National Renewable Energy Laboratory Innovation for Our Energy Future

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

Sarah Kurtz

CDMA Conference: 

Opportunities for 

Chemicals and 

Materials in Wind and 

Solar Energy

December 4, 2009

Philadelphia, PA

NREL/PR-520-46876

Material Needs for Thin-Film and 

Concentrator Photovoltaic Modules
Outline

• Solar – a huge success, but still a long way to go

• Material requirements:

• Low cost

• High performance

• Excellent reliability

• Materials for thin-film and concentrator PV

• Support (substrate, superstrate)

• Encapsulation

• Electrical connections (inside and out)

• CPV – optics, heat sinking, etc.

National Renewable Energy Laboratory 2 Innovation for Our Energy Future
Solar energy is abundant

Convenient truth: small area can supply our energy needs

Sunlight reaching earth in 1 hour is enough to power the world for 1 year

5-6 kWh/sq m/day

>10 kWh/sq m/day

At 10% 

efficiency, 

area needed 

for US 

electricity
National Renewable Energy Laboratory Innovation for Our Energy Future

Growth of photovoltaic (PV) industry

4

Area of Si passes 

microelectronics 

Tons of Si passes 

microelectronics 

Maintaining growth rate requires reduction in cost & adequate availability
Growth of PV industry

If we can maintain the 

current growth rate, PV 

will reach major 

milestones in < 10 yrs

1

10

100

GW of PV shipped worldwide annually

2000 2005 2010 2015 2020

Year

Annual new electricity capacity 1996-2006*

*www.eia.doe.gov/emeu/international/electricitycapacity.html (4012-2981 GW)/10 yr

Annual replacement of electricity capacity for 20 yr cycle
Ongoing needs motivate use of new materials

National Renewable Energy Laboratory 6 Innovation for Our Energy Future

Performance Reliability

Low cost

Manufacturability Availability
Three key approaches to photovoltaic (PV) panels

Front Solar cell

Back

2. Thin film

3. Concentrator

1. Silicon

Two strategies to reduce 

semiconductor material Conventional approach

Reduce cost by reducing use 

of semiconductor
National Renewable Energy Laboratory Innovation for Our Energy Future

Many technology choices

8
One “winner” or many technologies?

National Renewable Energy Laboratory Innovation for Our Energy Future

Nickel cadmium

Lithium ion

Lead acid

Lithium

Alkaline Nickel metal hydride

Different technologies for different applications

9
Thin-film approaches on the market

National Renewable Energy Laboratory Innovation for Our Energy Future

CuIn(Ga)Se CdTe Amorphous silicon

10
Typical thin-film structures require many materials

National Renewable Energy Laboratory Innovation for Our Energy Future

Glass

ITO or TCO

CdS

CdTe

Metal

Glass for strength

CdTe uses superstrate

Glass for protection

ZnO or TCO

CdS

CuInGaSe

Molybdenum

Glass

CuInGaSe uses substrate

Not to 

scale

11

EVA

EVA
Monolithic module integration

National Renewable Energy Laboratory Innovation for Our Energy Future

+

Contact

Cell

(Inactive)

Contact

Cell

(Inactive)

Active Cell

Glass

5 x 120 μm

Conductor

Device

Conductor

– w

12
Substrate/superstrate

Glass is transparent, strong, inert, and relatively cheap

Glass is relatively heavy, breakable, and more 

expensive than desired

Light-weight, flexible substrates are desired, especially 

for building-integrated applications

- strong (mechanically, durable to UV, moisture, etc.)

- withstand processing of solar cells

- inexpensive 

National Renewable Energy Laboratory 13 Innovation for Our Energy Future
Packaging is essential to PV reliability

Packaging needs:

- Keep water out

- Resistant to UV

- Resistant to temperature cycling and high 

temperatures

- Inexpensive

- Front packaging must be transparent (~ 300-1100 nm)

- Adequate adhesion

- Easily processed

National Renewable Energy Laboratory 14 Innovation for Our Energy Future
Flexible CIGS requires reduced sensitivity to moisture

ZnO (and other transparent conductors) react with 

moisture, causing increase in series resistance

Two strategies:

- Harden the cell (e.g. Sundaramoorthy, et al 34th PVSC)

- Harden the packaging (barrier coatings)

National Renewable Energy Laboratory 15 Innovation for Our Energy Future
Electrical connections

Monolithic interconnections avoids need to tab every ‘cell’

Need two wires coming out:

- Connect to end cells

- Bring contacts to junction box

- Junction box (and mounting) has similar requirements to 

silicon modules

- Pressure-sensitive adhesives are controversial

National Renewable Energy Laboratory 16 Innovation for Our Energy Future
Variety of material needs for thin-film PV

• Edge seal may allow water into glass/glass module

• Desiccant may be useful

• Adhesion to glass can be problem – add 

materials/coatings to improve adhesion

• Role of sodium is important in CuInGaSe modules, 

but sodium can move; may intentionally add sodium

• Currently, the biggest effort with CuInGaSe is to try to 

put it on a flexible substrate – requires excellent 

barrier coating unless cell can be hardened to 

moisture

• Organic PV uses mostly low-temperature processes
National Renewable Energy Laboratory Innovation for Our Energy Future

Range of concentrator approaches

High concentration

• 35% - 40% cells

• 400X – 1500 X

Low concentration

• 15% - 25% cells

• 2X – 100 X

Amonix JX Crystals

18
UV transmission depends on design

Analysis of transmitted optical spectrum enabling accelerated testing of 

CPV designs

SPIE 2009 David Miller, et al
Bonds to heat sink and optics

Cell

Heat sink

Optic Optical bond with 100% 

Transmission

Electrical 50 W/sq cm

contact

Small ∆T (<10°C)

Electrical isolation

No voids

T cycle OK

•DBC (direct bonded copper) performs well, but is 

expensive

• Intense UV may be a substantial problem, but 

optics may not transmit UV

IR image of void in 

die attach

Bosco, et al 34th

PVSC
CPV – many parts

CPV has many parts

• Optics – lens, mirrors, (primary & secondary); glass, 

plastic, metal, film, etc.

• Cell encapsulation – water out, resistant to UV

• Thermal contact – low cost; electrical isolation

• Heat sink – must be electrically isolated, but excellent 

thermal contact

• Strategy for letting air in, while keeping dirt out 

(filters?)

National Renewable Energy Laboratory 21 Innovation for Our Energy Future
Concentrator technology

Creative 

designs?
Summary

• Solar is growing rapidly and could become a 

significant source of electricity within our lifetimes

• Packaging is essential to success of PV

• Key material needs for thin-film and CPV include:

• Structural support (substrate/superstrate, frames, etc.)

• Electrical connections to cells

• Electrical connections to outside world

• Packaging to keep water out 

• Encapsulants

• Barrier coats

• Edge seals (maybe even desiccant)

• Thermal contacts for CPV

• Optical materials for CPV

National Renewable Energy Laboratory 23 Innovation for Our Energy Future
Planet powered by 

renewable energy

By year 2100 or before?

Thank you for your attention!