**ANNUAL REPORT**

**to**

**National Renewable Energy Laboratory**

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

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

**Deliverable D.1.6**

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

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

**PRINCIPAL INVESTIGATORS:** 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 2003

**Table of Contents**

[**Abstract** ii](#__RefHeading___Toc57187409)

[**Acknowledgements** iii](#__RefHeading___Toc57187410)

[**1. Introduction** 1](#__RefHeading___Toc57187411)

[**2. Deposition Reactor Development** 2](#__RefHeading___Toc57187412)

[**2.1 Distributor Development** 2](#__RefHeading___Toc57187413)

[**2.1.1 Alternative Distributors** 3](#__RefHeading___Toc57187414)

[**2.1.2 Distributor Modeling** 6](#__RefHeading___Toc57187415)

[**2.2 Process Control and Development** 7](#__RefHeading___Toc57187416)

[**2.3 Film Characterization** 9](#__RefHeading___Toc57187417)

[**2.3.1 Optical Reflectance** 10](#__RefHeading___Toc57187418)

[**2.3.2 Time-Resolved Photoluminescence**
10](#__RefHeading___Toc57187419)

[**2.3.3 Expanded Device Measurements** 12](#__RefHeading___Toc57187420)

[**2.3.4 IV Mapping** 13](#__RefHeading___Toc57187421)

[**2.3.5 Non-uniformity loss estimates**
14](#__RefHeading___Toc57187422)

[**3. Accelerated Life Testing Development**
15](#__RefHeading___Toc57187423)

[**3.1 Development of predictive ALT Protocols**
15](#__RefHeading___Toc57187424)

[**3.2 Stress-Induced Change Mechanisms**
19](#__RefHeading___Toc57187425)

[**3.2.1 Monitoring of the electrical response of stressed devices**
20](#__RefHeading___Toc57187426)

[**3.2.2 Analysis of degraded devices** 20](#__RefHeading___Toc57187427)

[**3.2.3 Device modeling** 23](#__RefHeading___Toc57187428)

[**3.2.4 Investigation of material changes due to impurity ingress**
25](#__RefHeading___Toc57187429)

[**3.2.5 Identification and modification of back-contact chemistry**
26](#__RefHeading___Toc57187430)

[**4. Environmental, Health and Safety Improvements**
28](#__RefHeading___Toc57187431)

[**4.1 Dust Emissions** 28](#__RefHeading___Toc57187432)

[**4.2 Recycling** 30](#__RefHeading___Toc57187433)

[**4.2.1 Module Recycling** 30](#__RefHeading___Toc57187434)

[**4.2.2 Recycling of bulk CdS/CdTe** 30](#__RefHeading___Toc57187435)

[**4.2.3 Personal protective equipment disposal**
30](#__RefHeading___Toc57187436)

[**4.2.4 Reduction of Cd leaching risk**
31](#__RefHeading___Toc57187437)

[**4.3 Training** 32](#__RefHeading___Toc57187438)

[**4.4 Industrial Hygiene Program Validation**
33](#__RefHeading___Toc57187439)

[**5. Future Plans** 34](#__RefHeading___Toc57187440)

[**List of Figures** 35](#__RefHeading___Toc57187441)

[**Glossary of Abbreviations** 36](#__RefHeading___Toc57187442)

[**References** 37](#__RefHeading___Toc57187443)

[]{#__RefHeading___Toc57187409 .anchor}**[Abstract]{.underline}**

First Solar is committed to commercializing CdTe-based thin film
photovoltaics. This commercialization effort includes a major addition
of floor space and equipment as well as process improvements to achieve
higher efficiency and greater durability. This report presents the
results of phase I of the subcontract entitled **"**Research Leading to
High Throughput Manufacturing of Thin-Film CdTe PV Modules". The
subcontract supports several important aspects needed to begin high
volume manufacturing including further development of the semiconductor
deposition reactor, advancement of accelerated life testing methods and
understanding, and improvements to the environmental, health and safety
programs.

Progress in the development of the semiconductor deposition reactor was
made in several areas. First, a new style of vapor transport deposition
distributor with simpler operational behavior and the potential for
improved cross-web uniformity was demonstrated. Second, an improved CdS
feed system that will improve down-web uniformity was developed. Third,
the core of a numerical model of fluid and heat flow within the
distributor was developed including flow in a 3 component gas system at
high temperature and low pressure and particle sublimation.

Progress in several measurement systems that support both deposition
reactor development and accelerated life testing includes device
performance mapping and bandgap determination from optical reflectance.
Progress on the core of numeric model for the quantitative assessment of
the effects of macro-non-uniformity was also made.

A rapid (3 day) accelerated life test procedure is under development and
initial correlation results indicate that the current 56 day test could
be replaced. Advancement in understanding stress induced change
mechanisms included 1) the elimination of several potential mechanisms,
2) increased sophistication of capacitance-voltage interpretation, 3)
identification of several potential degradation markers, and 4) the
selection of improved interface control procedures. We did not find a
clear mechanism and marker for the stress induced changes investigated
however.

Several significant improvements in our Environmental , Health and
Safety program were achieved including 1) a reduction in solid waste
generation, 2) development of methodology for testing hazardous dust
filtration apparatus, 3) investigation of a leaching mitigation
strategy, and 4) initiation of a third party verification of our
industrial hygiene program. Solid waste reduction was significant with
the conversion from disposal to recycling of over 14,000 pounds in 4
months.

Work under this subcontract contributes to the overall manufacturing
operation. During phase I, average module efficiency on the line was
improved from 7.1 to 7.9% due primarily to increased photocurrent
resulting from a decrease in CdS thickness. At the same time production
volume for commercial sale increased from 1.5 to 2.5 MW/yr.

[]{#__RefHeading___Toc57187410
.anchor}**[Acknowledgements]{.underline}**

We gratefully acknowledge the contributions of First Solar employees
including, Anke Abken, Dave Berger, Eugene Bykov, Ken Cherry, John
Christiansen, Todd Coleman, Tony Draper, Andy Gray, Upali Jayamaha,
Peter Meyers, Anne Moser, Nick Reiter, Ken Smigielski, 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 Dave Albin, Sally Asher,
Pat Dippo, Tim Gessert, Tom McMahon, Glenn Teeter, Harin Ullal, Suhuai
Wei, Xuanzhi Wu, and Ken Zweibel. Contributions from Steve Hegedus and
Brian McCandless of Institute of Energy Conversion; Jim Sites and Samuel
Demtsu of Colorado State University; Alan Farhenburch of ALF; Alvin
Compaan, Victor Karpov, and Diana Shvydka of University of Toledo; Scott
Feldman, Scotty Gilmore, Victor Kaydanov and Tim Ohno of Colorado School
of Mines; and Chris 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, Robert Kee, and Mark 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 is being performed by Victor Karpov and Diana
Shvydka at University of Toledo.

[]{#__RefHeading___Toc57187411 .anchor}**[1. Introduction]{.underline}**

First Solar has developed proprietary thin-film PV module manufacturing
technology and has employed that technology in a new module
manufacturing plant with an expected capacity of 25 MW/yr. First Solar's
plan for rapid, large-scale commercialization of thin-film PV requires
significant efforts in a number of areas. Support from this contract
addresses three major components -- 1) research on enhancements to the
high-throughput CdS/CdTe deposition reactor, 2) Accelerated Life Test
(ALT) development and research of stress-induced changes in devices, and
3) enhanced environmental, health and safety (EH&S) technology.

Motivation for research on the deposition reactor stems from a
relatively young deposition technique for the main semiconductor
junction, the need for consistent operation during the production of
\~10^4^ plates between maintenance cycles, and persistent uniformity
issues over large areas.

ALT testing is critical to assure the 20+ year performance that
underlies the economics of PV systems.

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.

Related work that is supported with other funding sources (primarily
internal) includes:

-   Improved transparent conducting layers with buffer layers for use
    with thin CdS to increase module efficiency

-   Improved encapsulation processes to eliminate the risk of
    encapsulation failure

-   Next-generation scribing for reduced costs and increased module
    efficiency

-   New recrystallization approaches, possibly providing improved
    process robustness and product reliability

-   Production process development for increased module efficiency,
    yield and throughput

During this contract year two additional items are worth noting. First
as part of an expansion of the production facility, it was decided to
build a new semiconductor deposition system based on the success of the
semiconductor deposition reactor. This decision was taken primarily to
improve the uptime of the semiconductor coating equipment. Second, in
partnership with NREL, work on the VTD method has been recognized with a
2003 R&D 100 award.

