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

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Cost and Reliability 

Improvement for CIGS-Based 

PV on Flexible Substrate 

May 24, 2006 — July 31, 2010

Scott Wiedeman

Global Solar Energy

Tucson, Arizona

Subcontract Report

NREL/SR-5200-51604

May 2011
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

National Renewable Energy Laboratory 

1617 Cole Boulevard 

Golden, Colorado 80401 

303-275-3000 • www.nrel.gov

Contract No. DE-AC36-08GO28308 

 

Cost and Reliability 

Improvement for CIGS-Based 

PV on Flexible Substrate 

May 24, 2006 — July 31, 2010 

Scott Wiedeman 

Global Solar Energy 

Tucson, Arizona 

NREL Technical Monitor: Harin S. Ullal 

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

Subcontract Report 

NREL/SR-5200-51604 

May 2011 
This publication was reproduced from the best available copy 

submitted by the subcontractor and received no editorial review at NREL.

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1

Table of Contents 

Executive Summary .................................................................................................................................... 2

List of Figures ........................................................................................................................................... 10

List of Tables ............................................................................................................................................. 13

Task 1: Enhanced Module Reliability (Objectives) ............................................................................... 14

Product Types and Testing .......................................................................................................14 

Portable Flex Product ...............................................................................................................14 

Rigid PV Product .....................................................................................................................15 

Flexible Power Product - Testing and Reliability ....................................................................22 

Intrinsic Product Stability ........................................................................................................27 

Bending Tests for Flexible Power Product ..............................................................................28 

Improved Product Appearance, Uniformity ............................................................................29

Task 2: CIGS Coating Cost Reduction (Objectives) ........................................................................... 32

Production Scale-up (New Manufacturing Facilities) .............................................................32 

Economy of Scale – Factory Completion ................................................................................36 

Enhanced CIGS Deposition Rates ...........................................................................................40 

Absorber (CIGS) Deposition Process ......................................................................................42 

Process Chemistry Relating to Optimization ...........................................................................46 

Absorber Thickness Variation .................................................................................................47 

Alternate Na Incorporation ......................................................................................................48 

Thermal Coefficients for IV Parameters and “Bandgap Engineering” ....................................49 

Process Development for Large Scale Production ...................................................................52

Task 3: Front Contact Cost Reduction (Objectives) ............................................................................. 54

Novel TCO Deposition Process ...............................................................................................55 

Incremental Improvement of the Existing TCO Process: ‘Gen1’ Process ..............................58 

‘Gen2’ (TCO) Process Scale-Up and Cost Improvement ........................................................61

Task 4: Back Contact Cost Reduction and Efficiency Improvement (Objectives) ........................... 63

Back Contact Process Improvements.......................................................................................64 

Alternate Substrate Vendors and Materials .............................................................................64 

Materials Substitution and Reduced Cost ................................................................................65 

“Gen2” Back Contact Equipment Installation and Characterization .......................................66

References ................................................................................................................................................. 73
2

Executive Summary 

When this TFPPP subcontract was partially completed, Global Solar Energy (GSE) was charged 

with simultaneously executing a dramatic increase in production capacity, with the planning and 

construction of two entirely new production facilities for thin film CIGS PV (in Tucson, AZ and 

Berlin, Germany). The expansion was the major focus of the organization for about 24 months, 

and in some cases diverted resources that had been allocated to development, including this 

subcontract. However, since the expansion required entirely new production equipment for all 

phases of the thin film deposition and “back-end” operations, it also provided a unique 

opportunity to execute on the goals of this subcontract in ways we had not envisioned. Major 

improvements in cost and reliability, goals of this subcontract, could now be addressed in the 

design stages for the ‘Gen2’ plants and equipment, opening up large opportunities, and 

leveraging what had been learned at GSE with the existing ‘Gen1’ production equipment and 

processes. 

The expansion 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. 

Extensive architectural and design work allowed efficient, balanced production in all processes, 

in a fully facilitated, climate-controlled, state-of-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 process complexity, reproducibility, higher 

materials utilization, and less labor and maintenance cost. 

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. Product 

reliability and product design for durability has also been a successful focus of this effort. 

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 

TFPPP subcontract. 

Introduction of thin film PV into the solar market place has accelerated dramatically in the past 

year, delivering on the promise of performance, stability and low cost, which in turn were 

promoted by advances in efficiency, manufacturing cost reduction, and by design improvements 

for stability. Significant potential for further improvement in the factors driving thin film PV 

into the commercial market is enabled by several key concepts which were first initiated by 

Global Solar Energy (GSE). The key concepts pioneered by GSE include roll-to-roll deposition, 

thin stainless steel foil substrate, in-situ real time process monitoring for intelligent control, and 

highly automated backend assembly using CuInxGa(1-x)Se2 (CIGS)[1,2]. The selection of CIGS 

enables very high ultimate efficiency, using very thin layers of a semiconductor that is 

fundamentally stable. Steel foil substrate enables low cost and low materials usage in high 
3

temperature processes with rapid thermal equilibration times, and roll-to-roll processes with 

flexible product. Roll-to-roll methods allow continuous production with real-time control in 

very compact, low cost deposition equipment, minimizing capital expenditure and required 

factory size. Real-time, in-situ sensors with intelligent process control enable high 

manufacturing yields at rapid rates. Both rigid and flexible end products, addressing multiple 

markets, can be made by starting with flexible substrate. Successful implementation of these key 

concepts, with economies of scale, maximize commercial potential. 

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 cell conversion efficiency, higher processing 

rates, greater automation and improved control in all process steps. These improvements are 

integral to this TFPPP program, and all realized with the ‘Gen2’ plants, processes and 

equipment. 

Dramatic increases in module efficiency, which can now exceed 13%, are evident at GSE 

factories in two countries with a combined capacity greater than 75 MW. During 2009 the 

average efficiency of cell strings (3780 cm2

) was increased from 7% to over 11%, with “best” 

results exceeding 13% (Fig. s1). These results reflect all the cell production in the Tucson plant 

on the Gen2 equipment (commissioned in 2008) for all steps. 
4

Fig. s1. Efficiency vs. time of full “strings” from the manufacturing line at GSE, showing all results 

and best results over the course of the factory ramp-up. 

 

The “Gen2” design is substantially larger (210x100 mm) than earlier Gen1 product. The Gen2 

design also entailed a different interconnect method, using tabbing and stringing equipment 

similar to that used commonly for crystalline silicon PV. The basic product, shown in Fig. s2a, 

is intended for applicability to several markets, including rigid product and “Building-Integrated” 

(BIPV) applications. Standard format rigid GSE panels using the Gen2 cells have been certified 

by NREL at over 190 W at standard conditions. These cells from GSE have been used in rigid 

product to erect the largest CIGS rooftop installation in the world, comprising 820 kWp of CIGS 

PV on 7+ acres of roof space, in Vincenza, Italy (see figure s2b). The earlier Gen1 cells 

continue to operate successfully in large power fields, as shown in figure s3 for the 750 kWp

field in Tucson, AZ. 

Production week

Efficiency, total area (%)

2009-022009-042009-062009-082009-102009-122009-142009-162009-182009-202009-222009-242009-262009-282009-302009-322009-342009-362009-382009-402009-422009-442009-462009-482009-502009-522010-032010-05

3

4

5

6

7

8

9

10

11

12

13

14

 New Champion String

 All Strings (median, 25%-75%, 10%-90%)
5

Fig. s2a,b. A “Gen2” cell at GSE (210 cm2), and modules made with Gen2 cells at the largest 

rooftop CIGS installation in the world (820 kWp at Vincenza, Italy). (Credit: Global Solar Energy)

Fig. s3. The GSE factory in Tucson, showing the ground-mounted 750 kWp CIGS power field in 

AZ. (Credit: Global Solar Energy) 

Product reliability continues to evaluate favorably even for ‘Gen1’ product, operating with 

excellent field reliability on glass-encapsulated CIGS products. In particular, a 750 kW PV 

system on the grounds of the GSE Tucson facility was commissioned in late 2008 (fig. s3), and 

has been consistently producing at higher than rated expectation. 
6

Actual product cost is intimately linked to production yields and thus have also been a major 

focus of effort. Production yield at both factories, shown in figure s4, has similarly progressed 

and now can exceed 95% in electrical test. The rapid advance shown in yield relate to progress 

in control, reproducibility and process development during this subcontract. In this respect 

reliance on in-situ monitoring for real-time intelligent process control, and operator training is 

crucial for reproducibility. The data also demonstrate the successful bi-directional transfer of 

necessary technology and information between GSE factories operating at differing locations, so 

that the same processes and equipment produce essentially the same results despite the 

complexity involved. 

Fig. s4. Production yields for GSE “Gen 2” strings in 2009, at both factories. Electrical yield is the 

fraction of all strings that pass electrical performance tests (> 8% Efficiency). 

Another representation of the progress made in overall efficiency, concurrent with gains in 

control and reproducibility that translate to improved yield is shown in figure s5. 

