National Renewable Energy Laboratory 

Innovation for Our Energy Future 

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

Office of Energy Efficiency & Renewable Energy

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

Research Leading to HighThroughput Manufacturing of 

Thin-Film CdTe PV Modules 

Annual Subcontract Report 

September 2004—September 2005 

R.C. Powell 

First Solar, LLC 

Perrysburg, Ohio 

Subcontract Report 

NREL/SR-520-39669 

April 2006 
Research Leading to HighThroughput Manufacturing of 

Thin-Film CdTe PV Modules 

Annual Subcontract Report 

September 2004—September 2005 

R.C. Powell 

First Solar, LLC 

Perrysburg, Ohio 

NREL Technical Monitor: H.S. Ullal 

Prepared under Subcontract No. RDJ-2-30630-20 

Subcontract Report 

NREL/SR-520-39669 

April 2006 

National Renewable Energy Laboratory 

1617 Cole Boulevard, Golden, Colorado 80401-3393 

303-275-3000 • www.nrel.gov 

Operated for the U.S. Department of Energy 

Office of Energy Efficiency and Renewable Energy 

by Midwest Research Institute • Battelle 

Contract No. DE-AC36-99-GO10337
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Table of Contents 

Table of Contents...........................................................................................................................iii 

List of Figures................................................................................................................................ iv 

Abstract........................................................................................................................................... v 

Acknowledgements........................................................................................................................ vi 

1 Introduction............................................................................................................................. 1 

1.1 Project Background......................................................................................................... 1 

1.2 Technical Approach........................................................................................................ 1

1.3 Implementation Approach .............................................................................................. 3 

2 Advanced Front Contact and Window Layers (AFCW)......................................................... 3 

2.1 AFCW background ......................................................................................................... 3 

2.2 TCO Deposition Processes ............................................................................................. 4 

2.3 AFCW development ....................................................................................................... 5 

3 CdS/CdTe Deposition Reactor Development ......................................................................... 5 

3.1 VTD Background............................................................................................................ 5

3.2 VTD Technical Obstacles............................................................................................... 6 

3.3 VTD Technical Approach............................................................................................... 7 

3.4 Distributor Development ................................................................................................ 7 

3.5 CdS Powder Feed............................................................................................................ 8 

4 Distributor Modeling .............................................................................................................. 9 

5 Impurity control .................................................................................................................... 11 

5.1 Chemical analysis ......................................................................................................... 11 

5.2 Impurities and Chlorine Issues...................................................................................... 11 

6 Identification and modification of back-contact chemistry .................................................. 12 

7 Accelerated Life Test Development ..................................................................................... 13 

7.1 Physical model underlying ALT Technical Approach ................................................. 13 

7.2 Correlation to 56 day LS............................................................................................... 14 

7.3 Correlation with field data ............................................................................................ 15 

7.4 National CdTe R&D Stability Subteam........................................................................ 16 

8 Environmental, Health and Safety Improvements................................................................ 17 

8.1 Technical Obstacles ...................................................................................................... 17

8.2 EH&S Tasks.................................................................................................................. 17 

9 Summary............................................................................................................................... 18 

9.1 First Solar Growth......................................................................................................... 18 

9.2 Subcontract Accomplishments...................................................................................... 18 

10 Publications:...................................................................................................................... 19 

Glossary of Acronyms .................................................................................................................. 20 

References..................................................................................................................................... 21 

iii
List of Figures 

page

Figure 1 Schematic cross section of First Solar polycrystalline thin film CdTe PV module 

showing films and interconnect scribes ......................................................................2 

Figure 2. Dependence of cell efficiency on Jsc for devices produced on various 

thicknesses of CdS - with and without a ZTO buffer layer ........................................4 

Figure 3 Champion module achieved using a buffer layer. ......................................................5 

Figure 4 Conceptual drawing of First Solar proprietary VTD system......................................5 

Figure 5 Cross-web thickness uniformity of CdTe films deposited with different 

distributors. .................................................................................................................7 

Figure 6. Box plot of relative cross-web thickness deviation ....................................................8 

Figure 7 Box plot of relative cross web surface roughness deviation.......................................8 

Figure 8 Primary elements of the distributor simulation model developed by CSM 

researchers...................................................................................................................9 

Figure 9 Design parameters (arbitrary units) for Taguchi computer simulation 

experiment.................................................................................................................10 

Figure 10 Optimized distributor design parameters for each output variable based on 

computer simulation..................................................................................................10 

Figure 11 Representative data showing relative changes in efficiency with lightsoak for 

devices with and without intentionally introduced impurities..................................11 

Figure 12 Relative efficiency as a function of added impurity level of devices after 7 and 

19 days light soak at 120 C.......................................................................................12 

Figure 13 A. Initial quantum efficiency for devices with varying impurity levels and B. 

quantum efficiency improvement after light soaking at 120 C for 7 and 19 days....12 

Figure 14 Correlation of predicted baseline performance of various test structures, stressed 

at different temperatures, with baseline performance predicted using 

measurements made in the stress ambient as opposed to after the test structure 

had been cooled to room temperature.......................................................................14 

Figure 15 Variation of relative baseline performance as predicted by curve-fitting for 

strips from the same plate that were stressed at elevated temperature. ..................15 

Figure 16 Normalized histograms showing distribution of A. Fielded module baseline 

efficiency and B. Relative change in efficiency of fielded modules over time for 

each of 12 process conditions of a designed experiment..........................................16 

Figure 17 Correlation between change in performance as indicated by ALT and observed 

change in field data. ..................................................................................................16 

iv
Abstract 

Cost of photovoltaic (PV) modules is a major impediment to widespread use of solar energy for 

commercial production of electricity. Thin film technologies are expected to enable significant 

cost reductions compared to conventional wafer silicon PV technologies. First Solar’s Thin Film 

PV partnership program is directly targeted to reduction of PV module costs ($/W) through a 

combination of improved module efficiency (W/m2

) and improved process control leading to 

lower material cost ($/m2

). Competitive pricing is an essential element in development of a 

vibrant US PV industry that will bring clean, reliable, and affordable energy technologies to the 

marketplace in support of the EERE mission; cost reduction of development and production are 

specific elements of the SETP Program Goali

. 

Specific overall objectives of this subcontract are improvement in baseline field performance of 

manufactured CdTe PV modules while reducing environmental, health and safety risk in the 

manufacturing environment. Project objectives focus on four broad categories: 1) development 

of advanced front contact window layers, 2) improved semiconductor film deposition, 

3) development of improved accelerated life test procedures that indicate baseline field 

performance, and 4) reduction cadmium-related environmental, health and safety risks. 

During performance of this subcontract First Solar accomplished the following: 

• Designed, built, tested and evaluated for production a new generation of advanced 

distributors for semiconductor vapor transport deposition. More than an optimization, these 

advanced distributors were developed around a new operating principle and represent a 

change in the fundamental design that circumvents fundamental deficiencies in earlier 

designs and produce films that are more uniform across a plate, more uniform plate-to-plate 

within a run, and exhibit simplified deposition control and increased material utilization. 

• Developed an advanced semiconductor powder feeder suitable for vapor transport deposition 

of ultra-thin CdS films. 

• Generated five patent applications – four on advanced distributor design; one on advanced 

powder feeder 

• Through a Colorado School of Mines subcontract - developed a three stage model for 

distributor operation including a) injector model of powder feed into heated vaporizer, 

b) distributor model of vaporization and flow of gas within the distributor, and c) deposition 

model of distribution of gas coming out of the distributor and deposition onto glass. The 

detailed distributor model was used to carry out a Taguchi design of experiments that 

identified key features of distributor design essential for producing uniform films. 

• Demonstrated module scale application of buffer layer and sputtered CdS films. A module 

produced with ultra-thin CdS and buffer layers displayed 9.6% efficiency (unconfirmed) 

compared to <9% efficiency obtained with modules with the same CdS thickness but without 

a buffer layer. 

• Demonstrated ability to reproducibly deposit 900 Å CdS films in manufacturing 

• Demonstrated ability to reproducibly deposit 2.5 μm CdTe films in manufacturing 

• Developed equipment and ALT methodology to establish good correlation among nonlaminated cell, strips and submodules and performed preliminary correlations with field data 

• Virtually eliminated hazardous waste generated from off-spec WIP plates and modules. 

• Reduced operator exposure to Cd-containing ambients 

v
Acknowledgements 

Many people at First Solar contributed to the success of this subcontract. We gratefully 

acknowledge the contributions of Anke Abken, Dave Berger, Eugene Bykov, John Christiansen, 

Todd Coleman, Douglas Dauson, Nelson DeVoe, Tony Draper, Marcus Gloeckler, Andy Gray, 

Akhlesh Gupta, Roger Green, Upali Jayamaha, Peter Meyers, Anne Moser, Mike Steele, and 

Syed Zafar, as well as personnel from First Solar production, engineering and management. 

