Solid State Theory of Photovoltaic Materials: Nanoscale Grain Boundaries and Doping CIGS Page: 4 of 5
This article is part of the collection entitled: Office of Scientific & Technical Information Technical Reports and was provided to Digital Library by the UNT Libraries Government Documents Department.
The following text was automatically extracted from the image on this page using optical character recognition software:
along the series CuGaS2 CuGaSe2 CuGaTe2.
Finally, comparing all compounds, the FM stability
decreases along GaN GaP GaAs CuGaS2
GaSb CuGaTe2. This work was done in collaboration
with ONR support.
3.3 Quantum-dot solar cells [3,4]
Many optoelectronic devices could achieve much
higher efficiencies if the excess energy of electrons
excited well above the conduction band minimum could
be used to promote other valence electrons across the gap
rather than being lost to phonons. It would then be
possible to obtain two electron-hole pairs from one. This
will possibly lead to high-efficiency solar cells. In bulk
materials, this process is inherently inefficient due to the
constraint of simultaneous energy and momentum
conservation. We calculated the rate of these processes,
and of selected competing ones in CdSe colloidal dots,
using our semi-empirical non-local pseudopotential
approach. (This methodology was developed over the
past seven years via BES-SC support). We find much
higher carrier multiplication rates than in conventional
bulk materials for electron excess energies just above the
energy gap Eg. We also find that in a neutral dot, the
only effective competing mechanism is Auger cooling,
whose decay rates can be comparable to those calculated
for the carrier multiplication process. Figure 1 shows our
results. We find that: (1) the DCM rates are of the order
of 1010 s, whereas in the usual bulk materials, rates of
this magnitude are obtained only for excess energies
about leV above Eg; (2) the lifetime of the competing
Auger cooling (AC) mechanism is of about the same
order of magnitude as that of the DCM process. For
higher excess energies, the presence of an energy gap
within the hole manifold slows DCM considerably
compared to AC, which is unaffected by it, leading to
inefficient DCM in an energy window of the size of such
a gap. As in the case of Auger multi-exciton
recombination rates, the main contribution to the DCM
rates is found to come from the dot surface. We conclude
that hot-electrons can live for only picoseconds-v-relaxing
by at least six orders of magnitude faster than the "phonon
bottleneck" will suggest. Furthermore, direct carrier
multiplication is not an efficient process, having Auger
cooling as a strong competitor.
10 20 30 40
Fig. 1. DCM lifetimes with (filled squares and solid line)
and without empty squares and dashed line) a hole
present, compared to AC lifetimes (filled circles and solid
line), for different (initial) impacting electron levels eth+1
as a function of the photon energy hv-2Eg at room
temperature. Inset: detail of the curve crossings in the
low-energy region of the graph.
 C. Person and A. Zunger, "Anomalous Grain
Boundary Physics in Polycrystalline CuInSe2: The
Existence of a Hole Barrier," PhysicalReview Letters, 91,
 Y.J. Zhao and A. Zunger, "Site preference for Mn
substitution in spintronic CuMX2 chalcopyrite
semiconductors," Phys. Rev. B, 69, 75208 (2004).
 M. Califano, A. Zunger and A. Franceschetti,
"Efficient Inverse Auger Recombination at Threshold in
CdSe Nanocrystals," NanoLetters, 4, (2004).
 M. Califano, A. Zunger and A. Franceschetti, "Direct
Carrier Multiplication due to Inverse Auger Scattering in
CdSe Quantum Dots," Appl. Phys. Lett., 84, 2409 (2004).
- U x Dircct
-' . ......0 0 ... c
- - - --__---A-e oo
Here’s what’s next.
This article can be searched. Note: Results may vary based on the legibility of text within the document.
Tools / Downloads
Get a copy of this page or view the extracted text.
Citing and Sharing
Basic information for referencing this web page. We also provide extended guidance on usage rights, references, copying or embedding.
Reference the current page of this Article.
Zunger, A. Solid State Theory of Photovoltaic Materials: Nanoscale Grain Boundaries and Doping CIGS, article, January 1, 2005; Golden, Colorado. (digital.library.unt.edu/ark:/67531/metadc779814/m1/4/: accessed December 12, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.