[]{#__RefHeading___Toc57187412 .anchor}**[2. Deposition Reactor
Development]{.underline}**

[]{#__RefHeading___Toc57187413 .anchor}**[2.1 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 \[1, 2\]. 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. As
currently employed, CdTe films are deposited with average growth rates
\> 1 µm/s and peak rates estimated to be \~20 μm/s. 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 \~1.9 m/min 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. In fact, a recent distributor
was used to coat \~17500 plates without maintenance.

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.

Once the overall system reaches steady state, very consistent films are
produced. The uniformity of film properties is remarkably good given the
simplicity of the distributor and the width of the substrate. CdS and
CdTe films with adequate thickness uniformity (±10%) can be produced
with the current system. However, device performance varies as a
function of position across a plate. In addition, differences between
nominally identical distributors and distributor aging effects have been
observed. Moreover, in the base distributor design thickness uniformity
depends on carrier gas flow. Consequently, carrier gas flow is adjusted
to produce the best possible film thickness uniformity and cannot be
altered to change dilution or improve input powder transport.

Research on enhancements to the high-throughput CdS/CdTe deposition
reactor has 3 main components: the design of improved distributors,
process development, and thin film characterization. It is well known
that spatial variation in thin film coatings can lead to a number of
parasitic losses. Consequently significant effort has been directed
towards measuring, improving, and assessing the impact of film
uniformity.

[]{#__RefHeading___Toc57187414 .anchor}**2.1.1 Alternative
Distributors**

At the heart 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 design is simple both mechanically and thermally.
Film thickness uniformity depends on carrier gas flow and feed injection
location. Moreover the dependence is not intuitive and experience is
required for uniformity adjustment. A number of possible alternative
designs have been investigated, but lack of understanding of distributor
operation was one major obstacle limiting progress. In other words, the
rudimentary modeling that had been employed to aid the distributor
design process was incapable of describing the central features of base
distributor operation.

Therefore we began an effort to provide a solid theoretical base for
distributor operation as will be described below. The first major result
of this modeling highlighted some of the subtleties of vapor flow in a
low-pressure, three component gas system. This insight helped guide the
selection of alternative distributor designs.

During this contract period, three general types of alternative
distributors were tested. The first type involves variation of the base
distributor, primarily in the manner in which powder and carrier gas is
injected into the vaporizer. The distributor modeling strongly suggested
that if two different configurations could be experimentally
implemented, that significant improvements were possible. The
theoretically most desirable configuration was built and tested. The
result was a minor change to base distributor behavior. Apparently the
needed conditions were not achieved with the particular experimental
implementation. Since no other straightforward arrangement was obvious,
the design path was abandoned. The second variation on the base
distributor theme is still under investigation. Early tests indicate
some significant change to distributor behavior but a clear advantage
given the added complexity has not yet been demonstrated.

The second general type of alternative distributor will be designated as
VTD-2. In this concept, the problems that arise in a low-pressure, three
component gas system are practically eliminated in a robust manner. The
VTD-2 idea essentially separates the vaporization and distribution
functions of the system. It is intuitively appealing and has been a
candidate for some time. Input from the theoretical modeling effort has
bolstered confidence for this concept. Several different configurations
of the VTD-2 idea have been tested. The basic operation of a VTD-2 style
distributor is fundamentally different from the base distributor
behavior. Here the film thickness uniformity is controlled by
distributor geometry and is nearly independent of carrier gas flow (see
Figure 1 and Figure 2). The VTD-2 concept necessitates distributors with
a greater degree of mechanical and thermal complexity however. As will
be described below, the thermal configuration now becomes the central
challenge for a practical distributor.

![](media/image1.wmf)

[]{#__RefHeading___Toc57187444 .anchor}Figure 1. Cross-web thickness
uniformity for VTD-1 type distributor.

![](media/image2.wmf)

[]{#__RefHeading___Toc57187445 .anchor}Figure 2. Cross-web thickness
uniformity for VTD-2 type distributor.

The third type of alternative distributor is a combination of VTD with a
batch load source crucible. This type of distributor is under
consideration for CdS only since the material usage rates are low. For
very thin films of CdS, input powder metering becomes a significant
issue for VTD systems (see below). In this case a batch load system may
be a practical alternative, however very tight thermal control is
required to control film thickness. For CdTe the greater material usage
rate favors continuous material input systems. A batch load VTD
distributor system for CdS was built and tested. Good quality films were
easily obtained although film thickness uniformity was not adequate. The
experiment pointed the path to improved uniformity, however further work
will be delayed indefinitely. The direct control of film thickness via
powder input rate provided by the standard VTD system is very convenient
and even for thin CdS films powder metering has not been shown to be too
cumbersome.

![](media/image3.wmf)

[]{#__RefHeading___Toc57187446 .anchor}Figure 3. Cross-web thickness
uniformity for Batch Load Type CdS distributor.

Several versions of the VTD-2 concept were built and tested. The greater
mechanical and thermal complexity of the VTD-2 concept raises additional
issues. For example, mechanical tolerances between distributor
components at temperature must be considered. The most significant issue
however is a thermochemical one. Te~2~ and S~2~ vapors at high
temperature are very aggressive and can attack high temperature
components. The corrosion per se is not troublesome, but volatile
impurities can form and contaminate the semiconductor layers or disrupt
deposition. While this impurity pathway can occur in a base distributor,
significant effects do not occur until operation at \~200ºC above normal
operating conditions.

Several of the VTD-2 designs tested had regions where the temperature
limit was exceeded and thus film contamination was observed. Newer
designs are careful to obey the temperature limit.

[]{#__RefHeading___Toc57187415 .anchor}**2.1.2 Distributor Modeling**

Despite the simplicity of the basic VTD system, the heat and mass
transport processes that occur within it are complex. A program to
provide a solid theoretical understanding of the distributor operation
was begun in collaboration with a group from Colorado School of Mines.
The goal of this effort is to guide the experimental work of distributor
design and operation. The distributor function separates into 4 major
components: vaporization, fluid flow, heat transfer and deposition. The
approach is to first model each functional component separately and then
integrate the components.

The first function addressed was fluid flow. Two complementary models
have been employed: plug flow and the "Dusty Gas Model". Plug flow is a
quasi-one-dimensional approximation within each layer of the distributor
and accounts for the flow from one layer to the next using source and
sink terms. With the appropriate form of the source and sink terms, both
sublimation and geometrical aspects of the distributor can be handled.
The Dusty Gas Model is used where appropriate for source and sink terms.
The overall fluid flow model has been used without sublimation to study
the flow distribution in several distributor designs and to explore the
effects of dimensions, geometries, and flow rates. The fluid flow model
has also helped in understanding fluid flow in a 3 component gas at high
temperatures and low pressure. Elements of the fluid flow model will
also be used to treat the mass transport in the gas above the deposition
surface.

Heat transfer within the distributor has been modeled using commercially
available finite element software. Two aspects of the heat flow have
been investigated with 2-dimensional approximations. The first was
investigation of axial heat flow at the distributor ends. This was done
to simulate thermal edge effects and to investigate the possibility of
sourcing impurities from components at the edge of the hot zone. We also
used thermal modeling to evaluate excessive temperature conditions for
various heating options in a VTD-2 distributor.

Using the fluid flow model, we have found that the vapor source location
can have a strong influence on the axial vapor density uniformity for
some distributor types. We also know that the thermal budget in the
distributor has a variety of effects. Therefore a sublimation model of
the powder injection is being developed. When complete, this model will
track the path the powder particles take through the distributor as they
sublime. This model will be capable of using a non-uniform size
distribution of particles and allow for multiple injection points of
particles and carrier gas. The flow field calculations in the model will
be coupled to the particles by way of mass, momentum, and energy.
Currently, the model has the ability to track a single particle through
a specified three-dimensional flow field using an approximation for the
sublimation rate. We are currently looking at ways to determine the
sublimation rate based on the reaction rates, particle size,
temperature, velocity, and the conditions in the surrounding gas.

