Acrylic Liquid Encapsulant for Photovoltaic Modules 10-20 20-10 Melinda Einsla The Dow Chemical Company 1
Role of Encapsulants in Photovoltaic Devices Frontsheet Encapsulant Cells Encapsulant Backsheet Film encapsulants designed for crystalline silicon modules must be vacuum laminated and do not allow module manufacture by entirely rollto-roll process 2-part, curable liquid encapsulant enables new manufacturing processes for flexible PV modules 2
Liquid Encapsulation Approach Liquid Encapsulant enables new manufacturing process for flexible PV modules Reduce total cost of ownership for the module manufacturer by increasing throughput and decreasing capital expense Non-EVA encapsulant eliminates module failure due to acid corrosion Requires a change both in process equipment and in material set Discussions with interested module manufacturers indicate they want a ready-to-use use, turnkey system Dow is working with equipment manufacturers to bring equipment, process, and material set to market 3
Market Share of Thin-Film PV Technologies is Growing Ann nual PV cell/m module produ uction (GW) ROW Taiwan China Europe Japan U.S. Thin-Film PV Market Share 2003 5% 2008 14% 2012 (projected) 16-34% 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 From: 2008 Solar Technologies Market Report, U.S. Department of Energy, January 2010 4
Photovoltaic Encapsulants Have Stringent Property Requirements General Requirements: Optical clarity (no yellowness, high transmission over visible/uv range) UV/temperature stability (including thermal cycling: -40 to +110 C) Compatibility & adhesion with system components (Si, Ag, Glass, PET, fluoropolymers) Mechanical integrity over wide temperature range Challenges for Liquid Encapsulant: Low viscosity (for pumping, coating) Coatability Long pot/die life Short cure time 5
Liquid Encapsulant Meets or Exceeds Critical Specs Specification Dow LE Development Candidate STR EVA 15420P/UF Product Specs Dow Corning 6010 Liquid Encapsulant Benefit / Comment Refractive Index 1.478 1.482 1.42 Higher RI increases module power output. Light transmission >98%, 400-1100nm (0.5mm thickness) ~90% unspecified Higher transmission allows more light to reach the cell. Viscosity, mpa-s <2000 cp @ 60C NA 925 @ 25C Viscosity will be tailored to equipment Hardness Viscosity @ RT 38s (55% solids, Din 4) Target: <50s (din 4 cup) for 15s (39% solids, Din 4) solvent coating 45, Shore A 88, Shore 00 69, Shore A 21 Shore D 34, Shore 00 Cure Temperature 25C-150C 150C 80C-150C Cure time and temperature will be tailored to equipment via catalyst level Tensile strength at break, 1.4 MPa / 18.1 MPa 0.022 MPa / Higher strength improves MPa / Elongation at break, % 44% 800 900% 554% module durability 6
Color Development Upon Accelerated UV Exposure Encapsulant Photopackage YI (0h) YI (500h) YI (1000h) YI (2000h) YI (3000h) YI (4000h) Dow Development Candidate Yes, unoptimized None 1.1 1.3 2.0 1.2 1.5 2.0 2.0 2.0 2.1 3.8 1.6 3.3 ASTM G115-05a, 63 C, 50% RH, NO H 2 O spray 7
Haze Development Upon Accelerated UV Exposure Photostabilizer Haze (0h) Haze (3000h) Yes, Unoptimized 1.4 1.7 none 09 0.9 11.6 ASTM G115-05a, 63 C, 50% RH, WITH H 2 O spray 8
