**STUDIES OF BASIC ELECTRONIC PROPERTIES OF CdTe-BASED SOLAR CELLS &
THEIR EVOLUTION DURING PROCESSING & STRESS.**

**Quarterly Technical Status Report for the period from June 01, 2004,
to September 30, 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:**

**Professor Reuben T. Collins**

**Assistant Research Professor J. Beach**

**Grad. Research Associates: S. Feldman, F. Seymour, J. Yahrbough and L.
Mansfield**

**Contract technical monitor: Kenneth Zweibel**

In this report we present some of the results of our continued studies
of CdTe/CdS obtained in the past four months in the following areas:

-   Studies of electroluminescence intensity and pattern in differently
    processes and treated cells

-   Studies of deep electronic states (DES) with admittance spectroscopy
    and transient measurements; analysis and classification of the DES
    signatures

-   Mapping photocurrent collection over a cell area with the high
    resolution near field scanning optical microscopy (NSOM).

Besides, a copy is attached of the paper published at this period: S.D.
Feldman, R.T. Collins, and T.R. Ohno, "Effects of Cu in CdS/CdTe solar
cells with patterned doping and spatially resolved luminescence", *Appl.
Phys. Lett.*, **85** (9), 1529 (2004).

**1. Effect of Bias Light on the Electroluminescence Intensity and
Pattern**

Here we present the studies of the electro-luminescence (EL) on CdTe/CdS
cells that seem to provide an independent evidence of photoconductivity
in CdS and its influence on the cell properties. Measurements also
revealed a persistent effect of illumination by "blue" light with long
term transients lasting at least for hours.

**1.1 Low bias light intensity**

The studies were conducted on cells fully processed at First Solar, LLC
(FS). Light with wavelength, , of 400, 500, 550, 600 and 630 nm was
provided by using a halogen bulb with narrow-band pass filters in front
of it. The light was also filtered out using a high-pass filter that
allows the IR EL image to pass to the CCD camera but blocks the visible
light. EL images were measured with the nominal current density 140
mA/cm^2^, which did not damage the cell. To refine the EL data from the
effects of stray light, an EL image was taken after taking the image
without current (a dark frame), and then the latter was subtracted from
the former.

At room temperature the PL signal caused by the bias light was
undetectable due to very low excitation level provided by the light
passing through a narrow-band filter. However the visible bias light
caused a slight but measurable increase, up to 5-6 %, in the mean EL
intensity. More significant changes were observed in the EL
nonuniformity (NU) which is defined as the standard deviation of EL
intensity normalized by the mean one. The bias light decreased NU value
by 20-30 %, most significantly at = 500 and 550 nm. J-V characteristics
were also affected which led to reducing the forward bias voltage
applied to keep the current magnitude in light the same as in dark
("constant current" forward bias, CCFB). The CCFB decrease was most
significant also at 500. It is reasonable to suggest that the observed
effects, at least partially, relate to the bias light absorption in CdS.
Indeed the most significant changes in the EL NU and in the CCFB take
place at 500 nm (E~ph~ 2.5 eV), just above the CdS bandgap.

The problem of a photoconductive CdS layer (PC-CdS) in CdTe/CdS solar
cells has been discussed in numerous publications, see, e.g. \[1\], and
at several National CdTe Team Meetings. Based on experimental data it
has been suggested that while the films deposited on glass demonstrate
free electron density up to 10^16^-10^17^ cm^-3^, the donor states are
strongly compensated for the CdS in a cell, supposedly by the Cu-related
deep hole traps. Our previous Hall measurements on differently treated
CdS films, including the Cu-doped ones, and PL studied both on cells and
films \[2, 3\], strongly support this model. Blue light modulates
electron concentration which makes CdS rather conductive and decreases
its contribution to the depletion width as well as to the cell "series
resistance". In particular, "The enhanced PC was often correlated with a
large light-to-dark crossover"\[1\]. This effect is probably responsible
for the observed CCFB reducing by the bias light. It is reasonable to
expect that the high CdS dark resistance is spatially non-uniform, thus
causing non-uniformity in current density and hence in EL intensity. If
so, one can attribute the observed decrease in the EL NU in the bias
light to reducing the CdS contribution to the cell resistance. Thus, the
EL pattern could be influenced by nonuniform resistance not only of the
back contact \[4, 5\], but also of the window layer (TCO/CdS). Indeed
our recent studies \[6\] demonstrated a significant effect of the
bi-layer front contact (TCO/HRT) on the EL pattern.