[]{#__RefHeading___Toc57187416 .anchor}**[2.2 Process Control
and]{.underline}** **[Development]{.underline}**

Testing of alternative distributors involves both process development
and new process control protocols. In addition, there are a number of
process tasks that are common to all VTD deposition systems. This
section addresses the optimization and control of film thickness.

The use of thinner CdS films is a well-known path to improve device
efficiency. During this contract year the production target for CdS
thickness has been reduced from 3800 to 1200 Å. This has resulted in a
significant gain in efficiency through improved photocurrent. Further
reductions in CdS thickness are expected to yield more efficiency gains.

The on-line measurement system for CdS thickness via optical absorption,
first developed under the previous Thin Film Partnership contract, was
improved. The primary source of noise was found to be absorption from
stray particulates in the input optical beam path. Shields and
condensation traps were added to eliminate this noise source. In
addition, the input laser stability was improved by the addition of
temperature control modules and optical feedback. Despite the
improvements in the on-line system, output is not reliable enough to
incorporate into a simple closed loop control scheme. Thus the system
currently monitored by operators.

The cross-web uniformity of VTD films is primarily controlled by the
distributor while the consistency of thickness over time (i.e. down-web)
is determined largely by the control of input powder feed rate. For
thick films, regular feed pulses common to many powder feeders are
frequent enough compared to the characteristic time to sublimate that no
effect is seen in the deposited film. Effectively the feed pulses are
smoothed out in the vaporization section of the distributor. However,
for low feed rates needed for thin films the time between powder pulses
can be longer than the characteristic sublimation time. For example,
Figure 4 shows variations in CdS film thickness due to feeder pulses.

![](media/image4.png){width="4.896527777777778in"
height="3.5083333333333333in"}

[]{#__RefHeading___Toc57187447 .anchor}Figure 4. CdS thickness map from
a typical film deposited onto a 60 cm x 120 cm substrate using the
commercially available powder feeder currently employed in production.

The powder feed rate needed to form CdS films with thickness \<1000Å at
our production line speed is \~1gm/min. Controlling the powder feed at
this level is a formidable task. We have developed a custom powder
feeder based on commercial components in order to eliminate feed pulses.
Figure 5 illustrates the effectiveness of this equipment. Work continues
however to ensure that longer term fluctuations in feed rate can be
managed. Several options for in-line monitoring of powder flow are under
investigation.

![](media/image5.png){width="4.947916666666667in"
height="3.527083333333333in"}

[]{#__RefHeading___Toc57187448 .anchor}Figure 5. CdS thickness map from
a film deposited onto a 60 cm x 120 cm substrate using the alternative
powder feeder under development.

Typically we use a target CdTe film thickness \~3.5µm. This value was
determined from early experimental optimization. Theoretically no
additional optical absorption within CdTe will occur after the first 1
to 1.5µm thickness. Thus we conducted several production scale tests
using CdTe films from \<2 to 5 µm thick. Some conflicting results were
found but essentially based on initial efficiency 2µm thick layers
produced similar devices to normal thickness films. More extensive
experiments are in progress to determine the behavior of thin absorber
devices in accelerated life tests.

[]{#__RefHeading___Toc57187417 .anchor}**[2.3 Film
Characterization]{.underline}**

Film characterization is the essential third component of the
development of high-throughput CdS/CdTe deposition reactor. It is also
needed to support the work of accelerated life testing described in the
next section. Film measurements that have been established previously
include automated CdS thickness mapping, automated CdTe roughness
mapping, and manual CdTe thickness. Techniques that have been developed
or investigated during this contract include time-resolved
photoluminescence, bandgap determination via optical reflectance, and
automated IV mapping. Many measurement techniques can detect differences
in samples but establishing a quantitative correlation to module
performance is frequently lacking. As part of the goal to reduce
parasitic losses, we have begun working with a group from the University
of Toledo on assessing the impact of film non-uniformity.

[]{#__RefHeading___Toc57187418 .anchor}**2.3.1 Optical Reflectance**

Optical spectral reflectance of CdTe is a potential parameter to assess
large-area film uniformity. The optical reflectance from the front
(glass) side of the substrate exhibits a step change as the energy of
the probe light is scanned over the CdTe bandgap energy, E~g~. An
estimate of the CdTe bandgap energy can be found from the inflection
point of the step change. We have found consistent differences in
bandgap as a function of CdCl~2~ heat treatment. These are interpreted
as shifts due to alloying with CdS. For small samples, a Varian CARY 500
spectrophotometer with an integrating sphere and an enclosed sample
chamber is used. In order to map a full size plate, we use a CVI
Spectral Products SM-GT2 spectrometer with a spectral range of 1.42 to
1.58 eV. This spectral range includes the fundamental transition of the
as-deposited and CdCl~2~-treated CdTe. The equipment can operate in
ordinary room light and spectra are acquired in a few seconds. Figure 6
shows an example of the bandgap energy inferred from optical reflectance
data as a function of cross-web position. This data suggests that some
alloying (E~g~\~1.47 eV) has occurred at some locations in as-deposited
films.

![](media/image6.wmf){width="4.2131944444444445in"
height="3.417361111111111in"}

[]{#__RefHeading___Toc57187449 .anchor}Figure 6. Bandgap energy of CdTe
films as a function of cross web position.

[]{#__RefHeading___Toc57187419 .anchor}**2.3.2 Time-Resolved
Photoluminescence**

Measurements of electronic properties are important in order to analyze
CdS and CdTe films deposited and processed under various conditions.
Measurement of the excess minority carrier lifetime of CdS and/or CdTe
via photoluminescence (PL) decay is an attractive metric as it is
non-destructive and does not require a finished device. Recombination
dynamics of the excess carriers are influenced by many factors including
material quality, purity, band alignment, doping, stress and disorder.
For many devices minority carrier lifetime is expected to directly
relate to device performance. The technique is established for Si-based
technologies but is not common for thin-film heterostructure PV.

A preliminary evaluation of the excess minority carrier lifetime of CdTe
indicated a correlation between device V~oc~ and τ~n~ of CdTe measured
after CdCl~2~. We thus prepared a set of nominally identical CdS & CdTe
samples and varied only the CdCl~2~ anneal temperature to affect a
change in device V~oc~ and PL decay. The lowest anneal temperature
resulted in inadequate treatment, and the highest anneal temperature
resulted in damaged devices.

For PL decay measurements, the CdTe was excited from the sunny side with
a 636 nm light at a repetition rate of 20 MHz and an average excitation
power of 0.1 mW. A band pass filter at 850±25 nm was used to suppress
background and scattered excitation light. The time-resolved
luminescence decay data was fit with a 3-exponential decay model to
obtain fast and bulk decay lifetime. Since the excitation was from the
CdS/CdTe interface, we expected that the fast decay would be a
representative of the quality of the material within the depletion width
of CdTe.

Error: Reference source not found shows average dot-cell V~oc~ versus
minority carrier lifetime obtained from the PL decay measurements. The
device V~oc~ increased monotonically with increasing lifetime for the
lower temperature CdCl~2~ anneals. However, there seems to be an optimum
lifetime above which the V~oc~ drops. This set of data suggests that the
device V~oc~ is not dominated by minority carrier lifetime limitations.
The unexpected behavior was confirmed by measurements from the sister
samples at NREL.

In general PL lifetime is useful to investigate factors that relate to
recombination dynamics such as impurity density, recombination center
density, grain boundary passivation, and electric field strength. The
interaction between these factors and the PL decay measurement factors
(excitation energy, emission energy, excitation intensity, excitation
history and substrate temperature) is complex and is not uniquely
correlated with device properties. In other words, the PL decay lifetime
did not qualify as a useful metric and we plan no further activity at
this time.

![](media/image7.wmf){width="5.595138888888889in"
height="2.5305555555555554in"}

[]{#__RefHeading___Toc57187450 .anchor}Figure 7. Dot-cell V~oc~ versus
excess minority carrier lifetime of CdTe

[]{#__RefHeading___Toc57187420 .anchor}**2.3.3 Expanded Device
Measurements**

Standard device performance measurements (light IV) constitute the vast
majority of all measurements taken. We have occasionally used other
techniques including IV as a function of temperature (IVT), quantum
efficiency (QE), QE as a function of voltage (QEV), and capacitance
voltage (CV). After consultation with CdTe team partners at IEC, CSU,
USF, and ALF, we settled on an essential set of measurements that are
expected to yield a relatively complete assessment of device operation.
The goal of the assessment is to quantify sources of efficiency loss and
to relate these to specific process steps. One example would be to
quantify macroscopic spatial non-uniformity differences. The essential
set includes IVT and QEV. Additional IV measurements as a function of
intensity may be included.

