Thin-Film Photovoltaic Partnership ---

> **Process R&D for CIS-Based Thin-Film PV**

####### **Annual Technical Report**

####### **April 24, 2002 -- January 31, 2003**

####### 

###### **Dale E. Tarrant, Principal Investigator**

###### **Robert R. Gay, Program Manager**

###### **Harin S. Ullal, NREL Technical Monitor** 

###### 

###### 

###### Siemens Solar Industries

###### 4650 Adohr Lane

###### Camarillo, California 93011-6032

###### **Prepared under Subcontract** 

###### **NREL Subcontract #ZDJ-2-30630-16** 

###### 

###### **National Renewable Energy Laboratory**

###### **1617 Cole Blvd.**

###### **Golden, Colorado 80401-3393**

# Preface

Shell Solar Industries (SSI), formerly Siemens Solar Industries has
pursued the research and development of CuInSe~2~-based thin film PV
technology since 1980. At the start of subcontract activities with NREL,
SSI had demonstrated a 14.1% efficient 3.4 cm^2^ active-area cell,
unencapsulated integrated modules with aperture efficiencies of 11.2% on
940 cm^2^ and 9.1% on 3900 cm^2^, and an encapsulated module with 8.7%
efficiency on 3883 cm^2^ (verified by NREL).

SSI began a 3-year, 3 phase cost-shared subcontract (No. ZN-1-19019-5)
on May 1, 1991 with the overall project goal of fabricating a large
area, stable, 12.5% aperture efficient encapsulated CIS module by
scaleable, low-cost techniques on inexpensive substrates. Subcontract
accomplishments were facilitated by addressing module reproducibility
using small area test devices and mini-modules. Statistical process
control disciplines were adopted to rigorously quantify process
reproducibility. SSI addressed uniformity and reproducibility of
absorber formation, interactions of the substrate with the absorber, and
performance losses near interconnects. Subcontract accomplishments
included demonstration of encapsulated module efficiencies that were at
that time the highest reported mini-module efficiencies for any thin
film technology (encapsulated 12.8% efficient mini-module on 68.9 cm^2^
and an NREL-verified 12.7% efficient unencapsulated circuit on 69 cm^2^
with a prismatic cover), demonstration of a champion large area
(3860 cm^2^) encapsulated module efficiency of 10.3% (verified by NREL)
that was the first thin film module of its size to exceed the 10%
efficiency level, and delivery to NREL of a one kilowatt array of large
area (\~3890 cm^2^) approximately 30 watt modules \[[^1]\].

From September 1995 through December 1998, SSI participated in a 3-year,
3 phase cost-shared TFPPP subcontract (No. ZAF-5-14142-03). The primary
objective of this subcontract was to establish reliable high-throughput,
high-yield thin film deposition processes in order to make CIS a viable
option for the next generation of photovoltaics. Outdoor testing,
accelerated environmental testing, and packaging development progressed
throughout all phases of this subcontract. During Phase 1, SSI
rigorously demonstrating process reproducibility and yield for a
10x10-cm monolithically interconnected \"mini-module\" baseline process
and demonstrated a 13.6% aperture area efficient mini-module (verified
by NREL). During Phase 2, SSI demonstrated the need to replace an
existing large area reactor with a reactor based on a more direct
scale-up of the baseline reactor, built a new large area reactor, and
demonstrated comparable performance for the mini-modules baseline and
28x30-cm circuit plates. SSI developed products and prototype large area
modules using a new package designed to integrate small circuit plates
into larger modules. A one kilowatt array of Cu (In,Ga)(S,Se)~2~ modules
was delivered to NREL replacing a previously installed array based on an
older absorber formation technology without sulfur incorporated in the
absorber (Cu(In,Ga)Se~2~). This array demonstrated significant
improvements in efficiency and the temperature coefficient for power.
SSI introduced two new 5-watt (ST5) and 10-watt (ST10) CIS-based
products designed for use in 12 V systems, and NREL confirmed a new
world-record efficiency of 11.1% on a SSI large area (3665 cm^2^)
module. During subcontract Phase 3, substrate size was scaled from
\~30x30 cm to \~30x120 cm and good process control was demonstrated with
an average efficiency of 10.8%. Commercial product samples were
delivered to NREL and a second set of \~30x120 cm modules (32 modules
totaling \~1.2 kW) was delivered to the NREL Outdoor Test Facility. The
NREL measured average efficiency at standard test conditions of 11.4%
was at that time the highest large area efficiency for any thin-film
technology and NREL confirmed a world-record 11.8% large area (3651
cm^2^) efficiency for the champion module \[[^2]\].

From August 1998 through November 2001, SSI participated in a 3-year, 3
phase cost-shared TFPPP subcontract (No. ZAF-5-14142-03). The primary
objectives of this "Commercialization of CIS-Based Thin-Film PV"
subcontract were to scale-up substrate size and to increase production
capacity of the baseline CIS module process while introducing CIS-based
products. These objectives were pursued to fabricate efficient and
stable thin-film modules made by scaleable, manufacturable, low-cost
techniques. An additional mid- to longer-term objective was to advance
CIS based thin-film technology thereby assuring future product
competitiveness by improving module performance, cost per watt produced,
and reliability. Throughout this subcontract, SSI capabilities were
leveraged as a Technology Partner participating in NREL team oriented
TFPPP activities to address near-term to longer-term R&D topics. SSI's
approach to this work was to apply design of experiment and statistical
process control methodologies. Siemens Solar was the first company in
the world to start production of PV modules based on CIS thin-film
technology and this major milestone in the development of PV was
recognized by R&D Magazine by awarding the prestigious R&D 100 Award to
the Siemens Solar family of CIS solar modules. NREL, the California
Energy Commission and SSI shared this award. SSI expanded the CIS
product line in 1999 to include 20-Watt "ST20" modules and 40-Watt
"ST40" modules. Also during the first subcontract phase, a
record-breaking efficiency of more than 12% was verified by NREL for an
ST-40 module. This result in 1999 far surpassed the DOE year 2000 goal
for a commercial CIS module above 10%. During the second subcontract
phase, SSI delivered 20 ST-40 large area modules, all with efficiencies
over 11%, to meet the subcontract deliverables defined as large area
modules with efficiencies of over 10%. The average efficiency based on a
Gaussian fit to the main portion of the circuit plate efficiency
distribution was increased from 10.8% prior to this subcontract to 11.6%
for this subcontract period. These advancements were due to continuous
improvement of all process along with particular attention to process
research for two critical processes -- CIS formation in new large area
reactors and the quality of molybdenum deposited in new high capacity
sputtering equipment. Process development improved adhesion, decreased
breakage, addressed control of raw materials, and decreased failures
associated with patterning. Further R&D of all CIS processes for part
size and capacity scale-up was pursued during the third subcontract
phase. Major accomplishments included addressing process issues for
implementation of high quality high throughput Mo deposition and
patterning, high throughput precursor deposition, and higher throughput
reaction of the precursor. Circuit plate production capacity was
increased by more than an order of magnitude from the beginning of this
subcontract while circuit and module efficiencies were steadily
improved. The second subcontract milestone -- to achieve a pilot
production rate 500 kW per year by the end of subcontract -- was first
achieved in March of 2001 \[[^3]\].

