A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

National Renewable Energy Laboratory 

Innovation for Our Energy Future 

Cost and Reliability Improvement 

for CIGS-Based PV on Flexible 

Substrate 

Annual Technical Report 

24 May 2006 – 25 September 2007 

S. Wiedeman 

Global Solar Energy, LLC 

Tucson, Arizona 

Subcontract Report 

NREL/SR-520-43840 

August 2008 

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 
Cost and Reliability Improvement 

for CIGS-Based PV on Flexible 

Substrate 

Annual Technical Report 

24 May 2006 – 25 September 2007 

S. Wiedeman 

Global Solar Energy, LLC 

Tucson, Arizona 

NREL Technical Monitor: H.S. Ullal 

Prepared under Subcontract No. ZXL-6-44205-13 

Subcontract Report 

NREL/SR-520-43840 

August 2008 

National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393 

303-275-3000 • www.nrel.gov 

Operated for the U.S. Department of Energy 

Office of Energy Efficiency and Renewable Energy 

by Midwest Research Institute • Battelle 

Contract No. DE-AC36-99-GO10337
This publication was reproduced from the best available copy 

submitted by the subcontractor and received no editorial review at NREL

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Executive Summary 

Global Solar Energy (GSE) has completed many of the tasks planned under the 

first phase of this subcontract while simultaneously executing a dramatic increase 

in production capacity in the planning and construction of two entirely new 

production facilities for thin film CIGS PV (in Tucson, AZ and Berlin, Germany). 

GSE has made tangible progress, as detailed in the following pages, in 

quantifying and enhancing module reliability, reducing the cost and improving the 

performance of the CIGS deposition process, and in reducing the cost of the 

materials and processes used for the front and back contacts. Each of these 

areas is a major thrust of this Thin Film Photovoltaic Partnership Program 

subcontract, and an important opportunity to improve thin film PV product. 

The largest share of the effort over the past months, however, has gone into the 

activities necessary to bring this countries’ first large scale facilities for the 

production of CIGS PV into operation. A major endeavor, this effort entailed the 

selection, negotiation and purchase of a 120,000 sq. ft. facility on 22 acres in 

Tucson, and planning for another complete factory in a leased facility in Berlin. 

Both facilities entailed extensive architectural and design work to attain efficient, 

balanced production in all processes, in a fully facilitated, climate-controlled, stateof-the-art thin film manufacturing plant. 

Simultaneously, lines of entirely new production tools for all processes were taken 

from concept, through design, vendor selection and into fabrication and 

installation for both plants. Previous experience gained by GSE in bringing the 

world’s 1st generation of roll-to-roll CIGS processing equipment to 4 MWp/year 

production was fully leveraged in the design of the 2nd generation tools for the 

new plants in Tucson and Berlin. The 2nd generation thin film production tools are 

designed to run faster, longer, better and with greater flexibility, reproducibility, 

higher materials utilization, and less labor and maintenance cost. 

Much of the design effort to attain these goals for the production equipment also 

represents progress toward the major thrusts of this TFPPP subcontract. For 

examples, we fully expect that the concepts incorporated into the 2nd generation 

CIGS deposition equipment will enable GSE to easily meet, and exceed, the “hibar” goal for CIGS processing rate in the TFPPP milestones. We fully expect that 

the flexibility and improved in-situ measurement capability built into the new 

machines will allow rapid progress toward TFPPP goals of thin absorber layers, 

improved uniformity, etc. 

The 60 MWp/yr capacity of the 1st phase represents greater than an order of 

magnitude expansion for GSE. The 2nd phase expansion, already commenced, is 

to 140 MWp/yr capacity between Tucson and Berlin. Global Solar is committed to 

the successful large-scale manufacture of thin-film CIGS solar products. 

iii
Table of Contents 

Executive Summary .......................................................................................... iii 

List of Figures ..................................................................................................... v 

List of tables ....................................................................................................... v 

1. Task 1: Enhanced Module Reliability ............................................................ 1 

1.1 Product Testing and Reliability ............................................................... 1 

1.2 Flexible Product ..................................................................................... 1 

1.3 Glass-based Product ............................................................................. 2 

1.4 Reliability and Performance – Related Tests ......................................... 5 

2. Task 2: CIGS Coating Cost Reduction .......................................................... 6 

2.1 Production Scale-up (New Manufacturing Facilities) .............................. 6 

2.2 Enhanced CIGS Deposition Rates ....................................................... 10 

3. Task 3: Front Contact Cost Reduction (Objectives) .................................. 14 

3.1 Incremental Improvement of the Existing TCO Process....................... 14 

3.2 Novel TCO Deposition Process ........................................................... 17 

4. Task 4: Back Contact Cost Reduction and Efficiency Improvement ............ 19 

4.1 Back Contact Process Improvements .................................................. 19 

4.2 Alternate Substrate Vendors and Materials .......................................... 20 

Summary .......................................................................................................... 21 

Future Plans ..................................................................................................... 21 

References ....................................................................................................... 22 

iv
v

List of Figures 

Figure 1: A comparison of glass module performance in damp heat between 

first and second-generation string technologies ..................................... 3 

Figure 2: An example of accelerated testing in progress at GSE. Various 

product configuration and materials are tested in damp heat 

(85°C, 85% relative humidity) for extended time .................................... 4 

