**ANNUAL REPORT**

**to**

**National Renewable Energy Laboratory**

**"THIN-FILM PHOTOVOLTAICS PARNERSHIP PROGRAM"**

**Contract** **NO. RDJ-2-30630-20**

**Deliverable D.2.6**

**PROJECT TITLE: "**Research Leading to High Throughput Manufacturing of
Thin-Film CdTe PV Modules"

**NAME OF ORGANIZATION:** First Solar, LLC

**PRINCIPAL INVESTIGATOR:** Dr. Rick C. Powell

**ADDRESS:** First Solar, LLC

28101 Cedar Park Blvd.

Perrysburg, OH 43551

**PHONE:** 419-662-7549

**FAX:** 419-662-8525

**e-mail:** rcpowell@firstsolar.com

**CATEGORY:** Technology Partner

27 September 2004

**Table of Contents**

[**Abstract** ii](#__RefHeading___Toc87416792)

[**Acknowledgements** iv](#__RefHeading___Toc87416793)

[**1. Introduction** 1](#__RefHeading___Toc87416794)

[**2. Advanced Front Contact and Window Layers (AFCW)**
2](#__RefHeading___Toc87416795)

[**2.1 Zinc Stannate (ZTO) Deposition** 3](#__RefHeading___Toc87416796)

[**2.2 Oxygenated Amorphous CdS (a-CdS:O)** **Deposition**
3](#__RefHeading___Toc87416797)

[**2.3 AFCW Device Optimization** 4](#__RefHeading___Toc87416798)

[**3. CdS/CdTe Deposition Reactor Development**
7](#__RefHeading___Toc87416799)

[**3.1 Distributor Development** 7](#__RefHeading___Toc87416800)

[**3.1.1 Alternative Distributors** 9](#__RefHeading___Toc87416801)

[**3.1.2 Distributor Modeling** 12](#__RefHeading___Toc87416802)

[**3.2 Process Control** 14](#__RefHeading___Toc87416803)

[**3.3 Film Characterization** 15](#__RefHeading___Toc87416804)

[**3.3.1 Device Analysis** 15](#__RefHeading___Toc87416805)

[**3.3.2 IV Mapping** 16](#__RefHeading___Toc87416806)

[**3.3.3 Non-uniformity loss estimates**
17](#__RefHeading___Toc87416807)

[**3.3.4 Chemical Analysis** 18](#__RefHeading___Toc87416808)

[**4. Accelerated Life Testing Development**
19](#__RefHeading___Toc87416809)

[**4.1 Development of predictive ALT Protocols**
19](#__RefHeading___Toc87416810)

[**4.2 Stress-Induced Change Mechanisms**
19](#__RefHeading___Toc87416811)

[**4.2.1 CdCl~2~ treatments** 20](#__RefHeading___Toc87416812)

[**4.2.2 Back Contact Processing** 22](#__RefHeading___Toc87416813)

[**4.2.3 Humidity effects during device processing**
22](#__RefHeading___Toc87416814)

[**5. Environmental, Health and Safety Improvements**
23](#__RefHeading___Toc87416815)

[**5.1 Updated Safety Program** 23](#__RefHeading___Toc87416816)

[**5.2 Ongoing Monitoring** 24](#__RefHeading___Toc87416817)

[**5.3 Material recovery** 24](#__RefHeading___Toc87416818)

[**6. Future Plans** 26](#__RefHeading___Toc87416819)

[**List of Figures** 27](#__RefHeading___Toc87416820)

[**Glossary of Abbreviations** 28](#__RefHeading___Toc87416821)

[**References** 29](#__RefHeading___Toc87416822)

[]{#__RefHeading___Toc87416792 .anchor}**[Abstract]{.underline}**

First Solar is actively commercializing CdTe-based thin film
photovoltaics. During the past year major additions of production
capability have been completed as well as process improvements to
achieve higher throughput and efficiency and greater durability. This
report presents the results of phase II of the subcontract entitled
**"**Research Leading to High Throughput Manufacturing of Thin-Film CdTe
PV Modules". The subcontract supports several important aspects needed
to for high volume manufacturing of high efficiency modules including
exploration of large area advanced front contact window layers,
improvements of the semiconductor deposition system, advancement in
understanding of post deposition processing steps and accelerated life
testing methods, and progress to the environmental, health and safety
programs.

Advanced front contact and window layers (AFCW) are believed to be
necessary for efficiency improvements, particularly when using very thin
CdS window layers. Tasks in this area were added to the statement of
work in Phase 2. Adaptation of laboratory deposition processes to large
area and initial tests on full scale modules were completed. The
benefits of the AFCW have not yet been realized in full modules.

Progress continued in the development of the vapor transport deposition
(VTD) system. A number of advanced system designs, designated as VTD-2,
with improved operational characteristics were demonstrated. One design
in particular has been selected for future concentration. The improved
system results in better film uniformity and material utilization while
also opening new regions of parameter space for coating optimization. We
continued to improve and characterize a new CdS powder feed system that
will be increasingly important for high efficiency modules which employ
very thin CdS window layers. We have documented the superiority of the
new system, but further optimization is underway. We expect to introduce
the improved powder metering system in concert with an improved VTD-2
deposition system for CdS.

Significant progress has also been made on a numerical model of mass and
heat flow within the semiconductor deposition system. The sophistication
of the model has grown to include details of particle injection,
particle sublimation, and vapor transport. Attention is now focused on
modeling the film growth process. Validation with experimental data is
ongoing as many experimental observations can now be semi-quantitatively
predicted.

Progress was made in understanding several steps in module processing.
Improvements in back contact formation and understanding of the
influence of humidity on device processing advanced.

A new faster accelerated life test has been shown to be equivalent to
and in some respects better than our old 8 week test. This new procedure
is being implemented in our standard quality assurance program and work
continues to determine how the new test correlates with field data.

Our Environmental, Health and Safety program has increased to include
all aspects of the factory expansion. New engineering controls have
further reduced worker exposure risk within the factory from already low
levels. We also continued to progress in methods to handle our waste
streams.

Work under this subcontract contributes to the overall manufacturing
operation. During phase II, average module efficiency (total area) on
the production line was improved from 7.9 to 8.6% due primarily to
process optimization. At the same time production volume for commercial
sale increased from 2.5 MW in 2003 to an estimated 6 MW in 2004. Much of
the new 25 MW/yr production line has been qualified and production
volume is steadily increasing.

[]{#__RefHeading___Toc87416793
.anchor}**[Acknowledgements]{.underline}**

Many people at First Solar contributed to the success of the Phase 2
work. We gratefully acknowledge the contributions of Anke Abken, Dave
Berger, Eugene Bykov, John Christiansen, Todd Coleman, Nelson DeVoe,
Tony Draper, Andy Gray, Roger Green, Upali Jayamaha, Peter Meyers, Anne
Moser, Mike Steele, and Syed Zafar, as well as personnel from First
Solar production, engineering and management.

We are also grateful for the support that we have received from National
Renewable Energy Laboratory staff, including D. Albin, S. Asher, T.
Gessert, T. McMahon, H. Ullal, S. Wei, X. Wu, and K. Zweibel.
Contributions from S. Hegedus and B. McCandless of Institute of Energy
Conversion; J. Sites, K. Barth and A. Enzenroth of Colorado State
University; A. Fahrenbruch of ALF; A. Compaan, V. Karpov, and D. Shvydka
of University of Toledo; S. Feldman, S. Gilmore, V. Kaydanov and T. Ohno
of Colorado School of Mines; and C. Ferekides of University of South
Florida are gratefully acknowledged.

Two lower-tier subcontracts are directly contributing to this work. The
first project, titled "Direct Integration of Solid-Prescursor Powders
into Chemical Vapor Deposition Systems", involves modeling of the heat
and mass transfer in the deposition reactor. This work is being
conducted by J.P. Delplanque, R. Kee, and M. Pavol of Colorado School of
Mines. The second project, titled "Nonuniformity Loss in Photovoltaics",
involves the modeling of performance loss due to spatial variation. This
work was performed by V. Karpov and D. Shvydka at University of Toledo.