# Acknowledgments

Siemens Solar Industries wishes to acknowledge the contributions of the
following people and organizations.

> The Siemens Solar Industries CIS Team:

  ----------------------------------- -----------------------------------
  M. Dietrich                         A. Ramos

  O. Frausto                          C. Rischmiller

  R. Gay (Program Manager)            U. Rühle

  D. Heinemann                        A. Seapan

  C. Köble                            J. Schmitzberger

  J. Maher                            D. Tarrant (Principal Investigator)

  P. Norum                            S. Voss

  H. Nguyen                           R. Wieting

  D. Pollock                          D. Willett

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

> Brookhaven National Laboratory
>
> Colorado State University
>
> Florida Solar Energy Center
>
> Fraunhofer Institute / GSF
>
> Institute of Energy Conversion / University of Delaware
>
> National Renewable Energy Laboratory
>
> Pennsylvania State University
>
> Showa Shell Seikyu, K.K.
>
> Shell Solar GmbH
>
> Washington State University

# Summary

Compared to traditional wafer-based crystalline silicon technologies,
monolithic integration of thin film solar cells can lead to products of
comparable performance but with significant manufacturing advantages:
lower consumption of direct and indirect materials, fewer processing
steps and easier automation. Monolithic integration is required to
achieve these advantages since this eliminates multiple process steps
and handling operations during formation of the absorber and during
module assembly. The basic module elements for all thin-film
technologies (alloys of amorphous silicon, cadmium telluride and CIS)
are the same; the module elements are a circuit-glass/cover-glass
laminate, a frame, and a junction box. The basic circuit elements are
also very similar; they each have a base electrode, an absorber, a
junction, a top electrode and three patterning steps for monolithic
integration. While the details of these module elements or equivalent
module elements differ, the basic cost structures are very similar on an
area-related basis. Since the cost per unit area is the same, the cost
per watt is inversely proportional to the module efficiency. CIS cells
and monolithically integrated modules have demonstrated the highest
efficiencies of any candidate thin-film technologies; therefore, CIS is
expected to have the lowest manufacturing cost/watt.

The primary objectives of this subcontract are to:

-   Address key near-term technical R&D issues for continued CIS product
    improvement

-   Continue process development for increased production capacity

-   Develop processes capable of significantly contributing to DOE 2020
    PV shipment goals

-   Advance mid- and longer-term R&D needed by industry for future
    product competitiveness including improving module performance,
    decreasing production process costs per watt produced, and improving
    reliability

-   Perform aggressive module lifetime R&D directed at developing
    packages that address the DOE goal for modules that should last up
    to 30 years while retaining 80% of initial power

Outstanding progress has been made in the initial commercialization of
high performance thin film CIS technology. During this subcontract
period, predictability of SSI's CIS process was demonstrated by
continuously executing the process while increasing throughput.
Cumulative production for 2002 exceeded 1 MW - about twice the
production rate for 2001. Average laminate efficiency for 2002 is 10.8%
with a full width of only 12% of the average. Dramatic increases in line
yield were achieved from improved production protocols and by addressing
disparate special causes for process variation. Line yield increased
from about 60% in 2000 to about 85% in 2002. NREL confirmed a champion
12.8 percent aperture area conversion efficiency for a large area (3626
cm^2^) CIS production module. Field failure mechanisms for prototype
modules were clearly demonstrated. Additional circuit plate or packaging
process variables, although not as clearly established, were also found
to effect long-term stability for pre-production modules. Significant
progress was made toward developing a "glass/glass" package that
eliminates the TPAT backsheet for decreased cost, simplification of the
package and decreased operating temperature. Very promising preliminary
results were demonstrated for edge seals developed in collaboration with
the new NREL sponsored National Thin-Film PV Module Reliability Team.
Long-term outdoor stability has been demonstrated at NREL where \~30x30
cm and \~30x120 cm modules with multiple prototype package designs have
undergone testing for over fourteen years.

The demonstrated high line yield is the major accomplishment. No one
major process improvement was responsible for the yield improvements.
Judicious application of manufacturing engineering disciplines such as
SPC, analysis of variation and design of experiments led to a clear
definition of near term yield issues. Dramatic improvements in yield
were the result of improving production protocols and addressing
disparate special causes for process variation. This major
accomplishment supports attractive cost projections for CIS. Process R&D
at successive levels of CIS production has led to the continued
demonstration of the prerequisites for commitment to large-scale
commercialization. SSI\'s thin-film CIS technology is poised to make
very significant contributions to DOE/NREL/NCPV long-term goals - higher
volume, lower cost commercial products.

# Table of Contents

Preface [i](#preface)

Acknowledgments [iii](#acknowledgments)

Summary [iv](#summary)

Table of Contents [vi](#table-of-contents)

List of Figures [vii](#list-of-figures)

Introduction [1](#introduction)

Overview [1](#overview)

SSI CIS Process [2](#ssi-cis-process)

SSI's R&D Approach [5](#ssis-rd-approach)

Subcontract Activities and Milestones
[5](#subcontract-activities-and-milestones)

Background [5](#background)

Objectives [6](#objectives)

Milestones [6](#milestones)

Deliverables [6](#deliverables)

Technical Review [7](#technical-review)

Process R&D [7](#process-rd)

National CIS R&D Team [11](#national-cis-rd-team)

Package Development [12](#package-development)

Glass/Glass Package [12](#glassglass-package)

Outdoor testing [14](#outdoor-testing)

Conclusions [23](#conclusions)

References [24](#references)

# List of Figures

Figure 1. Structure of SSI's monolithically integrated thin-film
circuits. [1](#__RefHeading___Toc61866269)

Figure 2. SSI's CIS cell structure (not to scale).
[3](#__RefHeading___Toc61866270)

Figure 3. SSI CIS Circuit Processing Sequence.
[3](#__RefHeading___Toc61866271)

Figure 4. Typical elemental profile for the SSI graded absorber (SIMS
from NREL). [4](#__RefHeading___Toc61866272)

Figure 5. Single circuit plate module configuration with a TPAT
backsheet. [4](#__RefHeading___Toc61866273)

Figure 6. Champion ST40 module from the upper end of the production
distribution. [7](#__RefHeading___Toc61866274)

Figure 7. Production distribution for 1x4-ft. laminates produced during
2002 [8](#__RefHeading___Toc61866275)

Figure 8. Historical production rate including the first phase of this
subcontract (2002). [8](#__RefHeading___Toc61866276)

Figure 9. Yield improvements. [9](#__RefHeading___Toc61866277)

Figure 10. Laser scribed readable serial number and a 2-dimensional
barcode. [11](#__RefHeading___Toc61866278)

Figure 11. Present and future package designs.
[13](#__RefHeading___Toc61866279)

Figure 12. Darkening and broadening of the interconnects due to moisture
ingress. [14](#__RefHeading___Toc61866280)