From these measurements, we extract parameters that characterize the
main junction, the back contact barrier, and voltage dependent current
collection. Many devices that have been exposed to extreme stress, made
with processing extremes, or failed for various reasons, exhibit very
non-ideal device performance. The analysis algorithms for meaningful
parameter extraction must be robust, and work continues in this area.
For example, Figure 8 shows a device with a significant back contact
barrier which was determined to be 0.36 eV in magnitude. Figure 9 shows
the QEV characteristics of the same device. This data suggests that red
response of the device could be improved. We are beginning to
systematically make these measurements and they are expected to provide
significant insight for the characterization the deposition and post
deposition processes.

![](media/image8.wmf)

[]{#__RefHeading___Toc57187451 .anchor}Figure 8. IVT measurement of
sample with apparent contact barrier.

![](media/image9.wmf)

[]{#__RefHeading___Toc57187452 .anchor}Figure 9. Example of quantum
efficiency as a function of bias voltage.

[]{#__RefHeading___Toc57187421 .anchor}**2.3.4 IV Mapping**

IV mapping is a powerful technique that can be used to detect variation
patterns in processing and locate problem areas during failure analysis.
During this contract period, we completed a system to automatically
measure small area cells formed from standard unencapsulated modules.
Essentially any unencapsulated module can be divided into small areas by
adding additional laser scribes perpendicular to the standard
interconnect structure. These so-called interconnect cells ("i-cells")
make use of the standard interconnect via that connects the back contact
of a cell to the front contact of a neighboring cell. Consequently pogo
pin contactors can be used to probe any cell on the plate. Figure 10
shows an example output from this system. Since full IV curves are taken
at each point, summary maps of any of the standard IV parameters can be
obtained.

![](media/image10.png){width="4.016666666666667in"
height="2.873611111111111in"}

[]{#__RefHeading___Toc57187453 .anchor}Figure 10. Example IV map from a
module that failed light soak testing. In this case every 4^th^ cell was
measured.

[]{#__RefHeading___Toc57187422 .anchor}**2.3.5 Non-uniformity loss
estimates**

In principle, one should be able to reconstruct the full module
performance characteristics given the IV mapping data. Once the
numerical reconstruction methodology has been verified, it can be used
to quantitatively assess the impact of various types of non-uniformity.
Moreover, working together the mapping and reconstruction techniques
should also be able to assess the impact on non-uniformity on stability.
Thus we have begun a project working with a group at the University of
Toledo to develop the numeric reconstruction tools.

[]{#__RefHeading___Toc57187423 .anchor}**[3. 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___Toc57187424 .anchor}**[3.1 Development of predictive
ALT Protocols]{.underline}**

First Solar has done considerable research on accelerated life and field
testing of CdTe thin-film PV both independently and in cooperation with
other CdTe National Team members. Potential failure modes for thin-film
PV are generally thought to be associated with loss of encapsulation
integrity or exposure to some combination 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.

Previous work has primarily been directed toward the validation of
reasonable field performance and the correlation between existing light
soak procedure (continuous exposure, \~0.7 sun intensity, open circuit
bias, \~65ºC for 56 days) and field performance. This approach has a
number of deficiencies including the time, space and power needed to
conduct the light soak tests, the relatively few measurements obtained
during the test, and the lack of predictive capability. The length of
the light soak protocol necessarily limits the number of samples that
can be tested and thus many conclusions are drawn from an insufficient
population sampling. Moreover in the duration of the test the
performance of many samples does not approach an asymptote which further
complicates analysis. In addition the current light soak system has poor
temperature control over the long duration.

Previously we presented data which showed some correlation (with an
acceleration factor of \~60) between performance of cells in
elevated-temperature, open-circuit light soak and performance of modules
in the field \[3\]. While this correlation was taken as an indication
that light soak is a legitimate ALT, large scatter in the data and known
problems about the data prevented the result from being quantitative or
definitive. Known problems include poorly paired field and laboratory
samples and field data that was not corrected for time and temperature
in the field.

Consequently we have split ALT development into two stages. The first
step is to establish a test that would reliably give the same prediction
the existing 56 day protocol, except faster and more consistent. The
second step is to select a rapid test protocol that has good correlation
to field performance. Presumably the rapid, field-correlated protocol
will be similar to the first rapid test, but may have different applied
stresses. To perform this work, we have set aside laboratory samples
that are paired with modules that have been field deployed.

The ALT apparatus includes a Xe light source and electronics for
measuring 16 isolated by laser scribe devices (i-cells) simultaneously
in the temperature range between 40º and 135ºC with applied bias in the
range from -2 to +2 Volts. I-V data sets are recorded continuously
during the stressing at temperature. The key to accelerate degradation
processes is testing at elevated temperature with continuous monitoring
of the data.

In order to correlate the existing light soak procedure with a new ALT
protocol, several modules from different batches were cut in half and
the first half was light soaked using current procedure while the second
half was used to make set of coupons. Each of the coupons has at least
16 small area devices (\~1 cm^2^) for ALT. To resolve issue of the
spatial difference in degradation and performance, coupons were taken
from several locations across the half-module.

Characteristic decay of the efficiency during ALT and corresponding
exponential decay fits are presented on the Figure 11 for cells stressed
at 125ºC and open circuit (OC) for 3 days. The frequently sampled data
allows one to reliably fit an analytic form to the data. Typically the
fit used is an exponential decay to an asymptotic (saturation) value.
The value of the asymptote is used to summarize the data. The
distribution of saturation values from small area cells tested at high
temperature is used to compare to data from interconnected modules.

![](media/image11.jpeg){width="5.790277777777778in"
height="4.519444444444445in"}

[]{#__RefHeading___Toc57187454 .anchor}Figure 11. Characteristic decay
of the relative efficiency during ALT and exponential fits.

Light soak testing of modules consists of exposure to continuous
illumination at \~65ºC under OC conditions for 56 days. Modules are
removed from the stress and their performance tested every 7 days. Again
the data is fit to an exponential decay with an asymptotic value form.
The saturation value of the module data from standard light soak is then
compared to the distribution of saturation values found in the high
temperature ALT test for small cells. Figure 12 shows an example of this
comparison. In this case, the saturated module efficiency extracted was
0.957 of the maximum value during the test while the distribution median
for the high temperature small area cells was 0.94.

![](media/image12.jpeg){width="5.2444444444444445in" height="4.0875in"}

[]{#__RefHeading___Toc57187455 .anchor}Figure 12. Current light soak vs.
ALT prediction comparison for the same module

The correlation between the existing light soak procedure and the new
ALT protocol is presented in the Figure 13. While more data is clearly
needed, the new ALT protocol appears promising and suggests that a high
temperature 3 day test could replace the current 56 day test.

![](media/image13.jpeg){width="5.1194444444444445in"
height="3.926388888888889in"}

[]{#__RefHeading___Toc57187456 .anchor}Figure 13. Current light soak
procedure prediction vs. ALT prediction for the saturated efficiency
value.

In addition to the current ALT analysis procedure as described above, we
estimated an activation energy for the degradation process for different
devices, which can be used to relate to different failure mechanisms
\[4\]. As an example, Figure 14 shows activation energy obtained for
three devices from the same module. In this case voltage bias was
applied in cyclic manner with different frequencies as denoted on the
graph. An arbitrary power loss point of 10% was chosen and the time for
each sample to reach this loss point was calculated from the fit
equation.

![](media/image14.jpeg){width="5.999305555555556in"
height="4.601388888888889in"}

[]{#__RefHeading___Toc57187457 .anchor}Figure 14. Activation energy
estimates from temperature dependence of cyclic bias stress samples.

