Electrically conductive adhesives (ECA) with low silver ...

www.kit.edu

Institute for Mechanical Engineering and Mechanics

Applied Mechanics Department

KIT – The Research University in the Helmholtz Association

Electrically conductive adhesives (ECA) with

low silver content for the interconnection of solar cells

Katrin Dyhr, Nils Klasen (Fraunhofer ISE) and Prof. Dr. Norbert Willenbacher

Electrically conductive adhesives with low silver content based on

capillary suspensions - Katrin Dyhr

Institute for Mechanical Process Engineering and Mechanics

Applied Mechanics Group

The market share of conductive adhesives for the interconnection of solar cells is expected to rise from 3 to over 20%

Reasons are new cell concepts, other interconnection technologies such as shingling and the reduction of solders containing lead

02.11.20202

Conductive Adhesives for Cell Interconnection

ITRPV 11th Edition,

April 2020





Ternary solid/liquid/liquid–system

Secondary and bulk liquid must be immiscible

Attractive capillary forces lead to the formation of particle networks

The rheology changes yield stress tunable in a wide range

02.11.20203

Capillary Suspension

Hydrophobically CaCO3 in diisononyl phthalate (r = 0.8 mm,

f = 0.111) with added water (q = 139.2°)

E. Koos and N. Willenbacher,

Science 331(6019), 897 (2011)





02.11.20204

Pendular vs Capillary State

Pendular state Capillary state

secondary liquid

is better wetting

bulk phase is

better wetting

Θ < 90° 90°< Θ < 151°

Capillary bridges

between

particles

Particle cluster

around liquid

droplet

F. Bossler and E. Koos, Langmuir 2016

Model system based on fluorescent glass particles

and index-matched solvents





Formation of capillary-force-driven particle network reduces percolation threshold

02.11.20205

Reduction of Silver Volume Fraction

Low silver content

“Capillary state“ capillary suspension

high compatibility between silver

particles and bulk fluid

High cyclic

stability





Adhesive A

Commercially available epoxy based adhesive

Curing < 3 min at 180 °C

Sec. liquid to solid ratio 0.1

Silver Flakes with

x50,3 = 5.7 µm

x90,3 = 12.5 µm

Adhesive B

Commercially available epoxy based adhesive

Curing < 1 min at 160 °C

Sec. liquid to solid ratio 0.06

Silver Flakes with

x50,3 = 6.0 µm

x90,3 = 15.0 µm

6

Materials

5 µm

gylcerol based hydrophilic

secondary fluid

5 µm

02.11.2020





Yield stress for capillary suspensions more than two decades higher

Regular suspensions show no yield stress below 8 vol%

02.11.20207

Flow Properties of Adhesive Formulations

101 102 103 104

10-4

10-3

10-2

10-1

100

101

102

103

shear stress t / Pa

deformation g / %

sy= 220 Pa

regular

suspension

capillary

suspension

0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20100

101

102

103

104

particle volume fraction fsol / -

A B

capillary suspension

regular suspension

yield stress sy / Pa





Capillary suspensions show lower percolation threshold than regular suspensions

Capillary suspensions exhibit high conductivity

at low particle concentration

02.11.20208

Percolation Threshold

0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20101

102

103

104

105

106


regular suspension


electrical conductivity k / S × m-1

B

percolation treshhold via power law fit:

fsol,c=0.04 fsol,c=0.08

0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16101

102

103

104

105

106


regular suspension


electrical conductivity k / S × m-1

A

percolation treshhold via power law fit:

fsol,c=0.03 fsol,c=0.08

VSenseI I

adhesiveglass slide

4 point conductivity

measurement





Contact resistivity is not influenced by the capillary suspension concept

All samples have a very low contact resistivity,

even if they are non-conductive in lateral direction

02.11.20209

Contact Resistivity of Adhesive A

I

V

adhesive

ribbon

0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.1610-5

10-4

10-3

10-2

10-1



regular suspension

contact resistivity rc / mW · cm²

Distance between ribbons because of different

flow properties not constant

greek cross method





Capillary network does not alter mechanical properties

E-Modulus slightly lower for lower particle concentrations

02.11.202010

Mechanical Properties of the Adhesive A

regular

suspension,

10vol% silver

regular

suspension,

5vol% silver

capillary

suspension,

5vol% silver

0

200

400

600

800

1000E-Modul / N/mm2

regular

suspension,

10vol% silver

regular

suspension,

5vol% silver

capillary

suspension,

5vol% silver

0

20

40Tensile strength / N/mm²





Solar cells where connected to mini modules via shingling technology

02.11.202011

Proof of Concept

Capillary suspension Regular Suspension

Silver content 5 vol % 10 vol %

Yield stress 272 ± 38 Pa 15 ± 1Pa

Efficiency 17.0 ± 0.5 % 18.1 %

Poor contact formation between

bus bar and adhesive

Capillary suspension with

lower yield stress = reduced

secondary fluid content

Adhesive B: work in progress





Capillary suspension concept applicable for ECA formulation

Percolation threshold, i.e. silver consumption can be significantly reduced

Shingled mini-module as proof of concept

further refinement of contact formation, i.e. adhesive rheology necessary

02.11.202012

Summary and Conclusion





Financial support by…

the foundation 100 prozent erneuerbar

Thank you for your attention

https://100-prozent-erneuerbar.de/

Electrically conductive adhesives (ECA) with low silver ...

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