Figure 13. Demonstrated long-term outdoor stability.
[15](#__RefHeading___Toc61866281)

Figure 14. Normalized changes in FF, ranging from virtually no loss to
significant losses, for six representative array modules.
[16](#__RefHeading___Toc61866282)

Figure 15. Recovery after the thermal exposure of damp heat accelerated
tests with or without including high humidity in the tests.
[17](#__RefHeading___Toc61866283)

Figure 16. Dirt buildup in a narrow strip at the edge of a wide frame.
[18](#__RefHeading___Toc61866284)

Figure 17. Improperly used tape inside the package.
[19](#__RefHeading___Toc61866285)

Figure 18. A laser scan of a thin-film module showing shunting at the
edges (Figure 8. from reference 19 ). [20](#__RefHeading___Toc61866286)

Figure 19. 32 Module Arrays: Average Annual Decline
[21](#__RefHeading___Toc61866287)

# Introduction

## Overview

Multinary Cu(In,Ga)(Se,S)~2~ absorbers (CIS-based absorbers) are
promising candidates for reducing the cost of photovoltaics well below
the cost of crystalline silicon. CIS champion solar cells have exceeded
18% efficiency for devices fabricated at NREL \[[^4]\]. Small area,
fully integrated modules exceeding 13% in efficiency have been
demonstrated by several groups \[[^5]\]. Record breaking efficiencies of
over 12% for a commercial large area module have been verified by NREL
\[[^6]\]. Long-term outdoor stability has been demonstrated at NREL by
\~30x30 cm and \~30x120 cm SSI modules which have been in field-testing
for over fourteen years. Projections based on current processing
indicate production costs well below the cost of crystalline silicon
\[Error: Reference source not found\].

Compared to traditional wafer-based crystalline silicon technologies,
new thin film technologies yield products of comparable performance but
with significant advantages in manufacturing \[Error: Reference source
not found, [^7]\]:

-   Lower consumption of direct and indirect materials

-   Fewer processing steps

-   Easier automation

Lower consumption of direct and indirect materials results in part from
the thin-film structure for the semiconductor used to collect solar
energy. All three of these manufacturing advantages are in part due to
an integrated, monolithic circuit design illustrated in Figure 1.
Monolithic integration eliminates multiple process steps that are
otherwise required to handle individual wafers and assemble individual
solar cells into the final product.

![](media/image1.wmf){width="5.217361111111111in"
height="2.092361111111111in"}

[]{#__RefHeading___Toc61866269 .anchor}Figure 1. Structure of SSI's
monolithically integrated thin-film circuits.

A number of thin film photovoltaic technologies have been developed as
alternatives to the traditional solar cells based on crystalline silicon
wafers \[Error: Reference source not found\]. The technologies with the
greatest potential to significantly reduce manufacturing costs are based
on alloys of amorphous silicon (a‑Si), cadmium telluride (CdTe), CIS,
and film silicon (Si-film). These photovoltaic thin film technologies
have similar manufacturing costs per unit area since all share common
elements of design and construction:

-   Deposition of typically three layers on a suitable substrate --
    window/electrode, absorber, and back electrode

```{=html}
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```
-   Patterning to create monolithically integrated circuit plates

-   Encapsulation to construct modules

Cost per watt is a more appropriate figure of merit than cost per unit
area \[Error: Reference source not found\]. All thin film technologies
have similar manufacturing costs per unit area since they all use
similar or equivalent deposition, patterning, and encapsulation
processes. About half of the total module cost -- material, labor, and
overhead -- originates in the encapsulation scheme which is for the most
part independent of the thin film technology. Costs for alternative
encapsulation schemes are typically similar or even higher. The average
efficiency of large, \~30x120 cm modules in pilot production at Siemens
Solar is approximately 11%. This performance is comparable to many
modules based on crystalline silicon, and is substantially better than
the performance reported for competing thin-film technologies. The
lowest cost per peak watt will result from the technology with the
highest efficiency, CIS technology, since most thin film technologies
have similar cost per unit area.

## SSI CIS Process

Most terrestrial photovoltaic products today are designed to charge a
12-volt battery, however the output voltage of an individual solar cell
is typically about 0.5 volts. Wafer-based technologies build up the
voltage by connecting individual solar cells in series. In contrast, CIS
circuits are fabricated monolithically (Figure 1); the interconnection
is accomplished as part of the processing sequence to form the solar
cell by alternately depositing a layer in the cell structure and
patterning the layer using laser or mechanical scribing.

The structure of a SSI CIS solar cell is shown in Figure 2. The full
process to form CIS circuit plates, including monolithic integration, is
outlined in Figure 3. This process starts with ordinary sodalime window
glass, which is cleaned and an SiO~2~ barrier layer is deposited to
control sodium diffusion and improve adhesion between the CIS and the
molybdenum (Mo) base electrode. The Mo base electrode is sputtered onto
the substrate. This is followed by the first patterning step (referred
to as "P1") required to create monolithically integrated circuit plates
-- laser scribing to cut an isolation scribe in the Mo electrode.
Copper, gallium and indium precursors to CIS formation are then
deposited by sputtering. Deposition of the precursors occurs
sequentially from two targets in an in-line sputtering system, first
from a copper-gallium alloy target (17 at% Ga) and then from a pure
indium target. CIS formation is accomplished by heating the precursors
in H~2~Se and H~2~S to form the CIS absorber. Beginning at room
temperature, furnace temperature is ramped to around 400ºC for
selenization via H~2~Se, and ramped again to around 500ºC for subsequent
sulfidation via H~2~S, followed by cool-down to room temperature. This
deposition of copper and indium precursors followed by reaction to form
CIS is often referred to as the two-stage process. A very thin coating
of cadmium sulfide (CdS) is deposited by chemical bath deposition (CBD).
This layer is often referred to as a "buffer" layer. A second patterning
step (P2) is performed by mechanical scribing through the CIS absorber
to the Mo substrate thereby forming an interconnect via. A transparent
contact is made by chemical vapor deposition (CVD) of zinc oxide (ZnO).
This layer is often referred to as a "window layer" or a transparent
conducting oxide (TCO). Simultaneously, ZnO is deposited on the exposed
part of the Mo substrate in the interconnect via and thereby connects
the Mo and ZnO electrodes of adjacent cells. A third and final
patterning step (P3) is performed by mechanical scribing through the ZnO
and CIS absorber to isolate adjacent cells.

![](media/image2.wmf)

[]{#__RefHeading___Toc61866270 .anchor}Figure 2. SSI's CIS cell
structure (not to scale).

![](media/image3.wmf)

[]{#__RefHeading___Toc61866271 .anchor}Figure 3. SSI CIS Circuit
Processing Sequence.

The CIS-based absorber referred to in this report is composed of the
ternary compound CuInSe~2~ combined with sulfur and gallium to form the
multinary compound Cu(In,Ga)(S,Se)~2~. Gallium and sulfur are not
uniformly distributed throughout the absorber but the concentrations are
graded; hence, this structure is referred to as a "graded absorber." The
graded absorber structure is a graded Cu(In,Ga)(Se,S)~2~ multinary with
higher sulfur concentration at the front and back and higher Ga
concentration at the back. Elemental profiles typical of the SSI graded
absorber structures are presented in Figure 4. Efficiency, voltage, and
adhesion improvements have been reported for the SSI graded absorber
structure \[Error: Reference source not found, [^8], [^9]\].

![](media/image4.wmf){width="3.8131944444444446in"
height="2.6006944444444446in"}

[]{#__RefHeading___Toc61866272 .anchor}Figure 4. Typical elemental
profile for the SSI graded absorber (SIMS from NREL).

Figure 5 illustrates the module configuration used for prototypes and ST
products during this subcontract period. EVA is used to laminate circuit
plates to a tempered cover glass and a Tedlar/polyester/Al/Tedlar (TPAT)
backsheet provides a hermetic seal. Aluminum extrusions are used to
build frames for the modules. In addition to providing a hermetic seal,
the combination of the TPAT backsheet and the offset between the circuit
plate and the frame provides electrical isolation from the frame.

![](media/image5.wmf)

[]{#__RefHeading___Toc61866273 .anchor}Figure 5. Single circuit plate
module configuration with a TPAT backsheet.

## SSI's R&D Approach

From the industrial perspective, the full process sequence anticipated
for use in large‑scale production must be mastered and rigorously
demonstrated. The SSI research approach is composed of two main
elements:

-   Experimentation and development using device structures that
    exercise all aspects of large area module production \[[^10]\]

-   Application of statistical process control (SPC) as the discipline
    to rigorously quantify process reproducibility, and application of
    statistical methods such as analysis of variation (ANOVA) to
    rigorously quantify experimental results \[[^11], [^12]\].

Process predictability is a prerequisite for commercialization of
thin-film PV since product performance ratings, yields and costs must be
known before committing to produce products. Also, process
predictability is essential for proper interpretation of process
development efforts since experimental results may be ambiguous or
misleading if compared to an unpredictable baseline process. SSI has
adopted SPC methodologies because SPC was developed to rigorously
quantify process reproducibility and process capability; the essence of
SPC is predictability. Equally significantly, SPC provides the measure
of systematic progress as processes are developed. Communication of this
progress is typically best expressed in the language of the SPC
discipline \[[^13]\]. For example, process characterization results are
demonstrated to be "statistically significant" based on knowledge of
process repeatability measured using the SPC discipline and compared to
a predictable baseline process. Confidence in the appropriate
interpretation of experimental results is gained through application of
statistical methods such as ANOVA to demonstrate statistically
significant results.

## Subcontract Activities and Milestones

### Background

The purpose of the Thin-Film Photovoltaics Partnerships Program (TFPPP)
is to accelerate the progress of thin film solar cells and module
development as well as to address mid and long-term research and
development issues. The long-term objective of the TFPPP is to
demonstrate commercial, low-cost, reproducible, high yield and robust
modules of 15% aperture-area efficiency. Further more, this research is
directed at making progress toward this objective by achieving interim
goals in thin film module efficiencies; cell and module processing; cell
and module reliability and the necessary fundamental research needed to
build the technology base that support these key areas. Participation in
the National R&D Teams is paramount to the success of this project. The
DOE/NREL/NCPV strategy in undertaking this R&D effort is to maintain the
good coupling between laboratory results from fundamental materials and
processes research to manufacturing R&D, pilot-line operation, and early
entry of advanced thin-film PV products to the ever-growing marketplace
worldwide.

The purpose of this subcontract, as part of the Technology Partners
Category, is to accelerate the progress of thin film solar cell and
module development as well as to address mid and long-term research and
development issues by achieving aggressive interim goals in thin film