**1.2 High light intensity**

In these experiments an Ar-Ion and a He-Ne lasers were used as sources
of the high intensity light. The experimental design was as follows: an
EL images before and after exposure to the laser irradiation were taken
from a cell displaying visible non-uniformity in the form of
high-contrast bright spots. Using a cylindrical lens we could increase
the irradiated area and change its shape so that it included several
bright EL spots. An image of the laser reflection clearly showed the
irradiated area which usually was of about 5001000 m^2^. The mean
irradiation power density ranged from 3 to 30 W/cm^2^. Duration of
exposure to laser varied in the range of 1 to 3 minutes.

The experiments were first conducted on the cells grown at CSM with the
gas jet deposition (GJD) technique. No significant change in the EL mean
intensity and pattern was observed after exposure to the laser beam with
the photon energy of E~ph~=1.92 eV (He-Ne laser) and 2.41 eV (Ar-Ion
laser). The situation radically changed for irradiation with E~ph~=2.54
eV and 2.71eV, both provided with Ar-Ion laser. Fig 1 shows that the
mean EL became much brighter in the exposed area than in the surrounding
(before irradiation it was practically the same). It is also seen that
the brightness of the exposed area decays very slowly: the effect could
still be seen many hours later.

![](media/image1.png){width="0.9137795275590551in"
height="0.9098425196850394in"}![](media/image2.png){width="0.9090551181102362in"
height="0.9216535433070866in"}![](media/image3.png){width="0.9255905511811023in"
height="0.9027559055118111in"}![](media/image4.png){width="0.8799212598425197in"
height="0.9283464566929134in"}

\(a\) (b) (c) (d)

**Fig.1.1** Cell exposure to the 2.54 eV laser irradiation. (a) Laser
reflection, zero current.

EL images: (b) just after turning laser off, (c) in 1.5 hours, (d) in
2.5 hours.

There is strong evidence that the observed changes in EL are due to the
light absorption in CdS and generation of electron-hole pairs. The
photons with E~ph~ =1.92 eV (below the CdS bandgap) are not practically
absorbed, and for those with E~ph~ =2.4 eV (only a little above the
bandgap) the absorption coefficient is so small that the most of these
photons pass through CdS and are absorbed in CdTe not significantly
contributing to the CdS photo-conductivity. The latter, caused by
absorption of the energetic photons, reduces the CdS contribution to the
"series resistance" only in the laser-exposed region. Since the total
current is maintained constant, the current density redistributes over
the cell area becoming higher (along with the EL intensity) in the
exposed area at the expense of the current and EL in the un-exposed one.
The CdS photoconductivity is usually ascribed to excess electrons (see,
e.g., ref. \[1\]). Based on a slow decay of photoconductivity, one must
conclude that the recombination rate is very slow. This is consistent
with the idea means that photogenerated holes are deactivated, captured
by very slow deep traps (see Sec. 1.1).

The same persistent increase in the EL intensity after exposure to the
laser beam was observed on many other differently processed cells. In
particular, Fig. 2 shows the data for a cell fabricated at FS that
demonstared low EL non-uniformity before exposure to laser.

![](media/image5.wmf){width="5.997916666666667in"
height="2.732638888888889in"}

**Fig 1.2.** Linear scan of EL across the FS cell irradiated by 2.71 eV
laser beam. Black - EL scan before irradiation' red -- EL scan after
irradiation; green the laser light reflection scan, indicating the
irradiated region. Current maintained the same for EL measurements
before and after irradiation.

To compare the effects of the laser-induced non-uniformity in the CdS
and the back contact non-uniformity, a cell was studied with the CdTe
back surface non-uniformly doped with Cu evaporated through a shadow
mask \[5\]. This "Cu dot" cell was made on the FS material with a back
contact completed at CSM. Before exposing to the laser, this cell showed
EL primarily originating from the Cu dots. The ratio of EL intensity
between Cu dots and non-Cu doped regions was about a factor of 2. After
exposure to the laser, the EL still showed the Cu dot pattern, but the
areas under the laser were much brighter than areas not under the laser.
This effect was so large that non-Cu doped regions exposed to laser
light showed brighter EL than not exposed Cu dot regions. The results
are summarized in the table below with the numbers representing
approximate normalized intensities using arbitrary units:

**Table 2.1.** Influence of irradiation with laser on the EL intensity
in Cu dot cell

+-------------+-------------+-------------+-------------+-------------+
|             | Exposed     | Exposed     | Not exposed | Not exposed |
|             |             |             |             | non-Cu      |
|             | Cu region   | non-Cu      | Cu region   | region      |
|             |             | region      |             |             |
+-------------+-------------+-------------+-------------+-------------+
|             | Approximate |             |             |             |
|             | normalized  |             |             |             |
|             | EL          |             |             |             |
|             | intensity,  |             |             |             |
|             | arb. units  |             |             |             |
+-------------+-------------+-------------+-------------+-------------+
| Before      | 2.5         | 1           | 2.5         | 1           |
| laser       |             |             |             |             |
+-------------+-------------+-------------+-------------+-------------+
| After laser | 6           | 2.5         | 1.5         | 0.8         |
+-------------+-------------+-------------+-------------+-------------+

Fig.3 illustrates effect of exposure to laser light on the EL pattern of
the Cu dot cell.