Figure 3: Performance data taken over an extended time for GSE 

glass-based product in actual outdoor use ............................................. 5 

Figure 4: One section of the GSE Tucson building under demolition .................... 7 

Figure 5: A northeast section of the GSE Tucson building under demolition ......... 8 

Figure 6: The building exterior for the Tucson facility before renovation ................ 8 

Figure 7: Interior structures, equipment chases and HVAC under renovation 

in the Tucson facility .............................................................................. 9 

Figure 8: Interior rooms, overhead lighting and HVAC under renovation in the 

Tucson facility ........................................................................................ 9 

Figure 9: Auger electron spectroscopic analysis done at NREL comparing 

the composition of cells made at GSE at web speeds of 12 and 

15-in/min.............................................................................................. 11

Figure 10: Cell efficiency results for a controlled study comparing CIGS material 

made at 15-in/min. vs. 12-in/min ......................................................... 12 

Figure 11: A representative I-V characteristic for a large area cell made at an 

increased web speed of 15-in/min ....................................................... 13 

Figure 12: A comparison of the optical transmission as a function of wavelength 

for the standard production TCO process and a new process under 

evaluation ............................................................................................ 15 

Figure 13a,b: A comparison of the sheet resistance behavior over the course of 

single runs for the previous process (upper plot) and the improved 

process (lower plot). ............................................................................ 16

Figure 14: The external optical transmission of a ZnO film on glass deposited 

using the “ECS” method ...................................................................... 18 

List of Tables

Table 1: Pre-UL testing – a battery of tests run at GSE representing many 

of the procedures included in UL testing ................................................ 1 
Task 1: Enhanced Module Reliability (Objectives) 

1. Identify, characterize and quantify degradation and failure mechanisms in the PV 

stack and cell interconnect as well as encapsulation structure and complete 

module package. 

2. Design meaningful stress tests for flexible and rigid thin film CIGS modules. 

3. Develop a finite element model predicting mechanical post-lamination stresses at 

module operating conditions (daily and seasonal temperature cycles). 

4. Explore solutions to eliminate failure & degradation mechanisms via process 

changes, advanced alternate encapsulation, protective coatings, structural 

elements and complete package. 

5. Verify and optimize long-term product reliability. 

6. Improve product appearance and cost. 

1.1 Product Testing and Reliability 

For rigid PV products intended for rooftop and commercial bulk power generation, long 

term reliability (20 – 30 years) is crucial. For flexible PV product in intermittent use, 

features such as durability, portability and appearance are valued over long term 

reliability. In either case, real-time and accelerated testing is important to determining 

reliability, durability, failure modes and their causes. 

GSE uses multiple methods to evaluate suitability, including actual outdoor exposure 

testing, accelerated testing in dedicated chambers or heightened stresses, and specific, 

standardized tests. 

1.2 Flexible Product 

A battery of tests is shown below, which was devised in this example to pre-qualify 

flexible product for UL testing. 

Table 1 - Pre-UL testing – a battery of tests run at GSE representing many of the 

procedures included in UL testing for 6 and 12 watt flex product. 

Bonding Path resistance 

Leakage Current 

Dielectric Voltage Withstand 

Water Spray 

Leakage Current 

Dielectric Voltage Withstand 

Pull Test 

Push Test 

Wet Insulation Test 

Dielectric Voltage Withstand 

1 
After continued improvement of procedures and materials used in product assembly, 

GSE product tested using the sequence above passed all tests. 

For flex product a failure mode associated with lengthy cycles of folding and unfolding 

was observed. An evaluation of alternative wiring schemes (between sub-modules) for 

the portable foldable module product line was conducted with the objective of increasing 

the field durability of the modules against repeated deployment. Equipment was set up 

to accelerate and monitor failure in the fold zone. Microscopic analyses of the 

conductors in the folding areas after stressing were performed and the source of failure 

was determined to be metal wire fatigue and separation. An alternative wiring scheme 

was developed that increased the MTBF by over 5X. The alternative scheme was 

implemented for all models anticipated to require significant flexing in typical use. 

Outdoor testing of flex modules for long term exposure under actual outdoor conditions 

has also been valuable in relative comparisons of product variations, and in identifying 

modes of failure. When a new failure mode is identified, specific tests are usually 

designed to isolate that failure mode under controlled conditions using quantitative 

means. Experience gained in identifying failure modes in outdoor testing and in 

accelerated testing has allowed GSE to establish protocols for test article submission 

for both modules and submodules of flexible and glass based products. 

1.3 Glass-based Product 

As an example, after early field observations in outdoor testing identified some failures 

relating to delamination in glass product, standardized “pull tests” were initiated to 

gauge adhesion strength as a function of the laminating materials and procedures used. 

The adhesion strength of several laminating materials to glass was tested versus 

several different cleaning methods. The cleaning method giving the best results was 

affirmed as the standard method, and used for all glass product. 

Another use of the same standardized adhesion “pull test” was to evaluate a method of 

chemical priming with current laminating materials for adhesion strength. Pull tests 

indicated consistently higher adhesion without using this priming compound. 

Standard tests and patterns were also instituted for electrical performance measures. 

