advanced thin-film silicon solar cells

Miro ZemanDelft University of Technology, The Netherlands

Advanced Thin-Film Silicon Solar Cells

Acknowledgments:• Members of PVMD group at TUD• Nuon Helianthos, TU/e, UU, ECN, IPV Julich, OM&T, Ljubljana University• SenterNovem for financial support

Outline

Helianthos project

Status of thin-film Si solar cell technology

Improvement and technology issues

Conclusions

Si films with suppressed degradation

Photon management

World of PhotovoltaicsPV industry: the fastest growing industry in the world

0

1000

2000

3000

4000

5000

1999 2000 2001 2002 2003 2004 2005 2006 2007

202 287 401560 750

1256

1815

~3800

MW Solar cell production 1999-2007

42% 40% 39% 34%68%

45%

50%

Photon International, March 2007

2006: 90% wafer-type c-Si technology

Estimation market:

2007Cumulative installed capacity of PV systems

~ 9200 MW

Turnover (modules+BOS)

~ 15x109 €~ 70 000 jobs

2536

40%

World of PhotovoltaicsPV industry: announced increase in capacity

0,0

5.000,0

10.000,0

15.000,0

20.000,0

25.000,0

30.000,0

35.000,0

40.000,0

45.000,0Pr

oduc

tion

Cap

acity

[MW

]

2006 2007 2008 2009 2010 2012

Crystalline SiliconThin Films

Arnulf Jäger-Waldau, EU-PVSEC-23, Valencia, 2008

Thin-Film PhotovoltaicsPV industry: announced increase in capacity

0

2.000

4.000

6.000

8.000

10.000

12.000

[MW

]

2006 2007 2008 2009 2010 2012

silicon basedCdTeCISDye + others

Oerlikon

Applied Materials

Arnulf Jäger-Waldau, EU-PVSEC-23, Valencia, 2008

Thin-Film Si solar cell producers

EUROPE

Schott Solar Thin Film (Germany) a-Si/uc-Si.Ersol Thin Film (Germany) a-Si Single, a-Si/uc-SiSontor GmbH (Q-Cells) (German) a-Si/uc-SiSunfilm AG (Germany) a-Si/uc-SiMalibu GmbH (Germany) a-Si/uc-SiInventux Technologies AG (Germany) a-Si/uc-SiSignet Solar (Germany) a-SiMasdar PV (Germany) a-Si/uc-SiHelioGrid (Hungary) a-Si TandemSolar Plus (Portugal) a-Si TandemHeliodomi (Greece) a-Si TandemOerlikon (Switzerland) (equipment manufacturer) a-Si/uc-SiPramac SpA (Italy) a-Si/uc-SiT-Solar Global (Spain) a-Si/uc-SiNuon Helianthos (The Netherlands) a-Si/uc-SiIntico Solar (Switzerland/Germany) a-Si/uc-SiEnergo Solar (Hungary) (equipment manufacturer) a-SiFlexcell (Switzerland/Germany) a-Si

USA

United Solar Ovonic(ECD) (US) a-Si/a-SiGe/a-SiGeEPV Solar (US) a-Si TandemSignet Solar (US) a-Si SingleXsunX (US) a-Si/a-SiCOptisolar (US) a-SiXunlight (US) a-SiGe single, a-Si/a-SiGe/nc-Si tripleApplied Materials (US) (equipment manufacturer) a-Si/uc-Si

JAPAN

Kaneka Solartech (Japan) a-Si/Poly-SiSharp Thin Film (Japan) a-Si/uc-Si/a-Si GaAsSanyo Amorton(Japan) a-Si SingleMHI (Mitsubishi Heavy Industries) (Japan) a-Si/uc-SiFuji Electric Systems (Japan) a-Si/a-SiGe

