Metallization Workshop 5 th workshop on metallization of crystalline solar cells 2014 Combined microstructural and electrical characterization of metallization layers in industrial solar cells P. Kumar, B. Willsch, M. Dürrschnabel, Z. Aabdin, R. Hönig, N. Peranio, F. Clement, D. Biro and O. Eibl Institute of Applied Physics Eberhard Karls Universität Tübingen Auf der Morgenstelle 10, D-72076 Tübingen, Germany Contact us on : www.uni-tuebingen.de/elmi
Outline Introduction Structure of the solar cells and microstructure of metallization interface Textured vs. planar cells, electrical properties, pastes used Methodology Sample preparation in plan-view and cross-sectional view Electron microscopy: SEM/TEM Imaging EDX chemical composition analysis of the Silicon metallization interface (SMI) Correlation of microstructure with the contact resistance and conclusions for the current path Summary and conclusions
Goals of the investigation Goals of the investigation Motivation: (i) Correlation of microstructural features with paste used and the contact resistance of the cells (ii) what is the relevant current path? 1. Glass layer (Model I) Wetting behavior and thickness of glass layer Chemical composition of glass layer Size and density of Ag colloids SiN x layer present or not, thickness 2. Ag crystallites at SMI (Model II) Size and density of Ag crystallites Planar samples: effect of crystallographic orientation
Models for the current path at the metallization interface Model I nano-ag colloids assisted tunneling via thin glass layer Z.G.Li et.al., J.Appl Phys 105, 066102 (2009) Model II Current path via Ag-crystallites C. Ballif et.al., Appl Phys Lett 82, 1878-1880 (2003)
Process flow and strategy for characterization Solar cell processing at Fraunhofer ISE Cz-Si, 156 x 156 mm² psq 200mm, ρ base 1-3 Ωcm Randomization + Labeling Alkaline texture POCL 3 diffusion 75 Ω/sq PSG etch PECVD SiN X -ARC Rear side screen print (full Al-BSF), ISE reference paste Front side screen print of FSP1 / FSP2 (paste supplier A / C) Fast firing in inline belt furnace @ optimum firing temperature Edge isolation (by laser) IV & SunsV OC measurement (Specific) contact resistance measurement (by TLM) Electrical and microstructural characterization in Tuebingen Talk #4 this session
Paste and Si orientation affecting the electrical properties of solar cells FSP1 Front side pastes FSP2 FSP3 All cells processed within MikroSol project, at Fraunhofer ISE Supplier A Supplier C Carefully selected cells out of more than 2000!! Geometry Textured (optimally fired) planar Solar cells Substrate FSP Paste T FF0 [ C] η (%) ρ c [mω.cm 2 ] Rs [Ω*cm²] Rp [Ω*cm²] #1 (2129) Cz-Si <100> FSP1 900 16.9 21 0.90 4451 #2 (2017) Cz-Si <100> FSP2 900 17.8 4.7 0.74 35506 #3 (1004) Cz-Si <100> FSP2 900 18.0 5 0.61 32123 #1 (1236) Fz-Si <100> FSP1 930 7.8 33 #2 (1204) Fz-Si <100> FSP3 930 16.1 - #3 (1111) Fz-Si <111> FSP3 930 17.1 4.9 Czochralski grown Si for textured samples, Float zone for planar samples R. Hoenig, M. Duerrschnabel, W. Mierlo, Z. Aabdin, J. Bernhard, J. Biskupek, O. Eibl, U. Kaiser, J. Wilde, F. Clement, D. Biro; Energy Procedia 43 (2013) 27-36
Advanced methodology for microstructural analysis For SEM SEM (SE, BSE and EDX) (TEM, EFTEM EELS &EDX) Mechanical grinding & polishing followed by ion milling is the essential tool for high quality images and chemical analysis For each sample more than 6 areas were analyzed in detailed HRSEM images about 70,000 magnification achieved