module efficiencies; cell and module processing; cell and module
reliability; and in the technology base that supports these key areas.

### Objectives

The primary objectives of this subcontract are to:

-   Address key near-term technical R&D issues for continued CIS product
    improvement

-   Continue process development for increased production capacity

-   Develop processes capable of significantly contributing to DOE 2020
    PV shipment goals

-   Advance mid- and longer-term R&D needed by industry for future
    product competitiveness including improving module performance,
    decreasing production process costs per watt produced, and improving
    reliability

-   Perform aggressive module lifetime R&D directed at developing
    packages that address the DOE goal for modules that should last up
    to 30 years while retaining 80% of initial power

### Milestones

SSI shall perform each of the above tasks with the goal of meeting the
following targets:

-   Scale the substrate size from 1 ft. X 4 ft. to approximately 2 ft. X
    5 ft. by the end of the subcontract.

-   Achieve pilot production rates of 9,000 kW per year by the end of
    the subcontract.

-   Demonstrate commercial, low-cost, reproducible, high yield and
    robust module process that achieve the DOE goal for 15%
    aperture-area efficiency

-   Deliverables for the subcontract include CIS-based products and
    representative modules delivered to the NREL Module Testing Team for
    outdoor testing and evaluation.

### Deliverables

SSI will deliver 10 representative CIS-based module products at the end
of each phase of the subcontract.

SSI will deliver 10 representative CIS modules to the NREL Module
Testing Team by the end of each phase of the subcontract.

# Technical Review

## Process R&D

NREL has confirmed a champion 12.8 percent aperture area conversion
efficiency for a large area (3626 cm^2^) CIS module (Figure 6). The
aperture area for this champion module was defined by taping off the
approximately 1 cm inactive boarder surrounding the monolithically
integrated CIS circuit in a ST40, 40 W, production module. Other than
definition of the aperture area, this module is simply one module from
the upper end of the production distribution for standard modules.

![](media/image6.wmf){width="6.073611111111111in"
height="3.104861111111111in"}

[]{#__RefHeading___Toc61866274 .anchor}Figure 6. Champion ST40 module
from the upper end of the production distribution.

Figure 7 is the production distribution for approximately sixteen
thousand \~1x4-ft. laminates produced during 2002. The average
efficiency of this distribution is 10.8%. A Normal distribution fit to
the main portion of the distribution yields a standard deviation of
0.66%; over 88% of this production output is over 10% efficiency.

![](media/image7.wmf){width="5.2555555555555555in"
height="3.6256944444444446in"}

[]{#__RefHeading___Toc61866275 .anchor}Figure 7. Production distribution
for 1x4-ft. laminates produced during 2002

These champion efficiency and production distribution results are
particularly notable since they were achieved along with a cumulative
production for 2002 of over 1 MW; almost twice the production rate for
2001. Historical production rates and the production rate achieved
during 2002, nominally the first phase of this subcontract, are charted
in Figure 8.

![](media/image8.wmf){width="5.245833333333334in"
height="3.5930555555555554in"}

[]{#__RefHeading___Toc61866276 .anchor}Figure 8. Historical production
rate including the first phase of this subcontract (2002).

Dramatic increases in CIS line yield have been demonstrated \[[^14],
[^15]\]. Line yield is defined as the ratio of two areas -- the area of
product produced divided by the area of glass started through the
production line. This is total yield including both electrical yield and
mechanical yield through all processing required to produce products.
Line yield has increased from about 60% in 2000 to about 85% in 2002.
Process development, guided by SPC approaches, for all process steps was
required to achieve this high yield. This major accomplishment supports
attractive cost projections for CIS.

Figure 9 illustrates yield improvements over approximately the last
three years. No one major process improvement was responsible for
demonstrated yield improvements. Instead, these advancements were due to
continuous improvement of multiple processes.

![](media/image9.wmf){width="5.179166666666666in"
height="3.5416666666666665in"}

[]{#__RefHeading___Toc61866277 .anchor}Figure 9. Yield improvements.

Judicious application of manufacturing engineering disciplines such as
SPC, analysis of variation and design of experiments led to a clear
definition of near term yield issues. Based on this guidance, the
dramatic improvements in monthly yield through this timeframe were the
result of improving production protocols and addressing disparate
special causes for process variation. Significant improvements were made
by developing protocols and procedures for sun soaking modules prior to
final testing. Yield improvements as the result of process development
included:

-   Decreased breakage at multiple process steps related to handling and
    process definition

-   Elimination of shunting along the laser scribe in the Mo base
    electrode related to both the Mo deposition and laser patterning
    processes

-   Decreased loss due to equipment failures by improving control
    systems and production procedures

-   Decreased peeling related to glass cleaning, precursor deposition,
    the reaction process, reactor uniformity, and ZnO deposition

-   Decreased peeling related to glass cleaning

-   Decreased peeling related to precursor deposition

-   Decreased peeling related to reaction process conditions

-   Decreased peeling related to reactor uniformity

-   Decreased peeling related to ZnO deposition

-   Decreased loss of process runs by improving infrastructure for
    higher capacity and yield

These yield improvements were obtained through repeated cycles of
learning. For example, improper execution for multiple process steps can
lead to peeling. The drop in yield through the first half of 2002,
Figure 9, was due to peeling. This was found to be due to setup and wear
issues for a glass washer; issues that are obvious and trivial in
hindsight. Finding and addressing this special cause lead to the
immediate and permanent return to high yield.

As an example of subcontract work leading to yield improvements, process
development led to yield improvements for newly implemented equipment
\[[^16]\]. SSI defined the requirements and procured laser barcode
scribing and barcode reading equipment with the goals of increasing
capacity by improving productivity and providing high quality data for
integrated manufacturing infrastructure. Figure 10 illustrates the
corner of a circuit plate with both a readable serial number for humans
and a 2-dimensional barcode for machines.

![](media/image10.wmf){width="4.70625in" height="3.584722222222222in"}

[]{#__RefHeading___Toc61866278 .anchor}Figure 10. Laser scribed readable
serial number and a 2-dimensional barcode.

Process development demonstrated reduced breakage when laser bar codes
scribing replaced hand scribed serial numbers. A study of yield loss due
to breakage during the absorber formation process demonstrated that
breakage associated with hand scribed serial numbers was one of the
major causes of breakage. The use of laser scribed bar codes has reduced
this kind of breakage by 88%.

## National CIS R&D Team 

Prior to and during this subcontract phase SSI contributed to TFPPP
National CIS R&D Team. Summarizing results for all of the extensive team
member activities is not attempted in this report since the expertise
for most team activities resides with the team members. Instead, the
following joint NREL/SSI experiments are summarized as examples of
teamwork where SSI has had major involvement in sample preparation or
data analysis. Team activities included:

-   SSI supplied two types of SSI thin-films to TFPPP absorber team
    members - complete through Cu(In,Ga)(Se,S)~2~ absorber formation and
    complete through ZnO deposition. These films were supplied for
    electrical characterization at NREL and IEC, and for compositional
    analysis at NREL, FSEC, and the University of Illinois.

-   SSI supplied thin-films complete through Cu(In,Ga)(Se,S)~2~ absorber
    formation for buffer layer experiments at NREL and IEC.

-   SSI deposited CVD ZnO on multiple sets of samples from Larry Olsen,
    WSU.

-   SSI supplied samples of precursors for reaction pathway measurements
    at Oak Ridge National Laboratories to Suku Kim who is in Tim
    Anderson's group at the University of Florida.

-   SSI received data from the CIS array at the NREL OTF from Jill
    Adelstein. A summary of this data was included in the final report
    for the previous subcontract.

-   SSI attended the National CIS R&D Team Meeting held in conjunction