[]{#__RefHeading___Toc87416794 .anchor}**[1. Introduction]{.underline}**

First Solar (FS) has developed proprietary thin-film PV module
technology and is rapidly transitioning this technology into a
significant manufacturing capability. Production volume has increased
substantially in the last several years from 1.5 MW in 2002 to 2.5 MW in
2003. Approximately 6 MW will be produced in 2004. The production
facility has been expanded to 131,000 ft^2^ and an upgraded and fully
automated production line with an expected capacity of 25 MW/yr is
currently being installed and qualified. Full production on this new
line is expected in 2005.

Concurrent with the increase in production volume, average module
performance (total area) has increased from 7.9% in 2003 to 8.6% in
2004. Our current champion module has an NREL-confirmed performance of
9.4% (10.2% aperture area).

In addition, quality assurance procedures within the factory have been
developed and documented. ISO 9001 certification was achieved in May
2004. The module testing program has been greatly expanded and now
includes indoor accelerated light soaking and durability testing using a
number of protocols as well as field testing. We are currently closely
tracking \~1,800 modules at 3 locations.

Market development has kept pace with the production volume increase.
Our basic market strategy remains the production of high-quality,
high-volume, best-value solar modules for large field installations.
Market opportunities in Europe justified the establishment of a German
subsidiary in June 2003 to better serve customers. More information can
be found at our website www.firstsolar.com.

The support of First Solar through the Thin Film Photovoltaic
Partnership Program (TFPPP) has been instrumental in the progress over
the past several years. Phase 2 of the present contract (RDJ-2-30630-20)
has assisted in 1) exploration of production-compatible front contact
window layers, 2) continued development of our unique high-rate vapor
transport deposition (VTD) process for the semiconductor layers, 3)
optimization of post-deposition processing, 4) improvements in
accelerated life testing, and 5)  progress in environmental, health and
safety (EH&S) programs.

Advanced front contact and window layers (AFCW) are believed to be
necessary for efficiency improvements, particularly when using very thin
CdS window layers. An expanded program in this area was added into the
contract statement of work. Specifically the work includes the
adaptation of demonstrated laboratory deposition processes to large area
and the evaluation of AFCW layers when incorporated into our module
manufacturing process.

Motivation for continued development of our high-rate VTD process stems
from the fact that VTD is a relatively immature process used to form the
main junction and the thickest layer in the device. The VTD system is
required to continuously and consistently produce uniform films for long
durations.

ALT testing is critical to assure the 20+ year performance that
underlies the economics of PV systems. Continued development of rapid
tests which differentiate the stability of experimental devices and
which can be correlated to long term performance is essential. Further
understanding of degradation mechanisms is also vital.

Large scale manufacturing of a product that contains cadmium, a
regulated material, requires development of procedures for further
reduction in emissions and solid wastes. This includes further
identification, characterization, monitoring and reduction of cadmium
emissions and improvements in module recycling.

[]{#__RefHeading___Toc87416795 .anchor}**[2. Advanced Front Contact and
Window Layers (AFCW)]{.underline}**

During Phase 2 we added to the statement of work tasks related to the
development of advanced front contact and window layers (AFCW). AFCW
layers are believed to be central to improving device efficiency and are
an integral part of the record efficiency CdS/CdTe small area devices.
The purpose of AFCW layers is to reduce the optical losses incurred in
the front contact and the CdS window layer. Optical losses can be cut by
thinning inactive absorbing layers or by making those inactive layers
more transparent. In CdS/CdTe solar cells the CdS is an absorbing
inactive layer. CdS is inactive in the sense that the collection of
photogenerated carriers within it is very inefficient. When CdS is
thinned below a certain critical thickness, although the current
increases as expected due to reduced optical losses, no net gain in
efficiency is observed because of the loss in open circuit voltage (Voc)
and fill factor (FF). Inclusion of a suitable buffer layer between the
transparent conducting oxide (TCO) and the CdS has been a way to
overcome this problem. Modification of the CdS layer to decrease its
absorption is also possible.

The goal of our AFCW work is to establish methods to form AFCW layers on
full size plates using techniques which could be transferred to
production and to demonstrate the benefit of these layers on the module
scale in our overall process.

Our approach has been to start with the front contact process developed
at NREL by Dr. Wu for record efficiency small area devices and to adapt
elements of it for large area production use. The NREL process employs a
transparent conductive layer of cadmium stannate (Cd~2~SnO~4~), a buffer
layer of zinc stannate (Zn~2~SnO~4~ abbreviated as ZTO), and an n-layer
of amorphous oxygenated CdS (a-CdS:O) \[1, 2\]. All layers in the NREL
process are sputtered. The ZTO is believed to be a strong candidate for
the buffer layer and a-CdS:O has been shown to be more transparent
potentially lowering optical losses. Rather than tackle all three layers
used in the NREL process, we began by investigating only the ZTO buffer
layer and the a-CdS:O layer using our normal commercially-available
SnO~2~ as the transparent conductor.

Thus during Phase 2 we investigated the two AFCW combinations:

(1) SnO~2~:F /sputtered ZTO/sputtered a-CdS:O

(2) SnO~2~:F /sputtered ZTO/VTD-CdS .

    []{#__RefHeading___Toc87416796 .anchor}**[2.1 Zinc Stannate (ZTO)
    Deposition]{.underline}**

The NREL process uses rf-sputtering from a compound target to form the
ZTO layers. It is difficult to use rf-sputtering to deposit uniform
large area films at high rates. Instead we chose to deposit films using
an A/B pulsed DC sputter mode.

ZTO films were deposited onto 60cm x 120 cm full size plates using
sputtering from a compound target. The ZTO deposition rates were low due
in part to the large fraction of oxygen used in the sputter gas. The
deposition rates are adequate for thin buffer layers. ZTO films were
characterized using optical absorption/reflection and resistivity
measurements. The ZTO films undergo drastic changes during the high
temperature device fabrication process. To understand the electro
optical properties of ZTO films within a high efficiency device, single
layer ZTO films are annealed at 570ºC for 2 minutes in the presence of
CdS to simulate the changes that undergo during the device fabrication.
For ZTO films to act as a good buffer layer the resistivity of the
annealed films should be within a specific range and the optical
absorption should be low.

In order to optimize the large area deposition process, we deposited
several sample films on Corning 1737 glass substrates at various
sputtering pressures and gas mixtures. The results are shown in Table 1.
The sample ZTO-15 had properties similar to those considered optimal for
high efficiency NREL devices.

Table 1. Optical and electrical properties of ZTO films before and after
anneal. Measurements and annealing provided by Dr. Wu at NREL.

+---------+-------+--------+-----------+-----------+-----------------+
| Sample  | Eg \* | Eg †   | Sheet R   | Re        | Absorption at   |
|         |       |        | \*        | sistivity | 400 nm †        |
|         | (eV)  | (eV)   |           | †         |                 |
|         |       |        | (Ohm/sq)  |           | (%)             |
|         |       |        |           | (Ohm-cm)  |                 |
+---------+-------+--------+-----------+-----------+-----------------+
| ZTO-7   |       |        |           | High      |                 |
+---------+-------+--------+-----------+-----------+-----------------+
| ZTO-12  | 3.46  | 3.51   | 5x10^8^   | 0.05      | 15              |
+---------+-------+--------+-----------+-----------+-----------------+
| ZTO-14  | 3.57  | 3.47   | \>10^9^   | 0.29      | 25              |
+---------+-------+--------+-----------+-----------+-----------------+
| ZTO-15  | 3.70  | 3.64   | \>10^9^   | 1.4       | \<10            |
+---------+-------+--------+-----------+-----------+-----------------+

\* as grown film † annealed at 570ºC for 2 minutes in the presence of
CdS

The inherent thermal annealing of ZTO buffer layers within the FS
semiconductor deposition process may differ from the simulated anneals.
Thus the above deposition condition for optimal in-device ZTO properties
is only a starting point.

[]{#__RefHeading___Toc87416797 .anchor}[**2.2 Oxygenated Amorphous CdS
(a-CdS:O)** **Deposition**]{.underline}

We also deposited a-CdS:O films on full size plates using the pulsed
sputter mode. The a-CdS:O deposition rates were comparable to metal
deposition rates in the same system. The uniformity of one of the early
films was measured on our in-house CdS thickness mapping system and the
standard deviation in thickness over the full plate was \~4%. We expect
uniformity to improve as the sputtering target is conditioned over time.

