**STUDIES OF BASIC ELECTRONIC PROPERTIES OF CdTe-BASED SOLAR CELLS**

**& THEIR EVOLUTION DURING PROCESSING & STRESS**

**Quarterly Technical Status Report for the Quarter ended January 31,
2004**

**Under Prime Contract No. DE-AC36-99-GO1033**

**Subcontract No. ADJ-2-30630-05**

**Subcontractor: Colorado School of Mines**

**Principal Investigators: V. Kaydanov and T. R. Ohno**

**Contributors:**

**Professors Reuben T. Collins**

**Post Doc. J. Beach**

**Grad. Research Associates: S. Feldman, F. Symour and L. Mansfield**

**Contract technical monitor: Kenneth Zweibel**

1.  **Characterization of cells with electroluminescence (EL)**

Our previous studies demonstrated high sensitivity of EL intensity and
pattern to the CdTe cell processing and stressing (see, e.g., \[1\]). In
particular, it was found that even a "gentle" stress can significantly
effect measured EL whereas the J-V characterization detects only slight
changes. EL imaging of a cell proved useful to reveal
micro-nonuniformities in electronic properties and clarify mechanisms
behind them. Some new results of continued studies of EL obtained in the
past quarter are presented in this section.

1.  **Buffer layer in the cell front contact**

EL was studied and compared for cells with a single-layer (SL) TCO front
contact and those with a buffer layer between TCO and CdS (BL) prepared
at NREL. Cells prepared with CSS technique were supplied by D. Albin
(NREL). Cells fabricated at CSM (J. Kestner and J. Beach) were processed
with the gas jet deposition (GJD) technique. Usually BL cells
demonstrated improved efficiency. Replacement of a SL contact by the BL
led to a significant change in the EL pattern (see. Fig. 1.1) and
decrease in the EL non-uniformity (NU) for both CSS and GJD cells. The
NU, defined as a standard deviation of local EL intensity normalized by
mean EL intensity, was several times lower for BL cells.

![](media/image1.jpeg){width="3.3506944444444446in"
height="1.7166666666666666in"}![](media/image2.jpeg){width="3.0902777777777777in"
height="1.6916666666666667in"}

**Fig.1.1.** EL pattern in NREL cells: left picture - SL, right picture
-- BL. Both images

> are magnified representative areas (vertical dimension of \~350 m).

The mean EL intensity results are less unambiguous. With the BL front
contact it was lower for CSS and some GJD cells, however some CSM cells
showed increased mean intensity. Further studies are needed to make
reliable conclusions about the BL effect on mean EL intensity. Two JGD
cells were also characterized with LBIC at CSU (T. Nagle). The BL cell
with higher mean EL intensity than the SL control cell also demonstrated
higher mean apparent quantum efficiency (AQE). Similarly to EL,
non-uniformity of the LBIC pattern was lower for the BL cell.

Although we made some attempts to explain the observations, the results
obtained so far may not be representative and detailed enough to make
any certain conclusions about the mechanisms of the BL effect on EL
pattern and mean intensity as well as on cell performance and stability.

**1.2 Effect of etching of the CdTe surface**

EL was studied on differently etched cells and an unetched cell, all
supplied by D. Albin (NREL). One cell was prepared with the NP etch of
CdTe surface, another with the Br:Methanol etch. While no noticeable
difference in EL (mean intensity and non- uniformity) was observed
between the NP- and Br:Meth.-etched cells, there was a pronounced
difference between the etched and unetched cells. The etched cells
showed greater mean EL intensity as well as greater EL NU. Both effects
originated from bright spots in EL from the etched cells. Etched cells
had higher efficiency than the control ones, mostly due to enhanced fill
factor/reduced series resistance R~se~. It could be suggested that a
decrease in R~se~ due to etch occurs rather non-uniformly. Bright spots
in EL probably correspond to the regions with low resistance, hence with
enhanced current density/injection rate.

This suggestion is in agreement with our previous comparison of EL from
as-prepared and stressed cells, reported in \[1\]. Stressing, especially
under reverse bias, led to a significant raise in measured series
resistance of a cell, being accompanied with a drastic change in EL
pattern. The majority of EL came from the small-size, very bright spots
on the dark background. This effect was explained by strong
nonuniformity of the back contact resistance. Thus, while changes in EL
due to etching are generated by non-uniform reduction of the back
contact resistance, stressing caused non-uniform degradation of back
contact.

