1 / 16

Solar Cells based on Quantum Dots: Multiple Exciton Generation

Solar Cells based on Quantum Dots: Multiple Exciton Generation. Dave Cone UC-Davis Physics CM Journal Club Spring, 2008. Significance of Solar. Solar PVs $15B market 25% of US energy from alternatives by 2025 Grand Plan- US free of oil/carbon by 2050. How Solar Cells Work.

shepry
Download Presentation

Solar Cells based on Quantum Dots: Multiple Exciton Generation

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. Solar Cells based on Quantum Dots:Multiple Exciton Generation Dave Cone UC-Davis Physics CM Journal Club Spring, 2008

  2. Significance of Solar • Solar PVs $15B market • 25% of US energy from alternatives by 2025 • Grand Plan- US free of oil/carbon by 2050

  3. How Solar Cells Work Based on “photoelectric effect” • Photon excites valence electron • Electron-hole pair created • Electrons & holes “separate” • Band gap determines what is absorbed

  4. 1.1eV=1100nm (Si) 1.4eV- 870nm (GaAs) 1.7eV- 710nm (CaSe) 2.0eV- 600nm (CaSeQD) 3.2eV- 400nm (TiO2)

  5. “Ideal” Solar Cell

  6. Strategies for Improvement(with nanostructures- QDs) • Capture more sunlight - Tune energy band gaps of materials “Stacked Architecture” Potential Advantages: • Cheaper Materials • Capture more of Solar Spectrum • Generate more electrons per photon: “Multiple Exciton Generation” Potential Advantages: • Dramatic Efficiency Improvements • Greater Solar Spectrum

  7. MEG- Multiple Exciton Generation Basic Solar Cell: 1 photon = 1 exciton (1 electron/ 1 hole + excess energy) MEG Solar Cell: 1 photon = 2+ excitons (2+ electron/holes –no heat!) Potential Efficiency Improvement

  8. Experimental EvidenceMEG using PbSe Thin Film Size - 8nm diam Thickness- 85 nm Band gap- .7eV 4-7X response- 3-5eV laser pulse Kim, Kim, et al- APL Jan, 2008

  9. Bulk vs NC Effect on MEG

  10. What causes MEG ? • Bulk vs Nano-crystal • More than scale • Quantum confinement • Impact Ionization • Too slow ? psec (-12)vs fmsec(-15) • Virtual states

  11. Klimov: MEG via Virtual Exciton States Proposed Mechanism • Two valence band electrons interact (Coulomb potential) - Creates virtual biexciton from “vacuum state”. • Absorbs photon (> 2Eg ) to become real, energy-conserving biexciton( 2 electrons/ 2 holes). • Possible in QDs (5-10 nm) due to relaxation of momentum conservation. Approach • Model PbSe nanodots with TD 2nd order perturbation theory

  12. Virtual Exciton States Transition Rates (FGR-2nd order) Possible Transitions Hxx = Coulomb –interaction operator – creates excitons from vacuum Hev = Electron-photon interaction operator

  13. MEG as function of Photon Energy Experimental Evidence Theory vs Experiment F= W2,xx/ W biexciton gen rate/total photo-gen rate

  14. Conclusions • MEG is a real phenomenon • Impact ionization seems unable to account for level of carrier generation (some disagree) • Virtual excitons “explains” MEG in nanodots. • Proposed mechanism is valid for any quantum-confined structure • Jury’s still out

  15. Thank you

More Related