Factory Production Yield for 3780cm2 

GSE "Gen 2" Strings

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

6 13 20 27 34 41 48 4

Week (2009)

Electrical Yield (%)

Electrical Yield, Tucson Plant

Electrical Yield, Berlin Plant
7

Fig. s5. A histogram of string (210 cm2 cells x 18 cells each) production from the start of 2008 

compared with 2009 from the GSE factory in Tucson. 

The shift in “string” efficiency to a higher average, concurrent with a more desirable distribution 

is evident, reflecting improvements in multiple processes at GSE during 2009. 

Figure s6 shows a champion module made using standard production processes and cells. This 

3880 cm2

 module was measured by NREL, producing over 50 Watts at 13.2% aperture area 

efficiency, and was made using a string of 18 cells (210 cm2

 each). The cells were not specially 

selected or matched, but resulted from standard manufacturing processes and equipment at GSE. 

18-Cell Strings (3780 cm2)

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Efficiency (%)

Count (Arbitrary Units)

Jan-2009

Jan-2010
8

Fig. s6. An NREL measurement of a module made at GSE from a string using all standard, 

production processes and equipment (51.1 Watts, 13.2% efficiency, 3883 cm2

 area). 

Beyond further improvements in reproducibility and control, there are significant power losses 

present in the current cell and interconnection design. Figure s7 shows the IV characteristic of a 

15.4% small area device using standard GSE production CIGS on the flexible foil substrate, but 

finished using front contacting layers and grid made at NREL. Analysis of the results affirms 

that a significant increase in efficiency is available through improvement in the current collection 

and cell interconnection components. 
9

Fig. s7. The IV characteristic of a 15.4% efficient small area device made with GSE production 

material using a grid and contacting layers made at NREL. 

Acknowledgements 

We gratefully acknowledge support from NREL under subcontract ZXL-6-44205-13 and 

valuable discussion, measurements and technical guidance by Drs. Harin Ullal, Ingrid Repins, 

Rommel Noufi, Steve Rummel, and Keith Emery. We also acknowledge assistance from HZB 

Berlin under a joint work package with GSE. We would also acknowledge all the GSE 

employees and contractors, both in the U.S. and in Germany, who worked together to make these 

GSE advancements possible. 
10

List of Figures 

Fig. s1. Efficiency vs. time of full “strings” from the manufacturing line at GSE, showing all 

results and best results over the course of the factory ramp-up. ................................................4 

Fig. s2a,b. A “Gen2” cell at GSE (210 cm2

), and modules made with Gen2 cells at the largest 

rooftop CIGS installation in the world (820 kWp at Vincenza, Italy). ......................................5 

Fig. s3. The GSE factory in Tucson, showing the ground-mounted 750 kWp CIGS power field in 

AZ. .............................................................................................................................................5 

Fig. s4. Production yields for GSE “Gen 2” strings in 2009, at both factories. Electrical yield is 

the fraction of all strings that pass electrical performance tests (> 8% Efficiency). .................6 

Fig. s5. A histogram of string (210 cm2 cells x 18 cells each) production from the start of 2008 

compared with 2009 from the GSE factory in Tucson. .............................................................7 

Fig. s6. An NREL measurement of a module made at GSE from a string using all standard, 

production processes and equipment (51.1 Watts, 13.2% efficiency, 3883 cm2

 area). .............8 

Fig. s7. The IV characteristic of a 15.4% efficient small area device made with GSE production 

material using a grid and contacting layers made at NREL. ......................................................9 

Fig. 1.1. A comparison of glass module performance in damp heat (85°C, 85% Relative 

Humidity) using first and second-generation string technologies with identical packaging. 

The modules (15 and 10 in each group, respectively) were light-soaked between 4 and 10 

hours prior to each measurement. ............................................................................................16

Fig. 1.2. An example of accelerated testing in progress at GSE. One combination of materials 

and assembly procedures is 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. .....................................................................................................17 

Fig.1.3. 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. .................................18 

Fig.1.4. The mean relative power output of 2 groups modules (with and without edge sealant) vs. 

exposure time in damp heat (85/85). The regular point-point, 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. ...................................................................................19 

Fig. 1.5. Mean Pmax of modules packaged in glass/backsheet; forward-biased near Vmax and 

controls (non-biased) in damp heat testing in the dark. ...........................................................20 

Fig. 1.6. Mean relative Pmax of CIGS strings laminated in glass, non-biased and biased at Vmax, 

during damp heat treatment. ....................................................................................................21 

Fig. 1.7. Mean relative Pmax of flexible modules fabricated by three stringing techniques and 

deployed outdoors in Tucson. ..................................................................................................23 

Fig. 1.9. Schematic of peel test set-up. The film is pulled vertically to separate from the 

substrate which is held on a mandrel. ......................................................................................25 

Fig. 1.10. Normalized efficiency vs time in 85°C, 85% relative humidity for a variety of CIGS 

module constructions. All modules use a flexible frontsheet, and over half shown above also 

use a flexible backsheet (the remainder use a glass backsheet). ..............................................27 

Fig. 1.11. Unprotected CIGS device efficiency (relative to starting efficiency) as a function of 

time exposed to 85°C / 85% humidity. These cells were completely unencapsulated during 

exposure and measurement. .....................................................................................................28 

Fig. 1.12. Mean efficiency of cells after being bent around mandrels with decreasing diameter.29 
11

Fig. 1.13. An early large area product based on the Gen1 cell fabrication, and showing typical 

color variation due primarily to thickness variation in the initial CdS process. ......................30 

Fig. 1.14. A color photo of more recent large area product based on the Gen2 cell fabrication, 

and showing the more uniform color appearance attained with improved control of the CdS 

process......................................................................................................................................31 

Fig. 1.15. Relative QE measurements on GSE samples made at NREL showing variation in the 

wavelengths near 460 nm where variation due to CdS thickness changes would be 

prominent. ................................................................................................................................32 

Fig. 2.1. One section of the GSE Tucson building under demolition. ..........................................33 

Fig. 2.2. A northeast section of the GSE Tucson building under demolition. ..............................34 

Fig. 2.3. The building exterior for the Tucson facility before renovation. ...................................34 

Fig. 2.4. Interior structures, equipment chases and HVAC under renovation in the Tucson 

facility. .....................................................................................................................................35 

Fig. 2.5. Interior rooms, overhead lighting and HVAC under renovation in the Tucson facility.35 

Fig. 2.6. The new GSE manufacturing facility in Tucson (10,220m2). ........................................38 

Fig. 2.7. A GSE tool for roll-to-roll absorber (CIGS) deposition at the new manufacturing 

facility in Tucson. ....................................................................................................................38 

Fig. 2.8. Roll-to-Roll equipment for one of the “back-end” processes in the new factory (front 

contact collection grid printing). ..............................................................................................39 

Fig. 2.9. A view of the thin film CIGS-based PV strings on metal foil (and nominal string 

electrical characteristics) that represent the initial’ Gen2’ product of the new GSE factory. .39 

Fig. 2.10. Auger electron spectroscopic analysis done at NREL comparing the composition of 

cells made at GSE at web speeds of 12 and 15-in/min on ‘Gen1’ equipment. ........................40 

Fig. 2.11. A representative I-V characteristic for a large area cell made at an increased web speed 

of 15-in/min on the ‘Gen1’ equipment. ...................................................................................41 

Fig. 2.12. Equivalent thicknesses of copper, indium, and gallium in a typical CIGS film (across 

the web width) as measured by ex-situ XRF. ..........................................................................43 

Fig. 2.13. Atomic ratios and total thickness of a CIGS film (across the web width) as measured 

by ex-situ XRF. ........................................................................................................................43 

Fig. 2.14. 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. .............................................44 

Fig. 2.15. Discrepancy in the elemental layer thickness (for Cu, In, Ga and Mo) made in several 

locations before (square columns) and after (cones) upgrades were made to automatically 

correct measurement errors in XRF real-time readings. ..........................................................45 

Fig. 2.16. XPS survey spectra of the front (top panel) and back side (bottom panel) of the 

investigated test structures .......................................................................................................46 

Fig. 2.17. Average efficiency, fill factor and Voc of “strings” of full sized Gen2 cells produced 

using 2 different methods of Na incorporation, or none at all. The strings were fabricated 

using standard manufacturing techniques otherwise, and consisted of 18 serially connected 

large area cells (210 cm2

) each. ...............................................................................................48 

Fig. 2.18. Average efficiency, fill factor and Voc of “strings” produced using two different 

methods of Na incorporation, or both simultaneously. The strings were fabricated using 

standard manufacturing techniques otherwise, and consisted of 18 serially connected large 

area cells (210 cm2

) each. ........................................................................................................49 

Fig. 2.19. The measured thermal coefficient for output power as a function of open circuit cell 

voltage. .....................................................................................................................................51 
12

Fig. 2.20. Open circuit voltage (per cell) for “strings” on the production line at GSE, showing the 

initial impact of the campaign to increase the CIGS bandgap. ................................................52 

Fig. 2.21. Parameters of a 1 km web. Efficiency of strings made using 3 locations across the 

web, and independent parameters of CIGS thickness, Cu/(In+Ga) and Ga/(In+Ga) are shown 

at two crossweb locations down the entire web length. A “string” is 18 serially connected 

large area cells, each 210 cm2 in area. .....................................................................................54 

Fig. 3.1. 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. ............................................................................................................................56 

Fig. 3.2. The optical transmittance of TCO made using the alternate TCO process compared to 

Al-doped ZnO made using a typical process having the same sheet resistivity. .....................57 

Fig. 3.3. Large area (68.8 cm2

) cell efficiency for GSE cells using the alternate TCO process. 

