Naß-chemische Konditionierung von Siliziumsubstraten ... · ¾¾Random pyramids for Random...

NaNaßß--chemische Konditionierung von Siliziumsubstraten: chemische Konditionierung von Siliziumsubstraten: Optimierung von optischen und elektronischen Optimierung von optischen und elektronischen

GrenzflGrenzfläächeneigenschaftencheneigenschaftenH. Angermann

H. Angermann, Workshop CiS Erfurt, 30.10.2008

Hahn Meitner Institut / Helmholtz-Zentrum Berlin für Materialien und Energie, Abt. Siliziumphotovoltaik Kekuléstraße 5, D-12489 Berlin, Germany

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- 70 Doktoranden - 40 Auszubildende

- etwa 500 wissenschaftliche Veröffentlichungen pro JahrH. Angermann, Workshop CiS Erfurt, 30.10.2008

GrGrüündung des Helmholtzndung des Helmholtz--Zentrum Berlin Zentrum Berlin ffüür Materialien und Energier Materialien und Energie

Standort Wannsee Standort Adlershof

Umbenennung

Juni 2008

Fusion

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programmatisch ausgerichtete Spitzenforschung in den

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10 Institute

ca. 1600 Beschca. 1600 Beschääftigte ftigte


Si

SiH

Fundamental Materialproperties

Solar cellsand prototypes

Large areaprocessing

Technology transfer: Technology transfer: Thin film PV competence Thin film PV competence centercenter BerlinBerlin((PVcomBPVcomB))

Systems



Thin film PV competence Thin film PV competence centercenter Berlin:Berlin:CIS and Si modules on 30 x 30 cmCIS and Si modules on 30 x 30 cm22

State of the art, flexible reference lines with alternative processes under development

Silizium-Oberfläche

Photovoltaik:

Sensorik:

Micromachining:

Mikroelektronik:

NaNaßß--chemische chemische KonditionierungKonditionierung von von SiliziumsubstratenSiliziumsubstraten: : OptimierungOptimierung von von GrenzflGrenzfläächeneigenschaftencheneigenschaften

FunktionalisierungFunktionalisierung

StrukturierungStrukturierung

GrenzflGrenzfläächenpassivierungchenpassivierung

ReinigungReinigung


NaNaßß--chemischechemische KonditionierungKonditionierung von von SiliziumsubstratenSiliziumsubstraten: : OptimierungOptimierung von von optischenoptischen und und elektronischenelektronischen

GrenzflGrenzfläächeneigenschaftencheneigenschaftenHelmholtz-Zentrum Berlin für Materialien und Energie, Siliziumphotovoltaik Kekuléstraße 5, D-12489 Berlin, Germany

J. Rappich (PL)I. Sieber (SEM)E. Conrad, a. Laades (PECV)L. Korte, M. Schmidt (solar cells)

W. Henrion, M. Rebien (UV-VIS SE)

Institut für Spektrochemie und angewandte Spektroskopie, Albert-Einstein-Str. 9, D-12489 Berlin, Germany

A. Röseler (FT-IR)

A.-D. Müller, F. Müller (AFM)Chemnitz University of Technology, Institut of Physics ,Solid Surface Analysis Group, 09107 Chemnitz

K. Hübener, J. Polte J. Hauschild, (AFM)

Freie Universitaet Berlin, FB Physik, 14195 Berlin


Electronic properties of silicon interfaces:Electronic properties of silicon interfaces:Effect of surface morphology and wetEffect of surface morphology and wet--chemical prechemical pre--treatmenttreatment

Dangling bond defects (db) and energetic distribution Dangling bond defects (db) and energetic distribution of rechargeable states of rechargeable states

HH--termination of flat Si:termination of flat Si:Effect of wetEffect of wet--chemical oxide, final etching chemical oxide, final etching solution and substrate orientation solution and substrate orientation

Native oxidation and wetNative oxidation and wet--chemical oxides: chemical oxides: Influence of surface microInfluence of surface micro--roughnessroughnessof Si substrate orientationof Si substrate orientation

