NCPV preprints for the 2. world conference on photovoltaic solar energy conversion Page: 17 of 144
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of the area is defect-free. The parameters for the defect-
free region are:
Jp= 0.035A/cm2, J01= 3.6 x 10-9 A/cm2 , J= 4.5 x
From the experimental data, the parameters for the
"defected" cell are:
Jph= 0.0245 A/cm2 , J01=3.6 x 10-8 Acm2 , J02
4.5x 10-11 A/cm2.
Figure 5 shows the calculated I-V characteristics of
these two cells. Their cell parameters are: Voc= 650
mV, Jsc= 34.45 mA/cm2, FF= 81.01, and the
efficiency=18.4 for defect-free; and Voc= 62 mV,
Jsc=32.7 mA/cm2, FF= 75.76 and Eff = 16.7 for
defected cells, respectively. It is seen that all the
parameters of the "defected" cell are lower than for the
"defect-free" cell. However, the major reduction is in
the Vo, and the FF. It should be pointed out that in an
"undefected" cell, a reduction of 30 mV would be
accompanied by a large reduction in Jsc in accordance
with the cell equation; shunting produces a
disproportionate reduction in the voltage.
0.1 0.2 0.3 0.4 0.5
Figure 5. Calculated I-V characteristics showing a
significant decrease in Voc by introducing defect
clusters in a solar cell.
7. EFFECT ON SOLAR CELL PROCESSING
Because defect clusters are decorated with
impurities, they are likely to make device performance
very sensitive to the process conditions. This effect
can happen via two mechanisms: (i) the inability to
getter precipitated impurities, and (ii) processing mc-Si
wafers at high temperature (>900*C) can causes partial
dissolution of the precipitated impurities leading to
increased impurity concentrations in the device.
We have shown that the clustering of defects can
occur in high-quality, multicrystalline Si solar cell
substrates. A mechanism for the formation of such
defects is proposed. Defect clusters can contain
precipitated impurities; as a result, impurity gettering
cannot work well in such regions . If device
fabrication is done at high temperatures, typically
exceeding 900 *C, some of the precipitates can start
dissolving which, in turn, can increase the
concentration of soluble impurities in the bulk of the
device. This behavior can make cell performance very
sensitive to the processing conditions. Defect clusters
act as shunts, degrading primarily the voltage-related
parameters of the device. By introducing device-
processing conditions that can rapidly dissolve the
precipitates and allow them to be gettered, one can
ameliorate the effect of defect clusters by processes
such as phosphorus diffusion and Al alloying. Such a
high-temperature process may also unpin the defects to
acquire a lower energy configuration and a lower
Detailed analyses have shown that the net reduction
in the photogenerated current is much smaller than the
fractional area of the defect clusters. This is because
the photocurrent can be quite high even for short
MCDL values. Consequently, the dominant effect of
the defect clusters is not via the reduction in the
photocurrent, but by affecting the voltage-related
In addition, as described in this paper, because
defect cluster propagates through the entire thickness
of the substrate, it is a "filamentary" junction shunt.
The shunting effect is further enhanced by the impurity
decoration during the crystal growth.
This work was supported by the US Department of
Energy under Contract # DE-AC36-83CH10093. The
authors are very grateful to Prof. Teh Tan of Duke
University and Prof. Sergei Ostapenko of University of
South Florida for many valuable discussions on the
formation and characterization of cluster defects.
 Bhushan Sopori, Procd. ICDS-19, Trans Tech
Pub., Edited by Gordon Davies and Maria Helena
Nazare, 527 (1997).
 B. L. Sopori, J. Electrochem. Soc., 131, 667
 B. L. Sopori, L. Jastrzebski, T. Y. Tan, and S.
Narayanan, Procd.12' PVSEC, 1003(1994).
 J. G. Fossum and F. A. Lindholm, IEEE Trans.
 B. L. Sopori, W. Chen, K. Nemire, J. Gee, S.
Ostapenko, Procd. MRS '98 Spring Meeting,
Symposium on Defect and Impurity Engineered
Semiconductors and Devices II, to be published.
- dolset lru cell
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NCPV preprints for the 2. world conference on photovoltaic solar energy conversion, article, September 1, 1998; Golden, Colorado. (https://digital.library.unt.edu/ark:/67531/metadc707815/m1/17/: accessed March 22, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.