Subcontract Report

NREL/SR-520-45213

March 2009

Cost and Reliability Improvement 

for CIGS-Based PV on Flexible 

Substrate

26 September 2007 – 25 September 2008

S. Wiedeman

Global Solar Energy, LLC

Tucson, Arizona
National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393

303-275-3000 • www.nrel.gov

NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency and Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

Contract No. DE-AC36-08-GO28308

Subcontract Report

NREL/SR-520-45213

March 2009

Cost and Reliability Improvement 

for CIGS-Based PV on Flexible 

Substrate

26 September 2007 – 25 September 2008

S. Wiedeman

Global Solar Energy, LLC

Tucson, Arizona

NREL Technical Monitor: Harin S. Ullal

Prepared under Subcontract No. ZXL-6-44205-13
This publication was reproduced from the best available copy

Submitted by the subcontractor and received no editorial review at NREL

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iii 

Executive Summary

Leveraging the experience gained in successfully developing and operating a 

4.2MWp/yr CIGS production line for several years, Global Solar Energy initiated an 

ambitious scale-up plan in 2006. Now in the final stages of the first phase of the 

plan, 75MWp/yr of combined manufacturing capacity is being installed at new 

production facilities in Tucson, AZ and Berlin, Germany. The physical plants in 

both Tucson and Berlin are complete, facilitated and capable of self-sufficient and 

independent operation with trained personnel now in place. All of the tool sets 

required for planned Phase 1 operation are complete in Tucson, and nearly 

complete in Berlin. 

While the scale of the plant expansion is rapid, the new production tools represent 

an evolutionary progression of the Global Solar CIGS technology. Manufacturing 

cost has been reduced by increased automation, higher materials utilization, and 

greater capacity with higher rates in all tools. These advancements garner

reduced capital expense, a smaller factory floor area requirement, and greater 

productivity in addition to reduced direct product cost. During Phase 2 of this 

TFPPP subcontract the production lines were completed with the “Gen2” 

equipment for these new factories, and the processes were brought on line. 

Successful transfer of the processes from the existing “Gen1” equipment was 

assured through exhaustive cross-testing and measurement using both “Gen1” 

and “Gen2” equipment during the transition. Significant process optimization at 

the higher production rates of the “Gen2” equipment and cost reduction as 

embodied in the goals of this subcontract have been accomplished during this 

phase of the TFPPP subcontract, as described in this report. Product reliability 

and product design for durability has also been a successful focus of the effort in 

Phase 2 of TFPPP.
iv 

Table of Contents

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

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

List of Tables .......................................................................................................... v 

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

Rigid Product – Testing and Reliability ............................................................1

Rigid Product – Determination of Thermal Coefficients for 

IV Parameters..................................................................................................5

Flexible Product – Testing and Reliability ........................................................5

Task 2: CIGS Coating Cost Reduction ...................................................................7

Economy of Scale – Factory Completion.........................................................7

Absorber (CIGS) Deposition Process ............................................................ 11

Process Chemistry Relating to Optimization .................................................13

Task 3: Front Contact Cost Reduction (Objectives)..............................................15

Front Contact (TCO) Process Scale-Up ........................................................15

Task 4: Back Contact Cost Reduction and Efficiency Improvement ..................17

Materials Substitution and Reduced Cost .....................................................17

“Gen2” Back Contact Equipment Installation and Characterization ...............19

Summary..............................................................................................................20

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

References ..........................................................................................................21
v 

List of Figures

Figure 1.1: Relative power output of glass modules with and without

edge seals ..................................................................................... 2

Figure 1.2: Power output of glass modules in accelerated test for 2 conditions 

of electrical bias ............................................................................. 3

Figure 1.3: Light-soak and power output behavior of glass modules in 

accelerated test for different bias conditions.................................. 4

Figure 1.4: Power output for modules having flexible packaging in outdoor 

test, using various interconnection methods................................. 6

Figure 2.1: New Global Solar Energy manufacturing facility in Tucson............ 9

Figure 2.2: GSE CIGS deposition equipment for roll-to-roll processing ......... 10

Figure 2.3: Roll-to-Roll equipment for one of the “back-end” processes in the 

new factory................................................................................... 10

Figure 2.4: A view of the thin film CIGS-based PV strings on metal foil..........11

Figure 2.5: Equivalent thicknesses of copper, indium, and gallium in a 

typical CIGS film (across the web width)...................................... 12

Figure 2.6: Atomic ratios and thickness of a CIGS film (across the

web width)................................................................................... 12

Figure 2.7: Equivalent thicknesses of copper, indium, and gallium in 

a typical CIGS film (down the web length) ................................... 13

Figure 2.8: XPS survey spectra of the front and back side of the device 

stack at various process stages ............................................................. 14

Figure 3.1: Front contact (TCO) deposition equipment at GSE ..................... 16

Figure 4.1: A contour plot showing back contact thickness uniformity............. 20

List of Tables

Table 1.1: Temperature coefficients for IV parameters of “Gen2” 

CIGS strings .................................................................................. 5

Table 3.1: A Statistical comparison of IV parameters for reduced 

power i-ZnO................................................................................ 17
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.