**1.3 Effect of inter-facial layer (IFL) on EL in FS cells**

Two sets of cells prepared at FS, with and without application of the
IFL treatment, were studied as a part of the non-uniformity sub-team
effort (coordinated by D. Shwydka, UT). The IFL treatment provided a
very strong effect on cell performance. Table 1.1 presents the cell
photo-electric parameters averaged over numerous cells of various
diameters, from 1.2 mm to 12 mm. Efficiency for the "no IFL" cells were
half of the IFL cells, mostly due to lower $V_{\text{oc}}$, but also due
to lower fill factor.

**Table 1.1.** Parameters of FS cells processed with and w/o IFL
treatment

  -------- -------- -------- ---------- -------- -------- -------- --------
  IFL      Eff., %  V~oc~,   J~sc~,     FF, %    R~sc~,   R~zb~    R~oc~
                    mV       mA/cm^2^                              

                                                 -cm^2^            

  Yes      12.8     819      23.3       66.8     660      6000     4.1

  No       6.2      472      23.3       55.2     414      2000     4.9
  -------- -------- -------- ---------- -------- -------- -------- --------

The EL data dramatically changed due to IFL treatment as illustrated by
the table below.

**Table 1.2.** Effect of IFL treatment on EL. J is the average injection
current density.

  ----------- -------------- -------------- ---------------- ---------------
  IFL         Diameter, mm   J, mA/cm^2^    Mean EL, counts  EL
                                                             nonuniformity

  no          3              25             163              2.2

  no          3              50             226              2.6

  no          3              125            1263             2.6

  yes         3              25             356              0.15

  yes         3              50             804              0.11

  yes         3              125            2878             0.11

  yes         12             25             385              0.24

  yes         12             50             1052             0.31

  yes         12             125            3605             0.34
  ----------- -------------- -------------- ---------------- ---------------

The IFL-cells showed relatively uniform emission whereas the no-IFL
cells were drastically non-uniform, much like stressed FS cells studied
previously. Mean EL intensity is higher in the IFL-cells by at least a
factor of 2 and up to more than an order of magnitude. IFL also
significantly decreases EL nonuniformity.

It was believed that one of the beneficial effects of IFL treatment is
blocking pin-holes and preventing them from filling with Cu during the
consequent deposition and diffusion of Cu. If so, one can suspect that
without IFL treatment we have a cell with a numerous "shunts" that (a)
decrease the V~oc~ value, (b) reduce R~sh~ (both effects are seen in the
table) and (c) make injection current density and EL intensity rather
non-uniform. With respect to the EL nonuniformity we might consider
another possibility: IFL improves the back contact quality, making it
less resistive and more uniform. However, as seen from the table, the
IFL treatment does not improve considerably R~oc~.

Studying the cells of different diameter revealed the size dependence of
measured mean EL and NU. As an example one can compare two IFL-treated
cells, 3 mm and 12 mm in diameter presented in Table 1.2. Increase in
the calculated EL NU reflects presence of non-uniformities with
different characteristic spatial scales. For the larger size cells the
probability is greater to find spots of significantly enhanced current
density, e.g. due to leaks. The EL intensity depends superlinearly on
the injection current density (compare the data in the table for J=25,
50 and 125 mA/cm^2^ ). Thus presence of a leak should increase EL
integrated over the cell area, or the mean EL intensity.

**1.4 Comparison of the EL and thermography images.**

This work was also a part of a study coordinated by the non-uniformity
sub-team (UT).

Previously it was suggested that the nonuniform brightness of both the
EL (CSM) and thermography (Max Plank Institute) images can be due to the
same reason: nonuniform distribution of current density over the cell
area. To verify this idea, the EL and thermography images obtained on
nominally identical cells from the same set were compared. It was hoped
that correlations (if any exists) could be more reliable if obtained not
on similar but on the exact same cells. For this purpose we used EL to
image cells fabricated at UT and FS and already imaged with
thermography.

Unfortunately, the correlation between the thermography and EL images
was very poor. The only reliable correlation to be reported was the
presence of a halo (ring of bright emission) around the edge of the cell
prepared in UT. A similar feature of the cell fabricated by FS was seen
only in the thermography, not in the EL image. One possible explanation
may be that the cells (especially the FS one) degraded significantly in
the period (months) between the thermography and EL measurements.
Whatever the reason for the failure, these studies should be continued.
They can provide better understanding of the mechanisms behind
micrononuniformity of the cell electronic properties which is believed
to be among the major factors affecting the cell performance and
stability \[2\].