We are also grateful for the support that we have received from National Renewable Energy 

Laboratory staff including D. Albin, S. Asher, T. Gessert, T. McMahon, H. Ullal, B. 

vonRoedern, S. Wei, X. Wu, and K. Zweibel. Contributions from S. Hegedus and B. 

McCandless of Institute of Energy Conversion; J. Sites, K. Barth and A. Enzenroth of Colorado 

State University; A. Fahrenbruch of ALF; A. Compaan, V. Karpov, and D. Shvydka of 

University of Toledo; S. Feldman, S. Gilmore, V. Kaydanov and T. Ohno of Colorado School of 

Mines; and C. Ferekides of University of South Florida are gratefully acknowledged. 

Two lower-tier subcontracts directly contributed to this work. The first project, titled “Direct 

Integration of Solid-Precursor Powders into Chemical Vapor Deposition Systems”, carried out 

modeling of the heat and mass transfer in the deposition reactor. This work was performed by 

J.P. Delplanque, R. Kee, and M. Pavol of Colorado School of Mines. The second project, titled 

“Nonuniformity Loss in Photovoltaics”, carried out modeling of performance loss due to spatial 

variation. This work was performed by V. Karpov and D. Shvydka at University of Toledo. 

vi
1 Introduction 

1.1 Project Background 

First Solar, through its predecessor Solar Cells, Inc. (SCI), began development of thin film CdTe 

manufacturing technology in 1990 and produced a 6% total area 0.72 m2 PV module in June 

1992. SCI was awarded NREL thin film PV partnership subcontracts in 1993, 1998 and 2002 

(the present project). In addition SCI was awarded PVMat subcontracts in 1995 and 1998 and 

US Department of Energy (DOE) Small Business for Innovative Research (SBIR) contracts in 

1995 and 1998. Private funding combined with DOE funding was used both to improve product 

performance and to develop high throughput manufacturing technology. Significant additional 

private investment obtained in 1999 resulted in the formation of First Solar, LLC and enabled 

construction of a manufacturing facility. Initial manufacturing of thin film polycrystalline CdTe 

PV modules began in 2001. In December 2004 First Solar surpassed 10 MW of cumulative 

production. Production in 2005 exceeded 20 MW. In May 2005 First Solar broke ground for a 

50 MW capacity expansion expected to come on line in Q3 2006. 

Throughout its history First Solar technical staff have worked closely with the wider CdTe 

technical community including researchers at NREL, SNL and the academic community. Since 

its founding ten years ago First Solar has been an active member of the National CdTe PV R&D 

Team. CdTe Team activities focus on fundamental issues that underpin and ultimately advance 

all CdTe PV module technology but are not specific to First Solar manufacturing technology. 

Similar to its predecessor subcontracts, the current NREL TFPV subcontract to First Solar 

enables First Solar technical staff to evaluate scientific insights – whether obtained from the 

broader technical community or through in-house studies- and to adapt those insights into 

(typically proprietary) equipment or process changes that can improve First Solar module 

performance and manufacturing technology. Development of equipment or process changes 

typically requires invention that results in creation of proprietary or patentable technology. 

1.2 Technical Approach 

First Solar’s approach to bringing down the cost of PV modules is to develop thin film PV 

technology to replace the current mainstream PV technology based on wafer Si. Of the various 

commercial or near commercial thin film technologies, CdTe PV has the most favorable 

combination of current performance, manufacturability, cost, and upside performance potential. 

The current program focuses onto four key areas: 

1. Advanced front contact window (AFCW) layer (See Figure 1.)- A key factor in improving 

CdTe/CdS device performance is increasing short circuit current density (Jsc) by thinning the 

CdS window layer. Although thinner CdS enables more photons to reach the CdTe where 

they can be converted to electrical current, too thin CdS results in dramatically lower open 

circuit voltage (Voc) and fill factor (FF). In practice the minimum CdS thickness that can be 

achieved in cells without loss in Voc and FF requires use of an additional high resistivity 

buffer layer between the high conductivity transparent conducting oxide (TCO) front 

electrode and the CdS. 

1
Champion cell 

performance 

achieved at NREL 

employed AFCW 

layers consisting of 

high conductivity, 

high optical 

transparency 

cadmium stannate 

(CTO) as the 

transparent front 

electrode, high 

resistivity zinc 

stannate (ZTO) as 

the buffer layer in 

combination with 

thin sputtered CdS 

or aCdS:Oii. During 

the second year of 

the program, program tasks were modified to include investigation of AFCW layers and 

identification of technical issues related to their possible incorporation into the First Solar 

production line. 

Glass, laminant

EVA, encapsulant

Metal, back electrode

Low resistivity “interfacial” layer 

CdTe, p-type absorber

CdS, n-type window

High resistivity “buffer” layer*

TCO, front electrode

Ion diffusion barrier

Glass, superstrate

Laser scribes

- metal isolation

- via (interconnect)

- TCO isolation

insulator

light

Glass, laminant

EVA, encapsulant

Metal, back electrode

Low resistivity “interfacial” layer 

CdTe, p-type absorber

CdS, n-type window

High resistivity “buffer” layer*

TCO, front electrode

Ion diffusion barrier

Glass, superstrate

Laser scribes

- metal isolation

- via (interconnect)

- TCO isolation

insulator

light

Figure 1. Schematic cross section of First Solar polycrystalline thin film 

CdTe PV module showing films and interconnect scribes. The TCO, 

buffer layer, and CdS window films are collectively referred to as the 

Advanced Front Contact Window layer (AFCW). 

2. Development of improved CdTe and CdS semiconductor deposition through improved 

distributor design, improved process control and improved material characterization metrics - 

In one sense CdTe PV technology has broad process latitude in that CdTe films deposited by 

a wide variety of procedures can be processed to produce >10% efficient devices; i.e., CdTe 

PV is a very manufacturable technology. Nonetheless, optimum device performance has 

been achieved for only for a relatively few CdTe film deposition processes. Furthermore, for 

any given film, optimum device performance depends upon film properties in ways that are 

not fully understood. Realization of CdTe PV’s full potential requires not only that 

semiconductor (CdTe/CdS) film properties be optimized, but also that both process and 

work-in-progress (WIP) product variation be minimized. 

Another factor is material utilization. Semiconductor material not deposited onto the glass 

plate is deposited onto chamber walls and rollers. Lost material results not only in higher 

direct material cost per plate but also in increased maintenance and reduced equipment run 

time. 

Finally, economics dictates that the equipment have high throughput, low capital cost, high 

uptime, and produce consistent coatings. 

3. Development of improved accelerated life test (ALT) procedures that indicate module 

baseline field performance - Changes in performance of CdTe PV modules in the field can be 

separated into packaging-related issues (such as breakage or moisture ingress) and devicerelated issues. In this project ALT focuses on device-related changes in performance. 

Device performance is monitored as devices are “stressed” using various temperature-timeillumination levels and sequences; changes in performance are used as indicators of field 

performance. 

2
4. Improved Environmental, Health and Safety (EH&S) procedures - CdTe PV has a spotless 

EH&S record with respect to cadmium toxicity issues. There are no documented cases of 

researchers, manufacturers or end users suffering any Cd-related ill effects associated with 

their work with CdTe PV modules. Similarly there is no evidence of increased Cd levels in 

the environment associated with CdTe PV research, manufacturing or deployment. First 

Solar has purchased an insurance policy to fund solar module reclamation and recycling 

expenses at end of product life; First solar is the only PV manufacturer of any technology to 

establish such a policyiii. First Solar is proud of its proactive EH&S record. 

That being said, the Cd-toxicity perception issue is real. Some customers and regulators 

express concern; some competitors use the Cd-toxicity issue for their competitive advantage. 

Any program can be improved and development of improved EH&S procedures related to 

Cd are part of this subcontract. Tasks within this program relate to reduction of hazardous 

manufacturing waste, development of engineering procedures that reduce the exposure of 

manufacturing personnel to Cd-containing environments and reduced detection response time 

for potential exposure of personnel to Cd-containing ambients. 

1.3 Implementation Approach 

Much subcontract work is carried out on the production line using a system of Engineering Test 

Authorizations (ETA) that provide access by researchers to production line equipment with 

minimal disruption of production. Additional equipment enables independent fabrication of cells 

– except for semiconductor deposition by vapor transport deposition. An off-line sputter coater 

capable of depositing metal, semiconductor, TCO and buffer layers onto 60 cm X 120 cm plates 

is used extensively for the AFCW program. 

First Solar engineers and researchers have had extensive training in and routinely employ 

Taguchi methods of robust engineering and Six Sigma methods for process optimization and 

control. Data collection and analysis are greatly facilitated by First Solar’s extensive online data 

collection and storage system that enables analysis of data using Microsoft Access and Minitab. 