![](media/image6.png){width="0.9507874015748031in"
height="0.9236220472440945in"}![](media/image7.png){width="0.9216535433070866in"
height="0.8862204724409449in"}![](media/image8.png){width="0.952755905511811in"
height="0.9251968503937008in"}

\(a\) (b) (c)

**Fig.1.3.** Effect of irradiation with laser on the Cu dot cell. (a)
and (c) EL before and after irradiation; (b) laser reflection image.

The next experiment was conducted on cells fabricated at CSM with GJD
method. The two cells studied differed by the window layer structure:
TCO/CdS (single-layer front contact) and TCO/HRT/CdS (bi-layer front
contact). A significant difference in the EL pattern between the two
before irradiation is seen from images (a) and (b) in Fig. 4: the EL NU
is much higher in the single-layer. Exposure to the laser beam increases
significantly brightness of the exposed region in the "single-layer"
cell and makes much darker the rest of the cell (image (c)). Changes in
the EL pattern after irradiation are much less significant in the
bi-layer cell(images (b) and (d)). However, it should be noted that the
bi-layer cell also had a CdS layer that was thinner by a factor of \~2
than the single layer cell.

  ----------------------- ----------------------------------------------------- -----------------------------------------------------
                          Single layer front contact                            Bi-layer front contact

  Before irradiation      ![](media/image9.png){width="1.3229166666666667in"    ![](media/image10.png){width="1.1145833333333333in"
                          height="0.9583333333333334in"} (a)                    height="1.0208333333333333in"} (b)

  After irradiation       ![](media/image11.png){width="1.2395833333333333in"   ![](media/image12.png){width="1.1145833333333333in"
                          height="1.0in"} (c)                                   height="0.9791666666666666in"} (d)
  ----------------------- ----------------------------------------------------- -----------------------------------------------------

**Fig. 1.4**. EL pattern in the "single-layer" and "bi-layer" cells and
changes caused by irradiation

To explain the data above in the context of the discussion at the end of
Sec. 1.1, one may suggest:

\(1\) Presence of HRT reduces the CdS dark resistance, both its mean
value and non-uniformity thus reducing the EL NU.

\(2\) Contribution of CdS in a cell "series resistance" in the
"bi-layer" cell is rather small as compared to other components,
therefore an additional light-induced decrease of CdS resistance due to
irradiation cannot significantly change the current's spatial
distribution and therefore the EL pattern.

\(3\) Contrary to what we thought earlier, the effect of the window
layer resistance and its non- uniformity on the EL pattern may be
comparable to or even greater than the effects of a nonuniform back
contact.

We are not able now to discuss possible mechanisms of HRT influence on
the CdS properties, however we believe that better understanding of the
phenomena could provide a significant progress in searching new
approaches to improvement of the CdTe/CdS cell performance. Further
studies of this problem with a variety of independent experimental
methods along with theoretical analysis and modeling are of significant
importance.

**2. Detection and Studying Deep Electronic States**

Deep electronic states (DES) in CdTe/CdS cells were detected and studied
using admittance spectroscopy (AS) and capacitance transient (CTr)
measurements. A combination of two methods allowed us to cover a broad
range of characteristic times,$\tau$, ranging from \~
$0\text{.}1\mu s$to minutes. Characteristic time was determined from the
position on the frequency scale of the peak in
$\text{dG}(\omega\frac{)}{d\omega}$ or $\frac{- \text{dC}}{d\omega}$
functions for the AS method ($G$, C and $\omega$ are the AC conductance,
capacitance and angular frequency of testing voltage, respectively).
When measuring transients, the DLTS function,
$\frac{\text{dC}}{d}\text{ln}t$, was analyzed. For the exponential time
dependence
$\Delta C(t) = {\Delta C}_{o}\text{exp}( - \frac{t}{\tau})$the peak of
this function is located on the time scale at $t_{\text{peak}} = \tau$.
The origin of the time scale, $t = 0$, corresponds to the bias or light
pulse switch on or off. The signatures of the DES: activation energy,
$E_{a}$, and apparent capture section, $\sigma$, were estimated from the
Arrhenius plot $\left( \tau \cdot T^{2} \right)^{- 2}$vs.
$\frac{\text{1000}}{T}$ regression slope and y-intercept.

