SLIDE 1
Characterisation of the contacting interface ISC: Sara Olibet, - - PowerPoint PPT Presentation
Characterisation of the contacting interface ISC: Sara Olibet, - - PowerPoint PPT Presentation
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 Gtz, Gunnar Schubert ECN: Anna
SLIDE 2
SLIDE 3
Introduction: Screen-printed Ag contact
Fast co-firing
rature (°C) Silver paste Silver & glass Olibet et al, ACPV workshop, Oslo, June 20th, 2012
Ag-paste consisting of Ag-powder and glass frit for SiN etching, adhesion to Si and Ag melting temperature reduction
3 Temperatu ° Time (s)
SLIDE 4
Introduction: Contact formation process
T˂ 550°C Organics burn out Glass-frit etches SiNx Redox reaction Si & glass 550°C<T˂ 700°C 700°C<T˂ 800°C
Schubert, PhD thesis, Konstanz, 2006
Olibet et al, ACPV workshop, Oslo, June 20th, 2012 4 T˂ 550°C Liquid Pb melts Ag Ag-Pb melt reacts with Si Ag recrystallises on cool-down 550°C<T˂ 700°C 700°C<T˂ 800°C 700°C<T˂ 800°C 700°C<T˂ 800°C Room temperature
SLIDE 5
Introduction: Microscopic view on resulting contact
Ag-crystallite
Glass
Ag-finger Ag-colloids Ag-finger
Glass
Olibet et al, ACPV workshop, Oslo, June 20th, 2012 5
Si
Ag-crystallite
Si
Ag-colloids TEM by Per Erik Vullum, Sintef 100 nm
SLIDE 6
Introduction: Possible current flow paths
Olibet et al, ACPV workshop, Oslo, June 20th, 2012 6 Hilali, PhD thesis, Georgia Tech USA, 2005
SLIDE 7
Introduction: Beyond contact resistivity
Contact induced recombination
Rfront-surface
Rfront-contact
R
Rfront-contact
Remitter
Rfront-surface
Rfront-contact
Old Ag-paste Either: Or: New
Remitter
Olibet et al, ACPV workshop, Oslo, June 20th, 2012 7
RBSF
Rbulk
Rback-contact
Remitter
Rbulk
Rfront-surface
Rback-contact Rfront-contact
New Ag-paste
RBSF
SLIDE 8
» 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
Introduction: Beyond contact resistivity
Remitter
Rfront-surface
Rfront-contact
Remitter
Rfront-surface
Rfront-contact
Olibet et al, ACPV workshop, Oslo, June 20th, 2012 8
Rbulk
Rback-contact
RBSF
Rbulk Rfront-surface
Rback-contact
RBSF
SLIDE 9
Characterisation
Characterization of contacting interfaces from real solar cell devices Current-Voltage (IV) incl SunsVoc
Olibet et al, ACPV workshop, Oslo, June 20th, 2012 9
incl SunsVoc Electroluminescence (EL) » Series resistance (and recombination) Transfer Length Method (TLM) » Contact resistance ρC
13 14 27 23 ρc (mOhmcm2)
SLIDE 10
Characterisation: SEM after etch-back
Selective silver/glass etch
sel Ag etch sel Ag+sel glass etch full etch
Olibet et al, ACPV workshop, Oslo, June 20th, 2012 10 »Glass under Ag-finger » Imprints of etched-away directly connected Ag- crystallites » Imprints of directly connected Ag-crystallites » Ag-crystallites that were before underneath glass » Imprints of all Ag- crystallites
SLIDE 11
Reconnection of contact elements by liquid Ag after selective etch-back
Application: Identification of dominant current path
Original All Ag grown Ag grown in Si, direct contacts Glass, direct contacts Bare emitter, all Ag and glass Cabrera et al., JAP 2011 direct current current through glass Olibet et al, ACPV workshop, Oslo, June 20th, 2012 11
Surface Ag-paste ρC (mΩ cm2)
Pyr text New 6 ≤ 1 1.5 700 1300 Old 8 ≤ 1 3.5 20 1500 Flat (NaOH) New 50 3.5 3.5 700 1300 Old 200 20 25 200 2000
grown into Si contacts removed contacts removed glass removed
»Dominant current through Ag-crystallites in direct contact with Ag-finger
Cabrera et al., JAP 2011
SLIDE 12
Application: Direct contact frequency
Selective Ag-etch: View on glass, direct contacts removed
Textured, new paste: ρC = 6 mΩ cm2 Flat, old paste: ρC = 200 mΩ cm2
Olibet et al, ACPV workshop, Oslo, June 20th, 2012 12
2 µm
