Solar Cells based on Quantum Dots: Multiple Exciton Generation

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Presentation transcript:

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 Based on “photoelectric effect” Photon excites valence electron Electron-hole pair created Electrons & holes “separate” Band gap determines what is absorbed

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

“Ideal” Solar Cell

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

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

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

Bulk vs NC Effect on MEG

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

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

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

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

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

Thank you