In order to get desired film properties the deposition pressure and the
oxygen/argon ratio of the process gas was varied systematically. Films
were characterized at First Solar and NREL using optical
transmission/reflection, dark and photo conductivity and XRD
measurements. Although lower sputtering pressures are desired from a
manufacturing point of view, films grown at lower sputtering pressures
had undesirable optical properties (see Figure 1a).

+-----------------------------------+-----------------------------------+
| \(a\)                             | \(b\)                             |
|                                   |                                   |
| ![](media/image1.                 | ![](media/image2                  |
| wmf){width="3.0006944444444446in" | .wmf){width="2.998611111111111in" |
| height="1.9979166666666666in"}    | height="2.001388888888889in"}     |
+-----------------------------------+-----------------------------------+

[]{#__RefHeading___Toc87416823 .anchor}Figure 1. Optical absorption
curves of a-CdS:O films.

Films grown at a higher deposition pressures had better optical
properties (see Figure 1b). Further characterization of films was done
using XRD and dark/photo conductivity measurements at NREL by Dr. Wu. As
indicated by XRD patterns, film grown without oxygen exhibits a
polycrystalline structure with the preferential orientation along the
(002) axis. The intensity of the (002) peak is reduced progressively as
the oxygen/argon gas ratio is increased as the films become amorphous.
The optical band gap of the amorphous material is about 2.53eV. The
ratio of photo/dark conductivity of the amorphous films was also quite
high which is an indication that this material is suitable as a window
layer for CdTe based solar cells.

[]{#__RefHeading___Toc87416798 .anchor}**[2.3 AFCW Device
Optimization]{.underline}**

To date two tests have been run using the SnO~2~:F/ZTO/a-CdS:O front
contact structure. In the first test, fourteen plates were coated with
six different combinations of ZTO/a-CdS:O. CdTe deposited by VTD in the
production system. Dot cells were made on several plates while the rest
were processed through our standard production stream to make modules.
Although the device efficiencies were somewhat lower than that produced
using our optimized standard process, this very first attempt proved
that that the room temperature sputtered ZTO/a-CdS:O layers are
basically compatible with our VTD coating and module fabrication
process. Representative device results are shown in Table 2.

Table 2. Summary Results from SnO~2~:F/ZTO/a-CdS:O AFCW structure.

+------------------+--------+-------+-------+-----+--------+--------+
| **Type**         | *      | **    | **    | **F | *      | *      |
|                  | *Total | Voc** | Jsc** | F** | *Roc** | *Rsc** |
|                  | Area** |       |       |     |        |        |
|                  |        | **(   | **(m  | *   | **(    | **(    |
|                  | **     | mV/ce | A/cm^ | *(% | ohm-cm | ohm-cm |
|                  | Effici | ll)** | 2^)** | )** | ^2^)** | ^2^)** |
|                  | ency** |       |       |     |        |        |
|                  |        |       |       |     |        |        |
|                  | *      |       |       |     |        |        |
|                  | *(%)** |       |       |     |        |        |
+==================+========+=======+=======+=====+========+========+
| Condition 2,     | 10.7   | 811   | 20.3  | 6   | 6.8    | 1180   |
| Test 1           |        |       |       | 4.7 |        |        |
|                  |        |       |       |     |        |        |
| Cell average     |        |       |       |     |        |        |
+------------------+--------+-------+-------+-----+--------+--------+
| Condition 2,     | 11.4   | 824   | 21.0  | 6   | 6.5    | 1020   |
| Test1            |        |       |       | 5.6 |        |        |
|                  |        |       |       |     |        |        |
| Best Cell        |        |       |       |     |        |        |
+------------------+--------+-------+-------+-----+--------+--------+
| Best Module Test | 8.4    | 772   | 21.1  | 6   | 7.5    | 811    |
| 1                |        |       |       | 0.9 |        |        |
|                  |        |       |       |     |        |        |
| E03230670        |        |       |       |     |        |        |
+------------------+--------+-------+-------+-----+--------+--------+
| Best Module Test | 8.8    | 804   | 23.0  | 5   | 9.5    | 588    |
| 2                |        |       |       | 6.0 |        |        |
|                  |        |       |       |     |        |        |
| E07140274        |        |       |       |     |        |        |
+------------------+--------+-------+-------+-----+--------+--------+

One of the sub-modules (E03230670) with the SnO~2~:F/ZTO/a-CdS:O AFCW
structure underwent our standard 56 day light soak. The stability of
that module was comparable to standard product.

A second, more elaborate experiment based on Taguchi methodology of
optimization was conducted. For this test 54 plates were made using
various processing condition for ZTO, a-CdS:O, CdTe, and subsequent
processing . I-V parameters of the best sub-module are shown in Table 2.

Two experiments with the SnO~2~:F/ZTO/VTD-CdS AFCW structure have also
been completed. For the preliminary test, CdS and CdTe films were
deposited by VTD in the production system onto fifteen SnO~2~:F plates
coated with ZTO films. Device results are shown in Table 3.

Again a second more comprehensive designed experiment involving 54
plates and variations in ZTO, CdS/CdTe and subsequent processing was
conducted. I-V parameters of the best module are shown in Table 3.

Table 3. Summary Results from SnO~2~:F/ZTO/VTD-CdS AFCW structure.

+-----------------+---------+-------+-------+-----+---------+---------+
| **Type**        | **Total | **    | **    | **F | **Roc** | **Rsc** |
|                 | Area**  | Voc** | Jsc** | F** |         |         |
|                 |         |       |       |     | *       | *       |
|                 | **Effic | **(   | **(m  | *   | *(ohm-c | *(ohm-c |
|                 | iency** | mV/ce | A/cm^ | *(% | m^2^)** | m^2^)** |
|                 |         | ll)** | 2^)** | )** |         |         |
|                 | **(%)** |       |       |     |         |         |
+=================+=========+=======+=======+=====+=========+=========+
| Best condition  | 11.5    | 794   | 21.9  | 6   | 6.3     | 870     |
| Test 1          |         |       |       | 6.3 |         |         |
|                 |         |       |       |     |         |         |
| Cell average    |         |       |       |     |         |         |
+-----------------+---------+-------+-------+-----+---------+---------+
| Best condition  | 12.0    | 817   | 21.9  | 6   | 6.0     | 19800   |
| Test 1,         |         |       |       | 7.3 |         |         |
|                 |         |       |       |     |         |         |
| Best Cell       |         |       |       |     |         |         |
+-----------------+---------+-------+-------+-----+---------+---------+
| Best Module     | 8.3     | 731   | 22.5  | 5   | 9.2     | 746     |
| Test 1          |         |       |       | 9.7 |         |         |
|                 |         |       |       |     |         |         |
| E04210171       |         |       |       |     |         |         |
+-----------------+---------+-------+-------+-----+---------+---------+
| Best Module     | 9.2     | 793   | 22.6  | 6   | 8.5     | 778     |
| Test 2          |         |       |       | 0.4 |         |         |
|                 |         |       |       |     |         |         |
| E07230523       |         |       |       |     |         |         |
+-----------------+---------+-------+-------+-----+---------+---------+

While good modules have been made with both AFCW structures, on balance
the SnO~2~:F/ZTO/VTD-CdS sequence seems to be more compatible with our
process at the present time. This sequence is also simpler as it
involves the addition of only one layer into our device. Thus we are
concentrating on this AFCW structure and have already run a third test.

Without a buffer layer, thinning the CdS increases efficiency and
photocurrent until the photocurrent reaches about 23 mA/cm^2^. Further
reductions in CdS thickness yield higher photocurrent but reduced
efficiency due to loss in Voc and FF (see Figure 2). The use of a buffer
layer has been shown to extend the maximum photocurrent limit to \>25
mA/cm^2^.

![](media/image3.wmf){width="4.835416666666666in"
height="3.223611111111111in"}

[]{#__RefHeading___Toc87416824 .anchor}Figure 2. Efficiency as a
function of photocurrent for a set of dot cells made on a graded CdS
thickness plate without a buffer layer.

