1 / 9

Space-Separated Quantum Cutting

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)

norman-kemp
Download Presentation

Space-Separated Quantum Cutting

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related