Adhesives in Photovoltaics Dr. Hartmut Henneken Berlin, 23.11.2011
Scale-up Issues towards a 20 MW (250 m Rotor Blade Dimension) Wind Turbine To compare dimensions: 20MW PV plant near Cagliari Photo from www.rechargenews.com Picture taken at ECN, Petten, NL
nolax belongs to the Collano Group Independent specialist for adhesive bonding technologies headquarted in Switzerland (Sempach-Station). Worldwide, the Group employs 328 persons at several locations in Switzerland, Germany, France, and the USA.
History and Key Figures 1947 Dr. Marcel Ebnöther establishes the adhesive company Ebnöther AG in Olten/Switzerland 1997 50th anniversary, name change from Ebnöther AG to Collano Ebnöther AG (and 2001 to Collano AG) 2008 Incorporation of the group companies: nolax AG, Collano Adhesives AG, Collano Services AG
nolax Core Competencies R&D Technologies Reactive systems Hotmelts Adhesive films Water-based systems Services Adhesive evaluation System development Adhesive development The Collano group and especially nolax is involved in several commercial and research projects in the field of photovoltaics.
Adhesives Used in Photovoltaics Encapsulants Protection of solar cells from environment and adhesive binding of all module components Framing adhesives Edge sealing of PV modules; connecting the module with the frame Junction box adhesives Adhere the junction box to the module Back sheets Connecting different layers to each other Other (e.g. conductive adhesives, adhesives to connect wiring, labels, ) In this presentation, the encapsulants will be covered in detail.
Encapsulants are adhesives, that are playing an important role in the PV module setup. J-Box Encapsulant Encapsulant Backsheet Frontsheet PV-film are connecting all interfaces of the PV module to each other and have to maintain good optical, mechanical and electrical performance over decades of operation. are a key player in the lamination process, which is besides the solar cell production, significantly influencing the final cost of the module.
Known Encapsulant Types Ethylene vinyl acetate copolymer (EVA) Siloxanes (PDMS) Plasticized polyvinyl butyral (PVB) Polyethylene ionomers Thermoplastic Polyolefins (TPO) Thermoplastic polyurethanes (TPU) UV curable high vinyl acetate containing EVA (EVM) other thermoplastic blockcopolymers
Encapsulant Structures R O O O H n m o PVB O O n TPO m O CH 3 Si n R N O O n O O CH 3 PDMS H TPU O O n m EVA Na, Zn n m Ionomers H 3 C Si O H 3 C H 3 C n Si CH 3 or combinations, e.g. PDMS and TPU blockcopolymers R N H O O m
Production of Thermoplastic Encapsulant Cast extrusion of encapsulants at nolax (Collano Services GmbH)in Buxtehude/ Germany
EVA Encapsulant The dominant encapsulant for c-si PV today More than 25 years in the field, lots of reliability data available Copolymer of ethylene and vinyl acetate (VA) with a VA content of about 33 % by weight High VA content guarantees a greater than 90% light transmission to the solar cells Must be cross-linked to seal the cells and prevent flow/creep Issues with EVA degradation: generate acetic acid, yellowing in the dark, residual peroxides decompose into volatiles
Encapsulant Example Formulation In general PV encapsulants are highly formulated variations of the base resin. An example EVA formulation for PV was published as followed (EVA base resin, 96% to 98%): Benzoltriazole (0.2% to 0.35%) UV Absorber Peroxide(1% to 2%) Crosslinker Trialkoxy Silane (0.2% to 1%) Adhesion Promoter PhenolicPhosphonite (0.1% to 0.2%) Peroxide Decomposer/ Radical Scavenger Hindered Amine Light Stabilizer (HALS) (0.1% to 0.2%) Decomposes Peroxide Radicals Photovoltaics International (9th Edition; August 2010; page 172)
Thermoset against Thermoplastic Thermoplastic encapsulants: Potential to decrease cycle times Roll-to-Roll lamination possible Better reprocessing (e.g. for repairs) Easier to recycle Require higher process temperatures Thermoset encapsulants: Additional process time for curing No danger of creeping Lower process viscosity possible Shelf life restrictions
Thermoset against Thermoplastic Generally, most resins could be prepared as thermoset or thermoplastic formulations. Thermoplastic materials are better for R2R processes, as junction boxes and connectors could be assembled and reprocessed offline. Though thermoplastics do not need curing time, they still need to melt, which can take some time, depending on laminator setup and layer thicknesses. UV-curable resins are also available, they behave as a thermoplastic during lamination and could be cured in a second step.