Rigid Product - Testing and Reliability 

Glass-based product usually exhibits better outdoor stability than flexible PV 

product, as the glass itself is impervious to moisture and oxygen ingress. 

However, potential degradation mechanisms do exist for glass-based product, 

and stability must be carefully evaluated to achieve the required 20-40 year 

service life. Outdoor field tests and accelerated tests are important to determine

durability, failure modes and their causes. [1] 

At an installation in Springerville, AZ a variety of PV products have been installed 

and are monitored for daily performance over the long term by Tucson Electric 

Power (TEP). The earliest CIGS PV using glass-glass fabrication by GSE 

continues to generate power in the original array, with no replacements or dropouts of individual modules. The performance over increasing time has shown 

substantially stable behavior, although recent data is unavailable due to the 

failure of a large transformer connecting the inverter to the grid. TEP has 

advised that they plan to replace the failed transformer and restart data 

collection.

One potential failure mechanism for glass-based PV is moisture and O2 ingress 

at the edges of the module. Various “edge seals” have been considered to avoid 

this effect. An evaluation of the selected edge seals was conducted on strings 

laminated in glass/backsheet modules. Edge seals were applied to six test 

modules, and six additional test modules that were otherwise identical were used 

as controls. Modules were qualified by light-soaking outdoors for three days 

followed by pulsed-light measurement. Module efficiencies at this stage ranged 

between 8.3% and 9.9%. All modules were stressed in damp heat and 

periodically removed for measurement followed by six hours of light-soaking and 

re-measurement. After 1960 hours damp heat treatment, the samples with edge 

sealant were on average unchanged in power from their starting power, while the 
2 

mean power from the controls degraded to approximately 94% of their starting 

power. The difference between the two sets was statistically significant with 90% 

confidence. The two sets primarily differed in open-circuit voltage. The results 

were interpreted to indicate a real effect that justifies further investigation for 

edge seals. The results are shown graphically in Fig. 1.1.

Hours in 85C/85%RH

R 

0

140

280

420

560

700

840

920

980

1120

1260

1400

1540

1680

1820

1960

70

75

80

85

90

95

100

105

110

115

120

 controls

 w/edge sealant

Figure 1.1. The mean relative power output of 2 groups modules (with and 

without edge sealant) vs. exposure time in damp heat (85/85). The regular pointpoint, low-high variation during the measurement is due to sequential 

measurement before, and then after light soaking upon each removal from the 

damp-heat exposure.

The potential impact of voltage bias, and possible interactions with corrosion 

mechanisms as a mode of degradation has also been studied at GSE. Modules 

stressed in damp heat (85°C/85%RH) were maintained in the dark under opencircuit conditions. A test was conducted to determine if modules stressed in 

damp heat under forward bias are subject to accelerated corrosion and failure. 

This information is valuable for better understanding module reliability under 

actual operating conditions. It has been frequently observed that modules 

stressed by damp heat require a period of light-soaking for maximum power 

output. Consequently, another goal of the test was to determine whether 

continuous forward-bias during accelerated testing had an affect on light-soaking 

characteristics.
3 

When no clear difference in response due to bias in the dark under accelerated 

(85/85) conditions were noted, the test was repeated for longer times, nearly 

2000 hours) to look for accelerated corrosion or related failure. The mean Pmax 

of modules in each group is shown in Fig. 1.2. Modules that were forward-biased 

during damp heat stress were found to deteriorate more slowly than non-biased 

modules. The results were statistically significant to greater than 99% 

confidence. 

Time in Damp Heat (hrs.)

Pma (W)

0 567 707 1265 1547 1967

10.0

10.5

11.0

11.5

12.0

12.5

13.0

13.5

14.0

 controls (non-biased)

 forward-biased

Figure 1.2. Mean Pmax of modules packaged in glass/backsheet; forward-biased 

near Vmax and controls (non-biased) in damp heat testing in the dark.

Since electrochemical corrosion mechanisms are driven by differences in 

potential, failure mechanisms may be different in light as compared to dark 

conditions. Other studies were conducted at GSE to determine the impact of 

corrosion under illuminated conditions using accelerated tests. 

Twelve glass modules were fabricated and randomly assigned to two groups. 

Modules in one group were maintained at their individual Vmax (under AM1.5 

illumination) during damp heat (85/85) treatment. Modules in the second group 

were not voltage-biased. After 560 hours of treatment, no statistical difference 

was observed between the two groups (Fig. 1.3). However, the two groups 

differed in their response to light-soaking following each treatment interval. 

Modules maintained at forward bias declined in Pmax by 1-4% upon lightsoaking. 

Modules not biased generally increased in Pmax upon lightsoaking, and the 
4 

magnitude of improvement increased with increasing treatment time. In any 

case, no difference in light-soaked performance under standard conditions was 

noted in the two groups held at different voltage biases under illumination in this 

accelerated test.

