© Fraunhofer ISE

Fundamental Reactions During the Formation of Fired Silver Contacts and Solar Cell Results

M. Hörteis, S. W. Glunz

2nd workshop on Metallization for Crystalline Silicon Solar Cells

Konstanz, 15.04.2010

© Fraunhofer ISE

2

Materials used for the front side metallizationIngredients

Glas frit (Lead oxide, Bismuthoxide…)

Front side silver paste Silver

© Fraunhofer ISE

3

Development of ink/pastes Preparation

© Fraunhofer ISE

4

Paste deposition on the front sideUsed technologies

Printing techniques

Screen printing

Stencil printing

Pad printing

Direct write

Inkjet

Aerosol jet

Extrusion/Dispensing

© Fraunhofer ISE

5

Formation of the contactSchematic

Printed layer

Fired at T= 800°C

optionally enhanced by a plating step

n-emitter

p-base

d=18 µm

© Fraunhofer ISE

6

Reactions during contact formationInk – Solar cell

n-emitterDeposited ink

Opening of the ARC (SiNx) Ink reacts with Silicon Nitride

Contact formation Ink reacts with Silicon

n-emitter

© Fraunhofer ISE

7

Investigated materialsRelated to the contact formation

Silicon

Silicon nitride (ARC)SilverGlas frit

(Lead oxide, Bismuth oxide…)

© Fraunhofer ISE

8

Basic reactions TG-DTA measurement – Si + PbO

M. Hörteis, Adv. Funct. Mater. 2010, 20, 476–484

10 20 30 40 50 60 70 80 90 10011012013099.0

99.2

99.4

99.6

99.8

100.0

mas

s [%

]

time [min]

Si + PbO

0

100

200

300

400

500

600

700

800

900

temperature

tem

pera

ture

[°C

]

mass-signal

Balance

Furnace

Sample

TC1TC2

© Fraunhofer ISE

9

Basic reactions TG-DTA measurement – Si + PbO

-1.5

-1.0

-0.5

0.0

0.5

DTA

[μV

/mg]DTA-Signal

tempe

rature

656°C

10 20 30 40 50 60 70 80 90 1001101201300

100

200

300

400

500

600

700

800

900Si + PbO

time [min]

tem

pera

ture

[°C

]Balance

Furnace

Sample

TC1TC2

© Fraunhofer ISE

10

1

2

3

1: Silicon

2: Lead

3: Glass

Si

Pb

Glas

Si + 2 PbO SiO/PbOglas + 2 Pb>600°C

Inte

nsity

[a.u

.]

Energy [keV]

Si

Inte

nsity

[a.u

.]

Energy [keV]

Pb

Pb

Inte

nsity

[a.u

.]

Energy [keV]

PbSi

O

Basic reactions SEM + EDX analysis – Si + PbO

© Fraunhofer ISE

11

-0.50

-0.25

0.00

DTA

-sig

nal [μV

/mg]

20 40 60 80 100 120 140 160 1800

100

200

300

400

500

600

700

800

900

Tem

pera

tur [

°C]

Si + PbO + Ag

Zeit [min]

Reaction under the presence of silver

Re-crystallization during cooling

Basic reactions TG-DTA measurements – Si + PbO + Ag

© Fraunhofer ISE

12

Basic reactions - Si + PbO + Ag Phase diagram Ag-Pb

800 700 600 500 400 300 200 100

600°C

300°C

DTA

-sig

nal [

a.u.

]

temperature [°C] Ag [at. %]

Eutectic atca. 304°C

600°C

304°C

962°C

tem

pera

ture

[°C

]

© Fraunhofer ISE

13

1

2

Basic reactions - Si + PbO + Ag SEM + EDX analysis

0 2 4 6 8 10 12 14 16 18 20

Ag Pb In

tens

ity [a

.u.]

Energy [keV]

Pb

0 2 4 6 8 10 12 14 16 18 20

Inte

nsity

[a.u

.]

