The a-Si:H/c-Si interface - key for high efficiency heterojunction

The a-Si:H/c-Si interface -key for high efficiency heterojunction cellsLars Korte, Erhard Conrad, Heike Angermann, Rolf Stangl, Tim Schulze,Manfred Schmidt

Weierstraß-Institut Berlin, 24. November 2008

Weierstraß-Institut Berlin – 24.11.2008Lars Korte

The a-Si:H/c-Si interface -key for high efficiency heterojunction cells

2

Outline• a-Si:H/c-Si heterojunction solar cells• impact of the heterointerface: passivation and transport• experimental results:

– “soft deposition” of a-Si:H, initial growth on c-Si– optimization of a-Si:H:

• optimum emitter thickness• measuring the density of states in <10 nm a-Si:H• optimum doping

– a-Si:H/c-Si band offsets• (older) cell results & summary



3

The a-Si:H/c-Si solar cell

a-Si:H(n), ~ 10 nm

c-Si(p) (~300 µm)

Al grid

a-Si:H(p+), ~ 20-30 nmAl

sputtering/e-beam

PE/HW-CVD

back surface field

front contact

back contact

TCO

base(absorber)

emitter

PE/HW-CVDsputtering

function deposition by

ZnO:Al, 80 nm

a-Si:H(i), < 10 nm PE/HW-CVD

PE/HW-CVDa-Si:H(i), < 10 nm



Progress in a-Si:H/c-Si cell efficiency(incomplete) list of R&D on a-Si:H/c-Si cellsJapan

Sanyo:• (p,i)aSi on (n)cSi (22.3%)• production: 100cm2, 19.5%, 350MWp in ‘08

USNREL: p/i on n-type, 18.2%, HW-CVDUDEL: rear contacted cell, w/ SunPower

AustraliaUNSW: p/i on n-type, 17.6% (EPVSEC 22)

EuropeHZB (D):

• n on p-type, 18.4%, and• p on n-type, 19.8%, no i-layer

CEA-INES (F): n, (i)pmSi on p, 16.8%, 25cm2

U Neuchâtel (CH): n,i on p, VHF-PECVDothers:

CNRS (F), ENEA (I), IMEC (B),U Utrecht (NL), IPE (D), FUH (D), ...

1996 1998 2000 2002 2004 2006 20086

8

10

12

14

16

18

20

22

cell

effic

ienc

y [%

]

year

p/n n/p Sanyo R&D Sanyo Production

NREL R&D HMI R&D

Others (Europe)

Korte et al., 22nd EPVSEC, Milan (2007) p. 859



Impact of the heterointerface: Recombination and transport

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a-Si:H(n/p)

c-Si(p/n)

metallization

ZnO

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

back contacta-Si:H(n)

Ev

EF

EC

10nm

band scheme

+

-

-

+

c-Si(p)

transport

passivation

Tasks: • minimize recombination losses at/near a-Si:H/c-Si interface• maximize efficiency of charge carrier transport over heterointerface



simulation using AFORS-HET

• strong influence of interface recombination at front (and rear) side• Dit <1010cm-2eV-1 → IF recombination plays no role in cell

1010 1011 101216

18

20

2237

38

39600

640

680

η

η [%

]Dit [cm-2 eV-1]

Isc

I sc[m

A cm

-2]

Voc

[mV]

Voc

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

Ev

EF

EC

~10nm

LD=400µm

parameter:interface state density Dit(E)

influence of interface defects on cell parameters - simulation study

Schmidt et al., 4th IEEE WCPEC (2006) 1433



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– “soft deposition” of a-Si:H, initial growth on c-Si– optimization of a-Si:H without intrinsic layer:





Maruyama et al., 4th IEEE WCPEC (2006)and also• Taira et al., 22nd EPVSEC (2007),• H. Kanno et al., 23rd EPVSEC (2008),• ...

low damage deposition?- RF-PECVD at low power - VHF-PECVD

at low pressure - hot-wire-CVD- remote plasma - …?

soft deposition of a-Si:H - the Sanyo „mantra“

HZB

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Influence of epitaxy at the a-Si:H/c-Si interfacedetrimental effect of epi-growth at the a-Si:H/c-Si interfacereported many times:• E. Centurioni et al., IEEE Trans. El. Devices & 19th EPVSEC, Paris (2004) 1285• T.H. Wang et al., ibd., p. 1269• de Wolf & Kondo, Appl. Phys. Lett. 90 (2007) 042111• ...

usual result: epitaxy up to doped layer → Voc < 600mV

possible explanations:• epitaxy extends through i-layer

→ surface defect passivation by doped a-Si:H:much less effective than (i)a-Si:H

• partial epitaxy/mixed phase (i)a-Si:H→ increased interface area betw. a-Si & c-Si

