Advanced Thin-Film Silicon Solar Cells Miro Zeman Delft University of Technology, The Netherlands

Acknowledgments: • • •

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

Outline ¾ Status of thin-film Si solar cell technology ¾ Improvement and technology issues ¾ Si films with suppressed degradation ¾ Photon management ¾ Helianthos project ¾ Conclusions

World of Photovoltaics PV industry: the fastest growing industry in the world MW

Solar cell production 1999-2007

5000

2006: 90% wafer-type c-Si technology

~3800

4000 3000

1000

202

42%

40%

39%

34%

40%

Cumulative installed capacity of PV systems ~ 9200 MW

45%

750

560

401

287

1256

2007

50%

1815

2000

0

Estimation market:

2536

68%

Turnover (modules+BOS) ~ 15x109 €

1999

2000

2001

2002

2003

2004

2005

2006

2007

Photon International, March 2007

~ 70 000 jobs

World of Photovoltaics PV industry: announced increase in capacity 45.000,0

Crystalline Silicon Thin Films

Production Capacity [MW]

40.000,0 35.000,0 30.000,0 25.000,0 20.000,0 15.000,0 10.000,0 5.000,0 0,0 2006

2007

2008

2009

2010

2012

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

Thin-Film Photovoltaics PV industry: announced increase in capacity 12.000

10.000

silicon based CdTe CIS Dye + others

Applied Materials

[MW]

8.000

6.000 Oerlikon

4.000

2.000

0 2006

2007

2008

2009

2010

2012 Arnulf Jäger-Waldau, EU-PVSEC-23, Valencia, 2008

Thin-Film Si solar cell producers EUROPE

USA

Schott Solar Thin Film (Germany) a-Si/uc-Si. Ersol Thin Film (Germany) a-Si Single, a-Si/uc-Si Sontor GmbH (Q-Cells) (German) a-Si/uc-Si Sunfilm AG (Germany) a-Si/uc-Si Malibu GmbH (Germany) a-Si/uc-Si Inventux Technologies AG (Germany) a-Si/uc-Si Signet Solar (Germany) a-Si Masdar PV (Germany) a-Si/uc-Si HelioGrid (Hungary) a-Si Tandem Solar Plus (Portugal) a-Si Tandem Heliodomi (Greece) a-Si Tandem Oerlikon (Switzerland) (equipment manufacturer) a-Si/uc-Si Pramac SpA (Italy) a-Si/uc-Si T-Solar Global (Spain) a-Si/uc-Si Nuon Helianthos (The Netherlands) a-Si/uc-Si Intico Solar (Switzerland/Germany) a-Si/uc-Si Energo Solar (Hungary) (equipment manufacturer) a-Si Flexcell (Switzerland/Germany) a-Si

United Solar Ovonic(ECD) (US) a-Si/a-SiGe/a-SiGe EPV Solar (US) a-Si Tandem Signet Solar (US) a-Si Single XsunX (US) a-Si/a-SiC Optisolar (US) a-Si Xunlight (US) a-SiGe single, a-Si/a-SiGe/nc-Si triple Applied Materials (US) (equipment manufacturer) a-Si/uc-Si JAPAN Kaneka Solartech (Japan) a-Si/Poly-Si Sharp Thin Film (Japan) a-Si/uc-Si/a-Si GaAs Sanyo Amorton(Japan) a-Si Single MHI (Mitsubishi Heavy Industries) (Japan) a-Si/uc-Si Fuji Electric Systems (Japan) a-Si/a-SiGe

Thin-Film Si solar cell producers CHINA

TAIWAN

Topray Solar(Shenzhen China) a-Si Tandem Soltechpv (Beijing China) a-Si Tandem Jinneng Solar (Tianjin China) a-Si Tandem Polar PV (Anhui China) a-Si Single, Tandem Trony (Shenzhen China) a-Si Single Sumoncle (Shenzhen China) a-Si Single hksolar (Harbin China) a-Si Single Xinao Group (Hebe China) a-Si Tandem Suntech (Shanghai China) a-Si/uc-Si BSTRPV (Weihan China) a-Si Tandem China Solar Power (Yantai China) a-Si Single QS Solar (Nantong China) a-Si Tandem Yuanchang (Changzhou China) a-Si Tandem Ganneng Huaji (Jiangxi China) a-Si Tandem GS Solar (Quanzhou China) a-Si Tandem Zhongshang Quanxin (Zhongshan China) a-Si Tandem Cineng PV (Hangzhou China) a-Si Tandem Shenyang Hanfeng (Shenyang China) a-Si Tandem Uni-Solar Jinneng (Tianjin China) a-Si/a-SiGe/a-SiGe

Green Energy Technology (Taoyuan Taiwan) a-Si Single CMC (Taoyuan Taiwan) a-Si Single Yutong Light Energy (Tainan Taiwan) a-Si/uc-Si Nexpower (Central Taiwan) a-Si Single Sunner Solar (Central Taiwan) a-Si Single Formosun (Hsinchu Taiwan) a-Si Tandem Kenmos PV (Tainan Taiwan) a-Si Tandem NanoWin (Tainan Taiwan) a-Si/uc-Si Sinonar (Hsinchu Taiwan) a-Si Tandem OTHER Bangkok Solar (Thailand) a-Si Tandem SolarMorph (Singapore) a-Si/uc-Si Moser Baer Photo Voltaic (India) a-Si Single Lambda 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)