Time in 85C/85%RH (hours)

0 140 146 280 286 420 426 560 566

92

96

100

104

108

112

116

 Not biased

 Forward-biased

6hr. LS

6hr. LS

6hr. LS

6hr. LS

Figure 1.3. Mean relative Pmax of CIGS strings laminated in glass, non-biased 

and biased at Vmax, during damp heat treatment. 

Other testing at GSE included special test structures that were developed to 

evaluate the reliability of the cell interconnects within strings or modules. The 

focus of this evaluation was finding changes in series resistance. Test structures 

were exposed to thermal cycling and damp heat. Variable results were obtained 

and further experimentation is required for better understanding. 

Changes in construction materials or processes used for module fabrication 

usually demand reliability testing, so reliability tests were initiated for strings of 

cells fabricated using a new TCO process for instance, to confirm the absence of 

ill effects. Generally, cells or modules made with the standard process and 

materials are used as “controls” for comparison to results from the “new” process 

or method using accelerated testing (usually 85/85 damp heat testing). Other 

examples of the rigor required for process changes include the tests done for 

new materials for electrically connecting cells, which were procured and 

evaluated against the standard material for adhesion and module performance, 

before and after stress testing. 
5 

Long-term testing of strings and modules stored in inventory was also initiated. 

Rigid Product – Determination of Thermal Coefficients for IV Parameters 

Thermal coefficients for IV characteristics were measured for glass modules 

fabricated from Gen2 strings. The evaluations were conducted outdoors under 

cloudless conditions. A portable IV tester, calibration cell, and a transparent 

window box (to limit wind cooling) were utilized. Module temperature was 

monitored by an adhesive thermocouple attached to the back of the module. 

Utilizing specially-fabricated structures, tests were conducted to determine the 

temperature differential between a thermocouple mounted directly on the back of 

the string and the exterior surface of the backsheet. The difference was found to 

be less than 2°C. Average thermal coefficients are shown in Table 1.1. 

Table 1.1. Thermal coefficients for Gen2 strings in glass modules 

Vmax 

(%/C)

Imax 

(%/C)

Pmax 

(%/C)

Voc 

(%/C)

Isc 

(%/C)

FF 

(%/C)

-0.49 -0.13 -0.59 -0.39 0.03 -0.24

Flexible Product - Testing and Reliability 

Several new fabrication methods for flexible PV product, using various 

approaches for interconnecting cells were evaluated outdoors for their application 

in flexible modules. Six modules of each stringing approach (A-C) were 

fabricated into ~7W, 10% efficient flexible modules; a standard GSE product 

design. The modules were deployed outdoors in early summer in Tucson. One 

group experienced extreme power degradation (C), a second group experienced 

rapid degradation followed by slow degradation to 85% of initial power (B), and a 

third group (A) varied somewhat during the period but generally maintained 

output without degradation (Figure 1.4). The degradation of groups B and C was 

dominated by fill factor reduction. Analysis of the IV curves indicated that 

modules from groups B and C experienced increased series resistance. After 10 

months outdoors, one of the stringing approaches, type A, continues to 

demonstrate stable performance Modifications have been made to the production 

processes to incorporate the important aspects learned in this test. 
6 

Days Outdoors

R 

0

1

3

17

29

43

50

59

66

80

94

108

122

137

151

175

197

309

20

30

40

50

60

70

80

90

100

110

120

 String type A

 String type B

 String type C

Figure 1.4. Mean relative Pmax of flexible modules fabricated by three stringing 

techniques and deployed outdoors in Tucson.

Accelerated testing was used on a comparative basis to look at GSE product 

made using larger “Gen2” cells vs. “Gen1” cells. Flexible modules were 

fabricated from Gen2 cells with the same packaging as a standard product at 

GSE. Six modules were stressed in damp heat and six modules were deployed 

outdoors. The modules deployed in damp heat failed at a rate similar to the 

standard product fabricated from Gen1 cells. After 62 days outdoors, the other 

module set deployed in typical outdoor use still averages over 100% of initial 

power. Further time is required for any effects to become apparent. These types 

of tests eventually serve to build a link between accelerated and typical outdoor

testing in terms of implied failure rates. 

Although performance under “standard” conditions typically warrants the greatest 

study, some degradation mechanisms tend to show effects more rapidly at nonstandard conditions. For example, failure mechanisms owing to increasing 

shunts in the PV product might be expected to impact weak light performance 

more rapidly than that under AM1.5 intensity. To identify any response of this 

nature, GSE fabricated modules using prototype Gen2 cells and characterized

module performance at variable intensity before and after stress testing in damp 

heat. For exposure times less than 1000 hours, no significant change in the 

weak light module performance was observed.
7 

Again, all major process changes are generally tested for reliability and 

environmental durability. For the transition at GSE from “Gen1” to the larger 

“Gen2” cells, tests were conducted to evaluate the reliability of flexible and glass 

modules fabricated from Gen1 cells, with the back contact deposited either in the 

Gen1 or Gen2 coaters. Results will be forthcoming in many studies such as this 

involving the significant elapsed time often required to observe any differences in 

behavior.

