Space-Separated Quantum Cutting

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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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