Thin-film Si solar cell technology Present 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

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 technology Thin-film Si solar cells on glass

Power plant

Roof integration and new designs

Thin-film Si solar cell technology Flexible thin-film Si solar cells Stand-alone system

Consumer electronics

Flexible module

Roof integration

Thin-film Si solar cell technology Al

Al

SiO

n+

2

Glass superstrate

Thin film Si (0.3 - 5 μm)

TCO p-type

-

• Material usage strongly reduced + -

+

Intrinsic a-Si:H

p-type sc Si p++

p++ Al

c-Si (200-300 μm)

• Energy and cost strongly reduced n-type Metal electrode

a-Si:H (0.3-0.5 μm)

Thin-film Si solar cell technology Plasma Enhanced CVD

heater

reaction chamber

+ + + +

electrode substrate plasma electrode

Gas system

rf generator

Pump system

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

Thin-film Si solar cell technology Single-junction a-Si:H solar cells: Crucial parts of a-Si:H solar cell: 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

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

Thin-film Si:H solar cell issues Degradation of a-Si solar cells Current density [mA cm-2]

5 0 -5 -10

p-i-n a -Si:H solar cell

Initial

Jsc [mA/cm 2]

16.2

15.7

0.75 Voc [V] fill factor 0.69 efficiency [%] 8.4

0.74 0.64 6.3

Degraded

initial

-15

degraded -20 -0.2

0.0

0.2

0.4 Voltage [V]

0.6

0.8

1.0

• 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 cells challenges Suppressing degradation

Increasing efficiency

Stable material

Photon management

• • •

• • •

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

Textured substrates - scattering Back reflector Novel approaches

Multi-junction concept

Multi-bandgap concept

•

•

Tandem solar cells

Low band-gap materials

Efficient use of solar spectrum Suppress degradation

Increase efficiency Photon energy [eV]

p

i

np i

i

4.13

n

2.48

5.0

a-Si a-SiGea-Si or μc-Si

1.38

1.13

0.95

0.83

AM1.5 global solar spectrum

4.0

3.0

27

EF

Photon flux [10

Energy

3

ph / m s ]

a-Si a-Si

1.77

2.0 1.0

a-Si

a-SiGe

0.0 300

Multi-junction solar cell concept

500

700 900 1100 Wavelength [nm]

1300

1500

Multi-bandgap solar cell concept

Thin-film Si:H solar cell structures single-junction amorphous (a-Si:H) microcrystalline (uc-Si:H)

p i n

double-junction micromorph a-Si:H/uc-Si:H

triple-junction e.g. a-Si:H/a-SiGe:H/ uc-Si:H

glass

glass

glass

surface-textured TCO

surface- textured TCO ZnO:Al

surface- textured TCO ZnO:Al

uc-Si:H layers

a-Si:H absorber

a-Si:H top absorber

ZnO

surface-textured TCO interlayer

a-Si Ge:H middle absorber

back metal contact (Ag)

uc-Si:H absorber

ZnO Ag

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

11.7% (a-Si/ μc-Si) Kaneka

10.1% (μc-Si) Kaneka

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

uc-Si:H bottom absorber

ZnO Ag

13.0% (Si/SiGe/SiGe) USSC*

Si:H films from hydrogen diluted silane Proto-crystalline Si growth regime: •

Effect of high H dilution of silane, dilution ratio R=[H2]/[SiH4]

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 Micro-

Spectral Response

a-Si:H

Micro morph

µc-Si:H

a-Si:H/ a-Si:H

ZnO morph

400 glass

600 800 Wavelength [nm]

1000

- Optimal bandgap combination

- μc-Si:H cell (1-3 μm) stable

-1.7 eV (a-Si:H) / 1.1 eV (μc-Si:H)

- a-Si:H cell (0.2-0.3 μm) unstable

Degradation of pc-Si:H solar cells Degradation conditions: 670 nm laser, 300 mW/cm2

Halogen lamp, 100 mW/cm2 1.00

1.05

0.95

0.95 0.90 0.85 0.80 0.75 0.70

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

0.65 0.60 0.1

normalized efficiency

Normalized efficiency

1.00

0.90 0.85 0.80 0.75 0.70

1

10

100

1000 10000 100000

Illuminatino time (min)

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

1

1x10

2

3

1x10 1x10 time (minutes)

4

1x10

5

1x10

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

Modelling ASA (Advanced Semiconductor Analysis) program: • ASA program has continuously been developed since 1987 • Focus on amorphous semiconductors and amorphous 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

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

Light trapping Standard techniques: • Random surface-textured substrates Asahi U-type AP CVD SnO2:F, Julich wet-etched ZnO:Al