Task 2: CIGS Coating Cost Reduction (Objectives)

1. Increase processing rate for CIGS deposition by at least 25% with a highbar 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 InGa 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.

Economy of Scale – Factory Completion

As scalability is a key advantage of the thin film approach, particularly using the 

roll-to-roll processes for thin film coating that GSE has pioneered, overall cost 

reduction is heavily dependent on factory scale-up to significant size. During the 

2nd phase of this Thin Film Partnership subcontract substantial progress has 

been made toward completion of the GSE plants in Tucson and Berlin (40 MWp 

and 35 MWp capacities respectively). A rough chronology of major events 

throughout the 2nd phase include:

• Further required planning and construction of the GSE production facilities 

in both Tucson and Berlin and related equipment were carried out 

concurrently, saving time but increasing risk.

• Demolition of the Tucson (Rita Road Site) 103,000 sq. ft. building interior 

was completed.

• Construction of the new interior started on 6/15/2007. The objective was 

to prepare the facility for installation of new tools that began arrival in 

October. Building completion was staged to accommodate the tools as 

they arrive.
8 

• The building was completed generally from north to south, with temporary 

barriers erected to separate active construction areas from completed 

areas receiving production tools. 

• Facilities such as chilled water, compressed air, electricity, exhaust, and 

air conditioning were brought up and distributed dynamically as required 

for the production tools as they arrived. 

• Production tools were generally checked and accepted once at the 

suppliers’ sites and then checked and accepted again after final 

installation at GSE (either Tucson or Berlin).

• Production tools were installed and facilitated as they arrived from diverse 

fabricators. Complete sets of thin film coating equipment (back contact, 

absorber, heterojunction formation and front contact roll-to-roll coating 

tools) were operational midway through phase 2.

• Transfer and development of the process for the thin film coating steps 

was initiated (back contact, absorber, heterojunction and front contact 

steps), concurrently with the installation of additional tools. For instance, 

the majority of the hardware for the first installed CIGS system (CIGS5) 

was demonstrated to be robust, reliable and proven capable for 

depositions extended to 600m web lengths while the second system 

(CIGS6) began installation.

• Remaining process tools were installed and qualified, including tools for 

printing, slitting, “tabbing”, “stringing” and measurement functions. 

• Evaluations were conducted as necessary to successfully increase web 

lengths from 300 meters to 600 meters in length, from 12.5” to 13” in width 

and from the smaller “Gen1” to the larger “Gen2” cell format. All of these 

changes capitalize on the economies of scale built into the design of the 

new factory processes and equipment. An equipment integration plan 

required the Gen2 processes to be individually qualified against Gen1 

processes with known metrics. Wherever the approach required web 

interchangeability between the old and new production tools, modifications 

were made to the Gen1 equipment to allow it to process the wider web.

• Tests were conducted in CIGS deposition tools to determine the accuracy 

of the in-situ sensors and develop the control parameters that lead to 

CIGS with characteristics similar to the films deposited in the Gen1 

production line. All deposition tests were conducted at a web speed of 

0.61 m/min (24 inches/min). The metrics evaluated for these first 

optimizations were coating thickness, composition, adhesion, morphology, 

visual appearance, and solar cell performance. Initially, achieving a 

sufficient and controllable supply of selenium was a notable problem. 

• Cell performance was evaluated by applying other coatings (besides 

CIGS) in the Gen1 line, and fabricating Gen1 cells (68cm2

). By the fourth 

such test, gross control set points had been derived that resulted in a 
9 

maximum efficiency of 8.6%. Ongoing optimization of the thin film and 

other factory processes continue to increase measured cell efficiencies 

and reproducibility. 

Figure 2.1. The new Global Solar Energy manufacturing facility in Tucson (10,220m2).
10 

Figure 2.2. A GSE tool for roll-to-roll absorber (CIGS) deposition at the new 

manufacturing facility in Tucson.

Figure 2.3. Roll-to-Roll equipment for one of the “back-end” processes in the new 

factory (front contact collection grid printing).
11 

Figure 2.4. A view of the thin film CIGS-based PV strings on metal foil (and nominal 

string electrical characteristics) that represent the typical product of the new GSE 

factory.

Absorber (CIGS) Deposition Process

Successful CIGS deposition invokes a multidimensional parameter space that 

must be well understood and well controlled. Multisource co-evaporation offers 

perhaps the greatest flexibility and potential in device engineering on an atomic 

level. This potential comes at the cost of severe engineering challenges in 

scaling a process requiring multivariate control under extreme conditions in a 

harsh and difficult environment.

