Inorganic Thin Films:Future Perspectives

Global Climate Energy ProjectSolar Energy Workshop: Thin-Film Photovoltaics

October 19, 2004John P. Benner

Division Manager Electronic Materials and DevicesNational Center for Photovoltaics

Future Perspectives from 1975

2004 CdTe

25.90.845

0.75516.5

Effic

iency

(%)

CuInSe2

CdTe

Amorphous silicon

(stabilized)

Univ. of Maine

Boeing

Boeing

Boeing

Boeing

ARCO

AMETEK

NREL

Boeing

EuroCIS

Univ. of So. Florida

Univ. of So. FL

BP Solar

Kodak

KodakMonosolar

Matsushita

12

8

4

0200019951990198519801975

United Solar

16

20

NREL

2005

RCA

ECD

NREL

Photon Energy

026587181

Effic

iency

(%)

CuInSe2

CdTe

Amorphous silicon

(stabilized)

Univ. of Maine

Boeing

Boeing

Boeing

Boeing

ARCO

AMETEK

NREL

Boeing

EuroCIS

Univ. of So. Florida

Univ. of So. FL

BP Solar

Kodak

KodakMonosolar

Matsushita

12

8

4

0200019951990198519801975

United Solar

16

20

NREL

2005

RCA

ECD

NREL

Photon Energy

026587181

Best ResearchBest Research--Cell EfficienciesCell EfficienciesThinThin--Film Film InorganicsInorganics

World PV Cell/Module Production (MW)World PV Cell/Module Production (MW)

1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 20010

100

200

300

400

Rest of worldEuropeJapanU.S.

33.6 40.2 46.5 55.4 57.9 60.1 69.4 77.6 88.6125.8

154.9201.3

287.7

390.5

2002

500

Source: PV News, March 2004

600 561.8

700

800744.1

2003

~50 MW Thin-Film in 2003

U.S. Thin-Film Manufacturing

0

5

10

15

20

25

1996 1999 2002 2005

a-Si CIS CdTe

Projected

Think Big -- Very Big 2-5 GW Factories

05

10152025303540

1990 1995 2000 2005 2010 2015 2020

GW

Ann

ual M

odul

e Pr

oduc

tion

Ground Rules for GW Scale Factories

• Dedicated float glass line• 5x reduction in glass cost ~$4/m2 finished

• Redundant cluster production tools• 8x reduction in capital cost • ~100 20MW deposition clusters @ $2M each

• Recycled Effluents• 75% utilization

• Advanced Packaging Factory• Aluminum Extruding and Fabrication

M. Keshner, et al, Hewlett Packard Final Rpt NREL#ADJ-3-33631-01

page 10September 20, 2004 hp confidential

Solar Factory ModuleCost Comparisons for Completed Solar Panels

Cost Summary 20 M W Plant 2 GW Plant N et Gain(all numbers are per sq. meter)

Coated Glass $ 23 .62 $ 4 .62 5 x

Operating Ex penses $ 4 .00 $ 1 .50 2 .5x

M ateria ls and deprecia tiona - Si $ 2 .33 + $ 13 .35 $ 0 .31 + $ 2 .67 5xCdTe $ 3 .46 + $ 10 .00 $ 2 .31 + $ 2 .00 7 .5xCuInGaSe2 $ 13 .96 + $ 1 3 .35 $ 9 .3 1 + $ 2 .67 7 .5x

Assembly, Packaging $ 41 .71 $ 1 0 .50 4x & Interconnect

Overa ll process yield 60 % 93 % 1 .55x

Tota l manufacturing cost per w atta - Si ( 7%) $ 2 .02 $ 0 .30

CdTe (11%) $ 1 .25 $ 0 .21CuInGaSe2 (12%) $ 1 .34 $ 0 .26

+

225 volts_

Complete solar panel ready for simple a ttachment onto a roof

Notes: If CdTe and CuInGaSe2 could use effective light trapping and be reduced in thickness to 0.4 um like a-Si, then their cost per W p would be $.19 and $.19, respectively.If a-Si could use a second junction of a-SiGe or uc Si, its efficiency would be circa 10% and its cost per W p would be $.21.

page 10September 20, 2004 hp confidential

Solar Factory ModuleCost Comparisons for Completed Solar Panels

Cost Summary 20 M W Plant 2 GW Plant N et Gain(all numbers are per sq. meter)