Each group of cells was processed at different conditions during TCO deposition. The 

standard TCO process is shown also for the same cell lot as dashed lines for the average cell 

efficiency ± 1 standard deviation. ............................................................................................58 

Fig. 3.4. A comparison of the optical transmission as a function of wavelength for the standard 

production TCO process and a new process under evaluation. ...............................................59 

Figs. 3.5a,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). ....................................61 

Fig. 3.6. The front contact (TCO) is deposited by pulsed DC magnetron sputtering. ...................62 

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

(distance weighted least squares fit ..........................................................................................67 

Fig. 4.2. Three coupon samples of substrate material with various coatings after 24 hours 

exposure to damp heat (85°C/85% RH), visually showing a differing response. ...................68 

Fig. 4.3. The series resistance of ribbon contacts made to the molybdenum coated PV substrate 

after only 16 hours of damp heat (85°C/85% RH) exposure, indicating a dependence of 

magnitude and variability on the test group. Each test group represented different deposition 

conditions for the molybdenum overlayer. ..............................................................................69 

Fig. 4.4. The mechanical strength of ribbon contacts made to the molybdenum coated PV 

substrate after only 8 hours of damp heat (85°C/85% RH) exposure. The test groups 

correspond to those in fig. 4.3., representing different deposition conditions for the 

molybdenum overlayer. ...........................................................................................................70 

Fig. 4.5. Visual appearance of the ribbon contact test structures made to the molybdenum coated 

PV substrate was observed to vary significantly after damp heat (85°C/85% RH) treatment 

depending on Mo deposition conditions. This figure shows samples from groups 4 and 5 

after 8 hours of damp heat exposure. .......................................................................................71 

Fig. 4.6. A plot of the change in series resistance vs damp heat exposure time for ribbon 

connections made under different conditions to the backside contact. All conditions 

including the GSE standard process show Resistance increases of < 15% after 600 hrs in 

DH. ...........................................................................................................................................72 
13

List of Tables 

Table 1.1. 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. ............................................................15 

Table 1.2. Module performance changes after stress testing. .......................................................22 

Table 2.1. Mean electrical characteristics of cells with CIGS deposited at 51 and 61 

cm/minute. ...............................................................................................................................42 

Table 2.2. Mean electrical characteristics of cells with “thin” and “thick” CIGS ........................47 

Table 2.3. Thermal coefficients for Gen2 strings in glass modules. .............................................50 

Table 2.4. Expected cell efficiencies with CIGS absorbers of large and small bandgaps at 

operating temperature. Cells are 13% efficient at standard conditions of 25°C. ....................51 

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

controls. ....................................................................................................................................63 
14

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. 

Product Types and Testing 

The specific tests required depend on the product type. For GSE, product is separated into three 

broad categories: portable flexible product, rigid PV for power use, and flexible power product 

for power or BIPV use. Portable flex product is intended for intermittent use where features of 

durability, portability and appearance are valued over long term reliability. Rigid PV products 

are intended for rooftop and commercial bulk power generation where power density and long 

term reliability (20 – 30 years) are crucial. Flexible power product has all the requirements of 

rigid PV products, but has added requirements for flexibility, weight and appearance. 

Both real-time and accelerated testing is important to determining reliability, durability, failure 

modes and their causes. Specific to the product type above, GSE uses multiple methods to 

evaluate suitability, including actual outdoor exposure testing, accelerated testing in dedicated 

chambers or heightened stresses, and standardized tests. 

Portable Flex Product 

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

product for UL testing. 
15

Table 1.1 - 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 

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

portable product tested using the sequence above passed all tests. 

As a typical example of iterative product reliability improvement, a failure mode due to lengthy 

cycles of folding and unfolding was noted for portable flex product, traced to the conductors 

used for internal circuitry, determined to be metal wire fatigue and separation. An alternative 

wiring scheme for the portable folding product was proposed and evaluated. Accelerated tests 

were made with equipment set up to continuously fold and unfold the product and monitor for 

failure in the fold zone. The alternative wiring scheme showed a 5x increase in the MTBF, 

which was sufficient for the expected product lifecycle. The alternative scheme was 

implemented for all product 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. 

Rigid PV Product 

This product has a structure generally using glass / PV / backsheet, where the backsheet can be 

glass or a combination of polymer and metal foil. Early field observations in identified some 

delamination in glass product under outdoor tests. “Pull tests” were incorporated into 

standardized testing at GSE to gauge adhesion strength as a function of the laminating materials, 

procedures and product exposure history. For example, the adhesion strength of several 

laminating materials to glass was tested versus several different cleaning methods. A method of 

chemically priming the current laminating materials was evaluated using the same standardized 

adhesion “pull test”. In this case pull tests indicated consistently higher adhesion without using 
16

this priming compound. Although other factors are considered (material availability, process 

speed, reproducibility, etc.), generally the best settings and materials as indicated by the 

standardized test are selected for further testing, including outdoor tests. 

Standard tests for electrical performance measures have also been devised. Curing profiles and 

multiple types of electrically conductive inks were iteratively tested to improve the resistance of 

printed cell grid patterns. Process settings can have dramatic impact; one test demonstrated an 

adjusted curing profile that resulted in a printed pattern resistance 55% lower than the “standard” 

curing profile. 

Reliable stringing methods, using compatible materials 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. 

 

Fig. 1.1. A comparison of glass module performance in damp heat (85°C, 85% Relative Humidity) 

using first and second-generation string technologies with identical packaging. The modules (15 

and 10 in each group, respectively) were light-soaked between 4 and 10 hours prior to each 

measurement. 

Naturally, changes in construction materials or processes used for module fabrication invariably 

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. 

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
17

Moving to ‘Gen2’ from ‘Gen1’ cell size and fabrication lines for the expanded GSE production 

required thorough testing for performance and reliability. 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 1.1). 

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. 

Fig. 1.2. An example of accelerated testing in progress at GSE. One combination of materials and 

assembly procedures is 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. 

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 
18

The “Damp Heat” test at 85°C and 85% relative humidity is typically one of the most demanding 

tests for module reliability, thus it is used frequently to gauge progress in reliability. Iterative 

testing of alternative materials and assembly techniques 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. 1.2 for glass product). 

While 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 the validation of models relating 

accelerated test data and life expectancy under actual conditions of use. 

The rigid product that has shown reliability in accelerated tests continues to show stable 

operation under actual outdoor use. Early glass-based product, deployed at Springerville, AZ for 

long-term outdoor testing functioned with little degradation over several years (Fig 1.3). The 

performance over increasing time has shown substantially stable behavior, although recent more 

data is unavailable due to the failure of a large transformer connecting the inverter to the grid. 

Fig.1.3. 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. 

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)
19

The edges of the module potentially introduce unique failure mechanisms for glass-based PV due 

to moisture and O2 ingress. Methods and materials for “edge seals” have been developed to 

counter this effect. The impact of one such edge sealing method on strings laminated in 

glass/backsheet modules is shown in Fig. 1.4. 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 after 

stress in damp heat for those under test. After almost 2000 hours in damp heat, the samples with 

edge sealant were on average unchanged in power from their starting power, while that of the 

controls (without edge seals) degraded to approximately 94% of the 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 indicate that the impact of edges and seals 

on the product must be considered. 

Fig.1.4. The mean relative power output of 2 groups modules (with and without edge sealant) vs. 

exposure time in damp heat (85/85). The regular point-point, 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. 

Possible interactions with corrosion mechanisms as a mode of degradation and voltage bias have 

also been studied at GSE. Normally, modules stressed in damp heat (85°C/85%RH) are 

maintained in the dark under open-circuit conditions. A test was run in which groups of modules 

were subjected to either damp heat under forward bias or under open circuit bias. Another goal 

of the test was to determine whether continuous forward-bias during accelerated testing had an 

effect on light-soaking characteristics. 

Hours in 85C/85%RH

Relative Pmax (%)

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
20

Initially no clear difference in response due to bias in the dark under accelerated (85/85) 

conditions was seen, but at times approaching 2000 hours a difference emerged (Fig. 1.5). 

Modules that were forward-biased during damp heat stress were found to deteriorate more 

slowly than un-biased modules, to a statistical confidence level greater than 99%. 

Fig. 1.5. 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 also 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. 1.6). 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 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 Damp Heat (hrs.)

Pmax (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
21

Fig. 1.6. Mean relative Pmax of CIGS strings laminated in glass, non-biased and biased at Vmax, 

during damp heat treatment. 

By nature, PV product requires many cell interconnections, each of which represents a potential 

point of failure. Accordingly, special test structures 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 when exposed to thermal cycling and damp heat. The variable 

results that were obtained clearly indicated that a better understanding was required. At length, 

further investigations indicated that several modes of failure were involved, including 

electrochemical corrosion between dissimilar surfaces, simple mechanical failure due to 

differential thermal expansion, and other mechanisms. The reliability and failure mode was 

highly dependent on the materials and construction methods. Despite the complexity, and an 

understanding that is still incomplete, workable solutions appear to have been identified which 

indicate relatively stable series resistance in a glass package when subjected to damp heat, 

thermal cycling and humidity-freeze tests. 

