NCPV preprints for the 2. world conference on photovoltaic solar energy conversion Page: 56 of 144
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E =- ( Nb (5)
00 4r m*s)
The energy separation between the Fermi level and valence
bands, EK, near the grain boundary, but in the absence of a
potential barrier, can be expressed in terms of E,. The
value of F,, or doping density, determines how far up the
barrier the peak in thermally assisted emission occurs, and
the energy spread of this emission is related to the barrier
To confirm the existence of a dominate grain-barrier
resistance and the model as shown in Fig. 1, the resistance
frequency response was performed on etched and non-
etched samples. Fig. 2 shows an example of the resistivity
dependence on frequency in the light, and thus, confirms
the model. As can be seen from the figure, the resistance is
reduced by several orders of magnitude at high frequencies
and is indicative of the dominant barrier resistance. Also,
the value of high-frequency resistance is nearly constant in
all samples. This indicates that the intragrain resistance, or
bulk doping, remains unchanged. However, there is a
dependence on the low-frequency resistance, or barrier
resistance, between etched and non-etched samples.
Figure 2: Resistance frequency response of non-etched and NP-
etched CdTe under 1-sun illumination. Breakpoint is approx.
equal to (RlC)l.
The barrier potentials, Vb, were determined by measuring
the current vs. temperature for each sample and fitting each
sample to each conduction model as described earlier. The
results of each fit for a typical non-etched sample are
shown in Fig. 3. As can be seen from the figure, the
thermally assisted tunneling model fits best. It should be
noted that the maximum value of Nb was limited to 10s
cmr for drift-diffusion, as at this point v, = vd. A
maximum value was also set for the thermionic emission
case of Nb = 101 cm3, as at this point the effects of
tunneling may be significant. If the best fit for thermionic
emission went to this maximum, then the data should be fit
with the thermally assisted tunneling modeL This was the
case for the samples herein. The barrier height, Vb, as
determined by the thermally assisted tunneling model was
772 meV and E. was 7.9 meV. This barrier height
correlates well with values of about 0.8 eV for the valence
band offset between an unreacted metal on a "pinned"
Fermi level CdTe surface . For comparison, Vb as
determined from thermionic emission was 624 meV using
the maximum value of Nb. Nb as determined from E~ is
about 7x10" cm. Thus, previous studies, which used bulk
values for Nb, would yield even lower and more erroneous
values for Vb.
300 320 340 360
Figure 3: Comparison of fits to current vs. temperature data for
different models of conduction.
Similar good fits can be obtained for the NP-etched
samples. In this case, the thermionic model fits almost as
well as the thermally assisted tunneling model. It was
observed that varying the conduction thickness over a
couple of orders of magnitude had little effect on the
resultant barrier height of about 275 meV. For
comparison, a Vb of 302 meV was obtained for the
thermionic-only fit and using the maximum Nb value. The
thermally assisted tunneling model value of V compares
well with the value of 0.26 eV for the valence-band offset
between evaporated Te on CdTe . It was shown
previously that the N-P etch preferentially etches grain
boundaries and creates Te-rich grain boundaries .
The results of the fit and high-frequency resistance
measurements for a non-etched sample allow us to
calculate the shape of the band diagram, as seen in Fig. 4:
E Electron barrier
E= 1.s eV
x =.25 eV
Figure 4: Grain-Boundary Band Diagram of non-etched CdTe.
Vb and Nb as determined from thermally assisted tunneling theory
fit to current vs. temperature data.
The bulk doping level was estimated to be about 7x10" cnr
3 and was determined from the value of the high-frequency
intragrain resistance, device geometry, and a nominal value
of 60 cmv-s as the bulk hole mobility in crystalline CdTe.
--- Thermally Assisted
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NCPV preprints for the 2. world conference on photovoltaic solar energy conversion, article, September 1, 1998; Golden, Colorado. (digital.library.unt.edu/ark:/67531/metadc707815/m1/56/: accessed October 17, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.