Characterisation of the contacting interface ISC: Sara Olibet, Enrique Cabrera, Dominik Rudolph, Radovan Kopecek ISC: Sara Olibet, Enrique Cabrera, Dominik Rudolph, Radovan Kopecek Sunways: Daniel Reinke, Anne Götz, Gunnar Schubert ECN: Anna Carr, Martien Koppes, Jaap Hoornstra, Arthur Weeber, Kees Broek Olibet et al, ACPV workshop, Oslo, June 20th, 2012
Outline Introduction: Screen-printed thick-film silver contact Characterisation of the contacting interface » Identification of the dominant current path » Topography dependent contact formation » Topography dependent contact formation » Hipersol model paste testing results Conclusions Outlook Olibet et al, ACPV workshop, Oslo, June 20th, 2012 2
Introduction: Screen-printed Ag contact Silver Silver & paste glass Fast co-firing rature ( ° C) ° Temperatu Time (s) Ag-paste consisting of Ag-powder and glass frit for SiN etching, adhesion to Si and Ag melting temperature reduction Olibet et al, ACPV workshop, Oslo, June 20th, 2012 3
Introduction: Contact formation process Schubert, PhD thesis, Konstanz, 2006 Organics burn out Redox reaction Si & glass Glass-frit etches SiNx T˂ 550 ° C T˂ 550 ° C 550 ° C<T˂ 700 ° C 550 ° C<T˂ 700 ° C 700 ° C<T˂ 800 ° C 700 ° C<T˂ 800 ° C Liquid Pb melts Ag Ag-Pb melt reacts with Si Ag recrystallises on cool-down 700 ° C<T˂ 800 ° C 700 ° C<T˂ 800 ° C Room temperature Olibet et al, ACPV workshop, Oslo, June 20th, 2012 4
Introduction: Microscopic view on resulting contact Ag-finger Glass Ag-finger Glass Ag-crystallite Ag-colloids Ag-colloids Ag-crystallite Si 100 nm Si TEM by Per Erik Vullum, Sintef Olibet et al, ACPV workshop, Oslo, June 20th, 2012 5
Introduction: Possible current flow paths Hilali, PhD thesis, Georgia Tech USA, 2005 Olibet et al, ACPV workshop, Oslo, June 20th, 2012 6
Introduction: Beyond contact resistivity Contact induced recombination R front-contact Old Ag-paste R front-surface Or: Either: R emitter R front-contact R front-surface R emitter New New R front-contact R front-contact R R front-surface Ag-paste R emitter R bulk R bulk R BSF R back-contact R BSF R back-contact Olibet et al, ACPV workshop, Oslo, June 20th, 2012 7
Introduction: Beyond contact resistivity » Ag-pastes no more limiting standard c-Si solar cells, but in advanced cell concepts with passivated rear, recombination under contact fingers becomes again dominant R front-contact R front-contact R front-surface R front-surface R emitter R emitter R bulk R bulk R BSF R BSF R front-surface R back-contact R back-contact Olibet et al, ACPV workshop, Oslo, June 20th, 2012 8
Characterisation Characterization of contacting interfaces from real solar cell devices Current-Voltage (IV) incl SunsVoc incl SunsVoc Electroluminescence (EL) » Series resistance (and recombination) ρ c (mOhmcm 2 ) Transfer Length Method (TLM) » Contact resistance ρ C 13 14 27 23 Olibet et al, ACPV workshop, Oslo, June 20th, 2012 9
Characterisation: SEM after etch-back Selective silver/glass etch sel Ag etch sel Ag+sel glass etch full etch »Glass under Ag-finger » Imprints of directly » Imprints of all Ag- » Imprints of etched-away connected Ag-crystallites crystallites directly connected Ag- » Ag-crystallites that were crystallites before underneath glass Olibet et al, ACPV workshop, Oslo, June 20th, 2012 10
Application: Identification of dominant current path Reconnection of contact elements by liquid Ag after selective etch-back direct current current through glass Ag grown in Glass, Bare emitter, All Ag direct all Ag and Si, direct Original Cabrera et al., JAP 2011 Cabrera et al., JAP 2011 grown grown contacts contacts contacts contacts glass glass into Si removed removed removed cm 2 ) Surface Ag-paste ρ C (mΩ New 6 ≤ 1 1.5 700 1300 Pyr text Old 8 ≤ 1 3.5 20 1500 New 50 3.5 3.5 700 1300 Flat (NaOH) Old 200 20 25 200 2000 »Dominant current through Ag-crystallites in direct contact with Ag-finger Olibet et al, ACPV workshop, Oslo, June 20th, 2012 11