1.  **Studying of Supposedly Identical Cells**

The goal of this study was to reveal and analyze the variances in the
detected DES signatures, $E_{a}$ and $\sigma$, between the cells
fabricated with supposedly identical processing procedures. This was
intended to serve as a baseline for determining the statistical
significance of measured differences between cells with different
processing treatments. The cells for studies were supplied by Dr. D.
Albin (NREL). A single large cell was grown on 1mm thick glass with a
standard procedure except some changes in the Br-Methanol solution used
for the CdTe surface etching. The individual small cells were prepared
by selectively scraping the thin film with a razor blade to the TCO
layer. The geometry of the cell layout with the surface area for each
cell is shown in figure 2.1.

**Fig. 2.1.** Plate received from D. Albin with 8 completed cells G,
through N and their respective surface areas in cm^2^. The indium front
contact is shown in grey.

Standard *J-V* measurements performed on CSM equipment indicated
efficiencies approximately 2% lower than the readings taken at NREL.
With this correction the eight cell efficiencies varied between \~ 8%
and \~ 10% with a strong correlation to the $J_{\text{sc}}$variation.
The zero bias dark resistance for all cells was above 10 M-cm^2^
indicating an absence of significant current shunts. The QE spectra were
similar for all cells with some variations in magnitude from cell to
cell and crossing of some graphs.

Temperature dependence of capacitance (measured with 100 kHz testing
frequency) demonstrated a significant decrease with cooling down from
room temperature and saturated below -100^o^ C at the level
$C \approx 1 \cdot \text{10}^{- 9}\frac{F}{\text{cm}^{2}}$for all cells.
The insulating (depleted) layer thickness was estimated based on the
$C$(-110^o^C) values and it varied between 8.2 and 9.9$\mu$m with an
average of 9.2$\mu$m. It was notably close to the 9.4 m average CdTe
thickness estimated at NREL with profilometry. Like our estimates, the
NREL data show thinner CdTe towards the top and bottom of the large cell
in Fig. 2.1.

It is likely that a strong (seven to eight times) decrease in
capacitance with cooling from room temperature is mainly due to the
holes freezing out meaning that hole concentration is controlled by not
too shallow acceptor levels. However, reducing of the trap-related
capacitance also might be partially responsible for the effect.

A very significant variation in the signatures of supposedly same DES
was detected by AS and CTr measurements performed on all cells. AS was
conducted at zero bias with frequencies ranging from 25Hz to 1MHz. CTr
was studied by measuring and recording capacitance during a 5 minute
0.5V forward bias (FB) pulse followed by a 5 minute zero bias (ZB)
recovery period. Temperatures ranged from --110C to 85C for all cells
since (a) preliminary measurements did not reveal any DES manifestations
at lower temperatures and (b) heating beyond 85^o^C leads to
irreversible changes in cell characteristics according to our previous
studies.

The DES signatures:$E_{a}$ and $\sigma$ were estimated from the
temperature dependence of the characteristic time, $\tau$. The latter
was derived from AS spectra on the frequency scale or peaks of the DLTS
function on the time scale for CTr's. It should be noted that both could
be observed in a limited ranges of temperature, different for different
DES. Although the determined signatures initially appear to be randomly
scattered, we have separated them to four groups based on the technique
(AS or CTr), bias applied at TCr measurement (FB or ZB), and the
temperature interval where the Arrhenius plots
$(\tau \cdot T^{2})^{- 1}$vs. $\frac{\text{1000}}{T}$ could be
constructed (see Table 2.1).