Plates run through the production line are individually bar coded prior to entering the 

semiconductor coater. Bar code readers at various downstream process locations enable 

engineers to determine many process conditions for any given plate. Module and submodule IV 

curves – not just efficiency - of each plate every time it is measured is stored in the data base and 

can be readily retrieved by researchers at their desks. In addition cell IV measurements are 

stored in the data base and QE and C-V-f data is stored and is readily transferred to portable 

storage devices. 

2 Advanced Front Contact and Window Layers (AFCW) 

2.1 AFCW background 

Initial studies focused on adapting the NREL small cell technology to First Solar manufacturing 

procedures. Process parameters were varied in designed experiments using Taguchi 

methodology. When initial trials failed to demonstrate the advantages of ZTO (see Figure 2iv), 

researchers began trials with alternative buffer layer materials. It should be noted that 

requirements placed on TCO/buffer layer in a manufacturing environment are not the same as 

those of a laboratory setting. Some additional desirable properties that the TCO/buffer layer 

should have are: 

3
Jsc (mA/cm2)

Efficiency (%)

20.5 21.0 21.5 22.0 22.5 23.0 23.5 24.0 24.5

10

9

8

7

6

5

Buffer

Layer

None

ZTO

Efficiency vs. Photocurrent

Figure 2. Dependence of cell efficiency on Jsc for 

devices produced on various thicknesses of CdS - with 

and without a ZTO buffer layer4

. 

• Ability to adhere to the substrate 

under severe conditions. 

Submodule processing requires the 

TCO to be heated to high 

temperatures. The TCO has to be 

stable at these high temperatures. 

Also, throughout the life of the 

module, the TCO will be subject to 

severe temperature and electrical 

cycling. The interfaces with glass 

and with the other thin films have 

to maintain their integrity. 

• Ability to block the migration of 

elements through the TCO. The 

glass has elements in it that are 

dopants (e.g., Na) and that can 

diffuse into the TCO. 

• Ability to hide imperfections in processing. Certain thickness ranges of the TCO are more 

susceptible to color changes than other ranges. The TCO color sensitivity can be altered by 

adding additional transparent oxides to the stack. 

• Reproducible surface texture. TCO texture has a strong impact on optimum semiconductor 

deposition and other downstream processing parameters. For example, rough topography of 

the TCO requires that the subsequent coating be thicker to assure complete coverage. 

2.2 TCO Deposition Processes 

In order to achieve optimum TCO and buffer layer properties suitable for the production 

environment, existing commercial TCO deposition processes were reviewed. The three major 

forms of deposition are: 

1. Sputter Deposition. This process is done by magnetically enhanced cathodic sputtering 

under vacuum. The vacuum coater can deposit many different materials including oxides. 

Sputter deposition has advanced so that deposition can occur at higher temperatures, 

although not as high as are employed in the VTD process. The major advantages of vacuum 

deposition are that many different types of materials can be deposited and that the control 

and uniformity of the coatings is very good. The major disadvantage of vacuum deposition is 

that the coatings are weakly bonded to the substrate. 

2. Spray Pyrolysis: This coating technique was the predecessor of the chemical vapor 

deposition (CVD) techniques. The coatings are applied at elevated temperatures by spraying 

droplets of liquid precursors onto hot substrates. The major advantages of spray pyrolysis 

are that the coatings are more durable than vacuum deposited coatings, the variety of 

precursors is quite large, and the cost of the system is cheaper than CVD or vacuum 

deposition. The disadvantage of spray pyrolysis is that the coatings are not uniform in 

thickness as becomes more evident when the coatings are above 1000 Angstrom in thickness. 

3. Chemical Vapor Deposition (CVD): The process consists of vaporizing the precursors and 

directing the resultant gasses onto a hot substrate. There are many types of CVD processes 

but the major one, in terms of square feet of material made, is Atmospheric Pressure 

Chemical Vapor Deposition (APCVD). The advantages of this process are that the TCO 

4
chemistry has good adhesion with 

the substrate and the coatings can be 

more uniform than spray pyrolysis. 

The disadvantages of CVD are: 

a. The chemical usage 

efficiencies run between 10 

and 20%. 

b. The uniformity of the 

coatings, although better than 

spray pyrolysis, is not as 

good as the vacuum coatings. 

c. Capital costs are high. 

d. A limited number of 

chemistries can be applied 

depending on desired film 

composition and the 

substrate. 

e. Switching chemistries could 

take time and capital. 

As of the conclusion of this subcontract 

researchers had not made a final decision regarding choice of TCO deposition technology. In 

order to take advantage of the ability to investigate a large number of material systems, 

experimental trials during this subcontract were limited to investigation of sputter-deposited 

films. 

-25

-20

-15

-10

-5

0

5

10

15

-0.25 0.00 0.25 0.50 0.75 1.00

J vs. V

J [mA/cm2]

Volts per Cell

Total Area Efficiency (%) 9.71

Voc/cell (mV) 814

Jsc (mA/cm2

) 21.9

FF (%) 64.0

Area Ratio 0.85

-25

-20

-15

-10

-5

0

5

10

15

-0.25 0.00 0.25 0.50 0.75 1.00

J vs. V

J [mA/cm2]

Volts per Cell

Total Area Efficiency (%) 9.71

Voc/cell (mV) 814

Jsc (mA/cm2

) 21.9

FF (%) 64.0

Area Ratio 0.85

Figure 3. Champion 7200 cm2 module achieved using 

a buffer layer.

2.3 AFCW development 

Figure 4. Conceptual drawing of First Solar proprietary VTD 

system. First Solar’s High Rate Vapor Transport Deposition 

system was awarded an IR&D 100 award in 2003.

Researchers have been investigating 

various sputter-deposited buffer 

layers. Best results achieved to date 

are 9.71% total area efficiency 

(unconfirmed) with the IV curve 

displayed in Figure 3. Films made 

during the same coater run and 

processed during the same period 

using the same CdS thickness had 

efficiency below 9%. 

3 CdS/CdTe Deposition 

Reactor Development 

3.1 VTD Background 

At the heart of the First Solar 

manufacturing technology is the 

semiconductor coater. CdS and 

CdTe are deposited sequentially in 

the same piece of equipment using 

5
separate distributors that both employ the vapor transport deposition (VTD) method. VTD is a 

high-throughput, large- area coating technique pioneered and patented by First Solarv,vi. The 

system capitalizes on the ease with which CdS and CdTe can be vaporized and on the rapid rates 

at which quality films can be formed from re-condensing vapors. As depicted conceptually in 

Figure 4, the method utilizes flash-sublimation of injected semiconductor powders in an inert 

carrier gas stream to create a dense vapor cloud in a modest vacuum. Inert carrier gas flow 

directs and controls the dense vapor cloud. Semiconductor films are formed on glass substrates 

at temperatures >450 oC at high rates. The inert carrier gas and some vapors not deposited are 

then exhausted from the chamber. 

Since the VTD deposition rates are high, the length of the deposition zones needed to form the 

relatively thick absorber layer at production line speeds is relatively short. Compact high 

temperature deposition zones are ideal from a manufacturing equipment perspective as short 

deposition chambers can be used (lower capital equipment costs) and small critical areas are 

easier to manage. The production vapor generator and vapor distributor system in use at the 

beginning of this subcontract and still used today is relatively simple, robust, and durable. 

3.2 VTD Technical Obstacles 

VTD has been shown to be capable of deposition of device-quality CdS and CdTe films at 

several microns per second in a production environment, nonetheless VTD is a relatively 

immature process. At the start of this program film uniformity was relatively good (± 10%), but 

film properties – as measured by physical characteristics such as roughness and XRD pattern – 

varied across the plate. Although there were indications that several process factors affected 

uniformity, the root causes and their relationship to non-uniformity were unknown. As variation 

in as-deposited film properties are known to result in variation of the optimized downstream 

process procedure, to the extent that film properties vary across a plate it is impossible to identify 

a single downstream process that will optimize all areas of a plate. As the relationship between 

as-deposited film properties and optimized downstream processing is not known, the primary 

“non-uniformity” metric is variation of I-cell1

 performance across a plate. In addition, although 

it seems “intuitively obvious” that module performance could be improved if films were uniform 

across a plate, there was no quantitative measure of the “non-uniformity” loss. 

Another technical barrier relates to VTD of thin (<1000 Å) CdS films. As response time of VTD 

is fast, any variation in input CdS powder feed rate results in non-uniformity in film thickness. 

CdS deposition is complicated by the particulate nature of the source material; the desired film 

thickness requires that only a few grains per second of CdS source powder be evaporated. 

Commercial powder feeders are not available to consistently supply powder at the low rates 

necessary. Thus one subtask was directed toward development of powder feeders appropriate for 

controlled deposition of uniform <1000 Å CdS films. 