Application: Direct contact frequency Selective Ag-etch: View on glass, direct contacts removed Textured, new paste: ρ C = 6 mΩ cm 2 Flat, old paste: ρ C = 200 mΩ cm 2 2 µm 1 µm » Glass-free pyramid tips with imprints of » Homogeneous glass-layer etched-away directly contacted Ag-crystallites covering whole Si surface » ~25% of pyramid tips contain direct contacts Olibet et al, ACPV workshop, Oslo, June 20th, 2012 12
Application: Study topography dependence of contact formation Cabrera et al., submitted for publication ρ c Si pyramid height variation FF [mΩ cm²] 1.5 1.7 1.2 1.2 [%] 0 30 80 80 79.9 79.9 79.8 40 75 76 80 80 10 µm 10 µm 10 µm 10 µm 70 10 µm 120 10 µm 65 160 150 60 200 60 Flat Ultra small Small Std Large Very small Olibet et al, ACPV workshop, Oslo, June 20th, 2012 13
Application: Study topography dependence of contact formation Si pyramid tip rounding ρ c FF [mΩ cm²] [%] 0 82 1.2 1.6 2.3 2 81 4 80 80 80 79.9 6 6 79 79 10 µm 10 µm 10 µm 10 µm 8 78 10 77 12 76 14 75 14 74.4 16 74 Std Slightly rounded Rounded Strongly rounded Olibet et al, ACPV workshop, Oslo, June 20th, 2012 14
Application: Study topography dependence of contact formation Si pyramid height variation Si pyramid tip rounding ~ Glass layer thickness Olibet et al, ACPV workshop, Oslo, June 20th, 2012 15
Application: Study topography dependence of contact formation No emitter = no dead-layer: With emitter doping, strongly rounded: Glass-free pyramid tips with Homogeneous glass layer direct contacts 1 µm » Surface sharpness dependent wetting of highest importance for direct contact formation and thus low contact resistivity See also poster Cabrera tonight Olibet et al, ACPV workshop, Oslo, June 20th, 2012 16
Current conduction through Ag- crystallites grown into Si Doping dependent Emitter profile/Ag-crystallite depth contact resistivity Ag Ag Schubert, PhD thesis, Konstanz, 2006 » Importance of Ag-crystallite geometry for contact resistivity Olibet et al, ACPV workshop, Oslo, June 20th, 2012 17
Ag-crystallite geometry Crystal orientation dependence of Schottky-barrier Depth-dependence of Schottky barrier because of varying P-doping concentration in the emitter 500 nm Ag-crystallite density depends on crystal defects, such as dead-layer from emitter doping, sharp surface topography Olibet et al, ACPV workshop, Oslo, June 20th, 2012 18
Hipersol paste testing results Model paste consisting of Ag, PbO, B 2 O 3 and SiO 2 only Full contact etch Ref paste, ref firing profile Hipersol paste, lower T Hipersol paste, higher T 2 µm 2 µm 2 µm Voc = 623 mV Voc = 594 mV Voc = 587 mV ρ C = 7 mΩ cm 2 ρ C = 264 mΩ cm 2 ρ C = 23 mΩ cm 2 » Too aggressive Si etching reduces Voc, also at low temperature, where Ag-crystallite formation does not yet take place Olibet et al, ACPV workshop, Oslo, June 20th, 2012 19
Hipersol paste testing results » Best compromise between contact formation and glass etching away emitter Ag 2 µm Olibet et al, ACPV workshop, Oslo, June 20th, 2012 20
Hipersol paste testing results: Explanation » Best compromise between contact formation and glass etching away emitter From Koduvelikulathu et al., npv workshop, Amsterdam, 2012 700 700 680 680 660 660 640 640 Emitter Metal Penetration Emitter Metal Penetration 620 620 600 600 580 580 560 560 540 540 520 520 Voc (mV) Voc (mV) 500 500 Deep Emitter Deep Emitter p-Emitter 480 480 460 460 Shallow Emitter Shallow Emitter 440 440 420 420 400 400 100 100 50 50 0 0 0 0 100 100 200 200 300 300 400 400 500 500 600 600 700 700 Metal Penetration Depth (nm) Metal Penetration Depth (nm) Olibet et al, ACPV workshop, Oslo, June 20th, 2012 21
Conclusions Direct contacts are of highest importance for current conduction Trade-off Ag-crystallite formation ↔ Ag-crystallite penetration, Si etching 2 µm Olibet et al, ACPV workshop, Oslo, June 20th, 2012 22
Outlook Reduce contact recombination Replace silver ITRPV roadmap 2012 Olibet et al, ACPV workshop, Oslo, June 20th, 2012 23
Acknowledgements The research leading to these results has received funding from the Seventh Framework Programme under grant agreement n ° 228513 (HiperSol). ° Thank you for your attention! Olibet et al, ACPV workshop, Oslo, June 20th, 2012 24
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