Silicon Solar Cell Metallization and Module Technology

Contents
About the editors
Biographies
1 Introduction
References
2 Main requirements for solar cells
2.1 Operation of a PV device
2.2 The detailed balance limit of a p-n junction solar cell
2.3 Practical solar cells
2.3.1 Two-diode model
2.3.2 Solar cell loss categories
2.4 Practical considerations and requirements
2.4.1 Optics
2.4.1.1 Metallization grid shading
2.4.1.2 Texturing and ARC
2.4.1.3 Front surface escape and transmission
2.4.2 Parasitic absorption
2.4.2.1 ARC absorption
2.4.2.2 NIR parasitic absorption
2.4.3 Collection losses
2.4.4 Energy gap
2.4.5 Recombination
2.4.5.1 SRH recombination in the bulk
2.4.5.2 Surface recombination
2.4.5.3 Metal-induced recombination
2.4.6 Series resistance
2.4.6.1 Base resistance Rbase
2.4.6.2 Surface doped layer
2.4.6.3 Contact resistance
2.4.6.4 Metallization grid pattern
2.4.7 Shunt resistance
2.4.8 Non-ideality
2.5 Economic and environmental aspects of solar cell metallization
2.5.1 Production cost requirements
2.5.2 Sustainability requirements
References
3 Fundamentals of metallization
3.1 Introduction
3.2 Barrier height
3.3 Carrier transport
3.4 Selective contacts
3.5 Passivation contacts
3.6 Characterization of solar cell contacts
3.7 Summary
References
4 Metallization approaches
4.1 A brief history of metallization approaches for laboratory-type c-Si solar cells
4.2 A short history of the main industrial c-Si cell concepts
4.3 Metallization approaches based on PVD contacts
4.4 Metallization approaches based on screen-printed contacts
4.5 Metallization approaches based on alternative printing techniques
4.6 Metallization approaches based on plated contacts
4.7 Comparison of the different metallization approaches
4.8 Outlook on metallization trends
References
5 Evaporated contacts
5.1 Metallization by means of physical vapour deposition
5.1.1 Deposition
5.1.1.1 Evaporation
5.1.1.2 Sputtering
5.1.1.3 Degradation effects during deposition
5.1.2 Properties of PVD metal contacts
5.1.3 Structuring of PVD metallization
5.1.4 Conclusions
5.2 Transparent conducting oxides in crystalline solar cells
5.2.1 Transparent conducting oxides
5.2.2 Properties and characterization of TCOs
5.2.2.1 Structural properties of TCOs
Crystallinity and crystallographic orientation
Lattice parameters
Average grain size
5.2.2.2 Electrical properties of TCOs
5.2.2.3 Optical properties of a TCO
5.2.2.4 Thickness
5.2.3 TCO materials for c-Si-based solar cells
5.2.3.1 Requirements for TCO films in c-Si-based solar cells
5.2.3.2 In-based TCO films
5.2.3.3 In-free TCO films
5.2.4 Conclusions
5.3 Thin metal compound contact interlayers
5.3.1 Introduction
5.3.1.1 Contact carrier selectivity
5.3.1.2 Achieving asymmetric carrier profiles
5.3.1.3 Introducing asymmetries at the interface
5.3.2 Materials
5.3.2.1 Alkali and alkaline-earth metal compounds
5.3.2.2 Low/moderate work function n-type transition metal oxidesand nitrides
5.3.2.3 High work function n-type transition metal oxides
5.3.2.4 Dedicated passivating interlayers
5.3.3 Deposition techniques for metal compound interlayers
5.3.3.1 Thermal and electron beam PVD
5.3.3.2 Sputtered PVD
5.3.3.3 Atomic layer deposition
5.3.4 Remaining challenges
5.4 Production tools for evaporation and sputtering TCOs
5.4.1 Prerequisites for mass-volume industrial equipment
5.4.2 Comparison of several deposition techniques and technologies
5.4.2.1 Chemical vapour deposition (CVD)
5.4.2.2 Physical vapour deposition
(a) Thermal vacuum evaporation
(b) Cathodic sputtering
(c) Developments of sputtering equipment for production
(d) Ion plating
(e) Comparison of RPD versus MS
5.4.3 Essential metrics for a volume production equipment
A. Uptime
(b) Mean time between failure (MTBF)
(c) Mean time to repair
(d) Product quality issues/non-quality
(e) Target material utilization rate
(f) Time for the format change (or set up time)
5.4.4 Conclusions
Acknowledgement
References
6 Screen-printed contacts
6.1 Silver pastes
6.1.1 Introduction to silver pastes
6.1.2 Manufacturing process and composition
6.1.3 Market
6.1.4 Metallization and firing process
