Space separated quantum cutting
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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

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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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