Well Passivated a-Si:H Back C

Double-Heterojunction Silicon 

2006 IEEE 4th World Conference on Photovoltaic E

May 7-12, 2006

Hilton Waikoloa Village, Waikoloa, Ha

Matthew Page, Eugene Iwaniczko, Y

Qi Wang, Yanfa Yan, Lorenzo R

Howard Branz, and Tihu Wan

NREL, Golden, Colorado, US

NREL/PR-520-39968
Disclaimer and Government License

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Outline

ÖAdvantage of doing back contac

with Silicon Heterojunction (SHJ

ÖHot-Wire CVD (HWCVD) 

ÖSHJ back-contact better than all

3⁄4both n- and p-type wafers3⁄4good back-surface-field (BSF)

ÖCritical for good SHJ solar cell fa

3⁄4layer optimization- in brief, here3⁄4surface preparation
Advantages of a-Si:H/c-Si Hete

ÖLow temperature processing (< 250

o

3⁄4preserves high lifetime3⁄4compatible with gettering or hydrogenat3⁄4prevents bowing (< 200

μ

m wafers)

ÖExcellent passivation on c-Si

3⁄4LOW minority-carrier recombination velo3⁄4HIGH open-circuit voltage (

Voc

)

Öa-Si BSF better than alloyed or diffus

3⁄4both passivation and vertical current con3⁄4no direct metal/c-Si contact (impurity so
Advantages of Hot-Wire 

ÖHWCVD

3⁄4simple3⁄4scalable3⁄4fast deposition3⁄4no ion bombardment 

of c-Si surface

Plasma damage to c-Si a major issue 

F
Optimization of SHJ Dev

ÖCritical: 1. preparation, 2. depos

3⁄41. Very clean SHJ interface prepa

• stringent cleaning before a-Si:H depo• junction and contacts close to interfa

3⁄42. High quality intrinsic and doped 

• no epitaxy at interface (

Voc

)

• low interface defect density intrinsic a

• high dopant activation in emitter and • low blue absorption in a-Si:H (

Jsc

)

• good front/back contacts to ITO/meta
0

20406080

100

350 500 650 800 

Wavelength 

EQE (%)

SHJ Back-Contact Passivation 

Ti/Pd/AgTi/Pd/Ag

ITO

a-Si:H (i) a-Si:H (p+)

n c-Si

P-Diff (n+)

Ti/Pd/Ag

ITO

a-Si:H (i) a-Si:H (p+)

n c-Si

a-Si:H (n+) a-Si:H (i)

Ti/Pd/Ag

ITO

Phosphorus Diffused vs. 

Absolute External Quantu

Max Voc 710 Max Voc 630 
20

406080

100

950 975 1000 1025 

Wavelength 

IQE (%)

SHJ Back-Contact Passivation 

a-S

ai:H

(i)

Si:H

(n+)

ITO

p

c-Si

Al

Ti/Pd/Ag

a-S

ai:H

(i)

Si:H

(n+)

ITO

p

c-Si

Al

Ti/Pd/Ag

a-Si:H

(p+)

a-Si:H

(i)

Al-BSF

Absolute IQE- removing r

Alloyed Al-BSF vs. 

Max Voc 650 mV

Max Voc 680 mV
SHJ Back-Contact is Exc

ÖSHJ better than Al-BSF on p-type wa

3⁄4superior back surface passivation3⁄4Fill-Factor greater than 78% achieved3⁄4 minority-carrier recombination velocity

• 15 cm/s for SHJ 

Ö (i/p) a-Si:H/Al

• 1000 cm/s for Al-BSF

ÖSHJ better than Phosphorous diffuse

3⁄4superior back surface passivation3⁄4Fill-Factor greater than 74% achieved

ÖSHJ interfaces are more critical than 

alloyed or diffused junction surfaces

vs. 
Surface Preparation Impo

Ö4 Generations (GEN-1 through GEN

3⁄4increasing complexity3⁄4developed for SHJ3⁄4baseline deposition for each GEN’s max

ÖStable oxide and interface

3⁄4store in clean box3⁄4remove impurities trapped in oxide by fi

ÖProtective chemical oxide by RCA-2

3⁄46:1:1 

Ö

H

2O : HCl : H

2

O

2

3⁄42.5% HF strip before deposition
GEN-1 Simplest Cleaning Pr

DI Water Cascade

t = 10 min.

T = RT

Boiling DI Soak

t = 10 min.

T = 95 

o

C

Ultrasonic TCE

t = 10 min.

T = RT

n-FZ WaferMax Voc = 630 mV

H-TerminatedReproducible

N2

Dry
GEN-2 Aggressive Acid Cl

Dry Storage

< 1 Day

T = RT

Aggressive Acid

t = <1 min.

T = RT

!

DI Water Cascade

t = 10 min.

T = RT

N2

Dry

n-FZ WaferMax Voc = 680 mV

Full clean right 

before deposition

Not reproducible

DI Water

t = 10 min.

T = RT

Replaced Boiling DI Water

Ultrasonic TCE

t = 10 min.

T = RT

N2

Dry
GEN-3 Repeat Aggressive Aci

Dry Storage

< 1 Week

T = RT

n-FZ WaferMax Voc = 690 mV

Stable oxideNot reproducible

Ultrasonic TCE

t = 10 min.

T = RT

N2

Dry

Aggressive Acid

t = <1 min.

T = RT

! 

DI Water

t = 10 min.

T = RT

Repeat One Time-Fresh A

DI 

t = 

T 

N2

Dry

DI 

t = 

T 

N2

Dry
GEN-4 Improved Degrease and 

n-FZ WaferMax Voc = 710 mVStable oxideReproducible

!

Acid Degrease

t = 10 min.

T = 85 

o

C

Ultrasonic TCE

t = 10 min.

T = RT

N2

Dry

DI Water

t = 10 min.

T = RT

!

Dilute HF Etch

t = 1 min.

T = RT

Dilute HF Etch

t = 1 min.

T = RT

Dry Storage

< 1 Week

T = RT

DI Water Cascade

t = 10 min.

T = RT

N2

Dry

Dry Storage

< 1 Week

T = RT

DI Water Cascade

t = 10 min.

T = RT

N2

Dry

Dry Storage

< 1 Week

T = RT

DI Water Cascade

t = 10 min.

T = RT

N2

Dry

Aggressive Acid

t = <1 min.

T = RT

! 

Rep
Surface Preparation Sum

GEN-1 Simplest

3⁄4 H-terminated

GEN-2 Aggressive Acid 

Cleaning

3⁄4 just before deposition3⁄4 not reproducible

GEN-3 Repeated

Aggressive Acid

3⁄4 degrease interaction3⁄4 not reproducible

GEN-4 Improved Degrease 

& Oxide Purity

3⁄4 reproducible3⁄4 stable chemical oxide

630

680

600620640660680700720

Gen-1 Gen-2 

Max Voc (mV) 
Conclusions

ÖSHJ successfully replaced 

Al-BSF or P-diffused full area back contacts

ÖSurface preparation and 

a-Si:H optimization are both critical for device performance

ÖConfirmed Efficiency

3⁄418.2 %

AM1.5 1-cm2 Voc = 667 mV

Jsc = 35.5 mA

FF = 76.9 %η = 18.2 %
Acknowledgements

ÖScott Ward, Russell Bauer, Ann

Bobby To for their valuable techscientific contributions.

ÖAjeet Rohatgi, and Vijay Yelund

Tech for p-type Al-BSF wafers.

ÖThis work is supported or funded 

DOE Contract No. DE-AC36-99G