6.1.5 Review of the current models on contact formation
6.1.6 Possible current paths on printed contacts
6.1.7 The importance of electrons
6.2 Aluminium pastes
6.2.1 Aluminium paste composition
6.2.2 Forming p+ silicon by the aluminium alloying process
6.2.3 Applications of Al paste in conventional and advanced solar cells
6.3 Contacting of boron emitters with Ag and Ag-Al pastes
6.3.1 Model of contact formation
6.3.2 Recombination of Ag-Al contacts
6.3.3 Contact resistance
6.3.4 Contacting of boron emitters with Al-free Ag pastes
6.3.5 Further development of pastes and actual status
6.4 Screen printer machine technology
6.4.1 Solar cell process details
6.4.2 Screen printing automation design
6.4.3 Machine cycle time
6.4.4 Dual printing, double printing
6.4.5 Printing on large wafers
6.4.6 Drying ovens
6.5 Screen technology
6.5.1 The mesh
6.5.2 The emulsion on the mesh
6.5.3 Simulation of screens
6.6 Screen printing process mechanics
6.7 Stencil printing
6.7.1 Stencil fabrication and Ag finger grid design
6.7.2 Stencil printed Ag finger properties
6.7.3 Stencil-printed silicon solar cells
References
7 Alternative printing technologies
7.1 Parallel dispensing
7.1.1 Introduction: working principle of dispensing
7.1.2 Dispensing lines on solar cells
7.1.2.1 Historical outline of research activities in Si-PV
7.1.2.2 Previous industrial approaches to increase throughput rates
7.1.2.3 Parallel dispensing at Fraunhofer ISE and HighLine Technology GmbH
7.1.3 Improving dispensed contact geometries
7.1.3.1 Influence of paste rheology during dispensing
7.1.3.2 Other approaches
7.1.4 Dispensing applications in PV
7.1.4.1 Overview
7.1.4.2 Dispensing the front-side grid of PERC solar cells
7.1.4.3 Dispensing of AgAl and Al-FT Pastes on n-PERT and BiPERC
7.1.4.4 Dispensing low temperature pastes
7.2 Rotary printing
7.2.1 Classification of rotary printing methods
7.2.2 Rotary screen printing
7.2.2.1 Historical and technical background
7.2.2.2 Working principle and influence factors
7.2.2.3 Rotary screen/stencil cylinder
7.2.2.4 Squeegee system
7.2.2.5 Further influencing factors
7.2.3 Application of RSP for solar cell metallization
7.2.3.1 Machine platform and drying equipment
7.2.3.2 Properties of the metallization paste
7.2.3.3 Optimal specification of the rotary screen
7.2.3.4 Selection and adjustment of the squeegee
7.2.4 Rotary screen-printed solar cell metallization – results and future perspectives
7.2.5 Flexographic printing
7.2.5.1 Historical and technical background
7.2.5.2 Working principle and influence factors
Ink supply system and ink chamber
Anilox oller
Printing plate cylinder
The flexographic printing form
Impression cylinder/transport system
7.2.5.3 Application of flexographic printing for solar cell metallization
7.2.5.3.1 Machine platform and drying equipment
7.2.5.3.2 Anilox roller specification
7.2.5.3.3 Flexographic printing plate/sleeve
7.2.5.3.4 Ink properties
7.2.5.3.5 Printing pressure
7.2.5.4 Flexo-printed solar cell metallization – results and future perspectives
7.2.6 Rotogravure printing
7.3 Laser transfer printing
7.3.1 Introduction
7.3.2 The (A)LTC and LTF process
7.3.3 Pattern transfer printing
7.4 Chapter summary
References
8 Plated contacts
8.1 Fundamental principles and nomenclature
8.2 Direct plating on Si
8.2.1 Patterning methods
8.2.1.1 Wet chemical etching
8.2.1.2 Laser ablation
8.2.2 Deposition methods
8.2.3 Complete process sequence
8.2.4 Contact properties
8.2.4.1 Shape
8.2.4.2 Contact resistance
8.2.4.3 Line conductivity
8.2.4.4 Solderability/corrosion resistance
8.2.5 Challenges
8.2.5.1 Adhesion
8.2.5.1.1 Long-term stability
8.2.5.2 Parasitic plating
8.3 Plating on metal seed layer
8.3.1 Seed layer types
8.3.2 Deposition methods
8.3.3 Contact properties
8.3.4 Challenges
8.4 Production tools for plating and costs of plating processes
8.4.1 Typical tool design, process parameters
8.4.2 Single-side inline plating
8.4.3 Bifacial inline plating
8.4.4 Bifacial batch plating/rack plating/lead frame plating
8.5 Challenges
8.5.1 Market penetration/cost calculations
8.5.2 Wastewater treatment
