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.

Sarah Kurtz 

Principal Scientist; 

Reliability Group Manager

8.13.2010

Solar Instructor 

Training Network 

Webinar

NREL/PR-520-49176

PV Technology for Today and Tomorrow
Outline

• Winning technology or many technologies?

• Some history to put today and tomorrow in 

perspective

• Three primary approaches

- Silicon

- Thin film

- Concentrator

• Comparison of the three and looking at 

tomorrow

National Renewable Energy Laboratory 2 Innovation for Our Energy Future
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

Expect this for both PV and batteries

3
A little history

National Renewable Energy Laboratory 4 Innovation for Our Energy Future
National Renewable Energy Laboratory Innovation for Our Energy Future

Growth of photovoltaic (PV) industry

5

Area of Si passes 

microelectronics 

Tons of Si pass 

microelectronics 

The PV industry has been doubling every ~2 years

Sources: Prometheus/Navigant
National Renewable Energy Laboratory 6 Innovation for Our Energy Future

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
PV prices have decreased

National Renewable Energy Laboratory 7 Innovation for Our Energy Future

Source: PHOTON International

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Module price ($/Watt) 

February June October

2009 2010 

February

Crystalline silicon

Cadmium Telluride (First Solar)

Key to sustaining growth of the industry is price reduction

Contributors to lower costs:

• Thinner wafers

• Automation

• Standard equipment

• Optimized processes
Three approaches to PV (and lower cost)

National Renewable Energy Laboratory 8 Innovation for Our Energy Future

Front

Solar cell

Back

2. Thin film

3. Concentrator

1. Silicon

Reduce semiconductor 

material
Higher efficiency can reduce cost

National Renewable Energy Laboratory 9 Innovation for Our Energy Future

Upfront costs:

1. Semiconductor material

2. Area-related costs (glass, installation, real estate, wiring)

3. Power-related costs (inverter)

Front Solar cell

Back

2. Thin film

3. Concentrator

1. Silicon

Reduce semiconductor 

material

Reduce area by 

increasing efficiency

Two strategies for reducing cost:

Increasing efficiency may be a key path to reduced cost
Types of PV – currently available 

• Crystalline silicon

• Mono-crystalline

• Multi-crystalline

• Ribbon

• Thin film

• CdTe (Cadmium telluride)

• CIGS (Copper Indium (Gallium) Selenide)

• Amorphous silicon – usually combined with microcrystalline 

silicon layers in a multijunction stack; may contain Ge

• Organic

• Concentrator (may be classified in many ways)

• Refractive/reflective

• Multijunction III-V or silicon

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Historic PV Technology Mix

National Renewable Energy Laboratory 11 Innovation for Our Energy Future

• Historically, crystalline silicon has dominated the market

• Technology mix is becoming more diverse

• CdTe is primary new entrant; CIS may be 5-7 yr behind; CPV ~ 10 yr

• Long-term trends may take decades to establish

Source: 

PHOTON 

International

Calculators
Historic PV Technology Mix

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• Historically, crystalline silicon has dominated the market

• Technology mix is becoming more diverse

• CdTe is primary new entrant; CIS may be 5-7 yr behind; CPV ~ 10 yr

• Long-term trends may take decades to establish

Source: 

PHOTON 

International

What does this mean for you?

PV will become increasingly complex
0.001 0.01 0.1 1 10 100 1000

Installation size (MW)

A key factor affecting technology mix: Distributed vs Central

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PV

Solar Thermal

Modern Wind

Residential Commercial Utility

200

150

100

50

Maximum PV Plant Size (MW)

0

2004 2006 2008 2010 2012

Year

Planned

Recently PV has seen increases in:

• Maximum system size

• Average system size

• Ground mount (instead of roof mount)

• Connection at transmission instead of 

at distribution voltages

• Utility ownership

These changes may affect the technology mix
Within US, predictions are for large utility growth

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8

6

4

2

0

GW

2009 2010 2011 2012 2013

Year

Rogol

Lorenz

Projected Installed US Utility-Scale Capacity

All US installations in 2009

Source: PHOTON International

If utility growth is this large, it will change the technology mix
Three approaches to PV – 1. Silicon

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Front

Solar cell

Back

2. Thin film

3. Concentrator

1. Silicon

•Mono-crystalline

•Multi-crystalline

•Ribbon
Silicon modules

SiliconS

Glass

.Backsheet .

