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Space-Separated Quantum Cutting. Anthony Yeh EE C235, Spring 2009. 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)

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space separated quantum cutting

Space-Separated Quantum Cutting

Anthony Yeh

EE C235, Spring 2009

introduction
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, andT. Gregorkiewicz
    • Van der Waals-Zeeman Institute, University of Amsterdam

http://en.wikipedia.org/wiki/File:Solar_Spectrum.png

“Shockley-Queisser limit,” Wikipedia

motivation for quantum cutting
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

space separated quantum cutting1
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 aregenerated in different places
    • Compared toMultiple Exciton Generation (MEG):
      • Less interaction of excitons with each other
      • Longer lifetimes
      • Easier to harvest energy
experimental setup
Experimental Setup
  • Silicon Nanocrystals (Si NCs)
    • Embedded in SiO2 substrate by sputtering (4.1x1018 cm-3)
    • Average diameter: 3.1nm
    • Average distance between adjacent NCs: ~3nm
    • Bandgap: ~1.5eV
  • Some samples also doped with Er3+ 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
erbium doped ssqc system
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
silicon only ssqc system
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
theoretical mechanism
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
conclusions
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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