Finally, the film formation process must be integrated with the control of glass properties. The 

extremely high deposition rates of the VTD method require high substrate temperatures. From 

the glass perspective, the VTD system heats low-strength, annealed glass to temperatures in 

excess of the glass softening point. Quenching during venting of the vacuum exit load lock 

 

1

 I-Cells are ~1 cm2 cells used for quantification of spatial uniformity of device performance within a submodule. Icells are created by making a set of isolating scribe lines perpendicular to the three cell scribes (TCO isolation, via, 

and metal isolation) that are used to connect cells in series. Thus each of the 116 cells in a submodule is cut into 57 

I-cells whose individual I-V parameters can be measured. 

6
strengthens the glass. The quenching process must be closely coordinated with overall system 

thermal management in order to achieve glass flatness and strength uniformity. Glass strength is 

required for field deployment and glass flatness is needed for consistent laser scribing and 

module lamination. 

3.3 VTD Technical Approach 

The approach to improving semiconductor film properties while reducing non-uniformity losses 

involved: a) development of advanced distributors, b) sophisticated modeling and simulation of 

distributor operation focused on CdTe deposition, and c) quantification of the effect of nonuniform device performance on module performance. Quantification of non-uniform device 

performance was modeled at the University of Toledo. Modeling results were discussed in an 

earlier report4

 and will not be repeated here. Computer simulation of distributor performance 

was carried out by researchers at the Colorado School of Mines and is described in Section 4. 

3.4 Distributor Development 

Even award winning designs can be improvedvii, however, and during this contract we have 

developed an alternative distributor concept that has been designated as VTD-2. VTD-2 was 

built and tested and has demonstrated significantly reduced cross-plate variation.

VTD-2 is not an optimization of the earlier VTD design, rather it is based upon a fundamentally 

new concept that effectively circumvents the fundamental mechanisms responsible for spatial 

non-uniformity problems that can arise in a low-pressure, three component gas system. VTD-2 

systems are simpler to operate, but are mechanically and thermally more complex. Several 

different configurations of the VTD-2 concept were tested and one version was selected for 

production trials. Four patent applications have been filed on the VTD-2 systems. 

An example of the improved 

performance is given in Figure 5which 

shows improved cross-web thickness 

uniformity that remains constant 

throughout a long run. Figure 6 and 

Figure 7 show that improved thickness 

and roughness uniformity, respectively, 

is achieved within individual plates 

within a run. Moreover the uniformity 

obtained with a VTD-2 distributor is 

fixed by construction geometry unlike a 

VTD-1 distributor which requires 

operator fine-tuning. The VTD-2 

distributor also provides a broad 

operating window for carrier gas flow. 

The VTD-2 distributor also has benefits 

in material utilization. The original VTD-1 distributor was operated with poor material 

utilization. One variation of a VTD-1 distributor improved utilization by a factor of 1.8. The 

material utilization with VTD-2, however, improved by 15% over the best VTD-1 distributor and 

was a factor of 2.1 better than the original VTD-1 system. The improved utilization not only 

saved material, it increased run durations significantly due to slower build-up on furnace 

Sequence in Run

Relative Thickness Deviation (%)

0 1000 2000 3000 4000

25

20

15

10

5

0

GDS_run

2036

2037

2038

2039

2040

2041

2042

2043

2044

2045

2030

2046

2047

2048

2049

2050

2051

2052

2031

2032

2033

Relative Cross-web Thickness Deviation vs Sequence in Run

2048=VTD-2, 2051=Modified VTD-1

VTD-2

Modified VTD-1

Figure 5. Cross-web thickness uniformity of CdTe 

films deposited with different distributors. Relative 

deviation is lower and remains constant throughout a 

3600 plate run with the VTD-2 distributor design. 

7
components. Build-up on furnace components is one of the primary factors that cause the 

decrease in film uniformity throughout the duration of a long run. Tens of thousands of plates of 

2.5 μm CdTe plates have been routinely produced. The VTD-2 distributor design provides a 

straightforward method to control the location of the deposition flux. We thus expect further 

improvements in utilization as VTD-2 design is optimized. 

The VTD-2 system was successfully tested in a number of large production trials. Results have 

been compelling and the VTD-2 system is now operating 100% of the time for CdTe in one 

production coater, about 50% of the time in the original production coater, and is slated for 100% 

usage in new coaters. 

Distributor Type

Relative Cross-web Thickness Deviation (%)

VTD-2 VTD-1

10

8

6

4

2

0

6.82528

5.0022

Cross Web Thickness Deviation

Figure 6. Box plot of relative cross-web thickness 

deviation. VTD-2 distributors average about a 5% 

cross-web thickness deviation.

Distributor_Type

Relative Roughness Deviation (%)

VTD-2 VTD-1

50

40

30

20

10

0

35.375

10.6724

Cross-web Relative Roughness Deviation

Figure 7. Box plot of relative cross web surface 

roughness deviation.

3.5 CdS Powder Feed 

As discussed in the AFCW section, increased efficiency can be achieved in part through use of 

increasingly thin CdS films that enable higher Jsc. While sputter deposition of CdS is one option 

for depositing thin CdS films, tasks in this subcontract were also directed toward VTD 

deposition of ultra-thin CdS films. CdS film thickness uniformity is also important, as losses in 

FF and Voc associated areas of a module with too-thin CdS can easily outweigh the advantage of 

higher Jsc. High rate semiconductor film deposition combined with requirements for uniform 

deposition of increasing thin CdS films places significant constraints on the uniformity of CdS 

powder feed. In practice powder feeders must supply only a few CdS grains per second to 

produce CdS films of appropriate thickness and they must maintain that feed rate consistently 

over many hours. Evaluation of many powder feeders from several vendors indicates that 

commercially available powder feeders are not up to that task. 

Therefore during this subcontract a significant effort was devoted to development of improved 

CdS powder feeders that could consistently deliver small amounts of powder into the VTD 

distributor over the period of a deposition run. CdS powder feeders that operate on different 

principles from those in use at the beginning of the contract were developed and tested. CdS 

films <900 Å thick were produced on the production line. In addition, further improvements 

have been made in the new, improved design. Powder feeders can now be reloaded without 

interrupting production. Powder clogging has been greatly reduced. Off-the-shelf components 

are replacing custom components. Sensors have been incorporated to indicate powder level in 

the hopper. A patent application on the advanced powder feeder has been filed. 

8
4 Distributor Modeling 

Despite the simplicity of the basic 

VTD-1 system, the heat and mass 

transport processes that occur w

it are complex. Demands for 

meeting production goals make 

conducting experimental trials using 

production equipment increasingly 

difficult and inferring physical 

mechanisms operating within the 

distributor based on resulting film 

characteristics is very tricky. 

Therefore a program to provide a 

solid theoretical understanding of 

the distributor operation was carried 

out in collaboration with a group 

from Colorado School of Mines 

(CSM). The goal of this effort was to guide the experimental work of distributor design and 

operation. 

ithin 

Input

- Flow rates

- Particle size

- Internal pressure

e velo

- Particle temp

Input

- Flow rates

- Particle velocity

- Particle temperature

Input

- Slot mass flow rate

- Slot mass fractions

Input

- Flow rates

- Particle size

- Internal pressure

Output

- Particle velo

- Particle temp

Input

- Flow rates

- Particle velocity

- Particle temperature

Input

- Slot mass flow rate

- Slot mass fractions

Output

- Particl city

erature

Output

- Slot mass flow rate

- Slot mass fractions

- Internal pressure

Output

- Deposition rates

- Gas conditions

city

erature

Output

- Slot mass flow rate

- Slot mass fractions

- Internal pressure

Output

- Deposition rates

- Gas conditions

Input

- Flow rates

- Particle size

- Internal pressure

Output

- Particle velo

- Particle temp

Input

- Flow rates

- Particle velocity

- Particle temperature

Input

- Slot mass flow rate

- Slot mass fractions

Input

- Flow rates

- Particle size

- Internal pressure

Output

- Particle velo

- Particle temp

Input

- Flow rates

- Particle velocity

- Particle temperature

Input

- Slot mass flow rate

- Slot mass fractions

city

erature

Output

- Slot mass flow rate

- Slot mass fractions

- Internal pressure

Output

- Deposition rates

- Gas conditions

city

erature

Output

- Slot mass flow rate

- Slot mass fractions

- Internal pressure

Output

- Deposition rates

- Gas conditions

Figure 8. Primary elements of the distributor simulation model 

developed by CSM researchers.

The CSM team developed a three-stage numerical model of mass and heat flow within the 

semiconductor deposition system. The model includes details of particle injection, particle 

sublimation and vapor transport within the distributor, and vapor transport and film deposition of 

source gas exiting the distributor slot; see Figure 8. 

Within the distributor flow field calculations are coupled to the particles by way of mass, 

momentum, and energy. Convective sublimation is approximated using the extended film 

model, which describes the particle mass loss and the heat required by the sublimation process. 

