Screen printing on high sheet resistance epitaxial emitters...where the BSF and the base are grown...

Screen‐printing on high sheet resistance epitaxial emitters

M. Recamán, M. Norton, S. de Vecchi, H. Sivaramakrishnan, K. Van Nieuwenhuysen, Jan Van Hoeymissen, F. Dross, J. Poortmans

2nd Workshop on Metallization for Crystalline Si solar cellsKonstanz, 14th‐15th of April 2010

Outline

Screen-printing on high sheet resistance epitaxial emitters IMEC 2010

• Introduction

• Contacting epi emitters by screen‐printing

• Evaluation of Ag pastes

• Conclusions

2


• Introduction



• Conclusions

3

Introduction: diffused emitters

Screen-printing on high sheet resistance epitaxial emitters IMEC 2010 4

SIMS profiles (Ag and P)

Trend towards high Rsheet emitters.

Why? ...

Better blue response

Improved effective emitter surface passivation

Challenges...

Contact resistance

Junction shunting

Source:M. M. Hilali, B. To, A. Rohatgi, A review and understanding of screen‐printed contacts and selective‐emitter formation, presented at the 14th Workshop on Crystalline Silicon Solar Cells and Modules, Colorado, 2004.

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0

1E14

1E15

1E16

1E17

1E18

1E19

1E20

Conc

entr

atio

n (a

t/cm

3 )

Depth (μm)

Introduction: epi emitters


CVD n‐type emitter (As)

POCl3 diffused n+‐type FSF (P)

SRP profile after creation of the epi emitter

Versatile design of an epi emitter (n‐type As).

Thickness and doping can be independently tuned as needed


Investigation of the epi emitter profile.

N‐type (As) emitter: thickness and doping

N‐type POCl3 diffused (P) FSF

50 60 70 80 90 100 110 120 130 140 15029,0

29,5

30,0

30,5

31,0

31,5

J sc (m

A/cm

2 )

Rsheet

(Ω/square)


IV results: Jsc versus emitter Rsheet

300 400 500 600 700 800 900 1000 1100 12000

10

20

30

40

50

60

70

80

90

100

SCREEN‐PRINTED CONTACTS epi emitter + diffused FSF (140 Ω/sq.) epi emitter + diffused FSF (106 Ω/sq.) diffused emitter (60 Ω/sq.)

IQE (%

)

Wavelength (nm)

Spectral response

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0

1E14

1E15

1E16

1E17

1E18

1E19

1E20

Conc

entr

atio

n (a

t/cm

3 )

Depth (μm)



SRP profile after creation of the epi emitter

Versatile design of an epi emitter (n‐type As).

Thickness and doping can be independently tuned as needed

Lowly‐doped and thick profile is possible.

No compromise between shunting and dead‐layer

CVD n‐type emitter (As)

Diffused POCl3 n+‐type FSF (P)


Thick epi emitter.

Ag does not get close

to the p‐n junction

SIMS profile after metallization


Source:M. M. Hilali, B. To and A. Rohatgi

1.00E‐03

1.00E‐02

1.00E‐01

1.00E+00

1.00E+01

1.00E+02

1.00E+03

1.00E+04

1.00E+05

1.00E+14

1.00E+15

1.00E+16

1.00E+17

1.00E+18

1.00E+19

1.00E+20

1.00E+21

0 500 1000 1500 2000 2500 3000 3500

Intensity (Cou

nts/s)

Concen

tration (at/cm

3 )

Depth (nm)

POCl3 diffusion: P conc (at/cm3)

Epitaxy: As conc (at/cm3)

Screen‐printing paste: Ag (counts/s)

Asand P conc. are only trustable after the transient region



Epi cells (multi‐c Si) as “vehicle” on which the emitter performance is tested.

Why? ...

Creation of the epi emitter only requires few more seconds in the CVD step where the BSF and the base are grown (total active device thickness ∼20 µm)

However...