Compositional control and uniformity of copper, gallium and indium is crucial to 

achieving product performance and yield. Cross-web uniformity is governed 

chiefly by the design and control of the effusion sources and the geometry of the 

deposition zone. Down-web compositional uniformity is more a function of 

temporal control and stability of the effusion sources. In the design of the 

equipment for the new factories, GSE intentionally increased the degrees of 

freedom and also designed the systems for higher deposition rates and 

capacities. These factors presented further challenges in the engineering, and in 

the process development. At the end of the second phase, GSE had 

demonstrated satisfactory control and uniformity in CIGS composition, as shown 

below. Nonetheless, further improvement is anticipated in these areas. A typical 

cross-web composition is shown in Fig. 2.5. for the CIGS deposition equipment

at the new GSE factories operating at a 61-cm/min web rate.

Pmp (W): 39.5

Vmp (A): 7.3

Imp (A): 5.4

Voc (V): 10.3

Isc (A): 6.7
12 

Figure 2.5. Equivalent thicknesses of copper, indium, and gallium in a typical CIGS film 

(across the web width) as measured by ex-situ XRF.

Fortunately, compositional ratios of Cu/(In+Ga) and Ga/(In+Ga) are more critical than 

absolute equivalent thicknesses of the individual elements, or than total thickness of the 

CIGS layer itself. These quantities are plotted in Fig. 2.6 for the same web in Fig. 2.5. 

Figure 2.6. Atomic ratios and thickness of a CIGS film (across the web width) as 

measured by ex-situ XRF.

Considerable effort in process control methods at GSE have resulted in uniform 

compositional control both across and down-web at the high web coating rates required. 
13 

Fig. 2.7 shows the equivalent thicknesses for copper, indium and gallium as a function of 

down-web location for a typical deposition.

Figure 2.7. Equivalent thicknesses of copper, indium, and gallium in a typical CIGS film 

(down the web length) as measured by ex-situ XRF at the web center.

The cost reduction in the CIGS process (and other processes) during phase 2 

were largely addressed through improvements to process control, engineering, 

operational procedures and process setpoints that were all directed toward 

running at high deposition rates, and for increasing web lengths. High 

deposition rates and increasing web lengths are effective in reducing cost, but 

only if cell efficiency and yield can be maintained concurrently. Current plans call 

for increasing web length to 1 km through all tools within several months. 

Deposition rates for the CIGS process are slated to remain at levels between 19 

and 24-in/min. 

Process Chemistry Relating to Optimization

In optimizing process setpoints, including processing rates and temperatures, it is 

useful to gain as much insight into the materials interactions as possible. For 

instance, it is important to understand the state of the starting substrate, Mocoated stainless foil, in terms of its composition, contamination level, oxidation, 

tendency toward selenization as the process proceeds, etc. Knowing the relative 

changes in interfaces throughout the device under “standard” processing 

conditions allows better interpretation of how changes made to increase reaction 

rates might impact important interfacial reactions. Toward that end, through 

collaboration with the laboratory at UNLV directed by Dr. Clements Heske, GSE 

has obtained data on the surface condition at important steps in the device 

formation process using XPS. This data was obtained as a ‘baseline’ for 

standard conditions on both the front and backside of the substrate, shown in 

Fig. 2.8. 
14 

Fig. 2.8 XPS survey spectra of the front (top panel) and back side (bottom panel) of the investigated 

test structures.

Si 2p

Si 2s

Mo 3d*

Ga 2p

In 4s

In 4p Mo 4s

Mo 4p

Fe 2s

Cr 2p3/2

Fe LMM

Fe 2p

Fe 3s

Fe 3p

In 4d

S 2s

S LMM

S 2p

Mo MNV

Mo 3s

Mo 3p

Mo 3d

Cd 4d In 4d

Ga LMM

Zn 2p

Cu 3s

Cu 3p

Cu LMM

Se 3s

Se 3d

Se 3p

C KVV

C 1s

Cu 2p

Na 1s

Cd 3p

Sn MNN

O KLL

In 3p

In MNN

Cd MNN

Cd 3d

Sn 3d

In 3d

Normalized Intensity

O 1s

Mg Kα

* excited with O Kα

5

4

3

2

1sample

1200 1000 800 600 400 200 0 Cu 2p Ga 2p In 3p Si 2s Fe 3s Se 3d* Zn 3s Zn 3d Zn 3p Se MMN Mo 3d* Se 3p Se 3d In 3d Cr 2p3/2 Si 2p Fe 3p Fe 2s Fe 2p C KVV

Mo MNV

Mo 4s

Mo 4p

C 1s

Mo 3s

Mo 3p

Mo 3d

O KLL

O 1s

Normalized Intensity

Binding Energy [eV]

Mg Kα

Zn 2p

Zn LMM

sample

5

4

3

2

1
15 

More detailed interpretation of the meaning of the data is given by Dr. Heske in 

his report of 5-2008 to NREL under the TFPPP initiative. The data indicates 

some conversion of oxides of moly to selenides during processing, and the 

transport of some constituents of the CIGS deposit to the backside of the 

substrate.

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%.