This sublimation model is connected to a model for tracking particle motion through a specified 

flow field. Distributions of input particle sizes can be tracked as they enter the distributor and 

sublime. The model is fully coupled. It accounts for physical motion of the powders, radiative 

and convective heat transfer, and powder sublimation. The sublimed vapor significantly alters 

the pressure, velocity and composition of the gas phase which in turn affects the motion of the 

injected powder. Once the injected powder has sublimed, gas flow through the porous 

distributor is modeled using a combination of Darcy flow, Fickian diffusion, and Knudsen 

diffusion that defines the composition of source gas exiting the distributor slot. Finally 

deposition onto the glass substrate is modeled using a 3-dimensional Navier-Stokes analysis 

using commercially available computer code combined with equations that describe the film 

deposition reaction. 

Clearly the computer simulation involves many complex calculations based upon “best 

estimates” of conditions within the distributor and deposition chamber. In order to develop 

confidence in the simulation, initial simulations were performed to replicate experimental results 

obtained under various VTD-1 operating conditions4

. Upon successful replication of 

experimental results, a Taguchi design of simulations was carried out to evaluate the impact of 

various distributor design variations on expected film characteristics. Each simulation required 

entering a new set of distributor design parameters followed by calculation of the results using 

finite element analyses. Time for simulation of each case within the Taguchi design required 

9
about one man-day. In contrast, experimental evaluation of the same parameters would have 

required design, fabrication and testing of each distributor variation plus additional time for 

equipment installation, operation of the semiconductor coater, and experimental analysis of the 

resulting films. Direct lost production time would have been on the order of two weeks per trial. 

Details of the experimental design and of the output parameters monitored are proprietary, but 

“form, fit and function data” sufficient to demonstrate the procedure, analysis and results follow. 

Uniformity of film thickness, uniformity of film structure, and material utilization rate were the 

three variables selected for monitoring simulated distributor performance. Figure 9 contains the 

Taguchi design and output parameters. Uniformity is indicated by the signal to noise ratio of all 

data points generated across a plate during a simulation; material utilization is given in arbitrary 

units based on the ratio of combined mass of material deposited on the plate to mass of material 

injected into the distributor 

Analysis of the actual computer simulation data as represented in Figure 9 enabled calculation of 

optimized output parameters as displayed in Figure 10 as well as determining tradeoffs between 

parameters. Confirmation runs, not shown, verified the self-consistency of the approach. 

Run 

No. A B C D E F G H I J K L M N Thick S/N

Texture 

S/N

Material

Utilization 

(AU)

1 low low low low low low low low low low low low low low 43 7 2.2

2 low low low low low low low high high high high high high high 35 7 1.8

3 low low low high high high high low low low low high high high 20 1 1.8

4 low low low high high high high high high high high low low low 27 -3 2.4

5 low high high low low high high low low high high low low high 29 -3 1.2

6 low high high low low high high high high low low high high low 25 -4 1.7

7 low high high high high low low low low high high high high low 23 14 1.6

8 low high high high high low low high high low low low low high 28 14 1.0

9 high low high low high low high low high low high low high low 35 -3 2.2

10 high low high low high low high high low high low high low high 25 -5 1.7

11 high low high high low high low low high low high high low high 25 4 1.4

12 high low high high low high low high low high low low high low 29 5 2.1

13 high high low low high high low low high high low low high high 30 19 1.3

14 high high low low high high low high low low high high low low 35 3 2.0

15 high high low high low low high low high high low high low low 23 -3 1.7

16 high high low high low low high high low low high low high high 25 -3 1.2

Input Variables (Distributor Design) (Film Properties) 

Output Variables 

Figure 9. Design parameters (arbitrary units) for Taguchi computer simulation experiment. A through N 

are distributor design parameters; output parameters indicate signal to noise (S/N) of film thickness, film 

texture, and material utilization. 

Optimized Output 

Parameter A B C D E F G H I J K L M N

Thick 

S/N

Texture 

S/N

Material 

Utilization 

(AU)

"Baseline" low low low low low low low low low low low low low low 43 7 2.2

Thickness S/N low low low low low low low high low low high low low low 46 2 2.3

Texture S/N low high low high high low low low high high low low high high 26 22 1.2

Material Utilization low low low low high high high high low high high high high low 29 -4 2.6

Input Variables (Distributor Design) Output Variables 

Figure 10. Optimized distributor design parameters for each output variable based on computer 

simulation. (Calculated optimized output parameters are highlighted.) 

10
In practice experimental verification of the optimized VTD-1 design was “overcome by events” 

as the VTD-2 design employed fundamentally different approach, nonetheless, computer 

simulation of VTD-1 provided valuable insights into distributor operation and many features of 

the VTD-1 simulation could be applied to the VTD-2 design. 

5 Impurity control 

5.1 Chemical analysis 

Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used for the 

determination of trace impurities in deposited semiconductor layers after various processing 

steps. Correlations between semiconductor deposition, CdCl2 processing, back-contact 

application and total impurity levels in the device were established. Trace element analysis 

focused on elements that are believed to have a major impact on device performance and longterm stability. Measurements were performed as part of designed experiments on the production 

line and for standard production pulls. Data concerning the variation of impurity level after 

different processing steps support process optimization and failure analysis. The introduction of 

ICP-OES measurements on a regular base is now being evaluated as a standard production line 

metric. 

In addition, ICP-OES measurements of incoming raw materials are being used to define 

specifications and impurity limits. Historically, it has been seen that raw material batches from 

the same purity grade show differences in impurity level of critical elements. It is believed that 

these fluctuations in purity level are linked to observed differences in final device performance. 

5.2 Impurities and Chlorine Issues 

CdCl2 processing introduces a certain amount of impurities into CdTe and CdS material. Some 

impurities e.g. Cu, Sb, Bi, and As are known to act as p-type dopants, others like Al and Si may 

compensate the p-type doping in CdTe. CdS 

resistivity and Fermi level are also believed to 

be affected by impurity accumulation. In 

order to evaluate the impact of impurities on 

initial and long-term performance ultra-pure 

CdCl2 was used for experiments performed on 

small-area devices. The investigated 

impurities were blended in defined amounts 

into pure CdCl2. The total up-take of the 

impurities into the devices during CdCl2

processing was determined using ICP-OES. 

As shown \in Figure 11, light-soaking at 

elevated temperatures lead to an improvement 

in relative efficiency for devices showing a 

higher impurity level compared to devices 

with lower impurity levels. The improved 

relative stability was more than offset, 

however, by the reduced initial efficiency 

observed in intentionally-contaminated 

devices. That is, devices, which had the 

0.0 0.5 1.0 1.5 2.0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

efficiency / arb. unit

initial

7d ALT

19d ALT

impurity level / arab. unit

Figure 11. Representative data showing relative 

changes in efficiency with lightsoak for devices with 

and without intentionally introduced impurities. Note 

that absolute efficiency was higher for devices without 

impurities. 

11
lowest impurity level, exhibited the highest initial efficiencies accompanied by the best lightsoak stability. 

Further increases in impurity level resulted 

in further non-negligible losses in initial 

efficiency, open-circuit voltage, fill-factor 

and short-circuit current. Light-soaking of 

these devices at elevated temperatures 

(~120°C), however, actually lead to an 

improvement in all 1st and 2nd level 

metrics; see Figure 12. In general, a 

higher impurity level of the device leads to 

major improvements in device 

performance during light-soaking. 

Quantum efficiency measurements show 

that increasing impurity level results in an 

increase in losses of the red response; see 

Figure 13. Consistent with the improved 

device performance during light soak, light-soaking leads to an overall improvement in the red 

response that occurs in an early stage of light-soaking - in this case after 7 days. Continuation of 

light-soaking did not change the quantum efficiency response although the JV-response still 

changed. Samples, which were treated with ultra-pure CdCl2 and had therefore the lowest 

overall impurity level, also exhibited a similar quantum efficiency response of improving the red 

response in light-soaking. Re-distribution of the impurities in the device and re-arrangement of 

defect states may be the root cause for the observed QE performance. 

wavelength (nm)

QE

400 500 600 700 800 900

0.0

0.2

0.4

0.6

0.8

1.0

wavelength (nm)

400 500 600 700 800 900

-0.025

-0.020

-0.015

-0.010

-0.005

0.000

0.005

0.010

400 500 600 700 800 900

1st derivative

wavelength (nm)

-0.025

-0.020

-0.015

-0.010

-0.005

0.000

0.005

0.010

400 500 600 700 800 900

1st derivative

-0.025

-0.020

-0.015

-0.010

-0.005

0.000

0.005

0.010

400 500 600 700 800 900

1st derivative

1.0

Figure 13. A. Initial quantum efficiency for devices with varying impurity levels and B. Quantum efficiency 

improvement after light soaking at 120 C for 7 and 19 days. 

