Why Does Ga Addition to CIS Limit Its Cell Performance: The Amazing Physics of Grain-Boundaries and Killer-Defects in Chalcopyrites Page: 4 of 5
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at the GB, indicating the limited supply of one type of
carrier there. Scanning Tunneling Microscopy (STM)
scans at low voltage, (when only electrons are injected
from the tip into the GB) revealed9 a decrease in
photon emission intensity at the GB when compared to
GI, demonstrating a reduced hole density at the GB as
predicted by the model. Two dimensional device
simulations of the model of neutral offset at the GB/GI
interface indicate a strongly reduced recombination at
the GB (on account of a reduced n-p product), leading
to a significant increase in solar cell efficiency relative
to a cell having no band offset at the GB/GI interface.
Figure 1 shows further that: (i) anion-terminated
GB's have negligible AE~ in CIS and CGS; (ii) cation-
terminated CGS has a larger GB AE~ compared to
CIS; (iii) the cation-terminated CGS has a large GB
AE, than CIS; (iv) relative cell performance of CIS vs.
CGS: Three factors are pertinent here: (a) we have
shown10 that an important reason for the lesser
performance of Ga-rich (more than 30% Ga) CIGS
solar cells is due to different behaviors of their GI not
GB. Both materials exhibit pinning of the Fermi level
(a) CuInSe,
(I 1 2 ? y, . iurlc hull (II2)1L suzrfacC
Elg - '.0 E_". El 0.04
Au
E, E. E 0.04
E, -0.22
(b) CuGaSe,
Fig 1.2,,,, Bandoffset energie (i unt of eV) of (112)
cation metal andbulk (inate surfaces
fAE. C e e 0.0
.1 0.5 A
E, E. 0.0
Fig. 1. Band offset energies (in units of eV) of (112)
cation metal and (112) anion Se terminated surfaces
of CIS and CGS. The error bar is estimated to 0.05 -
0.10 eV.at about E~+0.8 eV due to the spontaneous formation
of electron-annihilating Vc, acceptors; however this
energetic position is 0.9 eV below the CBM of CGS,
whereas it is only 0.2 eV or less below the CBM of
CIS. Thus, the maximum attainable voltage is more
limited in Ga-rich material; (b) At abrupt GI/GB
interfaces, the carrier transport can be limited by
tunneling assisted electron-hole recombinations (as in
conventional charged pn-junctions4). Whereas in CIS
the energy difference Ag = EC(GB) - E~(GI) at the
charge-neutral GB/GI interface (Fig. 1) nearly equals
the bulk CIS band gap, in CGS the smaller Ag = 0.6 eV
at the (112) GB/GI interface will increase
recombination; (c) The larger AE, in CGS can affect
Voc: In the space charge region near the CdS/CIGS
interface, the band profile restricts Voc and thus the cell
performance. In CIS AE, = 0 (Fig. 1) which does not
limit Voc. However, in CGS the strong downward band
bending AE, at the GB will affect Voc adversely. Effects
(a) - (c) lead to a lesser performance of CGS relative
to CIS.
ACKNOWLEDGEMENTS
The work was performed under DOE contract DE-
AC36-99-GO10337 in collaboration with C.Persson
and S.Lany.
REFERENCES
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4J.Y.W. Seto, J. Apple. Phys. 46, 5247 (1975).
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Jassim, Appl. Phys. Lett. 84, 3477 (2004).
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Contreras, A. Zunger and L.J. Brillson, Appl. Phys.
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For references to ALL NREL theoretical papers on PV,
arranged according to subjects, please see
http://www.sst.nrel.qov/topics/pv.html .
Please see pdf of many RECENT and relevant papers in
http://www.sst.nrel.gov/photovoltaics pub/pv.html2
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Zunger, A. Why Does Ga Addition to CIS Limit Its Cell Performance: The Amazing Physics of Grain-Boundaries and Killer-Defects in Chalcopyrites, article, November 1, 2005; Golden, Colorado. (https://digital.library.unt.edu/ark:/67531/metadc875685/m1/4/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.