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, and T. Gregorkiewicz  Van der Waals-Zeeman Institute, University of Amsterdam “Shockley-Queisser limit,” Wikipedia http://en.wikipedia.org/wiki/File:Solar_Spectrum.png

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 are generated in different places  Compared to Multiple 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 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

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