8.5.3 Inhomogeneous plating deposition
References
9 Metallization of specific solar cells
9.1 Metallization of industrial PERC and PERC+ solar cells
9.1.1 PERC and PERC+ solar cell design
9.1.2 Production process sequence
9.1.3 Metallization of the Ag front contact
9.1.4 Metallization of Al rear contact
9.1.5 Future Improvement opportunities
9.2 Silicon heterojunction solar cells
9.2.1 Introduction
9.2.2 Screen-printing of low-temperature pastes
9.2.2.1 Screen-printing process
9.2.2.2 Curing of LT paste
9.2.3 Metallization of heterojunction cells by electrodeposition of copper
9.2.3.1 Implications of the heterojunction structure for copper metallization
9.2.3.2 Processing and contacting options
9.2.3.3 Processes in development
9.2.3.4 There might not be enough silver for all the solar cells needed in the future
9.2.4 Inkjet printing
9.2.4.1 Introduction
9.2.4.2 Inkjet printing for SHJ
9.2.4.3 Optimization of the inkjet printing parameters for SHJ solar cell metallization
9.2.5 Cell design
9.2.6 Interconnection
9.2.7 Module reliability, bifaciality and energy yield
9.2.7.1 Reliability
9.2.7.2 Bifaciality
9.2.7.3 Energy yield
9.2.8 Summary
9.3 Metallization of poly-Si based passivated contacts solar cells
9.3.1 Introduction
9.3.2 Metallization of poly-Si-based passivated contacts
9.3.2.1 Thin a-Si/poly-Si layers (<100 nm)
a. Thermal evaporation
b. Plating
c. Screen-printing of LT NFT paste
9.3.2.2 Thick poly-Si layer (>100 nm)
9.3.2.3 Selection of pastes for FT metallization of passivated contacts
9.3.2.4 Other metallization methods
9.3.2.5 Summary
9.3.3 Selected passivated contact cell results from literature with respect to different metallization techniques
9.4 Interdigitated back contact solar cells
9.4.1 Introduction to IBC cells
9.4.2 PVD metallization for IBC cells
9.4.3 Electroplating metallization for IBC cells
9.4.4 Screen-printing metallization for IBC cells
References
10 Module interconnection technologies
10.1 Solder interconnection of solar cells
10.1.1 Materials for solder interconnection
10.1.1.1 Solar cell metallization
10.1.1.2 Interconnectors
10.1.1.3 Solder
10.1.1.4 Flux
10.1.1.5 Solder joints and intermetallic compounds
10.1.1.6 Thermomechanical properties of the interconnection materials
10.1.2 Soldering process
10.2 Electrically conductive adhesives
10.2.1 Introduction
10.2.2 Applications of ECAs for solar cell interconnection
10.2.2.1 Interconnection of silicon heterojunction solar cells
10.2.2.2 Shingle interconnection
10.2.3 ECA material systems
10.2.3.1 Operation principle and conduction types
10.2.3.2 Base polymers for ECAs
10.2.3.3 Filler
10.2.4 Mechanical and electrical properties of cured ECAs
10.2.4.1 Dynamic mechanical behaviour
10.2.4.2 Adhesion of ECA bonds
10.2.4.3 Electrical properties
10.2.4.4 Moisture ingress and mechanical/electrical ageing
10.2.5 Processing aspects of ECA
10.2.5.1 Comparison of ECA ribbon with tin–lead solder ribboninterconnection
10.2.6 Summary
10.3 Welding-based solar cell interconnection
10.3.1 Introduction
10.3.2 Parallel gap welding
10.3.3 Ultrasonic welding
10.3.4 Laser welding
References
11 Module design
11.1 Module design
11.2 Electrical design
11.2.1 Module circuit design
Series and parallel interconnection
Electrical power loss calculation
Current mismatch
11.2.2 Module-level electronics
Bypass diodes
Module optimizer and module inverter
11.2.3 Modelling and simulation
11.3 Optical design
11.3.1 Modelling approaches
Ray-tracing
Matrix formalisms
Module performance under realistic illumination conditions
11.3.2 Maximising packing density
11.3.3 Light-trapping in modules
Glass texturing and coatings
Shading reduction of interconnectors
Light-trapping in exposed non-cell areas
11.3.4 Bifacial module design
Bifacial module performance in the field
11.4 Mechanical design
11.4.1 Encapsulants
11.4.2 Implications of induced thermomechanical stress
Soldering
Lamination
Thermal cycling
11.4.3 Cell cracking
11.4.4 Solder joint failures
11.4.5 Rear module design
11.4.6 Module frame, mounting and mechanical load
References
Index
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