Curing profiles and multiple types of electrically conductive inks were iteratively tested 

to improve the resistance of printed cell grid patterns. Results of one test demonstrated 

an adjusted curing profile that resulted in a printed pattern resistance 55% lower than 

the “standard” curing profile. 

Given the emphasis on glass-based product in the production plans, GSE has ramped 

up design and test efforts to develop compatible stringing materials and methods for 

glass product. Reliable stringing methods are required to avoid interconnection failures. 

Again, failure modes were identified, and experiments to evaluate stress effects were 

completed, resulting in a preliminary selection of compatible stringing materials for glass 

product. 

2 
Another change required for the expanded GSE production plan is the use of larger cell 

sizes, called “Gen2” (210mm x 100 mm, and 115 mm x 76 mm). The cell design and 

interconnect methodology developed for the Tucson 40MWp expansion line was 

evaluated. Gen2 cell strings were fabricated by a manual technique to define the 

specifications for an automated stringing tool. Materials were down-selected for string 

connection based on conductivity, mechanical stress, and capability of application in 

high speed equipment. The new generation strings (laminated into glass modules) 

demonstrated satisfactory performance under accelerated testing (Figure 1). Several 

modules have been placed outdoors for periodic measurement. 

Time in 85C/85% RH (hours)

Efficiency relative to Cell (%)

0 500 1000 1500 2000 2500 3000 3500 4000

60

65

70

75

80

85

90

95

100

105

110

Structure: Gen1

Structure: Gen2

Fig. 1 A comparison of glass module performance in damp heat between first and secondgeneration string technologies. The packaging is identical. The modules (15 and 10 in each 

group, res.) were light-soaked between 4 and 10 hours prior to each measurement.

3 
Because the “Damp Heat” test at 85°C and 85% relative humidity is one of the most 

demanding tests for module reliability, it is used frequently to gauge progress in 

reliability. Iterative testing of new materials and assembly techniques described in this 

section has improved the expected reliability of glass-based product. GSE has been 

able to demonstrate results that bode well in this regard (one example shown below in 

Fig. 2 for glass product). 

Damp heat (85-85) Testing of Glass Product

0 500 1000 1500 2000 2500 3000 3500

Hours at 85% relative humidity and 85° Celsius

92

94

96

98

100

102

104

106

108

110

112

114

Pmax normalized to initial value 

 Mean Mean±0.95 Conf. Interval 

Fig. 2 An example of accelerated testing in progress at GSE. Various product configuration 

and materials are tested in damp heat (85°C, 85% relative humidity) for extended time. 

Normalized power output greater than 100% are often observed due to “light soak” effects 

typical of CIGS PV. 

Extended testing of different product configurations and laminates continues in the 

damp heat (85°C, 85% relative humidity) to build accelerated test data. Glass-based 

product also continues in outdoor test deployment to gather performance data under 

actual conditions. Ultimately the data gathered using accelerated and real-time testing 

will allow stronger correlation between the two and accurate assignment of the 

acceleration factors in the accelerated tests. 

4 
Accumulated performance data for early glass-based product, deployed at Springerville, 

AZ for long-term outdoor testing continues to function with little degradation. This data, 

monitored daily, is shown in Fig. 3 below. 

CIGS Module Array Efficiency 

(monitored daily, weather permitting, corrected to STC)

0

1

2

3

4

5

6

7

8

9

10

0 100 200 300 400 500 600 700 800 900 1000

Days Deployed

Efficiency (Corrected via model, to 

STC)

(8/18/2005 Installation Date)

Fig.3 Performance data taken over an extended time for GSE glass-based product in actual 

outdoor use. These modules are early versions of glass-based product, and thus did not 

incorporate many of the improvements developed under this program. 

1.4 Reliability and Performance – Related Tests 

The reliability of two screen-printable Ag inks was compared to the standard production 

ink applied as the collection grid. Various formulations of conductive inks are constantly 

under consideration for cost reduction and to improve the printing characteristics (line 

definition, pot life). Reliability was evaluated by accelerated testing (85 ̊C/85% RH) of 

glass-laminated and flexible modules. No statistically significant difference between the 

inks was observed. 

Inventory storage of finished large area cells in itself may introduce questions of 

stability. The electrical parameters of large area cells stored in a humidity-controlled 

environment up to 1024 hours were evaluated, to understand whether changes in the 

characteristics of inventoried solar cells can occur. The test was configured to eliminate 

light-soaking (during testing) from affecting the results. One lot from each of four CIGS 

deposition systems were evaluated. Over the duration of 1024 hours, three lots 

increased in efficiency (+3%, +3%, +11%), and one lot decreased (-12%). Further 

investigation will be performed to understand the differences observed. 

5 
Production control and analyses of reliability tests are frequently complicated by lightsoaking effects. Conditioning outdoors is subject to the vagaries of weather and 

season. Various light sources were evaluated for their utility for light-soaking nonencapsulated solar cells and thereby bringing the cells to a reproducible state for 

testing. Emphasis was placed on light sources with strong UV and/or blue spectrum 

components. One of the wider spectrum sources was ultimately identified to meet the 

requirement for indoor light soaking required for accurate test measurements. 

Task 2: CIGS Coating Cost Reduction (Objectives) 

1. Increase processing rate for CIGS deposition by at least 25% with a high-bar 

goal of 50% (from 12-in/min to 15-in/min and potentially 18-in/min). 

