17 October 2003 1

High performance silicon solar cells

Gabriela Bunea Ph.D.

SunPower Corporation

17 October 2003 2

Outline

• Background • SunPower brief history• High efficiency solar cells• High volume manufacturing• Future directions

17 October 2003 3

Questions often heard from the general public

• Why have solar cells never become a substantial source of energy?”

• “Too bad solar never made it, it seemed so promising back in the 1970s.”

• “When will the big breakthrough come that will make solar cells practical?”

17 October 2003 4

Answers and fun facts

• Solar cell manufacturing is a vital and rapidly growing industry, enjoying over 30% annual growth over the last 10 years.

• In 2002, more square inches of silicon was used by the solar cell industry than the IC industry.

• There will be no big breakthrough that impacts the industry for at least 10 years, and probably 20 years.

• Instead, the existing technologies will evolve to where they will be cost effective in most distributed applications in 10 years, and will be competitive with fossil fuel generation in 20 years.

17 October 2003 5

Solar Cell Price Exhibits a Classic Experience Curve Behavior

1

10

100

1 10 100 1000 10000 100000

Cum ulative Production (MW)

Mo

du

le P

rice

($/

W)

($20

02)

Historical

Projected1980

$21.83/W

1985

$11.20/W1990

$6.07/W 1995

$4.90/W2000

$3.89/W2005

$2.70/W2010

$1.82/W2013

$1.44/W

2002$3/W

17 October 2003 6

Solar Cell Rules of Thumb

• The annual production of solar modules increases ten-fold every decade

• The price of solar cell modules decreases by half every decade– 2002: $3.00/W– 2012: $1.50/W– 2022: $0.75/W

17 October 2003 7

Silicon Module Cost Components

Ingot Growth

30%

Wafering20%

Module Assembly

30%

Cell Processing

20%

Higher efficiency leverages cost Higher efficiency leverages cost savings throughout the value savings throughout the value chainchain

Investing in high efficiency cell Investing in high efficiency cell processing makes economic processing makes economic sensesense

17 October 2003 8

Factors Driving PastCost Reduction

• Poly silicon price: $300/kg → $30/kg • Wire saws: now < $0.25/W • Larger wafers: 3” → 6”• Thinner wafers: 15 mil → 8 mil • Improved efficiency: 10% → 16%• Volume manufacturing: 1MW → 100MW• Increased automation: none → some• Improved manufacturing processes

17 October 2003 9

The Renewable Energy Revolution• Renewable energy will capture a

meaningful share of the Global Energy Market in the next 25 years.

• Key drivers will be:– Falling costs for renewable

energy– Declining fossil fuel

production– Increasing energy demand

worldwide– Environmental concerns

Source: C.J.Campbell “World Oil Resources” Dec 2000

Oil industry consensus: production will peak between 2004 and 2010

17 October 2003 10

The Future of Renewables Projected World Energy Production

0

100

200

300

Co

al Oil

Gas

Nu

clea

r

Bio

mas

s

Hyd

ro

Win

d

So

lar

Geo

19992020

20402060

Exa

jou

les

Source: Royal Dutch Shell Group

17 October 2003 11

SunPower company history• 1985: Record efficiency Silicon Solar Cell developed at Stanford

Univ.

• 1988: SunPower formed to commercialize technology for concentrator applications

• 1993: SunPower supplies solar cells for Honda Dream, winner of World Solar Challenge

1994: Opto product line introduced1996: Honda invests1998: HP selects SunPower for IrDA detectors1998: Pegasus product line introduced.

17 October 2003 12

Company History (cont.)• 2000: SunPower ships 35 kW to

AeroVironment for Helios solar airplane.

• 2001: Helios flies to 96,500 ft.

• 2001: Low-cost, back-contact cell manufacturing process developed

• 2002: Cypress Semiconductor invests

• 2002: 21.1% efficiency one-sun in Austin, TX pilot line

17 October 2003 13

Solar cell operation

I

V

Isc

Vocdark

light

LInkT

qVII

1)exp(0

17 October 2003 14

Solar cell parameters

SCOC

MPMP

IV

IVFF

IN

MPMP

P

IVFill Factor: Efficiency:

17 October 2003 15

Solar spectrum

17 October 2003 16

SunPower solar cells• One-sun• Concentrator

Building integrated Remote industrial Remote for habitat

17 October 2003 17

SunPower one-sun Si solar cellA-300

5” semi-square

17 October 2003 18

Efficiency Losses in SiliconPractic

al Efficiency Limit

14.7%

24.6%

14.3%

4.4%

4.0% 4.0%

Silicon material intrinsic loss(Auger recombination, non-optimum bandgap)

