**Manufacturing Process Optimization to Improve Stability, Yield and
Efficiency of CdS/CdTe PV Devices**

September 2005 - January 2006

Thin Film Partnership Program

Subcontract XXL-5-44205-02

National Renewable Energy Laboratory

Phase I, Annual Report

W. S. Sampath, Al Enzenroth and Kurt Barth

*Materials Engineering Laboratory*

*Department of Mechanical Engineering*

*Colorado State University*

*Fort Collins, Colorado 80523*

**1. SUMMARY**

Under Phase I of the Thin Film Partnership Program, we have made
significant progress. During this year, the reproducibility of device
efficiency and stability has been demonstrated during long duration
operation of the system with a single charge loading for the CdS, CdTe
and CdCl~2~ process. Currently efforts are underway to advance the back
contact processing to the same level as outlined in the SOW. In order to
accomplish this, a significant upgrade to increase the processing space
in our pilot scale system has been designed, fabricated and is currently
being installed. A series of detailed studies on the effect of oxygen
during the CdCl~2~ treatment has been performed. Oxygen has been shown
to improve the effectiveness of the CdCl~2~ treatment. Mathematical
modeling of a package design has yielded results that show that moisture
can be kept from our device for 22 years at 65C temperature with
saturated humidity. Hardware to perform steady state photocapacitance
(PHCAP) has been designed, built and tested in our laboratory. The
effect of processing on the deep states in CdTe devices can be seen
quantitatively. Optimum process conditions lead to lower
photocapacitance signals. The feasibility of cooling the substrate in
vacuum for optimum processing of the back contact has been demonstrated.
A large number of experiments have been performed on laser scribing of
CdS/CdTe films. Mechanical scribing has yielded the lowest contact
resistance between the scribed region and the metallization, to date.
Researchers at National Starch and Chemicals (NSC) have performed many
experiments to screen print carbon, nickel and silver films in patterns.
The adhesion of these films was tested according to ASTM 3359B and
achieved a 5B or \"excellent\" rating.

**2. INTRODUCTION**

The aim of this Thin Film PV Partnership Program (TFPP) effort is to
advance the understanding of device stability, efficiency and process
yield for CdTe PV devices. Methodologies for reliably processing larger
areas and increasing device efficiencies are being developed. We will
actively participate in the National CdTe R&D Team activities. Our
ongoing collaborations with the PV Testing Lab at CSU, NREL, Pacific
Northwest Natl. Lab and other CdTe Team Members will are resulting in
further advances. Successful completion of the three phases of this
project will be a significant step towards realizing the full potential
of CdTe PV.

**3. RESEARCH RESULTS**

[3.1. Improving Performance and Manufacturability]{.underline}

3.1.1 Process Reproducibility:

Reproducibility of efficiency and stability has been demonstrated during
long duration operation of the system with a single charge loading for
the CdS, CdTe and CdCl~2~ process \[Sampath 2004\]. Currently efforts
are underway to advance the back contact processing to the same level.
An average efficiency of 11.4% with a standard deviation of 0.54% was
demonstrated during nine hour processing with a single charge loading
for the CdS, CdTe and CdCl~2~ process \[Sampath 2004\]. The stability of
these devices is shown in Figure 1.

![](media/image1.wmf){width="3.626388888888889in"
height="2.703472222222222in"}

Figure 1: Average efficiency vs accelerated stress time for three groups
of cells (12 cells total). Inset shows substrate IDs. The stress
conditions are 65C, OC, 5 hours ON/ 3 hours OFF.

As shown in Figure 1, groups of cells fabricated with the same process
conditions in three separate fabrication runs exhibit nominally the same
stability performance during accelerated stress testing. Devices
processed identically to the ones in Figure 1, but without the Cu back
contact had Voc's ≥ 700 mV. We have shown earlier that devices with Voc
greater than 700 mV without a Cu back contact have good stability
\[Barth 2005, Sampath 2004\].

3.1.2. Oxygen and the CdCl~2~ Treatment:

The presence of oxygen during the CdCl~2~ treatment leads to better
devices. In order to study the effect of oxygen during the CdCl~2~
treatment, a system for introducing controlled amounts of oxygen
directly into the CdCl~2~ vapor source and the CdCl~2~ annealing station
was designed, built and installed. The system incorporated a mass flow
controller for precise control and modification to the graphite sources.
A series of studies were under taken which involved nearly 225
substrates.

These studies have shown that oxygen during the CdCl~2~ treatment
improves the device performance. The JV performance of two example cells
is shown in Table 1 and Figure 2. A Cu containing back contact was not
applied to either of the cells in Table 1 or Figure 1. However, the JV
performance of devices with out intentional copper is predicative of the
stability of fully processed devices \[Barth 2005\]. These studies are
continuing; will be statistically validated.

Table 1

  -------------- -------------- -------------- -------------- --------------
  Cell           Voc            Jsc            ff             Eff\[

                 \[mV\]         \[mA/cm\^2\]   \[%\]          \[%\]

  17638-3: 0.6%  685            19.68          40             5.55
  O2                                                          

  17650-3: 2% O2 745            20.34          48             7.32
  -------------- -------------- -------------- -------------- --------------

Figure 2: Light JV traces for devices with varied oxygen during the
CdCl~2~ treatment. Red trace: 0.6% O~2~ ; blue trace: 2% O~2~.

3.1.3. Process Optimization:

The process has been optimized with CdS from ASARCO. The earlier
supplier (Balzers/Unaxis) discontinued the manufacturing of CdS pellets.
In addition, the CdS from Unaxis was more expensive. The efficiency and
stability of devices with CdS from ASARCO is comparable to device
efficiency and stability with CdS from Unaxis. In addition, the process
has been optimized with purer material for the back contact processing
from a different supplier. The earlier supplier had lower purity. The
efficiency and stability results with the newer material are comparable
to the results with the older material. These optimization studies
involved nearly 400 substrates. Efforts to obtain uniformity of back
contact processing over long duration are currently underway. X-Ray
Photoelectron Spectroscopy (XPS) at CSU has been used in these process
optimization studies.

