UNDER

PRIME CONTRACT NO. DE-AC36-99-GO10337

**SUBCONTRACT NO. ADJ-2-30630-05**

## STUDIES OF BASIC ELECTRONIC PROPERTIES

OF CdTe-BASED SOLAR CELLS & THEIR EVOLUTION DURING PROCESSING & STRESS

###### **Quarterly Technical Status Report**

### For Quarter Ended January 31, 2003

#### 

#### 

#### **Subcontractor: Colorado School of Mines**

### **Principal Investigator: Victor Kaydanov**

**Principal Investigator: Tim R. Ohno**

##### Professor: Peter Sutter

# **Grad. Research Associates:** **James Kestner, Scott Feldman,** 

# **Fred Seymour**

**Contract technical monitor: Kenneth Zweibel**

In this project, we emphasize the relationship between structural and
electronic material properties and various processing procedures as well
as the microscopic mechanisms responsible for cell performance and
degradation. Our activities in the past Quarter included further
development of both the CdTe deposition techniques and methods for
studies of cell electronic and structural properties. The latter was
accompanied with application of these methods for characterization of
differently processed and stressed cells.

1.  **Optimization of CdTe processing procedure**

In Phase I of the project, the Jet Vapor Deposition (JVD) technique
replaced the initial APCVD system. The JVD setup results in growth rates
up to two orders of magnitude greater and material utilization of at
least an order of magnitude better than APCVD. Currently our efforts are
aimed at optimization of all cell processing steps for this new
deposition technique. Our previous Report \[1\] provided the results of
variation of the CBD CdS layer thickness and pre- CdTe heat treatment.
These activities lead to a significant improvement in cell performance.
Our recent efforts were focused mostly on CdTe growth conditions and
CdCl~2~ heat treatment. CdTe films were grown at the "nominal" substrate
temperatures of 150, 300, and 450ºC. The actual substrate surface
temperature was likely 50-100ºC higher due to additional radiative
heating by the CdTe source during deposition. CdTe was deposited on the
surface of the 300 nm thick CBD CdS annealed for 50 minutes at 450ºC.
These were the substrates that were previously determined to yield the
best devices for vapor-deposited CdTe. Following CdTe deposition, films
grown at each substrate temperature were annealed at 370, 390, and 410ºC
in the presence of CdCl~2~ as described in \[1\]. The cells were
completed with application of a Cu/Au back contact.

The substrate temperature of 150°C yielded the poorest devices with the
most variability as expected. Devices on the 300°C films appeared to
show the most consistent performance as a function of anneal
temperature. The best cells were obtained with the 450°C films annealed
at 390°C. The films grown at 450^o^C showed a dark colored,
rough-looking ring around the perimeter of the substrate. SEM images
reveal a rather different grain structure and high roughness in this
region as compared to the smoother, lighter-colored film in the center.
Cells made near the center of the substrates consistently yielded better
performance than those near the edge. It is thought that this variation
is likely due to gradients in substrate temperature across the sample,
and further studies are ongoing to confirm this hypothesis.

The results of these and previous studies show that there is room for
further optimization of processing and improvement of the cell
performance (currently the efficiency reached is about 9%). In
particular, we continue investigating the influence of substrate
temperature combined with the temperature and duration of the CdTe
post-deposition treatment. Simultaneously, we are developing the PECVD
technique for deposition of a high resistance SnO~2~ buffer layer on the
top of commercial soda lime glass substrates coated with TCO.
Supposedly, the buffer layer will allow for the further thinning of CdS
that so far was shown to be beneficial for our cells.

2.  **Electroluminescence (EL)**

Our previous studies \[1\] indicated that CdTe/CdS cells emitted light
under forward bias in the photon energy range in the vicinity of the
CdTe bandgap. The EL spectra were similar to the PL ones, both at room
and at low temperatures. Using a CCD camera and a microscope objective
we obtain spatially resolved EL images with the average current
densities through the cell on the order of J­­~sc~ or even less. It was
found that both average EL intensity and the bright spots pattern are
significantly influenced by cell processing and stressing.

In the reported Quarter we continued our efforts to develop further and
improve the procedure of EL measuring/imaging and data treatment, as
well as studying EL on the cells fabricated by various institutions and
stressed under different conditions.

