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
Electrically conductive adhesives with low silver content based on
capillary suspensions - Katrin Dyhr
Institute for Mechanical Process Engineering and Mechanics
Applied Mechanics Group
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)
Electrically conductive adhesives with low silver content based on
capillary suspensions - Katrin Dyhr
Institute for Mechanical Process Engineering and Mechanics
Applied Mechanics Group
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
Electrically conductive adhesives with low silver content based on
capillary suspensions - Katrin Dyhr
Institute for Mechanical Process Engineering and Mechanics
Applied Mechanics Group
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
Electrically conductive adhesives with low silver content based on
capillary suspensions - Katrin Dyhr
Institute for Mechanical Process Engineering and Mechanics
Applied Mechanics Group
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
Electrically conductive adhesives with low silver content based on
capillary suspensions - Katrin Dyhr
Institute for Mechanical Process Engineering and Mechanics
Applied Mechanics Group
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
Electrically conductive adhesives with low silver content based on
capillary suspensions - Katrin Dyhr
Institute for Mechanical Process Engineering and Mechanics
Applied Mechanics Group
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
particle volume fraction fsol / -
regular suspension
capillary 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
particle volume fraction fsol / -
regular suspension
capillary 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
Electrically conductive adhesives with low silver content based on
capillary suspensions - Katrin Dyhr
Institute for Mechanical Process Engineering and Mechanics
Applied Mechanics Group
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
particle volume fraction fsol / -
capillary suspension
regular suspension
contact resistivity rc / mW · cm²
Distance between ribbons because of different
flow properties not constant
greek cross method
Electrically conductive adhesives with low silver content based on
capillary suspensions - Katrin Dyhr
Institute for Mechanical Process Engineering and Mechanics
Applied Mechanics Group
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
Electrically conductive adhesives with low silver content based on
capillary suspensions - Katrin Dyhr
Institute for Mechanical Process Engineering and Mechanics
Applied Mechanics Group
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
Electrically conductive adhesives with low silver content based on
capillary suspensions - Katrin Dyhr
Institute for Mechanical Process Engineering and Mechanics
Applied Mechanics Group
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
Electrically conductive adhesives with low silver content based on
capillary suspensions - Katrin Dyhr
Institute for Mechanical Process Engineering and Mechanics
Applied Mechanics Group
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