In preparation for IEC certification, glass-backsheet modules constructed from production strings 

were evaluated in damp heat tests (85°C/85% Relative Humidity) at GSE. Thermal cycling and 

humidity-freeze tests were conducted by a subcontractor. IEC 1646 protocol was used with an 

exception to the IEC standards in the thermal cycling test (only 155 cycles were conducted, not 

200 cycles as specified in IEC 1646). Results are summarized below. 

Using two 18-cell strings in each module, six modules were submitted for humidity-freeze tests, 

and five identical modules for thermal cycling. Since the GSE damp heat chambers are unable to 

Time in 85C/85%RH (hours)

Relative Pmax (%)

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
22

accommodate modules fabricated from 18-cell strings due to size, 12 modules were manually 

fabricated with 12 cells each in a 4x3 configuration (cut from 18-cell strings) for damp heat 

testing. The initial efficiencies of all tested modules were 9-10%. 

Prior to initial and final measurements light soaking was conducted for 3-4 sunny days in Tucson 

(March 2009). The mean results, summarized in Table 1.2, indicate no significant losses after 

thermal cycling and humidity-freeze stress tests. A 10% loss in Pmax after damp heat testing was 

observed, primarily due to fill factor loss. Steps required for the manual reconfiguration of the 

modules for damp heat testing may have been responsible for a portion of the loss observed in 

the damp heat test, as other damp heat tests on similar product has shown less power loss over 

longer exposure times. 

Table 1.2.- Module performance changes after stress testing. 

Relative % Change upon Accelerated Stress 

Parameter DH-1000 TC-155 HF-10 

Vmax -7.6 3.9 1.7 

Imax -2.4 -1.1 -1.0 

Pmax -10.0 2.6 0.4 

Voc -0.7 1.5 1.1 

Isc 0.8 -0.9 -0.1 

FF -10.1 1.9 -0.4 

In the data above, “DH-1000”, “TC-155” and “HF-10” respectively refer to the damp heat test in 

85°C/85% humidity for 1000 hours, the thermal cycling test for 155 cycles, and the humidityfreeze test. The results of the accelerated tests for thermal cycling and humidity-freeze are 

intended to simulate real-world conditions for product in the field, which is expected to undergo 

daily cycles in temperature and (potentially) cycles of freezing and thawing involving moisture 

in and around the package. The stresses imparted during these tests might be expected to impact 

module degradation through corrosion, or differential thermal expansion and contraction of 

different components in the module, leading to mechanical stresses on components and joints, 

particularly internal electrical connections. 

Flexible Power Product - Testing and Reliability 

Glass-based product more easily exhibits outdoor stability compared to 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.[3] Thus, testing requirements to assure 

stability for flexible product are yet more demanding. Additionally, flexible power product was 
23

introduced more recently and development has taken place over a limited time during this 

subcontract. Despite this, significant progress has been made with good results indicated. 

In initial approaches, several approaches for interconnecting cells were evaluated in outdoor tests 

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 for flex 

product. 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 (Fig. 1.7). 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 were made to the production processes to incorporate the important 

aspects learned in this test. 

Fig. 1.7. Mean relative Pmax of flexible modules fabricated by three stringing techniques and 

deployed outdoors in Tucson. 

In other initial tests a transition to the larger ‘Gen2’ cells for flexible power product was studied 

to rule out special factors relating to that change alone. 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 

Days Outdoors

Relative Pmax (%)

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
24

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 non-standard 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. 

Bending and peel strength tests take on much more importance in flexible as compared to rigid 

product. A multitude of ongoing study in these areas are underway. The peel strengths of 

various adhesives to materials applied inside CIGS PV modules were evaluated as a function of 

time in damp heat (85C, 85% RH). Two constructions using EVA films available from distinct 

suppliers (C and D), and another two using non-EVA adhesives with characteristics satisfactory 

for use in PV modules (A and B) were tested, results shown in fig 1.8. Peel test samples were 

prepared by laminating the adhesives to EVA-primed backsheets, PET, and Molybdenum-coated 

steel, and then separated using an Imada push/pull tester to pull on the prepared samples (Fig. 

1.8). A strain gauge attached to the tester is used measure the force. The peel strength of EVAbonded PET samples was found to drop significantly during the first several hundred hours of 

damp heat. 

 
25

Fig. 1.8. Peel strength of various adhesives to PET film. 

Fig. 1.9. Schematic of peel test set-up. The film is pulled vertically to separate from the substrate 

which is held on a mandrel. (Credit: Global Solar Energy) 

Backsheet adhesion is also an issue requiring extensive testing. Several backsheets used 

previously with glass modules were evaluated for adhesion and module performance in damp 

Hours at 85C/85% RH

Peel Strength (lb/in)

0 140 280 420 560 700 840 980

-5

0

5

10

15

20

25

30

35

40

 A

 B

 C

 D
26

heat. The adhesion of the test backsheets to the standard adhesive was poor, and rapid damp heat 

test failure of those same backsheets in live modules was subsequently observed. Several items 

of note in these studies are that accelerated test results correlated to results in actual use, and 

materials that are successfully used in rigid power product are not necessarily applicable to 

flexible power product. 

The frontsheet is more problematic, as materials having optical clarity with low water vapor 

transmission rates are desirable. In one study, transparent polymer films with coatings designed 

to act as water vapor barriers were received from multiple suppliers. The films were laminated 

over flexible CIGS solar cells (with leads attached) and subjected to damp heat stress and 

intermittent testing of the solar cells. Some of these initial results showed <20% degradation of 

Pmax after 1000 hours, but further improvements in stability are necessary to enable a +20 year 

warranty. 

Consequently samples of revised or alternate vapor barrier material were procured from four 

suppliers for another evaluation. Vapor barriers from three of the suppliers passed an adhesion 

peel test, and were subsequently evaluated in damp heat as the cover sheet for tabbed CIGS solar 

cells. An impermeable back sheet was applied to all samples opposite the “sunny-side”. At the 

end of the reporting period, two of the vapor barriers are showing encouraging performance 

compared to the control samples after several hundred hours in damp. Samples were stressed up 

to 2000 hours in damp heat before concluding the tests. 

Many similar tests have been undertaken, and a multitude are ongoing presently for the flex 

power product. Reliability (as represented by stability in 85/85 conditions) continues to 

improve, with some recent results shown in fig. 1.10. The stability of the flexible CIGS product 

is evolving toward the level required for market acceptability more rapidly recently, owing to a 

better understanding of degradation mechanisms, appropriate test procedures, and also to an 

increasing availability of flexible polymer sheets incorporating vapor barrier layers. 
27

Fig. 1.10. Normalized efficiency vs time in 85°C, 85% relative humidity for a variety of CIGS module 

constructions. All modules use a flexible frontsheet, and over half shown above also use a 

flexible backsheet (the remainder use a glass backsheet). 

Intrinsic Product Stability 

One of the most remarkable aspects of CIGS product stability is that it appears to depend heavily 

on complex interactions between materials in most cases. As an example, although many 

module constructions have evidenced significant degradation in damp heat testing, instances 

have been noted of complete stability for naked (unprotected) CIGS cells exposed directly to the 

same damp heat conditions. 

The relative efficiency of some unencapsulated small area devices as a function of exposure time 

in the 85-85 test is shown in Fig. 1.10. The devices were fabricated using standard GSE 

materials and processes, but consisted of only the device stack on the stainless foil substrate. 

Contacts for testing were made using evaporated metal grids with probes or indium solder. 

 

Normalized Efficiency

0

20

40

60

80

100

0 500 1000 1500 2000 2500 3000

Hours 85C/85% RH

Efficiency normalized to 3day LS (%

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22
28

Fig. 1.11. Unprotected CIGS device efficiency (relative to starting efficiency) as a function of time 

exposed to 85°C / 85% humidity. These cells were completely unencapsulated during exposure 

and measurement. 

This remarkable result indicates that the basic CIGS materials system, with heterojunction layer 

and front and back contacts of TCO and molybdenum on stainless foil, can be fundamentally 

stable. Thus it appears that many cases of observed degradation must be linked with other 

materials used in the module fabrication, or their interaction. Selective replacement tests in 

which various materials within the package are changed out with other materials clearly indicate 

the strong impact of the materials outside the thin film layers on the reliability of the total 

product. 

Bending Tests for Flexible Power Product 

Bending tests were designed to determine the bend radius below which cells are mechanically 

damaged, specifically at the junction between the printed fingers and the main collection buss. 

Gen1 cells (dimensions: 7.25 in. x 1.65 in.) with printed silver collection grid were bent around 

polished steel mandrels of decreasing diameter. Cells were oriented with the collection fingers 

perpendicular to the axis of the steel mandrel. To avoid non-uniform stress, the force required to 

conform the cell to the mandrel was applied through a silicone foam rubber. 