CharacterisationCharacterisation methodsmethods

IntroductionIntroduction

HH++

MeMe++

FF––HH22OO22

HH22OO

Cl Cl ––--OHOH

X Xδ−

Si SiX 3Δ−δ

Si SiΔ Si SiΔ

Si SiΔ

Si(111)Si(111)

Si(100)Si(100)

1X 1δ−

Si SiΔ

Si Si21 2Δ−δ+δ+

Si SiΔ

2X 2δ−

COCO22OO22

NN22

Random pyramids for Random pyramids for aa--Si:H/cSi:H/c--Si hetero solar cells:Si hetero solar cells:Minimisation of Minimisation of DDitit and interface recombination lossand interface recombination lossby wetby wet--chemical smoothingchemical smoothing

ConclusionConclusionH. Angermann, Workshop CiS Erfurt, 30.10.2008

1. 1. IntroductionIntroduction

Fast surface sensitive

characterization methods

Theoretical modelof chemical reactions,defects and electronicstates on the Si surface

Reliable cleaningand passivation

technology

Prototype-material:flat Si(111) and Si(100)substrates ssOptimised Optimised

surface cleaningsurface cleaningand wetand wet--chemicalchemical passivation passivation

of rechargeable interface states of rechargeable interface states on on

structured structured SiSi


Si surface preparation: wetSi surface preparation: wet--chemical standard treatmentschemical standard treatmentsFirst investigations T. M. Buck and F. S. McKim,

J. Electrochem. Soc. 105, 709 1958Texturising and polishing solutions alkaline, saline and acidic solutions

A.F. Bogenschütz, Ätzpraxis für Halbleiter, München,1967Removal of metallic, organic andparticle contaminations RCA standard cleaning process

W. Kern, Surf. Sci. 31, 207, 1970.Removal of native oxide HF-treatment

E. Yablonovitch et. al, Phys. Rev. Lett. 57, 249, 1986

Passivation by Hydrogen wet-chemical H-TerminationA. Chabal, et. al, J. Vac. Sci. Technol. A7 (3), 2104, 1989

electro-chemical H-TerminationH.J. Lewerenz, T. Bitzer, J. Electrochem. Soc. 139,L21, 1992

1967

1970

1986

19891992

1958


Preparation of Si interfaces Preparation of Si interfaces wwithith special crystallographic configurations special crystallographic configurations and well defined electronic propertiesand well defined electronic properties

Challenge: Challenge: Minimized density of rechargeable interface statesMinimized density of rechargeable interface stateson atomically flat or structured Si substrateson atomically flat or structured Si substrates

Solar cell substrates:Solar cell substrates:

WetWet--cchemicalhemical conditioning of Si surfacesconditioning of Si surfaces

polycrystalline zone

fissure zone

transition zone

elastical strain zone

density of interface states Dit,min : 1010... 1013 cm-2eV-1

Sis -dangling bonds

Sis−O−H Sis−H Sis -dangling bondSis−H2


SiSi--wafer:wafer:

Preparation conditionsPPreparation conditionsreparation conditions wetwet--chemical solutions:chemical solutions:

cleanclean--room:room: composition

etching time

DOC (O2-content)

pH leveletch stop

concentrationillumination

humidity

temperature

drying process

fabrication

doping typedoping level

orientation

ssSurface electronic propertiesSSurfaceurface eelectroniclectronic ppropertiesroperties

surface morphology surface covering


2.2. CharacterisationCharacterisation methodsmethods

Surface PhotoVoltage

(SPV)

(SEM) (AFM)Electron microscopy

Atomic force microscopy

(SE)FTIR- and UV-VISSpectroscopic Ellipsometry

Photoluminescence(PL)

ssPreparation-induces

surface electronic properties and morphology


Surface morphology: (AFM)Surface morphology: (AFM)

Atomic force microscopyAtomic force microscopy

PSIA XE-100.

Contact mode

Standard Silicon Cantilever,

20 nN.

