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EVALUATION OF FLEXOGRAPHIC PRINTING TECHNOLOGY FOR MULTI BUSBAR SOLAR CELLS

A. Lorenza, A. Senneb, J. Rohdec, S. Kroha, M. Wittenberga, K. Krügera, F. Clementa, and D. Biroa

aFraunhofer ISE, FreiburgbContiTech Elastomer-Beschichtungen GmbH, Northeim

cZecher GmbH, Paderborn

Metallization WorkshopConstance, October 21st 2014

www.ise.fraunhofer.de

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MotivationSolar Cell Metallization Costs

Screen printing still state-of-the-art technology

Drawbacks: Limited throughput, cost-intensive screens, high silver consumption andlimited finger widths

Objectives: Higher throughput rates

Reduce costs of printing consumables

Reduce silver consumption

Reduce shading lossesShare of cell production costs without Si wafer calculated with CoO Calculation Tool SCost [1], Ag-Price 562.00 €/Kg, 14.10.2014. [1] S. Nold et al., 27th EU PVSEC 2012

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ApproachFlexographic Printing Technology

Front and rear side metallizationusing rotational printing

Flexography - relief printing technique, flexible printing plate

High-speed roll-to-roll machines used in package printing

FhG ISE and TU Darmstadt introduced this technology for Si solar cells in 2011 [2]

[2] M. Frey et al., Energy Procedia 11 (2011)

Flexographic printing platform for solar cell metallization

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TechnologyFlexographic Printing Technology

Significantly higher throughputthan screen printing possiblevFX 50 - 600 m/min.vSP 12 m/min.

Low costs of printing plates(10 – 25 € per plate)

Low amount of ink transferred

Approx. 7-10 mg Ag per Wafer

Thin layer thickness (2-4 µm)

SEM images of flexo double-printed contact finger (top) and screen printed contact finger (bottom)

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TechnologySeed & Plate vs. fully flexo printed Metallization

Flexo-printed seed layer + Light-induced plating (LIP) Solar cell results up to 18.8 %

demonstrated by ISE [3]

CoO-Calculation demonstratedconsiderable cost saving potential

Fully flexo-printed metallization Single or double printing

process

Highly interesting for Multi Busbar Solar cells

Solar cell with flexo printed seed layer front side metallization + Ag-LIP[3] A. Lorenz et al., JPMTR (2014)

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TechnologyFlexographic Printing Technology – Schematic

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TechnologyFlexographic Printing Technology – Schematic

500 µm

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TechnologyFlexographic Printing Technology – Schematic

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TechnologyFlexographic Printing Technology – Schematic

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TechnologyFlexographic Printing Technology – Schematic

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TechnologyElastomer Printing Plates

New Approach: Laser-engraved elastomer-based

Plates

Nominal widths down to wn 5 µm can be realized

Considerably longer operation time (than polymer plates)

Resistant against mostsolvents

Low costs per plate

Flexographic printing form with H-patterncell layout / SEM image of Finger-Busbar cross-section

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Experimental ApproachTest layout

Experimental approach: Test layout with finger elements

wn,min = 5µm to wn,max = 50 µm3 identical sections

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Experimental ApproachAnilox roll with differently engraved band sections

Experimental approach: Test layout with finger elements

wn,min = 5µm to wn,max = 50 µm3 identical sections

Evaluation of optimum anilox roll 3 band sections with differentdip volumes (in cm³/m²)

DHex1 = 11.8 cm³/m²

DHex2 = 8.7 cm³/m²

DSquare = 10.1 cm³/m²

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Experimental ApproachPrinting process

Experimental approach: Test layout with finger elements

wn,min = 5µm to wn,max = 50 µm3 identical sections

Evaluation of optimum anilox roll 3 band sections with differentdip volumes (in cm³/m²)

Double printing with intermediatedrying

Printed test form on silicon wafer during theexperiment

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Experimental ApproachOptical and electrical characterization

Experimental approach: Test layout with finger elements

wn,min = 5µm to wn,max = 50 µm3 identical sections

Evaluation of optimum anilox roll 3 band sections with differentdip volumes (in cm³/m²)