Cells were tested in both compressive (grid toward mandrel) and tensile stress (grid away from 

mandrel). Controls were re-measured along with each of the two stressed sets to rule out 

Efficiency vs. Exposure in 85-85

0.92

0.94

0.96

0.98

1.00

1.02

1.04

1.06

1.08

1.10

1.12

0 400 800 1200 1600 2000

Damp Heat Exposure Time (Hours)

Normalized Efficiency

cell 1

cell 2

cell 3

cell 4
29

potential effects of repeated measurements. There were ten samples in each of the stressed 

groups and five control samples. 

The results, shown in Fig. 1.11, indicate that compressive stress results in greater damage than 

tensile stress for a given bend radius. Little or no damage was apparent for either stressed group 

down to a bend diameter of 0.25 inches. The controls changed little during the duration of the 

test. 

Fig. 1.12 Mean efficiency of cells after being bent around mandrels with decreasing diameter. 

Improved Product Appearance, Uniformity 

Color variation of the CIGS cells, which is primarily due to thickness variations in the CdS layer 

is of concern for aesthetic considerations. Efforts to improve the control and uniformity of key 

variables, including reactant introduction in the CdS process resulted in significantly improved 

CdS uniformity and process control. The more uniform appearance in finished product that 

resulted is evident in the comparison of product photos in figures 1.12 and 1.13 below. 

Diameter (inches)

Efficiency (%)

Baseline

1.00000.87500.81250.75000.68750.62500.56250.50000.43750.37500.31250.25000.18750.12500.09380.0625

0

2

4

6

8

10

12

14

 Tensile

 Compressive

 Control (no bend)
30

Fig. 1.13. An early large area product based on the Gen1 cell fabrication, and showing typical 

color variation due primarily to thickness variation in the initial CdS process. 

(Credit: Global Solar Energy) 
31

Fig. 1.14. A color photo of more recent large area product based on the Gen2 cell fabrication, and 

showing the more uniform color appearance attained with improved control of the CdS process. 

(Credit: Global Solar Energy) 

After these improvements the CdS thickness variation appeared not to exceed limits required for 

good electrical performance. However, more careful measurements of quantum efficiency made 

at NREL subsequently indicated that a variation in electrical performance that was linked to the 

CdS layer (see Fig. 1.14). An analysis of the QE in the wavelength range near 460 nm done at 

NREL indicated that the CdS thickness may vary from 360Å to 700Å, which was correlated with 

variation in local J-V parameters (specifically Voc and FF). Thus, despite the improvement in 

uniformity, it appears that further improvements in uniformity would be beneficial. 
32

Fig. 1.15 Relative QE measurements on GSE samples made at NREL showing variation in the 

wavelengths near 460 nm where variation due to CdS thickness changes would be prominent.

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. 

Production Scale-up (New Manufacturing Facilities) 

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

thin film PV, and “economies of scale”, coupled with improved capability in production 

equipment are key to those goals, we will report on changes impacting these factors here. 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 contract goal and many of the specific process improvements are 

related to, or dependant on, the design and installation of improved plant and equipment. In 

0

10

20

30

40

50

60

70

80

90

100

300 400 500 600 700 800 900 1000 1100 1200

Wavelength (nm)

4526#2 GSE CdS + wrap

4526#7 GSE CdS + wrap

4539#2 GSE CdS + wrap

4539#8 GSE CdS + wrap

4539#4 GSE ITO

4526#3 GSE ITO

4539#7 GSE CIGS

4526#3 GSE CIGS

4526#3 GSE CdS
33

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. Subsequently, 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. 

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 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 2.1-2.5 below show the Tucson facility in early stages of 

demolition. 

Fig. 2.1. One section of the GSE Tucson building under demolition. (Credit: Global Solar Energy) 
34

Fig. 2.2. A northeast section of the GSE Tucson building under demolition. 

(Credit: Global Solar Energy) 

 

Fig. 2.3. The building exterior for the Tucson facility before renovation. 

(Credit: Global Solar Energy) 
35

Fig. 2.4. Interior structures, equipment chases and HVAC under renovation in the Tucson facility. 

(Credit: Global Solar Energy) 

Fig. 2.5. Interior rooms, overhead lighting and HVAC under renovation in the Tucson facility. 

(Credit: Global Solar Energy) 
36

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. 

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 was 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. 

• The building was completed generally from north to south, shown in Fig. 2.6, 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. 
37

• 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 (examples shown in Figs. 2.7 and 2.8). 

• 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 insitu 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 maximum efficiency of 8.6%. 

• Ongoing efforts were then focused on ‘Gen2’ processes and form factors for 

continuing process optimization of the thin film and other factory processes, 

increasing measured cell efficiencies and reproducibility, and thus yield. 
38

Fig. 2.6. The new GSE manufacturing facility in Tucson (10,220m2

). (Credit: Global Solar Energy) 

Fig. 2.7. A GSE tool for roll-to-roll absorber (CIGS) deposition at the new manufacturing facility in 

Tucson. (Credit: Global Solar Energy)
39

Fig. 2.8. Roll-to-Roll equipment for one of the “back-end” processes in the new factory (front 

contact collection grid printing). (Credit: Global Solar Energy) 

Fig. 2.9. A view of the thin film CIGS-based PV strings on metal foil (and nominal string electrical 

characteristics) that represent the initial’ Gen2’ product of the new GSE factory. 

(Credit: Global Solar Energy)

Pmp (W): 39.5 

Vmp (A): 7.3 

Imp (A): 5.4 

Voc (V): 10.3 

Isc (A): 6.7 
40

Enhanced CIGS Deposition Rates 

For CIGS formation the maximum rate possible 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. High CIGS formation rates are desirable to maximize throughput for 

the CIGS deposition, and is one of the goals of this program. Limited CIGS deposition speed 

trials were run early in this TFPPP program, demonstrating that deposition speed can be 

increased from 12 to 15-in/min in the ‘Gen1’ equipment 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. 2.10. 

Fig. 2.10. Auger electron spectroscopic analysis done at NREL comparing the composition of 

cells made at GSE at web speeds of 12 and 15-in/min on ‘Gen1’ equipment. 

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

Fig. 2.10 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. More importantly, ‘Gen1’cells made at the 15-in/min speed were equivalent in 

performance to those made at the 12-in/mid CIGS deposition rate. Representative electrical data 

for a large area ‘Gen1’ cell made at 15-in/min is shown in Fig. 2.11. 

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)
41

Fig. 2.11. A representative I-V characteristic for a large area cell made at an increased web speed 

of 15-in/min on the ‘Gen1’ equipment. 

Upon moving to all ‘Gen2’ equipment in the new plants, cost reduction in the CIGS process (and 

other processes) was largely addressed through improvements to equipment design, process 

control, engineering, operational procedures and process setpoints that were all directed toward 

running at higher deposition rates, and for increasing web lengths. 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. 

High deposition rates and increasing web lengths are effective in reducing cost, but only if cell 

efficiency and yield can be maintained concurrently. Internal goals called for increasing web 

length to 1 km through all tools, with CIGS deposition rates of between 19 and 24-in/min. 

CIGS deposition rates in this range were evaluated for the impact on solar cell electrical 

characteristics. In the test, the deposition rate was varied by moving the web at two different 

speeds (51 and 61 cm/minute) and adjusting the effusion source fluxes at each web speed to 

supply identical thicknesses of Cu, In, and Ga. Diffusion was not controlled in this test since the 

web temperature is dependent on residence time near heating sources (including the effusion 

sources). 

Gen1 cells (area: 68cm2

) were prepared on panels extracted from each test region. Mean 

electrical characteristics for each web speed are shown in the table 2.1. Cells with CIGS 

deposited at the lower web speed exhibited higher efficiency, primarily due to higher fill factor. 

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 0.7

Voltage V

Current A

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
42

Table 2.1. Mean electrical characteristics of cells with CIGS deposited at 51 and 61 cm/minute. 

CIGS at 

51 

cm/min.

CIGS at 

61 

cm/min.

Voc (mV) 569 566 

Isc (mA) 2183 2194 

Fill Factor 

(%) 55.7 53.5 

Efficiency 

(%) 10.5 9.8 

This and other tests indicated that good efficiency could be attained while meeting the internal 

goals of between 19 and 24-in/min. All ‘Gen2’ results cited henceforth were achieved at a CIGS 

deposition rate of 20-in/min, unless otherwise noted. This CIGS deposition rate exceeds both the 

goal and “stretch goals” of the TFPPP program of 15 and 18-in/min respectively. 

Fundamental improvements were also 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 were made to ensure adequate cross-web composition 

uniformity, in addition to improvements in materials utilization. This objective was 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 by instituting a scheme for more flexible delivery of CIGS reactants, but also requires 

optimization to find the optimal delivery profile. Steady progress has been made in this regard 

leading to increasing efficiencies and yields for the large area ‘Gen2’ cells. 

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 - stability of the effusion 

sources and efficacy of the real-time process monitors and closed-loop control algorithms. 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 
43

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. 2.12. for the CIGS deposition equipment at the 

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

Fig. 2.12. 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. 2.13 for the same web in Fig. 2.12. 

Fig. 2.13. Atomic ratios and total thickness of a CIGS film (across the web width) as measured by 

ex-situ XRF. 
44

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. Fig. 2.14 shows the 

equivalent thicknesses for copper, indium and gallium as a function of down-web location for a 

typical deposition. 