HITACHI S-4100 .

cold field emission cathode

Electron microscopyElectron microscopy (SEM)(SEM)transparent ZnO:Al layer on top of

the a-Si:H emitter a-Si:H ~ 4…8 nm

Atomic steps on Si(111)


repeatedly during the preparationrepeatedly during the preparation

NonNon--destructivedestructive surface csurface characterisationharacterisation::

FTFT--IRIR

H-terminationNrel

Si−H and Si(−H)2 vibration

SESE

Surface roughness

Oxide thickness< dr>, <dox>

ε2(E)

complex effective dielectric function

MorphologyMorphology

SPVSPV

Interface state distribution

Dit(E)

surface band bendingY(UF)

PLPL

Density of states Recombination

luminescenceintensity

Electronic propertiesElectronic properties


UVUV--VIS spectroscopic ellipsometryVIS spectroscopic ellipsometry

Variable Angle Spectroscopic Ellipsometer(V.A.S.E.) (J. A. Woollam Co.)

• quasi in-situ measurements around E2 critical point 3.2 to 4.5 eV Angle of incidence : 77°

imaginary part of the dielectric functionDetermination of

Bruggeman effective medium(50 % c-Si, 50 % voids)

bulk c-Si [1]

• effective surface roughness < deffective surface roughness < drr>>

SiO2

c-Si (H-terminated)

•• effective thickness of oxide <deffective thickness of oxide <doxox>>

[1] T. Yasuda, D.E. Aspnes, Appl. Opt. 33 (1994), 7435.

3.9 4.0 4.1 4.2 4.3 4.4

38

40

42

44

46

48

Photon energy [eV]

Si(111)(0)

(2)

(3)

(4)

(5)

(6)

(7)

(1)

E2

(0) reference data [26] (1) H-termination (i) (2) H-termination (ii) (3) 30 min air (4) 100 min air (5) 190 min air (6) 26 h air (7) 123 h air

< ε 2>

E2

Initially H-terminated Si(111) surface during native oxidation in clean-room air


0.3545

0.3550

0.3555

0.3560

0.3565ta

nΨ

-175.0°

-174.8°

-174.6°

Δ

2000 2050 2100 2150-5

0

5

10

15 ε2

ε1

ε 1, ε 2

wave number [cm-1]

directly detected by directly detected by IRIR--SE single reflection!SE single reflection!

δmax

FTIR:FTIR: HH--Termination: SiTermination: Si--H and SiH and Si--HH2 resonance:resonance:

•• Si(111)Si(111) --SiSi––H H δmax ≈ 0.4o 2083 cm-1

(a) Typical tan-Ψ ,

(b) Δ-spectrum of the SiH bonds(c) gives the ε2-spectrum of the best fit

Lorentz oscillator.

W. Henrion, A. Röseler, H. Angermann, M. Rebien, phys. stat. sol. (a) 175 (1999), 121

• Si(100) -Si(–– H)2

δmax ≈ 0.1° 2102 cm-1

H. Angermann, CiS Erfurt Seminar 22.09.2008

Pulsed Photoluminescence (PL):Pulsed Photoluminescence (PL):recombination loss at Si and recombination loss at Si and aa--Si:H/cSi:H/c--Si interfacesSi interfaces

Eel

Si

Pulsed excitation (500 nm, 6 ns)

Surfacelayer

+ +

-

-

Nonradiative

Relaxation

Radiative Recombination

PL

•• Determination of defect concentration at SiDetermination of defect concentration at Si--surfaces and interfacessurfaces and interfaces

EC

EV

Calibration by CV-techniques Defect

ContactContact--less measurement less measurement

of recombination behavior of recombination behavior

of light inducedof light induced

charge carrierscharge carriers

(at room(at room--temperature)temperature)

1J. Rappich, P. Hartig, N.H. Nickel, I. Sieber, S. Schulze, Th. Dittrich, 80 (2005) 62-65.H. Angermann, Workshop CiS Erfurt, 30.10.2008

Surface Surface PhotoVoltagePhotoVoltage method (SPV)method (SPV)

Surface potential as a function of bias voltages Uf:• surface Fermi-level position EFNS = E-Ei• energetic distribution of interface states Dit(E)• minimal value of interface state density Dit, min