Coated Glass $ 23 .62 $ 4 .62 5 x

Operating Ex penses $ 4 .00 $ 1 .50 2 .5x

M ateria ls and deprecia tiona - Si $ 2 .33 + $ 13 .35 $ 0 .31 + $ 2 .67 5xCdTe $ 3 .46 + $ 10 .00 $ 2 .31 + $ 2 .00 7 .5xCuInGaSe2 $ 13 .96 + $ 1 3 .35 $ 9 .3 1 + $ 2 .67 7 .5x

Assembly, Packaging $ 41 .71 $ 1 0 .50 4x & Interconnect

Overa ll process yield 60 % 93 % 1 .55x

Tota l manufacturing cost per w atta - Si ( 7%) $ 2 .02 $ 0 .30

CdTe (11%) $ 1 .25 $ 0 .21CuInGaSe2 (12%) $ 1 .34 $ 0 .26

+

225 volts_

Complete solar panel ready for simple a ttachment onto a roof

Notes: If CdTe and CuInGaSe2 could use effective light trapping and be reduced in thickness to 0.4 um like a-Si, then their cost per W p would be $.19 and $.19, respectively.If a-Si could use a second junction of a-SiGe or uc Si, its efficiency would be circa 10% and its cost per W p would be $.21.

Time to Production:Processes must be better characterized

CIGS - Shellα-Si:H - BP

CdTe – First Solar

0%

20%

40%

60%

80%

100%

120%

Feb-97

Nov-97

Sep-98

Jan-99

Mar-99

Dec-99

Mar-00

Sep-00

Dec-00

1-Jul

Dec-01

Month / Year

Perc

enta

geof

Cap

acity

0.0%10.0%20.0%30.0%40.0%50.0%60.0%70.0%80.0%90.0%100.0%

Yiel

d

16.5% Efficient CdTe Solar Cells

Back-contact (C:HgTe:CuTe)

CdTe (~10 µm)

CdS (0.07 - 0.1 µm)

Zn2SnO4 (0.1 -0.2 µm)

Cd2SnO4 (0.15 -0.3 µm)

Glass substrate

Front-contact (In) CdS – ZTO interdiffuse

CdSxTe1-xO content in CdS

Cu diffusionAnneal in CdCl2

Thin Film Cells are….. ThinAdvantages•Low material consumption

•High throughput potential

•Module patterning

•Improved carrier generation profile

•Drift collection

•Flexible

•Semi-transparent

Challenges•Unique Materials

•Interdiffusion

•Grain size

•Low-lifetime

•Drift collection

•Characterization

Effect of Back Contacts Deposition Temperature on Thin-Film CdTe Solar Cell Performance

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⊗⊗

⊗

⊗

25

35

45

55

65

75

200 240 280 320 360 400

Fill

Fact

or (%

)

Contact Deposition Temperature (°C)

⊗⊗⊗⊗⊗⊗⊗⊗

⊗

⊗

⊗⊗⊗⊗⊗⊗⊗

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⊗

⊗

⊗

450

550

650

750

850

200 240 280 320 360 400

Ope

n C

ircui

t Vol

tage

(mV)

Contact Deposition Temperature (°C)

Cu Diffusion from CdCl2 and Contact Processes

High-Resolution SIMS of Cu Concentration

Quantified

1016

1017

1018

1019

1020

1021

Con

cent

ratio

n (a

t/cm

3 )

6543210Depth (µm)

Cu Profiles As grown CdCl2 treated W5 USF W11 NREL W7 NREL ZnTe

ZnTe | CdTe | CdS | SnO2

Un-quantified

100

101

102

103

104

105

106

Seco

ndar

y io

n co

unts

543210Depth (µm)

no contact no CdCl2 no contact wet CdCl2 207¡C 285¡C 335¡C 390¡C

CdTe | CdS | SnO2

ZnTe:Cu

Combined EBIC of ZnTe:Cu Contacted Devices

Think High

• High efficiency– Multi junction– Highly ordered, oriented films– Single crystal

• High rate deposition

Polycrystalline Thin Film Tandem Solar Cell

7059 Cornning glassCTOZTO

S-CdS:O

CdTe

CuxTe back-contactITO

Ni/Al grids

In contact

c-ZTO / i-ZnOCBD-CdS

CISMo

Soda-lime glass

Ni/Al grids

In contact

CdTe top cellAchieved 50%transmission,12.7% efficiency

CIS bottom cellAchieved 14.5% efficiency

FY06 milestone: 15% efficient 4-terminal device will be met one year early

Red QE equals USSC bottom cell

1.0

0.8

0.6

0.4

0.2

0.0

QE

1000900800700600500400300Wavelength (nm)

Q_L1067Q_T2358

µC-Si

a-SiGe

Film c-Si on glass concept

glass

epitaxially thickened c-Si

c-Si seed layer

• Many approaches to both seed and epitaxy under study

See, review by Bergmann & Werner, Thin Sol Films 2002

Ni-seeded c-Si template / HWCVD c-Si

glassPoly c-Si (Ni)

300°C HW poly c-Si

• Si lifetime > ~10 µs– H in grain boundaries?