• poor conditions for epitaxy→ epi highly defective

PECVD @ 230°C on c-Si(100)

de Wolf & Kondo, Appl. Phys. Lett. 90 (2007) 042111

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Smoothing and passivation of pyramids

additional complication: random pyramidsurface texture(light trapping)

need optimizedchemical pre-treatment of c-Si wafer

Angermann et al., Proc. 23rd EU-PVSEC 2008, pp. 1422-6

optimized smoothinganisotropic etching

3 µm

0.5 µm



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12

a-Si:H(n/p)

c-Si(p/n)

metallization

ZnO

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


Ev

EF

EC

10nm

band diagram

c-Si(p)

cell structureoptimum?



finding the optimal a-Si:H emitter thickness

400 600 800 1000 12000.0

0.2

0.4

0.6

0.8

1.0

5nm a-Si:H(n)

30nm a-Si:H(n)

inte

rnal

yie

ld

λ [nm]

internal quantum efficiency

absorption and recombination in the a-Si:H emitter lead to a decrease in JSCoptimum a-Si:H thickness: 5-10nm – how to analyze its electronic properties?

0 5 10 15 20

28.0

28.5

29.0

29.5

30.0

30.5

31.0

615

620

625

630

635

640

J sc [m

A/cm

2 ]d [nm]

V oc [m

V]

solar cell parameters

Stangl et al., 3rd WCPEC (2003) 4P-A8-45

cell structure: TCO/(n)a-Si:H/(p)c-Si/Al

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14







15

a-Si:H(n/p)

c-Si(p/n)

metallization

ZnO

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


Ev

EF

EC

10nm

band diagram

c-Si(p)

cell structure

density of defects? EF position?



hν

Nocc(Ekin,hν)

Constant Final State Yield Photoelectron SpectroscopyEVac

‘classical’ UPS:hν = const.Ekin= var.

energy

hνdensity of states

Ekin

CFSYS:hν = var.Ekin= const.

energy

E'FEF

EF

Sebastiani et al.,PRL 75, 3352 ('95)

EVac

hν1

hν1 E'F,1Nocc(Ekin,hν1)

Ekin

hν2

E'F,2

Nocc(Ekin,hν2)

hν2

we obtain: photoelectron yielddensity of occupied states Nocc(E)Fermi level position EF

low excitation energy (4…7 eV)high electron escape depth (6-10 nm)high excitation cross section (at 4eV: 103 times higher than at 21.2eV!)

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workfunction



Nocc(E) = exp. tail (‘strained bonds‘) + Gaussian distribution (‘dangling bonds‘)

Urbach energy E0v integrated density of deep defects ND

density of a-Si:H gap statesnear-UV photoemission (CFSYS mode) → Nocc(E)

1014

1015

1016

1017

1018

1019

1020

1021

1022

Noc

c [cm

-3eV

-1]

1.81.61.41.21.00.80.60.40.20.0-0.2E-EV [eV]

Ce1-1

EF

Nocc (E) strained bonds ('Urbach-tail') dangling bonds

EF

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1014

1015

1016

1017

1018

1019

1020

1021

1022

dens

ityof

sta

tes

N(E

) [cm

-3eV

-1]

2.01.51.00.50.0-0.5E-EV [eV]

p-type (1·104 B2H6)intrinsicn-type (2·104 PH3)

EV

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

c-Si(p), <111>, 1Ωcm

10nm (p, i and n)a-Si:H on c-Si, Ts = 200°Cdoping dependence of N(E), EF

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n-type doping seriessamples: ~10 nm a-Si:H(n) on c-Si(p), [PH3]/[SiH4] = 0 – 2×104 ppm, Ts=170°C

Korte & Schmidt, J. Non-Cryst. Sol. 354 (2008) 2138

1014

1015

1016

1017

1018

1019

1020

1021

1022D

ensi

tyof

occ

upie

dst

ates

[cm

-3eV

-1]

2.01.51.00.50.0-0.5E-EV[eV]

0 ppm

2·104ppm

EV

EC

a-Si:H(n)c-Si(p), <111>,

1Ωcm

measured using UV-PES

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optimum doping:[PH3]/[SiH4] = 104 ppm?

no change in band gapwith doping (Eg~1.7eV)

same as for thick films(e.g. V. Chacorn, D. Haneman,

Solid State Comm. 65 (1988) 609)

0.7

0.6

0.5

0.4

0.3

E C -

E F [e

V]

2.52.01.51.00.50.0[PH3]/[SiH4] [x104 ppm]

1.5

1.4

1.3

1.2

1.1

1.0

EF

- EV [e

V]

1.9

1.8

1.7

1.6

1.5

1.4

Eg ≈

EC -

EV [e

V]