Silicon cell Tab

Common packaging materials

EVA - Ethylene vinyl acetate

PET - Polyethylene terephthalate

PVF - Poly vinyl fluoride

ETFE – Ethylene tetrafluoroethylene 

Si module cross section 

(not to scale)

Construction of silicon modules is simple in concept

EVA
Crystalline Silicon - history 

• Predictions of the demise of silicon PV have been 

voiced for decades:

• Silicon cells must be fairly thick, increasing material cost

• Shortage of silicon feedstock – in 2007, 2008 we saw this 

(fast-growing industries tend to develop shortages)

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Area of Si passes 

microelectronics 

Tons of Si pass 

microelectronics 

Despite the predictions, silicon PV is alive and well
Crystalline silicon

Advantages:

• Builds on strong industry

• Silicon is abundant and non toxic

• Efficiencies of 15%-20% are achievable

• Demonstrated > 20 years performance in field

• Warranties typically < 1% degradation/y

• Potential for further cost reduction

Disadvantages:

• Costs are higher than desired

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Three approaches to PV – 2. Thin film

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Front

Solar cell

Back

3. Concentrator

1. Silicon

2. Thin film

•CdTe

•CIGS 

•Amorphous Si 

•Organic

Reduce semiconductor 

material
Thin-film approaches on the market

National Renewable Energy Laboratory Innovation for Our Energy Future

CuIn(Ga)Se CdTe Amorphous silicon

20
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

21

Thin-film modules have a different construction than Si modules
Thin film products vary in their construction

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

22

EVA

EVA
Is glass/glass construction required?

• Moisture sensitive?

• No: use ETFE or ?

• Yes: use glass/glass construction with edge seal

• Strategies:

• Reduce moisture sensitivity (change cell design)

• Develop flexible moisture barrier

• If successful, opens many markets:

• Awnings

• Shingles

• Car roofs, etc.

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If moisture problem is solved, flexible packages can open new markets
Thin film vision – looking to the future

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Vision (advantages)

• ~1-μm-thick film on inexpensive substrate

• Materials requirement is small: reduced cost

• Organic PV Vision: a PV plastic wrap

• Dye-sensitized Vision: PV spray-on paint

• Low CapEx enables easy ramp up

• Can be integrated into building façade

Challenges (disadvantages)

• Growth on inexpensive substrates limits efficiency

• Sensitivity to moisture leads to glass/glass laminate

• Infrastructure is not as well developed as for silicon 

• Building integration increases operating temperature
First Solar demonstrated thin-film concept

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2000

1500

1000

500

0

Capacity (MW/y)

2005 2006 2007 2008 2009 2010 2011

Year

Projected

First Solar Production Capacity

#1 in world

First Solar grew to be #1 in world in just four years, demonstrating 

the benefit of using less semiconductor material 
Historic PV Technology Mix

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• First Solar has put CdTe on the map 

• Dozens of other thin-film companies hope to be the next “First Solar”

Source: 

PHOTON 

International

First 

Solar
Comparison of efficiencies

National Renewable Energy Laboratory 27 Innovation for Our Energy Future

In general, silicon outperforms thin film in terms of efficiency

Source: DOE EERE 2008 Solar 

Technologies Market Report

Si

Thin Film
Comparison of degradation rates

National Renewable Energy Laboratory 28 Innovation for Our Energy Future

Statistically, observed 

degradation rates are about 

0.5%/y

Degradation rates of recent 

thin-film products are smaller 

than for pre-2000 products

Source: Jordan, et al. PVSC 2010
Three approaches to PV

National Renewable Energy Laboratory 29 Innovation for Our Energy Future

Front

Solar cell

Back

2. Thin film

3. Concentrator

1. Silicon

Reduce semiconductor 

material
National Renewable Energy Laboratory Innovation for Our Energy Future

Two primary concentrator approaches

High concentration

• 35% - 40% III-V cells

• 400X – 1500 X

Low concentration

• 15% - 25% Silicon cells

• 2X – 100 X

Amonix JX Crystals

30
Concentrator technology

Maturity is 

similar to that 

of airplanes 

100 years ago

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CPV

Advantages:

• CapEx is typically smaller than for silicon

• Reduces use of semiconductor material, potentially 

enabling low cost

• Allows use of very high efficiency solar cells

• Module efficiency up to ~ 30% (verified)

• Is mostly an engineering project

Disadvantages:

• Only uses direct beam (no output on cloudy days)

• Not yet well established

• Difficult to integrate into buildings (was rejected in ‘90s)