Our optical system allows for imaging only small fragments of the cells
of the area that does not exceed 1.6x1.2 mm^2^. To provide images of a
greater area or the whole cell we have developed the procedure of
stitching together neighboring images using *Adobe Photoshop*.
Improvements also have been made to correct for differing contrast
levels. The ability to create EL images of entire cell is important if
comparative studies and mapping of non-uniformities are performed using
various methods such as EL, LBIC, thermography, etc., on the same cell.
It is also beneficial for repeated imaging of the same small fragment in
the course of stressing. Fig.1 shows mosaic EL images of the cells
fabricated at First Solar (FS) with vapor transport deposition,
University of South Florida (USF) -- close space sublimation, and
University of Toledo (UT) -- sputtering. These cells are to be
characterized periodically with EL and other methods in the course of
stressing.

![](media/image1.wmf){width="6.0in" height="2.533333333333333in"}

Figure 1. EL image mosaics. All are to scale in dimensions but not
brightness. Note, the FS cell

was scribed from a circle into a sector to approximate the area of the
UT cell while

> still showing the edge of the original cell.

With the newest imaging station, the resolution at the high
magnification setting was also improved. Utilizing the 10x microscope
lens, each CCD pixel images a \~0.87 m long square fragment of a cell.
This resolution is near the optical limit since the photons emitted have
a wavelength close to \~ 0.85 m. A high magnification image of the UT
cell from above is shown in Fig. 2. Note the difference in
non-uniformity: whereas the image of the entire cell (Fig.1) looks
rather uniform, the highly resolved image is visibly inhomogeneous.

![](media/image2.wmf){width="5.993055555555555in" height="1.44375in"}

Figure 2. High magnification image of UT cell shown in Fig. 1, above.
The calibration grid on

> the right has 20 m wide white lines on it.

In addition to viewing the images, particularly bright/dark spots
patterns, their sizes and distribution over an entire cell or a small
fragment area, we have also developed and used some quantitative
approaches to EL characterization.

Intensity data of each pixel in the CCD is imported into *Mathematica*.
The simplest piece of information that can be obtained is the total
integrated intensity, or equivalently the mean intensity. To numerically
characterize the inhomogeneity of EL emission, a parameter
"nonuniformity" is used. Its value is defined as the standard deviation
of the pixel EL intensity normalized by the mean EL intensity. In order
to characterize the nonuniformity in more detail, namely by the
distribution of brightness, a histogram of the pixel brightness is used.
Comparing the images in Figs. 1 and 2 provides an opportunity to show
how different some histograms hence EL patterns could be. Fig. 3
presents the histograms taken from the fragments in the interior parts
of the FS and UT low magnification images in Fig.1 as well as a
histogram of a small fragment from the high magnification image in Fig.
2. It is seen that "non-uniformity" is much greater for the FS than for
the UT cell, while the mean intensities do not differ significantly.
While the UT histogram demonstrates distribution close to that of the
gaussian, the FS one has a wide peak with a shoulder, indicating two
separate populations. This corresponds to the observed light and dark
regions of the cell.

![](media/image3.wmf){width="5.989583333333333in"
height="1.5354166666666667in"} Figure 3. Selected Histograms

The differences between the low- and high-magnification image histograms
for the UT cell indicate that there coexist several characteristic
non-uniformity spatial scales. To detect and study them we started to
use variogram analysis \[2\] which is a geostatistical technique used to
quantify the spatial correlation of data. A variogram consists of
determining all sample pair combinations and calculating the sample
value difference squared. These sample pairs are divided into classes,
or lag cells, based on sample pair separation distance and direction.
For each lag cell, the average sample pair value difference squared is
computed. Typically, the closely spaced pairs have a smaller mean
difference squared than the more widely spaced pairs. The extent of this
spatial correlation enables quantifying the scale and anisotropy in the
non-uniformity structures underlying the EL data. We tried to apply it
to the analysis of the CCD pixel counts for the image taken from a small
fragment of a cell. Two correlation structures with a separation of
about 10 m and 60 m were found. We are planning to apply this
technique to the higher area images, which we believe will detect the
additional correlation structures of greater separation. The first step
will be studying the mosaic images and then those taken from the pattern
of equidistant fragments of a large area cell/submodule.

