in different places Compared to Multiple Exciton Generation (MEG): Less interaction of excitons with each other Longer lifetimes Easier to harvest energy
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Experimental Setup Silicon Nanocrystals (Si NCs) Embedded in SiO 2 substrate by sputtering (4.1x10 18 cm -3 ) Average diameter: 3.1nm Average distance between adjacent NCs: ~3nm Bandgap: ~1.5eV Some samples also doped with Er 3+ ions Used as an example of a “receptor” for the down-converted energy Photoluminescence at 1535nm (excitation energy: ~0.8 eV) Pulsed laser excitation Tunable from visible (~650nm) to UV (~350nm) [2-3.5eV] 5ns pulse width, 10 Hz repetition rate, 1-10 mJ/pulse Observe output wavelengths with photomultiplier
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Erbium-Doped SSQC System Quantum efficiency vs. wavelength # photons out / # photons in HE photon in, LE photon(s) out QC threshold around 2.6eV Si NC bandgap + Er excitation: 1.5eV + 0.8eV = 2.3eV Quantum Cutting Si NC absorbs HE photon Hot exciton relaxes to CB edge, exciting a nearby Er ion Cool exciton recombines, exciting another nearby Er ion
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Silicon-Only SSQC System QC threshold around 3eV Si NC bandgap x 2: 1.5eV x 2 = 3eV Higher threshold than Er system Quantum Cutting Si NC absorbs HE photon Hot exciton relaxes to CB edge, exciting another nearby Si NC Now there are two, spatially- separated cool excitons Both recombine and emit LE photons
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Theoretical Mechanism Similar to Multiple Exciton Generation (MEG) One HE photon generates multiple LE excitons in the same NC Physical mechanism still under debate Authors’ best explanation: Impact ionization Hot electron in CB “collides” with electron in VB, exciting it Occurs in bulk also, but at a very low rate (~1%) Rate rises dramatically for NCs due to strong Coulomb interaction of confined carriers and decreased phonon emission due to discrete spectrum Er ion or second NC must be quite close to the first NC (~1nm), so a hot exciton in one crystal can interact with carriers in the receptor
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Conclusions First group to demonstrate quantum cutting in Si NCs Use of silicon is important for potential manufacturability Silicon’s indirect bandgap is actually beneficial here Unlike previous MEG-based experiments: Down-converted energy transferred to external ion/NC Shows improved potential for harvesting energy Can use different material (e.g. Er ions) as receptor, lowering QC threshold from 2x Bandgap to Bandgap + Receptor energy Can be tuned to specific applications NC size affects energy levels NC separation affects strength of QC effect Can be applied to both solar cells and light emitters
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