5.  **EL characterization of the large area cell/modules.**

> As shown by our previous studies imaging EL and consequent
> quantification of the EL

intensity and nonuniformity can be a convenient, inexpensive and
sensitive non-destructive method for testing quality of CdTe cells.
First Solar has recently started performing EL measurements on some of
their modules. Our current imaging system of high sensitivity and high
spatial resolution can only image a fragment 1.7 x 1.3 mm^2^ at a time.
Though these images can be stitched together, data acquisition and image
processing for the large cell/module area is time consuming. The
advantage of our measurement systems are they permit detailed
examination of short scale non-uniformities and spectral resolution.

In parallel with FS efforts to image modules we are testing an
inexpensive, black and white CCD security camera with high sensitivity
(Sony SSCM183 with a 3.5-8mm adjustable focus lens). This allows for
imaging areas from slightly larger than 1 square cm to a large module
size. The camera is limited to a very short exposure time (1/30 s,
\~0.003 of that for the current system). Increasing the injection
current density or reducing the signal to noise by image averaging can
reduce the problem. For example, about 150 mA/cm^2^ was needed to image
the first cell tested with the new camera, which is not desirable.
Averaging or 'stacking' several images allows us to see features with
currents comparable to typical short circuit current densities. This is
similar to our standard CCD system that can integrate for minutes at a
time. As a first step to test this system, FS will send unencapsulated
portions of modules that have shown non-uniformities in their
measurements. We will compare EL imaging with the new camera and
modified software, as well as conduct spectral EL and PL studies on some
portions.

**1.6. Effect of cooling on EL pattern in non-uniformly doped cells**

In our previous report \[1\] we described studies of cells doped
intentionally non-uniformly with Cu. The cells were prepared with the
Au/Cu back contact using a standard procedure with only one difference.
Namely, Cu was evaporated on the CdTe surface in small dots through a
mask. This patterned non-uniformity had numerous Cu dots underneath the
Au layer occupied \~15% of the cell area, and only a Au contact for the
remainder. The major goal was to provide a non-uniform back contact
since doping with Cu improves its quality. Indeed, the contact averaged
"series resistance" estimated from the dark I-V curve was much higher
for the Au back contact than for the Au/Cu contact and was somewhere in
between for the Au/C-dotted contact. The EL images taken from the Cu-dot
cells demonstrated much brighter luminescence from the Cu-doped areas at
room temperature whereas the measured PL intensity spatial distribution
was flat. These facts were considered as confirmation of our concept
that the nonuniformity in EL over the cell area comes mostly from
nonuniform current density which in turn is caused by non-uniform back
contact resistance.

Here we present a new finding that leads to conclusions beyond the
initial goal of the non-uniform doping. Cooling the sample, aimed at
increasing of luminescence intensity and improvement of spectral and
spatial resolution of measurements, revealed an unexpected effect. The
EL pattern inverts at low temperature: the Cu-dot areas are darker than
the inter-dot areas (see Fig.1.2), whereas PL spatial distribution does
not show any correlation with the Cu-dot location. The inversion occurs
about 100 K, close to the temperature where capacitance measured on the
Cu-doped cells stops to depend on bias, which can be attributed to the
freezing-out of free carriers.

Based on this observations, we have proposed an explanation of the EL
pattern inversion. As before it is assumed that the EL pattern is
dominated by electric transport effects, not by recombination. In the
absence of Cu, cadmium vacancies, V~Cd,~ are believed to be the major
acceptor defects that control free hole concentration. Doping with Cu
that occupies the Cd sites reduces concentration of V~Cd~, but overall
increases acceptor concentration, especially in the vicinity of the back
contact. Thus it provides higher conductivity, hence enhanced current
density and brighter EL in the Cu-doped areas. In the "Cu-free" region
the V~Cd~ density is higher and provides greater contribution to the
free hole concentration. Despite the scattered literature data, all
estimates indicate that the acceptor level for the Cu~Cd~ defect is
significantly deeper than that for V~Cd~. Thus with reducing sample
temperature the depletion of free holes occurs first in the Cu~Cd~ --
dominated regions, making the "Cu-free" regions with higher V~Cd~
density the dominant paths for current and EL sources.

If this model is verified, temperature dependence of EL patterns and PL
in non-uniformly doped cells can be useful for obtaining additional
information on electrically active defects, in particular on their
relative density, activation energy, and thus the role in controlling
free carrier concentration.

