Solar Cells based on Quantum Dots Multiple Exciton

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

Significance of Solar • Solar PVs $15 B 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. 1 e. V=1100 nm (Si) 1. 4 e. V- 870 nm (Ga. As) 1. 7 e. V- 710 nm (Ca. Se) 2. 0 e. V- 600 nm (Ca. Se. QD) 3. 2 e. V- 400 nm (Ti. O 2)

“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” 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 Evidence MEG using Pb. Se Thin Film Size - 8 nm diam Thickness- 85 nm Band gap-. 7 e. V 4 -7 X response- 3 -5 e. V laser pulse Kim, et al- APL Jan, 2008

Bulk vs NC Effect on MEG

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 (> 2 Eg ) to become real, energy-conserving biexciton( 2 electrons/ 2 holes). • Possible in QDs (5 -10 nm) due to relaxation of momentum conservation. Approach • Model Pb. Se nanodots with TD 2 nd order perturbation theory

Virtual Exciton States Transition Rates (FGR-2 nd 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= W 2, 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