Fig. 2.14. 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 forgoing also illustrates the importance of process monitoring tools. Consequently, 

upgrades were made to the XRF software and calibration to improve measurement accuracy. 

Greater accuracy in measuring Cu, In, Ga, and Mo enables better process reproducibility, and is 

required to optimize the absorber band gap with fine detail. Fig 2.15 shows the discrepancy in 

the elemental layer thickness (for Cu, In, Ga and Mo) between measurements of the same film 

made at different machine locations and in different machines. Results are shown before, and 

after the correction upgrades were added to the algorithms. Before the correction upgrades the 

measured elemental thicknesses at a 2nd location were consistently less than at the first location, 

and could disagree by 10-15%. After the upgrades to correct measurement errors automatically 

the same measurements typically differed by 2-5%, with an error more uniformly distributed 

about zero. These measurements were made “real-time” under “hot” or actual deposition 

conditions. 
45

Fig. 2.15. Discrepancy in the elemental layer thickness (for Cu, In, Ga and Mo) made in several 

locations before (square columns) and after (cones) upgrades were made to automatically correct 

measurement errors in XRF real-time readings. 

1

2

3

4 Cu

(w/o

Corr)

In

(w/o

Corr)

Ga

(w/o

Corr)

Mo

(w/o

Corr)

Cu

(with

Corr)

In

(with

Corr)

Ga

(with

Corr)

Mo

(with

Corr) -20.0%

-15.0%

-10.0%

-5.0%

0.0%

5.0%

10.0%

Discrepancy (%)

Meas. Location

Cu, In, Ga, Mo

Element Thickness Discrepancy

Cu (w/o Corr)

In (w/o Corr)

Ga (w/o Corr)

Mo (w/o Corr)

Cu (with Corr)

In (with Corr)

Ga (with Corr)

Mo (with Corr)
46

Process Chemistry Relating to Optimization 

Fig. 2.16. 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
47

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, Mo-coated 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. 2.16. 

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. 

Absorber Thickness Variation 

The sensitivity of the solar cell electrical characteristics to CIGS thickness was also evaluated on 

a continuous 330 m web. In this test, the film thickness was varied by moving the web at two 

different speeds (51 and 61 cm/minute) while maintaining constant effusion source fluxes. The 

deposition rates are identical, but the coating deposited at the lower web speed likely reached a 

higher temperature from increased residence time near heating sources. This simple test 

approach leads to a “stretching” of the entire band gap profile in the thicker film. 

The CIGS coating thickness for the two test conditions was evaluated by XRF. The mean 

thicknesses for the “thin” and “thick” coatings were 1.31μm and 1.69μm, respectively. 

Gen1 cells (area: 68cm2

) were prepared on panels extracted from each test region. Mean 

electrical characteristics for each coating thickness are shown in the table 2.2. Cells with thicker 

CIGS exhibited significantly higher efficiency, with all electrical parameters improved. 

Table 2.2. Mean electrical characteristics of cells with “thin” and “thick” CIGS 

“Thin” 

CIGS 

“Thick” 

CIGS

Voc (mV) 566 580 

Isc (mA) 2141 2275 

Fill Factor (%) 54.5 58.3 

Efficiency (%) 9.7 11.2 

These results indicate that further effort, likely involving significant changes to deposition 

conditions and composition profile will be required to achieve comparable efficiency cells with 

the use of thin absorber layers. 
48

Alternate Na Incorporation 

One alternate method for Na incorporation was completed through an initial evaluation. The 

benefits of this alternate method over the standard operating process (‘SOP’) would potentially 

be better dose control, better uniformity, improved yield and adhesion. A first experiment used 

one manufacturing lot to compare the standard method to the alternate method and also to no Na 

introduction (fig 2.17). 

Fig. 2.17. Average efficiency, fill factor and Voc of “strings” of full sized Gen2 cells produced 

using 2 different methods of Na incorporation, or none at all. The strings were fabricated using 

standard manufacturing techniques otherwise, and consisted of 18 serially connected large area 

cells (210 cm2

) each. 

The data reiterates the importance of Na incorporation, and indicates that either the standard or 

the proposed alternate methods are effective. The varying levels of Na used with the alternate 

technique suggest that a level of Na above 35% (arbitrary units) is required. 

A second experiment using mostly “Gen2” process steps and equipment, with augmentation or 

replacement of one step using modified “Gen1” equipment was conducted. Sodium 

incorporation was accomplished using the standard process, the alternate process (with varying 

quantities), and both processes, with results in fig 2.18, allowing a finer comparison of the two 

approaches. 

Impact of Na Process

0%

1%

2%

3%

4%

5%

6%

7%

8%

9%

10%

Std" Na = Off,

'Alt' Na = 0%

Std" Na = On,

'Alt' Na = 0%

Std" Na = On,

'Alt' Na = 0%

Std" Na = Off,

'Alt' Na = 0%

Std" Na = Off,

'Alt' Na = 65%

Std" Na = Off,

'Alt' Na = 0%

Std" Na = Off,

'Alt' Na = 35%

Std" Na = Off,

'Alt' Na = 65%

Test Condition

String Efficiency (%)

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

Fill Factor, Voc (Volts)

Efficiency (%)

FF (%)

Voc (mV)
49

Fig. 2.18. Average efficiency, fill factor and Voc of “strings” produced using two different methods 

of Na incorporation, or both simultaneously. The strings were fabricated using standard 

manufacturing techniques otherwise, and consisted of 18 serially connected large area cells (210 

cm2

) each. 

This data indicates that, although the alternate Na incorporation process is effective, the cell and 

string performance does not match the standard process for Na incorporation. Again, the loss in 

efficiency stems mostly from fill factor and Voc, as would be expected, and amounts to almost 1 

percentage point in efficiency between the two methods. Additional Na or a combination of both 

Na incorporation methods does not improve results above the standard process alone. In this 

first test appropriate quantities of Na for the alternate method were estimated, and may not have 

been high enough. 

Thermal Coefficients for IV Parameters and “Bandgap Engineering” 

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 2.3. 

Impact of Na Process

8.6%

8.8%

9.0%

9.2%

9.4%

9.6%

9.8%

10.0%

10.2%

10.4%

Std" Na = On,

'Alt' Na = 0%

Std" Na = On,

'Alt' Na = 0%

Std" Na = On,

'Alt' Na = 90%

Std" Na = On,

'Alt' Na = 65%

Std" Na = Off,

'Alt' Na = 65%

Std" Na = Off,

'Alt' Na = 90%

Std" Na = On,

'Alt' Na = 90%

Test Condition

String Efficiency (%)

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

Fill Factor, Voc (Volts)

Efficiency (%)

FF (%)

Voc (mV)
50

Table 2.3. 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 

The figures above pose an opportunity for enhanced power generation under field conditions 

where module temperatures are typically higher than 25°C dictated by STC conditions. Work 

started in 2010 to increase the effective bandgap in the active region of the absorber, thereby 

reducing the performance loss with increasing temperature. The CIGS materials system is 

tolerant to bandgap modification over a substantial range[4] via gallium or aluminum substitution 

for indium, or sulfur substitution for selenium. Higher open circuit voltage is attained, usually at 

the expense of short circuit current, so that if fill factor can be maintained, there may be no loss 

in overall efficiency. 

Even without any efficiency gain at STC, increasing the bandgap provides two benefits: a higher 

voltage with less current reduces I2R losses, and a higher bandgap material exhibits a reduced 

rate of power loss versus temperature. This latter effect is significant for operation in most 

climates where actual cell operating temperature is much above standard measurement 

conditions of 25°C. Figure 2.19 shows the variation in the temperature coefficient for power 

measured on large area cells whose CIGS bandgap had been changed by modification of the Ga 

content. 

Using the linear relationship derived from a fit to the data of figure 2.19 the reduction in 

efficiency at elevated operating temperature can be estimated. The cell operating temperature 

was calculated using methods outlined by D.L. King[5] for a location in the southwest US at 980 

W/m2 irradiation for conditions representative of operation at mid-day in April and June 

(corresponding cell temperatures of 49.5°C and 65.5°C respectively). 
51

Fig. 2.19/ The measured thermal coefficient for output power as a function of open circuit cell 

voltage. 

A comparison of the expected efficiency at temperatures corresponding to these two outdoor 

conditions is shown in table 2.4 for a low and high bandgap CIGS cell. The improvement in 

actual efficiency at elevated temperatures is significant, indicating the merit in this approach. 

Table 2.4. Expected cell efficiencies with CIGS absorbers of large and small bandgaps at 

operating temperature. Cells are 13% efficient at standard conditions of 25°C. 

