Solarzellen aus dünnen Siliziumschichten – Stand der ...Thin Film Solar Cell Technologies and...

Solarzellen aus dünnen Siliziumschichten –Stand der Technik und Herausforderungen für die Zukunft

Bernd RechHelmholtz-Zentrum Berlin (HZB) and Technische Universität Berlin

Many thanks to my colleagues from HZB and FZ-Jülich (Uwe Rau et al.), Michael Powalla from ZSW – Stuttgart and industry partners

WIAS 2008

2Bernd Rech, WIAS 2008

Outline

� Motivation and Background� Thin Film Solar Cell Technologies and Applications� Amorphous and Microcrystalline Based Silicon and Tandem Cells� Poly-Crystalline Si Thin-Films� R&D Challenges and Conclusions


Thermodynamic limits – Generation of electricity in a Carnot process:

Carnot efficieny = Tsun – Tearth

Tsun

= 95 %

This is an absolute upper limit, however, unavoidable lossesof entropy reduce the thermodynamic limit towards 85 %(see e.g. Würfel, Physik der Solarzellen)

Note: due to the Tsun of 5800 K solar radiation is of high energetic value

“Theoretical Efficiency”


++++

++++

----

----

x

E

pppp

nnnn

----

++++

Thermalisation

Recombination

Reflection

LBLBLBLB

VBVBVBVB

EEEEGapGapGapGap

---- --------

++++++++

++++

Working Principle and Losses (p/n-junction solar cell)

voltagevoltagevoltagevoltage

ηηηη====PPPPLichtLichtLichtLicht

PPPPmaxmaxmaxmax

Working point PPPPmaxmaxmaxmax

currentcurrentcurrentcurrentdenisitydenisitydenisitydenisity

jjjjSCSCSCSC

VVVVOCOCOCOC

FF=PPPPmaxmaxmaxmax

VVVVOCOCOCOC * * * * jjjjSCSCSCSC

losses


(η = 12 – 17 %)

c-Si solar cell

c-Si wafer technology a-Si thin-filmtechnology

(η = 5 – 7 %)Si-thickness200-300 µm

Si-thickness0.5 µm

“Some Commercial Efficiencies”


Source Sontor

Source Solar Integrated TechnologiesSource Sulfurcell

Thin film advantages

� Material usage/cost (1-5 vs 200 µm)

� High productivity (large area)

� Monolithic series connection

� Short energy pay back time

� New products (e.g.. flexible)


Sol

ar C

ells Compound

Semiconductor

Silicon

Organic

Dye cells

New Concepts

CdTe

ChalkopyrideCIGSSe

GaAs (conc)

Thin Film

Crystallinewafers Mono

Poly

kristallin

amorphmikrokristallin

CdTe

ChalcopyriteCIGSSe

aSi/µcSi

Crystalline

Based on M. Powalla,,ZSW

Overview of photovoltaic material classes

This talk

8Bernd Rech, WIAS 2008 Source: Brabec, MRS Bulletin, Jan. 2005

Evolution of Record Solar Cells

a-Si,µc-Si∼14%

CdTe∼17%∼20%

CIS


Thin film PV technologies

The primary idea is a tiny amount of expensive material (1 micron or so) and lots of cheap glass and wire and metal and plastic

Ken Zweibel, NREL, 2004

Frontkontakt

Rückkontakt

encapsulation

Substrat

Superstrate technology(transparent substrate)

Absorber / p/n

front contact - TCO

back contact

Substrat

absorber - p/n-structure

light

Rückkontakt

Frontkontakt

Substrat

encapsulation

Substrate technology

Absorber / p/nback contact

front contact -TCO

substrate

glass, foil + polymer

glass, foil + polymer

absorber / p/n-structure

light

thin SiCISCdTe


ZnO

Light

Glass (1-4mm)

TCO

n

Ag

p

i

n

pi

µµcc--Si:HSi:H

aa--Si:HSi:H∼ 3µm

a-Si technologyExample:a-Si/µc-Si tandem cell(„Micromorph“)