6 Identification and modification of back-contact chemistry 

Device performance is sensitive to the electrical properties of the CdTe to metal electrode 

interface, i.e., back-contact, and controlling back-contact chemistry during device processing is 

one of the key factors in achieving long-term stability of high efficiency devices. In an attempt 

to identify stable alloys that might be important in determining device stability, a series of 

light-soaking 

initial 

B 

QE

wavelength (nm)

400 500 600 700 800 900

0.0

0.2

0.4

0.6

0.8

impurity level / arab. unit

Stbi / arab. unit

0.0000 0.0002 0.0004 0.0006 0.0008 0.0010 0.0012 0.0014 0.0016 0.0018

180

160

140

120

100

80

60

40

20

0

Stbi 7d, stbi 19d vs. impurity level

7d ALT

19d ALT 

Figure 12. Relative efficiency as a function of added 

impurity level of devices after 7 and 19 days light soak 

at 120 C.

Increasing 

impurity level 

A 

12
experiments were carried out that included several process variations including heat treatments 

up to 100 C above normal process temperature. Working with researchers from NREL, films 

produced using these variations in device processing were analyzed by XRD. Based on the 

phase diagrams of metals involved, formation and precipitation of stable alloys was expected 

within this process space, but in fact no alloying was detected. It is speculated that either the 

energy barrier for any solid state reaction required to form the alloys was not surmounted or that 

reactants diffused away from the interfacial region prior to attaining the necessary temperature 

for alloy formation. In any case, phase formation that could affect device performance or 

degradation was not detected. 

Additional studies focused on back contact surface preparation prior to metal electrode 

application and specifically on variations in the CdCl2 activation step. Working with IEC, XRDmeasurements performed at different stages of the CdTe post-deposition processing indicate the 

presence of CdO and CdTeO3 and indicate that the humidity level of the CdCl2 heat treatment 

ambient affects oxide formation. In order to evaluate effects of humidity during CdCl2 heat 

treatment on device performance, samples were pulled from different CdCl2 manufacturing runs 

processed under different humidity levels. Although it might be expected that the variation in 

humidity would result in an altered surface chemistry that would affect device performance, in 

fact the difference in device performance for the humidity levels studied was not statistically 

significant. 

Ideally the various oxides that form during the CdCl2 activation step would be removed to ensure 

consistent back-contact formation, thus a series of experiments was initiated in order to explore 

effects of oxide removal prior to contacting. As acidic-oxidizing etches are not compatible with 

our back-contact process, various alkaline etches were investigated. Different etch times and 

concentrations were employed to remove surface oxides and oxy-chlorides. For the treatments 

investigated, however, neither initial nor long term PV performance was significantly altered 

compared to non-treated devices. Grazing incidence XRD (GIXRD) (supported by IEC) 

measurements indicate that none of the etches investigated effectively removed all of the oxide 

species. 

7 Accelerated Life Test Development 

7.1 Physical model underlying ALT Technical Approach 

Our working hypothesis is that, apart from encapsulation failure, changes in module performance 

over time are largely determined by changes in cell performance. Changes in cell performance, 

in turn, depend on changes in the electronic defect distribution within the device. Changes in 

electronic defects are suspected to be related to defect chemistry of dopants, impurities, 

vacancies, interstitials and their complexes and may exist within grains or at grain boundaries, 

interfaces or structural defects. Changes over time in defect type, concentration and location are 

believed to be driven by deviations between the existing defect state and that steady state 

associated with the local quasi-Fermi level and chemical potential. Stress on the device, 

therefore, would best be defined in terms of the magnitude and gradient of the difference 

between the existing quasi-electrochemical potential and it’s steady state value. External factors 

affecting this “stress” include illumination, external field, and temperature. Mechanisms for 

changing the defect state include diffusion, migration, electron occupancy, defect mutation and 

defect chemical reaction. 

13
This view of the electrical properties of CdTe PV devices is both complex and device specific, 

i.e., observed changes in device performance depend on device structure and material properties 

as well as on the illumination-bias-temperature history of the device. In principle monitoring 

and understanding the change mechanisms would provide concepts for tailoring materials 

properties that influence the stress-induced changes in device performance. Many mechanisms 

are possible in polycrystalline compound semiconductor thin films, however, and techniques for 

measuring many important physical, chemical, and electrical and properties have not been 

demonstrated. Thus although it us helpful to have a broad conceptual basis for explaining 

changes in device performance, practical application of this model entails sophisticated 

analytical, scientific and computational capabilities not available within First Solar. Thus we 

believe that detailed analysis of defect chemistry and its impact on device performance is best 

left to the larger academic community and the national CdTe Team. First Solar’s ALT program 

relies on empirical studies. 

At First Solar a 56 day light soak (LS) at approximately 70 mW/cm2

 illumination and 65 C has 

been the standard indicator of device field performance. The 56 day light soak has a serious 

shortcoming, however, in that test throughput is less than one module per day, i.e., <0.1% of 

modules produced are evaluated for baseline field performance compared to 100% of modules 

that have efficiency measured. ALT development addresses the throughput issue through 

investigation of ALT protocols that take advantage of thermal activation on defect mobility and 

chemistry reaction rates to significantly reduce response time and thereby increase test 

throughput. Additional studies are aimed at discovering the relationship between ALT and field 

test results. 

7.2 Correlation to 56 day LS 

The ALT task objective is 

development of an Accelerated Life 

Test that enables timely monitoring 

of production line performance and 

enables process optimization to 

improve baseline field performance. 

Stress studies focus on stresses 

induced by illumination, electrical 

bias and temperature where elevated 

temperature is also the primary 

accelerating factor. In the course of 

this subcontract, experimental trials 

were primarily performed on 

submodules pulled from the 

production line although some of 

these modules were parts of designed 

experiments. ALT studies were 

preformed on 1 cm2

 cells (I-cells) 

isolated from fully interconnected submodules (working module prior to lamination), “strips” (a 

section cut from a submodule consisting of 10 cells, each cell 1 cm long by 57 cm wide, 

connected in series), 1⁄2 plates (a submodule cut in half), and (whole) submodules. 

Test 

Device 1/2 plate Strip Strip Strip

Stress 

Temp

No 

external 

heat

No 

external 

heat

Elevated Elevated

Measurement 

Temp

Room Room Room Elevated

"Sister" 

(full) 

plate

No 

external 

heat

Room 0.79 0.67 0.57 0.76

1/2 plate

No 

external 

heat

Room 0.83 0.74 0.92

Strip

No 

external 

heat

Room 0.78 0.97

Strip Elevated Room 0.76

Correlation Matrix of Various Experimental 

ALT Stress Protocols

Figure 14. Correlation of predicted baseline performance 

of a) various test structures, b) various stress temperatures, 

and c) effect of measurements being made in the stress 

ambient as opposed to after the test structure had been 

cooled to room temperature. 

14
Lamination was found to have a significant impact on test results and in these ALT trials 

although lamination did not necessarily improve stability. Furthermore, degradation of the 

laminant at elevated temperature introduces a practical upper temperature limit to ALT of 

modules. Thus application of ALT to modules introduces the practical issue of separating 

device-related changes in performance from packaging-related changes, and results presented 

here (with the exception of field data) were obtained with non-laminated devices. 

A wide variety of ALT protocols is possible but in this program investigations focused on 

stressing under approximately 100 mW/cm2

 illumination with devices biased to open circuit. In 

general relatively good correlation was obtained between I-cells, strips, 1⁄2 plates, and 

submodules evaluated using similar ALT trials. Figure 14 displays correlations obtained using 

various test structures (sister plates, 1⁄2 plates, and strips), elevated and “no external heat”2

 and 

with baseline performance predicted using elevated (i.e., in situ) or room temperature IV 

measurements. While at least rough correlations are seen in all cases, there exists sufficient 

variability that it is not clear that any one ALT protocol is superior to the others. 

Variation of ALT results originates both 

from variation in the test itself and 

variation in the samples tested. As ALT is 

inherently a “destructive” test in that it can 

not be repeated on a given device, 

comparisons were made between ALT 

results obtained from devices that were as 

similar as possible. Figure 15 shows that 

for strips from the same submodule, 

combined variation in predicted ALT 

baseline performance was ~1%. Although 

the small test-related variation within a 

single ALT protocol provides an 

existence-proof of a reproducible ALT, the 

lack of correlation among the various ALT 

protocols displayed in Figure 1 indicates 

that results are protocol-specific. 

Var

Frequency

-2 -1 0 1 2

9

8

7

6

5

4

3

2

1

0

Figure 15. Variation of relative baseline performance as 

predicted by curve-fitting for strips from the same plate 

that were stressed at elevated temperature. Variation is 

defined as 100 X (P-Pavg)/Pavg, where P is the predicted 

value and Pavg is the average of P on one plate. 