Mechanical Properties of Acrylic Liquid Encapsulant Composition Tensile Strength (MPa) Elongation at Break (%) Dow Development 1.433 43.7% Candidate Experimental Encapsulant Experimental Encapsulant Experimental Encapsulant 0.159 26.3% 0.367 29.4% 0.758 28.0% Materials cured with stoichiometric amount of crosslinker, 0.005% catalyst at 60 C. 9
Lower Catalyst Extends Pot life > 90 min 8000 0.005% 7000 0.0025% 0025% 0.001% 6000 Vis scosity (cp P) 5000 4000 3000 2000 1000 0 0 20 40 60 80 100 120 140 160 Time (minutes) 10
Choice of Catalyst Affects Pot Life 7000 6000 0.0025% Catalyst DBTDA A 0.0025% Catalyst DBTDL B 0.0025% Catalyst 0025% Bi C Octoate 5000 Viscosity (cp) 4000 3000 2000 1000 0 0 20 40 60 80 100 120 140 Time (min) 11
Temperature Tunes Cure Time Cure G /G Mechanical Chemical Cure Temperature Crossover Point Cure Complete Complete 80 C 6.9 min ~50 min ~2 hr 100 C 4.0 min ~20 min ~1.5 hr 120 C 2.8 min ~10 min ~50 min 150 C 2.1 min ~6.5 min ~15 min Materials cured with stoichiometric amount of crosslinker, 0.005% catalyst at 60 C. 12
Using DMA to Measure Physical Cure 10 6 10 3 10 5 10 4 ) G" ( ) [Pa] 10 3 10 2 10 2 Temp ( [ C] G' ( [Pa] 10 1 ) 10 0 G' / G" Crossover Point:(125.85,1.3841) 10-1 10-2 0.0 200.0 400.0 600.0 800.0 time [s] 10 1 1000.0 13
Using FTIR to Measure Chemical Cure 0.09 MLE-0270-144 MLE-0270-0 MLE-0270-2 0.08 MLE-0270-6 MLE-0270-30 MLE-0270-47 0.07 MLE-0270-73 0.06 Ab bsorbance 0.05 0.04 0.03 uncrosslinked isocyanate crosslinked urethane 0.02 0.01 0.00-0.01 001 14
Frontier Technologies Trial Demonstrated Concept on Scalable Slot-die Coater Dispense pot 3-way valve Piston pump Slot die Test on Mylar Moving platen 1. Coat glass 2. Place cell 3. Second coat 15
Change in Encapsulant Composition Significantly Improved Adhesion Prototype #1 Dow Development Candidate Stainless Steel 6.3 N/in (AFB) 17.6 N/in (A) HDPE (Untreated) 0.5 N/in (A) 0.2 N/in (A) HDPE (Corona- Treated) 0.9 N/in (AFB) 12.0 N/in (AFB) AFB = Adhesive e failure at backing A = Adhesive failure from substrate 16
5 4 3 2 1 Adhesion 0 Newest Prototype has Excellent All- Around Properties Viscosity Optical Performance Adhesion to Glass Tensile Properties Adhesion HDPE or SS Experimental Encapsulant Dow Development Candidate *All properties other than viscosity it were measured on post-cured samples cured with stoichiometric amount of crosslinker 17
Dow Liquid Encapsulant Has Low Water Vapor Transmission Rate Film Thickness WVTR (g cm -2 s -1 ) Dow Development Candidate* 20 mil (0.5 mm) 124x10 1.24 10-8 *Cured with stoichiometric amount of crosslinker, 0.005% catalyst at 60 C. Freestanding film was obtained by curing on release liner and peeling film off once cured. 18
Electrical Properties Dielectric Strength Volume Resistivity Surface Resistivity (V/mil) (Ω-cm) (Ω 2 ) Dow Liquid Encapsulant 1280 2.97 x 10 15 >10 16 STR 15295 EVA 1600 8.17 x 10 14 N.M. Dow Corning PV 6010 425 2 x 10 14 N.M. 19
Hand-Lamination of Silicon Cells with Dow Liquid Encapsulant 20
Summary Dow Liquid Encapsulant demonstrates: Excellent light transmission and UV stability Good mechanical properties Coatability Good adhesion to a variety of substrates Low water vapor transmission rate Excellent electrical properties Temperature and catalyst can be used to effectively tune cure time Slot-die coating produces controlled, defect-free coatings onto glass, over cells 21