2. Modify effusion sources as necessary to ensure adequate cross-web uniformity 

at increased absorber formation speeds. 

3. Reconfigure In and Ga sources to allow improved homogenization of In-Ga at the 

higher CIGS deposition rates and reduced time for mixing by diffusion. 

4. Re-optimize CIGS process parameters for device efficiency at the high 

processing rates and altered In-Ga delivery. 

5. Evaluate alternate sodium delivery, for efficiency, control and uniformity; and high 

process rates, implement if successful into the standard process. 

6. Evaluate thinner CIGS layers. Reduce flux rates to achieve less than 1.0 μm 

CIGS thickness and re-optimize CIGS process conditions to maximize efficiency 

for thin absorber layers. 

2.1 Production Scale-up (New Manufacturing Facilities) 

The overall intent of this contract is to enable significant gains in cost and throughput for 

thin film PV. The specific process improvements to enable significant cost and 

throughput gains are listed in the objectives above (for the CIGS layer, and in the 

following sections for the front and back contact layers). However, the overall goal of 

this contract, and many of the specific process improvements are related to, or 

dependant on, the design and installation of improved plant and equipment. GSE has 

embarked, in earnest, on just such a massive expansion. For completeness, and 

because new plant and equipment is a prerequisite to implementation of many of the 

objectives in sections 2, 3 and 4, progress toward this endeavor will be summarized in 

this section. 

In March of 2006, GSE was charged with expanding its manufacturing capability by 

more than an order of magnitude. GSE was expected to bring on line a new plant (in 

Tucson), with re-designed equipment for all processes within 12 months. Subsequent 

to this decision, another plant in Berlin, Germany having a capacity of 35 MW/year was 

planned in addition to the 40 MW/year capacity slated for the Tucson plant. These 

planned capacities are for the “phase 1” construction. A “phase 2” of the expansion is 

planned to increase the capacity of the Tucson location to 140 MW, in addition to a 

6 
capacity increase in the Berlin plant. The “phase 2” expansion is slated for completion 

in 2010, with the required -facility improvements for phase 2 already complete in the 

Tucson plant, and additional deposition equipment already under contract for many 

process steps for the Berlin factory. 

During phase 1 of this TFPPP subcontract GSE located and negotiated the purchase of 

a building and land (110,000 sq. ft., and 22 acres, resp.) in Tucson, and has made 

lease arrangements for the factory space in Berlin. The Tucson building was formerly 

one of the largest magnetic tape coating facilities in the world, so in this sense the new 

PV factory continues the thin film coating legacy of the original building. Other 

similarities are absent, and thus the interior of the former building was entirely 

demolished, and then rebuilt. All existing equipment was removed, an extensive volume 

that filled a 250,000 sq. ft. lot 3 times over. Industrial architects and contractors were 

engaged to plan all aspects of the new plant – renovation, distribution of utilities such as 

electrical power, compressed air, process gasses, chemical distribution and 

containment, process cooling water, heating, ventilation and air conditioning, humidity 

control, storage and office space. Figures 4-6 below show the Tucson facility in early 

stages of demolition. 

Fig. 4 One section of the GSE Tucson building under demolition.

7 
Fig. 5 A northeast section of the GSE Tucson building under demolition.

 

Fig. 6 The building exterior for the Tucson facility before renovation.

8 
Fig. 7 Interior structures, equipment chases and HVAC under renovation in the Tucson facility. 

 

Fig. 8 Interior rooms, overhead lighting and HVAC under renovation in the Tucson facility. 

9 
Simultaneously, subcontractors were engaged to design and build deposition and 

production equipment for all the processes required for large scale CIGS PV 

manufacture. Deposition equipment was built to GSE specifications and concepts, 

based on the actual manufacturing experience GSE had gained with the 1st generation 

equipment at the existing plant in Tucson. In all cases, the re-designed equipment 

embodied advancements, including: 

• Deposition equipment built for higher deposition rates, 

• Reduced product cost, 

• Improved materials utilization, 

• Enhanced process control and reproducibility, 

• A high degree of automation, all processes are designed to run largely 

unattended, 

• High throughput, 

• Process flexibility. 

Specifically for the CIGS process, the deposition equipment was designed to handle 

rolls of substrate 3x the length of the existing, 1st generation equipment, at 2x the 

speed, with better cross-web and down-web uniformity, improved In, Ga and Se 

utilization, and more process flexibility. 

2.2 Enhanced CIGS Deposition Rates

The maximum rate possible for CIGS formation is hitherto unknown, being limited at 

some point by the time-at-temperature required for sufficient materials reaction and 

diffusion through the absorber layer thickness. Higher CIGS formation rates are 

desirable to maximize throughput for the CIGS deposition, and is one of the goals of this 

program. Feasibility trials for CIGS deposition speed were run under this TFPPP 

program, demonstrating that deposition speed can be increased from 12 to 15-in/min in 

the current reactors without any impact on performance or material characteristics. 

Absorber layer compositions evaluated with Auger electron spectroscopy indicated 

essentially no change in the desired profile at 15-in/min compared to 12-in/min in Fig. 9. 