Implementation loss

Resulting efficiency

Conventional Cell

29%SiliconLimit

Detailed balance limit 33%

17 October 2003 19

Conventional Solar Cell Loss Mechanisms

1.8%

0.4%

1.4%

1.54% 3.8%

2.6%

2.0%

0.4%

0.3%

I2R LossReflection Loss

RecombinationLosses

Back LightAbsorption

Limit Cell Efficiency 29.0%

Total Losses -14.3%

Generic Cell Efficiency 14.7%

17 October 2003 20

Popular Efficiency-Enhancing Processes

•Aluminium or boron back-surface field (BSF)Aluminium or boron back-surface field (BSF)•Silicon nitride ARCSilicon nitride ARC•Laser buried grid metallization. Laser buried grid metallization. •Selective emitterSelective emitter•Oxide passivation with restricted metal contact Oxide passivation with restricted metal contact openings.openings.•Rear surface reflector.Rear surface reflector.•Higher lifetime silicon wafersHigher lifetime silicon wafers

17 October 2003 21

Impact of High Efficiency Processes

14.7%14.8%

15.6%16.5% 16.8%

17.1%

18.3%

19.7%

21.2%

10.0%

12.0%

14.0%

16.0%

18.0%

20.0%

22.0%C

ell

Eff

icie

ncy

Conve

ntion

al Silic

on C

ell

High L

ifetim

e Bas

e

Back S

urfa

ce F

ield

(BSF)

Rear L

ocal

Conta

cts (

RLC)

Passiv

ated

Em

itter (

PE)

Selecti

ve E

mitt

er (S

E)

BSF+ PE

SE + R

CL

High L

ifetim

e +

SE + R

CL

17 October 2003 22

High-Efficiency Back-Contact Loss Mechanisms

Limit Cell Efficiency 29.0%

Total Losses -4.4%

Enabled Cell Efficiency 24.6%

0.5%

0.2%

0.8%

1.0%

1.0%

0.2%

0.3%0.2%

I2R Loss0.1%

17 October 2003 23

Efficiency vs Lifetime• A lower lifetime

– reduces the collection of minority carriers,

– increases bulk recombination.

• This effect is magnified in rear-contact solar cells.

• Conclusion: desire > 1 ms.

0

5

10

15

20

0.01 0.1 1 10

Minority-carrier lifetime (ms)C

ell e

ffic

ienc

y

(%)

17 October 2003 24

Efficiency vs Cell Thickness• A thinner cell

– increases the collection efficiency of minority carriers,

– reduces bulk recombination.

• But thinner cells lose photogenerated current because not all photons absorbed.

• Over range 160–280 um efficiency is about constant.

Simulated with t = 3 ms.

16

18

20

22

160 200 240 280 320

Cell thickness (um)

Cel

l eff

icie

ncy

(%

)

17 October 2003 25

Concentrators solar cells• Can achieve a higher efficiency because a higher carrier

density increases output voltage

Heda312 with cover glass Efficiency vs. Irradiance

10.00

15.00

20.00

25.00

30.00

0 5 10 15 20 25 30 35 40 45 50

Irradiance (W/cm2)

Eff

icie

ncy

(%

)

NREL

17 October 2003 26

Concentrator Solar Cells

HECO HEDA

17 October 2003 27

N+ N+ N+P+ P+ P+P+

One-sun Concentrator

n

1/ FSF FSF

n

1/

SiO2SiO2

17 October 2003 28

High efficiency Si Concentrators solar cellsCross section

Record efficiency=26.8% at 25W/cm2 Irradiance

Single Crystal Silicon

Front

Back Localized PointContacts

PassivatingOxide

Texture + ARC

N+ N+ N+P+ P+ P+P+

Gridlines

17 October 2003 29

Challenges in processing high efficiency Si solar cells

• Process thin wafers

• Anti-reflection coating

• Low temperature passivation

17 October 2003 30

Conclusions and future directions

• Solar generated energy will play a major role in energy generation

• One sun: high volume manufacturing of 20% efficiency solar cells

• Concentrators:– 30% Si cell– 6” wafers

17 October 2003 31

Acknowledgments

• Dr. Dick Swanson

• Dr. Akira Terao

• Dr. David Smith