Quantum Dot Solar Cells: High Efficiency through Multiple Exciton Generation
Impact ionization is a process in which absorbed photons in semiconductors that are at least twice the bandgap can produce multiple electron-hole pairs. For single-bandgap photovoltaic devices, this effect produces greatly enhanced theoretical thermodynamic conversion efficiencies that range from 45-85%, depending upon solar concentration, the cell temperature, and the number of electron-hole pairs produced per photon. For quantum dots (QDs), electron-hole pairs exist as excitons. We have observed astoundingly efficient multiple exciton generation (MEG) in QDs of PbSe (bulk Eg = 0.28 eV), ranging in diameter from 3.9 to 5.7nm (Eg = 0.73, 0.82, and 0.91 eV, respectively). The effective masses of electron and holes are about equal in PbSe, and the onset for efficient MEG occurs at about three times the QD HOMO-LUMO transition (its ''bandgap''). The quantum yield rises quickly after the onset and reaches 300% at 4 x Eg (3.64 eV) for the smallest QD; this means that every QD in the sample produces three electron-hole pairs/photon.
- Research Organization:
- National Renewable Energy Laboratory (NREL), Golden, CO.
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC36-99GO10337
- OSTI ID:
- 860690
- Report Number(s):
- NREL/CP-590-37036
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
36 MATERIALS SCIENCE
BAND GAP
Basic Sciences
EFFICIENCY
ELECTRONS
EXCITONS
IMPACT IONIZATION
IONIZATION
MULTIPLE EXCITON GENERATION (MEG)
Materials Science and Semiconductors
PHOTONS
PV
QUANTUM DOTS
QUANTUM DOTS (QDS)
SEMICONDUCTOR NANOPARTICLES
SOLAR CELLS
THERMODYNAMIC CONVERSION
THERMODYNAMICS