**Table 2.1.** The DES groups, methods and temperature interval of
detection,

the DES signatures variations

+------------+------------+------------+------------+---------------+
| Group      | Technique/ | T , ^o^K   | Ea, eV     | , cm^2^       |
|            |            |            |            |               |
| ID         | Bias       |            |            |               |
+------------+------------+------------+------------+---------------+
| AS1        | AS         | $$\text{20 | $$0\tex    | $$3 \cdo      |
|            |            | 3}\cdots\t | t{.}\text{ | t \text{10}^{ |
|            |            | ext{333}$$ | 33}\cdots  | - \text{15}}\ |
|            |            |            | 0\text{.}\ | cdots 1 \cdot |
|            |            |            | text{37}$$ |  \text{10}^{- |
|            |            |            |            |  \text{14}}$$ |
+------------+------------+------------+------------+---------------+
| ZB1        | CTr ZB     | $$\text{27 | $$0\tex    | $$1 \cdo      |
|            |            | 3}\cdots\t | t{.}\text{ | t \text{10}^{ |
|            |            | ext{308}$$ | 48}\cdots  | - \text{20}}\ |
|            |            |            | 0\text{.}\ | cdots 2 \cdot |
|            |            |            | text{78}$$ |  \text{10}^{- |
|            |            |            |            |  \text{15}}$$ |
+------------+------------+------------+------------+---------------+
| FB1        | CTr FB     | $$\text{33 | $$0\tex    | $$2 \cdo      |
|            |            | 3}\cdots\t | t{.}\text{ | t \text{10}^{ |
|            |            | ext{358}$$ | 80}\cdots  | - \text{17}}\ |
|            |            |            | 0\text{.}\ | cdots 1 \cdot |
|            |            |            | text{93}$$ |  \text{10}^{- |
|            |            |            |            |  \text{15}}$$ |
+------------+------------+------------+------------+---------------+
| FB2        | CTr FB     | $$\text{28 | $$0\tex    | $$1 \cdo      |
|            |            | 3}\cdots\t | t{.}\text{ | t \text{10}^{ |
|            |            | ext{318}$$ | 55}\cdots  | - \text{19}}\ |
|            |            |            | 1\text{.}\ | cdots 9 \cdot |
|            |            |            | text{01}$$ |  \text{10}^{- |
|            |            |            |            |  \text{12}}$$ |
+------------+------------+------------+------------+---------------+

Groups FB1 and FB2 originated from two distinct peaks in the DLTS
function observed in FB capacitance transients. Group ZB1 includes
signatures of a single peak we could see in ZB transients in the
available temperature range for 5 min. pulse duration. It is seen that
the signatures in each group are still widely spread, especially in ZB1
and FB2. Fortunately we had one more criterion helping us to distinguish
signatures belonging to different groups. Namely, as seen in Fig. 2.2,
the dependencies $\text{ln}\sigma(E_{a})$are linear for each group,
while locations and slopes of the linear plots are different.

![](media/image13.wmf){width="6.010416666666667in"
height="3.4631944444444445in"}

**Fig. 2.2.** The $\text{log}\sigma(E_{a})$linear dependencies -- an
additional criterion for the signature grouping

It is hard to say whether the alignment of the ZB1 and FB2 signatures
are due to some physical reason or simply due to coincidence. Note that
transients caused by the bias change of different polarity can be
dominated by various processes: capture of the injected carriers,
emptying of traps (carrier emission). Moreover, more than one trap can
be involved in transients. Until the issue is clarified we have to refer
to the ZB1 and FB2 signatures as provided by different DES.

2.  **Discussion**

We are prone to believe that signatures within each group correspond to
the same defect/DES. If it is true, we have to answer the question, why
signatures determined by our measurement and data treatment procedures
vary so widely between the eight studied cells. Character of the
$\sigma(E_{a})$dependencies can help to propose possible explanation.

Previously we have reported similar linear
$\text{log}\sigma = f(E_{a})$plots found in CdTe/CdS cells, see \[6,
7\]. A similar behavior was observed earlier for other materials, in
particular on the InGaAsN-based diodes \[8\]. The similarities of the
results in \[8\] to our data include:

(a) A significant variation in $E_{a}$ and $\sigma$ even for diodes of
    the same composition.

(b) While $E_{a}$ varied from 0.5 to 0.8 eV, $\sigma$varied by about
    five orders of magnitude

(c) The $\text{log}\sigma = f(E_{a})$plot is linear following
    Meyer-Neldel Rule (MNR).

The MNR type behavior observed in $\alpha -$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 \[9\], phonons in particular \[10\], at a specific lattice
site to produce a thermally activated transition over an energy barrier.
The second important point is that $\Delta S$ is proportional to the
enthalpy of the defect activation process:$\Delta S = \beta\Delta H$. It
follows from this model that the apparent capture cross
section,$\sigma$, is related to the "true" one,
$\overset{\sim}{\sigma}$, by the expressions:

$\frac{\sigma}{\overset{\sim}{\sigma}} = \text{exp}(\beta\Delta H) = \text{exp}(\frac{E_{a}}{\text{kT}_{\text{iso}}})$
$\text{ln}\sigma = \text{Const}\text{.} + \frac{E_{a}}{\text{kT}_{\text{iso}}}$

Here $T_{\text{iso}}$ is the temperature at which characteristic time is
the same for all studied members of the MNR family, CdTe/CdS cells in
our case.