Double printing with intermediatedrying

Characterization of finger width wf,finger height hf and lateral resistance RL

Flexo printed contact finger with nominal width wn = 10 µm

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Experimental ApproachSimulation of Solar Cell Results

Experimental approach: Test layout with finger elements wn,min = 5µm to wn,max = 50 µm

3 identical sections

Evaluation of optimum anilox roll 3 band sections with differentdip volumes (in cm³/m²)

Double printing with intermediate drying

Characterization of finger width wf,finger height hf and lateral resistance RL

Calculation of rs,f and FF according to [4] and [5]

[4] T. Fellmeth et al., IEEE J-PV 4 (2014)[5] A. Mette, Dissertation 2007

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Experimental ApproachSimulation of Solar Cell Results

Experimental approach: Test layout with finger elements wn,min = 5µm to wn,max = 50 µm

3 identical sections

Evaluation of optimum anilox roll 3 band sections with differentdip volumes (in cm³/m²)

Double printing with intermediate drying

Characterization of finger width wf,finger height hf and lateral resistance RL

Calculation of rs,f and FF according to [4] and [5]

Simulation of flexo printed multi busbar solar cells

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Results and DiscussionFinger widths (double printing + intermediate drying)

Experimental Results: wf mainly depending on printing

pressure, dip volume (anilox roll) and nominal width wn

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Results and DiscussionFinger widths (double printing + intermediate drying)

Experimental Results: Finger widths down to

wf,min = 33 µm could be achieved

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Results and DiscussionFinger height

Experimental Results: Finger widths down to

wf,min = 33 µm could be achieved

hf between 5 and 8 µm f

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Results and DiscussionLateral conducitivity of fingers

Experimental Results: Finger widths down to

wf,min = 33 µm could be achieved

hf between 5 and 8 µm

Finger resistance per unit lengthRL,FX 500 -1500 /m(Screen printed fingers:RL,SP 40 – 70 /m)

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Results and DiscussionContribution to series resistance / fill factor loss

No. of Busbars

Finger res.[/m]

# Fingers Contribution of fingers to Rs

[cm²]

Fill factorloss [%]

5 1000 200 0.629 3.5810 1000 130 0.240 1.3715 1000 115 0.120 0.6820 1000 110 0.070 0.40

Assumptions: Multi-busbar solar cell with 5, 10, 15 and 20 copper busbar wires

Uniform fingers on the whole cell

Contact finger width wf = 40 µm

Number of contact fingers has been optimized individually

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Results and DiscussionSimulation of Solar Cell Results

Assumptions: Cz-Si, RSH = 75 /sq, B = 2 cm Al BSF rear side Finger width wf = 40 µm Contact Resistance c = 3.5 mcm² Specific Resistance Copper

BB wires BB = 1.68 µcm Diameter BB wires dBB = 200 µm

Varied parameters: Lateral finger resistance RL Number of contact fingers

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Results and DiscussionSimulation of Solar Cell Results

Assumptions: Cz-Si, RSH = 75 /sq, B = 2 cm Al BSF rear side Finger width wf = 40 µm Contact Resistance c = 3.5 mcm² Specific Resistance Copper

BB wires BB = 1.68 µcm Diameter BB wires dBB = 200 µm

Varied parameters: Lateral finger resistance RL Number of contact fingersFlexo printed

contact fingers

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Flexographic Printing for Multi-Busbar Solar CellsSummary + Outlook

Experimental results:

Flexo double printed contact fingers down to 33 µm demonstrated

Lateral resistance RL 500 - 1500 /m

Contribution to rs / FF and Simulation of solar cell results underline the potential of flexographic printing for multi busbar solar cells

Challenges and further research:

Optimize process stability, reduce tolerances of materials and process

Verify the results of the fundamental research by realizing flexo printedmulti busbar cells

Profound CoO-Calculation

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Thank you for your attention!

… and all Co-workers at PVTEC … as well as our industry partners who supported this work:

Andreas.Lorenz@ise.fraunhofer.de