» Glass-free pyramid tips with imprints of etched-away directly contacted Ag-crystallites » ~25% of pyramid tips contain direct contacts » Homogeneous glass-layer covering whole Si surface
1 µm
SLIDE 13
30 1.5 1.7 1.2 1.2 76 80 79.9 79.9 79.8
75 80 40 80
Application: Study topography dependence of contact formation
Si pyramid height variation
ρc [mΩ cm²] FF [%]
Cabrera et al., submitted for publication
Olibet et al, ACPV workshop, Oslo, June 20th, 2012 150 60
60 65 70 80 120 160 200
13
10 µm 10 µm
Very small Large Ultra small Flat Small Std
10 µm 10 µm 10 µm 10 µm
SLIDE 14
Application: Study topography dependence of contact formation
Si pyramid tip rounding
1.2 1.6 2.3 79.9 80 80
79 80 81 82 2 4 6 FF [%] ρc [mΩ cm²]
10 µm 10 µm
Olibet et al, ACPV workshop, Oslo, June 20th, 2012 14 14 74.4
74 75 76 77 78 79 6 8 10 12 14 16
Strongly rounded Slightly rounded Std Rounded
10 µm 10 µm
SLIDE 15
Application: Study topography dependence of contact formation
Si pyramid height variation
Olibet et al, ACPV workshop, Oslo, June 20th, 2012 15
Si pyramid tip rounding
~ Glass layer thickness
SLIDE 16
With emitter doping, strongly rounded: Homogeneous glass layer No emitter = no dead-layer: Glass-free pyramid tips with direct contacts
Application: Study topography dependence of contact formation
Olibet et al, ACPV workshop, Oslo, June 20th, 2012
» Surface sharpness dependent wetting of highest importance for direct contact formation and thus low contact resistivity
16
1 µm
See also poster Cabrera tonight
SLIDE 17
Current conduction through Ag- crystallites grown into Si
Ag
Emitter profile/Ag-crystallite depth Doping dependent contact resistivity
Olibet et al, ACPV workshop, Oslo, June 20th, 2012 17
Ag
Schubert, PhD thesis, Konstanz, 2006
» Importance of Ag-crystallite geometry for contact resistivity
SLIDE 18
Ag-crystallite geometry
Crystal orientation dependence of Schottky-barrier Depth-dependence of Schottky barrier because of varying P-doping concentration in the emitter
Olibet et al, ACPV workshop, Oslo, June 20th, 2012 18 500 nm
Ag-crystallite density depends on crystal defects, such as dead-layer from emitter doping, sharp surface topography
SLIDE 19
Hipersol paste testing results
Model paste consisting of Ag, PbO, B2O3 and SiO2 only
Ref paste, ref firing profile Hipersol paste, lower T Hipersol paste, higher T
Full contact etch
Olibet et al, ACPV workshop, Oslo, June 20th, 2012 19
Voc = 623 mV ρC = 7 mΩ cm2 Voc = 594 mV ρC = 264 mΩ cm2 Voc = 587 mV ρC = 23 mΩ cm2
2 µm 2 µm 2 µm
» Too aggressive Si etching reduces Voc, also at low temperature, where Ag-crystallite formation does not yet take place
SLIDE 20
Hipersol paste testing results
» Best compromise between contact formation and glass etching away emitter
Olibet et al, ACPV workshop, Oslo, June 20th, 2012 20
Ag
2 µm
SLIDE 21
Hipersol paste testing results: Explanation
» Best compromise between contact formation and glass etching away emitter
From Koduvelikulathu et al., npv workshop, Amsterdam, 2012
Emitter Metal Penetration
100 200 300 400 500 600 700 50 100 400 420 440 460 480 500 520 540 560 580 600 620 640 660 680 700
Voc (mV) Metal Penetration Depth (nm) Deep Emitter Shallow Emitter
Olibet et al, ACPV workshop, Oslo, June 20th, 2012 21
p-Emitter
Emitter Metal Penetration
100 200 300 400 500 600 700 50 100 400 420 440 460 480 500 520 540 560 580 600 620 640 660 680 700
Voc (mV) Metal Penetration Depth (nm) Deep Emitter Shallow Emitter
SLIDE 22
Conclusions
Direct contacts are of highest importance for current conduction
Olibet et al, ACPV workshop, Oslo, June 20th, 2012
Trade-off Ag-crystallite formation ↔ Ag-crystallite penetration, Si etching
22 2 µm
SLIDE 23
Outlook
Reduce contact recombination Replace silver
Olibet et al, ACPV workshop, Oslo, June 20th, 2012 23 ITRPV roadmap 2012
SLIDE 24