Figure 3 shows the results of the comprehensive module level test using
the SnO~2~:F/ZTO/VTD-CdS AFCW structure with CdS in the range of
500-900Å. We expect a range of results for the plates which have the ZTO
buffer layer because a wide variety of conditions were used.
Nevertheless we also expect to see some evidence of an increase in the
maximum photocurrent before the efficiency falls off. Figure 3 does not
show such evidence. Thus although we have made reasonably good modules
using the ZTO buffer layer, we have not yet shown that it provides any
benefit.

![](media/image4.wmf){width="5.063888888888889in"
height="3.379861111111111in"}

[]{#__RefHeading___Toc87416825 .anchor}Figure 3. Efficiency as a
function of photocurrent for modules made with and without ZTO buffer
layer.

[]{#__RefHeading___Toc87416799 .anchor}**[3. CdS/CdTe Deposition Reactor
Development]{.underline}**

[]{#__RefHeading___Toc87416800 .anchor}**[3.1 Distributor
Development]{.underline}**

At the heart of the First Solar manufacturing technology is the
semiconductor coater. CdS and CdTe are deposited sequentially in the
same piece of equipment using the vapor transport deposition (VTD)
method. VTD is a high-throughput, large- area coating technique
pioneered and patented by First Solar \[3, 4\]. The system capitalizes
on the ease with which CdS and CdTe can be vaporized and on the rapid
rates at which quality films can be formed from re-condensing vapors.
Essentially the method utilizes flash-sublimation of injected
semiconductor powders in an inert carrier gas stream to create a dense
vapor cloud in a modest vacuum (\~5 Torr). The inert carrier gas flow
directs and controls the dense vapor cloud. The semiconductor films are
formed on glass substrates at temperatures between 500 and 600 ºC at
high rates. The inert carrier gas and some vapors not deposited are then
exhausted from the chamber.

Since the VTD deposition rates are rapid, the length of the deposition
zones needed to form the relatively thick (2-5µm) absorber layer at
production line speeds of is relatively short. These compact high
temperature deposition zones are ideal from a manufacturing equipment
perspective as short deposition chambers can be used (lower capital
equipment costs) and small critical areas are easier to manage. The
production vapor generator and vapor distributor system in use today is
relatively simple, robust, and durable.

The film formation process must be integrated with the control of glass
properties. The extremely high deposition rates of the VTD method
require high substrate temperatures. From the glass perspective, the VTD
system heats low-strength, annealed glass to temperatures in excess of
the glass softening point. Quenching during venting of the vacuum exit
load lock strengthens the glass. The quenching process must be closely
coordinated with overall system thermal management in order to achieve
glass flatness and strength uniformity. Glass strength is required for
field deployment and glass flatness is needed for consistent laser
scribing and module lamination.

The core component of the VTD method is the vaporizer/distributor
system. The design, materials, and methods of operation of the
distributor system fundamentally control film formation including
uniformity of thickness, microstructure and junction properties. The
distributor serves three main functions. First, the input powder must be
vaporized. Second, a uniform cross-web[^1] distribution of mass flow
must be achieved in order to form films of uniform thickness. Third,
presentation of the deposition flux to the glass controls film
nucleation and growth and thereby microstructure.

The base distributor system is simple both mechanically and thermally
and can consistently produce films with adequate thickness uniformity
(±10%). However, film thickness and microstructure uniformity depend on
carrier gas flow and the details of feed injection. Normally carrier gas
flow is adjusted to produce the best possible film thickness uniformity
and cannot be altered to change dilution, to improve input powder
transport, or to adjust film microstructure. In addition, optimum
operating parameters vary between nominally identical distributors, and
adjustments must be made over the life of the distributor.

Spatial variation in microstructure leads to spatial variation in device
performance and is thus undesirable. The effect of microstructure
non-uniformity can be accentuated by operation in an asymmetric mode. In
this mode, film thickness is nearly uniform, but a clear asymmetry in
film growth is observed (see Figure 4). Reflection characteristics can
be used to give a rapid indication of surface microstructure. Figure 5
shows a map of the surface roughness determined from the specular and
scattered infrared reflection.

  ----------------------- ----------------------- -----------------------
  ![](media/image5.png)   ![](media/image6.png)   ![](media/image7.png)

  ----------------------- ----------------------- -----------------------

[]{#__RefHeading___Toc87416826 .anchor}Figure 4. SEM micrographs showing
variation in CdTe film microstructure across a plate deposited using a
standard distributor operated in an asymmetric mode. The G2 and G6
positions correspond to locations x=60, y=20 and x=60, y=-20
respectively in Figure 5.

![](media/image8.png){width="3.673611111111111in"
height="2.623611111111111in"}

[]{#__RefHeading___Toc87416827 .anchor}Figure 5. Color contour map of
surface roughness determined from reflection characteristics for a film
deposited with a standard distributor operated in an asymmetric mode.
Note that Y is the cross-web dimension, i.e. the direction of motion of
the plate is in the X direction.

To compensate for the underlying variation in microstructure, standard
distributors with deposition zones significantly wider than the glass
are normally used. In other words, to avoid edge effects the stream of
glass is passed through only the central region of vapor deposition
zone. Obviously there is consequent lower material utilization.

[]{#__RefHeading___Toc87416801 .anchor}**3.1.1 Alternative
Distributors**

During this contract period, we have focused on one basic alternative
distributor concept that will be designated as VTD-2. In this concept,
the spatial non-uniformity problems that can arise in a low-pressure,
three component gas system are effectively circumvented. Several
different configurations of the VTD-2 idea have been tested and we have
now selected one version (designated as the G3 distributor) for future
concentration. While the VTD-2 concept resolves most of the problems of
the VTD-1 systems, the VTD-2 distributors are mechanically and thermally
more complex.

A recent test (ETA 807) clearly demonstrates the improved uniformity of
the G3 system for CdTe. In this test operating parameters of the G3
distributor were varied to explore the range of CdTe films possible and
their suitability for devices. Film thickness uniformity was excellent
and was independent of operating parameters. In other words the geometry
of the G3 distributor controlled the film thickness uniformity. Figure 6
shows that films made with the G3 distributor are significantly more
uniform in thickness than films made with the standard distributor. This
result was achieved while simultaneously reducing the width of the
deposition zone dramatically which led to a 40% reduction in the source
material requirements.

![](media/image9.wmf){width="6.0in" height="4.0in"}

[]{#__RefHeading___Toc87416828 .anchor}Figure 6. Reduction in CdTe
thickness variation cross web for G3 distributor. On average there is a
37% reduction in cross-web variation using the G3 system.

The intentional variation in operating parameters in the ETA 807
experiment did lead to significant variation in film microstructure.
Figure 7 shows a representative sampling of the as-deposited film grain
structures.

  ----------------------------------------------------- -----------------------------------------------------
  ![](media/image10.jpeg){width="2.497916666666667in"   ![](media/image11.jpeg){width="2.497916666666667in"
  height="2.497916666666667in"}                         height="2.497916666666667in"}

  ![](media/image12.jpeg){width="2.497916666666667in"   ![](media/image13.jpeg){width="2.497916666666667in"
  height="2.497916666666667in"}                         height="2.497916666666667in"}

  ![](media/image14.jpeg){width="2.497916666666667in"   ![](media/image15.jpeg){width="2.497916666666667in"
  height="2.497916666666667in"}                         height="2.497916666666667in"}
  ----------------------------------------------------- -----------------------------------------------------

[]{#__RefHeading___Toc87416829 .anchor}Figure 7. SEM micrographs of
films deposited using the G3 distributor. A variety of microstructures
can be formed. These micrographs are from centered locations but in
general the G3 distributor produces films with uniform structure. All
micrographs 5000x magnification.

In addition to the variety of microstructures that can be formed, the
spatial uniformity of the microstructure has also improved. Using the
standard deviation in cross-web surface roughness profiles as an
indicator, Figure 8 shows that despite the wide variety of experimental
conditions explored in ETA 807 nearly all films are more uniform across
the web than those produced with the standard distributor.

