AIMCAL 2008

2008 Fall Conference

Vacuum Web Coating

Brent P. Nelson

October 22, 2008

The Potential of Photovoltaics

NREL is a national laboratory of the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy operated by the Alliance for Sustainable Energy, LLC

NREL/PR-520-44106

Presented at the Association of Industrial Metallizers, Coaters and Laminators (AIMCAL) Fall Technical Conference 2008 and 22nd International Vacuum Web Coating Conference held October 19-22, 2008 in Myrtle Beach, South Carolina.
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The Potential of PV: Course Outline

1. The Energy Market

2. Introduction to Photovoltaics (PV)

3. Current PV Technologies

a) crystalline silicon (c-Si)

b) amorphous silicon (a-Si:H)

c) cadmium telluride (CdTe)

d) copper indium gallium selenide (CIGS)

e) others, concentrator PV, organic PV, sensitized cells, etc.

4. Technology Comparison

5. PV Technology Trajectory
Breakdown of US Energy Use

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We use Mostly Chemical Energy

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World’s Consumable Resources Breeder Reactors

“Conventional” “Future”

0

5,978

11,956

17,934

23,912

29,890

0

1,031

2,061

3,092

4,122

5,153

Quads BBoO
40

30

20

10

0

TW

Oceans

Hydro

Biomass

Geothermal

Wind

Solar

 Resource Limit

 Practical Potential

Sustainable Resource Potential

70 TW > 100 TW
Consumable Resources

0

16

32

48

64

80

What are these units of time

of sunlight hitting the earth?

Hours

“Conventional” “Future”
Solar Energy Technologies

Passive Solar

(space heating) Concentrating

Solar Power

Solar

Hot Water

Solar Electric - Photovoltaics
The Basic Solar Cell

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PV Systems Building Blocks

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Cell

Module

Array

System

Includes storage,

voltage regulation,

inverters, etc.
PV Technologies

Concentrators

Silicon

Multijunctions

(III-Vs)

Flat plates

Crystalline silicon

Thin films

New technologies

Generation

1

2

3

1

2
2007 Flat Plate Module Production

Thin-Films

3,733 MW Total Production

• 90% is crystalline silicon

• 10% is thin-films (pulled out)

• 95% contains silicon (greens)

Source: Paul Maycock, PV News, February 2005

Generation

1

2
Crystalline Silicon = Wafers

Single Crystal Si

Multicrystalline Si

Wafering

Ribbons ≠ Wafering

String Ribbon Si

Edge-Defined 

Film-Fed Growth

(EFG) Si
c-Si – Device Structure

Best Commercial

Sample Structure

Common Industrial

Cell Structure

P type wafer

N type diffusion AR coating

Back contact

Front Contact

Point-contact cell

SunPower – 21.5%
C-Si Modules
Crystalline Silicon (c-Si)

Pros

•Well understood material system 

because of IC industry

•Equipment to production is 

readily available from multiple 

vendors.

•Lower barriers to entry for new 

companies

•Elemental abundance

Cons

•Si wafers are energy intensive 

to manufacture

•Feed stock processing growth 

rate constrains growth

•Actually a family of several 

device structures, substrates, 

and production technologies

•Indirect band gap

•Not monolithically processed

•Wafer technology

(too thick ~ 200 microns)

•Not many web-coating 

applications
Amorphous Silicon: Very Thin

a-Si:H Transparent Top Contact

Back Reflecting Metal

p-layer

i-layer

n-layer

B or P dopants

can be fully satisfied

∴ need %-levels

hν
Typical Growth Techniques

Transparent Top Contact

Back Reflecting Metal

p-layer

i-layer

n-layer

• Encapsulation

• Laser Patterning

• Top Contacts

– Metal Grids... 

fingers by PVD or wire

– TCO’s... ZnO, ITO by PVD

• Semiconductor Layers

– CVD techniques

– PVD in research

• Bottom Contacts

– Metals... Ag, Al, by PVD

– Texturing... ZnO by PVD

Growth Direction
a-Si:H Modules
Amorphous Silicon (a-Si:H)

Pros

•Well understood material system 

– lots of science

•Leverage off TFT industry

•At least three companies selling 

“turn key manufacturing” facilities

•Elemental abundance

•Scalable manufacturing 

techniques

•Low temperature processes

•Very thin absorbers

•Many web-coating applications

Cons

•Doesn’t work well in red end of 

solar spectrum

•Low hole-mobility

•Light induced metastability

•Lowest efficiency of readily 

available technologies

•Many size “standards”

•Many substrate “standards”
Future of Film-Silicon PV

0.1 1.0 10 100 1,000

10

100

nc-Si:H

< 300°C

pc-Si

APIVT

pc-Si

CVD

~1000°C

pc-Si, MIC,

LTE, < 300°C

mc-Si

epi, ZMR

APIVT

> 1000°C

c-Si

epi, lift,

smart cut

> 1000°C

1

Grain Size (μm)

