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 156x156mm 2 7.5A at 1 sun 210x210mm 2 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/ o C) silicon (Si) 4 metals (Al, Cu, Ag) ~20 Thicker tabs mandatory to maintain high efficiencies and/or fill factors Lead-free soldering Sn 60 Pb 40 (183 o C) Sn 96.5 -Ag 3.5 (221 o C) 125µm tab tab 225µm 330µm cell cell 200µm 125µm tab tab 225µm 4
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 200 o C 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 85 o C TC200: 40 o C to +80 o C Observations Stable contact resistance Corroded magnets 8
Soldered and glued interconnections Tab Cell busbar Adhesive residue on tab Tab Cell busbar Adhesive residue on busbar Si-Ag alloy break-out residue on tab Si-Ag alloy break-out from cell 9
90 o peel strength test results mean stdev fracture Magnetic interconnections... 0.15 N n.a. Soldered interconnections 2.02 N 0.93 N break out Conductive adhesive Busbar without glass fiber brushing. 1.89 N 0.12 N cohesive Busbar with glass fiber brushing 1.91 N 0.10 N cohesive 10
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 Test Fill factor Magnetic Temperature cycling initial Fill factor intermed. 71.5 50-69 (30 cycles) Fill factor after test testing stopped Temperature cycling 71.6 71.4 70.6 Conductive adhesive UV exposure 68.9 69.3 68.3 Roof 71.4 71.3 71.0 Soldered reference Stored in office 71.1 71.0 71.0 13
Laminates using magnetic interconnections EVA underfills tab-busbar Tab Cell (Aluminium) EVA underfill 14
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