We accepted this explanation in \[6, 7\], but there remains a question
we need to answer: why does the activation energy of the same DES/defect
in supposedly identical materials varies so widely, by tenths of eV? The
CdTe/CdS cells studied in \[6, 7\] were fabricated at NREL with
intentional variations in several processing steps/procedures. These
variations did not change significantly photoelectric characteristics,
but one may speculate that for some reasons they might strongly affect
the defect structure and DES characteristics caused by interaction with
other defects whose concentration and location varied with processing
variation. However our new studies presented in this report were
conducted on eight cells produced by selective scribing of a single
large cell. Thus one may assume that the processing procedure, defect
structure and electronic properties should be identical. However a
significant variation in the$E_{a}$ value was observed for the DES
belonging to the same group (see Fig. 2.2).

Below we discuss some possible answers to the question formulated above.
These suggestions are rather trivial since they are based not on ~~the~~
fundamental physics issues but rather on ~~the~~ instrumental ones.

The first answer could be found in ref. \[11\]. As applied to our
problem of DES the major idea and results in this paper could be
formulated as follows. Let us have simultaneously two DES with
significantly different activation energies, hence different
characteristic times: $\tau_{s}$ for the shallower DES much shorter that
for the deeper one, $\tau_{d}$. If so, the characteristic time
determined with AS or CTr at lower temperatures is close to
$\tau_{s}$even if concentration of shallower DES, $N_{s}$, is much
smaller than that of the deeper ones, $N_{d}$. With increasing
temperature the influence of the deeper state gradually increases and at
some temperature range can become dominating so that measured
$\tau \approx \tau_{d}$. If the transition from $\tau_{s}$to
$\tau_{d}$occurs in the same temperature range where DES could be
detected and $\tau$ measured, the Arrhenius plot has some curvature.
When approximating plot with a straight line, the regression slope may
vary depending on the relative contributions of deeper and shallower
states.

We have conducted modeling of $\tau(T)$, $E_{a}$ and $\sigma$ with
varying $E_{a,d}$, $E_{a,s}$ and $\frac{N_{s}}{N_{d}}$, whereas
assuming$N_{d}\text{>>}N_{s}$and $E_{a,d}$-
$E_{a,s}$`<!-- -->`{=html}0.3eV. The ratio
$\frac{\sigma_{d}}{\sigma_{s}}$ was also varied to put the transition
region in the temperature range from 360 K to 260 K close to the real
experimental conditions. It was found that the $E_{a}$ value determined
from the regression slope can vary by tenths of eV due to variation
in$N_{s}$. Changes in the linear plot slope lead to the proportional
changes in the y-intercept (linear dependence between the two). This
creates corresponding changes the $\sigma$ value. Since the y-scale is
logarithmic on $\tau$ and $\sigma \propto \tau^{- 1}$, the dependence
$\text{ln}\sigma = f(E_{a})$ is the MNR-like, that is, it is linear and
correlates well with our data in Fig 2.2. As mention above, even a very
low concentration of shallower states can change significantly the
activation energy determined by measurements. It is likely that, along
with the "major" deeper state we are studying, there exist in the
material small amounts of the "additional" shallower states that vary
uncontrollably ~~varying~~ from cell to cell thus providing variations
in the apparent activation energy.

Another possible explanation is even more trivial. As we pointed out the
temperature range in which we are able to detect and characterize a DES
is limited by (a) the DES characteristics, (b) the instrumental
limitations, and (c) irreversible changes of a material at enhanced
temperature. Because of this we are frequently able to construct the
Arrhenius plot in a narrow range of temperatures. As seen from Table 2.1
this was the case for our transient measurements. Taking into account
all possible sources of errors when determining characteristic time and
its temperature dependence, it is unrealistic to provide a good accuracy
for linear approximation of the Arrhenius plot for such a narrow
temperature range.

Indeed, the temperature range in which AS detection of DES was available
is 4-5 times wider than that for Ctr's (Table 2.1). Correspondingly the
spread in activation energies is 6% for AS while for FB1 it reaches 30%.

Thus the MNR-like dependence $\sigma(E_{a})$could be explained in
different ways. Explanation based solely on the entropy contribution to
the enthalpy of the DES activation process is attractive, but leaves
open the question why activation energy of the same state could vary so
widely, especially in identically produced materials. Two other
explanations allows one to answer this question. All three
mechanisms/reasons for MNR-like behavior of DES characteristics may
coexist. Their individual contributions should depend on the material
properties, nature of DES under investigation, and methods/techniques
applied.