Cell Thickness (μm)

Wafers

< 800°C

1410°C

a-Si:H

< 300°C
CdTe – Device Structure

Best Laboratory 

sample structure

Common Industrial 

module structure

~1 μ

8-10 μ

GRAIN BOUNDARY

Process 

Direction

Ref: Tim Gessert, 3M Tech Forum, 7/19/06

Glass

SnO2 .2-.5 μm

CdS 600-2000Ǻ

CdTe 1.5-4 μm

C-Paste with Cu

Or metals
“Typical” CdTe Process

Ref: G. Braun & D. Skinner, Experience Scaling-Up Manufacturing of Emerging Photovolltaic 

Technologies, NREL SR-640-39165, Jan 2007, p 42
CdTe Modules
CdTe – Thin film

Pros

•Low Manufacturing Cost

•Highest deposition rate of 

absorber materials = good 

Manufacturability

•High efficiency laboratory cells 

= great promise 

•Best Laboratory: > 16% 

•Champion Modules at 12.5%

•Monolithic Module construction

•Nice Aesthetics

•2 component manufacturing 

that is very impurity tolerant

Cons 

•Not as well understood as Si 

materials

•No Industry standard size or 

fabrication techniques

•No one sells equipment to 

build these modules. 

•Commercial Modules: 8-10.5

•Cd toxicity issues are know, 

and CdTe toxicity issues are 

being debated. 

•Not currently many webcoating applications
CIGS – Device Structure

Process 

Direction

Ref: Tim Gessert, 3M Tech Forum, 7/19/06

• Zn - Zinc

• O - Oxygen

• Sn - Tin

• Cd – Cadmium

• S – Sulphur

• Cu - Copper

• In - Indium

• Ga - Gallium

• Se – Selenium

• Mo - Molybdenum
“Typical” CIGS Process

Sputter Mo

Co Evaporate 

CuGaInSe2

CBD of CdS

Sputter

undoped ZnO

Sputter Cu

Reactive anneal

In H2Se, S, vapor

Sputter InGa

Can also be:

•Electrodepositon 

•Thermal evaporation

•Electron beam evaporation

•Screen printing

•Spray jet

Ref: Handbook of Photovoltaic Science and Engineering, Luque and Hegedus,

chapter 13, Shafaman and Stolt, p 583.

Sputter CdS 

Sputter

Al-doped ZnO Sputter ITO

Can also be:

•ZnO, ZnS, ZnSe,

•InS, ZnInxSey

•Inx(OH,S)y

•In2S3

Can also be:

Annealed with vapor of

•H2S

•Se

•S

Research Standard Manufacturing Example
CIGS Modules
CIGS – Thin film

Pros

•Possible low manufacturing cost

•Possible high deposition rate of 

absorber materials

•Highest efficiency laboratory cells 

= great promise 

•Best Laboratory: > 19.9% 

•Monolithic Module construction

•Very Nice Aesthetics

•Three “turn-key factory” 

companies

•Many web-coating applications

Cons 

•Requires large area 

stoichiometry of 4 elements

•High efficiency processes 

require strict uniformity

•Not as well understood as Si 

materials

•No Industry standard for size or 

fabrication techniques

•Best Commercial Modules: 

13.4%

•Increasing deposition rates 

lowers efficiency
Concentrator Photovoltaics (CPV)

• Flat plate collectors

• cover large areas with low cost cells

• don’t require external optics

• Concentrator 

• high efficiency cells

• cover large areas with low cost external optics

• Mid to high-concentration PV systems

• high-efficiency III-V or Si cells

• trackers

• reflective optics or

• refractive optics

• CPV is inherently system-oriented

• CPV requires direct sun (SW USA)

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Exciton-Based Materials

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Traditional

Semiconductor Conducting

Polymer

Charge

Separation

at Interface
Organic Solar Cells

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Grätzel (Dye-Sensitized) Cells

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High surface area

for rutheniumpolypyridine dye

Ox

Red
All PV Technologies are Improving

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Flat Panel PV Modules & Cells 

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From Citigroup Global Markets, equity research,

Applied Materials, Inc, (AMT), 19 Feb. 2008
Worldwide PV Module Production

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Thin-Films
Thin-Film Market Share in the USA

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Conclusions

1. The sun is the BIG energy player

2. PV can (should) be a big part of the Energy Portfolio

3. Some PV types have web-coating applications

a) crystalline silicon (c-Si) - LOW

b) amorphous silicon (a-Si:H) - VERY HIGH

c) cadmium telluride (CdTe) - POTENTIAL

d) copper indium gallium selenide (CIGS) - HIGH

e) others (CPV, OPV, DSSC, etc.) – VERY HIGH

4. PV industry is “a mile wide and an inch deep”

5. PV production growing a 35%++ annually

but still very small