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Department of Mechanical Engineering

Fort Collins, Colorado 50523

970.491.6559

06-28-05

Ken Zweibel, MS 3212

National Center for Photovoltaics

National Renewable Energy Laboratory

1617 Cole Blvd.

Golden, Co. 80401

Dear Mr. Zweibel:

This is the second 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.1.2 of the
subcontract statement of work (SOW).

# Summary

During this quarter, we have progressed on the tasks outlined in our SOW
for Phase I. The reproducibility of device efficiency and stability
results during long duration operation of the pilot system with a single
charge loading for the CdS, CdTe and CdCl~2~ processes has been reported
earlier by us \[1\]. Currently, efforts are underway to advance the back
contact processing to this same level of reproducibility, 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. Preliminary results on developing a module
interconnection process are promising. Many devices and CdS/CdTe films
have been provided to many groups including Larry Olsen of PNNL. Based
on our current progress, no difficulties are anticipated in successfully
completing the future targets outlined in the SOW.

A loadlock system for the prototype production system to transport
16.5x16.5 inch substrates has been designed, constructed and
successfully tested. This activity is supported by another program from
DOE-EERE and cost shared by National Starch and Chemicals (NSC). Three
separate groups including Lisa Barnett from DOE headquarters, Bolko Von
Roedern of NREL, Dave Miles from NSC and other representatives from DOE,
Peter Meyers and Roger Green of First Solar, and members of the Fort
Collins business community were all given a detailed tour of our
facilities. Detailed presentations and demonstrations of our technology
were also given.

**[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 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.

An extension to the pilot system has been designed and fabricated. This
extension chamber has space for four process stations. Two process heads
(vapor sources and substrate heaters) have been designed and
constructed. In addition the air to vacuum to air (AVA) seal has been
redesigned and is being fabricated. The old seal employed a glass filled
Teflon structure for supporting the substrate transport belt. The lack
of dimensional accuracy in the Teflon structure caused scratching of the
deposited films. A new seal made entirely of stainless steel. The
substrate transport belt has been extended to accommodate the longer
vacuum chamber. It is expected that the extended pilot system will be
fully operational by the end of July 2005. 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. 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 is being tested. Also, preliminary heat transfer analysis
and experiments suggest that contacting the substrate with a copper
plate with a flat, smooth surface might be adequate for cooling the
substrate.

[2. Process Optimization]{.underline}

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 1. 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 \[2\]. 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
  -------------- -------------- -------------- -------------- --------------

![](media/image2.wmf){width="2.5944444444444446in"
height="2.5569444444444445in"}

Figure 1: 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. 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. 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 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.

[4. Encapsulation]{.underline}

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.

[6. Collaborative Studies]{.underline}

Our group has been actively participating in National CdTe Team
activities. 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.

Many devices and coated substrates have been provided to many groups
including:

1)  21 samples were sent to Larry Olsen at PNNL for studies on the
    effect of moisture and heat

2)  11 cells to Prof. Wilbur\'s group at Colorado State University for
    high altitude balloon studies

3)  126 substrates to NSC for metallization studies

4)  Solar Fields enquired about our metallization and they have been
    provided details of our metallization and they were put in touch
    with NSC.

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

[References:]{.underline}

1\. 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

2\. 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.