![](media/image16.wmf){width="6.0in" height="4.0in"}

[]{#__RefHeading___Toc87416830 .anchor}Figure 8. Reduction in cross-web
roughness variation for G3 distributor.

Devices have been made from CdTe films produced with the G3 distributor.
Not surprisingly the different microstructure films require different
post-deposition processing to produce the best device. We are now in the
process of determining which deposition parameters and post deposition
conditions produce the most efficient and stable devices.

We believe that the G3 system will also work well for CdS and
improvements in thickness uniformity are expected to facilitate
fabrication of devices with very thin CdS.

[]{#__RefHeading___Toc87416802 .anchor}**3.1.2 Distributor Modeling**

Despite the simplicity of the basic VTD system, the heat and mass
transport processes that occur within it are complex. Conducting
experimental trials using production equipment is becoming increasingly
difficult and inferring physical mechanisms operating within the
distributor based on resulting film characteristics is very tricky.
Therefore a program to provide a solid theoretical understanding of the
distributor operation was begun in collaboration with a group from
Colorado School of Mines (CSM) in Phase I. The goal of this effort is to
guide the experimental work of distributor design and operation.

The CSM team made significant progress on a numerical model of mass and
heat flow within the semiconductor deposition system. The sophistication
of the model has grown to include details of particle injection,
particle sublimation, and vapor transport.

Within the distributor flow field calculations are coupled to the
particles by way of mass, momentum, and energy. Convective sublimation
is approximated using the extended film model, which describes the
particle mass loss and the heat required by the sublimation process.
This sublimation model is connected to a model for tracking particle
motion through a specified flow field. Distributions of input particle
sizes can be tracked as they enter the distributor and sublime. The
model is now fully coupled. It accounts for physical motion of the
powders, radiative and convective heat transfer, and powder sublimation.
The sublimed vapor significantly alters the pressure, velocity and
composition of the gas phase which in turn affects the motion of the
injected powder.

The model demonstrates that non-uniformity can arise due to how
particles are injected and vaporized and due to mass transport in the
vapor phase. Attention is now being directed towards modeling the film
growth process. A figure of merit or growth factor that can be related
to the film growth rate is being developed. Validation with experimental
data is ongoing and many experimental observations can now be
semi-quantitatively predicted. For example, Figure 9 shows the
comparison of experimental and modeled data from a standard distributor
operated in a mode that yields non-uniform thickness films.

+-----------------------------------+-----------------------------------+
| \(a\)                             | \(b\)                             |
|                                   |                                   |
| ![](media/image17.wmf)            | ![](media/image18.j               |
|                                   | peg){width="2.8555555555555556in" |
|                                   | height="2.5in"}                   |
+-----------------------------------+-----------------------------------+

[]{#__RefHeading___Toc87416831 .anchor}Figure 9. Cross-web CdTe
thickness profile from a standard distributor operated in a non-uniform
mode (a) experimental data, (b) modeled case.

The modeling effort is currently focused on the base VTD system. Once
the growth factor is more refined, we plan to optimize the base
distributor with respect to distributor geometry and thermal operating
conditions using the overall numeric model and a partial factorial
designed experiment. This should allow us to quantify the sensitivity of
non-uniformity to various design parameters that would be impractical to
test experimentally. We envision this numeric optimization project to
help translate the results of a complicated theoretical model into
practical use.

While the newer VTD-2 systems are geometrically more complex, the
modeling of these systems is expected to be easier due to their innate
improved uniformity characteristics. Thus the extension of the numeric
model to alternative distributors is predicted to be very
straightforward and to yield higher confidence results.

[]{#__RefHeading___Toc87416803 .anchor}**[3.2 Process
Control]{.underline}**

The use of thinner CdS films is a well-known path to improve device
efficiency. Consistently controlling CdS thickness is thus an essential
task. The cross-web uniformity of VTD films is primarily controlled by
the distributor while the average thickness and the consistency of
thickness over time (i.e. down-web) are primarily determined by the
input powder feed rate. At the present time approximately half of the
variation within a plate is due to down web fluctuations.

In Phase I we developed a new feed system that was shown to reduce
variation in CdS thickness on a single plate. However the data suggested
that feed rate fluctuations over longer time scales were present. During
Phase II we continued to refine this feed system and focused on
characterization of the feed systems in bench tests rather than via CdS
film thickness maps.

Figure 10 shows some typical results of feeder bench tests comparing the
feed system in production use (the best commercially available system
that we have found) and the new feeder equipment over the course of
several hours. As in the earlier tests using CdS thickness maps, the new
feed system appears to give less fluctuation. However, the feed rate
variation in the new system is still significant and day to day
reproducibility in both feed systems remains a concern. Consequently we
have made some physical changes in the new feed system and are
conducting optimization tests aimed at reducing variation and increasing
feed rate resolution.

![](media/image19.wmf){width="6.0in" height="4.0in"}

[]{#__RefHeading___Toc87416832 .anchor}Figure 10. Performance comparison
of powder feed systems for test duration of several hours. A 56%
reduction in distribution width was achieved with the new system.

[]{#__RefHeading___Toc87416804 .anchor}**[3.3 Film
Characterization]{.underline}**

Film characterization is the essential component of the development of
high-throughput CdS/CdTe deposition reactor. Film measurements that have
been established previously include automated CdS thickness mapping,
automated CdTe roughness mapping, and manual CdTe thickness. During
Phase 2 we completed final debugging of an automated IV mapping system
and began to utilize the system for understanding spatial variation in
modules.

[]{#__RefHeading___Toc87416805 .anchor}**3.3.1 Device Analysis**

During Phase 2 work continued on expanding the use of
current-voltage-temperatures (IVT), capacitance-voltage (CV) and quantum
efficiency (QE) measurements. Ideally these measurements would aid in
the determination of loss mechanisms and related process steps.

Traditionally "equivalent circuit" parameters are extracted from IVT
data since these parameters can be related to standard semiconductor
junction models. We attempted to gain insight into loss mechanisms by
analyzing the IVT behavior of several well-differentiated device sets.
We consulted with a number of the CdTe team members including UT, CSU,
USF, ALF, and IEC. Generally the group concluded that the IVT signature
of current First Solar devices is problematic for conventional analysis
and thus extraction of equivalent circuit parameters is unreliable. We
continue to use these measurements for qualitative comparisons.

[]{#__RefHeading___Toc87416806 .anchor}**3.3.2 IV Mapping**

The IV mapping system allows spatial variation in device performance to
be determined. Any un-encapsulated module can be divided into an array
of small area cells using additional laser scribes. The cells can then
be probed automatically making use of the monolithic interconnect via.
We have used this system to identify processing problems and to assess
non-uniformity losses. For example, during the start up of a new piece
of equipment, some modules had poor performance, using the IV mapping
system we found that a consistent non-uniformity was due to the setup of
the new equipment (see Figure 11). Figure 12 shows a plate with
non-uniformity related to a wet processing step. In each case, the IV
mapping system allows us to quantitatively understand patterns of
variation within a module and is invaluable in locating the process the
source of the non-uniformity.

![](media/image20.png){width="4.883333333333334in"
height="3.4993055555555554in"}

[]{#__RefHeading___Toc87416833 .anchor}Figure 11. IV map from a poor
performing module produced during start-up of a new piece of equipment.

![](media/image21.png){width="4.883333333333334in"
height="3.4993055555555554in"}

[]{#__RefHeading___Toc87416834 .anchor}Figure 12. IV map of a plate with
non-uniformity caused in a wet processing step.

[]{#__RefHeading___Toc87416807 .anchor}**3.3.3 Non-uniformity loss
estimates**

Work on understanding non-uniformity losses has progressed on both the
experimental and theoretical fronts. Experimentally we have been
monitoring non-uniformity in thin CdS modules by mapping IV parameters
in select modules that are laser scribed into an array of isolated
cells. For example, Figure 13 shows efficiency and Jsc maps from a
moderate efficiency module. Some structure is apparent in both the X and
Y directions. We believe that the variations are from known sources. For
example in this case the vertical bands are caused by variations in CdS
thickness due to feeder pulses. A series of modules that exhibited a
range of spatial variations were carefully measured for power loss
analysis. This set included modules that were processed well off optimum
and thus some very poor modules are included. The goal of this analysis
was to quantify the amount of loss due to cell mismatch. In other words
how much loss can be attributed to the fact that local areas in an
interconnected module are not operating at their maximum power point.
Figure 14 shows the comparison between average active area cell
efficiency and the active area efficiency of the module before division
into isolated cells. The difference between the two is the mismatch
loss. Contrary to expectations, Figure 14 shows that the mismatch loss
is quite minimal in this set of modules. Consequently at the present
time, the magnitude of the improvement possible by improved uniformity
can only be attributed to the difference between the average cell and
the best cell on a plate.