Mo (0.5 µm)

ZnO:Al (1 µm)

CdS (0.05 µm)

Glass (3 mm)

CIGS (2 µm)

i-ZnO (0.05 µm)

p

n

∼ 4µm

CIGS-solar cells: CuIn1-xGaxSe1-ySy

Light

CdTe-solar cells: CdTe

Light

Glass (1-4mm)

TCO

CdS (0.1 µm)

CdTe (3-8 µm)

Metal

Thin film PV technologies

ZnO (n-type)

Cu(In,Ga)(S,Se)2 (p-type)

Mo

glass substrate

Flexible solar cell on titanium foil

Example CIGS-Solar Cell


HZB: Spin-off Company


0,1

1,0

10,0

100,0

10 100 1.000 10.000 100.000

akumulierte Gesamtproduktion (MWp)

PV

Mod

ulpr

eis

(€/W

p)

O. Hartley, J. Malmström, A. Milner,21st EUPVSC, Dresden 2006

Cost reduction has different options

80%

status 2005

source: T. Surek NREL

long term target

Required:� Mass production� Technology development� Fundamental R&D

accumulated production (MWp)

PV

mod

ule

pric

e(€

/Wp)

Cost reduction strategies


a view

years ago

Capacity (thin film Si)> 10 MWp< 10 MWpCapacity (CIS/CdTe)> 10 MWp< 10 MWp

Thin Film PV Fabs:under constructionor in operation

Example Germany


Source: A. Jäger-Waldau, PV-Status Report 2007

Announced capacity by thin film type - world

2010/2011: 6 GWp!

130 TF companiesof which 21 wereactive in 2006

Note: CdTe growth even higher(First Solar)


Quelle: A. Jäger-Waldau, PV status report 2007

Thin Film vs. Wafer Based Crystalline Si


Solarpark Buttenwiesen – amorphous siliconQuelle: Phönix SonnenStrom AG

Thin-Film PV applications


Gescher-EsternEntsorgungs-Gesellschaft Westmünsterland (EGW) ,put in operation August 2006.One of the biggest roof-top installations(1.4 MWp, CdTe, First Solar) 23 430 thin-film modules on an area of ca. 17 000 m² and an investment of € 5.6 Mio.

Source: Reinecke + Pohl Sun Energy AG


BIPV (3S AG Megaslate System)

Roof integration – family homes

Würth-Solar CIGS modules


Optic Center Wales, CIS-Module, 85 kWP, 2004source: AVANCIS (www.avancis.de)

Solar fassade


Semitransparent Modules

Schott Solar – a-Si

Würth Solar


Source: Unisolar

Flexible thin-film PV


Flexible thin-film PV


Outline

� Motivation and Background� Thin Film Solar Cell Technologies and Applications� Amorphous Silicon and Microcrystalline BasedSilicon and Tandem Cells� Poly-Crystalline Si Thin-Films� R&D Challenges and Conclusions


1 µm

a-Si:H

Ag

TCO

a-Si:H solar cell cross section

BIPV: Stillwell Avenue Terminal, New Yorkca. 210 kWp, installed 2004Source: SCHOTT Solar

Amorphous Silicon Based Solar Cells


Si Si

SiSi

Si Si

SiSi

Si Si

SiSi

Si Si

SiSi

single-crystalline Sic-Si

amorphous Sia-Si:H

mikro-crystalline Si

(µc-Si:H)

5-15 % Hydrogen

Structure of Silicon


EF

n+

p

n

pi

mono- and poly c-Si

2222----300 300 300 300 µµµµmmmm 0.3 0.3 0.3 0.3 –––– 2 2 2 2 µµµµmmmm

a-Si:H, µc-Si:H

“diffusion controlled”“interface limited”

“drift controlled”“bulk limited”