This emitter profile could be also implemented in other solar cell structure

Scheme of the epi emitter structure at solar cell level


• Introduction



• Conclusions

10

Contacting epi emitters by screen‐printing

11Screen-printing on high sheet resistance epitaxial emitters

IMEC 2010

SEM after Ag contact and glass frit layer removal

The FSF is necessary for contact formationMore Ag crystallites imprints

epi emit. (As): bulk and surface conc. 5∙1017 at/cm3

epi emit. (As): bulk conc. 5∙1017 at/cm3

+diffused FSF (P): surface conc. 2∙1019 at/cm3

Lowly‐doped emitter difficult contactNot enough Ag crystallites imprints

Ag crystallites imprints


IMEC 2010

Epitaxial solar cells (multi‐c Si) results

No cells BEST CELLS (∼106 Ω/sq)

Eff. (%) FF (%) Jsc (mA/cm2) Voc (mV)

1 14.5 77.7 30.3 614

2 14.5 78.6 30.2 612

epi emit. (As): bulk and surface conc. 5∙1017 at/cm3

epi emit. (As): bulk conc. 5∙1017 at/cm3

+diffused FSF (P): surface conc. 2∙1019 at/cm3



IMEC 2010

Ag contact

Si substrate

glass

SEM just after metallization: epi emitter (bulk As conc. 5∙1017 at/cm3)+ POCl3 diffused FSF (surface P conc. 2∙1019 at/cm3)


However...

The presence of Ag crystallites is not enough to have a good contact

Because...

Although high FFs (and Rs<1 Ω∙cm2) are achieved, not always reproducible

Probably too thick glass frit layer at the Ag/Si interface!


IMEC 2010

Evaluation of Ag pastes

6,0

7,0

8,0

9,0

10,0

11,0

12,0

13,0

14,0

15,0

870 890 910 920 930 940 950 960 970 990

efficien

cy (%

)

temperature in the 3rd heating zone (ºC)

A

B

C

D

E

reference

Commercial Ag pastes: A, B, C, D, E and reference

lR furnace: constant belt speed and variable set‐point T in the 3rd heating zone

Epi cells (multi‐c Si) IV results: efficiency versus firing T

Several pastes (A, B, E and reference) lead to similar solar cell performance

1 solar cell per condition

40

45

50

55

60

65

70

75

80

85

870 890 910 920 930 940 950 960 970 990

FF before HF dip (%

)


ABCDEreference


IMEC 2010


Too thick glass frit layer prevents good contact

So far…

No paste gives reproducible optimal results

Paste A shows a good potential for contacting these emitters

Epi cells (multi‐c Si) IV results: FF (before and after HF dip) versus firing T

40

45

50

55

60

65

70

75

80

85

870 890 910 920 930 940 950 960 970 990

FF after HF dip (%

)


A

B

C

D

E

reference

However…

Screen‐printing still possible on ≥100 Ω/square emitters after glass frit removal

Epi emitters

Diffused emitters

Average FF (38 cells) = 77.2±0.8 %


IMEC 2010


Paste A (∼106 Ω/sq)


14.2 76.8 30.3 611

Paste A (∼127 Ω/sq)


14.0 75.3 30.7 605


• Introduction



• Conclusions

17

Conclusions


• FF up to 77 % were obtained for screen‐printed solar cells epi cells (∼20 µm) with epi emitters ≥100 Ω/square

• The presence of Ag crystallites is not enough to have a good contact

• Too thick glass frit layer is responsible of lack of reproducibilityOptimization of the firing conditions and/or reformulation of the Ag pastes 18

BEST CELLS (∼106 Ω/sq)


1 14.5 77.7 30.3 614

2 14.5 78.6 30.2 612

Screen‐printing on high sheet resistance epitaxial emitters

M. Recamán, M. Norton, S. de Vecchi, H. Sivaramakrishnan, K. Van Nieuwenhuysen, Jan Van Hoeymissen, F. Dross, J. Poortmans

2nd Workshop on Metallization for Crystalline Si solar cellsKonstanz, 14th‐15th of April 2010

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Screen printing on high sheet resistance epitaxial emitters...where the BSF and the base are grown...

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