On the theoretical side, the PSPICE simulation software developed at UT
is now operational. The basic results to date suggest that the mismatch
loss is small (\<10%) given the rather large values of series
resistance. Moreover the simulation suggests that the geometric pattern
does not much matter except for a few severe cases. This result is
puzzling and more work is needed to explicitly validate the PSPICE
simulation. Nonetheless, it appears that mismatch will not be a large
contributor until the average fill factor (series resistance) of the
underlying devices is improved.

![](media/image22.png){width="3.0722222222222224in"
height="2.201388888888889in"}
![](media/image23.png){width="3.0722222222222224in"
height="2.201388888888889in"}

[]{#__RefHeading___Toc87416835 .anchor}Figure 13. Data from IV maps.

![](media/image24.wmf){width="3.2194444444444446in"
height="2.203472222222222in"}
![](media/image25.wmf){width="2.9784722222222224in"
height="2.227777777777778in"}

[]{#__RefHeading___Toc87416836 .anchor}Figure 14. Power loss analysis.
Mismatch loss is the difference between module and average cell
efficiency.

[]{#__RefHeading___Toc87416808 .anchor}**3.3.4 Chemical Analysis**

A used inductively coupled plasma atomic emission spectrometer (ICP-AES)
has been acquired and refurbished. The ICP is now used for analysis of
trace impurities that influence initial and long-term device
performance. The ICP will also be used in defining specifications and
developing procedures for quality control of in-coming raw materials
such as CdS, CdTe, and CdCl~2~. Special emphasis has been focused on
monitoring the dopant added during back-contact preparation.

[]{#__RefHeading___Toc87416809 .anchor}**[4. Accelerated Life Testing
Development]{.underline}**

Stable, predictable performance of PV modules is essential for
achievement of cost-effective solar electric fields. This requires the
development of predictive accelerated life testing (ALT) protocols and
increased understanding of mechanisms of stress-induced changes in
devices.

[]{#__RefHeading___Toc87416810 .anchor}**[4.1 Development of predictive
ALT Protocols]{.underline}**

First Solar has done considerable research on accelerated life and field
testing of CdTe thin-film PV. Potential failure modes for thin-film PV
are generally thought to be associated with loss of encapsulation
integrity or exposure to combinations of light, heat, bias, and time. In
the present ALT protocol we focus on the changes due to exposure to
combinations of light, heat, bias, and time.

Ideally ALT would be quantitatively predictive of field performance.
Correlation of fast ALT protocols with field data is a formidable task
especially in an environment of continuous fabrication process change.
We are currently closely tracking \~1800 modules at 3 outdoor locations.
An equally important goal of ALT is to rapidly differentiate any device
stability differences due to process changes.

To these ends, during Phase 2 we continued to build the correlation
between the current 56 day ALT procedure and a more rapid test. The new
ALT test provides much more timely information and allows a much greater
number of samples to be tested for a given area of test bed.

The rapid test consists of subjecting devices to continuous illumination
at higher temperature and frequently recording IV measurements
*in-situ*. The normal multi-vapor metal halide lamps used in the
standard light soak stations provide the illumination. Data analysis
consists of correcting for illumination fluctuation including 60Hz
flicker and extracting a saturated efficiency value from a fitted decay
curve. The technique has been applied to cells, module segments, and
both un-encapsulated and finished modules.

The correlation of the new ALT protocol with the 56 day ALT is quite
good. In many ways the new test is a better indicator of stability than
the longer test as the more frequent sampling leads to higher confidence
in how efficiency changes with time. The new test also has more tightly
controlled module temperature and much better intensity measurements. We
have begun to build additional stations to perform the new protocol on a
greater number of samples. We have found that the ALT protocol for
encapsulated modules must be altered slightly to avoid an unrealistic
failure mode related to overheating the EVA. Thus the required test
duration is somewhat longer for finished modules.

[]{#__RefHeading___Toc87416811 .anchor}**[4.2 Stress-Induced Change
Mechanisms]{.underline}**

During Phase 1 we had a concentrated effort on finding markers and
mechanisms for stress induced changes in the device. In that work a
clear marker did not emerge and thus we took a more pragmatic approach
during Phase 2. This practical approach addressed important questions
regarding the CdCl2 heat treatment and environmental effects.

[]{#__RefHeading___Toc87416812 .anchor}**4.2.1 CdCl~2~ treatments**

Alternative CdCl~2~ treatment methods, most notably vapor CdCl~2~, have
been frequently studied. To ensure that the best possible method be used
for production VTD films, a joint experiment with members of the
National CdTe team was undertaken. The objective of this experiment was
a comparison between wet and vapor CdCl~2~ activation treatments. Each
of the participating groups received a series of production VTD
as-deposited CdS/CdTe film stacks from FS. Samples with two different
CdS thicknesses were provided. Each group performed CdCl~2~ wet and
vapor activation treatments following procedures and techniques
established at their facilities. The participants completed devices with
their specific back-contact technologies. Basic device characterization
was performed. CdCl~2~ activated material was also returned to FS for
device completion using the FS back-contact method. FS tested the
devices for long-term performance under OC light soaking at 65°C.
Results of the joint experiment were presented at the National CdTe Team
Meeting in February 2004. At FS, wet and close spaced sublimation (CSS)
vapor CdCl~2~ treatments were performed. The CSS-vapor CdCl~2~ apparatus
operates at atmospheric pressure with the source and substrate
isothermal. In general, small area devices with efficiencies \>12% were
obtained using both wet and vapor CdCl~2~ methods. Small area devices
made from material with thinner CdS often showed lower values for the
open-circuit voltage V~OC~ and fill factor FF, which may be caused by
the local consumption of CdS. Small differences in the JV-performance of
devices that received a wet or a vapor CdCl~2~ treatment were noted (see
Table 4), but a clear advantage of the vapor treatment method was not
found.

Table 4. JV-parameter for small-area devices processed with wet and
vapor CdCl~2~ treatments.

+-----------+-----+-----+------+------+---------+------+------+------+
| **Device  | *   | **  | **   | **V  | **Jsc** | **   | **R  | **R  |
| ID**      | *Cd | CdC | Effi | oc** |         | FF** | sc** | oc** |
|           | S** | l~2 | cien |      | **(mA/c |      |      |      |
|           |     | ~** | cy** | **(m | m^2^)** | **(  | **(  | **(  |
|           | *   |     |      | V)** |         | %)** | Ohm- | Ohm- |
|           | *Th | *   | **(  |      |         |      | cm^2 | cm^2 |
|           | ick | *me | %)** |      |         |      | ^)** | ^)** |
|           | nes | tho |      |      |         |      |      |      |
|           | s** | d** |      |      |         |      |      |      |
+-----------+-----+-----+------+------+---------+------+------+------+
| D10300182 | st  | wet | 1    | 800  | 23.2    | 65.0 | 1064 | 5.6  |
| I4 52     | and |     | 2.08 |      |         |      |      |      |
|           | ard |     |      |      |         |      |      |      |
+-----------+-----+-----+------+------+---------+------+------+------+
| D10300180 | t   | wet | 1    | 800  | 23.8    | 64.3 | 1550 | 6.1  |
| B4 52     | hin |     | 2.21 |      |         |      |      |      |
|           | ner |     |      |      |         |      |      |      |
+-----------+-----+-----+------+------+---------+------+------+------+
| D06270138 | st  | va  | 1    | 831  | 23.1    | 66.2 | 840  | 5.8  |
| G4 52     | and | por | 2.73 |      |         |      |      |      |
|           | ard |     |      |      |         |      |      |      |
+-----------+-----+-----+------+------+---------+------+------+------+
| D10300179 | t   | va  | 1    | 813  | 23.8    | 64.2 | 832  | 6.6  |
| F3 52     | hin | por | 2.40 |      |         |      |      |      |
|           | ner |     |      |      |         |      |      |      |
+-----------+-----+-----+------+------+---------+------+------+------+

Residues left behind by the CdCl~2~ activation treatment have to be
removed prior to back-contact application. The chemical nature and
amounts of residues depend on treatment temperature, CdCl~2~/O~2~ ratio
and the humidity level of the ambient during the treatment.