10 
0

20

40

60

80

100

120

0 20 40 60 80 100 120 140 160 180 200

C1.ls1 [0.128]

O1.ls1 [0.247]

S1.ls1 [1.695]

Cu1.ls1 [0.307]

Zn1.ls1 [0.296]

Ga1.ls1 [0.338]

Se1.ls1 [0.123]

Mo1.ls1 [0.271]

Cd1.ls1 [1.280]

In1 [0.762]

Sn1.ls1 [0.643]

C1.ls1

O1

S1.ls1

Cu1.ls1

Zn1.ls1

Ga1.ls1

Se1.ls1

Mo1.ls1

Cd1.ls1

In1

Sn1.ls1

12 ipm = Thick lines

15 ipm = Thin lines

Composition vs. Depth 

(sputtering time) for GSE 

CIGS (run 2161SA). 

Cells made at 12 ipm and 15 ipm web 

speed (via Auger, at NREL)

Fig. 9 Auger electron spectroscopic analysis done at NREL comparing the composition of cells 

made at GSE at web speeds of 12 and 15-in/min.

With proper control adjustments to flux rates at different stages of deposition, it is 

apparent from Fig. 9 that the deposition rate (and web speed) can be increased by at 

least 25% over the standard rate for the Gen 1 process at GSE while maintaining a 

virtually identical composition profile. 

The equivalence of the CIGS material quality made at increased deposition rates is also 

evident in cell performance, as indicated by the large area (68 cm2

) sample cell 

efficiencies in Fig. 10. Both cells were deposited at different locations on the same roll 

of substrate, in the same run but with control adjustments made to flux rates to maintain 

material thickness and profile at the higher web speed. Fig. 11 shows the IV 

characteristic of a single large area cell, again, indicating that good performance can be 

maintained even at an increased CIGS deposition rate, in satisfaction of one of the 

deliverables under this program. 

11 
Cell Efficiency vs. Web Speed (2161SA )

0

1

2

3

4

5

6

7

8

9

10

11

12

200 300 400 500 600 700 800 900

Length, ft

Efficiency, %

12 in/min

15 in/min

Fig. 10 Cell efficiency results for a controlled study comparing CIGS material made at 15-

in/min vs. 12 in/min. 

Further gains in maximum CIGS deposition rate are expected in the new Tucson and 

Berlin plants owing to the design of the 2nd generation CIGS deposition equipment. 

Based on experience with the earlier equipment, significant changes were instituted in 

the design phase of the 2nd generation equipment specifically to accommodate higher 

deposition rates without sacrificing material properties. In addition, the 2nd generation 

equipment was designed to allow more control and flexibility over the composition 

profile, further enhancing possibilities for achieving even higher deposition rates and/or 

performance. Fundamentally, even after the tests described here, the maximum 

deposition rate for the CIGS process is not known. 

12 
Cell 402.05

0

0.5

1

1.5

2

2.5

0 0.1 0.2 0.3 0.4 0.5 0.6 0

Voltage V

Current A

.7

Vmax, mV 385.8154

Imax, mA 1924.347

Pmax, mW 742.4427

Voc, mV 542.2863

Isc, mA 2270.657

FillFactor, % 60.2952

Efficiency, % 10.7913

Fig. 11 A representative I-V characteristic for a large area cell made at an increased web speed 

of 15-in/min. 

Similarly, fundamental improvements have been made in the design phase for the 2nd

generation equipment to address the 2nd, 3rd and 4th objectives listed in this task. 

Significant changes to the effusion source design used for CIGS deposition have been 

made to ensure adequate cross-web composition uniformity, in addition to 

improvements in materials utilization. The re-designed metals effusion sources have 

undergone preliminary testing during this reporting period, and so far perform well. 

Confirmation that the new design sources meet this objective is expected after they are 

fully installed and the 2nd generation equipment becomes operational in the new plant. 

This objective is complicated by concurrent demands for operation at higher deposition 

rates (discussed above) and greater capacity demanded by the planned introduction of 

much longer runs for up to 3x length webs in the 2nd generation equipment. 

Improvements in In-Ga homogenization and compositional profile control, and 

performance improvements relating CIGS processing parameters (per the 3rd and 4th

objectives) have been addressed in the design phase by instituting a scheme for more 

flexible delivery of CIGS reactants, but must be optimized and proven through designed 

experiments once the 2nd generation equipment is operational in the new plants. 

Working toward ever-improving control over product quality, variables that are identified 

but have an unknown impact are evaluated as time and capability allow. In this regard, 

tests were conducted to determine the effect of humidity swings on the quality of CIGS 

deposited in the production coaters and the resulting effect on solar cell performance. 

13 
The impact of this factor was important to the architects responsible for the plant design, 

and to assure adequate environmental controls for the new plants. It was determined 

that significant quantities of water in the deposition chambers were required before 

process control difficulties were encountered. 

Task 3: Front Contact Cost Reduction (Objectives) 

1. Develop a low-cost process for the transparent front contact TCO coating. 

2. Improve deposition rate of the TCO process, while re-optimizing the process to 

maintain large-area cell efficiency above 12.5%. 

A parallel approach using two paths has been taken for this task; a low-risk pursuit of 

marginal improvements using the present sputtered methodology, and a high risk 

pursuit of a novel TCO deposition process potentially offering very large reductions in 

cost. Cost reduction is important, as the TCO process is currently the single, most 

expensive thin film layer in the device stack. 

