Web Release Date: July 24,
Multiple Exciton Generation in Colloidal Silicon Nanocrystals
and
National Renewable Energy Laboratory, Golden, Colorado 80401, Innovalight, Inc., Santa Clara, California 95054, Department of Applied Physics, Colorado School of Mines, Golden, Colorado 80401, and Department of Chemistry, University of Colorado, Boulder, Colorado 80309
Received June 22, 2007
Abstract:
Multiple exciton generation (MEG) is a process whereby multiple electron-hole pairs, or excitons, are produced upon absorption of a single
photon in semiconductor nanocrystals (NCs) and represents a promising route to increased solar conversion efficiencies in single-junction
photovoltaic cells. We report for the first time MEG yields in colloidal Si NCs using ultrafast transient absorption spectroscopy. We find the
threshold photon energy for MEG in 9.5 nm diameter Si NCs (effective band gap 
Eg = 1.20 eV) to be 2.4 ± 0.1Eg and find an exciton-production quantum yield of 2.6 ± 0.2 excitons per absorbed photon at 3.4Eg. While MEG has been previously reported in direct-gap semiconductor
NCs of PbSe, PbS, PbTe, CdSe, and InAs, this represents the first report of MEG within indirect-gap semiconductor NCs. Furthermore, MEG
is found in relatively large Si NCs (diameter equal to about twice the Bohr radius) such that the confinement energy is not large enough to
produce a large blue-shift of the band gap (only 80 meV), but the Coulomb interaction is sufficiently enhanced to produce efficient MEG. Our
findings are of particular importance because Si dominates the photovoltaic solar cell industry, presents no problems regarding abundance
and accessibility within the Earth's crust, and poses no significant environmental problems regarding toxicity.
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