3.1.4. Higher Efficiency:

We have demonstrated NREL verified efficiencies of 12.44% on unmodified
Pilkington Tech15 glass. In order to obtain higher efficiencies, low
iron sodalime (white glass) glass plates have been obtained and cut to
size suitable for our pilot system. Fifty substrates have been sent to
Sierratherm for APCVD (atmospheric pressure CVD) coating with TO:F. The
APCVD process is suitable for low cost high volume processing.
Sierratherm has recently informed us that the coated substrates will be
delivered in early spring 2006. Hence the deliverable D 1.7 will be
targeted for phase II. After the current batch of samples, the use of
barrier layer deposited by APCVD and other methods will be investigated.
Discussions have been held with Pillkington and they have indicated that
they may produce white glass with \"TEC\" type tin oxide coating in the
next two years.

[3. 2. Studies on Interconnection and Module Packaging]{.underline}

3.2.1 Studies on Interconnection:

Processes for creating a series interconnection for our modules is being
developed. This work includes film scribing and the application of
electrode materials for the \"rollover\" contact. A significant number
of experiments on laser scribing of many of our films for
interconnection has been performed at US Laser and Spectra Physics. As
we reported earlier, for scribing the TO:F, the Nd:YAG laser has been
found to be suitable. For scribing the CdS/CdTe films, experiments have
been performed with both a Nd:YAG laser and a vanadate laser. The
scribing experiments have been performed at 122 conditions. We have
observed that scribing from the glass side results in better scribe
quality and lower contact resistance for both flash lamp pumped Nd YAG
and the diode pumped Nd vanadate laser. The CdS/CdTe scribes are
characterized with optical microscopy, scanning electron microscopy and
energy dispersive spectrometry. Though significant improvements have
been demonstrated over previous experiments, the scribe parameters have
not yielded a sufficiently low contact resistance (target 1 ohm for 1
cm) using the current spray applied electrode material. These
experiments are continuing. Some of the experiments at Spectra Physics
are being performed with \"galvanometer scanning\" system and this may
lead to very high productivity.

Our group continues to collaborate with Dr. David Miles of NSC on
optimizing the back electrode (metallization) and rollover
interconnection process. The replacing of the current spray process for
metallization with a screen print process is being investigated. This
has advantages for process throughput, material utilization and lower
volatile organic compound emissions. Since screen printing allows the
metallization to be performed in patterns, a separate process for
isolating the metallization in not needed. NSC is developing optimum
formulations for screen printing for our CdTe PV application. It is
envisioned that the back electrode will consist of screen printing of
carbon and nickel in patterns. Many substrates have been provided to NSC
for these studies.

3.2.2. Modeling Moisture Transmission and Encapsulation:

Significant progress has been made in the conceptual design of a package
that keeps moisture away from our devices. Our 16.5 x 16.5 inch module
is designed to have a 1\" border around the edge. In this region, it is
planned to use the Truseal solar edge tape which contains desiccant.
Calculations based on the finite difference method have been performed.
The aim of the calculations is to see how long a 1\" edge seal will keep
moisture away from our devices. For this, we calculated how long a 1\"
thick disc of the Truseal solar edge material will take to saturate with
water if one side was exposed to saturated water at 65C and the disc was
at 65 C was determined. The data for water transmission rate was
obtained from Truseal. The finite difference calculations gave 22.3
years for the 1\" thickness to saturate. These calculations were
repeated by researchers at Truseal and similar results were obtained by
them.

This is a conservative model because PV modules rarely reach 65C for a
few hours in a year and this calculation is done assuming the side is
exposed to saturated water on one side. Experiments are currently
underway at Truseal to refine the data on water transmission. After the
updated water transmission is obtained, the calculations will be
refined, the model will be experimentally validated and results
published. We will continue our fruitful interactions with Tom Mcmahan
and Mike Kempe of NREL.

Preliminary studies on encapsulation with EVA have been performed. A
vacuum bag with an accumulator was used in these studies. In this
approach, the glass/EVA/glass is placed in the vacuum bag and the bag
with the accumulator is evacuated and placed in the furnace. The purpose
of the accumulator is to provide a vacuum reservoir for the gases that
are evolved during the encapsulation to accumulate. These experiments
have resulted in bonding between the glass and the EVA. However the
bubbles could not be fully removed. In addition when the temperature of
the EVA was measured with a thermocouple, a high degree of variability
was observed. A conceptual design of an encapsulator based on standard
methods has been developed and this system will be constructed in the
near future for 16.5 x 16.5 inch substrates. This equipment will be
fabricated under the DOE-EERE program mentioned earlier. In order to
minimize moisture ingress into modules, we have maintained an ongoing
collaboration with TruSeal Corporation.

A fixture to quantitatively measure the moisture ingress through the
encapsulation has been developed. The fixture consists of a closed glass
container placed in a temperature controlled furnace. The container has
a small amount of water to saturate the container to 100% relative
humidity (RH). This fixture is capable of testing encapsulated 3\" X 3\"
specimens at different temperatures and 100% RH. A measured quantity of
indicating desiccant is placed in the specimen. The indicating desiccant
with CoCl~2~ and CaSO~4~ changes color when a certain quantity of
moisture has penetrated through the encapsulation. By estimating
moisture ingress at different temperatures and using well known
mathematical approaches, the ability of encapsulants to protect the
devices from moisture under field conditions can be predicted.

3.2.3 Studies on Interconnection:

Processes for creating a series interconnection for our modules is being
developed. This work includes film scribing and the application of
electrode materials for the \"rollover\" contact. In addition to the
experiments reported in the last quarterly report, many experiments on
laser scribing of many of our films for interconnection have been
performed at US Laser and Spectra Physics. For scribing the TO:F, the
Nd:YAG laser has been found to be suitable. For scribing the CdS/CdTe
films, experiments have been performed with both a Nd:YAG laser and a
vanadate laser. Recent scribing experiments have been performed at
Spectra Physics application lab. Over 115 different conditions have been
explored. The results of the laser scribing conditions on the contact
resistance to the spray applied electrode is shown below in Figure
3.From the figure, it can be seen that the scribe parameters have not
yielded a sufficiently low contact resistance (target 1 ohm for 1 cm)
using the current spray applied electrode material. The laser wavelength
was 1064 nm. Experiments are continuing with laser light wavelength of
532 nm. Initial results show improved contact resistance values to
approximately 5 ohm -cm of contact. Experiments are continuing.