Device Structure of Si Solar Cells


0

0,2

0,4

0,6

0,8

1

300 400 500 600 700 800 900 1000wavelength (nm)

quan

tum

effi

cien

cy

top cella-Si:H

bottom cellµc-Si:H

ZnO

Licht

glass (1-4mm)

TCO

n

Ag

p

i

n

pi

Advantages and challenges :� „red/IR-response“ µc-Si:H� no/small SWE ⇒ high stability� preparation with PECVD� indirect semiconductor: light trapping!� high growth rate and process control!

a-Si/µc-Si tandem cell

µµcc--Si:HSi:H

aa--Si:HSi:H∼ 3µm

pioneered by University of Neuchatel 1994see: Uwe Rau next talk, tomorrowfirst solar modules by Kaneka, J (2001)

Microcrystalline Silicon (µc-Si:H)


a-Si:H/µc-Si:H development

0.1 1 10 100 10006

7

8

9

10

11

12

effic

ienc

y (%

)

degradation time (h)

a-Si:H/µc-Si:H module µc-Si:H module

10x10 cm2 substrate(aperture area 64 cm2)

NREL confirmed: 10.1 %

NREL confirmed: 8.1 %

BR et al. TSF 2006

source: FZ-Jülich


SiH4+H2

HF

HF-Electrode

Substrate

Silicon growth by PECVD

(plasma enhanced chemical vapour deposition)

process parameters: amorphous ⇔ µ-crystalline growth

� SiH4 / H2-gas flow ratio� Pressure, � RF power, � Excitation Frequency� Substrate temperature� ...

source: FZ-Jülich


L. Houben, Dissertation, FZJ (IFF/IPV), Uni Düsseldorf O. Vetterl et al., Sol. Energ. Mat. Sol. Cells 62 (2000) 97-108

microcrystalline amorphous

Structure of µc-Si:H


very narrow process window for optimised µc-SI:H

4

6

8

4 5 6 7 8 9 10400

500

600

700

4 5 6 7 8 9 1010

15

20

0,6 0,8 1,0

50

60

70

0,6 0,8 1,0

a-Si

a-Si

a-Si

a-Si

µc-Si

η (%

)

µc-Si

µc-Si

µc-Si

VO

C (

mV

)

deposition pressure (Torr) deposition pressure (Torr)

JS

C (mA

/cm2)

FF

(%)

[SiH4]/[SiH4+H2] (%) [SiH4]/[SiH4+H2] (%)

µc-Si:H Solar Cell Process: 13.56 MHz Regime

BR et al, TSF 2003


Rotational temperature

SiH4 dissociation

Optical Emission Spectroscopy (OES)


µc-Si:H – control of film growth

-10 0 10 20 30 40 50 60

140

160

180

200

Sub

stra

te te

mpe

ratu

re (

°C)

time (min)

200 W, 11 Å/s 60 W, 5 Å/s

Plasma-induced substrate heating

-10 0 10 20 30 40 50 600,5

1,0

1,5

Em

issi

on in

tens

ity (

a.u.

)time (s)

SiH* emission (414 nm) H* emission (656 nm)

Amorphous growth conditions @plasma start

plasmaoff

plasmaon

M.N. van den Donker et al., TSF 2006


HF

x (cm) 1

Pot

enzi

alU

(V

)

Si-Schicht

• high power⇒ high plasma density (ne)

⇒ high growth rate

• high pressure⇒ low plasma potential⇒ low ion bombardment

“soft“ deposition

very narrow process windowand drift of the plasma properties

during growth*!⇒⇒⇒⇒

up-scaling extremely challenging !

