Characterisation of Aluminium screen-printed local contacts a,b F. S. Grasso, a L. Gautero, a J. Rentsch, b R. Lanzafame a Fraunhofer Institute for Solar Energy Systems ISE b Dipartimento di Ingegneria Industriale e Meccanica, Faculty of Engineering, University of Catania, Second Metallisation Workshop Konstanz, 14. 04. 2010
Agenda Motivation Approach of the characterisation Results and first interpretation Case Study: LFC contacts Conclusions Outlook 2
Motivation Passivated Emitter and Rear Cell (PERC) High potential : >23% in lab scale () Local Contacts 1 Blakers et Al. 9th EUPVSEC, Freiburg, 1989 3 Passivated Emitter and Rear Cell (PERC) High efficiency potential: >23% 1
Motivation Contact resistance dependency on Surface doping 2 TE (Thermionic) Schottky barrier FE ρ c ln ρ c FE (Field) TFE (Thermionic Field) Emissions 1 N D Higher surface doping The Schottky barrier becomes Ohmic-like at High surface doping High surface doping decreases recombination at metal contact 3 2 Yu, Solid State Electronics, 1970 3 Altermatt, EUPVSEC, Hamburg, 2009 4
Alloying model: Total Contact From literature on alloying model 4 4 Huster, 20 th EUPVSEC,Barcelona, 2005 5
Motivation Modify, refit existing technology Screen print Passivation Total surface contact Aluminium Paste Aluminium/Silicon Eutectic Passivation p+ type Silicon Local surface contact Out of scale p type Silicon 6
What is so hard in making local back surface contacts? Examples of Aluminium to Silicon Contacts Total surface contact Local surface contact BAD example 7
Contact Sintering - Experimental Surface structuring Track amount of paste Flat + - Process OK! Characterisation Hollow 8
Contact Sintering - Experimental Surface structuring Track amount of paste Flat + - Process OK! Characterisation Hollow 9
Laser Firing - Experimental Track amount of paste LFC + - Process OK! Characterisation 10
Preparation- Experimental Surface structuring Flat KOH Etch Dielectric deposition Local Opening Hollow KOH Etch Hollow Flat 11
Preparation- Experimental Flat Local Opening KOH Etch Dielectric deposition Local Opening KOH Etch ~100µm Passivation Layer 12 Hollow Flat
Preparation- Experimental KOH Etch Dielectric deposition Local Opening Tilted view of a hollow opening KOH Etch The hollow is at maximum 10 μm deep Hollow Flat 13
Preparation- Experimental Amount of paste KOH Etch LFC process as reference Dielectric deposition Local Opening KOH Etch Al Paste printing Firing LFC Hollow Flat 14
Approach Optical characterisation Paste Max Depth amount (µm) (mg/cm²) 7.4 28 ± 5 KOH Etch Dielectric deposition Local Opening KOH Etch Al Paste printing Firing LFC Al removal Hollow Flat 15
Approach Optical characterisation Paste amount (mg/cm²) 6.2 7.9 Max Depth (µm) 45 50 ± ± 1 4 KOH Etch Dielectric deposition Local Opening KOH Etch Al Paste printing Firing LFC Al removal Hollow Flat 16
Approach Optical characterisation Paste amount (mg/cm²) 7.4 17 Max Depth (µm) 28 ± 5 Paste amount (mg/cm²) 6.2 7.9 Max Depth (µm) 45 ± 1 50 ± 4
Approach - Cross Sectioning and Stain Etch 6.2 mg/cm² Concentration (cm -3 ) 1E19 1E18 1E17 1E16 1E15 ECV Al doping meas. 5 Depth Fired Al Al-BSF Eutectic Total contact 6.2 mg/cm² wet Al paste firing peak temperature 900 C. Metal Si Stain etch is suitable due to the metallurgic junction of Aluminium doped Silicon 4 5 Huster, 20 th EUPVSEC,Barcelona, 2005 18
Approach - Cross Sectioning and Stain Etch 20 μm Stain etch applied to local contacts t etch = 80 s 20 μm local contact - Flat 7.9 mg/cm² wet Al paste 109x102µm opening firing peak temperature 900 C. 19
Approach - Cross Sectioning and Stain Etch A. Fused Spherical particles B. Lamellas C. Surrounding BSF layer D. Irregularities in the cavity HOLLOW local contact - Hollow 7.9 mg/cm² wet Al paste 109x102µm opening firing peak temperature 900 C. 20
Fused Spherical particles Approach - Cross Sectioning and Stain Etch 21
Lamellas Approach - Cross Sectioning and Stain Etch Fused Spherical particles Lamellas Lamellas 22
Lamellas Approach - Cross Sectioning and Stain Etch Fused Spherical particles Lamellas Surrounding BSF layer Surrounding BSF layer Lamellas Surrounding BSF layer 23
Lamellas Approach - Cross Sectioning and Stain Etch Fused Spherical particles Lamellas Surrounding BSF layer Irregularities in the cavity Surrounding BSF layer Lamellas Surrounding BSF layer 24
Lamellas Approach - Cross Sectioning and Stain Etch Fused Spherical particles Lamellas Surrounding BSF layer Irregularities in the cavity Surrounding BSF layer Lamellas Surrounding BSF layer 25
Approach - Cross Sectioning and Stain Etch 540nm local contact Flat 6.2 mg/cm² wet Al paste 109x102µm opening firing peak temperature 900 C. FLAT 26
Approach - Cross Sectioning and Stain Etch 50µm 20µm HOLLOW Dielectric Contact local contact - Hollow 7.9 mg/cm² wet Al paste 109x102µm opening firing peak temperature 900 C. Lamellas can be found more than 20µm away from contact Loss of -doped- Silicon 27
Approach - Cross Sectioning and Stain Etch Fusion of Particles Irregularities in the contact formation Thick BSF Thin BSF FLAT No BSF local contact Flat 4.7 mg/cm² wet Al paste 109x102µm opening firing peak temperature 900 C. 28
Interpretation Alloying model: Local contact Dry Al Paste Si T=RT Dielectric 1 T=660 C 2 T=700 C 3 T=825 C 4 5. Thin BSF formation T=700 C 5 6. Si is expulsed, in forms of lamellas, wherever it is T=577 C 6 similar in Beaucarne, First WS Metallization, Utrecht, 2008 29
Case Study: Laser Fired Contact on SP Aluminium 100µm 30
Case Study: Laser Fired Contact on SP Aluminium Before stain etch After stain etch Thin BSF between resin and Silicon 31
Case Study: Laser Fired Contact on SP Aluminium 1. Carbon: due to the presence of the protective layer. 1 2 2. Oxygen 3. Aluminium 3 4 4. Silicon 32
Conclusions Surface doping is necessary for good contacts A method has been developed to characterise doping on local contacts First understanding of results of present state of art Case Study showed the flexibility of the method + - Process OK! Characterisation 33
Outlook Different methods need to be developed to increase the statistical significance Its characteristic are More contacts at a time (averaging) Less invasive (Optical, maybe PL/EL?) More closely related to the functioning principle 34
Special thanks go to Federico Sebastiano Grasso, Jan Nekarda, Miroslawa Kwiatkowska, Aleksander Filipovic for the contribution to this work 35
Thank you for your attention Questions? Fraunhofer-Institut für Solare Energiesysteme ISE Luca Gautero www.ise.fraunhofer.de Luca.Gautero@ise.fraunhofer.de 36