As we reported earlier, we have demonstrated the feasibility of
mechanical scribing to produce contact resistance of 1 ohm for 1 cm.
Recently mechanical scribing at high speeds has been performed and the
measured contact resistance between the mechanical scribe and the
sprayed electrode material was measured at 1 to 2 ohms for 1 cm of
scribe length

Figure 3: Contact resistance as a function of laser pulse energy density
at 1064 nm wavelength. The different symbols denote different laser
power, repetition rate and scan speed

Researchers at NSC under the guidance of Dr. David Miles are optimizing
the back electrode (metallization) and rollover interconnection process.
Replacing of current spray process for metallization with a screen print
process is being investigated. This has advantages for process
throughput, material utilization and lower volatile organic compound
emissions. Since screen printing allows the metallization to be
performed in patterns, a separate process for isolating the
metallization in not needed. NSC is developing optimum formulations for
screen printing for our CdTe PV application. NSC has performed many
experiments to screen print carbon, nickel and silver films in patterns.
Initial samples have recently been received from NSC. The samples show
that the screen printing technique can provide adequate line width
resolution and accuracy. Adhesion was measured for all the materials on
Pilkington Tec 15 glass and of the carbon material on CdS/CdTe following
ASTM 3359B. The results were \"5B\" or excellent adhesion. The
resistance values for interconnection of the silver based ink are
excellent; however the resistance of the nickel and possibly the carbon
need further optimization. This is currently under investigation at NSC.

3.2.4 Hail Impact Studies:

Based on the literature, a model for calculating stresses in hail impact
test was developed. The residual stresses produced by tempering and the
effect of our process on the residual stresses were measured by Laser
Glancing Angle Surface Polarimeter (GASP). Many encapsulated glass
substrates were also tested at the Photovoltaic testing laboratory (PTL)
at Arizona State University (ASU) with one inch diameter ice balls
traveling at 52 mph. Based on this study, it is predicted that when 3 mm
tempered glass is used as the substrate in our process and a 3 mm
tempered glass is used as the bottom substrate, the module would be able
to physically withstand the hail impact test of IEC 61646. These
predictions will be tested experimentally on 16X16 inch substrates.

[3.3 Detailed Device Analysis, Steady State
Photocapacitance:]{.underline}

Hardware to do steady state photocapacitance (PHCAP) has been designed,
built and tested in our laboratory. The photocapacitance technique
measures changes in device capacitance under the illumination of
sub-bandgap monochromatic illumination. The measurements are done at
77K. The relative magnitudes of the capacitance changes in the PHCAP
spectrums are proportional to the defect densities. The PHCAP spectrums
for our devices processed at various conditions are shown in Figure 4.
The yellow curve with X markers in Figure 4(a) is a device that received
no CdCl~2~ treatment. The blue and pink curves are devices that received
poor and optimum CdCl~2~ treatment respectively. The poor CdCl~2~
treatment shows greater defect density than those with the optimum
CdCl~2~ treatment. As shown in Figure 4(b) the devices with Cu back
contact have lower photocapacitance than the devices without copper. A
correlation between initial device efficiency and defect densities has
also been shown. The photocapacitance has been normalized to the
intensity of illumination at different wavelengths.

![](media/image2.wmf){width="3.035416666666667in"
height="2.8645833333333335in"}
![](media/image3.wmf){width="3.0145833333333334in"
height="2.8534722222222224in"}

\(a\) (b)

Figure 4: Normalized PHCAP spectrum of deep states with (a) varied
CdCl~2~ treatment, (b) with and without the Cu back contact.

The capacitance increases with incident light in Figure 4 are related to
the emission of holes from deep states and the decreases in capacitance
are related to the emission of electrons from deep states. However due
to the complex interactions of multiple deep levels (both majority and
minority traps) with possible overlap of optical cross sections it is
difficult to assign exact emission threshold energies. Further work is
underway to understand the PHCAP spectrums. Taru Takamia has completed a
MS thesis on PHCAP and Jim Sites served in his committee. Some of the
thesis work is presented here and efforts are ongoing in this area. Two
cells processed identically to cells from the PHCAP study were sent to
Steve Johnston at NREL for DLTS characterization. Work is ongoing to
compare the DLTS and PHCAP results.

[3.4 Modification of the Pilot System.]{.underline}

The pilot system for processing 3.6 x 3.1 inch substrates is being
modified to more closely simulate the back contact processing for actual
industrial manufacturing. With the current configuration of the pilot
system, Cu is introduced immediately after the CdCl~2~ treatment steps.
The substrate enters the copper process at too high a temperature
resulting in devices with high Rs. To avoid this, a pause is introduced
after the CdCl~2~ treatment step to allow the substrates and process
sources to cool. After a nominal 3 hour wait, the substrate motion
direction is reversed and the now cool samples are exposed to a vapor
flux of the Cu compound followed by annealing. This completes the
formation of the Cu containing back contact.

To simulate the Cu back contact process for actual production the pilot
system is being lengthened to allow fabrication of devices without the
pause and change in substrate motion (\"all forward\" process). The
modified pilot system would also allow processing of devices in a manner
which emulates a continuous production system. In addition, new process
methodologies can be investigated which can not be performed on the
unmodified system. These include longer CdCl~2~ treatment times and
copper processing. The current system is not long enough to apply the
back contact with the longer CdCl~2~ processing. The longer CdCl~2~
processing times have resulted in devices with higher Voc and Jsc
without the Cu back contact processing. This milestone will enable
precise definition of conditions for all the semiconductor processing
steps (including the back contact) needed to fabricate high efficiency,
stable CdTe devices. This process definition will be implemented in the
2 MW/yr. production prototype system for 16.5 x 16.5 inch substrates.