Requirements for Encapsulation Weatherability (UV, heat, moisture resistant) High transparency (over module life time) UV-stability, low yellowing Good adhesion (with all components) Barrier properties (water vapour, oxygen) Electric insulation Flexibility, no brittleness No creeping Cost efficiency, availability Low shrinkage Fit process Recyclability Flame retardant properties (e.g. for BIPV)
Water Ingress due to Diffusivity Rates into Different Encapsulants M.D.Kempe, NREL, «Review of Polymer Use in Photovoltaics», Plastics in PV Conference, Philadelphia, USA, 20/09/2011
Heat and Moisture Resistance WVTR of some used frontsheet materials are relative high, NO barrier effect: ETFE 100 µm = 4 g m -2 d -1 EVA 500 µm = 20 g m -2 d -1 Short term HF cycling (20 times IEC requirements ) shows that a-si technology is very resistant to heat and moisture. IEC Norm 10 cycles Humidity freeze -40 C to 85 C, 85% Rh Courtesy: Flexcell
Heat and Moisture Resistance Very long term HF cycling (> 100 x times IEC requirements!) shows a gradual power degradation due to mainly 3 phenomena: Humidity freeze -40 C to 85 C, 85% Rh Al corrosion from CH 3 COOH formation: it starts early but little affects the performance ITO discoloring and degradation Hydrolysis of PEN and cracking: only at very long exposure time (better than PET) Courtesy: Flexcell
Transmission to cells through 3.18 mm glass and 0.45 mm encapsulant PDMS #1 94.8 ± 0.3% PDMS #2 94.8 ± 0.3% PVB 94.3 ± 0.4% EVA 94.3 ± 0.4% TPU 93.8 ± 0.3% Ionomer#1 92.7 ± 0.4% PDMS #3 92.1 ± 0.3% Ionomer#2 88.7 ± 0.4% M.D.Kempe, PVSC 37, Seattle, Washington, June 19-24, 2011
UV Stability M.D.Kempe, NREL, «Review of Polymer Use in Photovoltaics», Plastics in PV Conference, Philadelphia, USA, 20/09/2011
Shrinkage Not only encapsulants, but also polymeric frontsheets, as well as backsheets can show significant shrinking. This example shows an ETFE/encapsulant/ETFE setup after vacuum lamination, but even these materials could be used to laminate wrinkle less modules by using appropriate laminator settings, even without applying vacuum.