Front Contact (TCO) Process Scale-Up

Two approaches have been pursued to reduce front contact costs: a low-risk 

approach based on evolutionary changes to the proven, existing method, and a 

higher risk approach based on the development of novel techniques for TCO 

deposition. The initial approach for a high-risk tact in phase 1 produced TCO 

having suitable properties for sheet resistance, optical transparency and growth 

rate. Further, the estimated cost of the technique was very attractive. However 

the effort was stopped when evidence indicated incompatibility between this 

process and the device stack and substrate used at GSE in the roll-to-roll 

process. Some equipment has been located and placed in the factory to 

continue the high risk approach in phase 2, but the machinery is not yet 

operational. Most of the effort in phase 2 for the low-cost TCO task has been 

dedicated toward low-risk approaches, some of which are described below. 

The first TCO roll coater (TCO5) of the new design was delivered and installed at

GSE during phase 2 of this subcontract. Subsequently, multiple identical tools 

have been received, placed, facilitated and put into service at both the Tucson 

and Berlin facilities (Fig. 3.1). These TCO deposition tools were designed to be 

capable of well-controlled, uniform, high rate deposition using a dense array of 

cathodes in a roll-to-roll approach. 

Comprehensive plans were developed for bringing the tool on-line and 

integrating the process. The first major goal of the plan was to demonstrate 

coating capability and solar cell metrics comparable to those produced by the 

Gen1 TCO manufacturing equipment, applying a “hook and loop” approach

where webs could be split and processed on a comparative basis in both “Gen1” 

and “Gen2” process equipment. 
16 

Figure 3. 1 The front contact (TCO) is deposited by pulsed DC magnetron sputtering.

Subsequent to installation, campaigns have been initiated more fully utilize the 

equipment design to deposit at high rates, using high utilization sputtering 

targets, both factors being intended to reduce costs. 

One example was the modification to attain more powerful magnet packs on the 

TCO targets to determine if faster deposition rates could be achieved. Faster 

deposition rates enable increased line speed and increased productivity for lower 

costs. With the new magnet packs, it was demonstrated that TCO cathode 

power could be decreased from 1400W to 1100W and maintain identical 

deposition rates. At an applied power of 1400W, cathodes with the new magnet 

packs allowed web speeds to be increased nearly 30%, with no loss in solar cell 

performance. All production systems were subsequently converted to the new 

packs. 

These approaches are most often iterative, in that device performance is 

impacted by interactions so that changes in one parameter setting demand retest of other parameter settings to garner full benefit. An example was a test 

conducted to evaluate the effect on cell performance of reduced power applied to 

deposit the insulating ZnO coating. For test conditions, the pulsed power to the 

insulating layer targets was adjusted so that the power delivered to the targets 

was reduced by half, with no effect on the deposit thickness. Statistical analysis 

indicated no difference between the test samples and controls generated on the 

same lot (Table 3.1).
17 

Mean Mean t-value df p Valid N Valid N Std.Dev. Std.Dev. F-ratio p

Test Control Test Control Test Control Variances Variances

Vmax 392.006 388.207 1.2626 166 0.208504 96 72 20.4722 17.6049 1.352254 0.182483

Imax 1786.153 1801.885 -0.83086 166 0.407244 96 72 137.0707 96.6604 2.010909 0.002303

Pmax 701.409 699.42 0.21189 166 0.832454 96 72 68.4228 46.9965 2.119682 0.001075

Voc 558.676 556.575 0.8884 166 0.375612 96 72 16.1677 13.717 1.389254 0.146521

Isc 2164.221 2172.617 -0.70681 166 0.480674 96 72 77.3668 74.6076 1.075333 0.752715

Fill Factor 57.914 57.838 0.11926 166 0.90521 96 72 4.677 3.1294 2.233678 0.000484

Efficiency 10.195 10.166 0.21193 166 0.832421 96 72 0.9945 0.6831 2.119691 0.001075

Rsh 853.78 976.034 -1.54358 166 0.124595 96 72 489.7579 531.4812 1.177641 0.454469

Rse 3.806 3.88 -1.32529 166 0.186898 96 72 0.3333 0.3827 1.31795 0.208519

Table 3.1 Statistical comparison of IV parameters between reduced power i-ZnO and 

controls.

Some effort during phase 2 was applied toward evaluations of the alternative 

sputtered TCO material. The alternative material is being evaluated as both an 

insulator against the buffer and as the front electrode. A variety of sputtering 

conditions were evaluated for their affects on solar cell performance. A wide 

distribution of cell efficiencies resulted from all deposition conditions evaluated, 

although maximum efficiencies were comparable to the standard production 

process. No conclusions could be reached due to the diminished yield. The 

higher performing solar cells were fabricated into strings for reliability testing of 

flexible and glass laminated modules outdoors and in damp heat. The first results 

of the reliability tests will be described subsequently.

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.