> ![](media/image3.png){width="2.138888888888889in"
> height="2.076388888888889in"}![](media/image4.png){width="2.138888888888889in"
> height="2.076388888888889in"}

**Fig.1.2.** EL images of a cell fragment with non-uniformly Cu-doped
back contact.

Temperature of the sample: 150^o^K - left; 100^o^K -- right

2.  **Detection and studying of deep states in CdTe/CdS cells**

    1.  **Further development of measurement techniques and procedures**

In our previous report we have discussed our strategy for searching and
studying deep electronic states in CdTe/CdS states by using admittance
spectroscopy (AS) and transient effects. A new measurement system
developed for studying these effects in a wide temperature range was
described and some new data on transients in capacitance and AC
conductance were presented. Our activities in the past Quarter were
aimed mostly at improving techniques and procedures of measurements.

In particular, we were working upon protection from external
electromagnetic fields, reducing parasitic effects originated from the
LCR meters, electrical leads and contacts to the sample, optimizing
design of the sample holder and chamber where it is positioned,
improving the temperature control system and optimizing the temperature
variation rate when measuring temperature dependencies of effects.
Besides, based on our observations and studies of transients, we
estimated the time necessary to achieve (or at least to come close) to
the steady state after changing the bias applied or temperature and
before starting AS and transient measurements under new conditions.

We also have been varying conditions and procedures of measurements to
define those providing more visible manifestations of deep states, hence
more opportunities for their detection and studying. For example, as
discussed in \[1\], if the characteristic angular frequency ~t~ of the
trap is beyond (lower) the LCR frequency range, but not too low, it
could be brought into the measurable range by heating the cell. But
temperature range is also limited from above by possible irreversible
changes in the cell (for CdTe cells usually \~350 K). Another
opportunity is to cool the sample to make the trap slow enough to
manifest itself in the measurable transients.

Fig. 2.1 illustrates the second approach. Fig. 2.1(a) shows the
capacitance transients at different temperatures on one of the cells
supplied by D. Albin (see Table 2.1) recorded after switching from
reverse bias of -1 V to zero bias. Fig. 2.1(b) presents the so called
DLTS functions: $\frac{\text{dC}}{d}\text{ln}t = f(t)$ calculated for
these transients. For a single level trap or narrow band this function
must demonstrate an extremum at $t = \omega_{t}^{- 1} = \tau_{t}$(the
trap characteristic time). The minimum is well seen in DLTS function
that shifts right in the time scale with reducing temperature.

![](media/image5.wmf){width="3.240972222222222in"
height="2.626388888888889in"}![](media/image6.wmf){width="3.1979166666666665in"
height="2.623611111111111in"}

**(a) (b)**

**Fig. 2.1.** Capacitance transients measured at low temperatures on the
cell D220B. Recording

starts just after the bias switch from -1 V to 0 V. AC frequency
$f = \text{10}\text{KHz}$.

From the Arrhenius plot an activation energy and apparent cross section
were determined:$E_{a} \approx 0\text{.}\text{42}\text{eV}$;
$\sigma \approx \text{10}^{- \text{18}}\text{cm}^{2}$. Using these
values, the characteristic frequency was estimated for the
high-temperature
limit:$f_{t}(\text{80}^{o}C) = \omega_{t}(\text{80}^{o}C\frac{)}{2}\pi \approx \text{14}\text{Hz}$,
which is below the lower frequency limit of our LCR meters. Thus cooling
of a cell and measurement of transients allowed to detect a trap level
non-detectable with the AS method.

Another problem arises when the trap is so slow that at room temperature
even the reasonably long transient measurements cannot reveal a trap
level or band (DLTS function does not have any extrema). An effective
approach to the problem could be studying of transients at enhanced
temperatures. Since the trap characteristic time decreases exponentially
with temperature one can anticipate that the extrema could be found in
the measurements of reasonable duration. Fig.2.2. illustrates this
situation. While at 40^o^C and lower temperatures there is no hint on
the presence of extrema in the DLTS function, the latter becomes
non-monotonous at 50^o^C and demonstrates a well pronounced maximum at
60, 70 and 80^o^C. As expected it shifts left in the time scale with the
temperature increase. The Arrhenius plot based on three points provided
approximate estimates of activation energy and apparent capture cross
section: $E_{a} \approx 0\text{.}\text{85}\text{eV}$;
$\sigma\text{\textasciitilde}\text{10}^{- \text{18}}\text{cm}^{2}$. The
estimated characteristic time at room temperature
is:$\tau_{t}(\text{22}^{o}C) \approx \text{33}\text{hours}$.

