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Department of Mechanical Engineering

Fort Collins, Colorado 50523

970.491.6559

09-29-05

Ken Zweibel, MS 3212

National Center for Photovoltaics

National Renewable Energy Laboratory

1617 Cole Blvd.

Golden, Co. 80401

Dear Mr. Zweibel:

This is the third quarterly report of our research under the current
Thin Film Partnership Program (Subcontract No. XXL-5-44205-02 to
Colorado State University). A brief summary and details of our work are
given below. This report is in fulfillment of deliverable D.1.3 of the
subcontract statement of work (SOW).

# Summary

During this quarter, we have progressed on the tasks outlined in our SOW
for Phase I. A significant extension to our pilot scale system has been
designed, fabricated, installed and debugged. This will allow improved
process conditions for the back contact process steps. 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. Based
on our current progress, no difficulties are anticipated in successfully
completing the future targets outlined in the SOW.

The nominal 2 MW/yr production prototype system continues to be
advanced. This activity is supported by another program from DOE-EERE
and cost shared by National Starch and Chemicals (NSC). Three separate
groups have expressed interest in licensing our technology for 50 MW/Yr
plants. We are also in discussion with a large manufacturer for
partnership to manufacture large volumes of CdTe PV. Numerous documents
have been submitted to these groups for evaluating our technology.
Detailed presentations and demonstrations of our technology were also
given to many groups.

**[Detailed Description of the Results:]{.underline}**

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

The pilot system for processing 3.6 x 3.1 inch substrates has been
modified to more closely simulate the back contact processing for actual
industrial manufacturing. With the old configuration of the pilot
system, the Cu back contact process head is located immediately after
the CdCl~2~ treatment heads. If the substrate enters the copper process
at too high a temperature, the resulting devices have a high series
resistance. To avoid this, a pause to cool the substrate was introduced
in the old configuration of the pilot system. 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 completed the formation of the Cu containing back
contact in the old configuration.

To simulate the Cu back contact process for actual production the pilot
system has been 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. The extension to the
pilot system has been designed, fabricated, installed and debugged. This
extension chamber has space for four process stations. Two process heads
(vapor sources and substrate heaters) have been designed, constructed
and installed. The substrate transport belt has been extended to
accommodate the longer vacuum chamber. The substrate motion through the
extended system is also functional. The modified pilot system 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.

In order to reduce the length of the \"all forward\" system, it is
desirable to actively cool the substrate before back contact processing.
Numerous approaches to cool the substrate have been investigated. The
feasibility of contacting the substrate with a copper plate with a flat,
smooth surface for cooling the substrate in vacuum has been
demonstrated.

![](media/image2.wmf){width="4.75in" height="2.092361111111111in"}

Figure 1: Schematic of contact cooling test apparatus in vacuum chamber

A series of measurements of cooling rate of glass substrates were
performed with the test apparatus shown schematically in figure 1. 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 1 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 2. As
shown in figure 2 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 2 for atmospheric pressure.
Also shown in figure 2 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/image3.wmf){width="3.875in" height="2.9506944444444443in"}

Figure 2: 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.

[2. Studies on Interconnection:]{.underline}

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. 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. 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.

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

From the above 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.
Experiments on automated high speed mechanical scrbing are planned.

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 sample 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. Higher Efficiency]{.underline}

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. 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.

[4. Collaborative Studies]{.underline}

During this quarter, the following collaborative activities were
conducted:

1)  11 cells to Prof. Wilbur\'s group at Colorado State University 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.

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

3)  Teleconference with First Solar for possible consulting was held.

If you have any questions, please do not hesitate to call the laboratory
at (970)-491-8411. Thank you.

Sincerely,

W.S. Sampath

Associate Professor

Colorado State University

CC: C. Lopez