A large number of experiments have been performed to explore the
feasibility of reducing the time needed to cool the substrate before the
copper processing. This will reduce the length of the \"all forward\"
system. Active contact conduction cooling of the substrate (rather than
radiation cooling) has been investigated. This would lead to a reduction
in the length of the vacuum system and an increase in process control.
Many thermal interface materials were explored including silicone
polymers with fillers. These materials were not suitable for repeated
contact with the substrate. A structure made from water cooled copper
plate and multiple foils extending for contacting the substrate was also
tested. Preliminary heat transfer analysis and experiments suggest that
contacting the substrate with a copper plate with a flat, smooth surface
should be adequate for cooling the substrate.

![](media/image4.wmf){width="4.111111111111111in"
height="1.8083333333333333in"}

Figure 5: Schematic of contact cooling test apparatus in vacuum chamber

A series of measurements of the cooling rate of glass substrates were
performed with the test apparatus shown schematically in figure 5. The
apparatus was installed in our pilot system. This allowed for the
transport the glass substrates through the heating station and then
under the water cooled plate. During measurements, the pressure was
varied between 1 Torr and 40 mTorr and one measurement was done at
atmospheric pressure as a control. The spacing between the water cooled
plate and the glass substrate indicated in Figure 5 was a close contact
in most of the cases. In one case a space of 0.010 inch was maintained.
The substrate was heated to 170 C in the heating station and then
indexed under the contact cooling plate. Measurements of the substrate
temperature were performed with either a thermocouple or with two
infrared (IR) sensors. The water cooled plate was maintained at
approximately 20 C.

The results of the temperature measurements are shown in Figure 6. As
shown in Figure 6 there is an increase in cooling rate with and increase
in background pressure. The fastest cooling rate was at atmospheric
pressure. In this case convection from the bottom of the substrate can
not be neglected. However other tests at atmospheric pressure where the
bottom of the substrate was insulated to prevent convection (not shown)
closely match the result shown in Figure 6 for atmospheric pressure.
Also shown is the predicted cooling rate for radiation alone. The
contact cooling rate in all cases was faster than simple radiation. At a
background pressure of 1 Torr the substrate cooled to 37 C in two
minutes and 30 C after 4 min. Also shown is a case at 1 Torr pressure
where the cooling plate was spaced away from the substrate by 0.010
inches. In this case the cooling rate closely matches the rate of a
substrate in direct contact to the cooling plate at a pressure of 300 \~
400 mTorr.

![](media/image5.wmf){width="3.392361111111111in"
height="2.582638888888889in"}

Figure 6: Plot of substrate temperature with time for different
background pressure and cooling plate spacing. Open symbols indicate
thermocouple measurements; closed symbols indicate IR sensor
measurements. All substrates used for the IR measurements had a CdTe
film.

The results indicate that an adequate cooling rate can be obtained with
a minimum pressure of 1 Torr of nitrogen gas. The cooling plate is best
maintained in close contact to the top surface of the substrate. No
glass cracking was observed during any of the testing. A reasonable fit
to most of the data could be obtained by using a temperature difference
model dT = K\*(T1-T2)\*dt where the T2 is the cooling plate temperature
and T1 is the substrate temperature, dT is the change in substrate
temperature in time dt. The constant K is dependant on the ambient
pressure. A comparison of the substrate temperature measured by the two
IR sensors indicates that the cooling of the substrate is uniform within
approximately 4 C. Based on the above experiments a fixture to contact
cool the substrate that allows the contact cooled regions to be raised
to 1 Torr \~ 10 Torr pressure is being designed and will be fabricated
and implemented in the pilot system. Since the operating pressure is
typically 40 mTorr, the fixture will be sealed against the substrate
using O ring seals.

**4. LARGE AREA PROCESSING**

A production prototype system is being developed to process 10 W
modules. This system, if run in production mode, would have a nominal 2
MW/yr throughput. This activity is supported by DOE-EERE and cost shared
by National Starch and Chemicals (NSC). This is an Inventions and
Innovation Program award to AVA Technologies LLC and is the only
Category 3 award ever granted by this program. The results described in
this Section 4 are partially supported by this program and performed by
AVA Technologies in collaboration with CSU\'s Materials Engineering Lab.

[4.1. Processing Larger Substrates]{.underline}

The semiconductor deposition system for the 2MW/Yr production line was
designed to be fabricated in modular sections. One section of the vacuum
chamber including the substrate transport and four deposition stations
has been fabricated and two stations have been tested. Load locks to
bring substrates into and out of the vacuum space have also been
fabricated and substrate motion has been tested. In order to complete
the deposition system an additional modular section is needed and the
conceptual design of this section has been completed. Detailed design is
currently underway. The new modular section includes all pump ports,
feedthroughs, access doors, water cooling plates and a support frame is
currently being fabricated. Many 16.5 X 16.5 inch substrates have been
heated from 25C to \~500C in two minutes with no glass cracking.

[4.2 Thermal Uniformity Measurements in the 2 MW/yr. System]{.underline}

In order to obtain consistent device performance, a high degree of
thermal uniformity is desired. Steady state temperature measurements of
the graphite blocks in our deposition sources showed a ± 1.5° C
variation on a 400º C base across the 14.5 x 14.5 in. pocket. In order
to develop a model of the temperature distribution in the substrate,
finite element analysis (FEA) simulation of substrate temperatures have
been done at CU Boulder. The preliminary results predict that the
substrate temperature difference would be larger during transient motion
than during steady state conditions. A series of temperature
measurements were undertaken to provide feedback to the FEA model and to
give an initial map of thermal uniformity during transient substrate
motion.

The temperature measurements were performed by using thermocouples
mounted in the glass substrates. A diamond coated drill bit was used to
drill 0.040 in. diameter holes 0.125 in. \~ 0.25 in. deep into the glass
substrates and the thermocouples were inserted into the holes. The
measurements were performed at pressures and source temperatures that
would be used during prototype production. The experimental setup is
shown in Figure 7. It was not possible to pass the thermocouples through
the load locks so the substrate was placed in position 1) in Figure 7
during warm-up of the heater.

![](media/image6.wmf){width="3.84375in" height="1.6875in"}

Figure 7: Schematic of temperature measurement setup Figure 8: Measured
substrate temperatures

> at selected points.