Mean 6.167906E-18

StDev 1.008

N 36

Variation in Relative Stabilized Efficiency

S trips from same Plate

ALT

Normal 

7.3 Correlation with field data 

In an attempt to better understand the relationship between ALT and baseline field performance, 

comparisons were made between sister plates exposed to ALT and 36 modules mounted in an 

outdoor array. Fielded module performance was measured prior to fielding and after 

stabilization in the field. Fielded modules were part of a designed experiment that include 12 

variations in the module fabrication process. Each process variation was intended to “stretch but 

not break” production line process specifications, and should therefore reasonably reflect 

production line modules. In this case sister plates were defined by the fact that ALT strips were 

cut from modules that were produced with the same “process variation condition”. 

 

2

 Typical temperature of test structure with no external heat was 65 C. 

15
Both performance and change in performance over time in the field were well-behaved. As 

displayed in Figure 16 statistically 91% of variation in field performance and 85% of variation in 

change in performance was explained by process condition. Furthermore, as shown above, 

variation of ALT on strips within a single plate is ~1%, thus both the field data and results from 

ALT are well behaved. Nonetheless, as displayed in Figure 17, when change in performance as 

a result of ALT is compared to change between initial and baseline field performance there is no 

correlation between the two metrics. This result is interpreted to mean the specific ALT 

evaluated is not a reliable predictor of field performance. It does not follow, however, that an 

effective field performance predictive ALT does not exist. Development of an effective ALT 

remains an important goal of First Solar and of the CdTe PV community. 

A. Field Efficiency (St andardized Units) Frequency

-2 -1 0 1 2

14

12

10

8

6

4

2

0

Normal 

Histogram of Fielded Module Efficiency by Process Variation Condition

Rsq adj = 91%

 B. Relative Efficiency Stabilizat ion (Standardized Units) Frequency

-2 -1 0 1 2 3

4

3

2

1

0

Normal 

Relative Efficiency Stabilization by Process Variation Condition

Rsq adj = 85%

Figure 16 Normalized histograms showing distribution of A. Fielded module baseline efficiency and 

B. Relative change in efficiency of fielded modules over time for each of 12 process conditions of a 

designed experiment. Adj Rsq is a measure of the percentage of overall variation that is explained by 

process condition. Standardized Units are average divided by standard deviation. Positive values of 

Relative Efficiency Stabilization indicate smaller changes in relative performance. 

7.4 National CdTe R&D Stability Subteam 

Work on development of an effective 

ALT continues both within First solar 

and within the National CdTe R&D 

Team. Significant progress has been 

made by the CdTe Team. In February 

2004 the Stability Subteam was 

formed and at the most recent meeting 

(May 5-6, 2005) three modes of stressinduced change in device performance 

were identified: a) decrease in Voc, 

b) rollover in 1st quadrant, and 

c) degradation in the resistive terms Rs 

and Rsh. Several possible 

mechanisms have been identified that 

may explain the observed modes, but 

quantifiable relationships have not 

been established. Work is ongoing. 

Field Stabilization (Standardized Units)

ALT Efficiency Change (Standardized Units)

-0.75 -0.50 -0.25 0.00 0.25 0.50

1.0

0.5

0.0

-0.5

-1.0

-1.5

-2.0

-2.5

ALT versus Field Stabilization Correlation 

Rsq adj = 0%

Figure 17. Correlation between change in performance 

as indicated by ALT and observed change in field data. 

As indicated by R-sq(adj) there is no indicated 

correlation between ALT and Field Stabilization values. 

16
8 Environmental, Health and Safety Improvements 

8.1 Technical Obstacles 

The overarching EH&S technical issue is evaluating potential risks and balancing those risks 

against immediate costs of risk mitigation. Failure to protect worker health and safety or the 

environment have major negative moral, legal and economic consequences but ineffective, 

inefficient or costly risk mitigation procedures can cause an otherwise profitable company to fail. 

Specific issues addressed in this subcontract relate to: 

a. Reduction of toxic waste. At the start of the program off-spec plates or modules were 

crushed and Cd-containing materials were dissolved with acid. Output from the process was 

a relatively large quantity of clean glass that could be discarded in a landfill and a relatively 

small quantity of Cd-containing “cakes” that were disposed of as toxic waste. While the 

process was effective in reducing the amount of toxic waste, the Cd-containing cakes could 

not be readily recycled due to the relatively low cost (~$2/kg) of low-purity Cd. 

b. Reduction of exposure of personnel to Cd-containing dust. At the beginning of the program 

there were two process steps that generated significant amounts of Cd-containing dust: 

1) removal of “wrap around” CdTe and CdS from the glass surface opposite that intended 

for semiconductor deposition, i.e., cleaning the “sunny side” of the WIP plate and 

2) removal of the CdTe/CdS and TCO layers from a 1.5 cm border of the film in order to 

provide effective electrical and environmental isolation in the encapsulated module, i.e., the 

“edge delete” step. These steps occur at different points in the process and each step 

required personnel to dress in appropriate personal protective equipment (PPE) including 

respirators and to work in confined spaces. From a manufacturing perspective, both 

processes were slow and labor-intensive. 

c. Determination of inadvertent exposure of personnel to Cd-containing ambients. Cadmium 

concentration is monitored at several locations in the plant in order to ensure that workers 

are not exposed to airborne Cd in excess of Occupational Safety and Health Administration 

(OHSA) action levels. The present procedure includes collection of airborne particles 

obtained using small fans to pull air through filters and then sending the filters to an external 

lab for determination of airborne Cd levels – a process that requires 4-7 days. In the event 

of an unforeseen increase in airborne Cd levels, EH&S response time is limited to time 

required for determination of measured airborne Cd. 

8.2 EH&S Tasks 

EH&S tasks were carried out by members of the First Solar EH&S group independently from 

other subcontract tasks. Typical work flow began with collection of data and investigation of 

options. Typically solution of EH&S objectives were synergistic with cost reduction objectives 

so that EH&S personnel worked closely with manufacturing engineers. For example, reduction 

of personnel exposure to cadmium dust also reduced direct labor; recycling of spent coated 

modules reduced both labor and costs associated with disposal of hazardous wastes. With 

respect to specific objectives of this subcontract, the following EH&S objectives were 

accomplished: 

a. Objective - Reduction of toxic waste 

Whereas in the past off-spec CdTe-coated plates were shredded, the CdTe/CdS was stripped 

in acid and converted to “cakes” which were then disposed of as hazardous waste, now 

17
shredded modules are sent to a Cu smelter where Cd is recovered and recycled. Thus 

hazardous waste from off-spec plates has been virtually eliminated. 

b. Objective – Reduction of exposure of personnel to Cd-containing dust 

All processes have been evaluated and there are no module fabrication processes that 

requires operator use of PPE3

. Edge delete and sunny-side buffing steps have both been 

fundamentally re-designed including design, fabrication and environmental testing of 

automated equipment that virtually eliminates exposure of personnel to Cd-containing dust. 

c. Determination of inadvertent exposure of personnel to Cd-containing ambients 

Efforts to develop a practical procedure for routine reduction in industrial-hygiene sample 

turn-around time to 2 hours from the current 4-7 days were not successful. It is emphasized, 

however, that there have been no cases of unexpected exposure of personnel to Cd-levels 

beyond OSHA limits. Nonetheless, First Solar continues work to reduce any margin for 

error and to improve its unblemished record with respect to personnel health and safety. 

9 Summary 

9.1 First Solar Growth 

During the course of this subcontract First Solar has significantly increased manufacturing 

capacity from less than 2 MW/yr to more than 20 MW/yr while at the same time increasing the 

average module total area power conversion efficiency from 7% to >9%. First Solar currently 

manufactures and sells 50-65 W thin film CdTe PV modules at a rate of approximately 

1.9 MW/month. Sales backlog (booked sales less current inventory divided by production rate) 

is more than a year. First Solar is currently building new facilities and installing additional 

equipment to increase production capacity by 50 MW/yr; the additional capacity is expected to 

come on line in the third quarter of 2006. 

9.2 Subcontract Accomplishments 

Specific subcontract accomplishments include: 

• Designed, built, tested and evaluated for production an advanced distributor for 

semiconductor vapor transport deposition. More than an optimization, VTD-2 distributors 

were developed around a new operating principle and represent a change in the fundamental 

distributor design. VTD-2 distributors circumvent fundamental deficiencies in earlier 

designs and produce films that are more uniform across a plate, more uniform plate-to-plate 

within a run, and exhibit simplified deposition control and increased material utilization. 

• Developed an advanced powder feeder suitable for vapor transport deposition of ultra-thin 

CdS films. 

• Generated five patent applications – four on advanced distributor design; one on advanced 

powder feeder 

• Through a Colorado School of Mines subcontract - Developed a three stage model for 

distributor operation including a) injector model including powder feed into heated vaporizer, 

b) distributor model including vaporization and flow of gas within the distributor, and 

c) deposition model including distribution of gas coming out of the distributor and deposition 

onto glass. Computer-simulated distributor performance replicates observed VTD-1 

 

3

 PPE are required to perform certain maintenance procedures. 