e-

e-

e-

e-e-

e-

e-

SiH4+H2

e-

e-

e-

+

Summary: µc-Si:H Process Conditions

* see PL0001 van de Sanden et al.BR et al, TSF 2006


Starting point

10 x 10 cm2

30 x 30 cm2

>1 m2

Goal:cost-effective production technologyfor highly efficient solar cells

a-Si:H/µc-Si:H: From laboratory towards production

Development@IPV, FZ-Jülich

Joint development:Applied Materials, Sontor, FAP, FZJ


Successful Scale-Up @ Sontor

Module size : 1.8 m2

Status: ~7.5 % (stab. tot. area) production average


silver

TCO

superstrate

TCO

a-Si pin????c-Si pin

1-4 mm

700 nm

)1-

1-3 µm

100 nm

silver

TCO

superstrate

TCO

a-Si pin????c-Si pin

Light

silver

TCO

Glas

TCO

µc-Si pin

1-4 mm

700

)1

100 nm

200-300 nm

APCVD SnO2 - Asahi U

sputtered & etched ZnO

0.5 µm

„Light-Trapping“ by Surface-Textured ZnO

source: FZ-Jülich


Properties of ZnO:Al

• highly transparent & conductive

• c-axis oriented

• resistant against H2-plasmas

• smooth surface

• surface-texture by etching(depends on initial film properties!)

wurtzite-structure

c-axis

oxygen

zinc

Sputter techniques:

• rf/dc ceramic targets• mf metallic targets, high rate

Ar+RF ZnO

target: ZnO:Al 2O3

glass substrate

Sputtered ZnO:Al Films as Front TCO


1µm

smooth short dip optimised texture

• δrms up to 150 nm, high transparency and conductivity• crater size 100-2000nm

• cristallite size δ < 50 nm

• phenomenologic model(Kluth et al. TSF, 2003)

glass

ZnO

HCl 0.5 %

120°

c-axis

Schematic cross section

microscopicmodel?

Optimised ZnO for Si Thin-Film Solar Cells


Tailor-made surface roughness and surface features ⇒⇒⇒⇒

• AR-effect by index-matching• efficient light trapping for long wavelength light • low free carrier absorption

Licht

ZnO

Glas

µc-Si:H:1 µm

ZnO

Ag

“Light Trapping” in µc-Si:H Solarzellen

B R et al. TSF, 2006

400 600 800 10000,0

0,2

0,4

0,6

0,8

1,0

wavelength (nm)

qua

ntum

effi

cien

cy

2 µm low Al content 1 µm low Al content 1 µm rf standard 1 µm rf smooth

15,623,0

24,3

26,8

mA/cm2


Rsheet after etching:

Large Area Surface-Textured ZnO

Sontor / Applied Materials


Outline

� Motivation and Background� Thin Film Solar Cell Technologies and Applications� Amorphous Silicon Based Solar Cells� Microcrystalline Silicon and Tandem Cells� Poly-Crystalline Si Thin-Films� R&D Challenges and Conclusions


Towards High Efficiency Thin Film Si Solar Cells

today mid term very long term

efficiency

long term

20 %

15 %

Wide-gap Si(quantum size effects)

High EfficiencyTandem Cells

„nano“-scaledlocal contacts!

Materials Research +Materials Research +Materials Research +Materials Research +ProcessProcessProcessProcess DevelopmentDevelopmentDevelopmentDevelopment ++++DeviceDeviceDeviceDevice DevelopmentDevelopmentDevelopmentDevelopmentin parallel !in parallel !in parallel !in parallel !

„„„„RoadmapRoadmapRoadmapRoadmap““““

increase in grain size

„perfect“ passivationlight trapping concept“

„high growth rates“


a-Si:H (n, p)

a-Si:H (p, n)

1 cm²34.9 mA/cm²629 mV17.4 %Pyramidsa-Si(n)/c-Si(p)/a-Si(p)

1 cm²39.3 mA/cm²639 mV19.8 %Pyramidsa-Si(p)/c-Si(n)/a-Si(n)

0 100 200 300 400 500 600 700

5

10

15

20

25

30

35

40

a-Si:H(p)/c-Si(n)

a-Si:H(n)/c-Si(p)

Jsc(mA/cm²) 39.26 34.9 Voc(mV) 639.4 629FF(%) 78.9 79ηηηη(%) 19.8 17.4

|J| (

mA

/cm

2 )