• Back reflector Thin ZnO layer between Si and metal

AP CVD SnO2:F

Wet etched ZnO:Al

Light trapping State-of-the-art uc-Si:H solar cell: 180

di-uc-Si:H = 1 um

140

glass

10x

2

AM 1.5 spectrom (mW/(cm um))

160

120

surface-textured TCO

100 80

p i n

cell 23.22 mA/cm2

60

uc-Si:H layers ZnO back metal contact (Ag)

40 20 0

400

500

600

700

800

900

Wavelength (nm)

1000

1100

1200 Janez Krc, 2008

Light trapping State-of-the-art uc-Si:H solar cell: analysis of optical losses using modeling 1.0

Absorption losses

0.8

Rtot

p+n

glass

0.6

i-uc-Si:H

BR

0.4

TCO sub.

surface-textured TCO

p i n

uc-Si:H layers ZnO back metal contact (Ag)

0.2

0.0 400

600

800

Wavelength (nm)

1000 Janez Krc, 2008

Improving device performance Modelling of a-Si:H/μc-Si:H solar cells: Starting 1.0

200 nm

(ZnO)

2.2 um

Quantum Efficiency, QE

0.8

0.6

QEtop

QEbot

10.0 mA/cm2

14.3 mA/cm2

0.4 Rtot

0.2

0.0

400

600

800

Wavelength, λ (nm)

1000

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

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

M. Zeman

Optical improvements 1.0

100 98 %

Lambertian

0.8

ro u g h Z n O /A g

Reflectance, R (%)

95

ADFT

0.6 ZnO etched

0.4

σrms = 60 nm

85

0.2 0.0 -90

90

-60

-30

0

30

60

90

Scattering angle, ϕ, (degrees)

Lambertian angular distribution (cos) function of scattered light

80 600

700

800

900

W a v e le n g th , λ (n m )

1000

1100

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

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

Improving device performance Modelling of a-Si:H/μc-Si:H solar cells: Improved

1.0

200 nm

(ZnO)

2.2 um

Quantum Efficiency, QE

0.8

QEtop

0.6

QEbot

15.65 mA/cm2

16.48 mA/cm2

0.4

Rtot

0.2

0.0

400

600

800

Wavelength, λ (nm)

1000

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

Improving device performance Modelling of a-Si:H/μc-Si:H solar cells:

Current density, J (mA/cm2)

-2 -4 -6

opt. + el. improved

starting cell

JSC = 10.41 mA/cm2 15.45 mA/cm2 VOC = 1.35 V

1.45 V

FF = 0.71

0.71

Eff. = 10 %

15.8 %

-8 -10

starting cell

-12 -14 -16

optically + electrically improved

0.00 0.25 0.50 0.75 1.00 1.25 1.50

Voltage, V (V)

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

Rel. contribution to increased JSC(opt) (%)

55 0

top (a-Si:H) cell bottom (uc-Si:H) cell

50 45 40 35 30 25 20 15 10 5 0

H=1

Lamb. ADF

5x lower abs. BR 98% single interl. TCO p,n ARC (sel.)

Develop concepts and test them

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

Advanced concepts for light trapping •

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

1.0

100 %

PC_1

0.6

0.4

50/100 nm

0.2

0.0

TUD, Helianthos ECN, OM&T

100 %

PC_2

0.8 Reflectance

0.8 Reflectance

1.0

Light-In project

0.6

0.4

70/140 nm

0.2

600

800

1000 Wavelength (nm)

1200

1400

0.0

600

800

1000 Wavelength (nm)

1200

1400

Advanced concepts for light trapping Angle-selective manipulation of light scattered at the rough interfaces using 1-D and 2-D diffraction gratings Light-In project TUD, Helianthos ECN, OM&T 1.0

ϕinc = 0o

P = 700 nm h = 80 nm

Asahi U-type

0.8

ADFT (a.u.)

•

0.6

0.4

0.2

0.0 -90

ZnO:Al (40" etched) σr 110 nm -60

-30

0

30

Scattering angle (ϕscatt)

60

90

Advanced concepts for light trapping •

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

9.0

Average efficiency of 10 best cells plotted versus groove height

Average Efficiency (%)

8.8 8.6

Asahi reference

8.4 8.2 8.0 7.8 7.6 7.4

Period = 600 nm 0

50

100

150

200

Feature height (nm)

250

300

Light-In project TUD, Helianthos ECN, OM&T

Advanced concepts for light trapping •

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

Average Efficiency (%)

Light-In project 9.4 9.2 9.0 8.8 8.6 Asahi reference 8.4 8.2 8.0 7.8 Period = 600 nm 7.6 2D period = 500-800 nm 7.4 0 50 100 150 200 250 300 Feature height (nm)

TUD, Helianthos ECN, OM&T

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 + TCO

- Al foil

+ a-Si:H

+ series connect

+ back contact

+ carrier foil

+ contact wires + cutting

+ encapsulant

Status Helianthos project

By courtesy of Helianthos bv.

Flexible a-Si:H module: ready for production

Flexible lab-size tandem module

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

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

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