[]{#__RefHeading___Toc57187425 .anchor}**[3.2 Stress-Induced Change
Mechanisms]{.underline}**

Ideally the identification and understanding of the mechanisms leading
to device degradation would allow the development of a mitigation
program and thus would be an important step for improving the long-term
performance of CdTe thin film modules. Understanding the change
mechanisms should provide concepts for tailoring materials properties in
order to influence the stress-induced response in the electrical device
performance. Many mechanisms are possible in polycrystalline compound
semiconductor thin films and techniques for measuring many important
properties are lacking. Consequently the process of understanding is
quite complex. Our working hypothesis is that, apart from encapsulation
failure, module performance is largely determined by cell performance.
Cell performance in turn depends on the defect distribution within the
device. Defects concentrations are affected by the local quasi-Fermi
level (i.e. the chemical potential). Changes in quasi-Fermi due to bias
or illumination result in a redistribution of defects. The
redistribution can include defect mutation, defect migration, and defect
occupancy. The defect redistribution in turn changes the quasi-Fermi
level. This framework provides a conceptual understanding of how current
device performance depends on the totality of the device history. Our
approach to unravel the observed behavior has been divided into several
steps:

-   monitoring the electrical response of devices during stressing and
    identification of the most obvious changes in device performances

-   analyzing changes in the defect chemistry of CdS and CdTe due to
    stress induced material changes

-   modeling of the defect chemistry in CdTe and device modeling
    considering different stress induced defect levels

-   modifying the material properties of CdTe and identifying the defect
    chemistry

-   improving the back-contact chemistry

-   demonstrating improved long-term stability of modified devices

    []{#__RefHeading___Toc57187426 .anchor}**3.2.1 Monitoring of the
    electrical response of stressed devices**

While we have stressed and monitored devices repeatedly in the past, a
complete data set for current vintage devices was needed. For this
purpose we made a set of dot cells and exposed them to bias and elevated
temperature. Two different illumination levels (dark, 0.7 sun) and three
temperature levels (65ºC, 90ºC, 115ºC) were chosen for the stress
experiment. For each of those six combinations, twelve different bias
conditions including open and short circuit conditions as well as
forward and reverse bias were considered. For consistency three
individual dot cells were stressed under each of the specific
conditions. After 28 days of stressing, the recovery behavior under OC
conditions (light/dark) was also investigated.

The most obvious losses in efficiency η, open circuit voltage V~OC~,
short circuit current J~SC~ and Fill Factor FF were observed for devices
stressed under reverse bias. These devices recover partially under OC
conditions, but show significant permanent changes in PV-performance. In
some case the development of a "roll over" in the IV-curves is observed.
Slight changes in all parameters could be identified for devices
stressed under OC conditions.

[]{#__RefHeading___Toc57187427 .anchor}**3.2.2 Analysis of degraded
devices**

For specific analysis of material changes concurrent with changes in
PV-performance, we focused on two stress conditions: -2V and OC under
illumination at 90ºC. In order to identify the pathway of changes within
the semiconductor material itself, we examined and compared samples
taken from different stages of device degradation using various
analytical methods. Standard characterization of stressed devices
included IV, quantum efficiency (QE) and capacitance-voltage (CV)
measurements.

Further investigations on stressed dot cell devices were performed on a
series of devices showing "low efficiency" accompanied by "slow
degradation" and on a series showing "high efficiency" accompanied by
"fast degradation".

<table>
<colgroup>
<col style="width: 5%" />
<col style="width: 12%" />
<col style="width: 13%" />
<col style="width: 6%" />
<col style="width: 5%" />
<col style="width: 10%" />
<col style="width: 8%" />
<col style="width: 8%" />
<col style="width: 8%" />
<col style="width: 9%" />
<col style="width: 8%" />
</colgroup>
<tbody>
<tr class="odd">
<td>cell</td>
<td><p>stress</p>
<p>light, 90°C</p></td>
<td><p>recovery</p>
<p>dark, 25°C</p></td>
<td><p>η</p>
<p>%</p></td>
<td><p>V<sub>OC</sub></p>
<p>mV</p></td>
<td><p>J<sub>SC</sub></p>
<p>mA/cm<sup>2</sup></p></td>
<td>FF</td>
<td><p>R<sub>OC</sub></p>
<p>Ω−cm<sup>2</sup></p></td>
<td><p>R<sub>SC</sub></p>
<p>Ω−cm<sup>2</sup></p></td>
<td><p>J<sub>maxp</sub></p>
<p>mA/cm<sup>2</sup></p></td>
<td><p>V<sub>maxp</sub></p>
<p>mV</p></td>
</tr>
<tr class="even">
<td>1</td>
<td>OC, 12 h</td>
<td>no</td>
<td>9.56</td>
<td>716</td>
<td>19.81</td>
<td>67.39</td>
<td>5.98</td>
<td>1154</td>
<td>17.76</td>
<td>538</td>
</tr>
<tr class="odd">
<td>1</td>
<td>OC, 7 days</td>
<td>no</td>
<td>9.21</td>
<td>727</td>
<td>19.79</td>
<td>63.98</td>
<td>7.44</td>
<td>1461</td>
<td>17.39</td>
<td>529</td>
</tr>
<tr class="even">
<td>2</td>
<td>OC, 12 h</td>
<td>no</td>
<td>9.86</td>
<td>706</td>
<td>20.67</td>
<td>67.61</td>
<td>5.59</td>
<td>1836</td>
<td>18.74</td>
<td>526</td>
</tr>
<tr class="odd">
<td>2</td>
<td>-2V, 7 days</td>
<td>no</td>
<td>3.25</td>
<td>656</td>
<td>19.67</td>
<td>25.17</td>
<td>28.22</td>
<td>74</td>
<td>9.77</td>
<td>332</td>
</tr>
<tr class="even">
<td>2</td>
<td>-2V, 7 days</td>
<td>1 day</td>
<td>3.48</td>
<td>650</td>
<td>19.40</td>
<td>27.59</td>
<td>24.42</td>
<td>70</td>
<td>10.03</td>
<td>347</td>
</tr>
<tr class="odd">
<td>2</td>
<td>-2V, 7 days</td>
<td>7 days</td>
<td>4.03</td>
<td>653</td>
<td>19.48</td>
<td>31.70</td>
<td>18.91</td>
<td>112</td>
<td>10.81</td>
<td>373</td>
</tr>
<tr class="even">
<td>2</td>
<td>-2V, 7 days</td>
<td>20 days</td>
<td>4.50</td>
<td>671</td>
<td>19.58</td>
<td>34.23</td>
<td>17.49</td>
<td>278</td>
<td>10.92</td>
<td>412</td>
</tr>
<tr class="odd">
<td>2</td>
<td>-2V, 7 days</td>
<td>36 days</td>
<td>4.73</td>
<td>673</td>
<td>19.87</td>
<td>35.36</td>
<td>16.63</td>
<td>166</td>
<td>12.00</td>
<td>394</td>
</tr>
</tbody>
</table>

Table 1. Representative device performance of dot-cells under stress and
recovery

Stressing at OC conditions indicates a slow, but obvious tendency for
deterioration of significant PV-parameters. Devices stressed under
reverse bias exhibit a tremendous drop in performance that seems to
saturate with time and temperature (see Table 1). These devices
partially recover with storage under OC conditions in the dark at 25ºC
after removal from the external bias stress. This stress/recovery
behavior suggests that some of the degradation mechanisms that work in
parallel may initiate irreversible changes in materials properties.

Photoluminescence (PL) measurements (supported by University of Toledo)
were used to detect native defects and impurity related states within
the band gap of the CdTe absorber material. This method is particularly
suited for the identification of shallow trap levels but can be applied
to certain deep traps, provided that the recombination is radiative. A
rough comparison of the photoluminescence spectra obtained for an
unstressed and a sample of the "low efficiency series" stressed for 6
days under --2V reverse bias shows the development of a feature around
1.5 eV (see Figure 15). This PL-feature disappears during device
recovery under OC conditions in the dark and is believed to be a
donor-acceptor pair (DAP) presumably related to an impurity introduced
during device processing. The DAP PL feature is not observed for devices
stressed under OC conditions and illumination. Radiative recombination
at this DAP presumably is correlated to a defect equilibrium which is
shifted while exposed to biasing. The defect equilibrium returns to a
state similar to the original when the bias stress is removed. However
the device performance does not fully recover. Thus the 1.5 eV DAP PL
signature is not a complete degradation marker. While the DAP may be
involved in the significant deterioration of the PV-performance, other
degradation mechanisms must be operating in parallel.