Time decay of the photovoltage pulse: • yields information about interface recombination behaviour

laserpulse

Ip = 150W/cm2 tp = 150 ns

transient- digitizer

PC

pre- amplifier

needle contact UF

quartz

TCO-elektrod

mica

wafer

e


33. Results and discussion. Results and discussion

ssElectronic

properties of Si interfaces:Effect of surface morphology

and wet-chemicaltreatment

Dangling bond defects (db) and energetic distribution of rechargeable statesDit(E)


HF treatment: positive surface chargepositive surface charge p-Si: inversionn-Si: accumulation

0 200 400 600 800 1000

-0,4

-0,2

0,0

0,2

-0,2 0,0 0,2 0,4

1011

1012

1013

Pho

tovo

ltage

(V)

Fieldvoltage (V)

Dit (

cm-2 e

V-1)

E-Ei

Dit, min

p-Si (111)

0 200 400 600 800 1000

-0,4

-0,2

0,0

0,2

-0,2 0,0 0,2 0,4

1011

1012

1013

Phot

ovol

tage

(V)

Fieldvoltage (V)

Dit (

cm-2 e

V-1)

E-Ei

Dit, min

p-Si (111)

NH4F treatment: decrease of positive surface chargepositive surface charge

clean room conditions: decrease of interface state densityinterface state density

0 200 400 600 800 1000

-0,4

-0,2

0,0

0,2

-0,2 0,0 0,2 0,4

1011

1012

1013

Pho

tovo

ltage

(V)

Fieldvoltage (V)

Dit (

cm-2 e

V-1)

E-Ei

Dit, min

p-Si (111)

Determination of Determination of DDitit((EE) on ) on HH--terminatedterminated Si Si surfacessurfaces


Change of Change of DDitit((EE) during the initial phase of oxidation) during the initial phase of oxidation

1011

1012

1013

-0.2 0.0 0.2

1011

1012

1013

-0.2 0.0 0.2 -0.2 0.0 0.2

(5)

(4)(6)

PH

PL

PL

PH

(1)

(2)

(3)

H-term. (i) 48 h air H-term. (ii) 10 min air H-term. (ii) 60 min N2

Dit,

min

[cm

-2eV

-1]

<d ox> : 1.5 - 2.5 nm<d ox> : 0.4 - 1.2 nm<d ox> : 0.1 - 0.4 nm

oxidation in deionized water 80 - 90°C

oxidation in clean-room atmosphere

H-term. (i) 14 d air

HF-dip 15 months air H-term. (i) 6 months air

(7)

(8)

H2O 80°C 50 min H

2O 80°C 20 min

(10)

(9)

H2O 90° C 60 min H

2O 80° C 90 min

E-Ei [eV]

(12)

(11) H2O 80°C 120 min Si(100) H

2O 80°C 120 min Si(111)

Si

SiSiSiSi

SiSiO

H2O, O2 Si

SiOO

H2O, O2 H2O, O2 Si

OOO

Previously H-terminated Si(111):


DDit,min it,min and and DDitit((EE): effect of surface micro): effect of surface micro--roughnessroughness

-0.2 0.0 0.2

1011

1012

1013

n-Si

Dit (E

) [cm

-2Ev

-1]

Dit,min

HF-treatment

5 sec 10 sec 30 sec 90 sec 180 sec

H-termination

E - Ei [eV]

Increase of Increase of DDit,minit,min by HF treatmentby HF treatment

Narrowed Narrowed DDitit((EE) distributions) distributions

0 2 4 6 8 10 121010

1011

1012

(3)

(2) (1)

NH4F-treatment

HF-treatment

n-Si(111) p-Si(111)

300 s90 s

30 s

10 s

H-termination (ii) after H2SO4/H2O2

H-termination (i) after H2O (80°C)

H-termination (iii) after RCA

180 s 600 s

60 s

5 s

Dit,

min [c

m-2eV

-1]

<dr> [Å]

Dit,min as a function of micro-roughness

strong relation between strong relation between DDit,minit,min and and the effective surface roughnessthe effective surface roughness