Materials Substitution and Reduced Cost

Substantial opportunity to reduce total cost often exists in the form of material 

substitution, using either less expensive grades of a constituent from a current supplier, 

or equivalent materials that are available at better prices from new vendors. This 

opportunity is driven by a growing diversity of global sources for some materials, new 

and better techniques for meeting materials requirements, and a growing recognition of 

the importance of the PV market and a desire to enter that market as a materials 

supplier. 
18

In one case, an alternative grade of stainless steel became available from our primary 

supplier. Analysis revealed that some components of the alloy were found in higher 

concentration than in our baseline stainless steel. The advantage of the alternate 

stainless foil was less difficulty in processing into roll form, equating to lower price and 

better characteristics in some areas. However, the performance of production lots 

fabricated on the new alloy was significantly lower than that of the controls fabricated on 

the standard stainless steel. Several of the new alloy’s characteristics were quantified 

to determine assignable cause for the reduced performance.

In another instance two new suppliers of stainless steel and another supplier previously 

investigated were evaluated. The supplier previously investigated (and found 

unsuitable) was re-considered because new cleaning techniques had been applied to 

their product. Upon re-evaluation of the latter by processing several of the new coils 

through the production line, the product was again found to lead to inadequate 

performance. However, both of the new suppliers were found to provide performance 

comparable to the qualified supplier. Additional, more extensive qualification tests are 

planned for these new potential suppliers in the future.

Besides the substrate itself, the materials used for the back contact coating also 

represent a significant expense, and thus an opportunity to reduce costs. [2] Alternative 

suppliers of chromium and molybdenum were evaluated to better understand the 

potential effects of common impurities on the performance and yield of the resulting PV 

product. Four lots of each supplier and material were processed through standard

production. When compared to controls, no statistically significant difference in lot 

performance was seen between any of the alternative suppliers for either target type.

Preliminary studies like these are often used as a basis to evaluate still other suppliers 

and materials preparation methods, where potential cost savings warrant, as illustrated 

by the evaluations below that were made during phase 2. 

• Sputtering cathodes from two suppliers were received and evaluated. The 

simpler, more robust and serviceable of the two cathodes was down-selected for 

utilization in the new Gen2 sputtering systems.

• Sputtering targets compatible with the Gen2 molybdenum sputtering chambers 

were procured from multiple suppliers. The molybdenum targets, prepared by 

different techniques, were evaluated in one of the Gen1 sputtering chambers with 

a fixture to accommodate the new target geometry. The targets were installed on 

a Gen2 cathode and evaluated for compatibility with the cathode, coating 

deposition rates on a moving web, and the condition of the target surface after 

extended sputtering periods. A supplier for the new targets was down-selected.

• Molybdenum targets fabricated by a less costly technique (than the production 

standards) were evaluated in the Gen1 equipment. Deposition rates under 

standard conditions were evaluated first. No significant differences in deposition 

rates were observed. Test lots made with the alternative targets were processed 

in the production line into solar cells and their performance was compared to the 

baseline molybdenum targets. The conversion efficiency of cells made with the 

new targets was significantly lower than cells made from the standard targets. 
19

The performance difference was attributed to specific impurities not found in the 

standard targets.

During Phase 2 significant progress has been made using re-selection and substitution 

of materials and suppliers for the back contact. Many, although not all, of the trials 

yielded positive results and were down-selected for production use. 

“Gen2” Back Contact Equipment Installation and Characterization

As previously mentioned, the increase in deposition rate and length purposely designed 

into the “Gen2” deposition tools for all thin film coating steps is an integral part of the 

cost reduction plan at GSE. Installation of the first roll coater for the back contact (Mo5) 

was completed in early January of Phase2. Similar to the deposition tool for the front 

contact, the first major goal was to demonstrate coating and solar cell metrics 

comparable to those produced by the Gen1 back contact manufacturing equipment, 

applying the “hook and loop” approach. The capability of Mo5 to produce individual 

solar cells with efficiency greater than 10% was demonstrated within a few depositions.

However, the first test webs coated in Mo5 had significant mechanical defects in the 

form of scuffs, scratches, and impressions. These defects were correlated with shunted 

solar cells. Modifications were made to shielding in the web path to guarantee sufficient 

clearance. The rollers and other components controlling the web motion were adjusted 

to linearity with tighter tolerances. The mechanical defect density was dramatically by 

these actions. However, defects continued to be present at a reduced level down the 

web length. 

Non-uniform web tension was hypothesized to explain some physical defects. A 

number of tests were conducted to explore parameters that could provide more uniform 

tension, including web temperature and coating stress. Presently, this line of 

investigation is ongoing, and no means have been determined to completely eliminate 

these remaining defects.

At GSE, the Mo coating thickness is typically characterized by XRF. Along the majority 

of a typical web, the Mo thickness uniformity is +/- 3%, with the thinnest coating 

occurring in the web center (Fig. 4.1). Thinner Mo coating is frequently observed in the 

first 100m of deposition. The source of this effect is under investigation.
20

Figure 4.1. A contour plot of Mo thickness (a.u.) down and across the web as measured by XRF 

(distance weighted least squares fit).

