Solar Cells based on Quantum Dots:Multiple Exciton Generation

Dave ConeUC-Davis PhysicsCM 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”

Potential Advantages: – Dramatic Efficiency Improvements– Greater Solar Spectrum

MEG- Multiple Exciton GenerationBasic 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 diamThickness- 85 nm

Band gap- .7eV4-7X 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 – 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 vacuumHev = 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