![](media/image15.wmf){width="5.458267716535433in"
height="2.377952755905512in"}![](media/image16.wmf){width="0.552755905511811in"
height="1.547244094488189in"}

[]{#__RefHeading___Toc57187458 .anchor}Figure 15. PL spectra of a
"low-efficiency" device. 1) unstressed device, 2) device stressed at
--2V, 90ºC, light for 6d, and 3) device stressed at --2V, 90ºC, light
for 6d; recovery: OC, 25ºC, dark for 6d.

PL measurements were also performed on samples of the "high efficiency
series" (at University of Toledo). A preliminary interpretation suggests
that the luminescent centers that radiate at around 1.55 eV are
associated with donor-acceptor pairs involving Cd-vacancies as
acceptors. These defects (V~Cd~) interact with extrinsic impurities to
form defect-complexes whose occupancy and concentration may change due
to stressing that shifts the local defect equilibria. Recovery is
understood as a relaxation to the original defect species that occurs
when the local equilibria return to their non-stressed values. In
addition, defect migration may contribute to irreversible degradation
phenomena. Compared to the PL-spectra taken from "low efficiency"
devices, "high efficiency" devices do not exhibit major PL peaks within
the "DAP-region". Differences between the two cases may be associated
with the effect on the defect spectra attributable to differences in the
CdTe~x~S~1-x~ intermixing region of the two sets of devices. In any
case, PL-spectra seem to be very sensitive indicators of material
changes that correlate with degradation phenomena.

![](media/image17.wmf){width="5.401968503937008in"
height="2.749212598425197in"}![](media/image18.wmf){width="0.49094488188976376in"
height="1.5366141732283465in"}

[]{#__RefHeading___Toc57187459 .anchor}Figure 16. PL spectra of a
"high-efficiency" device. 1) unstressed device, 2) device stressed at
--2V, 90ºC, light for 7d, and 3) device stressed at --2V, 90ºC, light
for 7d; recovery: OC, 25ºC, dark for 5d.

Deep level capacitance profiling (DLCP) (supported by CSM) allows
probing the energy and concentration of traps at the point where the
Fermi-level crosses the trap energy. First measurements on the "low
efficiency series" suggest that a defect level at \~E~V~+0.6 eV and a
level at \~E~V~+0.4 eV are dominating the overall defect chemistry of
CdTe during stressing. In general, a decrease in the defect state
density is observed for devices stressed under --2 V reverse bias while
devices stressed under OC conditions seem to achieve an increase of the
defect state density. This counter-intuitive result indicates that DLCP
measurements do not provide a simple device degradation marker.

[]{#__RefHeading___Toc57187428 .anchor}**3.2.3 Device modeling**

We provided IV, QE, and CV from the "low" and the "high" efficiency
sample sets to a sub-group of the National CdTe Team for modeling and
interpretation. Device modeling of selected devices was performed by
Alan Fahrenbruch. SCAPS was first used to attempt to simulate the
CV-data observed for stressed low efficiency devices \[5\]. The
experimental data converted to apparent net hole density vs. depletion
width using a simple abrupt junction model is shown in Figure 17. A
simulation sequence is presented in Figure 18. The simulations produce
CV data which is then converted to apparent net hole density vs.
depletion width using the same simple abrupt junction model. The
sequence includes the case of a constant shallow hole density, the
addition of neutral or donor-like recombination centers, and the
addition of a back-contact barrier of 0.5eV. The addition of the back
barrier leads to a minimum in the apparent carrier concentration vs.
depletion width in both the dark and the illuminated curves. This is in
qualitative agreement with the experimental data (Figure 17) and clearly
questions the simple conversion of CV data to apparent hole
concentration.

![](media/image19.wmf){width="2.6708333333333334in" height="3.075in"}

[]{#__RefHeading___Toc57187460 .anchor}Figure 17. Apparent net hole
density vs. depletion layer width in First Solar cells stressed as
indicated in legend.

![](media/image20.wmf){width="2.5347222222222223in" height="2.925in"}

[]{#__RefHeading___Toc57187461 .anchor}Figure 18. SCAPS simulation of
simple CdS/CdTe device with constant doping. (from Alan Fahrenbruch)

For a refined model, a higher doped CdTe sub-layer (1 μm) adjacent to
the back-contact was introduced (Figure 19). In the dark the quasi-Fermi
level of the electrons is below the donor levels and the donor states
are positively charged, thus compensating the net hole density. In the
light, the net hole density is raised, but also the electron Fermi level
is above the donor levels and many donors have a neutral charge. Thus,
the apparent (electrical) end of the cell shifts from nominal 3.2 μm
towards 2 μm. This shift is in qualitative agreement with the
experimental results.

![](media/image21.wmf){width="3.5133858267716533in"
height="2.527165354330709in"}![](media/image22.wmf){width="2.65748031496063in"
height="1.4200787401574804in"}

[]{#__RefHeading___Toc57187462 .anchor}Figure 19. Simulation of model
with two CdTe layers. Light intensity gives J~sc~ of \~5mA/cm^2^. (from
Alan Fahrenbruch)

[]{#__RefHeading___Toc57187429 .anchor}**3.2.4 Investigation of material
changes due to impurity ingress**

QE measurements performed on devices stressed under reverse bias suggest
changes in CdS photoconductivity. It is known from the literature that
CdS generally acts as a "sink" for impurities and thus impurity
accumulation in CdS under reverse bias stress is possible. Therefore, we
treated polycrystalline CdS layers with different impurities present
during device processing and analyzed the films with PL and XPS (both
supported by NREL). CdS films were annealed both with and without
CdCl~2~ flux.

For annealed samples without intentional impurities, we observed a PL
emission at \~2.39 eV, which is believed to be caused by a transition
from Cd~i~ or V~S~ to the valence band. This feature increases in
intensity in CdCl~2~ annealed samples. The addition of impurities from
other device layers, which are known to in-diffuse, did not lead to
additional PL emission features. The formation of impurity sulfides at
dangling S-bonds on the CdS grain surfaces is plausible. However, XPS
measurements did not give any indication for the precipitation of
impurity sulfides.

![](media/image23.wmf){width="4.088188976377952in"
height="2.8602362204724407in"}

[]{#__RefHeading___Toc57187463 .anchor}Figure 20. PL spectra of CdS 1)
as grown, 2) air annealing 400°C, 45 min, 3) CdCl~2~ treatment 395°C, 20
min

[]{#__RefHeading___Toc57187430 .anchor}**3.2.5 Identification and
modification of back-contact chemistry**

As the back-contact area is assumed to be a major source for
"degradation", controlling back-contact chemistry during device
processing is one of the key factors in achieving long-term stability of
high efficiency devices. XRD-measurements (supported by NREL) along the
multi-layer back-contact stack were performed. The samples were annealed
at temperatures up to 100ºC greater than our normal post-metal heat
treatment (PMHT) temperature and for longer times. From the phase
diagram of metals involved, the formation and precipitation of stable
alloys was expected. However no alloying was detected with XRD. Either
an inter-diffusion zone providing the necessary stoichiometry for alloy
formation was not created or the kinetic barrier for any solid state
reaction has not been surmounted. Thus we consider the back-contact
electrode as an impurity source but unwanted phase formation that could
be a source of degradation is unlikely.

Surface preparation prior to back-contact application can impact initial
and long-term performance of devices. The chemical properties are
determined by the thermo-chemical history of CdTe especially the CdCl~2~
activation step. XRD-measurements performed at different stages of the
CdTe post-deposition processing indicate the presence of CdO and
CdTeO~3~ as main surface reaction products during the CdCl~2~-activation
step, where the relative quantities depend on the relative
concentrations of CdCl~2~ and O~2~ during annealing. Presumably these
residues exist as surface layers. Contact wetting angle measurements
(supported by IEC) indicate that CdCl~2~-activation raises the surface
energy compared to as-deposited CdTe.