CHINA

Topray Solar(Shenzhen China) a-Si TandemSoltechpv (Beijing China) a-Si TandemJinneng Solar (Tianjin China) a-Si TandemPolar PV (Anhui China) a-Si Single, TandemTrony (Shenzhen China) a-Si SingleSumoncle (Shenzhen China) a-Si Singlehksolar (Harbin China) a-Si SingleXinao Group (Hebe China) a-Si TandemSuntech (Shanghai China) a-Si/uc-SiBSTRPV (Weihan China) a-Si TandemChina Solar Power (Yantai China) a-Si SingleQS Solar (Nantong China) a-Si TandemYuanchang (Changzhou China) a-Si TandemGanneng Huaji (Jiangxi China) a-Si TandemGS Solar (Quanzhou China) a-Si TandemZhongshang Quanxin (Zhongshan China) a-Si TandemCineng PV (Hangzhou China) a-Si TandemShenyang Hanfeng (Shenyang China) a-Si TandemUni-Solar Jinneng (Tianjin China) a-Si/a-SiGe/a-SiGe

Thin-Film Si solar cell producers

TAIWAN

Green Energy Technology (Taoyuan Taiwan) a-Si SingleCMC (Taoyuan Taiwan) a-Si SingleYutong Light Energy (Tainan Taiwan) a-Si/uc-SiNexpower (Central Taiwan) a-Si SingleSunner Solar (Central Taiwan) a-Si SingleFormosun (Hsinchu Taiwan) a-Si TandemKenmos PV (Tainan Taiwan) a-Si TandemNanoWin (Tainan Taiwan) a-Si/uc-SiSinonar (Hsinchu Taiwan) a-Si Tandem

OTHER

Bangkok Solar (Thailand) a-Si TandemSolarMorph (Singapore) a-Si/uc-SiMoser Baer Photo Voltaic (India) a-Si SingleLambda Energia (Mexico) a-Si/a-SiC

In 2008 more than 60 companies

Strategic Research Agenda: EU roadmap

www.eupvplatform.org

Wim Sinke (ECN, Leader of WG 3 : Science, technology & applications of EU PV Technology Platform)

Current developments:• increase in TF Si solar-cell production (in 2010 ~ 8 GW capacity)• complete production lines available

Future developments:• short term: optimize tandem cell• long term: optimize triple cell, breakthrough concepts for high

efficiency (η>17%)

Thin-film Si solar cell technologyPresent status:+ Promising low-cost solar cell technology+ Industrial production experience (Flat panel display industry)- Relatively low stabilized efficiencies (η ≈6-7%)+ Double-junction micromorph solar cell (η>10%)

• ideal combination of materials (a-Si:H/μc-Si:H) for converting AM1.5 solar spectrum into electricity

Thin-film Si solar cell technology

Thin-film Si solar cells on glass

Power plant

Roof integration and new designs


Flexible thin-film Si solar cells

Roof integration

Consumer electronics

Stand-alonesystem

Flexible module


+-

p-typesc Si

Al

p++

Al Al

SiO2

n+

p++

c-Si (200-300 μm)

Thin film Si (0.3 - 5 μm)

•• Material usage Material usage strongly reducedstrongly reduced

•• Energy and cost Energy and cost strongly reducedstrongly reduced

Glass superstrate

TCO

p-type

Intrinsic a-Si:H

n-typeMetal electrode

+ -

a-Si:H (0.3-0.5 μm)


Gas system rf generator

electrode

electrode

substrate

heater

Pump system

reaction chamber

plasma

Plasma Enhanced CVD

+ Low deposition temperature+ Use of cheap substrates+ Large area deposition+ Easy doping and alloying

– Low deposition rate (1-2 Å/s)

High potential for low cost solar cells

Thin-film Si solar cell technology issues

1. Increase conversion efficiency

2. Eliminate light-induced degradation

3. Increase deposition rate

4. Choice of mass-production technology

5. Lower material costs

Single-junction a-Si:H solar cells:


Absorption of light• surface texture of the TCOs• ZnO back reflector

Extraction of the charge carriers• TCO/p interface• p-type window layer• p/i hetero-junction interface• quality of the intrinsic layer

Crucial parts of a-Si:H solar cell:

First a-Si solar cell made in 1974 by David Carlson.