One can find in the literature reports on numerous DES in CdTe.
Frequently several different DES signatures are attributed to the same
defect or, vice versa, the same signatures are ascribed to different
defects. We think our classification of DES based on belonging to a
specific MNR families (groups like those in Fig.2.2) may bring some
order in the widely, sometimes random, distribution of the detected DES
signatures. We believe that different MNR families represent different
defects/DES's, while signatures within the family/group belong to the
same defect. The discussion above provides possible explanations to the
wide spread of $E_{a}$ and $\sigma$ within a family.

**2.3 Preliminary Studies of Light Induced Capacitance Transients**

Slow light induced capacitance transients have been observed at room
temperature at wavelengths below and above the CdTe bandgap (wavelength
cutoff 825nm). Lighting intensities were very low, of the order of
0.1mW/cm^2^. This excludes any considerable changes in temperature,
which was verified by the thermocouple readings that did not change with
lighting within the accuracy of 0.1^o^ K. Based on preliminary
measurements of the temperature coefficient of capacitance, this may
lead to thermally induced capacitance variations not exceeding
0.01nF/cm^2^, whereas the observed light induced changes were in the
range of 0.2 to 1 nF/cm^2^.

Figure 2.3 shows capacitance versus log time for a 30 second initial
period with no illumination, a 45 minute (2700s) light pulse, and a 45
minute recovery period on the CdTe/CdS cell 217C supplied by D. Albin.
The changes in capacitance indicate changes in depletion width due to
changes in net space charge density (SCD). The SCD variations are
inferred to result from net charging and discharging of DES.

A pronounced difference can be seen between the above band gap (680nm)
and below band gap (1064nm, 1300nm) wavelengths. With the above band gap
wavelength, following an initial spike, the capacitance drops below the
pre-illumination level before gradually increasing. Both during and
following the illumination the capacitance does not reach saturation
indicating very long characteristic times ($\tau > \text{1000}s$).
Differences in starting capacitance during the initial 30 s period prior
to lighting are due to remnant trap charges from previous testing. To
start with the same capacitance the sample needed to be kept in the dark
overnight indicating $\tau\text{\textasciitilde}$ hours.

The light source was a 150W halogen bulb with a rheostat control for
intensity. The light is conveyed to the cell through a ½ inch diameter
optical fiber bundle that is 3 feet long. One inch diameter narrow band
(\~20nm) Corian filters were placed at the end of the fiber bundle to
select the wavelength incident upon the solar cell. Light intensities
for the 680nm (1.82eV) and 1064nm (1.17eV) wavelengths were measured
with a silicon based optical power meter. For the 1300nm (0.95eV)
wavelength that is below the 1.1 eV silicon band gap, no direct light
intensity measurements were made. In this instance the rheostat setting
was left the same as for the 1064nm wavelength and the light intensity
was assumed to be approximately the same.

![](media/image14.wmf){width="4.75in" height="3.254861111111111in"}

**Fig. 2.3**. Light induced capacitance transients for 0.1mW/cm^2^
lighting intensity and wavelengths of 680nm, 1064nm, and 1300nm. The
sample D217C was suppliedby D. Albin. Testing voltage frequency is
100kHz, sequence is 30s dark, 2700s light, 2700s dark.

Figure 2.4 shows capacitance versus log time for light intensities of
0.025, 0.050, 0.100, 0.200, and 0.400 mW/cm^2^. For each test, the
sample was kept in the dark overnight prior to the test so as to start
with a comparable capacitance value. In spite of this, there remained
minor starting capacitance differences of up to 0.1 nF. On a couple of
tests the sample was kept in the dark for 60 hrs over a weekend (not
shown in figure 2.4) and this caused the starting capacitance to drop by
an additional 0.2nF. Even with these offsets, the structure of the light
induced CTr curves was essentially the same. Thus, overnight in the dark
was determined to be sufficient for these low light CTr tests.

![](media/image15.wmf){width="5.120138888888889in"
height="3.7423611111111112in"}

**Fig. 2.5.** Capacitance transients induced by the sub-bandgap (1064
nm) light of various intensities.

In all cases there was no measurable potential generated by the cell
from this sub- bandgap light source. There was an initial increase in
capacitance indicating a net increase in the negative space charge
density (SCD) for the p-type CdTe material. There is a subsequent
decrease in capacitance followed by a gradual increase. The drop in
capacitance never reached the pre-illumination level. During the 45
minute illumination period the capacitance increase never saturates.
Since the light source is sub-bandgap, these phenomena are a result of
direct interaction of photons with deep traps. This involves the
promotion of electrons from the valence band to the trap or from the
trap to the conduction band. This direct photon interaction is
superimposed on the thermal emission and capture of holes and electrons
by the trap.

