Non-contacting Busbars for Advanced Cell Structures using Low Temperature Copper

PasteDon Wood, Nick Powell, Adriana Zambova, Brian Chislea, Pierre Chevalier, Caroline Boulord, Alexandre Beucher, Nicolas Zeghers, Guy Beaucarne

Izabela Kuzma‐Filipek, Richard Russell, Filip Duerinckx, Jozef Szlufcik

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Dow Corning Cu paste : good conductivity at low cost

Substrate

HEAT

Substrate

Copper powder

Solder powder

Polymer

250 °C

ρ = 20 – 40 µΩ cm

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Cure in N2

Combination Ag screen printing – Cu pasteTexturing

PECVD SiNx front

Emitter formation

Junction isolation

Printing rear

Co-firing

Screen printing Ag fingers

Screen printing low-T Cu BB

Screen printing Ag fingers and BB

Wood et al.Silicon PV 2014Energy Procedia Vol.55

• Equivalent cell performance• No detrimental impact due to Cu presence• Full size modules passing 1 x IEC Wood et al. , Energy Procedia Vol.55

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Cure in N2

Cu paste can also be combined with plated cells

Rear metallization

Front end processing

Laser patterning and plating (only fingers)

Screen printing low-T Cu BB

Laser patterning and plating

• Cure at 200 to 300 ºC temperature-sensitive structures can be preserved

• Ni silicide formation/anneal and busbar curing can be done simultaneously

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Does a hybrid Cu plating – busbar printing process make sense ?

YES !• Capex and maintenance

o No laser capacity needed for ablation of busbar areao Screen printer much less capital intensive than laser systemo Fewer lasers → less maintenance

• Expected performanceo Less Si – metal interface area → less recombination → higher Voco Lower probability of shunting caused by laser damage

• Interconnectiono Traditional soldered interconnection can be appliedo Alternative to new, emergent but as of yet unproven interconnection

technologies

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p-type PERC cells with plated fingers

Texturing front and polishing rear

Clean and oxidation (target 120 Ohm/sq)

POCl3 target 130-140 Ohm/sq emitter + junction isolation

Rear laser ablation, Al sputtering and co-firing

PECVD SiNx on the front + PECVD stack on the rear

Full stack sintering

Front laser ablation – only fingers

Front laser ablation fingers and BB

LIP Ni-LIP Cu-LIP Ag plating

p-Cz (Ga), 156mm-sq, 180um, 1.8 Ohm-cm

Cu BB Printing

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Nett performance increase with Cu paste busbar

6 mV higher Vocthanks to floating busbar

0.4 % rel. decrease. Due to bleeding ?

0.8 % abs. higher FF thanks to higher Rshunt

0.1 % abs. higher efficiency. All cells with Cu paste BB have eff. > 20.2 %

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n-PERT cells with plated fingers

TMAH SDR/polishingTMAH SDR/polishing

BSG removal +Wet oxidationBSG removal +Wet oxidation

POCl3 diffusion ‐ FSFPOCl3 diffusion ‐ FSF

TMAH texturingTMAH texturing

PSG removal +Dry oxidationPSG removal +Dry oxidation

Front oxide removalFront oxide removal

Rear PECVD SiOxRear PECVD SiOx

Integration flow- front -end

Laser ablation ‐ rearLaser ablation ‐ rear

ARC front‐PECVD SiNxARC front‐PECVD SiNx

Integration flow- back-end

AlSi PVDAlSi PVD

FG‐anneal‐ contact formationFG‐anneal‐ contact formation

Ni/Cu/Ag platingNi/Cu/Ag plating

Boron diffusion‐ emitterBoron diffusion‐ emitter

Contact co‐sinteringContact co‐sintering

Front Laser: fingers and BBs

Front Laser: fingers and BBs

Front Laser: fingers only

Front Laser: fingers only

Cu BB printingCu BB printing

n-Cz (P), 156mm-sq, 180um, 3.5 Ohm-cmn-type SiN-type FSF

P-type emitter

Front passivationAntireflection coating

Rear passivation

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N-PERT cells with Cu paste busbar: cell results

∆ 5mV average gain for floating BB

Drop in Jsc leads to same efficiency

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Suspected reason for lower Jsc : bleeding

…

180 m

Shaded region

Unaffected regionPrinted Busbar

Plated fingers (LIP)

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…[The busbar is to the left]

‘Shaded Region’

Extent of copper

‘Extent of carbon’

• Wider regions between pyramids in carbon shaded region vs unaffected region• Small copper particles present at the base of the pyramids, carried by solvent • Both copper and carbon cause shading loss• Bleeding issue has been addressed in subsequent formulations and does not appear a fundamental problem

Bleeding : a closer look

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Summary

• A low-temperature screen printing paste based on Cuparticles was developed

• The Cu paste was applied to front busbars in PERC and n-PERT solar cells with Ni/Cu-plated fingers

• Consistent Voc increase thanks to lower recombination under busbars

• Efficiencies up to 20.5 % (PERC) and 20.9 % (n-PERT) have been obtained for 156 mm Cz wafers

• Cu paste busbar process results in solderable and easily measurable cells

• Good control of bleeding desired to make use of full efficiency increase potential