Snail Trail Formation Mechanism

Transcription

1 Snail Trail Formation Mechanism Modeling and Accelerated Aging Tests May 25, 2016 Dr. Jing Fan Research Scientist DuPont Electronics & Communications For over 40 years our material innovations have led the photovoltaics industry forward, and helped our clients transform the power of the Sun into power for us all. Today we offer a portfolio of solutions that deliver proven power and lasting value over the long term. Whatever your material needs, you can count on quality DuPont Photovoltaic Solutions to deliver the performance, efficiency and value you require, day after day after day

2 Snail trails are dark irregular lines First observed in Spain in March 2007, occurrence has increased rapidly over past few years Snail trails occur at the interface of cell and EVA. 2

3 Snail trails usually follow cell cracks and form in a reaction between silver and EVA chemistry Optical image EL image Snail trails occur mostly on microcracks, sometimes near cell edges or in cell gaps Dark nanoparticles in the snail trail region are silver compounds OM 3

4 Snail trails contain silver acetate Raman spectra of snail trail- outdoor exposure Nothing on normal region Acetic acid from EVA reacts with silver to form dark silver acetate AgC 2 H 3 O 2 Silver acetate detected in snail trail region Ag + CH 3 COOH UV AgC 2 H 3 O 2 (dark) Silver acetate reference White silver acetate powder darkens when irradiated Before radiation After radiation 4

5 Finite element method is used to simulate the production of silver acetate in PV module Geometry of Modeling/Simulation 2*2 mini-module 1 module has micro-crack Conclusions Production of acetic acid: depends on encapsulant & environment (UV, T, RH): Production of silver acetate: depends on acetic acid concentration High temperature transmission property of backsheets and encapsulant: literature, or calculated based on Arrhenius law, or estimated, WVTR is not significant. Results: distribution of moisture, acetic acid and silver acetate in different locations and changing with time. Calculations used Comsol Multiphysics FEM (Finite element method) tool 5

6 Simulation shows micro-crack acts as an efficient transfer path and concentrates the formation of silver acetate Moisture Acetic acid Silver acetate Silver acetate forms at the micro-crack 6

7 EVA degradation has the largest influence on acetic acid concentration, and thus silver acetate production Acetic acid content above crack Silver acetate content above crack Silver acetate concentration significantly decreases when acetic acid production rate drops, stable EVA can eliminate snail trails. Doubling acetic acid production rate significantly accelerates silver acetate formation 7

8 Simulation indicates that backsheet WVTR has negligible influence on silver acetate production Moisture content above crack Silver acetate content above crack Increasing WVTR by a factor of 100 has a negligible effect on silver acetate formation. 8

9 DuPont has developed accelerated aging methods to induce snail trails Snail trail on optical image Snail trail from outdoor aging Snail trail from QUV/Weatherometer aging Silver acetate reference Micro-crack on EL Accelerated test shows excellent match to outdoor aged module with snail trail QUV 0.55 W/m nm 70 C BPT Atlas Weatherometer 0.55 W/m nm 70 C BPT, 50%RH 9

10 Lab tests demonstrate that encapsulant free of acetic acid is the key to solve snail trail issue Encapsulant Normal EVA POE Ionomer Vinyl Acetate (VA) content /% 28~30% 0% 0% Snail Trail Status Within 1 ~ 7 week No snail trail No snail trail Normal EVA Ionomer Snail trail on optical image Micro-crack on EL image No snail trail on optical image Micro-crack on EL image 10

11 Snail trail formation rate Days Lab tests demonstrate that backsheet WVTR has no influence on snail trail formation rate Several PVF, PVDF, PET backsheets are evaluated Snail trail vs Backsheet WVTR WVTR g/ [ m² - day ] No correlation of backsheet WVTR to snail trail formation 11

12 Summary Reaction between acetic acid and silver is identified as a root cause of snail trails Simulation results indicate - Micro-crack acts as necessary pathway for transport - Encapsulant plays determining role in snail trail formation - Backsheet WVTR has negligible influence on snail trail. Accelerated aging test results match simulation work Proposed solution: - Encapsulant free of acetic acid is a key to solve snail trail issue. - The occurrence of micro-cracks must be reduced. 12

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