When the light is switched off, as the cell returns to its
pre-illumination equilibrium, there is a spike in capacitance followed
by a gradual decrease in capacitance. This is a more typical transient
response. It takes at least several days in the dark for the cell to
return to equilibrium. The magnitude of the effect increases with
intensity but the basic structure remains the same. The spikes at the
beginning of illumination and after the light is switched off are
limited by the 3 second capacitance reading interval of our
instrumentation and the approximately 2 seconds for the bulb to fully
illuminate and extinguish.

We expect that further study of these light induced CTr phenomena will
lead to a better understanding of CdTe/CdS solar cell basics, in
particular properties of the defect related DES.

3.  **High Spatial Resolution Studies of Photocurrent Collection**

**in Thin Film Solar Cells**

**3.1 CdTe/CdS cell.**

CdTe, being the primary PV material under investigation, has been
undergoing characterization using the in-house built Near Field Scanning
Optical Microscope (NSOM). Our previous NSOM studies on CdTe cell cross
sections revealed significant non-uniformities of photocurrent
collection (PCC) on the grain size scale \[12\].

The plane view NSOM image obtained once on a CdTe cell, after an access
for the optical probe was provided to the TCO surface, demonstrated in
some locations higher PCC for the intergrain regions than for the
grains. This result could be of significant importance providing better
understanding of the grain boundary (GB) effect on the cell performance.
The PCC studies in plane were partially initiated by speculations on
possible beneficial effects of GBs later presented in \[13\].

It was very difficult to repeat these in plane studies because CdTe
cells are usually of a superstrate configuaration. The technique we used
for the TCO exposure was complicated and usually damaged a cell. The
problem was solved recently after the University of Toledo provided us
with a cell of a substrate configuration. The CdTe/CdS device has been
grown on a Molybdenum substrate using an RF sputter deposition, then
went through a CdCl~2~ treament followed by depositing ITO via RF
sputtering. A helium neon laser with wavelength of 632.8 nm was used as
the excitation source in the photocurrent measurements. Results of this
experiment (Fig. 3.1) consistently show a reduced PCC from grains as
compared to the intergrain regions. Several images taken at random
points on the device show qualitative agreement with one another and
demonstrate on average a 5% difference in the photocurrent between these
structures. The images shown in figure 3.1 were generated with two
different programs to aid in the visualization of the topography vs.
photocurrent where the topography is on the left and photocurrent is on
the right. As one can see, the photocurrent in the two higher grains is
lower with an increase in photocurrent around the grains.

![](media/image16.png){width="5.997222222222222in"
height="4.846527777777778in"}

**Fig. 3.1.** Correlation between topography and photocurrent on CdTe

2.  **CIGS/CdS cell**

As a substrate configuration is very suitable for in-plane imaging of
PCC, a CIGS/CdS cell was also studied but under different circumstances.
The primary reason for investigating this sample was to determine
whether or not the area where the ITO and CdS were lifted off generated
enough photocurrent to provide a higher efficiency value then the
traditional scribing technique. These CIGS samples were prepared
normally then went through the new etching process in order to measure
their efficiency. Several scans were performed in different regions of
the etched layer and show that there is very little to no PCC in this
region Figure 3.2 shows an image obtained with the 26x26 m^2^ scan and
the excitation wavelength of 632.8 nm. In this image, the bottom left
corner is the actual device with the CdS and ITO still attached, and as
moving toward the top right corner, the ITO and CdS have been lifted
off. The image on the left is the topography and the image on the right
is photocurrent.

![](media/image17.png){width="5.9944444444444445in"
height="2.657638888888889in"}

**Fig. 3.2.** Correlation between topography and photocurrent on CIGS at
the etch step.

Further experiments were performed using multiple wavelengths
simultaneously as the excitation source with wavelengths of 457 nm and
632.8 nm. The results from these experiments show a difference in
photocurrent response as seen in Fig. 3.3 with 632.8 nm wavelength on
the left and 457 nm wavelength on the right.

![](media/image18.png){width="5.995833333333334in" height="2.5375in"}

**Fig. 3. 3** Difference in PCC at the etch stop for wavelengths

of 632.8 nm (on the left) and 457nm (on the right).

Further investigation of these two samples is needed to better interpret
the results shown here. One of the next step is to use a Ti-Saphire
laser simultaneously with the 632.8 nm as the excitation sources to
probe deeper into the sample.

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