• Poly-Si growth rate 1 Å/s --> slow at ~3 hr per µm– heavy H2 dilution

glass

Ni

glassSolid-phase crystallized a-Si

Richardson et al, MRS Spring A, 2004

Single Grain Si Films Induced by Hydrogen Plasma Seeding

Single nucleus achieved for holes <0.6 µm

Bo et al, JVST B. May 2002

Ta wire improves epitaxy

(100) substrate

• Ta filament: about 350 nm epitaxy

• W filament:50 to 100 nm epitaxy

• ~ 3Å/s at 270°C a-Si:H cone strained c-Si

Think Small• Defects and nanostructure

• Thinner Devices

CIGS, Ga/(In+Ga)=28.5%SKPMAFM

5 µm 5 µm

G1 G1

G2 G2

100 Pote

ntia

l (m

V)

0 2 4 6 8 10Distance (µm)

Hei

ght (

nm)

100

• Potential height: ~150 mV• Depletion width: 150~400 nm.

Cu0.9(In1-xGa0.30)1.1Se2

Hei

ght o

f pot

entia

l pea

k (m

V)Ef

fi cie

ncy

η (

%)

0

50

100

150

200

0 20 40 60 80 100Ga Content Ga/(In+Ga) (%)

8

12

16

20

24

(a)

(b)28

32

Measured efficiency

Predicted from band gapM. A. Green, Solar Cells, P. 89

1.00E-06

1.00E-05

1.00E-04

1.00E-03

1.00E-02

1.00E-01

1.00E+00

0 0.2 0.4 0.6 0.8 1 1.2X = Ga/(In+Ga)

Jo (m

A/c

m^2

)

1

2

3

4

5

Idea

lity

Fact

or A

Jo A

Difficulty for Ga>30%

• Difficult to dope n-type• Difficult to form n-type Cu-poor layers

S.-H. Wei et al APL 1998

Quantum Efficiency of CIGS Solar Cells

How is a Crystal 10% Cu Poor?Cu2Se

CuInSe2

CuIn3Se5

Cu2In4Se7

CuIn5Se8•••

Zhang et al Phys Rev B 1998

Neutral Defect Complex

(2VCu– + InCu

2+)

Phase SegregationMaterial immediately surrounding dislocations and grain boundaries in device-quality CI(G)S will have higher bandgap

The α/β hole mirror disappears at [Ga]/[Ga+In] ≅ 35%

Stanbery TBP

Million M2 per Year

10

100

10000

1000

1

10 100 1000 10000

Low Cost Processes

Large-Area Optical and Electronic Materials

Glass

Coated Glass

Paint

FPD

PV

SolarFuels

Electrode

Fuel Cell Bipolar plate

$/M2

1

United Solar Shingles

Advances in PV System Design Achieve Cost Advantages

Uni-Solar Amorphous Silicon Field Applied Roofing Products in units to 128W (18’x16”, 17 lbs, 33V & 3.88 A)

Summary• Inorganic Thin-Film PV is on the threshold of

increasing market presence. • Potential for further improvement

• 10x reduction in $/m2• 2-3x increase in module efficiency

• Current fundamental understanding in all material systems contains large gaps

• Large entry investments will demand improved understanding and predictive capability.

• Shared production infrastructure simplifies start-up and growth

Presented with Great Appreciation for the original work, contributions, discussions

and figures from my colleagues:• Mowafak Al-Jassim• Sally Asher• Howard Branz• Miguel Contreras• Tim Coutts• Tim Gessert• Falah Hasoon• Chun-Sheng Jiang• Rommel Noufi

• K. Ramanathan• Manuel Romero• Su-Huai Wei• Xuanzhi Wu• Yanfa Yan• Alex Zunger• Ken Zweibel• Marvin Keshner (HP)• B. Stanbery (HelioVolt)