Drivers for the cracking of multilayer polyamide‐based backsheets in field photovoltaic modules: In‐depth degradation mapping analysis

Abstract

There's a lack of understanding on the root cause of cracking of photovoltaic (PV) backsheets due to the challenge of multilayer characterization and the complicated failure modes at the sub-module level. In this work, in-depth degradation mapping of field-exposed polyamide-based (PA-based) PV module backsheets was studied, with the major focus on the identification of underlying drivers for the through cracking (in between the solar cells). PV modules were retrieved from five different locations, comprising a variety of climates, including humid subtropical, hot-summer Mediterranean, tropical savanna climate and hot arid. A suite of microscale cross-sectional characterizations, including chemical changes, fluorescence intensity, and modulus as a function of distance from the air surface of the backsheet, were performed. Results showed more advanced signs of degradation of the inner layer than the outer layer in the cracking region. Increases in the modulus was identified as the major indicator for the cracking. Moreover, a rudimentary test by immersion in acetic acid, which forms during photodegradation of the ethylene-vinyl acetate copolymer (EVA) encapsulant, showed the first-time direct evidence that acetic acid can largely accelerate the chemical degradation and facilitate the cracking of PA inner layer. This study suggests that the field cracking of PA-based backsheet can be attributed to the combined effects of chemical degradation and physical reorganization (chemi-crystallization) under cyclic thermomechanical stresses. Therefore, it is important to consider effects of local

Towards validation of combined-accelerated stress testing through failure analysis of polyamide-based photovoltaic backsheets

“…In the case of the C-AST-aged samples, we see that C-AST-1 (which exhibited no macrocracking) demonstrated a higher imide-to-carboxylic acid ratio than C-AST-2 (which exhibited extensive macrocracking). Similar peak intensity increases have also been widely reported for field-aged and some laboratory aged AAA samples 27 , 33 , 34 .…”

“…5 b), where Tonopah saw a higher ratio of imide groups than the other samples since there would have been insufficient moisture to convert the imides to carboxylic acids 31 . A similar correlation was observed in the elastic modulus of individual layers of AAA backsheet from modules fielded in the same location, where in this case the elastic modulus was used as a proxy for fracture toughness 33 .…”

“…The lower humidity and moisture conditions of Tonopah, AZ could explain the more pronounced imide group peak, where the lack of moisture means the imide groups do not break down into carboxylic acid as in the other samples, which experience more humid environments. Previous work has demonstrated that imide groups are favoured at the inner layer of fielded modules, except in the case where macrocracking is present and moisture can more easily penetrate the backsheet 33 . In the case of the C-AST-aged samples, we see that C-AST-1 (which exhibited no macrocracking) demonstrated a higher imide-to-carboxylic acid ratio than C-AST-2 (which exhibited extensive macrocracking).…”

“…Figure 5 c shows the change in normalized peak intensity for the carbonyl peak 1715 cm −1 for the C-AST- and field-aged samples from the outer layer of the backsheet. Previous studies used the height ratio of the carbonyl peak at 1715 cm −1 to the normalization peak at 1465 cm −1 as a measure of polymer decomposition 33 . The peak intensity could be interpreted as the degree of UV photodegradation.…”

“…Acetic acid is a known by-product in EVA resulting from the hydrolysis of the vinyl-acetate monomers 35 . A recent study demonstrated that the chemical degradation of free-standing AAA exposed to the outdoors could be accelerated in the presence of acetic acid 33 .…”