low-stress interconnection of solar cells

LOW-STRESS INTERCONNECTIONS OF SOLAR CELLS
Paul C. de Jong, D.W.K. Eikelboom, J.A. Wienke, M.W. Brieko, M.J.H. Kloos
Outline
• How to maintain €/Wp reduction with standard cells
• How to cope with thermo-mechanical stresses
• Test results of stress-free interconnections
• Mechanically clamped (magnetic) interconnections
• Interconnections with conductive adhesives
• Conclusions
2
Maintaining €/Wp reduction with standard cells
• Reduction of materials usage
• Larger cells
– 156x156mm2 → 7.5A at 1 sun
– 210x210mm2 → 13A at 1 sun
• Thinner cells (from 330µm to 150-200µm)
• Simplification of the module manufacturing process
• No extra yield losses
• Improving cell and module efficiencies
3
Yield problems
• Thermo-mechanical stresses caused by TCE mismatches
material
TCE (ppm/oC)
silicon (Si)
4
metals (Al, Cu, Ag)
~20
• Thicker tabs
– mandatory to maintain high efficiencies and/or fill factors
• Lead-free soldering
– Sn60Pb40 (183oC) → Sn96.5-Ag3.5 (221oC)
4
125µm
tab
tab
225µm
330µm
cell
200µm
125µm
tab
cell
tab
225µm
Stress-free interconnections
LARGE STRESS
IR Soldering
Spot Soldering
Ultrasonic Welding
Thermal Spraying
Conductive Adhesives
Mechanical Clamping
LOW STRESS
5
Interconnections
• Interconnection
• Soldered tabs onto silver busbars (reference)
• Silver-filled adhesives (types still under development)
– Optional glass-fiber brushing of busbars
• Mechanically clamped with attracting magnets
• Tab material
• Soldered reference: Lead-Tin (PbSn) plated copper
• Adhesive/magnetic: Silver (Ag) plated copper
– Silver-Silver contact on cell
– Silver-oxide is electrically conductive
6
Magnet selection
• Requirements
• Small size
• Large mechanical force
• No degradation after lamination
• Low-cost
• Neoflux (NdFeB) magnets
• Disk: 2mm diameter, 0.5mm height
• Typical attractive force of about 0.15N
• Operational up to 200oC
• High-volume price of 0.01-0.015 €/magnet,
or roughly 0.10 €/Wp
7
Magnetic interconnections
• Interconnection
• 3 pairs of Neoflux disk magnets
• 2mm diameter, 0.5mm height
• Zinc coating
• Tests
• 1000 hours DH 85% RH 85oC
• TC200: –40oC to +80oC
• Observations
• Stable contact resistance
• Corroded magnets
8
Soldered and glued interconnections
Tab
Cell busbar
Si-Ag alloy break-out
residue on tab
9
Adhesive Tab
residue
on tab
Si-Ag alloy
break-out from cell
Cell busbar
Adhesive
residue
on busbar
90o peel strength test results
mean
• Magnetic interconnections………………... 0.15 N
• Soldered interconnections………………… 2.02 N
• Conductive adhesive
Busbar without glass fiber brushing……. 1.89 N
Busbar with glass fiber brushing………… 1.91 N
10
stdev
0.93 N
fracture
n.a.
break out
0.12 N
0.10 N
cohesive
cohesive
Laminates using magnetic interconnections
• Ni-Cu-Ni coated (anti-corrosion) Neoflux magnets
• Minimum of 4 contacts per busbar
• Standard glass-EVA-cell-EVA-backsheet laminates
• Reliability testing
– Temperature cycling (–40/85°C)
11
Laminates using conductive adhesives
• Standard glass-EVA-cell-EVA-backsheet laminates
• Reliability testing
– UV exposure (Suntester) 1000 hours at 60°C
300 h @ 1 SUN + 700 h @ 1.5 SUN
– Temperature cycling
150 cycles – 40°C / + 85°C
– Outdoor
25 days on the roof
12
Reliability test results
Interconnect
Magnetic
Conductive
adhesive
Soldered
reference
13
Test
Fill factor Fill factor Fill factor
initial
intermed. after test
Temperature
cycling
71.5
50-69
(30 cycles)
testing
stopped
Temperature
cycling
71.6
71.4
70.6
UV exposure
68.9
69.3
68.3
Roof
71.4
71.3
71.0
Stored in
office
71.1
71.0
71.0
Laminates using magnetic interconnections
• EVA underfills tab-busbar
Cell (Aluminium)
EVA underfill
14
Tab
Conclusions
• Mechanically clamped interconnects
• Standard glass-EVA-cell-EVA-backsheet laminates
– Proven not to be reliable because of EVA underfill
– Application of magnets does not lead to reliability improvements
– Mechanical contacting may only be successful in encapsulantfree module concepts (e.g. Appolon NICE)
• Joining of the cell-tab interconnection is mandatory
• Conductive adhesives
– Excellent adhesion onto silver-plated copper tabs
– No requirement for pre-treatment of busbar
– Small variations in peel strength; indicator for low stress
– First sequence of reliability tests show no performance
degradation
15