Major emphasis has been focused on understanding the formation and
removal of surface species such as oxides and oxy-chlorides stemming
from CdCl~2~ processing (supported by IEC). In addition, the humidity
level of the ambient can change the oxide formation. CdTe annealed at
550°C and 600°C in dry or humid air without CdCl~2~ exhibits CdTeO~3~ as
a dominating surface oxide. In this case a weak time and moisture
dependence was found. Vapor CdCl~2~ treatment in dry air leads to CdO
between CdTeO~3~. Increasing the humidity, the appearance of CdTe~2~O~5~
between CdTeO~3~ was observed; no indication for CdO was found. Samples
pulled from different CdCl~2~ manufacturing runs processed under
seasonal different humidity levels suggest a slightly altered surface
chemistry. Further studies will be carried out if these differences can
be attributed to variations in the plant humidity or may have its roots
in variations of other processing parameters. However, dot-cell devices,
which were exposed to different humidity levels during CdCl~2~
activation show overall similar values of the PV parameters. The
difference in efficiency is not significant for both humidity levels,
but marginally lower for a humid CdCl~2~ treatment compared to the
standard CdCl~2~ process. A slight increase in R~OC~ and a decrease in
FF for devices activated under humid conditions are observed.

Ideally the various oxides that form during the CdCl~2~ activation step
should be removed to ensure consistent back-contact formation.
Acidic-oxidizing etches are not compatible with our back-contact
process, so various alkaline etches were investigated. Different etch
times and concentrations were employed to remove surface oxides and
oxy-chlorides. For the treatments investigated, neither initial nor long
term PV performance was significantly altered compared to non-treated
devices. Grazing incidence XRD (GI-XRD) (supported by IEC) measurements
indicate that, for the etch investigated, some oxide species are removed
but other species remain. Thus this improvement pathway still holds some
promise and further studies of surface treatment before back-contact
application are underway.

**, pp. 749-756, 1995.** []{#__RefHeading___Toc57187431 .anchor}**4.
Environmental, Health and Safety Improvements**

First Solar is committed to producing a safe product and operating a
safe factory. Thus Environmental, Health, and Safety (EH&S) programs are
a top priority. Large scale manufacturing of CdTe-based photovoltaic
modules involves handling a significant amount of material that contains
cadmium. Two pathways must be managed: airborne dust and solid waste.

Airborne dust can be generated from the CdTe film coating and film
annealing operations as well as processing steps where material is
removed, for example, laser scribing for interconnects and module
perimeter creation prior to encapsulation. Current emission levels are
significantly below OSHA Permissible Exposure Limits, but as production
volume increases, improved methods for identifying and capturing dusts
are needed.

Solid waste is generated from the coating operation, from module
handling during manufacture, from the disposal of broken or scrapped
modules, and from waste water treatment. Previously most waste was
disposed as a hazardous waste.

In addition maintaining a safe operation environment requires proactive
employee training program as well as ongoing verification of the current
Industrial Hygiene methodology.

[]{#__RefHeading___Toc57187432 .anchor}**[4.1 Dust
Emissions]{.underline}**

Dust emissions within the factory are primarily controlled using
high-efficiency particle air (HEPA) filters. The quality of the filters
and the installation method determine the actual effectiveness of
operating filters. Several programs are underway to verify the operation
of HEPA filters.

First, a HEPA filter testing apparatus has been designed and built to
evaluate the effectiveness of HEPA filtration and to evaluate the
quality of different vendor's filters (see Figure 21). Intentional
injection of known particulates to "challenge" the filter is used. An
ARTI airborne particle counter is the primary sensor used. Testing is
underway to determine the proper aerosol challenge including particulate
size, particulate material, and concentration for accurate HEPA filter
testing. As a baseline, documented standards for the HEPA testing in the
nuclear industry will be used.

![](media/image24.png){width="6.021653543307087in"
height="3.808267716535433in"}

[]{#__RefHeading___Toc57187464 .anchor}Figure 21. HEPA filter test
apparatus.

Development of an operational test for installed filters is also
underway. Using UV fluorescent tracer particles introduced via an
aerosol generator, detection of HEPA leaks may be possible with black
light illumination. Direct visual observation, microscopic observation
on capture samples, and specialized vision cameras are planned detection
methods. Initial data indicates that the fluorescent particles on
capture samples can be viewed under a microscope.

First Solar also attended a HEPA filter Workshop at Harvard's School of
Public Health. This program provided educational lectures and hands on
experience in testing HEPA filters.

[]{#__RefHeading___Toc57187433 .anchor}**[4.2 Recycling]{.underline}**

Over the past year, First Solar has made significant progress toward
developing methods for recycling solar modules and reducing hazardous
waste. Four examples follow.

[]{#__RefHeading___Toc57187434 .anchor}**4.2.1 Module Recycling**

For broken and scrap modules, we had previously developed a process in
which the modules are first crushed into glass cullet. The
semi-conductor, back electrode, and bus bars are dissolved into solution
and reformed into a solid, filter cake material. The clean glass cullet
is shipped off site to a facility where the glass is reprocessed.
Previously the filter cake was disposed as hazardous waste. We have
investigated a number of recycling options for both whole modules and
filter cake.

We have now begun to work with company that can recycle our filter cake.
This company blends our filter cake with waste from other sources and
returns it to an input stream of a smelting operation. Since May 2003,
over 14,000 pounds of filter cake waste has been recycled in this manner
rather than disposed. Thus we have now established a true recycling
process for all of the metal components and glass cullet generated from
scrap module within the manufacturing operation.

Currently the filter cake contains both Cd and Te. Recycling the filter
cake would be more economic if these major constituents could be
separated into two enriched filter cakes. A 2-step selective
precipitation process has been demonstrated in the laboratory
previously. Further development of this process is underway for use in
our recycling operation.

Two additional companies for recycling whole modules have been
identified and pilot scale tests are in progress. If the pilot tests are
successful, processing scrap modules into filter cake could be
eliminated at our facility.

[]{#__RefHeading___Toc57187435 .anchor}**4.2.2 Recycling of bulk
CdS/CdTe**

Another waste stream, bulk cadmium sulfide and cadmium telluride
material that is deposited onto chamber walls and transport rollers
during the deposition process has been evaluated by a CdTe manufacturer.
Testing has been confirmed that this material can be recycled and
negotiations are currently underway to complete this process.

[]{#__RefHeading___Toc57187436 .anchor}**4.2.3 Personal protective
equipment disposal**

First Solar employees wear personal protection equipment, PPE, including
Kevlar gloves, arm guards, and aprons when handling glass substrates
(modules). This protective equipment becomes soiled with material that
potentially contains Cd. Previously the soiled protective fabrics were
disposed as hazardous waste. We have now contracted with a supplier of
uniforms, CINTAS, to laundering the fabrics. CINTAS treats the wash
water and thus does not discharge Cd. The laundering process saves
significant purchase expense of protective equipment and reduces
hazardous waste disposal. In the first 4 months of operation, as savings
of \$6000 has been realized.

[]{#__RefHeading___Toc57187437 .anchor}**4.2.4 Reduction of Cd leaching
risk**

The designation of a hazardous solid waste depends in part on what might
leach into the ground water from a product disposed in an unregulated
landfill. The risk of ground water contamination is estimated using
various leaching protocols. Some Cd leaching from CdTe based modules can
occur depending on sample preparation and leaching conditions. The exact
designation of CdTe based modules as a hazardous waste has not been
determined and depends on prevailing regulations. However, we have begun
to explore a concept of chemical stabilization of the Cd within the
module to reduce leaching risk.

The addition of agent to bind Cd into an insoluble form has been
developed for Ni-Cd batteries \[6\]. Such an agent could be incorporated
into a module as a layer within the encapsulated structure or in an
external package that would release whenever the module itself was
broken.

For the initial concept test, three agents known to reduce the
solubility of Cd, iron sulfide, (FeS), sodium borohydride (NaBH~4~), and
starch xanthate (ISX) were tried. A common precipitating agent FeCl~3~
was also tested. These agents were added directly to the extracting
solution. The extracting solution used was that designated in the
Environmental Protection Agency's Toxic Leachate Protocol (TCLP) test.
This extracting solution contains acetic acid and NaOH. The exploratory
tests conducted differ from the TCLP protocol in that samples were
crushed to a much smaller size (x mm compared to the \<9.5mm specified
in the TCLP test) and a different mechanical agitation was used. The
tests conducted were significantly more aggressive than the actual TCLP
test due primarily to the small crushed particles included and the fact
that the crushing method causes complete delamination of encapsulated
package. The leaching test was conducted for 24 hours.