![](media/image7.wmf){width="3.595138888888889in"
height="2.9965277777777777in"}

**Fig. 2.2.** DLTS function for transients measured at enhanced
temperatures on the cell D115B.

Variation of polarity of the "charging pulse" can significantly
influence transients and provides additional opportunities to detect and
study traps. Capacitance transients at opposite polarities of the
applied bias are compared in Fig. 2.3. It is seen that magnitude of a
transient is much greater at forward bias (FB) than at reverse bias
(RB). Difference between the capacitance values at the beginning and end
of the FB transient is 24nF/cm^2^ (maybe higher, because there is a
delay between the bias switch and start of recording, and also the FB
steady state was not reached). It is significantly greater than the
initial, 6nF/cm^2^, and final, 9nF/cm^2^, measured capacitance values.
If we attribute the transient to some slow traps, we must conclude that
their density is greater (maybe much greater) than concentration of free
carriers and faster traps.

![](media/image8.wmf){width="3.2625in"
height="2.509027777777778in"}![](media/image9.wmf){width="3.084722222222222in"
height="2.522222222222222in"}

**(a) (b)**

**Fig.2.3.** Capacitance transients measured after switching bias from 0
V to V=+0.5V (red)

and from 0 V to -1 V (blue). Cell D220B, T22^o^C, f=100KHz.

The DLTS function for FB (Fig. 2.3 (b))demonstrates a well pronounced
minimum at $\text{280}s$ that can be considered as a manifestation of a
very slow single level (narrow band) trap. No extremum is seen in the RB
DLTS function. Generally, this should not be surprising. Indeed,
transients at FB and RB, even if controlled by the same trap, are
provided by opposite processes: emptying and filling traps and vice
versa depending on the trap type (for holes or electrons), while the
emission and capture rates can be significantly different.

In this respect it is very useful to compare transients recorded under
applied bias and after switching back to zero bias (relaxation stage).
DLTS functions look different for the two stages, sometimes they are
rather complex revealing several extrema, both minima and maxima. Thus,
comparing transients at different sequence of bias applied and at
different polarity one can obtain more detailed and reliable information
on the trap type/s, emission and capture rates.

It also should be mentioned, that a practically total recovery of the
capacitance value occurs in several hours after switching back to zero
bias in transients presented in the figures above. Reversibility may be
considered as an additional argument that the observed transient are
caused by the change in the slow trap filling with carriers, but not by
generation, transformation or migration of the charged defects.

It is to be said in conclusion that not all of our exploratory findings
are presented here. For example, we have started studying of transients
during and after light pulses at varying bias voltages. This work is
being continued, and more results will be presented in the following
progress reports and publications.

2.  **Trap states detected using AS and transients**

Although in the past Quarter we did not focus on the systematic study of
deep states, analysis of the results of AS and transient measurements
allowed for detection of some energy levels in the gap and determine
their signatures (activation energy, E~a~, plus apparent capture cross
section, ). The results are presented below for the cells supplied by D.
Albin (NREL). They were prepared on CdTe deposited with CSS and some
variations in the postdeposition treatments (see Table 2.1). All cells
were fabricated with bi-layer front contact and Ag paste back contact.

**Table 2.1.** Variations in cell processing and parameters

+-----+------+-------+------+---------+-------+------+------+------+
| Sam | CBD  | CSS   | CdC  | Pre     | Eff.  | Voc  | Jsc  | FF   |
| ple | CdS  | CdTe  | l~2~ | contact |       |      |      |      |
|     |      |       |      |         | \%    | mV   | mA/c | \%   |
|     | T    | P     | time | etch    |       |      | m^2^ |      |
|     | ime, | ~O2~, | ,min |         |       |      |      |      |
|     | min  | torr  |      |         |       |      |      |      |
+-----+------+-------+------+---------+-------+------+------+------+
| D1  | 37.5 | 0.8   | 10.0 | Std NP  | 11.58 | 808  | 21.3 | 6    |
| 33D |      |       |      |         |       |      |      | 7.26 |
+-----+------+-------+------+---------+-------+------+------+------+
| D1  | 35   | 0.8   | 6.0  | 20      | 10.87 | 817  | 20.3 | 6    |
| 15B |      |       |      | 0:50BrM |       |      |      | 5.48 |
+-----+------+-------+------+---------+-------+------+------+------+
| D1  | 35   | 0.8   | 13.5 | 20      | 11.63 | 822  | 20.4 | 6    |
| 16D |      |       |      | 0:50BrM |       |      |      | 9.22 |
+-----+------+-------+------+---------+-------+------+------+------+
| D2  | 37   | 0.8   | 8.0  | 20      | 11.42 | 830  | 19.7 | 6    |
| 17C |      |       |      | 0:50BrM |       |      |      | 9.81 |
+-----+------+-------+------+---------+-------+------+------+------+
| D2  | 37   | 1.4   | 7.0  | 20      | 11.25 | 829  | 19.3 | 7    |
| 20B |      |       |      | 0:50BrM |       |      |      | 0.35 |
+-----+------+-------+------+---------+-------+------+------+------+