    with the IEEE PV Specialists Conference.

-   Steve Voss represented SSI at the first meeting of the newly formed
    National Thin Film Module Reliability Team Meeting which started
    with a workshop on moisture ingress. Steve presented a talk titled
    "Accelerated Environmental Testing of Shell Solar CIS"

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-   SSI participated in team experiments related to identifying the
    reasons for inconsistent performance for SSI absorber only when
    devices are made with buffer and ZnO depositions by NREL or IEC. SSI
    supplied additional samples of thin-films complete through
    Cu(In,Ga)(Se,S)2 absorber formation for buffer layer experiments at
    NREL and IEC.

-   SSI prepared to supply samples for work on barrier coatings by Larry
    Olsen at PNNL.

## 

## 

## Package Development

### Glass/Glass Package

SSI is now developing a "glass/glass" package that eliminates the TPAT
backsheet for decreased cost, simplification of the package and
decreased operating temperature. Figure 11 is a sketch of the present
production package and the proposed glass/glass package. Simplification
of the package should increase yield. Decreasing the operating
temperature will lead to higher efficiency for modules in the field.

![](media/image11.wmf){width="5.6in" height="2.707638888888889in"}

[]{#__RefHeading___Toc61866279 .anchor}Figure 11. Present and future
package designs.

No detectable humidity ingress occurs during damp heat testing of the
current SSI products with a TPAT backsheet where damp heat testing
refers to an exposure for 1,000 hour at 85ºC and 85% relative humidity.
The new glass/glass package is required to pass accelerated
environmental tests such as this damp heat test; however, package
development indicates that this will require additional moisture
protection. Figure 12 is a photo of the edge of a glass/glass module
where the interconnect patterning for monolithic integration is visible
as vertical lines above the ruler. Darkening and broadening of the
interconnects at the edge of the circuit plate is observed for
glass/glass packages after damp heat testing and has been found to be an
indicator of power loss due to moisture ingress \[[^17]\]. This is not
observed for the glass/glass package after the same thermal exposure but
without high humidity. Similarly, the darkening and broadening of the
interconnects is not observed for production products, that employ a
TPAT backsheet and frame, after the same thermal exposure with or
without high humidity. Edge seal and barrier coatings to block humidity
ingress are being explored at SSI and in collaboration with the new NREL
sponsored National Thin-Film PV Module Reliability Team. Preliminary
results are very promising.

![](media/image2.wmf)

[]{#__RefHeading___Toc61866280 .anchor}Figure 12. Darkening and
broadening of the interconnects due to moisture ingress.

### 

### Outdoor testing

FM and UL approval has been obtained for the present package used for
the ST series of products and the performance of the ST family of CIS
thin-film products is backed by a 10-year warrantee. Present and
potential packages are subjected to both accelerated testing and
long-term outdoor testing to develop and improve low cost yet durable
packages. One way NREL supports SSI in developing products for long-term
outdoor reliability is through long term testing of arrays and
individual modules at the NREL Outdoor Test Facility (OTF). As depicted
in Figure 13, long-term outdoor stability has been demonstrated at NREL
where \~30x30 cm and \~30x120 cm modules with multiple prototype package
designs have undergone testing for over fourteen years.

![](media/image3.wmf){width="4.496527777777778in"
height="3.0805555555555557in"}

[]{#__RefHeading___Toc61866281 .anchor}Figure 13. Demonstrated long-term
outdoor stability.

SSI has supplied modules to the NREL OTF for three 1kW arrays. In each
case, a newer generation of modules has been used to replace older
designs using the same test site. The first two arrays demonstrated
stability and that thermally induced transients, which are observed
after exposure to high temperatures during accelerated environmental
testing, are not observed in the field despite daily and seasonal
changes in module temperature. This is entirely reasonable since known
transient effects are temperature dependent and the module temperatures
reached for actual deployment are less than the temperatures defined for
accelerated testing.

Data acquisition began on November 18, 1998 for the third 1kW array of
modules. The system is comprised of 28 modules with an average
efficiency of 11.4% at STC. The aperture area of each module is
0.3651m^2^ and of the total array is 10.2 m^2^. The array is fixed at a
40° tilt aligned true south and is connected to a resistive load through
three maximum power trackers. Continually logged data is corrected for
temperature. Only data for incident solar irradiance of between
950‑1050 W/m^2^ is used for array characterization. NREL measurements
indicated array performance, normalized to standard test conditions,
over 1kW.

NREL data from late February of 2000 indicated stability within 2% of
the measurements made shortly after array deployment. Similarly, data
for operation through June 30, 1999 indicated that the array remained
stable with array performance over 1kW. As for previous arrays, both
module and the array data show good stability with no seasonal variation
in performance. It can not be overemphasized that multiple studies have
demonstrated that thermally induced transients, which are observed after
exposure to accelerated environmental testing conditions, are not
observed in the field.

More recent continuously logged data from the OTF indicates that the
latest array is now exhibiting changes with time. To follow-up on this
observation, NREL made outdoor measurements (Daystar) on each module in
the array. Six modules were selected for further study as representative
of the varying of degrees power loss for all array modules. NREL
provided pulsed simulator (SPIRE), continuous illumination simulator
(LACSS), and outdoor measurement (SOMS) data for the six selected
modules (). This data indicates that changes in performance are
primarily changes in FF. This data also indicates, as is typical of CIS
transient effects, observed changes are smaller for measurements made
with continuous illumination than for pulsed simulation. For this sample
of six representative modules, the change in FF is highly variable
ranging from 4% to 18%. The low end of this range is virtually no loss,
i.e. within measurement variability. The high end of this range is
significant and indicated the need for additional follow-up.

![](media/image4.wmf){width="4.496527777777778in"
height="3.0805555555555557in"}

[]{#__RefHeading___Toc61866282 .anchor}Figure 14. Normalized changes in
FF, ranging from virtually no loss to significant losses, for six
representative array modules.

Packaging development from early work with mini-modules through ST
products has demonstrated that thermally induced losses from lamination
or accelerated testing will recover with extended outdoor exposure
\[Error: Reference source not found\]. SSI has also demonstrated
multiple packaging designs, with and without EVA, that protect circuit
plates from water vapor ingress during damp heat accelerated testing;
exposure for 1000 hour at 85ºC and 85% relative humidity \[Error:
Reference source not found, Error: Reference source not found\]. CIS
circuit plates in packages that provide protection from water vapor
ingress will pass accelerated environmental tests with an outdoor
exposure to reverse thermally induced transient effects. This conclusion
was supported by studies by the NREL TFPPP "Transient Effects Group."
The team defined a repeatable measurement methodology for systematic
study of thermal transient effects that was used to demonstrate
reversibility through multiple thermal cycles and thereby support
outdoor testing results that indicate long term stability for normal
operating conditions \[Error: Reference source not found, [^18]\]. Based
on this foundation, SSI testing indicates that no significant humidity
ingress occurs during damp heat testing of current SSI products with a
TPAT backsheet. Figure 15 is a chart of average efficiency for two
groups of 8 modules. One group went through the damp heat environmental
test and the other went through the same temperature cycle but without
humidity \[Error: Reference source not found\]. Both groups were then
placed outdoors for 8 weeks of recovery. The two groups show the same
thermally induced loss and recovery with light exposure; humidity has no
impact on the results for current SSI products.

![](media/image5.wmf){width="4.999305555555556in"
height="3.4180555555555556in"}

[]{#__RefHeading___Toc61866283 .anchor}Figure 15. Recovery after the
thermal exposure of damp heat accelerated tests with or without