ISX was found to be difficult to handle and did not produce significant
consistent reductions in the cadmium levels found in the tests.

![](media/image25.wmf){width="5.993055555555555in"
height="4.478472222222222in"}

[]{#__RefHeading___Toc57187465 .anchor}Figure 22. Aggressive Leach Test

Figure 22 shows that the addition of FeS or NaBH~4~ to the extraction
solution reduces the soluble Cd concentration to less than 10% of the
values obtained when no stabilization agent is used. Although this test
was aggressive, Cd concentrations well below the current regulatory
level of 1 mg/l were found with the use of binding agents. Thus this
pathway to mitigate Cd leaching appears promising.

[]{#__RefHeading___Toc57187438 .anchor}**[4.3 Training]{.underline}**

Training is a key element of operating a safe workplace. We have
converted our new employee training materials into an electronic format
that is easier to manage. These training materials including *Hazard
Communication, Cadmium Compliance,* *Lockout Tagout*, and *Forklift
Operation*, are used in a full day session. We have also added three new
safety training programs: *Radiation Quality Assurance Plan* (required
for Scanning Electronic Microscope operation), *Department of
Transportation Awareness Training* (for personnel who ship and receive
or handle hazardous materials), and *Department of Transportation
Function* *Specific Training* (for the employees responsible for
labeling and signing manifest for hazardous waste disposals).

We are also working to create a training film for new employee and for
the required annual training sessions for all employees. This will allow
for a more visual retention of the material. Currently a video using
florescent particles and a black light to show how particulate matter
travels through the air is being developed. Particulate matter is a
significant part of First Solar training program due to the use of
cadmium compounds. This training film will be complete by January 2004.

[]{#__RefHeading___Toc57187439 .anchor}**[4.4 Industrial Hygiene Program
Validation]{.underline}**

We are currently working with Occupational Health and Industrial Hygiene
personnel from the Medical College of Ohio (MCO) for cadmium air and
medical sampling verification. At our request, MCO is statistically
analyzing all of the industrial hygiene air sampling data generated at
the First Solar Cedar Park facility between 2000 and 2003. The purpose
of this study is to have a third party verify the sampling data and to
make recommendations for future sampling plans.

[]{#__RefHeading___Toc57187440 .anchor}**[5. Future Plans]{.underline}**

First Solar is committed to commercializing CdTe-based thin film
photovoltaics. This commercialization effort includes a major addition
of floor space and equipment as well as process improvements to achieve
higher efficiency and greater durability. The following activities are
planned for phase II of this subcontract.

Work on the deposition reactor will include 1) start-up of a new
production coating line, 2) continued work on VTD-2 type distributors,
3) investigation of film nucleation and microstructure control, 4)
exploration of CdS powder flow detectors, 5) implementation of more
reliable CdS powder feeder, and 6) integration of flow modeling
components to include powder sublimation

Work on measurement systems will include 1) improvements to the IVT and
QEV hardware and increased sophistication of analysis algorithms, 2)
verification of the use and implementation of bandgap mapping system, 3)
completion of software and methodology to quantify of non-uniformity
losses

For ALT development work we plan to correlate a rapid,
continuously-monitored, high temperature ALT test with the current 56
day protocol. We will also begin to correlate this test with field data.
While we firmly believe that fundamental understanding of material
changes is very important, we plan to revert to a more of an empirical
approach to understanding stress induced changes. This is primarily due
to a lack of a clear link between the investigated material measurements
and device performance.

EH&S work will consist of 1) development of HEPA filtration testing
protocols, 2) continued improvements to hazardous waste recycling and
reduction procedures, and 3) third party validation of our Industrial
Hygiene program.

[]{#__RefHeading___Toc57187441 .anchor}**List of Figures**

[Figure 1. Cross-web thickness uniformity for VTD-1 type distributor.
4](#__RefHeading___Toc57187444)

[Figure 2. Cross-web thickness uniformity for VTD-2 type distributor.
5](#__RefHeading___Toc57187445)

[Figure 3. Cross-web thickness uniformity for Batch Load Type CdS
distributor. 6](#__RefHeading___Toc57187446)

[Figure 4. CdS thickness map from a typical film deposited onto a 60 cm
x 120 cm substrate using the commercially available powder feeder
currently employed in production. 8](#__RefHeading___Toc57187447)

[Figure 5. CdS thickness map from a film deposited onto a 60 cm x 120 cm
substrate using the alternative powder feeder under development.
9](#__RefHeading___Toc57187448)

[Figure 6. Bandgap energy of CdTe films as a function of cross web
position. 10](#__RefHeading___Toc57187449)

[Figure 7. Dot-cell V~oc~ versus excess minority carrier lifetime of
CdTe 12](#__RefHeading___Toc57187450)

[Figure 8. IVT measurement of sample with apparent contact barrier.
13](#__RefHeading___Toc57187451)

[Figure 9. Example of quantum efficiency as a function of bias voltage.
13](#__RefHeading___Toc57187452)

[Figure 10. Example IV map from a module that failed light soak testing.
In this case every 4^th^ cell was measured.
14](#__RefHeading___Toc57187453)

[Figure 11. Characteristic decay of the relative efficiency during ALT
and exponential fits. 16](#__RefHeading___Toc57187454)

[Figure 12. Current light soak vs. ALT prediction comparison for the
same module 17](#__RefHeading___Toc57187455)

[Figure 13. Current light soak procedure prediction vs. ALT prediction
for the saturated efficiency value. 18](#__RefHeading___Toc57187456)

[Figure 14. Activation energy estimates from temperature dependence of
cyclic bias stress samples. 19](#__RefHeading___Toc57187457)

[Figure 15. PL spectra of a "low-efficiency" device. 1) unstressed
device, 2) device stressed at --2V, 90ºC, light for 6d, and 3) device
stressed at --2V, 90ºC, light for 6d; recovery: OC, 25ºC, dark for 6d.
22](#__RefHeading___Toc57187458)

[Figure 16. PL spectra of a "high-efficiency" device. 1) unstressed
device, 2) device stressed at --2V, 90ºC, light for 7d, and 3) device
stressed at --2V, 90ºC, light for 7d; recovery: OC, 25ºC, dark for 5d.
23](#__RefHeading___Toc57187459)

[Figure 17. Apparent net hole density vs. depletion layer width in First
Solar cells stressed as indicated in legend.
24](#__RefHeading___Toc57187460)

[Figure 18. SCAPS simulation of simple CdS/CdTe device with constant
doping. (from Alan Fahrenbruch) 24](#__RefHeading___Toc57187461)

[Figure 19. Simulation of model with two CdTe layers. Light intensity
gives J~sc~ of \~5mA/cm^2^. (from Alan Fahrenbruch)
25](#__RefHeading___Toc57187462)

[Figure 20. PL spectra of CdS 1) as grown, 2) air annealing 400°C, 45
min, 3) CdCl~2~ treatment 395°C, 20 min 26](#__RefHeading___Toc57187463)

[Figure 21. HEPA filter test apparatus. 29](#__RefHeading___Toc57187464)

[Figure 22. Aggressive Leach Test 32](#__RefHeading___Toc57187465)

[]{#__RefHeading___Toc57187442 .anchor}**Glossary of Abbreviations**

ALF Alan Farhenbruch consulting

ALT Accelerated Life Test

CV Capacitance-Voltage measurement

CSM Colorado School of Mines

CSU Colorado State University

DAP Donor-Acceptor Pair

EH&S Environmental, Health and Safety

FF Fill Factor

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

USF University of South Florida

UT University of Toledo

V~Cd~ Cadmium vacancy

V~oc~ Open circuit voltage

VTD Vapor Transport Deposition

**[References]{.underline}**

**\[1\] 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.**

**\[2\] 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.**

**\[3\] D. Rose, \"Appendix 10,\" presented at National CdTe R&D Team
Meeting, 2001.**

**\[4\] T. J. McMahon and G. J. Jorgensen, \"Progress toward a CdTe cell
life prediction,\" presented at 15th NCPV Photovoltaics Program Review,
Denver, CO, 1998.**

**\[5\] M. Burgleman, \"SCAPS v. 2.3,\" Univ. of Gent, 2001.**

**\[6\] K. F. Cherry, *Plating Waste Treatment*: Ann Arbor Science
Publishers, Inc., 1982.**

[^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.