Fig. 2.4 shows the state signatures obtained with a common procedure
from the Arrhenius plots of the characteristic time dependence on
temperature. Characteristic times were determined with the admittance
spectroscopy (marked in the figure as AS) or with the DLTS function
analysis for the capacitance transients recorded after switching from
zero bias to reverse bias (marked as RB) and after switching back to
zero bias (marked as ZB).

![](media/image10.wmf){width="5.541666666666667in" height="3.90625in"}

**Fig. 2.4.** Signatures of the trap states determined for the cells
listed in Table 2.1.

We do not feel prepared enough to classify and interpret these
signatures properly. As said above we currently are focused on varying
and optimizing procedures of measurements and approaches to their
analysis. Hopefully, in the course of these studies which include now
measurements under different polarities of bias, illumination with light
of varying intensity and spectrum, we will be able to make more reliable
and detailed conclusions. However even the data presented in the Figure
demonstrate some interesting features to be discussed right now:

**(1)** All the points in the figure (excluding two "extra" points for
the cell D220B) could be divided into three distinct groups each
containing data for all five cells. Signatures in the group marked as AS
belong to "fast states" and were obtained with admittance spectroscopy.
Signatures in two other groups (RB abd ZB) belong to rather "slow
states" that were detected and studied using transients.

Specific features of the$C(f)$and $G(f)$dependencies that provided
signatures in the AS group could be explained equally by effect of fast
trap and of back contact Schottky diode with the potential barrier
height in the range of 0.3 to 0.37 eV. So far we don't have a reliable
approach to distinguish these two effects. Therefore we discuss below
only the results of transient studies (groups ZB and RB).

**(2)** Data vary significantly within each group that is for different
cells. However, dependence $\text{log}\sigma = f(E_{a})$ could be
represented by a straight line with a not too high standard deviation. A
similar behavior was observed previously for other materials. The most
recent study of this kind was conducted on the GaAs-based diodes \[3\].

The ***similarities*** of the results in \[3\] to our data include:

\(a\) A significant variation in activation energy for the diodes
prepared with varying processing procedures, as well as containing small
amounts of In and Ga.

\(b\) While the activation energy varied in the range of 0.5 to 0.8 eV,
the apparent cross section varied by about five orders of magnitude.

\(c\) The dependence $\text{log}\sigma = f(E_{a})$ could be represented
by a straight line with a slope close to that for our groups ZB and RB.

The ***differences*** are:

\(a\) All the states detected in \[3\] were rather "fast" so that
standard DLTS measurements were used for their studies. In our studies
the most interesting and novel data are obtained from transients.

\(b\) The devices studied in \[3\] were much more numerous than studied
by us. The multitude of signatures was spread rather uniformly along the
same straight line. That brought the authors to the conclusion that all
of them belong to the same defect (hole trap) with the electronic
properties varying with variations in processing and composition. Our
data are much less representative, hence it is reasonable so far to
consider two separate groups, especially because the data are obtained
at essentially different conditions (RB and ZB), and assume, at least
for a while, that we deal with two different defects.

**(3)** The linear dependence $\text{log}\sigma = f(E_{a})$(following
the so called Meyer-Neldel rule, MNR) observed in -Si, GaAs and other
materials is usually explained by a considerable change in the system
entropy,$\Delta S$, associated with the assembly of large number of
system excitations \[4\], phonons, in particular \[5\], at a specific
lattice site to produce a thermally activated transition over an energy
barrier. The second important point is that $\Delta S$ proportional to
the enthalpy of the defect activation process:

> $\Delta S = \beta\Delta H$ (2.1)

This is a physical explanation of the MNR established empirically for
various thermally activated processes, not only for trap
filling/emptying. As to the latter, the emission rate must be described,
see, e.g., \[6\] by the equation (we specified it for holes):