H. Angermann, HMI Seminar 22.11.2007

dbdb--defects and electronic states on Si/SiOdefects and electronic states on Si/SiOxx--interfacesinterfaces

Defect structure

Stage of oxidation

Energetic Distribution Dit (E)

of interface states

Si Si

Si

Si Si Si

Si

Si

Si Si Si

Si Si Si

Streched Bond

Intrinsische Zustände Extrinsische Zustände

0 0Si +1 Si +2

Si +3

Dangling Bond - Defekte

E V E 0 E C

U T U T

U M

E V E C

PL

PHP OX

Si

SiSi O

Si

Si O O

Si

OO O

Oxidladung

strained bonds Silicon dangling bond defects

Si 0 Si 0 Si +1 Si +2 Si +3

intrinsic states, Pb0 and Pb, extrinsic states oxide charge

H. Angermann, Th. Dittrich, H. Flietner, Appl. Phys. A 59 (1994)H. Angermann, Workshop CiS Erfurt, 30.10.2008


ssElectronic



Dangling bond defects (db) and energetic distribution of rechargeable states Dit(E)

H-termination of Silicon Surfaces


-0.2 0.0 0.2

1010

1011

1012

1013

(4) H2O 80°C

(3) H2SO4/H2O2

(2) RCA II

(1) RCA I

wet-chemical oxides

H-termination

(ii)

(iii)

(i)

Dit

[cm

-2 e

V -1

]

E - Ei [eV]

obtained on wet-chemically oxidized Si(111) interfaces after RCA I, RCA II, H2SO4:H2O2 and hot water treatment

and on the resulting H-terminated Si(111) surfaces immediately after removing the oxide layers in NH4

solution.

DDitit((EE) on Si(111) interfaces) on Si(111) interfaces

H. Angermann, W. Henrion, M. Rebien, A. RöselerAppl. Surf. Sci. 235 (2004), pp. 322-339.


HH--terminated Si(111): effect of final etching solutionterminated Si(111): effect of final etching solution

WetWet--chemical chemical oxideoxideNHNH44F F solutionsolutionHF HF dipdip 30 sec30 sec

G. W. Trucks, K. Raghavachari, G. S. Higashi,Y. J. Chabal, Phys. Rev. Lett. 65 (1990), 504

RMS roughness: 0.94 nm

500 nmA

HH--terminated Si(100): effect of final etching solutionterminated Si(100): effect of final etching solutionNHNH44F F solutionsolution

500 nmB

RMS roughness: 0.21 nm

WetWet--chemical oxide + chemical oxide + 1%1% HFHF

+ 3 HF-SiF4

SiF

F −

Siδ+

Oδ −

Hδ+

+HF-H2O

F δ−

Siδ+

Si

SiF−

H+

F

FFH

H

O

Si H

Si

Si

Si Si

Si

Si

Si Si

Si

Si

Si Si

Si

H+

H. Angermann, A.D. Müller, F. Müller,13. Tagung Festkörperanalytik Chemnitz 26.-29.06.2005


HH--terminated Si(111) and Si(100): effect of smoothing proceduresterminated Si(111) and Si(100): effect of smoothing procedures

-0,2 0,0 0,2 -0,2 0,0 0,21010

1011

1012

(1) RCA + HF 1% 60 s (2) H2SO4 : H2O2 + NH4F (3) H2O, 80°C + NH4F (4) H2SO4 : H2O2 + HF 1% 60 s

E-Ei [eV]

FZ Si(100)

(1)

(2)

(3)(4)

b c

E-Ei [eV]

EFG Si

(1)

(2)

(4)

wet-chemical oxidation: oxide removal:

-0,2 0,0 0,21010

1011

1012

Dit (

E) /

cm

-2eV

_1

FZ Si(111)

(1)

(2)

(3)

Dit (

E) /

cm

-2eV

_1

E-Ei [eV]

a

Decrease of initial interface roughness by nonDecrease of initial interface roughness by non--aggressive oxidationaggressive oxidationOxide removal by HF or NHOxide removal by HF or NH44F solution considering the Si orientationF solution considering the Si orientation