-20

-15

-10

-5

0

5

-0.2 0.0 0.2 0.4 0.6 0.8 1.0Voltage [V]

Cur

rent

den

sity

[mA

cm-2]

initial

degraded

p-i-n a -Si:H Initial Degradedsolar cell

Jsc [mA/cm2] 16.2 15.7Voc [V] 0.75 0.74fill factor 0.69 0.64efficiency [%] 8.4 6.3

• Creation of extra metastable defects in a-Si:H under illumination

• Extra trapping and recombination centres

• Initial versus stabilized efficiency

Thin-film Si:H solar cell issues

Degradation of a-Si solar cells

Thin-film Si:H solar cells challenges

Increasing efficiency

Photon management• Textured substrates - scattering• Back reflector• Novel approaches

Multi-bandgap concept• Low band-gap materials

Suppressing degradation

Stable material• pc-Si:H, μc-Si:H or poly c-Si• New deposition techniques• Hydrogen diluted silane

Multi-junction concept• Tandem solar cells

300 500 700 900 1100 1300 1500Wavelength [nm]

Pho

ton

flux

[1027

ph

/ m3 s

]

4.13 2.48 1.77 1.38 1.13 0.95 0.83

Photon energy [eV]

5.0

4.0

3.0

2.0

1.0

0.0

a-Si a-SiGe

AM1.5 global solar spectrum

Increase efficiency

Multi-bandgap solar cell concept

Efficient use of solar spectrum

EFEner

gy

p ni

a-Si

EF

p ni

a-Si

EF

p nin pi

a-Si a-Si

Multi-junction solar cell concept

EF

p nin p

a-Si

i

a-SiGe or μc-Si

Suppress degradation

Thin-film Si:H solar cell structures

uc-Si:H bottom absorber

Ag ZnO

surface textured - TCOZnO:Al

glass

a-Si:H top absorber

a-Si :HGe middle absorber

surface textured TCO-

uc-Si:H absorber

Ag ZnO

surface textured - TCOZnO:Al

glass

a-Si:H absorber

interlayer


glass

uc-Si:H layers

back metal contact (Ag)

pin

ZnO

single-junctionamorphous (a-Si:H)microcrystalline (uc-Si:H)

double-junctionmicromorpha-Si:H/uc-Si:H

triple-junctione.g. a-Si:H/a-SiGe:H/uc-Si:H

Record ηst (confirmed) 9.5% (a-Si) Un. Neuchatel

10.1% (μc-Si) Kaneka

11.7% (a-Si/ μc-Si) Kaneka

12.4% (a-Si/a-SiGe) USSC* 13.0% (Si/SiGe/SiGe) USSC*

Proto-crystalline Si growth regime:• Effect of high H dilution of silane, dilution ratio R=[H2]/[SiH4]

Si:H films from hydrogen diluted silane

Si:H films from hydrogen diluted silane

Proto-crystalline Si growth regime:Glass substrate

Micromorph Si tandem solar cell

University of Neuchâtel (1994)

ZnO

µc-Si:H

a-Si:H

ZnO

glass

Micro-

morph

- Optimal bandgap combination

-1.7 eV (a-Si:H) / 1.1 eV (μc-Si:H)

400 600 800 1000

Spec

tral

Res

pons

e

Wavelength [nm]

morph

a-Si:H/a-Si:H

Microa-Si:H

µc-Si:H

- μc-Si:H cell (1-3 μm) stable

- a-Si:H cell (0.2-0.3 μm) unstable

Degradation of pc-Si:H solar cells

Degradation conditions:

0.1 1 10 100 1000 100001000000.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

1.05

Nor

mal

ized

effi

cien

cy

Illuminatino time (min)

R=0, 4.0 W, 0.7 mbar R=20, 4.0 W, 2.0 mbar

0 1x101 1x102 1x103 1x104 1x1050.70

0.75

0.80

0.85

0.90

0.95

1.00

norm

aliz

ed e

ffici

ency

time (minutes)

R=0 R=10 R=20 R=30 R=40

670 nm laser, 300 mW/cm2 Halogen lamp, 100 mW/cm2

Photon management

Proper handling of incident photons which have to

be trapped and absorbed in the absorber layers

of a solar cell

Photon management

Light trapping techniques:• Manipulation of light propagation: multiple passes

Engineering of optically-active layers(back and intermediate reflectors, layers for optical matching)