including high humidity in the tests.

A significant effort has gone into exploring the apparent inconsistency
between the most recent results from the most recent array of prototype
modules deployed at NREL, the most recent demonstration of stability for
production modules based on accelerated testing, and the combined
knowledge from multiple sets of accelerated test results, multiple
generations of modules deployed at NREL and the two previous arrays
deployed at NREL that were stable. SSI has deployed modules, typically
in 1 kW arrays, at test sites throughout the country and, to a lesser
degree, throughout the world. Modules from multiple sites were retrieved
for nondestructive and destructive testing to explore the observations
from the most recent NREL array.

Early prototype module designs included a wide frame that extended
across the front of the modules to cover buss bars. CIS modules have an
esthetically pleasing uniform nearly black appearance. However, the
simplest form of bus bar is a shinny tin-coated copper ribbon. This
might be an esthetically pleasing combination if the bus bars were
uniform but the shinny solder used to attach the bus bars yields ragged
rather than uniform lines. Wide frames were used in pre-production
modules to cover the shinny bus bars and solder. The inside edge of
these wide frames is very close to the outer cells that are narrow
strips extending the length of the module; parallel with and close to
the wide frame. This geometry proved to be very sensitive to dirt
buildup, Figure 16, in a narrow strip at the edge of frame that shadowed
the outer cells. Improved performance was demonstrated after scraping
away the dirt. However, not all of the subset of modules that showed
degradation fully recovered after scraping away the dirt. Dirt clearly
can degrade performance but this mechanism does not explain all
degradation. It is assumed that the wide frames also tended to trap
water, which also could have lead to degradation.

![](media/image6.wmf)

[]{#__RefHeading___Toc61866284 .anchor}Figure 16. Dirt buildup in a
narrow strip at the edge of a wide frame.

Additional testing demonstrated that errors during fabrication caused
degradation for some modules and may account for some of the observed
OTF array results. In another generation of modules, a uniform dark
appearance for the front of modules was achieved with narrow frames by
covering the shinny bus bars with black decorative tape prior to
encapsulation. For wide or narrow frames, additional tape is used during
lay-up for encapsulation to preposition and hold the circuit plate, EVA,
glass cover sheet and TPAT backsheet. The proper approach is to tape
together the external components, backsheet to the cover glass, thereby
holding all components together and allowing the tape to be removed
after lamination. However, destructive testing found that this tape was
at times improperly used to hold internal components i.e. internal
components were taped to the circuit plate or cover glass before laying
up the complete package. Therefore, this improperly used tape was partly
inside the package during lamination and the tape could not be removed
after lamination. Instead, after lamination the tape was trimmed off
with the excess EVA and backsheet. The bond between EVA and the adhesive
has never been tested for it's ability to prevent water vapor ingress
and EVA does not stick at all to the other side of this tape. Therefore,
the improperly used tape for lay-up provided a path for water vapor
penetration past the hermetic seal formed by the combination of the TPAT
backsheet and an offset between the circuit plate and the frame (see
Figure 5). The combination of improperly used tape during laminate
lay-up and dirt buildup on obsolete wide frames could explain
performance variation from virtually no loss to significant losses.

![](media/image7.wmf)

[]{#__RefHeading___Toc61866285 .anchor}Figure 17. Improperly used tape
inside the package.

One of the modules from the array at NREL that showed a degraded FF was
dissected expecting that lay-up tape would be found and explain the loss
in FF. The frames were removed and the edge of the laminate was
inspected. Although indentations in the backsheet due to proper use of
lay-up tape were found, inspection of the edge gave no indication of
improperly used lay-up tape. The EVA was cut and removed from the step
formed by the edge of the circuit plate and the front plate thereby
exposing the edge of the laminated circuit plate. No areas of poor
adhesion between the EVA and the back edge of the circuit plate were
observed by attempting to lift the EVA with a utility knife. This
testing indicted no improper use of lay-up tape for this particular
module.

Adhesion between the backsheet and the circuit plate was also tested by
cutting into the EVA parallel to the circuit plate to release a section
of the backsheet and then peeling back the backsheet. Lower adhesion of
the TPAT along one of the long edges of the circuit plate was observed
along with the odor of EVA. These subjective observations may indicate
incomplete curing of the EVA leading to lower interface adhesion and
thereby higher moisture ingress. Also, glass to EVA adhesion is know to
be dependent on EVA curing and the bond between EVA and glass is
mechanical rather than chemical although "primers" are added to the EVA
to improve adhesion. In addition to EVA curing, EVA quality, priming,
glass surface quality or other lamination and packaging issues could
have lead to the lower adhesion of the TPAT along one edge and the odor
of EVA. In addition, the ability of the frames to help maintain a
physical bond through compression, directly inhibit water vapor
penetration or avoid trapping water may depend on these packaging
details and the indentations in the backsheet from "proper" use of
lay-up tape. Therefore, packaging issues, the use of lay up tape and
other details of the lamination and framing processes, may have
contributed to failure for some but not all modules.

Laser scanning results published by NREL indicate degradation due to
shunting primarily at the edge of modules, which is consistent with an
interpretation that packaging issues lead to degradation \[[^19]\].
Reverse bias hot spot measurements made at NREL using a technique based
on observing discoloration of heat sensitive sheets indicated hot spots
throughout some degraded modules. However, a correlation between the
qualitative hot spot data and the modules that degraded could not be
established since similar data before degradation was not available, the
link between forward bias FF and reverse bias hot spots is tenuous, and
a comparison could not be made between hot spots and modules with and
without degradation. These hot spot measurements do not seem to clearly
contradict the laser scan results indicating a packaging issue.

![](media/image8.png){width="3.275in" height="3.4208333333333334in"}

[]{#__RefHeading___Toc61866286 .anchor}Figure 18. A laser scan of a
thin-film module showing shunting at the edges (Figure 8. from reference
Error: Reference source not found ).

Information generated in conjunction with the development of a new
glass/glass package, with no TPAT backsheet, was considered to explain
degradation for some modules made using the package design incorporating
a TPAT backsheet. As discussed in the preceding section, broadening and
darkening of interconnects, Figure 12, has been found to be an indicator
of power losses characterized by lower current due to gross moisture
ingress. No darkening and broadening of the interconnects was observed
for the dissected module from the array at NREL that lost FF. Smaller
(ST10) modules retrieved for nondestructive and destructive testing may
have had barely detectable corrosion along the edge of cells but not the
darkening and broadening observed for the prototype glass/glass package.
Also, the electrical performance change with darkening and broadening of
the interconnects is a drop in current rather than an decrease in FF.
Therefore, high water vapor ingress and a decrease in Isc does not seem
to be the cause of degradation for the subset of modules deployed with a
TPAT backsheet. This does not contradict the possibility that a lower
degree of water vapor ingress could be responsible for the decreases in
FF.

Array data from test sites throughout the country is consistent with and
augments the information obtained from individual modules and the NREL
array. Figure 19 is a chart of average annual decline in power for 32
module arrays deployed at:

> "FSEC" \` - Florida Solar Energy Center, Cocoa, FL
>
> "Wisconsin" - University of Wisconsin, Madison, Wisconsin
>
> "Gumbo" - Gumbo Limbo Environmental Education Center & Florida
> Atlantic    University, Boca Raton, Florida
>
> "NREL" - National Renewable Energy Laboratory, Golden, Colorado
>
> "Austin" -University of Texas at Austin, Austin, Texas
>
> "SWTDI" - Southwest Technology Development Institute, Las Cruces, New
> Mexico

