Process Development for a High-Throughput Fine Line Metallization Approach Based on Dispensing Technology

Transcription

1 Process Development for a High-Throughput Fine Line Metallization Approach Based on Dispensing Technology Rheology, CFD-Simulation and Print-Head Design M. Pospischil, M. Klawitter, M. Kuchler, J. Specht, H. Gentischer, R. Efinger, C. Kroner, M. Luegmair, M. König, M. Hörteis, C. Mohr, L. Wende, J. Lossen, M. Weiß, O. Doll, I. Koehler, R. Zengerle, F. Clement and D. Biro 4 th Workshop on Metallization for Crystalline Silicon Solar Cells, May 2013, Konstanz, Germany

2 Motivation Front-Side Metallization Requirements: Robust contact formation High conductivity Few shading losses Little (Ag-) paste consumption Source: Mette, A Source: Mette, A., Dissertation Thesis, University of Freiburg,

3 Screen Printing as Industrial Standard Metallization Process Established, reliable single step metallization approach Robust contact formation Relatively high throughput rates Finger homogeneity (meshmarks and paste spreading) Low aspect ratio (= height : width) Screen wear Mechanical load on wafer 3

4 Dispensing Technology for Front Side Metallization Possibilities and Challenges Non-impact, single step process High resolution, high aspect ratio using similar Ag-pastes Improvement of cell-efficiencies +0.3%abs (1) record: 20.6% (2) Challenges: Process stability p p Accuracy (precise line start/-stop) v Throughput Parallelization 4 * Sources: (1) Specht et al., EUPVSEC 2010, (2) Lohmüller et al., IEEE Electron, 2011

5 Developing an Industrial Metallisation Process Interacting Fields of Research and Engineering Rheology Simulation Paste Development (external) Hardware Design & Construction Process Development Contact Formation Technology & Material Evaluation Integration in PV Production Process 5

6 Rheological Characterisation Shear Flow of Non-Newtonian Fluids Velocity Profile (Laminar pipe flow) τ < τ y Herschel Bulkley Newtonian n = 1 Non Newtonian n 1 n > 0 n < 0 n = 0 y k y ( ) k n n1 6 Herschel, W.H. and R. Bulkley, Konsistenzmessungen von Gummi-Benzollösungen. 1926: p

7 Rheological Characterisation Determination of Characteristic Yield Stress Increasing shear stress until paste starts to yield yield stress τ y Strong influence on aspect ratio 7,cone M Source: Macosko, C.W., Rheology: principles, measurements, and applications. 1994: VCH. cone Paste A 10 1 Paste B y,a = 613 Pa y,b = 1882 Pa (Pa)

8 Rheological Characterisation Capillary Viscosimeter Measurement at high shear rates Recording pressure drop depending on flow rate w p c 4 D L w Q R n Paste A A 10 5 Paste B B 10 2 (Pa) (Pas) Paste A A Source: Macosko, C.W., Rheology: principles, measurements, and applications. 1994: VCH. Paste B (s -1 ) B

9 Developing an Industrial Metallisation Process Interacting Fields of Research and Engineering Rheology Simulation Paste Development (external) Hardware Design & Construction Process Development Contact Formation Technology & Material Evaluation Integration in PV Production Process 9

10 Computational Fluid Dynamics - Theory Laws of Conservation Mass Momentum t v t v. const 0 v 2 v v p ij r g q( x) E( f a Influenced by Rheology Energy Dissipative heat due to friction f p f Navier-Stokes Equations f V x) 10 U Uv q T : D t. const Source: Macosko, C.W., Rheology: principles, measurements, and applications. 1994: VCH. DT 2 cp kt T T : Dt D

11 Volume Flow Q (µl/s) Verification of Simulation Comparison with Reality Rheology + CFD- Simulation Flow measurements in experiment Experiment Conical Nozzle (Ø=80µm) Variation of Dispensing Pressure p 3.5 Rheologic Modelling Herschel-Bulkley y k n Paste A dispensed simulated Paste B dispensed simulated Simulation Pressure p depending on flowrate Target Pressure p (10 5 Pa)