3.1 Incremental Improvement of the Existing TCO Process 

Lower cost ceramic targets were evaluated in first-cut experiments to reduce the cost of 

the TCO process. Arcing problems were pervasive, and thought to result from 

excessive clearance under the dark space shields coupled with excessive flake and 

debris generation. However further experiments revealed that the actual cause of the 

variation in sputter voltage was intermittent plasma extinction due to a weak magnetic 

field at the target surface. 

In spite of this problem, generation of production solar cells was attempted for 

measurement and reliability testing. Compared to the baseline top contact process, 

solar cells fabricated with the alternative TCO material had significantly lower yield, and 

maximum conversion efficiency was 8.6% compared to the maximum 11% for the 

control cells. The yield and quality of the solar cells generated was deemed insufficient 

for reliability evaluation. Further deposition experiments were planned using stronger 

cathode magnet arrays to stabilize the plasma. 

Four new magnetic cathode arrays were installed to better evaluate the alternative TCO 

which had been evaluated previously. The improved magnetics resulted in a 640% 

improvement in voltage standard deviation. Once process control was achieved, 

coatings of the new TCO were applied to create solar cells for first reliability testing. 

The flexible modules produced were found to degrade more quickly than ITO control 

modules under accelerated testing (85 ̊C/85%RH). Further effort is planned using the 

lower cost TCO targets, however, it is clear that the TCO deposited from these low cost 

targets must be modified to achieve better resistance to degradation in the “damp heat” 

(85 ̊C/85%RH) test. 

The TCO thickness has been modified in the current production process and 

documented to give slightly better I-V performance concurrent with thinner TCO. 

Improved I-V performance arose due to a controlled shift of optical transmittance 

14 
maxima to more desirable ranges of the solar spectrum, giving an Isc gain (shown in 

Fig. 12). 

0.25

0.35

0.45

0.55

0.65

0.75

0.85

0.95

300 500 700 900 1100 1300 1500

Wavelength (nm)

Transmission (%)

Process under evaluation

Standard Process

Fig. 12 A comparison of the optical transmission as a function of wavelength for the standard 

production TCO process and a new process under evaluation. 

Further optimization of the baseline production TCO process was conducted in this time 

period. One goal was to improve bulk resistivity and thus enable a reduction in 

thickness. This has two key advantages; reduced cost by thickness reduction and 

increased optical transmission at higher energy wavelengths. Process variables 

including the composition of the sputtering gas, pulsing frequency, and pressure were 

systematically varied in designed experiments. The resulting process enabled a 50% 

increase in line speed with an equivalent sheet resistance and integrated transmission. 

Optical measurements of coatings produced by the new process show reduced 

thickness shifting the transmission peak closer the 550nm. Large area cells were 

produced to demonstrate equivalence with the baseline process. 

The modifications made to the gas composition and pressure results in 100% higher 

bulk conductivity. Moreover, the improved process also exhibits a better controlled 

sheet resistance over the course of a run (Figs.13a,b). Sheet resistance regulation of 

the new process is + 32% absolute with a standard deviation of 14%. 

15 
Rsh Probe Tracking

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previous process (upper plot) and the improved process (lower plot). 

16 
The improved TCO process optimization nets GSE significant cost savings. Higher bulk 

conductivity enables the process to run 50% faster, enhancing throughput. Sputter 

power is reduced by 33%. Higher bulk conductivity also reduces the coating thickness 

required, increasing target material utilization. 

3.2 Novel TCO Deposition Process 

The novel TCO deposition process is intended to avoid the requirement for sputtering 

targets, and those attendant costs with expensive target fabrication, limited target 

utilization and lifetime, maintenance and turnover labor thus yielding a cost 

improvement of an order of magnitude or more. An internal effort at GSE was initiated, 

as well as an effort carried out at a lower-tier subcontractor. 

Toward that end in this period, a dedicated web coating chamber has been recommissioned and made operational for development of the low cost TCO. The 

chamber has been fit with specialized mass flow control for two vapor-phase 

precursors, as well as up to 3 conventional gas feedstocks. Provisions were made for 

the addition of dopant in the dedicated chamber at GSE (called the “PE” chamber). 

Required ancillary provisions for a substrate heater and other monitoring instruments 

have also been installed in this roll-roll development chamber. A thermal decomposition 

reactor has been designed and fabricated to safely dispose of gas effluent expected of 

the process. 

Exploratory experiments were completed depositing the low cost TCO on polyimide and 

stainless test webs. Sample TCO films (without any dopant) were also deposited on 

glass substrates for optical and electrical characterization. Some of the conditions 

yielded TCO films that were dense, uniform and well-adherent. The sheet resistivity of 

all films to date is high, as may be expected without any dopant. 

Exploratory runs introducing varying levels of Al as a dopant to ZnO films were started. 

A first round of tests results show some reduction in sheet resistance, but still not 

approaching the low resistivity desired for TCO in films that are transparent and well 

adherent. Test depositions were primarily on glass witness slides. 

A 2nd round of statistically designed experiments have been completed in the “PE” 

chamber at GSE, showing some additional reduction in sheet resistivity while 

maintaining high transparency and adhesion, however a sheet resistivity in the desired 

range has still not been attained. Although reasonable growth rates have been attained 

in the “PE” chamber, the resistivity must be reduced by at least another factor of 4x, with 

reproducibility demonstrated. 

