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DECREASING METALLIZATION RELATED

RECOMBINATIONS FOR SCREEN PRINTED N-PERT CELLS BY SELECTIVE LASER DOPING AND REDUCTION IN CONTACT FRACTION

SUKHVINDER SINGH, PATRICK CHOULAT, LOIC TOUS, FILIP DUERINCKX,

IVAN GORDON AND JOZEF SZLUFCIK

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MOTIVATION

❖ Rear junction nPERT offers easier transition from p to n-type.

❖ For such rear emitter nPERT device, selective Front Surface

Field (FSF) is required

❖ Present work is focussed on reducing metallization related

recombinations by:

❖ Achieving selective FSF using laser doping from PSG layer

❖ Reducing metallization fraction for front side by employing dash-

printing

❖ Reducing line width of screen printed fingers

2

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OUTLINE

▪ Development of laser doped FSF from PSG layer

▪ Reducing contact fraction by dash printing

▪ Pushing the limits of standard screen printing by double print

▪ Cell results

▪ Conclusions and outlook

3

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OUTLINE

▪ Development of laser doped FSF from PSG layer

▪ Reducing contact fraction by dash printing

▪ Pushing the limits of standard screen printing by double print

▪ Cell results

▪ Conclusions and outlook

4

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PROCESS FLOW AND TEST STRUCTURES

5

FOR SELECTIVE FSF DEVELOPMENT

Screen-Printed Ag

Laser doped

Selective FSF n++

Laser doped

Selective FSF n++

Laser doped

Selective FSF n++

n-and p-type textured Cz-Silicon wafers Shallow POCl3 diffusion (> 200 Ω/sq) Laser doping from PSG (with different

laser power)

Thermal oxide + SiNx ARC Screen print Ag pastes, Varying metal fraction for J0n++metal

Laser doped

Selective FSF n++

FSF n+

Etch metal to measure QSSPC

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CHARACTERIZATION OF LASER DOPED REGIONS

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SHEET RESISTANCE OF LASER ABLATED AREA

Laser doped

Selective FSF n++

Laser doping from PSG on p-type wafers Four different sets of laser doping

parameters used

Doping

parameter

set

Average

Rsheet(Ω/sq)

Standard

deviation

(±)

1 71 4

2 102 9

3 116 9

4 155 21

No laser 308 18

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CHARACTERIZATION OF LASER DOPED REGIONS

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REFLECTANCE OF LASER ABLATED AREAS

Laser doped passivated wafers

Laser doped

Selective FSF n++

Doping

parameter

set

Average

Rsheet(Ω/sq)

Relative

Jsc loss

front*

(mA/cm2)

1 71 ± 4 0.036

2 102 ± 9 0.016

3 116 ± 9 0.013

4 155 ± 21 0.009

No laser 308 ± 18 0 (ref.)0

0.05

0.1

0.15

0.2

0.25

0.3

350 550 750 950 1150

Reflect

ance

Wavelength (nm)

Reflectance curves for laser doped

passivated surfaces

71Ω/sq

102Ω/sq

116Ω/sq

308Ω/sq

Higher laser power → More doping → but, more reflection loss

limited Jsc loss (< 0.02 mA/cm2 ) for laser doping down to 100 Ω/sq

*Using 10 % laser doped

surface area

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CHARACTERIZATION OF LASER DOPED REGIONS

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SEM OF LASER ABLATED AREAS

Average

Rsheet(Ω/sq)

Relative

Jsc loss

(mA/cm2)

71 ± 4 0.036

102 ± 9 0.016

116 ± 9 0.013

155 ± 21 0.009

308 ± 18 0 (ref.)