These test site locations represent diverse climates and, as indicated
in the figure, two main module configurations, wide and narrow frames,
are included in the data. The two module configurations are equally well
described as two production timeframes. The average annual decline
ranges from slightly negative, an improvement, through minimal changes
to significant losses. If there is a climatic component of the changes,
it is obscured by other variation. However, a hot humid environment,
which would be expected to be the harshest, is not necessary for changes
and, in fact, the array showing a small improvement is from an area
(Austin) selected for array deployment because of the hot and humid
environment.

![](media/image9.wmf){width="4.856944444444444in"
height="3.314583333333333in"}

[]{#__RefHeading___Toc61866287 .anchor}Figure 19. 32 Module Arrays:
Average Annual Decline

The presence of any significant loss is best correlated with module
configuration or timeframe \[Error: Reference source not found\]. This
correlation is also consistent with long-term outdoor stability testing
for individual modules, Figure 13, where multiple prototype package
designs have demonstrated long-term stability. The possible exceptions
are the two modules deployed in 1998, which is the same timeframe as the
arrays that exhibit losses. Losses are not inherent since multiple past
and present arrays have demonstrated stability, a range of array
performance from improvement through losses is observed for the
timeframe where arrays might show significant losses and within the
arrays that show losses there is a range of module performance including
no loss. Some failure mechanisms, such as improper use of lay-up tape
and dirt buildup for wide frames, have been clearly demonstrated. Other
subjective observations imply that additional circuit plate or packaging
process variables may have effected long-term stability for
pre-production modules produced in the timeframe when wide frames were
standard.

## Conclusions

Outstanding progress has been made in the initial commercialization of
high performance thin film CIS technology. The following are highlights
of accomplishments during this subcontract period:

-   Executing SSI's process continues to demonstrated predictability of
    the CIS process.

-   Cumulative production for 2002 exceeded 1 MW - about twice the
    production rate for 2001

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-   Average laminate efficiency for 2002 is 10.8% with a full width of
    only 12% of the average.

-   Dramatic increases in line yield from improved production protocols
    and addressing disparate special causes for process variation.

-   Line yield increased from about 60% in 2000 to about 85% in 2002.

-   NREL confirmed a champion 12.8 percent aperture area conversion
    efficiency for a large area (3626 cm^2^) CIS production module.

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-   Significant progress was made toward developing a "glass/glass"
    package that eliminates the TPAT backsheet for decreased cost,
    simplification of the package and decreased operating temperature

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```
-   Field failure mechanisms for prototype modules have been clearly
    demonstrated. Additional circuit plate or packaging process
    variables also effected long-term stability for pre-production
    modules.

-   Very promising preliminary results for edge seal developed in
    collaboration with the new NREL sponsored National Thin-Film PV
    Module Reliability Team.

-   Long-term outdoor stability has been demonstrated at NREL where
    \~30x30 cm and \~30x120 cm modules with multiple prototype package
    designs have undergone testing for over fourteen years.

The demonstrated high line yield as the result of process R&D is the
major accomplishment. No one major process improvement was responsible
for the yield improvements. Dramatic improvements in yield were the
result of improving production protocols and addressing disparate
special causes for process variation. This major accomplishment supports
attractive cost projections for CIS. SSI\'s thin-film CIS technology is
poised to make very significant contributions to DOE/NREL/NCPV long-term
goals - higher volume, lower cost commercial products.

Further device and production R&D can lead to higher efficiencies, lower
cost, and longer product lifetime. Prerequisites for commitment to
large-scale commercialization have been demonstrated at successive
levels of CIS production. Remaining R&D challenges are to scale the
processes to even larger areas, to reach higher production capacity, to
demonstrate in-service durability over longer times, and to advance the
fundamental understanding of CIS-based materials and devices with the
goal of improvements for future products. SSI\'s thin-film CIS
technology is poised to make very significant contributions to
DOE/NREL/NCPV long-term goals - higher volume, lower cost commercial
products.

# References

[^1]: D. Tarrant, R. Gay, "Research on High-Efficiency, Large-Area
    CuInSe~2~-Based Thin-Film Modules, Final Subcontract Report, 16
    August 1993 -- 30 June 1995", NREL/TP-413-8121, 1995.

[^2]: D. Tarrant, R. Gay, \"Thin-Film Photovoltaic Partnership Program -
    CIS-based thin film PV Technology. Final Technical Report, September
    1995 through December 1998\", (October 1999) NREL report
    NREL/SR-520-27148.

[^3]: D. Tarrant, R. Gay, \"Thin-Film Photovoltaic Partnership Program -
    Commercialization of CIS-Based Thin-Film PV. Final Technical Report,
    August 1998 - November 2001\", August 2002, (NREL/SR-520-32772).

[^4]: M. Contreras, et. al. "Progress Toward 20% Efficiency in
    Cu(In,Ga)Se~2~ Polycrystalline Thin-Film Solar Cells". Progress in
    Photovoltaics: Research and Applications. Vol. 7, 1999; pp. 311-316;
    NICH Report No. JA-520-26111.

[^5]: R. Gay, "Prerequisites to Manufacturing Thin-film Photovoltaics",
    Thin Film Photovoltaic Symposium, IEC, May 1997. Published in
    Progress in Photovoltaics: Research and Applications, 5 (1997) pp.
    337-343.

[^6]: Dale E. Tarrant and Robert R. Gay, "Early Experience with
    Manufacturing CIS Products", National Center for Photovoltaics,
    Program Review Meeting, April 16-19, 2000, Denver, Colorado.

[^7]: Robert R. Gay and Franz H. Karg, "Experience with Manufacturing
    CIS Products", Euroconference "Photovoltaic Devices: Thin Film
    Technology", Berlin (Teltow), Germany.

[^8]: C. L. Jensen, D. E. Tarrant, J. H. Ermer, G. A. Pollock, \"The
    Role of Gallium in CuInSe~2~ Solar Cells Fabricated by a Two-Stage
    Method\", Proc. 23rd IEEE Photovoltaic Specialists Conference,
    Louisville, Kentucky, May 10-14, 1993, pp. 577-580.

[^9]: D. Tarrant, J. Ermer, \"I-III-VI2 Multinary Solar Cells Based on
    CuInSe~2~\", Proc. 23rd IEEE Photovoltaic Specialists Conference,
    Louisville, Kentucky, May 10-14, 1993, pp. 372-378.

[^10]: R. Gay, M. Dietrich, C. Fredric, C. Jensen, K. Knapp, D. Tarrant,
    D. Willett, \"Efficiency and process Improvements in CuInSe~2~-Based
    Modules\", Proc. 12th European Photovoltaic Solar Energy Conference,
    Amsterdam, The Netherlands, April 1994, pp. 935-938.

[^11]: R. Wieting, D. DeLaney, M. Dietrich, C. Fredric, C. Jensen, D.
    Willett, "Progress in CIS-Based Photovoltaics through Statistical
    Process Control", Proc. 13th European Photovoltaic Energy
    Conference, Nice, France, October, 1995, 1627-1630.

[^12]: R. Wieting, \"The Application of Statistical Process Control to
    the Development of CIS-Based Photovoltaics\", 13th NREL
    Photovoltaics Program Review, Lakewood, Colorado, May 1995.
    Published in AIP Conference Proceedings 353, (1996), pp. 19-25.

[^13]: Wheeler, Donald J.; Chambers, David S. (1992). Understanding
    Statistical Process Control, 2nd edition. Knoxville, TN: SPC Press.

[^14]: D. Tarrant, R. Gay, "High Yield CIS Production - Progress &
    Perspectives", National Center for Photovoltaics and Solar Program
    Review Meeting -- 2003, March 24-26, 2003.

[^15]: D. E. Tarrant, R. R. Gay, V. Probst, F. H. Karg "CIS Thin Film
    Development and Production Status at Shell Solar, May 2003", 3rd
    World Conference on Photovoltaic Energy Conversion, May 11 - 18,
    2003, Osaka, Japan

[^16]: Supported by NREL Subcontract \# NDO-1-30628-06, "PV
    Manufacturing R&D, Integrated CIS Thin-Film Manufacturing
    Infrastructure".

[^17]: Voss, Steve, "Accelerated Environmental Testing of Shell Solar
    CIS", National Thin-Film Module Reliability Team Meeting, Florida
    Solar Energy Center, Cocoa, FL., February 27-28, 2003.

[^18]: J. R. Sites, "Device Physics of Thin-Film Polycrys-talline Cells
    and Modules, Phase I Annual Report", NREL/SR-520-27663, December,
    1999, p 16.

[^19]: Emery, Keith, "Nondestructive Performance Characterization
    Techniques for Module Reliability", NCPV and Solar Program Review
    Meeting 2003 NREL/CD-520-33586 Page 954.