Work at the lower-tier subcontractor to GSE with a specialized deposition method that 

may potentially be suitable for low cost TCO was also started, referred to as the “ECS” 

method. The ECS method for deposition of ZnO has produced high resistivity films with 

good transparency and adhesion in a first round of experiments. Hall-effect mobility 

was low (1.2 cm2

/V-sec), and examination indicated the films typically had a large 

amorphous volume content. All films were done at low substrate temperatures and 

showed good uniformity of deposition in one dimension. 

17 
A further round of experiments with the ECS method gave reproducible results for the 

conductive ZnO, having improved carrier mobility in the range of 10-20 cm2

/V-sec, sheet 

resistivity of ~ 20 ohms/square and good optical transmission (shown in Fig. 14). 

Several factors remain to be proven regarding the ECS approach for TCO based on 

ZnO, including adequate deposition rate and controllability, stability of the ZnO 

properties, and compatibility of the ECS approach for ZnO deposition on actual device 

material. The lower-tier subcontractor is confident of spatial uniformity with the 

approach, and is modifying the apparatus to allow the introduction of extrinsic doping. 

ZnO Optical Transmission (external)

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Fig. 14 The external optical transmission of a ZnO film on glass deposited using the “ECS” 

method at a lower-tier subcontractor to GSE. This film had a sheet resistivity of 22 ohms/square. 

18 
Task 4: Back Contact Cost Reduction and Efficiency Improvement 

(Objectives) 

1. Evaluate substrate properties and analyze resulting impacts on device 

performance. 

2. Quantify impacts of reduced Mo thickness and correlate to device performance. 

3. Examine low-cost alternate back contact materials for partial substitution of Mo 

and compare device performance and other properties against the established 

baseline. 

4. Demonstrate increased device efficiencies and reduced process costs through 

material savings and increased process speeds. 

5. Integrate alternate sodium delivery to maximize adhesion and efficiency at the 

higher CIGS web processing rate. 

4.1 Back Contact Process Improvements 

A first round of designed experiments using large-area cells indicated that cell 

performance is unchanged despite a reduction in the thickness of the back contact 

interface material of up to 50%, presenting an opportunity for reducing materials costs 

and process time. However, thickness changes in the molybdenum layer itself from the 

standard value resulted in a loss in cell performance. It may be possible, however, to 

change other process conditions to allow the use of thinner Mo. This prospect will be 

evaluated in upcoming tests. 

Another series of experiments were conducted to study the effects of deposition 

parameters on the morphology of the Mo layer and the subsequent effects on cell 

performance. Parameters included were temperature, pressure, and gas partial 

pressures. We evaluated morphology by SEM, adhesion of the Mo/CIGS interface by 

pull testing and cell performance. We observed improved cell performance at distinct 

points in the Mo process, and this improved understanding led to changes in processing 

parameters that yielded better process control and homogeneity over the length of the 

web. 

Prior to starting the back contact deposition, the chamber is run through a “conditioning” 

procedure. The conditioning procedure was changed in systematic experiments to 

improve deposition consistency. The results indicated no benefit to actual material 

consistency or quality, however the process reliability was improved due to reduced 

arcing over the course of production runs. Further tests to improve back contact quality 

and consistency are planned. 

Modifications to the start-up sequence and pump-down requirements were also 

investigated to increase throughput in the Mo sputter process. Several modifications 

were successfully implemented that reduced the time required between depositions. 

Flaking of deposited molybdenum during deposition shed from deposition shielding was 

observed to introduce defects on the web. Modifications were made to the deposition 

shields and process in the production coaters to minimize shedding. Other web defects 

caused by uneven tension of the metal foil during deposition were also identified. 

Interactions between web tension, deposition temperature, roller alignment in the 

19 
chamber and shape of the foil (camber) were investigated as a possible cause for the 

tension-related defects. 

4.2 Alternate Substrate Vendors and Materials 

Testing of the stainless steel foil used for substrate material supplied by alternate 

vendors to GSE has started. As the pricing and quality of the incoming substrate 

material vary significantly, this effort has fairly large potential impact on total cost. 

Sample steel coils were received from 3 alternate vendors, and orders placed for 

sample coils from another two during this period. Substrate cleaning methods are also 

important apart from surface and mechanical properties or chemical composition, 

however, and must be evaluated concurrently with the material from other 

manufacturers. Evaluation of the incoming substrate material from different vendors is 

ongoing. 

 The experience in comparative evaluation of substrate from different vendors identified 

a need for a standardized battery of tests for incoming metal foil. A survey of applicable 

techniques and equipment to characterize the important properties of the incoming 

substrate material has been put together. As a result, GSE has identified optical surface 

profilometry as a primary measurement capable of giving quantitative information about 

important properties of incoming metal foil. Several optical profilers were evaluated, 

with one selected for purchase. 

 New vendors were also tested and successfully qualified for sputtering targets of Mo 

and other required back contact materials, giving multiple redundancy in vendors and a 

cost savings in some cases. 

Additionally, a method was tested to join web sections in the roll-to-roll process 

established at GSE. The technique will facilitate more reliable and efficient testing and 

experimentation. A web can be split at any process and with each section undergoing 

separate treatment. After that step, the web can be joined together to complete identical 

processing for all other process steps. 