CFSYS

dark conductivity

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-0.7

-0.6

-0.5

-0.4

eVoc

, eφ 0

[eV

]

2.01.51.00.50.0[PH3]/[SiH4] [104 ppm]

band bending

Voc (solar cells)

optimum doping ~ 2000ppm - higher doping: enhanced recombination?

a-Si:H c-Si

Ev

EF

EC10nm

aSi

EvcSi

eϕ0

∆EV


Korte & Schmidt, J. Non-Cryst. Sol. 354 (2008) 213821



Band bending, Voc and recombination - optimized a-Si:H doping

600

400

200

0

τ ini[µ

s]

2.01.51.00.50.0[PH3]/[SiH4] [104 ppm]

SPV data:A. Laades, PhD Thesis, HMI/TU Berlin

-0.7

-0.5

-0.3

eVoc

, eφ 0

[eV

] calculated band bendingcell Voc

effective charge carrier lifetimeat high injection (from surface photovoltage)

doping > 2000ppm: enhanced recombination reduced charge carrier conc.Voc decreases

and also: hopping into a-Si:H via a-Si:H tail states possible, additional recombination (see also: Boehme et al., J. Non-Cryst. Sol. 352 (2006) 1113)

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23







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a-Si:H(n/p)

c-Si(p/n)

metallization

ZnO

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


Ev

EF

EC

10nm

band diagram

c-Si(p)

cell structure

band offset?



2.0

1.5

1.0

0.5

0.0

Y intC

FSY

S /(hν

R2 ) [x1

022cm

-3eV

-1]

1.51.00.50.0-0.5E-Ev [eV]

1014

1015

1016

1017

1018

1019

1020

1021

1022film thickness da-Si:2.9 nm6.6 nm10.8 nm103.6 nm

EF

c-Si

a-Si:H

ECEV EFz

hν

E

daSi < information depth

photo-electrons from substrate

assumption/model:

example: a-Si:H(i) / c-Si(p)a-Si:H/c-Si valence band offset

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fit: sum of a-Si:H- and c-Si spectrum

c-Si(E+∆EV)

a-Si:H(E)

1014

1015

1016

1017

1018

1019

1020

1021

1022

Yin

t/(hν

R2 ) [

w. E

.]

2.01.51.00.50.0-0.5-1.0E-Ev [eV]

-1000

100

Res

id. [

%]

∆Ev

c-Si

a-Si:H

ELEV

EF

z

EEV EL

a-Si:H/c-Si valence band offset

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fit results

• mean: ∆EV=0.458(6) eV (systematic error: ~50 meV)• no dependence on substrate- or film doping• (weak) trend: decreasing ∆EV with film thickness - possible explanation: decreasing

Si-H interface dipole, because Si-H bonds are substituted with Si-Si

0.8

0.6

0.4

0.2

0.0

∆ EV

[eV

]

151050da-Si:H [nm]

∆Ev = 0.458(6) eV

i on p (old) i on p, codep. withi on p i on nn on pp on n

a-Si:H/c-Si valence band offset

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– a-Si:H/c-Si band offsets• cell results & summary



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

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)

Solar cell results

areaJscVocηtexturedoping sequence

front contact

a-Si:H (n+/p+)d≈5-10nm

c-Si (p/n)

TCO - 80 nm

a-Si:H (p+/n+) d≈35nm

back contact

Conrad, Korte et al., EPVSEC21, 2DO.3.5

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

1 cm²18.4 %pyramids uncertified

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Summarya-Si:H growth• growth mode on c-Si: islands – coalescence – thickening• high hydrogen content at a-Si/c-Si interface ↔ enhanced defect density• optimum growth conditions:

• high c-Si surface quality prior to a-Si:H deposition• low plasma damage during growth• low defect density in a-Si:H “bulk”

reduced recombinationa-Si:H/c-Si interface• device simulation: (effective) interface DOS < 1010 cm-2

→ not detrimental to cell parameters• asymmetric band offset: ∆EV ~ 460meV (→ ∆EC ~ 150-200meV)

n-doped a-Si:H emitters• Urbach-Energy and ND increase with doping, comparable to thick films• optimum doping for device ~2000ppm, not at minimum of EF-EC –

trade-off: doping ↔ defect generation

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Thank youPeople

Walther FuhsBernd Rech

Thomas LußkyMatthias Schulz

Aziz LaadesKarsten von Maydell

Andreas SchöpkeKerstin Jacob

Brunhilde RabeDagmar Patzek

BMBF Netzwerk-Projekt Nr. 01SF0012„Grundlagen und Technologie von Solarzellen auf der

Basis von a-Si/c-Si Heterostrukturen“

Funding

EU FP7 project no. 211821“Heterojunction Solar Cells based on a-Si c-Si”

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The a-Si:H/c-Si interface - key for high efficiency heterojunction

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