Down to laser doping of 100 Ω/sq limited surface damage Therefore limited Jsc loss < 0.02 mA/cm2

High laser power

→Significant surface

damage

0

0.05

0.1

0.15

0.2

0.25

0.3

350 550 750 950 1150

Reflect

ance

Wavelength (nm)

Reflectance curves for laser doped

passivated surfaces

71Ω/sq

102Ω/sq

116Ω/sq

308Ω/sq

Optimum laser doping

→ 100-120 Ω/sq→ Only pyramid tip

melting

No laser- 308 Ω/sqReference doping n+

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SPECIFIC CONTACT RESISTIVITY

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TWO COMMERCIAL AG PASTES FROM 2 DIFFERENT SUPPLIERS

0

1

2

3

4

5

155 116 102 71

Sp

ecif

ic co

nta

ct

resi

stiv

ity

(mΩ

.cm

2)

Av. sheet resistance of laser doped n++

FSF (Ω/sq)

paste A

Paste B

▪ Contact resistivity increases with increase in sheet resistance of n++ doped regions (as

expected).

▪ Paste B seems slightly better for contacting, but difference is small.

Screen-Printed Ag

Laser doped

Selective FSF n++

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J0 MEASUREMENTS

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J0n++

pass

J0n++

metal

J0n+pass

J0total= J0n+

pass+J0n++

pass(1-Cfm)+J0n++

metal*Cfm

J0total= J0n+

pass + J0n++

pass +(J0n++

metal-J0n++

pass)*Cfm

intercept slope

▪ Plot J0 total vs metal

contact fraction

y = 448.27x + 77.586R² = 0.972

60

70

80

90

100

110

120

130

140

0 0.02 0.04 0.06 0.08 0.1 0.12J0

To

tal (

fA/c

m2

)

Metal contact fraction

Rheet ~ 100 ohm/sq

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J0 METAL

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RESULTS

50 100 150 200 250 300 350

0

20

40

60

80

100

120

J0n++,pass

J0n++,metal

n++ Sheet Resistance (Ohm/sq)

J0n+

+,p

ass (

fA/c

m2)

No-laser

J0n+ pass

400

500

600

700

800

900

1000

1100

1200

J0n+

+,m

eta

l (f

A/c

m2)

Rsheet

n++

(Ω/sq)

Sp. contact

resistivity (mΩ.cm2)

J0n++, pass (fA/cm2)

J0n++,metal(fA/cm2)

Area weighted

J0 (fA/cm2 )(laser + Ag paste)

Paste

A

Paste

B

Paste

A

Paste

B

Paste A Paste

B

155 3.1 2.9 39 618 815 21.3 27.2

116 2.4 2.0 53 507 597 18.9 21.6

102 1.6 1.4 68 512 546 20.1 21.1

71 1.2 0.9 111 454 567 21.4 24.8

▪ Laser doping with sheet resistance of 100-120 Ω/sq is chosen for device integration.

▪ For these doping conditions only tips of texture pyramids are molten, Jsc loss is limited

(< 0.02 mA/cm2)

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OUTLINE

▪ Development of laser doped FSF from PSG layer

▪ Reducing contact fraction by dash printing

▪ Pushing the limits of standard screen printing by double print

▪ Cell results

▪ Conclusions and outlook

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METHOD

▪ Use double printing

▪ First print dashes- contact and high J0

metal

▪ 2nd print non contact and low J0 metal

▪ Similar approach also reported

recently at

▪ At Silicon PV conference 2019*

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REDUCE EFFECTIVE CONTACT FRACTION/ J0 METAL

n+

Ag paste

(STD) for fire-

through

contact

Non-contacting paste

for low damage to SiNx

passivation

SiNx

n++

Silicon substrate n

or p type

*Zih-Wei Peng et. Al. Silicon PV 2019

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RESULTS DASH PRINTING

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SPECIFIC CONTACT RESISTANCE & JO GAIN

▪ J0 metal contribution can be reduced → iVoc gain upto 5 mV from n-side

▪ By reducing contact fraction of fire through paste by ~ 65 % (from 3 % to ~ 1 %)

▪ However, some increase in specific contact resistance from 2.8 to 3.3 mOhm.cm2

702

704

706

708

710

712

714

0 0.01 0.02 0.03 0.04

iVoc

(mV

)

Metal contact fraction of fire-through paste

2.0

2.5

3.0

3.5

4.0

4.5

5.0

0 0.2 0.4 0.6 0.8 1 1.2

Speci

fic

conta

ct r

esi

stan

ce

(m.o

hm

.cm

2)

Relative metal contact fraction of fire through

paste

Guide to the eye

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OUTLINE

▪ Development of laser doped FSF from PSG layer

▪ Reducing contact fraction by dash printing

▪ Pushing the limits of standard screen printing by double print

▪ Cell results

▪ Conclusions and outlook

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IMPROVEMENTS IN SCREEN PRINTING

▪ Knotless meshes not suitable for selective emitter/FSF

▪ Reported to deform significantly and have lower lifetime

[M. Galiazzo, 2017].