20 
Summary 

GSE has embarked on a program to further improve module reliability, reduce the cost 

and time of CIGS coating, and for both the front contact and back contact deposition 

processes and materials. Simultaneously GSE has executed on an aggressive plan to 

build and equip two new CIGS PV manufacturing plants (in Tucson, AZ and Berlin, 

Germany) with a capacity approaching 200 MWp at the end of the planned phase 2 

construction. Plant construction is well underway at both sites. Equipment for all 

processes is operational at the Tucson site and presently starting process adjustment 

and ramp-up. Phase 2 is scheduled for completion in 2010. Under this TFPPP 

subcontract from NREL, GSE has used standardized accelerated testing methods to 

evaluate alternative materials and methods of assembly to improve the reliability of both 

flexible and rigid (glass-based) product. CIGS coating process speed in GSE’s roll-roll 

process has been improved per the subcontract plan, reducing labor and capital costs 

for that critical step. The front contact has been subject to improvements in cost, quality 

and control/reproducibility under a low-risk program using the existing deposition 

technology. The part of the proposed front contact program using higher-risk approach 

offering dramatically lower costs has shown good material properties for the TCO, 

although more work is needed to demonstrate satisfactory rates and device results. 

Back contact studies under this subcontract have resulted in a better understanding and 

control of that process, along with viable reductions in the thickness of an interface 

layer, garnering modest cost improvement. Overall, this phase is characterized at GSE 

as a period of rapid change in plant, equipment and production capability along with 

continued process improvement in key areas for thin film CIGS PV. 

Future Plans 

The new deposition equipment and processes have been designed to enable, and 

make full use of the improvements proposed under this NREL TFPPP program. For 

example, the new CIGS deposition equipment at both plants will begin running 

immediately at the high process speeds (100% above previous standard rates) as set 

out as goals in the subcontract. Speed and cost improvements in the front and back 

contact layers are already enabled by the design of the new equipment for those 

processes at the new plant, as planned under the TFPPP contract. Cost reduction 

through materials substitution in the front contact layer remain as key areas of focus in 

this program, as well as efficiency enhancement. Improved materials utilization and 

reduced absorber thickness for the CIGS absorber layer will be pursued as near and 

longer-term efforts, respectively, under the program. The new production CIGS 

deposition equipment has been designed for more flexibility in the composition profile 

and control than ever before, partially with the intent to facilitate viable absorber 

thickness reduction. Foremost among the activities planned at GSE is completion and 

ramp-up of the largest roll-roll CIGS PV manufacturing plants in the world at the Tucson 

and Berlin locations. 

21 
22 

References 

[1] J.S. Britt, R. Huntington, J. VanAlsburg, S. Wiedeman, M. E. Beck, “Cost 

Improvement for Flexible CIGS-Based Product”, 4th World Conference on 

Photovoltaic Energy Conversion (IEEE), 2006, pp. 388-391. 

[2] J.S. Britt, E. Kanto, S. Lundberg, M. E. Beck, “CIGS Device Stability on Flexible 

Substrates”, 4th World Conference on Photovoltaic Energy Conversion (IEEE), 

2006, pp. 352-355 
F1146-E(09/2007) 

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PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION.

1. REPORT DATE (DD-MM-YYYY)

August 2008 

2. REPORT TYPE

Subcontract Report 

3. DATES COVERED (From - To)

24 May 2006 - 25 September 2007 

4. TITLE AND SUBTITLE

Cost and Reliability Improvement for CIGS-Based PV on Flexible 

Substrate: Phase 1 Annual Technical Report, 24 May 2006 – 

25 September 2007 

5a. CONTRACT NUMBER

DE-AC36-99-GO10337 

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

S. Wiedeman 

5d. PROJECT NUMBER

NREL/SR-520-43840 

5e. TASK NUMBER

PVB75401 

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

Global Solar Energy LLC 

5575 S. Houghton Road 

Tucson, Arizona 85747 

8. PERFORMING ORGANIZATION

REPORT NUMBER

ZXL-6-44205-13 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Renewable Energy Laboratory 

1617 Cole Blvd. 

Golden, CO 80401-3393 

10. SPONSOR/MONITOR'S ACRONYM(S)

NREL 

11. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NREL/SR-520-43840 

12. DISTRIBUTION AVAILABILITY STATEMENT

National Technical Information Service 

U.S. Department of Commerce 

5285 Port Royal Road 

Springfield, VA 22161 

13. SUPPLEMENTARY NOTES

NREL Technical Monitor: Harin S. Ullal 

14. ABSTRACT (Maximum 200 Words)

Global Solar Energy (GSE) describes its progress in quantifying and enhancing module reliability, reducing the cost 

and improving the performance of its CIGS deposition process, and reducing the cost of materials and processes for 

front and back contacts. GSE has been bringing its Tucson, Arizona, CIGS production plant into operation and 

planning another plant in Berlin, Germany. Much of the design effort to reach the goals for the production equipment 

also represent progress toward the major thrusts of this subcontract. 

15. SUBJECT TERMS

PV; module reliability; copper indium gallium diselenide; contacts; manufacturer; thin film; solar cells; cost reduction; 

flexible substrate; deposition; 

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a. REPORT 

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