Tiny Crystals Promise Big Benefits for Solar Technologies
< http://www.photonics.com/XQ/ASP/artid.334/QX/readart.htm >
LOS ALAMOS, N.M., Jan. 25 -- Los Alamos National Laboratory
scientists have discovered that a phenomenon called carrier
multiplication, in which semiconductor nanocrystals respond
to photons by producing multiple electrons, is applicable
to a broader array of materials than previously thought.
The discovery increases the potential for the use of nanoscrystals
as solar cell materials to produce higher electrical outputs
than current solar cells.
In papers published recently in the journals Nature Physics
and Applied Physics Letters, the scientists demonstrate that
carrier multiplication is not unique to lead selenide
nanocrystals, but also occurs with very high efficiency in
nanocrystals of other compositions, such as cadmium selenide.
In addition, these new results shed light on the mechanism
for carrier multiplication, which likely occurs via the
instantaneous photoexcitation of multiple electrons. Such
a process has never been observed in macroscopic materials,
and it explicitly relies on the unique physics of the
nanoscale size regime.
According to Richard Schaller, a Los Alamos scientist
on the team, "Our research of carrier multiplication
in previous years was really focused on analyzing the
response of lead selenide nanocrystals to very short
laser pulses. We discovered that the absorption of a
single photon could produce two or even three excited
electrons. We knew, somewhat instinctively, that
carrier multiplication was probably not confined to
lead selenide, but we needed to pursue the question."
The Los Alamos findings point toward practical
photovoltaic technologies that may utilize such traditional
solar cell materials as cadmium telluride, which is very
similar to cadmium selenide. Other interesting opportunities
may also be associated with the use of carrier multiplication
in solar-fuel technologies and specifically, the
production of hydrogen by photo-catalytic water splitting.
The latter process requires four electrons per water
molecule and its efficiency can be dramatically enhanced
if these multiple electrons can be produced via a
single-photon absorption event.
In addition to Klimov and Schaller, the Los Alamos
team includes Melissa Petruska, all of the Physical
Chemistry and Applied Spectroscopy group. Research
on carrier multiplication at Los Alamos is funded by
the DOE's Office of Basic Energy Sciences and by Los
Alamos' Laboratory-Directed Research and Development
(LDRD) program.
For more information, visit: quantumdot.lanl.gov online
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