April (Tcell 

= 49.5°C) 

June 

(Tcell = 

65.5°C) 

Pmax Tc 

(%/°C) 

Low Eg (Voc 

= 560 mV) 

11.39 10.42 -0.496 

High Eg (Voc 

= 680 mV) 

11.89 11.23 -0.340 

Absolute 

Efficiency 

Improvement 

0.5% 0.81% 

Temperature Coefficient vs Voc

y = 1.2996x - 1.224

R2

 = 0.8827

-0.6

-0.56

-0.52

-0.48

-0.44

-0.4

-0.36

0.475 0.500 0.525 0.550 0.575 0.600 0.625 0.650

Cell Voc

Pmax coeff. (%/°C)

Data

Linear (Data)
52

Accordingly, a campaign to alter the Ga content and profile of the CIGS layer has been initiated 

at GSE. Cells with Voc’s exceeding 700 mV have resulted recently from the bandgap 

engineering effort at GSE, without loss of overall efficiency. Thus, it seems feasible to reduce 

the temperature coefficient for power loss, attaining coefficients that are substantially below 

those typical for crystalline Si. The impact of the adjustments in the campaign to increase 

absorber bandgap is evident in the series of Voc measurements on sequential manufacturing lots, 

shown in fig. 2.20. 

Fig. 2.20.Open circuit voltage (per cell) for “strings” on the production line at GSE, showing the 

initial impact of the campaign to increase the CIGS bandgap. 

Process Development for Large Scale Production 

Multiple challenges existed to raising efficiency and yield on the full scale production 

equipment, complicated by complexity of the processes and equipment. The major obstacles 

included: 

1. Eliminating operational problems, deficiencies and design oversights in new equipment 

for all processes; 

2. Establishing adequate process monitoring and metrics for all steps, with the ability to 

record, store, correlate and report them; 

Week

Voc per Cell (V)

09-1909-0909-2209-1309-1209-0509-0409-3009-3409-3709-4209-4609-5009-5210-0710-1110-14

0.48

0.50

0.52

0.54

0.56

0.58

0.60

0.62

0.64

0.66

 Yielded Strings (median, 25%-75%, 10%-90%)
53

3. Assuring process reproducibility and control; 

4. Optimizing process settings for an entire manufacturing sequence in which many process 

steps are interactive; 

5. Training personnel, documenting critical procedures and establishing a qualified vendor 

network for critical parts and materials; 

6. Transferring technology, results and procedures between factories in multiple countries to 

maintain equivalent capability; 

7. Adaptation to new product introduction or major revision due to changing customer 

demand or technical capability. 

Reproducibility and process control is crucial in meaningful process optimization during 

development and high yields in production. Process control is common to all seven of the 

factors mentioned above as obstacles to overcome in this progression. An example web, shown 

in Fig. 2.21, illustrates the consistency of key parameters achieved during this effort for a 1 km 

length. The setpoints are monitored in real time and used with intelligent closed-loop control to 

maintain the process. The displayed dependent variable (efficiency) represents the efficiency of 

entire 18-cell “strings” having 3780 cm2

 total area made from all locations downweb, at each of 

3 locations crossweb. Independent variables are plotted to show the CIGS thickness and the 

composition rations of Cu/(In+Ga) and Ga/(In+Ga). The first 100 meters of the run is affected 

by a “start-up” effect – a phenomenon still under investigation that causes reduced efficiency 

despite proper setpoint values for all known process variables. Excepting this, there is very little 

variation over the entire course of the 1000 meter web in any of the parameters. All thin film 

deposition equipment typically runs unattended except where special intervention is necessary 

for special studies that require dynamic changes to parameter settings. 
54

Fig. 2.21. Parameters of a 1 km web. Efficiency of strings made using 3 locations across the web, 

and independent parameters of CIGS thickness, Cu/(In+Ga) and Ga/(In+Ga) are shown at two 

crossweb locations down the entire web length. A “string” is 18 serially connected large area 

cells, each 210 cm2

 in area. 

Once control and reproducibility were achieved, many CIGS process tests were conducted to 

better understand the process space for maximized efficiency and minimized sensitivity. Some 

of the parameters explored were substrate temperature by zone, in-process Cu-rich excursion, 

final Cu/(Ga+In), Se delivery, and bandgap profile. 

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%. 

Cost reduction in this step is important, as the TCO process was the single, most expensive thin 

film layer in the device stack upon initiation of this subcontract. A parallel approach using two 

paths has been taken for this task; a low-risk pursuit of marginal improvements using 

conventional sputtered methodology, and a high risk pursuit of a novel TCO deposition process 

potentially offering very large reductions in cost. 

1 km CIGS Run (4631SA)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

0 100 200 300 400 500 600 700 800 900 1000

Web Distance (m)

Composition Ratio, 

Thickness(μ)

0

2

4

6

8

10

12

Efficiency (%)

CIGS (A)

CIGS (C)

Ga/(Ga+In) (A)

Ga/(Ga+In) (C)

Cu/(In+Ga) (A)

Cu/(In+Ga) (C)

Eff. (A)

Eff. (B)

Eff. (C)
55

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 re-commissioned 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 a lower-tier subcontractor to GSE with a specialized deposition method that is 

potentially 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. 

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. 3.1). 
56

Fig. 3.1. 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. 

Materials properties have been evaluated and improved so that many key properties comparable 

to, or better than the standard TCO process used at GSE. The optical transmission of this 

alternate TCO process compares well with a typical process for ZnO having the same sheet 

resistivity (Fig. 3.2). 

ZnO Optical Transmission (external)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

300 400 500 600 700 800 900 1000 1100 1200 1300 1400

Wavelength (nM)

Transmission
57

Fig. 3.2. The optical transmittance of TCO made using the alternate TCO process compared to Aldoped ZnO made using a typical process having the same sheet resistivity. 

After materials studies confirmed the applicability of the alternate TCO process, several studies 

using standard CIGS PV were completed, encompassing designed experiments to optimize 

variables in a large parameter space. Device results for the most recent study are shown in Fig. 

3.3 for the best 3 experimental conditions for TCO deposition using large area cells. In all cases 

the decrease in efficiency was accounted for in Voc and fill factor loss. 

Optical Transmittance

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

250 450 650 850 1050 1250 1450

Wavelength (nm)

Optical Transmittance

Typical Al-doped Zinc Oxide (10 Ohm/sq)

Alternate TCO Process (10 Ohm/sq)
58

Fig. 3.3. Large area (68.8 cm2) cell efficiency for GSE cells using the alternate TCO process. Each 

group of cells was processed at different conditions during TCO deposition. The standard TCO 

process is shown also for the same cell lot as dashed lines for the average cell efficiency ± 1 

standard deviation. 

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. Although, 

under selected conditions, the alternate TCO process has produced large area cells that are 

comparable to the standard process, several aspects of the approach would require improvement, 

such as deposition rate, to make the process fully attractive. 

Incremental Improvement of the Existing TCO Process: ‘Gen1’ Process 

The incremental, low risk approach for TCO process improvement was implemented in two 

sequential efforts. Available remedies and improvements were applied to the ‘Gen1’ processes 

and equipments (designed around planar magnetrons). When the ‘Gen2’ equipment was 

designed and built, significant opportunities for process cost reduction were available, and acted 

on accordingly. 

For the ‘Gen1’ equipment, 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. 

Large Area Cells 

(GSE lot 4186SAB)

0.0

2.0

4.0

6.0

8.0

10.0

12.0

5 10 15 20 25 30 35 40

Panel

Efficiency (%), Gen1 Cells

Alternate TCO

Standard TCO

StDev

StDev
59

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). 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 maxima to more desirable ranges of the solar 

spectrum, giving an Isc gain (shown in Fig. 3.4). 

Fig. 3.4. A comparison of the optical transmission as a function of wavelength for the standard 

production TCO process and a new process under evaluation. 

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
60

One goal in the ‘Gen1’ process was to improve bulk resistivity and thus enable a reduction in 

thickness, thus providing reduced cost 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. 3.5 a, b). Sheet resistance regulation of the new process is + 32% 

absolute with a standard deviation of 14%. 

Rsh Probe Tracking

-10

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

0 100 200 300 400 500 600 700 800 900 1000 1100 1200

Web Position (ft)

Rsh Probe Reading (m

Rsh Probe (OP)

Rsh Probe (NOP)
61

Figs. 3.5 a,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). 

The data shown in Figs 3.5 a,b were taken in-situ, real-time for the TCO deposition using a 4-

point probe having rolling contacts made of a soft, electrically conductive material (for example, 

an electrically conductive polymer). The rolling conductive contacts are mechanically 

independent as they are held against the moving web, thus are tolerant of irregularities. 

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. 

‘Gen2’ (TCO) Process Scale-Up and Cost Improvement 

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 were received, placed, 

facilitated and put into service at both the Tucson and Berlin facilities (Fig. 3.6). These TCO 

deposition tools were designed to be capable of well-controlled, uniform, high rate deposition 

using a dense array of rotary cathodes in a roll-to-roll approach. 

Rsh Probe Tracking

-10

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

0 100 200 300 400 500 600 700 800 900 1000 1100 1200

Web Position (ft)

R s h P ro b e R

Rsh Probe (OP)

Rsh Probe (NOP)
62

Fig. 3.6. The front contact (TCO) is deposited by pulsed DC magnetron sputtering. 

(Credit: Global Solar Energy) 

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. 

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 re-test 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 
63

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). 

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 effects 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. 