MinimizationMinimization of Dof Ditit::


Native oxidation and wet-chemical oxides

ssElectronic




H-termination of Silicon surfaces


StabilityStability of of HH--terminationtermination: : effecteffect of surface of surface orientationorientation and and prepre--treatmenttreatment

101 102 103 104

0.0

0.2

0.4

0.6

0.8

1.0

1.2

initial phase of oxidation

(1) (2) (3)

NH4F after H2SO4/H2O2 Si(111) NH4F after hot water Si(100) NH4F after hot water Si(111)

native oxide growth

48 h6.5 h2 h

<dox

> [n

m]

storage time in clean-room air [min]

H. Angermann, W. Henrion, M. Rebien, A. Röseler SSP’2003 Sept. 14-16 Ustron, Poland

Conventional pre-treatment

Hot waterpre-treatment

UVUV--VIS SEVIS SE

Decrease of the relative number Nrel of the Si ≡ Si–H oscillators

Increase of the effective thickness <dox > of the native oxide film

Oxide Oxide thicknessthickness::

Strong increase in the density of interface states Dit,min of about two orders of magnitude

HH--terminationtermination::

Interface states:Interface states:

101 102 103 104 105

1010

1011

1012

0,0

0,5

1,0

1,5

FT-IR SE

uv-vis SE

Dit(E

) [eV

-1cm

-2]

minimal density of interface states

SPV

d

Si(111)

<dox

> [n

m] native oxide thickness

0,0

0,2

0,4

0,6

0,8

1,0

Nre

l hydrogen coverage

t air (min)101 102 103 104 105

1010

1011

1012

0,0

0,5

1,0

1,5

FT-IR SE

uv-vis SE

Dit(E

) [eV

-1cm

-2]

minimal density of interface states

SPV

d

Si(100) Si(111)

<dox

> [n

m] native oxide thickness

0,0

0,2

0,4

0,6

0,8

1,0

Nre

l hydrogen coverage

t air (min)

Storage time in clean-room air

Initial phase of oxidationInitial phase of oxidation

Native oxidation of Si Substrates: Native oxidation of Si Substrates: Effect of substrate orientationEffect of substrate orientation


Passivation of flat Si(111) by wetPassivation of flat Si(111) by wet--chemical oxideschemical oxides

AFM images (2µm x 2µm)

Initially H-terminated Si(111)after wet-chemical re-oxidation

Storage in ambient air during a few months

initially prepared surface morphology was reinitially prepared surface morphology was re--established by HF dip onlyestablished by HF dip only


H-terminated Si(111) without contamination

Oxide removalHF 1%


a-Si:H/c-Si hetero solar cells:

minimisation of Dit and interface recombination loss

Smoothing and passivation by wet-chemical oxides

ssElectronic




H-termination of flat Si:

Native oxidation and wet-chemical oxides


Increase of Increase of defectdefect densitydensity duringduring Si Si substratesubstrate texturisationtexturisationSPV

-0,4 -0,2 0,0 0,2 0,4

1011

1012

1013

flat Si(100) RCA + HF -dip

Dit [e

V-1cm

-2]

E-Ei[eV]

(1)

-0,4 -0,2 0,0 0,2 0,4

1011

1012

1013

(2)

pyramids RCA + HF - dip flat Si(100) RCA + HF -dip

Dit [e

V-1cm

-2]

E-Ei[eV]

(1)

-0,4 -0,2 0,0 0,2 0,4

1011

1012

1013 flat Si(100) etch polished pyramids (111) etch polished

(4)

(3)

(2)

pyramids RCA + HF - dip flat Si(100) RCA + HF -dip

Dit [e

V-1cm

-2]

E-Ei[eV]

(1)

DDitit((EE)) after RCA + HF dip:after RCA + HF dip:

(1)(1) flat Si(100) substrateflat Si(100) substrate

(2) anisotropic etching (KOH isopropanol)

randomly distributed upside pyramids (111)

(1-2 μm)

(3) subsequent isotropic etching of pyramids

(4) isotropic etching of flat Si(100)

(standard acid polishing solution:

HNO3, CH3COOH, H3PO4, HF)

Increase of Increase of DDitit

Narrowed Narrowed DDitit((EE) curves) curvesIncrease of tail states (strained bonds)Increase of tail states (strained bonds)


SEM

900 1000 1100 1200 1300

0,0000

0,0005

0,0010

0,0015

I PL [a

.u.]