+---------------------------+------------------------------------------+
| \(a\)                     | ![](media/image4.wmf)                    |
|                           |                                          |
| ![](media/image2.wmf)     |                                          |
|                           |                                          |
| ![](media/image3.wmf)     |                                          |
+---------------------------+------------------------------------------+
| \(b\)                     | ![](media/image4.wmf)                    |
|                           |                                          |
| ![](media/image2.wmf)     |                                          |
|                           |                                          |
| ![](media/image3.wmf)     |                                          |
+---------------------------+------------------------------------------+

[]{#__RefHeading___Toc87416837 .anchor}Figure 15. AFM micrographs of
CdTe surface after: a) wet CdCl~2~ treatment and rinsing, b) CSS-vapor
CdCl~2~ treatment in room ambient without rinsing. AFM measurements
provided by IEC.

For wet CdCl~2~ treatments the appearance of CdTeO~3~ as the main
surface residue was identified by XRD-measurements; CSS-vapor CdCl~2~
treatments leave CdO and CdTeO~3~ on the CdTe surface behind (XRD
measurements supported by IEC). Adequate etching procedures to remove
these oxides and to alter the CdTe surface prior to back-contact
application are still under investigation.

In general, it appears that none of the methods for CdCl~2~ treatments
(wet & vapor) used by other team members is clearly superior to the
method we use in production. The CdTe sub-team "Chlorine Issues" that
was formed to address this question has been formally dissolved.

[]{#__RefHeading___Toc87416813 .anchor}**4.2.2 Back Contact Processing**

During Phase 2 the Process Engineering group identified an important
back contact process parameter. Modest and simple changes to this
variable result in dramatic and reproducible improvements in device
performance and a significant reduction in performance fluctuation.
Figure 16 shows the effect for a significant population of modules. A
significant amount of effort has been devoted to understanding the
physical mechanism of this and surrounding process steps. We believe
that this work may help advance the understanding of stress-induced
changes.

![](media/image5.wmf){width="6.497222222222222in" height="3.7875in"}

[]{#__RefHeading___Toc87416838 .anchor}Figure 16. Effect of newly
identified back contact process parameter.

[]{#__RefHeading___Toc87416814 .anchor}**4.2.3 Humidity effects during
device processing**

Part of the effort to understand the improved back contact method
focused on the influence of humidity during several process steps. As
has been reported by others, the ambient during the high-temperature
CdCl~2~ annealing step affects device properties. Typically the best
devices are produced when the activation process is conducted in dry air
with humidity exposure resulting in poorer open circuit voltage V~OC~
and fill factor FF. High humidity levels during CdCl~2~ processing
increase the thickness and the amounts of oxide residues on the CdTe
surface (XRD measurements supported by IEC).

Humidity exposure at other stages during back contact formation was also
found to influence device performance in tests involving small area
devices. In general, humidity caused reduction in open circuit voltage
and fill factor. The worst cases involve a large amount of "roll-over"
and a significant efficiency loss. The changes persisted during further
light soak stress. In the exploratory tests, the humidity levels and
delay times used were extreme; however actual production line effects
have also been noted. Currently the production line has a number of
delay points and tests for each of these were conducted. In the new
production line, there will only be a few points where accumulators can
delay process flow, and thus concentration on these points is underway.

We experimented with process factors with the aim of reducing the
sensitivity of device performance to ambient variation. Using extreme
humidity levels for experiments on small area devices, we found that
humidity exposure can cause "roll-over" and high efficiency losses. We
also found that there were indeed process factors downstream from the
CdCl2 step that reduced the sensitivity to ambient humidity. Optimized
devices did not develop any "roll-over" in open-circuit light-soaking
experiments: they showed the typical long-term performance comparable to
devices, which had not seen any humidity exposure during processing (see
Figure 17). In addition, slight improvements in device performance could
be achieved.

+-----------------------------------+-----------------------------------+
| a\)                               | b\)                               |
|                                   |                                   |
| ![](media/image6                  | ![](media/image7.                 |
| .wmf){width="2.301388888888889in" | wmf){width="2.3027777777777776in" |
| height="2.2895833333333333in"}    | height="2.3229166666666665in"}    |
+-----------------------------------+-----------------------------------+

[]{#__RefHeading___Toc87416839 .anchor}Figure 17. JV-curves of
small-area devices a) standard devices with and without extreme humidity
exposure during processing, b) improved devices with and without extreme
humidity exposure during processing.

**[, pp. 749-756, 1995.]{.underline}** []{#__RefHeading___Toc87416815
.anchor}**[5. Environmental, Health and Safety
Improvements]{.underline}**

First Solar is committed to producing a safe product and to operating a
safe factory. Thus Environmental, Health, and Safety (EH&S) programs are
a top priority. Three main efforts were conducted during Phase 2:
updating the safety program to accommodate the factory expansion,
ongoing monitoring, and continued efforts for hazardous waste reduction
and processing.

[]{#__RefHeading___Toc87416816 .anchor}**[5.1 Updated Safety
Program]{.underline}**

With the factory expansion, much of the EH&S effort during Phase 2 was
devoted to updating our current safety program to include the new
production equipment. Safety inspections, review of standard operating
and lockout/tagout procedures for all new equipment, as well as air
sampling and sound monitoring has been conducted.

Training is a key element of operating a safe workplace. Increased staff
to handle the increase in production means increased training needs.
During Phase 2 we completed the conversion of all our training materials
to electronic format for easier management. These materials are now in
use for new employee training and ongoing recertification and include
*Hazard Communication, Cadmium Compliance,* *Lockout Tagout*, and
*Forklift Operation*, *Radiation Quality Assurance Plan*, *Department of
Transportation Awareness Training*, and *Department of Transportation
Function* *Specific Training*.

[]{#__RefHeading___Toc87416817 .anchor}**[5.2 Ongoing
Monitoring]{.underline}**

Air sampling is frequently performed. Air monitoring includes fixed
location sampling, mobile sampling attached to individuals, and
performance assessment of HEPA filtration units. During Phase 1, we
studied the effectiveness of HEPA filters and uncovered deficiencies in
commercially available units and installation methods. We have worked on
improved methods to alert us to any HEPA filter problems. However, we
continue to primarily rely on air sampling and biannual system
performance assessment by a consultant.

Previously we had three areas within the factory where operators were
required to wear respirators due to air sampling results. The three
associated process steps were CdTe material removal from the sunny-side,
CdCl~2~ heat treating, and edge delete prior to lamination. Engineering
controls were added to these processes as part of their production
upgrade for the automated factory. Two of the processes have now been
qualified for production without the need for respirator protection and
the third process is also expected to meet this goal once the new
automated equipment becomes operational.

[]{#__RefHeading___Toc87416818 .anchor}**[5.3 Material
recovery]{.underline}**

Large scale manufacturing of CdTe-based photovoltaic modules involves
handling a significant amount of material that contains cadmium. The
largest amount of material that must be handled is scrap plates and
off-spec modules.

Previously we have used a wet process that consists of crushing the
modules, etching off the CdS/CdTe, precipitating the metals and forming
a concentrated filter cake. The filter cake can be disposed of as
hazardous waste or can be blended into other larger-volume industrial
recovery streams. The filter cake contains copper from busbar ribbons
and lead wires and typically is the material of interest. In other
words, the filter cake can be blended into copper recycling streams. The
clean glass is recycled as glass cullet. The waste also contains
tellurium which has some value. We continue to investigate pathways to
separate the Te and thus access this value.