> $e_{h}(T) = {\overset{\sim}{\sigma}}_{h}(T) \cdot \left\langle v_{h}(T) \right\rangle \cdot N_{v}(T) \cdot \text{exp}\left\{ - \Delta G(T\frac{)}{\text{kT}} \right\}$
> (2.2)

where ${\overset{\sim}{\sigma}}_{h}$ is the capture cross section,
$\left\langle v_{h}(T) \right\rangle$ is the averaged thermal velocity
of holes,$N_{v}$is the effective density of states in the valence band,
and $\Delta G$ is the free energy change for ionization of the state
which is equal to the separation of the state energy level from the band
edge. We determined the activation energy and "apparent" cross section
from the Arrhenius plot $\frac{1}{T}$ vs.
$\text{ln}\frac{T^{2}}{\tau_{t}}$ based on the equation

> $\omega_{t =}\tau_{t}^{- 1} = \sigma \cdot \left\langle v(T) \right\rangle \cdot N_{v}(T) \cdot \text{exp}\left\{ \frac{- E_{a}}{\text{kT}} \right\}$
> (2.3)

which is a common procedure used also in \[3\]. Actually, the activation
energy determined from the slope of Arrhenius plot is the enthalpy of
the hole emission from the trap. Based on fundamental relation
$\Delta G = \Delta H - T\Delta S$ and using equations (2.1), (2.2) and
(2.3), one can find the relation between the "true" capture cross
section, $\overset{\sim}{\sigma}$, defined by Eq. (2.2) and the apparent
one,$\sigma$, determined based on Eq. (2.3) (the latter is presented in
Fig. 2.1):

$\overset{\sim}{\sigma} = \sigma \cdot \text{exp}\left( - \frac{\Delta S}{k} \right) = \sigma \cdot \left( - \beta\frac{\Delta H}{k} \right)$
(2.4)

If to assume that the signatures in a group (MNR family) belong to the
defect of the same nature, it is reasonable to suggest that the "true"
capture cross section should not vary significantly from cell to cell.
It was found in \[3\] that the characteristic time of the trap is the
same for all studied devices at
$T = \text{350}K = T_{\text{iso}}$("isokinetic" temperature), that
is$\beta = 2\text{.}9 \cdot \text{10}^{- 3}K^{- 1}$ . The "true" cross
section $\overset{\sim}{\sigma}$calculated with Eq. (2.4) for
this$\beta$ value, and using experimental data on $\sigma$ and
$\Delta H = E_{a}$, turned out to be almost the same for all studied
devices:$\sigma \approx \text{const}\text{.} \approx 1\text{.}5 \cdot \text{10}^{- \text{23}}\text{cm}^{2}$.
Assuming constancy of the "true" cross section for our states within the
group (ZB or RB) we have found
$\beta \approx 1\text{.}7 \cdot \text{10}^{- 3}K^{- 1}$for both groups,
and
$\overset{\sim}{\sigma} \approx 3 \cdot \text{10}^{- \text{23}}\text{cm}^{2}$for
the RB group and
$\overset{\sim}{\sigma} \approx 2 \cdot \text{10}^{- \text{25}}\text{cm}^{2}$
for the ZB group.

**(4)** An unusually small value of a capture cross section needs to be
discussed . The most probable explanation is the presence of the
repulsing potential barrier at the defect site for carriers to be
captured. The coulomb potential for a point defect with one or two extra
electrons/holes is not powerful enough to provide such a small cross
section. But the power of potential can increase dramatically when
created by the collective action of charged defects/states located close
to each other. The trap-cluster model was proposed R. Crandall and used
for explanation for unusual time dependence of slow trap charging caused
be the applied bias in $$$\alpha - \text{Si}$and CIGS cells \[7, 8\]. We
have discussed in \[1\] another model related to the grain boundary (GB)
states. The estimated density of these states per unit volume of CdTe
polycrystalline films studied by us is of the order of
$\text{10}^{\text{17}}\text{cm}^{- 3}$. Our experiments obviously
demonstrated powerful potential barriers for holes at GB's and their
very slow recharging after application of light or bias. Theoretical
analysis of kinetics of film resistance recovery was in a very good
agreement with the experimental results.

**(5)** The points (signatures) in a group/MNR-family correspond to the
cells prepared with at least one difference in the processing procedure.
Thus one may try to correlate the differences in the signatures with the
differences in procedures. For example, duration of CdCl~2~ treatment
was 6 min. for cell D115B whereas for cell D116D it was 13.5 min.
According to Table 2.1 it was the only difference in processing. The
defect signatures were rather different for these two cells.