National Renewable Energy Laboratory 32 Innovation for Our Energy Future
CPV

Current status

• Dozens of companies exploring CPV

• A handful of companies are setting up automated 

production

• These companies are likely to each install > 1 MW in 

2010

• Amonix just announced 30 MW project in Colorado

• Once bugs are worked out, could ramp quickly

• Not yet clear whether applications will be limited to 

utility-scale

National Renewable Energy Laboratory 33 Innovation for Our Energy Future
Future of PV

• Today’s technologies have lots of room for 

improvement – this is the near future of PV

• Other concepts have been studied for 

decades – need breakthrough:

- Intermediate band

- Multiple exciton

- Hot carrier

National Renewable Energy Laboratory 34 Innovation for Our Energy Future
Sorting the hype from reality

Questions to ask:

• What efficiency has been achieved? (Most 

concepts have high theoretical efficiencies 

but ‘real’ technologies present achieved 

efficiencies rather than theoretical)

• If high efficiencies are reported, are these 

for small/large cell/module?

• If high efficiencies have not been achieved, 

how much will it cost to install, etc?

National Renewable Energy Laboratory 35 Innovation for Our Energy Future
‘Breakthrough’ or ‘Revolution through Evolution’?

Two camps predicting future of PV:

- Need revolutionary breakthrough

- Achieve revolution through evolution

PV cost is already competitive for peaking 

power in locations with high electricity 

prices (Hawaii, California, etc.)

What fraction of penetration is achievable?

National Renewable Energy Laboratory 36 Innovation for Our Energy Future

Today’s technologies will contribute several % to energy supply

Will we need a breakthrough to achieve tens of %?
Which technology for which application?

Efficiency: important for area-constrained applications

Form factor: e.g. building integrated vs conventional 

racks

Weather: direct/diffuse; temperature; wind; stresses

Tracking: affects energy yield, packing density, dual use 

of land

National Renewable Energy Laboratory 37 Innovation for Our Energy Future

Many factors contribute to deciding which technology is best
National Renewable Energy Laboratory Innovation for Our Energy Future

Efficiency is key driver

38
Temperature coefficients affect output

Estimated temperature coefficients

Amorphous silicon: -0.2%/°C (variable)

CdTe: -0.2 to -0.25%/°C

CIGS (CIS): -0.4%/°C

Crystalline silicon: -0.4% to -0.5%/°C

If the temperature varies by 30°C, 

these translate to relative change in 

performance of ~6% to 15%

National Renewable Energy Laboratory 39 Innovation for Our Energy Future

Temperature coefficient can affect the choice of product in a 

small (~10%) way

Note: many of the thin-film products show changes in efficiency

110

100

90

80

70

Output of 100 W module (W)

0 20 40 60 80 100

Module temperature (°C)

Rated

Temperature

aSi

CdTe

CIGS

cSi
Off-axis and diffuse light

Fixed module receives 

light at variable angle

National Renewable Energy Laboratory 40 Innovation for Our Energy Future

Tracked module 

follows angle of sun; 

especially useful late 

in the day

Solyndra: unusual 

approach to collecting 

light from all angles

Tracking has > 20% effect; off-axis collection effect is smaller
To frame or not to frame

• Frames are useful to prevent damage to edges of 

modules

• Frames can provide mechanism for easy attachment

• Frames can collect dirt 

• Frames may be damaged by snow and ice

• Framed modules may require ground wires 

National Renewable Energy Laboratory 41 Innovation for Our Energy Future
Differentiation of technologies

• Area constrained = higher efficiency

• Peak shaving = use tracking

• BIPV = consider aesthetics

National Renewable Energy Laboratory 42 Innovation for Our Energy Future

Increasing temperature 

Sunny 



c-Si

CPV

CdTe, a-Si

CIGS

Technologies may show up to 10% advantage under these conditions
Summary

• As the PV industry grows, the technology 

mix will be increasingly diverse

• Silicon, thin-film, and concentrator 

approaches are all making progress

• When choosing between these:

• Cost is #1 driver

• Higher efficiency technologies have edge in areaconstrained applications

• Form factor can drive building integration

• Efficient conversion of diffuse light gives edge in 

cloudy/hazy regions

• Tracking can help meet peak loads late in the day

National Renewable Energy Laboratory 43 Innovation for Our Energy Future
Questions

1) With the recent flooding of the market with Chinese modules, what

are the trends in materials or processes that we can look for in a

module manufacturer that gives us a better idea of the durability and

life of the product they are producing. 

See Solar ABCs Recommendation to require qualification testing:

www.solarabcs.com/recommended_standards/Policy_recommondations_A

BCS-12B_1page-1.pdf

Ask company about their quality assurance program (if modules are 

manually assembled, how do you know they’re all the same?)

How have they assessed robustness to UV?

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