18
distributor performance over the operating parameter space that has been explored 

experimentally. The model provided important insight into the fundamental physical factors 

affecting device performance and thereby facilitated design of advanced distributors. The 

distributor model was used to carry out a Taguchi design of experiments that identified key 

features of distributor design essential for producing uniform films. 

• Demonstrated NREL-confirmed total area module efficiency of 9.32% on 7200 cm2

(67.12 W, 69.8 V Voc, 1.15 A Isc, 60.4% FF). 

• Demonstrated module scale application of buffer layer and sputtered CdS films. A module 

produced with ultra-thin CdS and buffer layers displayed 9.7% efficiency (unconfirmed) 

compared to <9% efficiency obtained with modules with the same CdS thickness but without 

a buffer layer. 

• Demonstrated ability to reproducibly deposit 900 Å CdS films in manufacturing. 

• Demonstrated ability to reproducibly deposit 2.5 μm CdTe films in manufacturing. Tens of 

thousands of plates with 2.5 μm CdTe have been routinely produced in production. 

Evaluation of the impact of CdTe thickness on baseline field performance is ongoing. 

• Developed equipment and ALT methodology to establish good correlation among nonlaminated cell, strips and submodules and demonstrated procedures for quantitative 

comparison between ALT and predicted baseline field performance. 

• Virtually eliminated hazardous waste generated from off-spec WIP plates and modules. 

• Reduced operator exposure to Cd-containing ambients. 

• Through a University of Toledo subcontract, developed initial statistical models indicating 

effects of non-uniformity of cell performance. 

With plants under construction to increase capacity to 75 MW/yr by 2007, First Solar is one of 

the fastest growing PV companies in the world. Technical accomplishments of the current 

subcontract have been important contributors to First Solar’s growth during this period; 

continued technical progress will be required to further expand the impact of CdTe PV on th PV 

market. 

10 Publications: 

1) R. C. Powell, “RESEARCH LEADING TO HIGH THROUGHPUT MANUFACTURING OF 

THIN-FILM CDTE PV MODULES”, Phase II annual technical report, NREL subcontract No. 

RDJ-2-30630-20, Feb. 9, 2005. Available at 

http://www.nrel.gov/ncpv/thin_film/pn_techbased_cadmium_telluride.html#annual_final_report

2) Peter V. Meyers, Anke Abken, Douglas Dauson, Upali Jayamaha, Rick Powell and Syed 

Zafar, “Technology in support of Thin Film CdTe PV Module Manufacturing”, Proceedings, 

DOE SET Program Review, Denver, CO (7-10 Nov. 2005) 

3) Other Information Sources: First Solar Website: http://www.firstsolar.com/

19
Glossary of Acronyms 

AFM atomic force microscopy 

ALF Alan Farhenbruch consulting 

ALT Accelerated Life Test 

CdS Cadmium Sulfide 

CdTe Cadmium Telluride 

CSM Colorado School of Mines 

CSU Colorado State University 

CTO cadmium tin oxide 

CV capacitance-voltage measurement 

C-V-f capacitance-voltage-frequency measurement 

DAP donor-acceptor Pair 

DOE US Department of Energy 

EERE Energy Efficiency and Renewable Energy, a program within DOE 

EH&S Environmental, Health and Safety 

FF fill factor 

FS First Solar, LLC. 

GIXRD glancing incidence X-ray diffraction 

HEPA high efficiency particle accumulator 

ICPOES inductively coupled plasma optical emission spectroscopy 

IEC Institute of Energy Conversion at the University of Delaware 

IFL interfacial layer 

Isc short circuit current (amps) 

I-V current-voltage measurement 

IVT current-voltage measurements as a function of temperature 

Jsc short circuit current density (mA/cm2

) 

LS light soak (relates to ALT) 

NREL National Renewable Energy Laboratory, operates under SETP 

OC open circuit 

OSHA Occupational Safety and Health Administration 

PL photoluminescence 

PPE personal protective equipment 

PV photovoltaic 

QE quantum efficiency 

Roc dynamic resistance at open circuit 

Rsc dynamic resistance at short circuit 

SC short circuit 

SCI Solar Cells, Inc (predecessor of First Solar, LLC) 

SETP Solar Energy Technologies Program, a program within EERE 

SNL Sandia National Laboratory 

TCO transparent conducting oxide 

TFPV Thin Film Photovoltaic Partnership, an NREL program 

USF University of South Florida 

UT University of Toledo 

VCd Cadmium vacancy 

Voc open circuit voltage 

20
VTD vapor transport deposition 

WIP work in progress 

XRD X-ray diffraction 

ZTO zinc tin oxide 

References 

 

i

 Solar Energy Technologies Program, Multi-year Technical Plan 2003-2007 and beyond, US Department of Energy 

Efficiency and Renewable Energy, www.eere.energy.gov. ii X. Wu and P. Sheldon, “Novel Manufacturing Process for Fabricating CdS/CdTe Polycrystalline Thin-Film Solar 

Cells”, 16th European PVSEC, Glasgow, UK, 2000. iii “First Solar announced Insurance Policy to Fund Solar Module Reclamation and Recycling Expenses at End of 

Product Life”, First Solar press release, 25 May 2005, http://www.firstsolar.com/

iv R. C. Powell, “RESEARCH LEADING TO HIGH THROUGHPUT MANUFACTURING OF THIN-FILM 

CDTE PV MODULES”, Phase II annual technical report, NREL subcontract No. RDJ-2-30630-20, Feb. 9, 

2005. Available at 

http://www.nrel.gov/ncpv/thin_film/pn_techbased_cadmium_telluride.html#annual_final_report v

 R. Powell, G. Dorer, N. Reiter, H. McMaster, S. Cox, and T. Kahle, "Apparatus and method for depositing a 

material on a substrate, U.S. Patent # 5,945,163." U.S.: First Solar, LLC., 1999. 

vi R. Powell, G. Dorer, N. Reiter, H. McMaster, S. Cox, and T. Kahle, "Apparatus and method for depositing a 

semiconductor material, U.S. Patent #6,037,241." U.S.: First Solar, LLC, 2000. 

vii First Solar’s High Rate Vapor Deposition System was awarded an IR&D 100 Award in 2003. 

21
F1146-E(05/2004) 

REPORT DOCUMENTATION PAGE Form Approved 

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PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION.

1. REPORT DATE (DD-MM-YYYY)

April 2006 

2. REPORT TYPE

Subcontract Report 

3. DATES COVERED (From - To)

September 2004–September 2005 

5a. CONTRACT NUMBER

DE-AC36-99-GO10337 

5b. GRANT NUMBER

4. TITLE AND SUBTITLE

Research Leading to High-Throughput Manufacturing of Thin-Film 

CdTe PV Modules: Annual Subcontract Report, September 2004–

September 2005 

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

NREL/SR-520-39669 

5e. TASK NUMBER

PVB65101 

6. AUTHOR(S)

 R.C. Powell 

5f. WORK UNIT NUMBER

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

 First Solar, LLC 

 28101 Cedar Park Blvd. 

 Perrysburg, OH 43551 

8. PERFORMING ORGANIZATION 

REPORT NUMBER

RDJ-2-30630-20 

10. SPONSOR/MONITOR'S ACRONYM(S)

NREL 

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

National Renewable Energy Laboratory 

1617 Cole Blvd. 

Golden, CO 80401-3393 11. SPONSORING/MONITORING 

AGENCY REPORT NUMBER

NREL/SR-520-39669 

12. DISTRIBUTION AVAILABILITY STATEMENT

National Technical Information Service 

U.S. Department of Commerce 

5285 Port Royal Road 

Springfield, VA 22161 

13. SUPPLEMENTARY NOTES

NREL Technical Monitor: H.S. Ullal 

14. ABSTRACT (Maximum 200 Words)

Specific overall objectives of this subcontract are improvement in baseline field performance of manufactured CdTe 

PV modules while reducing environmental, health and safety risk in the manufacturing environment. Project 

objectives focus on four broad categories: 1) development of advanced front-contact window layers, 2) improved 

semiconductor film deposition, 3) development of improved accelerated life test procedures that indicate baseline 

field performance, and 4) reduction of cadmium-related environmental, health and safety risks. First Solar has 

significantly increased manufacturing capacity from less than 2 MW/yr to more than 20 MW/yr, while increasing the 

average module total-area power conversion efficiency from 7% to >9%. First Solar currently manufactures and sells 

50-65-W thin-film CdTe PV modules at a rate of about 1.9 MW/month. Sales backlog (booked sales less current 

inventory divided by production rate) is more than a year. First Solar is currently building new facilities and installing 

additional equipment to increase production capacity by 50 MW/yr; the additional capacity is expected to come on 

line in the third quarter of 2006. 

15. SUBJECT TERMS

PV; thin film; solar cells; CdTe; modules; semiconductor; vapor transport deposition; manufacturing; facilities;

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