Wetting behaviour of textured cells 2129 (FSP1) and 2017 (FSP2) FSP1, Cell (2129) FSP2, Cell (2017) Glass Ag Si C Zn 2 SiO 4 BSE image in plan-view Disontinuous wetting (glass) layer for FSP1 Thickness of glass layer 0.2-0.7 µm Zn 2 SiO 4 rich phases present ρ c = 21 mω.cm 2 Glass Ag Si C BSE image in plan-view Continuous wetting (glass) layer for FSP2 Thickness of glass layer 0.07-2 µm No Zn 2 SiO 4 rich phases present ρ c = 5 mω.cm 2 R. Hoenig, M. Duerrschnabel, W. Mierlo, Z. Aabdin, J. Bernhard, J. Biskupek, O. Eibl, U. Kaiser, J. Wilde, F. Clement, D. Biro; Energy Procedia 43 (2013) 27-36
Wetting behaviour of textured cell 1004 contacted with FSP2 paste BSE image FSP2, Cell (1004) SE image (a) (b) 120 Ag colloids/µm² nano-ag colloids Ag-crystallites Si Ag Si Glass Continuous wetting (glass) layer for FSP2, Thickness of glass layer 100 nm-1 µm No Zn rich oxide phase detected Importantly, Ag crystallites are found which grow at the edges or intersection of {111} planes Large density of Ag colloids few tens of nm to 120 nm embedded in the glass layer Plan view analysis is essential to see the Ag crystallites at the edges of (111) Si faces
EFTEM-EDX-analysis of textured cells 2129 (FSP1 ) and 2017(FSP2) 2129 (FSP2) 2017 (FSP2) Comparison of glass layer spectra of both pastes FSP1 & FSP2 Quantitative TEM-EDX analysis of glass layer Phase Glass FSP1 Glass FSP2 Si [at.%] Ag Zn Pb Ti O [at.%] [at.%] [at.%] [at.%] [at.%] 26.7 4.9 8.3 12.3 1.4 46.6 35.1 3.5 1.7 11.9 0.0 47.8
Planar cells: orientation of the substrates affects the microstructure Low magnification BSE images High magnification BSE images Cell (1236),<100> Si, Paste : FSP1 ρ c = 33 mω.cm 2 7 Ag crystallites/µm² Cell (1204),<100> Si, Paste : FSP3 10 Ag crystallites/µm² Cell (1111), <111> Si, Paste : FSP3 ρ c = 4.9 mω.cm 2 110 Ag colloids/µm² No Ag crystallites!! For each sample more than 6 areas have been investigated in detail Planar solar cells serve as model systems for microstructural analysis
Ag crystallite formation: on (100) planes not on (111) planes Previous results: only (100) investigated planar textured Note: Ag crystallite formation appears on (100) planar surfaces but not on (111) faces of (100) textured Si : no Ag crystallites on (111) faces, only at edges of the pyramids!! S. Kontermann et.al., Appl Phys Lett 99, 191910-3 (2010) No contradictions in results, only in conclusions
MikroSol project: more than 2000 solar cells were processed and electrically characterized Textured and planar solar cells; planar cells serve as a model systems for correlating the microstructural features of the cells with the orientation of Si surface Mechanical polishing followed by ion milling: the essential tool for high quality cross sectional and planview sample preperation, Textured cells Contact resistance mainly depends on the paste used. Paste affecting properties: Clear correlation of electrical properties of cells with microstructure Paste FSP1 FSP2 Ag crystallites grow at the edges of (111) planes Planar cells Discontinuous glass phase Continuous glass phase Summary and conclusions Microstructure features for textured cells Zn-rich oxide phases No Zn-oxide phases Small density of nano-ag colloids Large density of nano-ag colloids ρ c [mω.cm 2 ] crystallographic orientation of the Si controls Ag crystallite formation: Ag crystallites impinging into the Si emitter found only for planar (100) Si surfaces not for planar (111) orientation lower contact resistance found for cells with planar (111) Si surfaces as compared to (100) Rp [Ω*cm²] 21 (High) 4451 (low) 4.7 (low) 35506 (High) (111)planar cells: Ag crystallites not necessary for low contact resistance. Statement also true for textured cells. Consequently: glass layer and Ag colloids are the relevant current path in textured cells.
Thank You Very much for your attention Nicola Peranio Praveen Kumar Benjamin Willsch The Baden-Wuerttemberg Stiftung is gratefully acknowledged for financing within the research project MikroSol (U23). Contact us on : www.uni-tuebingen.de/elmi