During Phase 2 we found several return avenues for dry, as-crushed
modules including the glass. This would greatly simplify our internal
operation.

Two different companies can blend our crushed scrap in with their
existing secondary feedstocks for copper smelting. The copper contained
in the scrap is the main target. The glass is an effective substitute
for silica sand, which smelters add in quantity for slag conditioning
and thus poses no problem as a feedstock component. Since cadmium and
tellurium are co-products of copper production, these materials rejoin
normal production streams and again become commodities. Cadmium and
tellurium are only acceptable since they occur in small quantities in
our input stream which is then blended with other material recovery
streams.

We are also working with a company that is developing a process for
handling electric arc furnace (EAF) dust. EAF dust is a hazardous waste
produced in very large quantities which contains large amounts of iron,
zinc, and cadmium, and other metals. The new proprietary
pyrometallurgical process melts the EAF dust at very high temperatures
driving off the volatile metals including zinc and cadmium which are
then collected as a dust in a baghouse. The baghouse dust is rich in
zinc and will be sent to a zinc smelter or processor for refinement.
Since nearly all cadmium is derived as a byproduct of zinc production,
cadmium in the zinc stream is normal. The other outputs from the EAF
dust process are pig iron which is returned for steel and a slag that is
used in road construction as it has some unusual friction
characteristics.

Crushed photovoltaic scrap (primarily glass) could be added to the EAF
dust melt as a slag conditioner, similar to copper smelting above,
offsetting silica sand use. Our cadmium would join the zinc material
stream where it is a normal component and end up back as a normal
cadmium commodity.

[]{#__RefHeading___Toc87416819 .anchor}**[6. Future Plans]{.underline}**

First Solar is committed to commercializing CdTe-based thin film
photovoltaics. This commercialization effort centers on bringing a 25MW
automated production facility on line in the next few months. Process
improvements to achieve higher efficiency and greater durability will
also continue. The following activities are planned for phase 3 of this
subcontract.

-   Continued work on large area AFCW layers to allow the use of thinner
    CdS for higher efficiency.

-   Continued work on VTD-2 type distributors

-   Equipment and process improvements to allow for consistent formation
    of thinner VTD CdS layers.

-   Use of numeric modeling to improve distributor designs and
    understanding of VTD process.

-   Expanded testing of devices using rapid ALT procedures and
    correlation of rapid ALT results with field data.

-   Continued investigation into post deposition processing, degradation
    mechanisms, and in-process measurements.

-   Continued vigilance in training and monitoring the safety program.
    Added work on ergonomic practices.

-   Continued investigations into material recovery options.

    []{#__RefHeading___Toc87416820 .anchor}**List of Figures**

[Figure 1. Optical absorption curves of a-CdS:O films.
4](#__RefHeading___Toc87416823)

[Figure 2. Efficiency as a function of photocurrent for a set of dot
cells made on a graded CdS thickness plate without a buffer layer.
6](#__RefHeading___Toc87416824)

[Figure 3. Efficiency as a function of photocurrent for modules made
with and without ZTO buffer layer. 7](#__RefHeading___Toc87416825)

[Figure 4. SEM micrographs showing variation in CdTe film microstructure
across a plate deposited using a standard distributor operated in an
asymmetric mode. The G2 and G6 positions correspond to locations x=60,
y=20 and x=60, y=-20 respectively in Figure 5.
8](#__RefHeading___Toc87416826)

[Figure 5. Color contour map of surface roughness determined from
reflection characteristics for a film deposited with a standard
distributor operated in an asymmetric mode. Note that Y is the cross-web
dimension, i.e. the direction of motion of the plate is in the X
direction. 9](#__RefHeading___Toc87416827)

[Figure 6. Reduction in CdTe thickness variation cross web for G3
distributor. On average there is a 37% reduction in cross-web variation
using the G3 system. 10](#__RefHeading___Toc87416828)

[Figure 7. SEM micrographs of films deposited using the G3 distributor.
A variety of microstructures can be formed. These micrographs are from
centered locations but in general the G3 distributor produces films with
uniform structure. All micrographs 5000x magnification.
11](#__RefHeading___Toc87416829)

[Figure 8. Reduction in cross-web roughness variation for G3
distributor. 12](#__RefHeading___Toc87416830)

[Figure 9. Cross-web CdTe thickness profile from a standard distributor
operated in a non-uniform mode (a) experimental data, (b) modeled case.
13](#__RefHeading___Toc87416831)

[Figure 10. Performance comparison of powder feed systems for test
duration of several hours. A 56% reduction in distribution width was
achieved with the new system. 15](#__RefHeading___Toc87416832)

[Figure 11. IV map from a poor performing module produced during
start-up of a new piece of equipment. 16](#__RefHeading___Toc87416833)

[Figure 12. IV map of a plate with non-uniformity caused in a wet
processing step. 17](#__RefHeading___Toc87416834)

[Figure 13. Data from IV maps. 18](#__RefHeading___Toc87416835)

[Figure 14. Power loss analysis. Mismatch loss is the difference between
module and average cell efficiency. 18](#__RefHeading___Toc87416836)

[Figure 15. AFM micrographs of CdTe surface after: a) wet CdCl~2~
treatment and rinsing, b) CSS-vapor CdCl~2~ treatment in room ambient
without rinsing. AFM measurements provided by IEC.
21](#__RefHeading___Toc87416837)

[Figure 16. Effect of newly identified back contact process parameter.
22](#__RefHeading___Toc87416838)

[Figure 17. JV-curves of small-area devices a) standard devices with and
without extreme humidity exposure during processing, b) improved devices
with and without extreme humidity exposure during processing.
23](#__RefHeading___Toc87416839)

[]{#__RefHeading___Toc87416821 .anchor}**Glossary of Abbreviations**

AFM Atomic Force Microscopy

ALF Alan Farhenbruch consulting

ALT Accelerated Life Test

CdS Cadmium Sulfide

CdTe Cadmium Telluride

CSM Colorado School of Mines

CSU Colorado State University

CV Capacitance-Voltage measurement

DAP Donor-Acceptor Pair

EH&S Environmental, Health and Safety

FF Fill Factor

FS First Solar, LLC.

IEC Institute of Energy Conversion at the University of Delaware

I-V Current-Voltage measurement

IVT Current-Voltage measurements as a function of temperature

J~sc~ Short circuit current density

NREL National Renewable Energy Laboratory

OC Open Circuit

PL Photoluminescence

PV Photovoltaics

Roc Dynamic resistance at open circuit

Rsc Dynamic resistance at short circuit

SC Short Circuit

TCO Transparent conducting oxide

USF University of South Florida

UT University of Toledo

V~Cd~ Cadmium vacancy

V~oc~ Open circuit voltage

VTD Vapor Transport Deposition

**[References]{.underline}**

**\[1\] X. Wu and P. Sheldon, \"Novel Manufacturing Process for
Fabricating CdS/CdTe Polycrystalline Thin-Film Solar Cells,\" presented
at Sixteenth European Photovoltaic Solar Energy Conference, Glasgow,
United Kingdom, 2000.**

**\[2\] X. Wu, R. G. Dhere, Y. Yan, M. J. Romero, Y. Zhang, J. Zhou, C.
DeHart, A. Duda, C. Perkins, and B. To, \"High-Efficiency
Polycrystalline CdTe Thin-Film Solar Cells with an Oxygenated Amorphous
CdS (a-CdS:O) Window Layer,\" presented at Twenty-Ninth IEEE
Photovoltaic Specialists Conference, New Orleans, Louisiana, 2002.**

**\[3\] R. Powell, G. Dorer, N. Reiter, H. McMaster, S. Cox, and T.
Kahle, \"Apparatus and method for depositing a material on a substrate,
U.S. Patent \# 5,945,163.\" U.S.: First Solar, LLC., 1999.**

**\[4\] R. Powell, G. Dorer, N. Reiter, H. McMaster, S. Cox, and T.
Kahle, \"Apparatus and method for depositing a semiconductor material,
U.S. Patent #6,037,241.\" U.S.: First Solar, LLC, 2000.**

[^1]: ^†^ Cross-web denotes transverse to the direction of motion in the
    deposition system and down-web refers to in the direction of motion
    in the deposition system.