![](media/image8.wmf){width="2.375in" height="2.1625in"} After the
heater and CdS source had reached operating temperature the substrate
was moved from position 1) to position 2) in 6 seconds. The substrate
was held in position 2) for 140 seconds to acquire the temperature data
for the substrate heating sequence. At the end of 140 seconds the
substrate was moved back to position 1). Figure 8 shows temperature vs.
time data for selected points on a substrate during the heating
sequence. The initial temperatures at t = 0 are for the substrate in
position 1) of Figure 1. The heating sequence begins at t = 10 sec when
the substrate is in position 2) of Figure 7. As can be seen in Figure 8
there is a large spread (\~ 125º C) in initial temperatures. This is due
to the fact that for the sources to reach operating temperature requires
about 2 hours. This initial temperature difference will be much smaller
during actual process operation when the pause time will be 2 minutes.

Figure 9: Predicted substrate temperature \[ºC\] exiting

heater, entering CdS station.

The most pertinent temperature difference in the raw data shown in
Figure 8 is between the center and the west edge since these points
start at t = 0 within 1º C of each other. The final temperature
difference is 3º C after 120 seconds. This is an acceptable amount of
variation.

As mentioned above the initial temperature difference between the
leading edge and the other points is large. One other factor that
affects the raw data shown in Figure 8 is that the leading edge has been
heated for six seconds more than the trailing edge. This slow indexing
time was used to eliminate damage to the thermocouple probes. In actual
operation the substrate index time to move from station to station will
be faster. In order to use the raw data to predict actual substrate
temperatures during a process run the raw data was "normalized". This
was done by using a data fit to the measured raw data. The data fit was
a bulk Euler integration model using glass parameters from the
manufacturer.

The initial substrate temperatures were predicted using calculations
based on radiation heat transfer. The time of heating the glass was
normalized to 120 seconds of heating time. This is valid since all
points on the substrate will have received 120 seconds of heating as the
substrate passes into the CdS station. The bulk model was then used with
the estimated initial starting temperatures and a uniform heating time
to predict substrate temperatures as the substrate enters the CdS
station. The center to edge temperature difference of 3º C was
preserved. The resulting substrate temperature uniformity map is shown
in Figure 9. As can be seen in Figure 9 the predicted temperature for
the center of the leading edge is 16º C hotter than the center of the
substrate. This is the greatest predicted variation.

The FEA model predicts that substrate uniformity will only increase as
the substrate passes through the CdS and CdTe stations. So the thermal
variation shown in Figure 9 should decrease as the substrate continues
to be processed. However, it is desirable to decrease the thermal
variation of the substrate in the heating station. Efforts are currently
underway to reduce the temperature variation. The radiation shielding in
the heater is being modified to reduce the heating of the leading edge
of the substrates before they enter the heater. The real time measured
data is also being used to refine the FEA model.

[4.3 Cost Estimates for Large Scale Manufacturing with our
Technology]{.underline}

During the last year, a detailed cost analysis was conducted to
determine the manufacturing and facility costs for manufacturing CdTe PV
by our technology. In addition the development costs to advance the
technology from the current pilot stage to a manufacturing stage was
estimated. These analysis were performed by AVA Technologies LLC of Fort
Collins. The methodology and results are summarized here.

4.3.1 Manufacturing Cost Analysis Background:

A detailed cost analysis was performed for a 20 MW/yr. PV manufacturing
facility using our technology. This analysis uses knowledge developed
from the operation of the pilot scale manufacturing facility since 1997
and the development of a production prototype system for processing
16.5x16.5 inch substrates. A manufacturing plant with an annual
throughput of nominally 20 MW/yr. processing 120x60 cm frameless modules
with a 63 watt rating is the basis for the analysis. This plant
throughput was chosen because it represents an incremental step towards
large scale manufacturing and is comparable to other facilities. Please
note that in this analysis no building or land capital costs are
included. However, equipment costs and rent are included.

-   Nominally 20 MW/yr. facility

-   Operating 2 shifts/day, 7 days/week, 49 weeks/yr.

-   120x60 cm thin film modules

-   Module rating of 63 watts

-   Glass/glass laminate package without frame (typical for CdTe PV)

[4.3.2 Methodology:]{.underline} Significant effort was taken to ensure
the accuracy of the data and the validity of the estimations used to
generate this analysis. With almost no exceptions, all the equipment and
materials costs and labor requirements used in this analysis were
obtained from direct quotes or consultation with reputed vendors. This
was augmented with the combined 35+ years experience of CSU\'s Materials
Engineering Lab in thin film PV and vacuum equipment design. The
analysis was performed using a spreadsheet which was provided to Ken
Zweibel of NREL.

-   The equipment and materials costs and labor requirements used in
    this analysis were obtained from direct quote or consultation with
    reputed vendors.

-   Detailed, automated spreadsheet was utilized to generate costs.

-   There are 25 processes to produce modules with this advanced
    technology. Most of the processes utilize commercially available
    standard industrial equipment optimized for high throughput.

-   Thin film modules are monolithically integrated and have no \"cell\"
    stage of processing. There are many significant differences in both
    the semiconductor and packaging processing between c-Si and thin
    film modules.

[4.3.3 Summary:]{.underline} The results of the analysis demonstrates
that this CdTe thin film PV manufacturing technology has significantly
lower manufacturing and equipment costs than other PV technologies.

-   The manufacturing costs for modules are less than \$1.00/Watt.

-   These manufacturing costs are significantly lower (approximately 3-4
    times) than other technologies according to data from another study
    \[NREL 2005\].

-   The one time equipment costs are less than \$10 million for 20
    MW/yr. facility.

-   These one time equipment costs are at least 5 times lower than the
    published costs for other technologies.

The equipment cost for the 10 MW Antec CdTe thin film facility in
Germany was estimated at \$27 million \[Bonnet 1992\]. This facility has
only half the throughput of the facility described here. A 30 MW Q-cells
factory (crystalline silicon cells and modules) is approximately \$75
million \[Solarbuzz 2005\]. Wurth Solar intends to invest approximately
\$66 million in a 15 MW thin film CIS facility \[Solarbuzz2 2005\].