VOC

(mV)

Independently confirmed at ISE Freiburg

Rear contact

TCO: 80nm

a-Si:H(n, p) ~ 5nm

c-Si(p, n)

FZ-Si

Front contact

a-Si:H(p, n) ~ 35nm

Note: Record efficiencies of these cell type > 22 % by Sanyo

M. Schmidt et al. TSF 2007

a-Si:H / c-Si Wafer Based Cells


single crystalline

Thin Film Approaches

grain size g (µm)

10-2 10-1 100 101 102 103 104

open

circ

uit v

olta

ge V

oc

350

400

450

500

550

600

650

700

750

∞

Neuchatel

pn

(mV

)

Sanyo

Kaneka*

BPTonen

ETL

ASE/ISFH

Daido

Astropower

ISE

Mitsu-bishi

Sony

ISEMPI

UNSW

ipe

IMEC

IMEC

ISI

pin

PHASE

ZAE

Canon

CanonETL

10-2 10-1 100 101 102 103 104

10-1

100

101

102

N

RST

Q

U

P

O

KM

L J

I

H

GF

E

CD

B A

limit Leff,mono

=102 µm

103

105

107

SGB

= 101 cm/s

diffu

sion

leng

th L

eff,p

oly [

µm

]

grain size g [µm]

well passivated grain bounderies

Recombination @ grain boundaries

R. Bergmann, J.H. Werner, TSF 202-204 (2002) 162K. Taretto, U. Rau, J.H. Werner, J. Appl. Phys. 93 (2003)

IPV


glasspoly-Si

Al

Epitaxial growthof absorber layer

by high rate deposition(e.g. E-Beam evaporation)

Growth rate >1.2µm/h

Glaspoly-Si

poly-Si

solar cellprocessing

Glasp+

p

TCOa-Si:H, n +

Glas Glaspoly-Si

Al

Seed layer conceptAluminium-induced layer exchange

(ALILE)

Ala-Si

Alternative Pathes: �solid phase crystallisation(first product by CSG Solar)�laser crystallisation�E-beam crystallisation

Poly Si thin film @ HZB


Outline

� Motivation and Background� Thin Film Solar Cell Technologies and Applications� Amorphous Silicon and Microcrystalline BasedSilicon and Tandem Cells� Poly-Crystalline Si Thin-Films� R&D Challenges and Conclusions


Si

SiH

Fundamental Materialproperties

Solar cellsand prototypes

Large area industrialprocessing

Technology Value Chain

In Thin-Film PV

Application in systems

Solar module development:From materials via devices towardscomplete systems

…and „vice versa“problems from application⇒ applied and basicR&D


� Large area PECVD (high rate, process control)

� Alternative techniques forabsorber deposition

� Quantitative understanding of materials interfaces and device

� improved/new materials (e.g. μμμμc-SiGe,SiC,…)

Goals for applied and fundamental R&Dprerequisites to realise the production roadmap !

� New deposition reactor concepts (very high growth rates, full gas usage)

� Incorporate quantum or spectrum-converting effects

� Combine thin-film Si with other PV technology

�Understand fundamental limitations of thin-film Si

hot topics

source: Strategic Research Agenda for PV (PV Technology Platform)

Concept forηηηη > 15 %

Demonstrateη > 12%

Prototype/test modules

2013-20202008-2013


� Thin-film solar modules will become major PV techno logies within the next decade. ⇒⇒⇒⇒ The proof of concept for a variety of technologies exists.

� The transfer of lab developments / prototypes into a cos teffective production is the challenge today.

� There is a strong need for broad R&D to improve existingconcepts and develop new thin-film technologies to openthe path for higher efficiencies and lower production cost s.

Conclusions


Scenario for the world´s primary energy mix in 2100


„ Key Product for the Bavarian Marketflexible thin-film solar cells for cold beer“

Solarzellen aus dünnen Siliziumschichten – Stand der ...Thin Film Solar Cell Technologies and...

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