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BACKGROUND AND MOTIVATION

M. Galiazzo, 7th Workshop on Metallization & Interconnection for c-Si Solar Cells (2017)

ITRPV roadmap: 2019

Predicts screen printed finger width:

~ 37 µm in 2019 & width ~20 µm in 2029

Finger width µm

2019

Screen deformation

Measured by finger interdistance

We focus on standard meshes with 22.5

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LINE WIDTH REDUCTION

Timeline March 2017 Dec 2017 March 2018 July 2018 May 2019 May 2019

Finger

opening45 µm 30 µm 25 µm 20 µm 15µm 10 µm

Print single print double print double print double print double print double print

Av. width 50 µm 40 µm 30 µm 26 µm 24 µm 17 µm*

Image

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WITH STANDARD SCREENS (22.5) AND FIRE THROUGH PASTES

* 17 µm lines still have some interruptions and some areas are very thin

▪ More work required to print < 20 µm lines without interruptions

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CONFOCAL MICROSCOPY

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BASELINE PROCESS

Finger width = 30 µm

Average height = 18 µmMinimum height = 12 µm

Max height = 21 µm

-30 µm print

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OPTICAL MICROSCOPY

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BEST PRINT

Average linewidth ~ 17 µm

- Analyzed by ImageJ software

- It is defined as:

20 X image 50 X image 50 X image

-17 µm lines using 10 µm line opening

Av. Line width (µm) =

area under printed finger

(µm2)

__________________

length of measured

finger (µm)

Are

a under

pri

nte

d fin

ger

Lengt

h o

f m

eas

ure

d fin

ger

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OUTLINE

▪ Development of laser doped FSF from PSG layer

▪ Reducing contact fraction by dash printing

▪ Pushing the limits of standard screen printing by double print

▪ Cell results

▪ Conclusions and outlook

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BEST CELL RESULTS

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USING LASER DOPED FSF AND SCREEN PRINTING DEVELOPED IN THIS WORK

Light I-V parameters (*externally confirmed) of champion rear-emitter nPERT Monofacial &

Bifacial cells. (Cell area=244.1 & 244.3 cm2 respectively)

▪ Efficiency > 23 % has been achieved with rear junction n-pert cells

without polysilicon passivated contacts

[1] L Tous EUPVSEC 2018 Brussels

[2] Press release imec & Jolywood April 2019

Cell type Busbars

Finger

width

[µm]

jsc[mA/cm2]

Voc[mV]

FF

[%]

eta

[%]

BiFi

[%]

Monofacial 0 40 41.1 683.3 82.0 23.03 [1] 0

Bifacial 12 30 40.3 693 83.1 23.2 [2] 80

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CONCLUSIONS AND OUTLOOK

▪ Process for laser doping from PSG to form selective FSF has been developed.

▪ The dash printing process could further reduce the effective J0n++metal

▪ Screen printed fingers down to 17 µm can be printed with 22.5 standard mesh

▪ However, more work needed to improve aspect ratio and reduce interruptions.

▪ Efficiency > 23 % has been achieved with rear junction n-pert cells without passivated

contacts

▪ The developed processes could also be implemented in PERC/PERT cells

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Acknowledgements

The authors thank Jolywood for funding part of this work.

This work was also partially funded by the Kuwait Foundation for the Advancement of Sciences

(KFAS) under the project number the project number P115-15EE-01.

Imec is a partner in EnergyVille (www.energyville.be), a collaboration between the Flemish research

partners KU Leuven, VITO, imec, and UHasselt in the field of sustainable energy and intelligent energy

systems.

Thank you for your attention!

http://www.energyville.be/

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▪ ANNEX slides

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LASER DOPING EXPECTED TO INCREASE MARKET SHARE

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P-TYPE