> [Facility cost]{.underline}
>
> [Technology (\$ million, normalized to 20 MW/yr.)]{.underline}
>
> **CdTe thin film reported here approx. \$10**
>
> Antec (CdTe) \$54
>
> Q-Cell (silicon) \$50 (May include building, not all modules, some
> cells)
>
> Wurth Solar (CIS) \$88 (May include building)

The module packaging and the substrate preparation processes are quite
similar between the different technologies. The difference is primarily
in the semiconductor processing. Since 1991, the focus of our efforts
has been to reduce the cost of the thin film semiconductor PV
processing. The cost advantages of our in-line, continuous, lean
manufacturing technology can be clearly seen from the above numbers.

# 5. PHASE II PLANS

As in Phase I, we will advance the tasks outlined in the statement of
work during Phase II. The focus of the efforts will be to further
develop the understanding of stability, yield and efficiency of CdS/CdTe
devices. We anticipate that further progress will be made towards
processing 16\" X 16\" substrates. We will continue our collaborative
activities with other research groups and provide assistance to
industries as in Phase I.

**6. REFERENCES**

Barth, K. L., R. A. Enzenroth, and W. S. Sampath, \"Consistent
Processing and Long Term Stability of CdTe Devices\", presented at the
31 IEEE PVSC, January 3-7, 2005, Lake Buena Vista, Florida.

Bonnet: Bonnet, D. Int. J. of Solar Energy 1992, Vol. 12. pp 1-14.

NREL: Study of PV manufacturing costs available at:
http://www.nrel.gov/ncpv/pv_manufacturing/cost_capacity.html, accessed
July 2005.

Sampath, W. S., K. L. Barth, R. A. Enzenroth, \"Progress in Continuous
In-Line Processing of CdS/CdTe Devices, Including Large Area (16" X 16")
Deposition\" presentation to the Solar Program Review Meeting, Denver
CO, November 2005. available at
:http://www.nrel.gov/ncpv/thin_film/docs/sampath2005solarreview.ppt

Solarbuzz: \"Evergreen Solar and Q-cells AG in Manufacturing Plant Joint
Venture\", Solarbuzz 1/14/05. Available
www.solarzuzz.com/News/NewsEUMA58.htm, accessed July 2005.

Solarbuzz2: \"Würth Solar to Invest €55 million in Thin Film
Manufacturing Plant\", Solarbuzz, 5/23/05,
http://www.solarbuzz.com/News/NewsEUMA68.htm , accessed July 2005.

Sampath, W. S., Kurt Barth and Al Enzenroth, "STABILITY, YIELD AND
EFFICIENCY OF CdS/CdTe DEVICES", Thin Film Partnership Program,
Subcontract ADJ-1-30630-07, National Renewable Energy Laboratory Final
Report. 2004. Available at
http://www.nrel.gov/ncpv/thin_film/pdfs/csu_sampath_final_report2004.pdf

Barth, K. L., R. A. Enzenroth, and W. S. Sampath, \"Consistent
Processing and Long Term Stability of CdTe Devices\", presented at the
31 IEEE PVSC, January 3-7, 2005, Lake Buena Vista, Florida.

**7. APPENDICES**

[7.1. Description of Activities with the National CdTe Team]{.underline}

Our group has been actively participating in National CdTe Team
activities. Two members of our group attended two sub-team (stability
and device physics) meetings.

Our group continues to support the basic research efforts of the CdTe
Team. We continue to supply devices, films and substrates as well as
experimental data to the Team members. As selection of the research
efforts supported are listed below:

1)  Nine cells and 24 samples were sent to Larry Olsen at PNNL for
    studies with the moisture barrier

2)  Two samples and two devices to CS Jiang of NREL for SKPM

3)  Two substrates to C Corwine of CSU PV group for PL

4)  Two cells to T. McMahon for Rsh characterization

5)  Four cells to A. Fahrenbruch for transient current characterization

6)  Twenty cells to A. Davies of CSU PV group for micro LBIC
    characterization

7)  One substrate to Tim Gessert

8)  12 cells with an outdoor fixture to N. Dehre of FSEC for outdoor
    condition testing

9)  Eight full 3.1 x 3.5 in. substrates to CSM for various studies

Three members of our group attended the National CdTe R&D Team meeting
and four presentations were given. Our group has continued an ongoing
study to understand the behavior of CdTe materials under the exposure to
moisture and heat. This effort has been laid out in a detailed
experimental matrix and involves multiple samples of both CdTe devices
and films. Analysis techniques include JV analysis, and XPS and
potentially XRD.

Larry Olsen of the Pacific Northwest National Laboratory has been a
frequent visitor to our laboratory and collaborator on research
projects. A collaborative research effort with our group and Larry Olsen
is currently in progress. A detailed series of experiments is being
conducted to investigate applying barrier coatings over our completed
devices. During the last year, 21 samples were sent to Larry Olsen at
PNNL for studies on the effect of moisture and heat after coating, a
number of these devices were be subjected to stress and humidity. These
ongoing studies have continued to result in presentations and
publications including at the IEEE PVSC.

Recently our group has begun collaborating with the PNNL group on
another application for thin film CdS/CdTe devices. We have supplied a
specialized sample with an etched tin oxide pattern. This was coated
with CdS/CdTe in our pilot system. Further experiments are planned in
the near future.

[7.2. Description of Interactions with Industry]{.underline}

Our CSU organization has a ongoing relationship with AVA Technologies
LLC of Fort Collins to advance CdTe manufacturing. The members of AVA
Technologies are also faculty at Colorado State University. AVA
Technolgies has been awarded an DOE Inventions and Innovation (I&I)
grant. A subcontract to CSU is part of this I & I grant. The close
relationship that AVA Tech has with a major research university is very
beneficial for both parties. This I & I grant then dovetails with the
CSU activities. On April 5, AVA Technologies LLC and our CSU
organization hosted a day long meeting on our technology. The members of
AVA Tech, Lisa Barnett of DOE, members of the DOE Golden Field Office,
NREL and Dave Miles of NSC/Acheson attended. In addition, one of the end
users of our PV modules, CSU Facilities, described their need for
potentially tens of megawatts of PV. This represents a significant
market for our technology. During this meeting a detailed power point
presentation was given which covered the PV market conditions,
description of our technology, and a review of this I &I project tasks
and timelines. This presentation was 43 pages long so it is not included
here but can be made available on request.