Energy [keV]

Ag

© Fraunhofer ISE

14

Basic reactions Summing-up

Reaction between Lead oxide and Silicon

Lead oxide is reduced to lead, and simultaneously Si is oxidized

Silver is liquefied far below its melting point

Silver – lead melt recrystallizes during cooling

During the reaction glass is produced

© Fraunhofer ISE

15

Basic reactions – Si3N4 + PbOOpening of the passivation-layer

Mass loss at T = 685°C

10 20 30 40 50 60 70 80 90 100 110 120 13098.0

98.2

98.4

98.6

98.8

99.0

99.2

99.4

99.6

99.8

100.0

mas

s [%

]

time [min]

mass

685°C

SiNx - PbO

0

100

200

300

400

500

600

700

800

900

tem

pera

ture

[°C

]

© Fraunhofer ISE

16

Basic reactions – Si3N4 + PbOOpening of the passivation-layer

10 20 30 40 50 60 70 80 90 100 110 120 13098.0

98.2

98.4

98.6

98.8

99.0

99.2

99.4

99.6

99.8

100.0

mas

s [%

]

time [min]

mass

685°C

-17.5

-15.0

-12.5

-10.0

-7.5

-5.0

-2.5

0.0

2.5

5.0

PbO - SiNx

SiNx - PbO

DTA

-Sig

nal [

µV/m

g]

Mass loss at T = 685°C

Exothermic reaction

© Fraunhofer ISE

17

Basic reactions – Si3N4 + PbOOpening of the passivation-layer

10 20 30 40 50 60 70 80 90 100 110 120 13098.0

98.2

98.4

98.6

98.8

99.0

99.2

99.4

99.6

99.8

100.0

6

8

10

12

mas

s [%

]

time [min]

mass

685°C

PbO - SiNx

SiNx - PbO

MS

-Sig

nal [

1x10

-9A]

N2 (28)

12 PbO + 2 Si3N4 6 SiO2 + 12 Pb + 4 N2 (↑)

Mass loss at T = 685°C

Exothermic reaction

Evolution of nitrogen

© Fraunhofer ISE

18

Basic reactions Summing-up

Reaction between Lead oxide and Silicon Lead oxide is reduced to lead, and simultaneously Si/SiNx are oxidizedSilver is liquefied far below its melting pointSilver – lead melt recrystallizes during cooling, forming an el. contactDuring contact formation additional glass is produced

Opening of the passivation Layer (SiNx)Similar reaction as with pure SiliconSiNx is oxidized and glass is formed

Contact formation Formation of an isolating glass layer

© Fraunhofer ISE

19

Ink Optimization Variation of the glass content

Density of contact-crystallites is increased with increasing glass content

For more than 10% of glass, the contact resistance is increased again, due to an increased glass layer

0 10 20 300.0

0.5

1.0

1.5

2.0

2.5

3.0

cont

act r

esis

tanc

e R

cxW [Ω

cm]

glass content [%]

© Fraunhofer ISE

20

Optimized ink, applied on high efficiency solar cellsPrinted and fired contacts vs. evaporated contacts

Is it possible to close the gap between screen printed contacts and high efficiency contacts?

© Fraunhofer ISE

21

Application on High efficiency solar cellsContact geometries

industrial-type contact vs. high-efficiency contact

37 µm

55 µm

© Fraunhofer ISE

22

Application on High efficiency solar cellsPrinted and fired contacts vs. evaporated contacts

Contact on shallow emitter

fine line printing

contact firing through ARC

Light induced plating

3 process steps for the front side metallization

Contact on a deep emitter

photolithographically opening of passivation layer

evaporation of seed contact TiPdAg

Light induced plating

8 process steps for the front side metallization

© Fraunhofer ISE

23

Application on High efficiency solar cellsCell structure

LIP-SilverPrinted and fired contact / evaporated contacts

SiNx-PECVD Antireflexioncoating / thermal grown SiO2

Shallow 110 Ω/sq. POCl3 –emitter / deep 120 Ω/sq. emitter

LFC point contacts

ALD-Al2O3 / PECVD – SiO2

PVD - Aluminum

© Fraunhofer ISE

24

Application on High efficiency solar cellsSolar cell results

Main difference in open circuit voltage

For higher solar cell efficiencies a passivated front side is necessary

Gap between high efficiency solar cells and “industrial-type” solar cells is going to be closed

21.779.839.86844deep120high efficiency type

21.180.140.26564shallow110industrial type

[%][%] [mA/cm²][mV][cm²][Ω/sq.]ηFFJSCVOCARshContacts

best cell results

© Fraunhofer ISE

25

Matthias Hörteis

www.ise.fraunhofer.dematthias.hoerteis@ise.fraunhofer.de

Fraunhofer-Institut für Solare Energiesysteme ISE