No best Encapsulant for all Type of Modules Until 1995 PV modules had a more or less similar setup Nowadays different types of solar cells are used or developed, such as e.g. CIGS, CdTe, DSC, organic solar cells, There are many new back sheets available As front sheets not only glass but also polymers such as ETFE, PVDF and PMMA are used the interfaces that need to be connected/sealed have significantly increased
Example: Back Sheet Types and Suppliers 2010: 17 suppliers offered 100 different back sheets; 2011: 22 suppliers offered already 112 types Source: K.Brust, Krempel GmbH, 5th SNEC, Shanghai, 2/2011; based on market survey Photon International 9/1010
Development of Solar Cell Technologies Source: Wikipedia «solar cells», scheme provided by NREL
Encapsulants Should be Selected According to the Specific Module Setup CIGS cells are much more sensitive to moisture than c-si C-Si wafers are much more sensitive to mechanical stress than flexible amorphous Si cells Not every encapsulant is suitable for every front- or backsheet (in terms of adhesion or process requirements) For R2R lamination thermoplastic encapsulants are better suited, liquid 2-component systems are in this case difficult to handle Maximum or minimum process temperatures and/or pressure requirements must fit for all components Some materials are more tacky than others, which can be a disadvantage, but sometimes also improving the handling Some encapsulants require vaccum lamination due to bubble forming, other ones could be laminated at standard pressure
There Cannot be a Single Encapsulant for all Type of Modules Just from the point of view of adhesion it is very unlikely that one type of encapsulant is the best choice (or even working) with every material combination. Source: SBM Solar 1. Frame 2. Junction box 3. Connectors and PV wire 4. Backsheet 5. Back encapsulant 6. Solar cell 7. Front encapsulant 8. Front sheet 9. Glass Organic solar module Fraunhofer ISE
Adhesive Testing Very small changes in formulation can have large effects on adhesive performance. The same applies for changes in surface chemistry of the substrates. It is very difficult to predict adhesion in detail, usually it is necessary to do intensive testing.
Adhesive Screening Example This graph shows a part of a large adhesive screening of base resins (different types/ grades/formulations) laminated on PVDF front sheet film. Generally more polar formulations perform better, but even within one resin, there are very good as well as very bad results found!
Adhesive Strength There is no official standard available yet, that describes what adhesive strength would be required for any of the interfaces! Some manufacturers use values in the range of 5-10 N/mm peel strength, others are working with 1 N/mm Important is that no delamination occurs during module lifetime, as in this case e.g. humidity could accumulate in cavities and induce rapid corrosion.
Module Lifetime (1) Solar panels should have a guaranteed performance of 20-30 years, but manufacturers are facing difficulties to do it. It is hardly possible to predict the real effects of outdoor exposure on modern at high efficiencies operating modules by doing indoor exposure or ageing test. There are a lot of standard test procedures that need to be passed in order to get a certified module, but a discussion would be to much for this presentation. Tests include Heat-Freeze Cycles, Damp-Heat Tests, Hail Testing, Flash Tests, Thermal aging tests,
Module Lifetime (2) Even though it is a wishful thinking that modules still deliver 80% or more power after 25 years, it is in my opinion maybe not really necessary: Research progress and costs might be reasons that people would like to replace even working modules after 15 years with modern types
www.pvgum.eu nolax is Taking Part in the EU-Projekt PV-GUM
www.pvgum.eu nolax is Taking Part in the EU-Projekt PV-GUM Bitumen: > 50% of all flat roofs in Europe Goal: 10m x 1.2m bituminous PV membrane Production by Roll-to-roll process EU FP-7 cost shared project: 01.11.2010 31.10.2013 Consortium: 9 partners Total budget 11 M, EU subsidy 6.2 M
R2R Lamination of Flexible Solar Cells The main scope of the PV-GUM project is to fully integrate flexible thin film PV modules in the bitumen-based roofing membrane. A roll-to-roll process and new equipment is being developed to encapsulate the solar cell directly on the roofing membrane. During this project e.g. Meyer Machines GmbH had build a prototype vacuum laminator that is capable to do R2R laminations (no pictures shown, the photograph shows the R2R lamination using a non-vacuum setup) and nolax has gained a lot of experience in continuous and batch lamination.
Production and application
Adhesion Testing for Batch and Roll Lamination T2 = T1 + 20 C; HS (High speed) = 6x LS (Low Speed); +O : add. heating without pressure WI: Water Immersion at 80 C ST: Special surface treatment of ETFE (standard is the commercially available (also surface treated) material)
Conclusions Adhesives are used in many parts of solar modules Encapsulation is one of the key adhesive applications Many different encapsulants are nowadays available Different modules and lamination procedures might require different encapsulants the formerly exclusively used EVA could be NOT suitable in some cases
Thank you for your attention.