Summary

GSE has executed on plans to build two new factories (Tucson and Berlin) with a 

combined capacity of 75 MWp and commence production of thin film CIGS PV using 

roll-to-roll processing of foil substrates, a first in PV accomplishments. Production 

equipment has been built, delivered, installed and started. Most production processes 

in the new factories have been successfully transferred from the “Gen1” equipment, with 

one remaining process nearing completion of its development on the “Gen2” equipment. 

Incorporating many of the goals of the NREL TFPPP program, cost and performance 

improvement campaigns have been re-initiated and have begun to accrue benefit. 

Continued testing of module reliability in rigid product has reaffirmed extended life 

expectancy for standard glass product, and has qualified additional lower-cost methods 

and materials. Expected lifetime for PV in flexible packages continues to increase as

failure mechanisms are elucidated, and resolved by better methods and materials. 

Significant cost reduction has been enabled in the front contact process through 

designs having better materials utilization, and in the back contact process through 

enhanced vendor and material qualification and selection. The largest cost gains have 

come as a result of higher processing rates, greater automation and improved control in 

all process steps, an integral part of this TFPPP program. The process speeds for 

which the new factory equipment has been designed have been realized. The new 

processes and equipment have been individually qualified with “Gen1” material of 

known properties to be capable of 10%-12% efficiency in large area cells. [3]
21

Future Plans

Some minor work remains to fully integrate one “Gen2” process step into full production, 

eliminating dependence on any of the “Gen1” processes and bringing factory production 

under all “Gen2” processes. Attention has already migrated to the optimization 

necessary to further increase yields in all steps and maximize production rates. This 

effort will continue, basically to balance rates in all steps, eliminate unscheduled 

interruptions to production and incrementally increase final product (assembled string) 

yields. GSE is slated to move to 3-shift, 7-day operation in March, 2009 in Tucson. 

The Berlin facility is scheduled to fully ramp production later in 2009.

Concurrent with these efforts, process optimization will continue to further increase 

performance (maximum and average large cell efficiency), along with continued cost 

reduction. A large opportunity for cost reduction remains at the TCO deposition step, 

and we have recently identified another novel pathway for a low cost initiative here. 

Identification (and elimination) of the causes of defect generation will also continue in 

order to reduce performance variability in final product. Defect generation is often 

minimized through minor modifications to the production equipment or operational 

procedures. 

Finally, evaluation of product durability and expected lifetime must continue for both 

rigid and flexible products. A high level of interest in flexible “building-integrated” 

concepts such as “rolled roofing” products incorporating PV, coupled with accelerating 

development of effective “barrier coatings” for polymer materials serve to maintain 

activity levels on lifetime evaluation in flexible packages. 

In brief, the planning and construction phase is complete. Installation, integration, 

process and equipment validation is nearing completion, and now effort will shift to 

issues primarily related to PV production rate, cost and quality – the end game. 

References

[1] 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

[2] 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.

[3] J.S. Britt, S. Wiedeman, U. Schoop, D. Verebelyi, “High-Volume Manufacturing 

of Flexible and Lightweight CIGS Solar Cells”, 33rd PVSC (IEEE), San Diego, CA, 

May 11, 2008,
F1146-E(10/2008)

REPORT DOCUMENTATION PAGE Form Approved

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PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION.

1. REPORT DATE (DD-MM-YYYY)

March 2009

2. REPORT TYPE

Subcontract Report

3. DATES COVERED (From - To)

26 September 2007-25 September 2008

4. TITLE AND SUBTITLE

Cost and Reliability Improvement for CIGS-Based PV on Flexible 

Substrate, Phase II: 26 September 2007 – 25 September 2008

5a. CONTRACT NUMBER

DE-AC36-08-GO28308

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

S. Wiedeman

5d. PROJECT NUMBER

NREL/SR-520-45213

5e. TASK NUMBER

PVB91210

5f. WORK UNIT NUMBER

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

Global Solar Energy, LLC

4475 S. Houghton Rd

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

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NREL

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13. SUPPLEMENTARY NOTES

NREL Technical Monitor: Harin S. Ullal

14. ABSTRACT (Maximum 200 Words)

Leveraging the experience gained in successfully developing and operating a 4.2-MWp/yr CIGS production line for 

several years, Global Solar Energy initiated an ambitious scale-up plan in 2006. Now in the final stages of the first 

phase of the plan, 75-MWp/yr of combined manufacturing capacity is being installed at new production facilities in 

Tucson, AZ, and Berlin, Germany. Both physical plants are complete, facilitated, and capable of self-sufficient and 

independent operation with trained personnel now in place. All of the tool sets required for planned Phase 1 

operation are complete in Tucson, and nearly complete in Berlin. While the scale of the plant expansion is rapid, the 

new production tools represent an evolutionary progression of the Global Solar CIGS technology. Manufacturing 

cost has been reduced by increased automation, higher materials utilization, and greater capacity with higher rates in 

all tools. 

15. SUBJECT TERMS

PV; manufacturing; scale-up; plant expansion; automation; materials utilization; capacity; process optimiization;

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