www.ecn.nl

WaferWafer--based silicon PV technologybased silicon PV technologyStatus, innovations and outlookStatus, innovations and outlook

Wim Sinke

ECN Solar Energy, Utrecht University & European PV Technology Platform

Contents

• Wafer-based silicon photovoltaics

- features

- market position

- history and state-of-the-art

• Cell and module efficiencies

- achievements so far

- limiting factors

- options for further improvement

• Towards integration of cell and module designs

• Cost reduction potential

• Outlook

Contents

• Wafer-based silicon photovoltaics

- features

- market position

- history and state-of-the-art

• Cell and module efficiencies

- achievements so far

- limiting factors

- options for further improvement

• Towards integration of cell and module designs

• Cost reduction potential

• Outlook

Wafer-based crystalline silicon

- ½ century of manufacturing experience

- huge technology base (materials, processes & device designs)

- extensive track record (performance, lifetime & reliability

- highest performance of flat-plate technologies

- further cost reduction (preserving efficiency) is main overall challenge

Bell

Cell & module technologies (“flat plate”)Commercial: wafer-basedcrystalline silicon- monocrystalline (cut)- multicrystalline (cut)- ribbons(80% of global market)

Commercial: thin films- silicon- copper-indium/gallium-diselenide (CIGS)- cadmium telluride (CdTe)(20% of global market)

Pilot production and laboratory:emerging and novel technologies- super-low-cost concepts

(printed organic & inorganic, etc.)- super-high-efficiency conceptsECN/Holst Centre

ECN

Nanosolar

Helianthos

Cell & module technologies (“flat plate”)

Commercial: wafer-basedcrystalline silicon

module efficiencies 13 ~ 19%

Commercial: thin films

module efficiencies 6 ~ 12%

Pilot production and laboratory:emerging and novel technologies

(various efficiencies;most not yet commercially available)ECN/Holst Centre

ECN

Nanosolar

Helianthos

The silicon PV value chain

Siliconfeedstock

Crystal WaferSolarcell

Solarmodule

PV-system

Solarelectricity

Typical current industrial silicon solar cell

• B-doped substrate (base)• P-doped front (emitter)• Al-doped rear (back surface field, BSF)

• SiN anti-reflection coating / passivation layer• Ag contacts

9

Cell design options

Standard:front emitter

Rear emitter / frontsurface field

Heterojunction

Metallisation WrapThrough (MWT)

Emitter Wrap Through(EWT)

Back Junction BackContact (BJBC)

Carrier collection at front

Carrier collection at rear

Front and rear contacted

All rear contacted

SunPower

Sanyo

Passivated Emitter and Rear Locally diffused (PERL) cell

Zhao, Wang & Green, UNSW (1999)

World record monocrystalline silicon cell(efficiency 25.0%)

based on Interdigitated Back Junction, Back Contact cells

Courtesy SunPower Corp.

World record monocrystalline silicon large-areamodule (efficiency 21.4%)

ancestor (1986):Point Contact Solar Cell

(>28% under concentration)

Swanson, Sinton & King

based on HIT (Heterojunction with Intrinsic Thin layer) cells

Courtesy Sanyo Electric Co., Ltd.

Very high efficiency monocrystalline siliconlarge-area module (efficiency …%)

Contents

• Wafer-based silicon photovoltaics

- features

- market position

- history and state-of-the-art

• Cell and module efficiencies

- achievements so far

- limiting factors

- options for further improvement

• Towards integration of cell and module designs

• Cost reduction potential

• Outlook

Historic efficiency development crystallinesilicon cells and modules (rounded values)

0

5

10

15

20

25

301

95

0

19

55

19

60

19

65

19

70

19

75

19

80

19

85

19

90

19

95

20

00

20

05

20

10

20

15

Eff

icie

nc

y[%

]

Year

best laboratory cell(multicrystalline Si)

best laboratory cells(monocrystaline Si)

typical commercial modules

Photovoltaic conversion:basic process and losses

energy gap(Si: 1.12 eV)

generation

recombination

X

X

X

Solar spectrum and spectral losses

wavelength [nm]

1.6

1.2

0.8

0.4

400 800 1200 1600 2000 24000.0

available for conversion in crystalline Si

infraredvisibleUV

solar spectrum (Air Mass 1,5; 1000 W/m2)

po

we

r[W

/(m

2.n

m)]

1100 nm 1.1 eV = Si bandgap

courtesy John Schermer, KUNCourtesy John Schermer, RUN, NL

X

X

Solar cell electrical characteristic:voltage and curve factor losses

LkTqV

0 J1eJJ

cu

r re

nt

Pmax

Vmax

Imax

voltage

cu

r re

nt

Pmax

Vmax

Imax

voltage

gapL E

J

J

q

kT

1lnV

0

OC

1max

P

JVFF

scoc

light

scoc

P

FFJV

Jmax

Crystalline silicon solar cell conversionefficiencies: limits and losses (indicative)

From lab to fab

• Trade-off between cost & performance

• Small area large area

• Best average

• Lower Si material quality, highly doped regions, surfacesand contacts additional recombination: Jsc, Voc

• Additional optical losses (reflection & transmission): Jsc

• Additional resistive losses: FF

• Efficiency range multicrystalline Si cells: ~14-17%

• Efficiency range monocrystalline Si cells : ~16-22%

Selected options for further improvement

• Minimize recombination:

- improve material quality manage defects and impurities, usen-type Si

- reduce heavy doping effects local doping, selective emitters

- effective surface passivation SiNx, SiO2, Al2O3, aSi, etc.

