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”
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 = excitons
(2+ electron/holes –no heat!)
Potential Efficiency Improvement

9. Experimental Evidence MEG 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

10. Bulk vs NC Effect on MEG

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

12. 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

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

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

15. 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

16. Thank you