3\. Scanning Tunneling Microscopy and Tunneling Spectroscopy (STM and
STS)

In \[1\] we have presented the results of STM imaging and STS
measurements on an almost completed CdTe cell (excluding back contact
metallization). The CdTe absorber layer was coated with a nominally
400-500 Å thick ZnTe:Cu back contact interlayer. ST microscopy was
performed on the surface of the ZnTe, but with the STM collector on the
TCO layer. In this configuration, tunneling electrons are injected into
the ZnTe layer, but have to travel into the CdTe absorber to contribute
to the tunneling current. Both constant-current 'topographic' STM images
and current images were obtained on ZnTe/CdTe. In constant-current STM,
the tip, controlled by a feedback loop, is scanned at constant bias
voltage and tunneling current, thus following and mapping the sample
topography. In current imaging, the feedback is switched off at each
pixel and a current voltage curve is obtained at this position. From
this data set, maps of tunneling current at a given bias voltage can be
obtained.

Following our demonstration of the possibility to use scanning tunneling
microscopy for spectroscopic imaging on CdTe thin film solar cells,
providing maps of current conduction and collection at nanometer
resolution, our activities have centered on two major focus areas:

i)  Modeling of local current injection from an STM tip and of the
    subsequent current transport

in the cell.

ii) Design/construction of a microscope dedicated to topographic and
    current imaging on thin

film solar cells.

Modeling of local current-voltage (I-V) characteristics will allow us to
build a solid framework for interpretation of STM current maps on entire
CdTe cells. Our model extends previous calculations of the STM I-V
spectra, accounting for carrier tunneling across the vacuum gap between
the STM tip and cell surface, and for current transport in the cell.
From a fit of the model to the experimental data, we should thus be able
to extract information on the spatial distribution of current in the
cell (with \~ 1 nm resolution), e.g., comparing grain boundaries with
intragrain material.

When completed, construction of a dedicated microscope will give us a
range of new capabilities that are important for current mapping on thin
film solar cells. Among the capabilities of the new microscope are:

-   large scans with fields of view of 20 x 20 μm^2^,

-   coarse tip positioning in two axes using a micrometer table with 1cm
    travel,

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-   optical access, allowing future combined experiments, e.g., EL
    mapping together with STM

current imaging.

We are planning to complete the new microscope and make it operational
by the end of February, 2003.

4.  **Team Activities**

We participate in two CdTe Team inter-institutional programs:
"Non-uniformity" and "Defect Chemistry". For the former we study
electroluminescence on the cells fabricated at FS, UT, FSU. They are
sent to us by Diana Shvydka from UT, the institution which manages the
program. We obtain and analyze the EL images as described in Sec.2. The
results obtained are sent back to the UT.

The second program is headed by FS. Cells at different stages of
stressing are sent to us by Anke Abken and are the subject for the I-V,
admittance spectroscopy (AS) and EL characterization. Recently we have
received and started characterization of the cells stressed for 43 days
under the open circuit and reverse bias conditions. There is some delay
in supplying FS with the results of characterization. The reasons are:
(a) Scotty Gilmore who was performing AS measurements has defended his
Ph.D thesis and left; his successor, a new graduate student Fred
Seymour, has to study in detail the measurement procedures; (b) we are
revising once more all our approaches for measuring and interpreting the
results of AS; (c) we are developing a system that will allow us to
perform the AS in a wide temperature range under varying illumination.
After completing I-V and AS measurements the cells are characterized
with EL. Currently, the EL characterization has been started on the
cells stressed for 7 days.

##### 5. References

1.  V.I Kaydanov, T.R. Ohno, "Studies of Basic Electronic Properties of
    CdTe-Based Solar Cells & Their Evolution during Processing and
    Stress", Annual Technical Report, Phase I, Oct. 16, 2001 -- Oct.
    15, 2002. Subcontract No. ADJ-2-30630-05 under Prime Contract No.
    DE-AC36-99-GO10337. Colorado School of Mines, Golden, Colorado.

2\. M. David, "Geostatistical Ore Reserve Estimation", Elsevier
Scientific Publishing , 1977, pages 73-90.