Optimisation of MWT cells and modules

Transcription

1 Optimisation of MWT cells and modules Lowering the cost of foil-based back-contact PV MWT Workshop Shanghai 19 th May

2 Why back-contact cells and modules? Limitations of H-pattern cells and modules: Increased performance of cells requires thicker and/or wider tabs for interconnection Wider tabs result in more shadowing Thicker tabs result in a stiffer interconnection and so more thermomechanical damage during module manufacture and operation Thinner cells and higher performance metallisation pastes result in cracking during cell and module manufacture Stringing process requires a lot of cell handling which will have an effect on manufacturing yield Zemen et al. 25 th EUPVSEC, Valencia, 2010

3 Current design MWT cells and modules 4x4 vias and 3x5 rear-side contacts Metallisation pattern optimised for highest conductivity with lowest coverage Balance of fill-factor and current generation Conductive back-sheet foil Copper as conductive layer Patterning by chemical etching or milling Surface protection on copper to prevent corrosion Encapsulant between cell and conductive back-sheet 200 µm Dictates minimum conductive adhesive dot height and diameter 4*4 via design Interdigitated Cu foil CA interconnection 3

4 MWT module manufacturing process Combined interconnection and lamination 4

5 FF25 [-] Outdoor testing: 36-cell MWT module 0.90 Module fill-factor Testing since April 2007 >5 years outdoors stable performance year

6 Conductive back-sheet foil optimisation No insulation lacquer Better reliability No Ag contact pad Lower cost Cheaper patterning Mechanical milling or roll-to-roll chemical etching 35 µm 250 µm 30 µm 6

7 Reduced use of conductive adhesive Current stencil for adhesive print 400 µm thick with 1.7 mm diameter holes for 200 µm encapsulant Volume conductive adhesive = 0.90 mm 3 Stencil for 100 µm encapsulant Volume conductive adhesive = 0.16 mm 3 >70% lower volume in conductive adhesive More contact points possible without cost penalty r h 7

8 Results of module manufacture with thin encapsulant Several encapsulants available in 100 µm thickness including EVA, ionomers and TPO Punching of encapsulant possible Lamination profile compatible with conductive adhesives Performance and reliability proven 8

9 Optimisation metallisation: reduction Ag consumption on cell Similar to multi bus-bar approach R fingers L 2 reduce finger length For MWT, reduce finger length with factor 2 by going from: 4 4 to 8 8 9

10 Optimisation metallisation pattern on cell Finite element model of efficiency as function of number of vias and resistance metallisation Optimal efficiency taking into account A open area and R series For narrow lines (35 µm) optimum above 10x10 vias Consequences for module cost? 10

11 Optimisation module design Addition of cost of conductive adhesive and conductive back-sheet foil Modelled using low-cost adhesive and thin encapsulant Approx. 4% cost reduction possible by optimisation front-side pattern 11

12 Optimisation of MWT conductive back-sheet foil Replacement copper by aluminium Contact to aluminium Native oxide increases contact resistance Contact coating needed Evaporation of copper over full area Hanita 4 th MWT Workshop Local application of copper at contact spots Copper cold-spray Cheap and well developed technique R.C. Dykhuizen e.a, J. Therm. Spray Technol., Vol 8 (No. 4), Dec 1999, p

13 Cold-spray technology R.C. McCune e.a., ASM International,1995, p 1 6; International Thermal Spray Association 13

14 Results 2x2 cell modules Patterned Al-foil Mask for CS-Cu Mask + CS-Cu Al-foil + CS-Cu Conductive foil Δfill-factor cell-to-module Soft aluminium + CS-Cu 3.0 Annealed aluminium + CS-Cu 2.1 Copper

15 Results 2x2 cell modules climate chamber tests Soft aluminium with CS-Cu Annealed aluminium with CS-Cu Copper with CS-Cu Copper 15

16 Potential cost reduction Copper foil Aluminium foil Aluminium foil + CS-Cu Mass 520 g 250 g 255 g Price % 16

17 Compatibility with copper plated cells Copper plated MWT cells compared to standard cells in climate chamber testing Reduced cell cost due to reduced silver consumption 17

18 Contact resistance [µohm] Results contact resistance and peel strength Interval Plot of OSP; Tin; Silver finished solar cells 95% CI for the Mean OSP Tin Silver Material finishing 90 (peel) 180 (shear) OSP 0,5-1 N N Tin 0,3-1 N N Silver 0,3-1 N 9-24 N Contact resistance lowest on tin coating, highest on OSP Compare well with results on fired silver metallisation Lower than contact resistance to copper back-sheet foils with OSP (approx. 500 µω) Peel test highest on OSP Comparable with peel test on fired silver metallisation (1-2 N typical for 90 test) Failure between tab and conductive adhesive

19 Further cost advantages to be found in: Use of thinner wafers <120 µm Lower cost interconnection pastes Further reduction in silver content Higher through-put and yield in manufacture Highly automated production line Higher cell and module power output Low CTM fill-factor loss only 0.8% for full-size modules 19

20 Conclusions MWT cells and foil-based back-contact module developments demonstrate competitive cost road-map Cost reduction shown for Cell metallisation Interconnection paste Conductive back-sheet foil 4% reduction of module cost by optimisation Potential saving of > 2 per module by use of Al foil MWT cells and modules: low cost, high performance, durable 20