12 Challenges of Dispensing Comparison of Nozzle Geometries (Operating Pressure) Metal Pastes Rheology: high viscous, Non-Newtonian, thixotropic, yield stress fluid Highly filled medium (agglomeration clogging) Process Demands Finest fingers (width~30µm) Nozzle diameter: Ø~40µm Printing velocity (~200mm/s) Low operating pressure p c L 4 D 12

13 Challenges of Dispensing Comparison of Nozzle Geometries (Local Pressuredrop) Metal Pastes Rheology: high viscous, Non-Newtonian, thixotropic, yield stress fluid Highly filled medium (agglomeration clogging) Process Demands Finest fingers (width~30µm) Nozzle diameter: Ø~40µm Printing velocity (~200mm/s) Low operating pressure 13

14 Challenges of Dispensing Comparison of Nozzle Geometries (Velocity Profile) Metal Pastes Rheology: high viscous, Non-Newtonian, thixotropic, yield stress fluid Highly filled medium (agglomeration clogging) Process Demands Finest fingers (width~30µm) Nozzle diameter: Ø~40µm Printing velocity (~200mm/s) Low operating pressure 14

15 Challenges of Dispensing Comparison of Nozzle Geometries (Shear strain rate) Metal Pastes Rheology: high viscous, Non-Newtonian, thixotropic, yield stress fluid Highly filled medium (agglomeration clogging) Process Demands Finest fingers (width~30µm) Nozzle diameter: Ø~40µm Printing velocity (~200mm/s) Low operating pressure 15

16 Challenges of Parallel Dispensing Robust Print Head Design Procedure: Homogeneous flow 10 nozzles with Ø = 40µm Variation of specific nozzles: Ø = 40µm Ø = 45µm w w 0 m m Impact on Dispensing Process: Deviation of mass flow rate 22% Resulting finger width ±4.2µm Higher silver consumption Possible line interruptions High impact of fab. tolerances! 16

17 Developing an Industrial Metallisation Process Interacting Fields of Research and Engineering Rheology Simulation Paste Development (external) Hardware Design & Construction Process Development Contact Formation Technology & Material Evaluation Integration in PV Production Process 17

18 Transfer into Prototype Parallel Dispensing Print-Head Modular Setup Print-head with paste distribution Exchangeable Nozzle-Adapter 10 x Ø = 40µm, Pitch: 1.55mm 18

19 Developing an Industrial Metallisation Process Interacting Fields of Research and Engineering Rheology Simulation Paste Development (external) Hardware Design & Construction Process Development Contact Formation Technology & Material Evaluation Integration in PV Production Process 19

20 Evaluation Printing Tests and First Cell Processing Printing Results Low system pressure (~4 bar) Homogeneous flow 10 nozzles achieved Processing of Cell Batch 156x156 mm² Cz p-type Si Standard Al + preprinted Busbars Efficiency increase 0.2% abs. compared to screen printing (grid not yet optimized) Finger widths on cells: 30±1µm Record finger widths < 30µm 20

21 Parallel Dispensing Process Summary + Outlook Rheology + Simulation: Successful integration of shear rheology into CFD-simulation Verification of paste model and steady state simulation Evaluation of different nozzle and print-head designs using CFD Development of a 10 nozzle parallel dispensing unit Hardware + Process: Launching of novel 10-nozzle print-head prototype successful First cell processing demonstrated, η ~+0.2%abs. Line width < 30µm demonstrated Outlook: Advanced cell processing + start & stop accuracy 21

22 Thank you for your attention! and all Co-workers within the Dispensing Project and at PVTEC as well as our industry partners: This work was supported by the German Federal Ministry of Environment, Nature Conservation and Nuclear Safety under contract number