Wawelength [nm]

PL -Intensity

(1)

(2)

(3)

NH4F

wet-chemical oxide

NH4F- treatment

anisotropic etching

wet-chemical oxide + HF

PL on pyramids: PL on pyramids: effect of weteffect of wet--chemical smoothingchemical smoothing

PL


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)

areaJscVocηtexturedoping sequence

front contact

a-Si:H (n+/p+) d≈5nm

c-Si (p/n)

TCO - 80 nm

a-Si:H (p+/n+) d≈35nm

back contact

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

Ev

EF

EC

10nm

+

-

-

+

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 79η(%) 19.8 17.4

|J| (

mA

/cm

2 )VOC (mV)

independently confirmed at ISE Freiburg

aa--Si:H/cSi:H/c--Si hetero solar cells on random pyramidsSi hetero solar cells on random pyramids

Standard surface preparationStandard surface preparation: RCA + HF dipRCA + HF dip


Solar cell efficiency: effect of smoothing and passivationSolar cell efficiency: effect of smoothing and passivation

Histograms of solar cell parameters(i) RCA + HF dip after texturisation(ii) additional wet-chemical smoothening and passivation

Lines: normal distributions fitted to the data.

TCO/aTCO/a--Si:H(n)/cSi:H(n)/c--Si(p)BSF/AlSi(p)BSF/Alsolar cellssolar cells

32 33 34 35 36 37 380

2

4

6

8

10

12

14

Voc [V]

efficiency [%]

fill factor [%]

no. o

f cel

ls

Isc [mA/cm²]

0.59 0.60 0.61 0.62

74 75 76 77 78 790

2

4

6

8

10

12

14

no. o

f cel

ls

15.5 16.0 16.5 17.0 17.5 18.0 18.5

conventional HF dip oxidation + etching

• narrow distribution of parameters

• FF, Isc, h increased

The mean value of of efficiency ηincreases from 16.5 to 17.8 %

The highest value ηincreasesfrom 17.4 to 18.4 %relative increase of 8 %.

H. Angermann, J. Rappich, L. Korte, I. Sieber, E. Conrad,M. Schmidt, K. Hübener, J. Polte, J. Hauschild, Appl. Surf. Scie. Vol 254/12 (2008) 3615H. Angermann, Workshop CiS Erfurt, 30.10.2008

In order tIn order to reduce interface state density on structured silicon substratesspecial sequences of wet-chemical oxidation and oxide removal were developed- avoid micro-roughness of the initial Si/SiOx interface- optimise final etching solution HF/NH4F with respect to Si surface orientations- complete native and wet-chemical oxide removal

ConclusionConclusion

The optimised surface state can be preserved by soft a-Si:H deposition.

For amorphous-crystalline hetero-junction solar cells on textured siliconwet-chemical smoothing and defect passivation result ina significant increase of short circuit current Isc, fill factor and efficiency η.

Electronic properties of Si surfaces and interfaces strongly depend on the initial surface morphology as well as on wet-chemical pre-treatments.

The preparation-induced micro-roughness and energetic distribution of surface states result from the course of two different chemical processes:

- the wet-chemical oxide formation of the surface - the final etching step of the silicon oxide and the silicon substrate.


Thank you for your kind

attention!

NaNaßß--chemischechemische KonditionierungKonditionierung von von SiliziumsubstratenSiliziumsubstraten::OptimierungOptimierung von von optischenoptischen und und elektronischenelektronischen

GrenzflGrenzfläächeneigenschaftencheneigenschaftenHelmholtz-Zentrum Berlin für Materialien und Energie, Siliziumphotovoltaik Kekuléstraße 5, D-12489 Berlin, Germany


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