Specialty electronic coatings are used as an electrode for our PV
devices. These materials are supplied by Acheson Industries which is a
part of National Starch and Chemical (NSC is the US subsidiary of the
approximately \$10 Billion/yr, ICI Group). NSC manufactures critical
products for the semiconductor, electronics and photonic industries. Our
group has a continuing involvement with NSC to advance CdTe PV under the
I and I program. During this last year, 126 substrates to NSC for
metallization studies.

During the last year, a large corporation capable of very large scale
manufacturing has expressed interest in pursuing a potential partnership
for PV manufacturing with our technology. Numerous teleconferences and
site visits have been conducted with high level representatives of the
company. Documentation was also provided which includes:

A. A detailed review of any environmental and occupational safety
aspects with our technology

B. A detailed break down of the cost to manufacturing large volumes of
PV with our technology.

C. A non-disclosure agreement.

D. Description of the advantages of our technology over competing
technologies.

E. Details on the intellectual property ownership.

These requests are in support of developing a potential partnership
relationship. The environmental analysis was prepared with the
assistance of an industrial hygienist and demonstrated that only simple,
industry standard environmental health and safety practices are needed
for to protect workers in a large scale manufacturing facility using our
technology. This effort was enabled by the materials available at
http://www.nrel.gov/cdte/.

On May 4, Peter Meyers and Roger Green of First Solar visited our
facility and was given a full tour and presentation on our activities.
This was followed with a conference call from Nelson Devoe for possible
consulting.

Southwall Technologies and an investor group represented by Jim Frantz
were given detailed presentations on the advantages of CdTe PV
manufacturing.

Based on their request, Solar Fields has been provided details of our
metallization and they were put in touch with NSC.

Five cells were provided to Trace Photonics for Beta radiation studies.

John Bleem from the local utility, Platte River Power Authority, was
given a facility tour.

[7.3. Additional Activities]{.underline}

The deputy director and two program managers of the Colorado Office of
Energy Management and Conservation (OEMC) visited our laboratory and
were given a demonstration and brief presentation. They have offered to
assist with the technology transfer efforts.

In November 2005, the well known CdTe PV researcher, Dr. D. Bonnet,
visited CSU. In addition to visiting J. Sites lab, Dr. Bonnet was given
a full tour of the Materials Engineering Lab. A demonstration of our
pilot scale CdTe manufacturing system was conducted. The current status
of the 2 MW/yr. prototype manufacturing line being developed by AVA
Tech. and CSU demonstrated. Dr. Bonnet gave a presentation to our group
on CdTe manufacturing facilities of ANTEC of Germany.

We are collaborating with Prof. Roop Mahajan of University of Colorado,
Boulder. He is an internationally renowned authority in the field of
heat transfer in processing semiconductors. This work through the
International Center for Science, a UN agency, in collaboration with our
group has been selected for funding. The funding will support two
students and demonstration projects in Mexico and India. This will
provide more opportunity for studying stability under field conditions.

A series of measurements of the residual stress in the glass substrates
after processing through our continuous pilot scale system have been
completed as per ASTM C1279. Glass substrates with different levels of
residual stress have been sent to the ASU Photovoltaic Testing Lab for
hail impact tests. These studies will establish the stress levels needed
to pass the hail impact test for modules as per IEC 61646.

Prof. Wilbur\'s group at Colorado State University was supported with 11
devices and characterization for high altitude balloon studies and the
devices performed well in these studies. These studies were part of the
Colorado Space Grant program and efficiencies of 15% were measured at
high altitudes at temperatures of -23 degrees C. The results were
presented at Jet Propulsion Laboratory.

[7.4. Student Activities]{.underline}

Graduate students, Tushar Shimpi, Marh Armijo and Rick Lueb were
supported through this project. They have assisted in the activities
described in this report. Tushar Shimpi and Toru Takamiya have performed
their graduate research on this project. The objective of Tushar
Shimpi\'s his thesis was to develop an understanding of the hail impact
test and Toru Takamiya conducted research on the Photocapacitance method
described earlier. Hershel Shelly is conducting his Ph. D. research on
this project. His research is to study the pilot scale process from an
industrial engineering perspective. Chetan Malhothra of CU Boulder is
pursuing his Ph.D. through thermal and deposition modeling of the
process. Marh Armijo and Rick Lueb have recently started their graduate
studies.

[7.5. Publications, Communications and Presentations]{.underline}

During the recent Solar Program Review Meeting, Sampath from our group
gave a talk entitled, \"Progress in Continuous In-Line Processing of
CdS/CdTe Devices Including Large Area (16" X 16") Deposition\" \[Sampath
2005\]. The manuscript \[Barth 2005\] was also drafted and is published
in the proceedings.

In addition to the tasks outlined above, two members of our team
attended the 31st IEEE PVSC. Two oral presentations (preprints were sent
to you earlier) were given. In addition, a poster was presented at the
ICTMC conference and this paper has been accepted for publication in a
journal.

7.5.1. Publications and Presentations

Sampath, W. S., K. L. Barth, R. A. Enzenroth, \"Progress in Continuous
In-Line Processing of CdS/CdTe Devices, Including Large Area (16" X 16")
Deposition\" presentation to the Solar Program Review Meeting, Denver
CO, November 2005. available at
:http://www.nrel.gov/ncpv/thin_film/docs/sampath2005solarreview.ppt

Barth, K. L, R. A. Enzenroth, and W. S. Sampath, \"Progress in
Continuous In-Line Processing of CdS/CdTe Devices Including LargeArea
(16 x 16 inch) Deposition\" paper for the Solar Program Review Meeting,
Denver CO, November 2005. Published in proceedings.

Malhotra, C. P., Mahajan, R. L., Sampath, W. S., "Use of a Commercial
Finite Element Software for Predicting Deposition Rates and Uniformity
in High Knudsen number PVD", accepted to IMECE 2005: ASME International
Mechanical Engineering Congress Orlando, FL, November 5--11, 2005

\"Mass Production of CdTe Solar Photovoltaics\" Presentation and
facility tour for the Fort Collins, Colorado Chamber of Commerce Chamber
Environmental Committee, Oct 12, 2005.