- low-recombination contacts heterojunctions

• Minimize optical losses

- reduce reflection, apply light trapping coatings & textures,plasmonic structures?

- reduce shadow losses rear contact designs

• Minimize resistive losses

- increase conductance advanced electrode architectures andmaterials, rear contact designs

Example:understanding and managing impurities

Understanding and managing impurities:effects of Fe, Ni, Cr added to Si feedstock

Bottom Middle Top

8

10

12

14

16

0% 20% 40% 60% 80% 100%Position in the ingot [%]

Eff

icie

ncy

[%]

Ref

Fe 50 ppm wt

Cr 40 ppm wt

Ni 40 ppm wt

Fe 200 ppm wt

• Major metal impurities positively charged in p-type, neutral in n-type.

• p-type dopant B forms B-O recombination center

Understanding and managing impurities:replace p-type Si by n-type Si

n-type siliconsubstrate

(n+) Phosphorus BSF

(p+) Boron emitter

Feo/+

Tio/+

p-type siliconsubstrate

(n+) Phosphorous emitter

Fe+/o

Ti+/o

Back contact

B-O2i

Example: the importance of low-recombinationcontacts

• Well-passivated emitter: ~30 fA/cm2 or less

• Ohmic contacts: ~1000-2000 fA/cm2

• With only 5% contact coverage, 50-100 fA/cm2 from contacts

1lnV

00OC

be

L

JJ

J

q

kT

passcontconteconte JfJfJ ,0,00 )1(

25 12-10-2010

Low-recombination contacts

• Transfer majority carriers without (resistive) loss

• Reflect minority carriers without recombination loss

“minority carrier mirrors”

• Practical solution:

- silicon heterojunction contacts

Graph: Miro Zeman, DUT, NL, 2010

Example: apply light trapping

• Full absorption (even) in very thin substrates

low bulk recombination (high Voc) combined with high Jsc

allow the use of low-quality materials

plasmon pictures Amolf

27 12-10-2010

Example: The best of both worlds (SunPower &Sanyo) – the IBC-HIT cell

• No shadow losses on front

• No optical absorption losses on front

• Very low contact recombination at rear (beyond aSi)

Work incollaboration withUniv. Rome, ENEA,ECN, Univ. Utrecht,and othersM. Tucci et al., “BEHIND” cell concept

Contents

• Wafer-based silicon photovoltaics

- features

- market position

- history and state-of-the-art

• Cell and module efficiencies

- achievements so far

- limiting factors

- options for further improvement

• Towards integration of cell and module designs

• Cost reduction potential

• Outlook

Anatomy of a standard module

• Module consists of:

- glass superstrate

- encapsulant (EVA)

- interconnected solar cells

- encapsulant (EVA)

- rear-side foil

• Finishing:

- framing

- junction box

- cabling and wiring

Base: pn+

Base: pn+

Base: pn+

Base: pn+

Base: pn+

A A

Cross-section AA

Example: Metallisation Wrap-Through (MWT)cell

inspired by Surface-Mount Technology (SMT)

Photo: GEC, Inc.

MWT cells & module:“single-shot” module manufacturing

Section of conductive foil

Rear of cell

Design and manufacture of MWT module

Equipment by Eurotron (NL)

Technology- 120 µm (Deutsche Solar)

and 160 µm (REC) mc-Si wafers

- Conductive adhesive (alternative: low-T solder)

- Patterned rear-side foil

- Novel module line; zero cell breakage

Module results- Aperture area efficiency:

- 16.0 % (120 µm cells)

- 17.0 % (160 µm; 17.8% cells)

MWT cells and modules

Contents

• Wafer-based silicon photovoltaics

- features

- market position

- history and state-of-the-art

• Cell and module efficiencies

- achievements so far

- limiting factors

- options for further improvement

• Towards integration of cell and module designs

• Cost reduction potential

• Outlook

How far can wafer Si module cost go down?

• h

36 12-10-2010

Cost structure of wafer Si PV (2009)

From Peter Fath, Centrotherm, 2009

Contents

• Wafer-based silicon photovoltaics

- features

- market position

- history and state-of-the-art

• Cell and module efficiencies

- achievements so far

- limiting factors

- options for further improvement

• Towards integration of cell and module designs

• Cost reduction potential

• Outlook

Crystalline silicon: first generation PV?

• picture

Mariska de Wild-Scholten, Environmental Sustainability of Thin Film PV2nd EPIA International Thin Film Conference, 12 November 2009, Munich

Energy payEnergy pay--back time of turnback time of turn--key PV systemskey PV systems

40

Carbon footprint of selected electricity generatingCarbon footprint of selected electricity generatingtechnologiestechnologies

Mariska de Wild-Scholten, Environmental Sustainability of Thin Film PV2nd EPIA International Thin Film Conference, 12 November 2009, Munich

9 March2010

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©Greenpeace