Overall, the cost reduction effort for TCO has been successful, mostly due to the improved 

design enabled by the transition to the ‘Gen2’ equipment. The use of rotary cathodes in a 

compact format allow a higher web coating rate in compact equipment (low capital cost) and 

much better materials utilization. The planar magnetron targets used in the ‘Gen1’ equipment 

often were expended after only 20% - 30% of the target material was used, due to the ‘racetrack’ 

wear pattern. The rotary cathodes implemented in the ‘Gen2’ equipment can approach 90% use 

of the target material before replacement. Initially the sputtering targets for the rotary cathodes 

were very expensive, but continual work with vendors has allowed a continual reduction in target 

costs. 

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. 

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
64 

Back Contact Process Improvements 

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. 

In subsequent tests, a 12% reduction in Mo thickness on string performance was evaluated using 

a test web prepared with control sections at the web ends and two interior sections with Mo 

thicknesses 94% and 88% of the controls. Slight reductions in string efficiency were observed 

for sections with thin Mo, but the differences were not statistically significant. Further tests were 

run to understand the process sensitivity to Mo thickness, and the potential for thickness (and 

cost) reduction. In general, the results indicated that Mo thickness reduction below a certain 

value caused a loss in efficiency that could not be avoided by other process changes. 

In another series of experiments the effects of deposition parameters on the morphology of the 

Mo layer and the subsequent effects on cell performance were evaluated. Parameters included 

were temperature, pressure, and gas partial pressures. We evaluated morphology by SEM, 

structure by XRD, adhesion of the Mo/CIGS interface by pull testing and cell performance. The 

morphology of the deposited Moly film was confirmed as columnar growth. XRD analysis 

revealed mixed orientations on the stainless steel substrate. We observed an impact on cell 

performance at distinct points in the Mo process study, 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. 

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 for preparation and pumpdown. 

Deterioration of roller surfaces as well as flakes of deposited molybdenum shed from zone 

shielding were 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 chamber and 

shape of the foil (camber) were investigated as a possible cause for the tension-related defects. 

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 
65 

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. 

Joining webs seamlessly is an important capability for direct comparison of different substrate 

material in a roll-to-roll process. A method was established for joining web sections at GSE that 

enables more reliable and efficient testing and designed experiments. Different sections of 

various substrates can be processed under identical conditions. Alternatively 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. 

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 

assembled. 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 redundancies in vendors and a cost savings in 

some cases.

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. A careful evaluation of current spec 

requirements was performed. Furthermore, 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 among materials suppliers of the importance of the PV market and a 

desire to enter that market. 

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 reconsidered because new cleaning techniques had been applied to their product. Upon reevaluation 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, one of the new suppliers 

was found to provide performance comparable to the qualified supplier. Additional, more 

extensive qualification tests are planned for the new potential supplier. 
66 

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

significant expense, and thus an opportunity to reduce costs.[6] Alternative suppliers of 

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 material from each supplier 

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 

different target types. 

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. 

From these results the new targets were down-selected from multiple suppliers. 

• A less costly technique for molybdenum target fabrication was evaluated in the Gen1 

equipment. Deposition rates under standard conditions were evaluated first, with no 

significant differences in deposition rates observed. However, test lots made using 

the alternative targets for production line solar cells showed significantly lower 

performance than cells using the standard molybdenum targets. The performance 

difference was linked to specific impurities not found in the standard targets. Since 

then an alternate target fabrication method that allows for cost reductions has been 

tested and has been qualified for production. 

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 downselected 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 (Moly 5) 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 Moly 5 to produce individual solar cells with efficiency greater than 10% was demonstrated 

within a few depositions. 
67 

However, the first test webs coated in Moly 5 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 aligned to tighter tolerances. 

Consequently, the mechanical defect density was dramatically reduced. However, defects 

continued to be present at a reduced level down the web length. 

Non-uniform web tension may be responsible for some physical defects. A number of tests were 

conducted to explore parameters that could provide more uniform tension, including web 

temperature and coating stress. The applied sputter power represents a considerable amount of 

the total energy contributing to web heating. High line speeds and therefore high sputter power 

are required for an economical manufacturing process. On the other hand, control of the substrate 

temperature and its effects on the structure and shape of the foil are important for good quality of 

the material. This line of investigation is continues with a goal of complete elimination of the 

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, 

with causes under investigation. 

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

(distance weighted least squares fit 

One aspect affecting reliability is the thickness and conditions used to deposit molybdenum 

layers on the stainless steel substrate, as eventually the large area cells are serially connected in 

the module fabrication step and electrical contacts are established to the molybdenum-coated 

stainless. 
68 

Fig. 4.2. Three coupon samples of substrate material with various coatings after 24 hours 

exposure to damp heat (85°C/85% RH), visually showing a differing response. 

(Credit: Global Solar Energy) 

Reliable electrical contacts to the molybdenum-coated substrate are required, yet the 

molybdenum is easily oxidized in the presence of moisture and oxygen. Because damp heat 

testing is an important part of the qualification regimen for the PV product, the impact of damp 

heat exposure on the reliability of the contacts to the substrate was investigated. Oxidation of the 

surface layers caused by damp heat (85°C/85% RH) exposure is clearly visible (fig. 4.2). Sets 

of tests were run comparing the mechanical and electrical characteristics of contacts made to 

substrate material as a function of the thickness and conditions used for the molybdenum 

deposition. One example of the electrical response after damp heat exposure of contacts made to 

substrates having molybdenum deposited with different characteristics is shown in fig. 4.3. In 

this test series resistance was measured in a standardized structure using ribbon contacts to the 

PV substrate, simulating actual module construction. Variation between different groups is 

evident, indicating that the change in electrical resistance at module interconnections during 

moisture exposure is affected by the characteristics of the molybdenum deposition for the PV 

substrate.
69 

Fig. 4.3. The series resistance of ribbon contacts made to the molybdenum coated PV substrate 

after only 16 hours of damp heat (85°C/85% RH) exposure, indicating a dependence of magnitude 

and variability on the test group. Each test group represented different deposition conditions for 

the molybdenum overlayer. 

Mechanical properties were also tested (fig. 4.4), looking at the impact of various molybdenum 

deposition thicknesses and conditions on ribbon bond strength. 

Box Plot (PTR060209B-NN Data 25v*493c)

group #

rseries

g-01 g-03 g-04 g-05 g-06 g-07 g-08 g-09 g-10 g-11 g-12 g-13 g-14 g-2R

0.012

0.014

0.016

0.018

0.020

0.022

0.024

0.026

0.028

0.030

0.032

0.034

0.036
70 

Fig. 4.4. The mechanical strength of ribbon contacts made to the molybdenum coated PV 

substrate after only 8 hours of damp heat (85°C/85% RH) exposure. The test groups correspond 

to those in Fig. 4.3., representing different deposition conditions for the molybdenum overlayer. 

Box Plot (PTR060209B-NN Shear Data 8v*122c)

Exclude condition: Order = 0

Group

Max Shear

g-01

g-03

g-04

g-05

g-06

g-07

g-08

g-09

g-10

g-11

g-12

g-13

g-14

g-02R

SS

17.4

17.6

17.8

18.0

18.2

18.4

18.6

18.8

19.0

19.2

19.4

19.6

19.8
71 

Fig. 4.5. Visual appearance of the ribbon contact test structures made to the molybdenum coated 

PV substrate was observed to vary significantly after damp heat (85°C/85% RH) treatment 

depending on Mo deposition conditions. This figure shows samples from groups 4 and 5 after 8 

hours of damp heat exposure. (Credit: Global Solar Energy)
72 

Fig. 4.6. A plot of the change in series resistance vs damp heat exposure time for ribbon 

connections made under different conditions to the backside contact. All conditions including the 

GSE standard process show Resistance increases of < 15% after 600 hrs in DH. 

Although work to understand the behavior and role in failure modes of the back contact continue, 

we have successfully reduced the increase of the series resistance from the back contact to less 

than ~15% (fig. 4.6). Recent tests show stable series resistance of a number of interconnects in 

unencapsulated cells out to 800 hrs in damp heat conditions. 

%Rs change of different back side in DH

-5%

0%

5%

10%

15%

20%

0 68 205 345 485 625

Hours Damp Heat (85/85)

% Change Resistance
73 

References 

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

Flexible and Lightweight CIGS Solar Cells,” 33rd IEEE PVSC, 2008, pp. 1670. 

[2] M.E. Beck, S. Wiedeman, R. Huntington, J. VanAlsburg, E. Kanto, R. Butcher, and J.S. Britt, 

“Advancements in Flexible CIGS Module Manufacturing,” 31st IEEE PVSC, 2005, pp. 211-214. 

[3] 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 

[4] W.N. Shafarman, R. Klenk, and B.E. McCandless, "Device and Material Characterization of 

Cu(In,Ga)Se2 Solar Cells with Increasing Bandgap," J. Appl. Phys. 79, 1996, pp. 7324. 

[5] D.L. King, J.A. Kratochviland, and W.E. Boyson, “Temperature Coefficients for PV 

Modules and Arrays:Measurement Methods, Difficulties, and Results,” 26th IEEE PVSC, 1997, 

pp. 1183-1186. 

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

for Flexible CIGS-Based Product,” 4th World Conference on Photovoltaic Energy Conversion 

(IEEE), 2006, pp. 388-391.