Malhotra, C. P., Mahajan, R. L., Sampath, W. S., Barth, K. L.,
Enzenroth, R. A., "Prediction of Spatial Evolution of Temperature across
Substrates during the Deposition Process for Manufacturing large area
CdTe Photovoltaic panels," submitted to IHTC-13: 13th International Heat
Transfer Conference Sydney, August 13-18, 2006

Barth, Kurt, W. S. Sampath, Robert \"Al\" Enzenroth, \"Project Update
Facility Tour\", Presentation, Discussion and Facility Tour with the
with representatives of US DOE, and Industry, April 5, 2005.

Enzenroth, R. Albert, K. L. Barth, and W. S. Sampath, \"Correlation of
Stability to Varied CdCl~2~ Treatment and Related Defects in CdS/CdTe PV
Devices as Measured by Thermal Admittance Spectroscopy\", Journal of
Physics and Chemistry of Solids, pp 1883-1886, 66 (2005).

Barth, Kurt, \"Mass Production of Thin Film Solar Photovoltaics\",
Invited seminar For US DOE Golden Field Office March 17, 2005.

Malhotra, C. P., Mahajan, R. L., Sampath, W. S., Barth, K. L.,
Enzenroth, R. A., "Control of Temperature Uniformity during the
Manufacture of Stable Thin-Film Photovoltaic Devices", submitted to
International Journal of Heat and Mass Transfer in March 2005

Sampath, W. S., Robert. A. Enzenroth and Kurt L. Barth, \"Mass
Production of Thin Film Solar Photovoltaics\", Presentation, Discussion
and Facility Tour with the Fort Collins Utilities, and CSU Facilities,
March 3, 2005.

Enzenroth, R. A., K. L. Barth and W. S. Sampath, \"Thermal Admittance
Spectroscopy Study: Preliminary Observations of a Meyer-Neldel
Relationship in CdTe Devices\", Invited talk to senior Researchers and
managers, NREL Golden CO, February, 16, 2005.

Barth, K. L., R. A. Enzenroth, and W. S. Sampath, \"Consistent
Processing and Long Term Stability of CdTe Devices\", presented at the
31 IEEE PVSC, January 3-7, 2005, Lake Buena Vista, Florida.

Enzenroth, R. A., K. L. Barth and W. S. Sampath, \"Thermal Admittance
Spectroscopy Study: Preliminary Observations of a Meyer-Neldel
Relationship in CdTe Devices\", presented at the 31 IEEE PVSC, January
3-7, 2005, Lake Buena Vista, Florida.

Barth, K. L., R. A. Enzenroth, and W. S. Sampath, \"Advances in
Continuous In-Line Processing of CdS/CdTe Devices: Stability and
Scale-Up\", DOE Solar Energy Technologies Program Review Meeting,
October 25-28, 2004, Denver, Colorado.

Sampath, W. S. K. L. Barth, \"Mass Production of CdTe Solar
Photovoltaics\", American Society of Mechanical Engineers (ASME),
Centennial Section Laboratory tour and presentation, Nov. 18, 2004.

Malhotra, C. P, R. L. Mahajan, W.S. Sampath, K. L. Barth, R. A.
Enzenroth, \"Control of Temperature Uniformity during the Manufacture of
Stable Thin-film Photovoltaic Devices\", ASME International Mechanical
Engineering Congress & Exposition, (IMECE 2004), November 13-19, 2004,
Anaheim, California.

Barth, K. L., \"Mass Production of CdTe Solar Photovoltaics\",
Mechanical Engineering Seminar Series, Colorado State University,
October 15, 2004.

Enzenroth, R. Albert, K. L. Barth, W. S. Sampath, \"Correlation of
Stability to Varied CdCl~2~ Treatment and Related Defects in CdS/CdTe PV
Devices as Measured by Thermal Admittance Spectroscopy\", 14th
International Conference on Ternary and Multinary Compounds, September
27-October 1, 2004, Denver, Colorado, USA.

7.5.1. National CdTe R&D Team Meeting Presentations

Sampath, W. S, R. Albert Enzenroth, and Kurt L. Barth, \"Advances in
Continuous In-Line Processing of CdTe PV Devices\" National CdTe Team
Meeting, Sponsored by the DOE through the National Renewable Energy
Laboratory, Golden, CO, May 5 and 6, 2005. Available at NREL website:
www.nrel.gov/ncpv/thin_film/docs/Sp-top-Sampath.ppt

Barth, Kurt L., R. Albert Enzenroth and W. S. Sampath, \"Progress at AVA
Technologies LLC\" National CdTe Team Meeting, Sponsored by the DOE
through the National Renewable Energy Laboratory, Golden, CO, May 5 and
6, 2005. Available at NREL website:
/www.nrel.gov/ncpv/thin_film/docs/Ind-AVA%20Tech.ppt#366,1,Progres

Enzenroth, R. A., K. L. Barth, and W. S. Sampath \"Estimation of
Acceleration Factors by Comparison of Performance During Accelerated
Stress and in Outdoor Conditions\", National CdTe Team Meeting,
Sponsored by the DOE through the National Renewable Energy Laboratory,
Golden, CO, May 5 and 6, 2005. Available at NREL website:
/www.nrel.gov/ncpv/thin_film/docs/Stab-Al%20Enzenroth.ppt

Enzenroth, R. A. T. Takamiya, K.L. Barth, and W.S. Sampath, \"Steady
State Photocapacitance (PHCAP) Study of CdS/CdTe PV Devices\", National
CdTe Team Meeting, Sponsored by the DOE through the National Renewable
Energy Laboratory, Golden, CO, May 5 and 6, 2005. Available at NREL
website: www.nrel.gov/ncpv/thin_film/docs/Mat-chem-Al%20Enzenroth.ppt

Enzenroth, R. Albert, K. L. Barth, W. S. Sampath, \"Stability Sub-Team
Meeting\" October 25, 2004, Denver, Colorado, USA.

Barth, K. L., R. A. Enzenroth, and W. S. Sampath, \"Advances in
Continuous In-Line Processing of CdS/CdTe Devices: Stability and
Scale-Up\", DOE Solar Energy Technologies Program Review Meeting,
October 25-28, 2004, Denver, Colorado.