• Light scattering: change direction of propagation

Design of surface texture (random or periodically textured surfaces)

Trap photons in the absorber layer and enlarge their average path

Modeling of thin-film Si solar cells

Optical modeling:• Increase photocurrent

Understand light trapping

Evaluate optical losses

Design efficient light-trapping schemes

Integrated optical nad electrical modeling:• Increase Voc and FF

Evaluate recombination losses

Design material and interface properties

ASA (Advanced Semiconductor Analysis) program: Users:Utrecht University, Eindhoven University of Technology, ECN, Helianthos bv, OM&T

Stuttgart University, Kaiserslautern University, University of Siegen, University of Neuchatel, Cenimat, University of Gent, Slovak Academy of Sciences

Fuji Electric, Kaneka, Tokyo Institute of Technology, LG Electronics, Samsung

Toledo University, Syracuse University, Iowa State University, Applied Material, Ultradots, OptiSolar

Modelling

• ASA program has continuously been developed since 1987

• Focus on amorphous semiconductors andamorphous silicon based solar cells

• Extended for crystalline semiconductor materials (comparable with PC-1D)

• Multi-junction solar cells with TRJ

• Genpro3 optical model: combination of coherent propagation of specular light and incoherent propagation of scattered light

Light trapping

AP CVD SnO2:F Wet etched ZnO:Al

Standard techniques:• Random surface-textured substrates

Asahi U-type AP CVD SnO2:F, Julich wet-etched ZnO:Al

• Back reflectorThin ZnO layer between Si and metal

Light trapping

State-of-the-art uc-Si:H solar cell:

Wavelength (nm)

400 500 600 700 800 900 1000 1100 1200

AM

1.5

spe

ctro

m (m

W/(c

m2 um

))

0

20

40

60

80

100

120

140

160

180

di-uc-Si:H = 1 um

cell23.22 mA/cm2

10x


glass

uc-Si:H layers


pin

ZnO

Janez Krc, 2008

Wavelength (nm)400 600 800 1000

Abs

orpt

ion

loss

es

0.0

0.2

0.4

0.6

0.8

1.0

Rtot

BR

TCOsub.

i-uc-Si:H

p+n

Light trapping


glass

uc-Si:H layers


pin

ZnO

State-of-the-art uc-Si:H solar cell: analysis of optical losses using modeling

Janez Krc, 2008

Improving device performance

200 nm

2.2 um

Wavelength, λ (nm)400 600 800 1000

Qua

ntum

Effi

cien

cy, Q

E

0.0

0.2

0.4

0.6

0.8

1.0

QEtop QEbot

Rtot

10.0 mA/cm2 14.3 mA/cm2

Starting

Modelling of a-Si:H/μc-Si:H solar cells:

(ZnO)

M. Zeman and J. Krc, Optical and electrical modeling of thin-film silicon solar cells, J. Mater. Res., 23 (4), Apr 2008

Assumptions:

1. Enhanced scattering parameters• ideal haze parameters H=1• broad (Lambertian) angular distribution function (ADF) of scattered light

2. Reduced absorption in optically non-active layers• decrease absorption in the front TCO (5×)• decrease absorption in doped layers (5×)

3. Improved back reflector• back reflector with an enhanced reflectance of 98 %

4. Improved light in-coupling• optimized single-layer ARC coating on glass• (a) an optimized single-layer intermediate reflector (interlayer)

(b) a (hypothetical) wavelength-selective interlayer• adjustment of the absorbers thickness for obtaining current matching

M. Zeman

Optical improvements


Scattering angle, ϕ, (degrees)-90 -60 -30 0 30 60 90

AD

F T

0.0

0.2

0.4

0.6

0.8

1.0

σrms = 60 nm

Lambertian

ZnOetched

W a ve le n g th , λ (n m )60 0 70 0 80 0 9 0 0 1 00 0 1 10 0

Ref

lect

ance

, R (%

)8 0

85

90

95

1 00

ro ug h Z n O /A g

98 %

Lambertian angular distribution(cos) function of scattered light

Improved back reflector R=98%(distributed Bragg reflectors)


Optical improvements

Wavelength, λ (nm)400 600 800 1000

Qua

ntum

Effi

cien

cy, Q

E

0.0

0.2

0.4

0.6

0.8

1.0

QEtop QEbot

Rtot

15.65 mA/cm2 16.48 mA/cm2

Improved

200 nm

2.2 um

(ZnO)




Rel

. con

tribu

tion

to in

crea

sed

J SC

(opt

) (%

)

0

5

10

15

20

25

30

35

40

45

50

55top (a-Si:H) cellbottom (uc-Si:H) cell

BR 98%5x lower abs.TCO p,n

H = 1 Lamb.ADF

single ARC

interl.(sel.)

