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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)
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
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 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
What causes MEG ? • Bulk vs Nano-crystal • 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