RB signatures: D115B - E~a~ = 0.88 eV, 10^-18^ cm^2^; D116D - E~a~ =
0.95 eV, 10^-11^ cm^2^. ZB signatures: D115B - E~a~ = 0.0.33 eV, 10^-22^
cm^2^; D116D - E~a~ = 0.26 eV, 10^-23^ cm^2^. Note that in Fig. 2.4 the
signatures for these cells are shifted with respect to each other in
opposite directions in the RB and ZB groups. One may attribute the
differences in the signatures between D115B and D116B to difference in
CdCl~2~ treatment and then speculate on the mechanism behind the
difference.

Unfortunately, there is more than one difference in processing for all
other pairs of cells. Thus the attribution of the signature differences
will not be reliable. In future studies it is desirable to have pairs
with only one difference, and also greater number of cells to make the
results statistically representative.

Even more interesting would be comparison of the cells with more radical
differences in processing, especially fabricated with different CdTe
deposition techniques. For this purpose we have started recently the
same studies on the cells fabricated at FS with the vapor transport
deposition technique. It would be instructive to see whether we can find
in these cells the defect signatures that can be positioned in the same
groups as for the NREL cells hence be attributed to the same defect;
what are the differences in a signature for the supposedly same defect;
what is the difference in its concentration etc; are there defects that
manifest themselves only in one type of cells but not in the other.

3.  **Team activities**

In the past Quarter we continued and broadened participation in team
activities. Our joint studies with the CdTe team members are presented
in previous sections. In this section we provide a brief list of these
studies.

-   **First Solar, LLC.**

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-   EL characterization of the cells prepared with and without IFL
    treatment

-   AFM studies of cells with different IFL treatments

-   Development of a system for EL imaging of large area cells and
    modules

-   Studying of defects/deep electronic states using EL, AS and
    transient measurements

    -   **University of Toledo**

> Participation in the micrononuniformity studies coordinated by UT:

-   EL imaging and analysis of the EL patterns in the cells processed
    with various techniques (at various facilities)

-   Studies aimed at clarification of mechanisms of nonuniformities:

> \- Preparing and studying of intentionally nonuniform cells (laterally
> nonuniform doping with Cu)
>
> \- Simultaneous spatially and spectrally resolved measurements of EL
> and PL

-   **Colorado State University**

Comparative EL and LBIC studies of the intentionally nonuniform cells
and cells with

single- and bi-layer front contact

-   **NREL**

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-   Comparative EL characterization of cells prepared at NREL with
    variations in the processing procedures: single- and bi-layer front
    contact; Br:M, NP etch, no etch; variation of CdS thickness

-   Studying deep electronic states using AS and transient measurements
    on the cells prepared at NREL with varying processing procedures

**References**

1.  V.I. Kaydanov, T.R. Ohno, "Studies of Basic Electronic Properties of
    CdTe-Based Solar Cells & Their Evolution During Processing and
    Stressing", Annual Technical Report, Phase II, October 25,2002 --
    October 15, 2003. Subcontract No. ADJ-2-30630-05 under Prime
    Contract No. DE-AC36-99-GO10337.

2.  V.G. Karpov, A.D. Compaan and Diana Shvidka, "Effects of
    nonuniformities in thin-film photovoltaics, *Appl. Phys. Lett*.
    **80**, 4256 (2002).

3\. S.W. Johnston and R.S. Crandall, "Evidence of the Meyer-Neldel Rule
in InGaAsN Alloys:

Consequences for Photovoltaic Materials", *Spring 2003 MRS Meeting*, S.
Francisco.

4\. R.S. Crandall, "Meyer-Neldel Rule in Charge-Trapping Metastability
in p-Type

Hydrogenated Amorphous Silicon", *Phys. Rev. B,* **66**, 195210 (2002).

5\. Engstrom, O. and Alm, A., *Solid St. Electron.* **21**, 1571 (1978).

6\. P. Blood and J. W. Orton, *The Electrical Characterization of
Semiconductors: Majority Carriers and Electron States.* (Academic Press,
1992), pp. 426-432.

7\. R.S Crandall, *Journ. of Electron. Mat.* **9**, 713 (1982).

8\. D.L. Young, K. Ramanathan, M. Contreras, J. Abushama, R.S. Crandall,
"A Barrier to Trap Filling in CuIn~1-x~Ga~x~Se~2~", *Spring 2003 MRS
Meeting*, S. Francisco.