- Broad ADF of scattered light (H=1)- BR and interlayer

Voltage, V (V)0.00 0.25 0.50 0.75 1.00 1.25 1.50

Cur

rent

den

sity

, J (m

A/c

m2 )

-16

-14

-12

-10

-8

-6

-4

-2

0

JSC = 10.41 mA/cm2 15.45 mA/cm2

VOC = 1.35 V 1.45 V

FF = 0.71 0.71Eff. = 10 % 15.8 %

starting cell

opt. + el.improved

starting cell

optically + electricallyimproved

Develop concepts and test them




Light-In projectTUD, Helianthos

ECN, OM&T

• Wavelength-selective manipulation of reflection and transmission of light at interfaces using 1-D photonic crystals

• Concept of modulated 1-D photonic crystals

• Applied as back and intermediate reflectors

Advanced concepts for light trapping

Wavelength (nm)600 800 1000 1200 1400

Ref

lect

ance

0.0

0.2

0.4

0.6

0.8

1.0 100 %

PC_1

50/100 nm

Wavelength (nm)

600 800 1000 1200 1400

Ref

lect

ance

0.0

0.2

0.4

0.6

0.8

1.0 100 %

PC_2

70/140 nm


ECN, OM&T

• Angle-selective manipulation of light scattered at the rough interfaces using 1-D and 2-D diffraction gratings

Scattering angle (ϕscatt)

-90 -60 -30 0 30 60 90

AD

F T (a.

u.)

0.0

0.2

0.4

0.6

0.8

1.0

P = 700 nmh = 80 nm

Asahi U-type

ϕinc = 0o

ZnO:Al(40" etched)σr � 110 nm


0 50 100 150 200 250 3007.4

7.6

7.8

8.0

8.2

8.4

8.6

8.8

9.0

Period = 600 nm

Asahi reference

Ave

rage

Effi

cien

cy (%

)

Feature height (nm)


ECN, OM&T

Average efficiency of 10 best cells plotted versus groove height


• Angle-selective manipulation of light scattered at the rough interfaces using 1-D diffraction gratings


ECN, OM&T

0 50 100 150 200 250 3007.47.67.88.08.28.48.68.89.09.29.4

Period = 600 nm 2D period = 500-800 nm

Asahi reference

Ave

rage

Effi

cien

cy (%

)

Feature height (nm)


• Angle-selective manipulation of light scattered at the rough interfaces using 2-D diffraction gratings

Helianthos project

• Development of low-cost roll-to-roll technology for fabrication of thin-film silicon solar modules (started in 1996)

• Dutch route: Temporary superstrate solar cell concept

By courtesy of Helianthos bv.

Helianthos manufacturing sequence

- Al foil

Al foil + TCO + a-Si:H + back contact + carrier foil

+ series connect + contact wires+ cutting

+ encapsulant

Status Helianthos project

Flexible lab-size tandem moduleFlexible a-Si:H module: ready for production

1st generation modulesSingle junction a-Si:H module ηin > 7%ηst = ~6%

2nd generation modulesTandem a-Si:H/μc-Si:H module ηin > 11%ηst = ~10%

Achieved: Challenge:

By courtesy of Helianthos bv.

Summary

Thin-film Si:H solar cell technology• Promising future option for large-area low-cost PV

• Expected large increase in production capacity

• Large scale of applications (rigid + flexible)

• Modules with 10% efficiency

Challenges:• Increase efficiency ηst>15% (photon management)

• Development and implementation of novel ideas

advanced thin-film silicon solar cells

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