Infrared Spectroscopy Analysis of Photovoltaic Backsheets: FTIR Material Identification and Degradat

Infrared spectroscopy analysis of photovoltaic backsheets is an essential analytical technique for verifying layer composition, confirming fluoropolymer content, and evaluating chemical degradation in solar module materials. Using FTIR (Fourier-transform infrared spectroscopy), accredited third-party laboratories can rapidly fingerprint each polymer layer of a backsheet and detect early signs of UV, damp-heat, and thermal aging before catastrophic module failure occurs. This article explains the principles, sample types, testing items, methods, and applications of infrared spectral analysis for PV backsheets in accordance with IEC and ASTM measurement practices.

Technical Overview

Photovoltaic backsheets are multilayer polymeric structures mounted on the rear side of solar modules, where they provide electrical insulation, moisture barrier protection, UV resistance, and mechanical support over a service life of 25 years or more. Typical constructions combine a fluoropolymer or coating-based outer layer (air side), a biaxially oriented polyethylene terephthalate (PET) core for dielectric strength and dimensional stability, and an inner layer engineered for adhesion to the encapsulant (cell side).

Infrared spectroscopy, most commonly implemented as Fourier-transform infrared (FTIR) spectroscopy, interrogates the molecular structure of these polymer layers by measuring the absorption of infrared radiation at characteristic vibrational frequencies between approximately 4000 and 400 cm⁻¹. Because every chemical bond and functional group absorbs at reproducible wavenumbers, the resulting spectrum acts as a molecular "fingerprint" that positively identifies the polymer type in each layer.

Beyond simple material identification, infrared spectral analysis is uniquely sensitive to the chemical changes that accompany backsheet aging. Photo-oxidation, hydrolysis of the PET core, chain scission, and fluoropolymer degradation all produce measurable changes in the spectrum—new absorption bands, intensity shifts, or band broadening—that allow engineers to quantify degradation long before visual defects such as yellowing, chalking, or cracking appear. This makes FTIR a cornerstone technique in backsheet qualification, quality control, and failure forensics, supporting requirements defined in IEC 61730, the IEC 61215 series, IEC 62788-2-1 for polymeric frontsheet and backsheet characterization, and general infrared practices such as ASTM E1252.

Testing Samples

Finished Backsheet Laminates

Complete multilayer backsheet films, typically 200–350 µm thick, are analyzed either as received or after controlled sectioning to expose individual layers. These samples are used for overall construction verification and incoming material inspection.

Individual Layer Specimens

The backsheet is delaminated or microtomed to separate the air-side outer layer, the PET core, and the cell-side inner layer. Each layer is then analyzed independently to confirm its polymer chemistry and assess its condition.

Aged and Weathered Samples

Samples retrieved after accelerated stress testing—damp heat exposure at 85 °C and 85% relative humidity, UV preconditioning, thermal cycling, and humidity-freeze sequences per IEC 61215 and IEC TS 62788-7-2—are compared against unaged references to quantify chemical degradation.

Field-Returned and Failed Modules

Backsheet fragments cut from modules recovered after outdoor service, particularly those exhibiting yellowing, cracking, delamination, or insulation loss, are examined to determine the root chemical cause of failure.

Coated Backsheet Surfaces

For coating-type backsheets in which a thin functional coating replaces a fluoropolymer film, the coated surface and the underlying PET substrate are analyzed separately to verify coating chemistry and integrity.

Testing Items

Infrared spectral analysis of photovoltaic backsheets typically covers the following testing items:

• Layer-by-layer material identification: confirmation of PVF, PVDF, PET, polyamide (PA), polyethylene (PE), ethylene-vinyl acetate (EVA), or co-extruded compositions within the laminate.
• Differentiation of fluoropolymer types: distinguishing polyvinyl fluoride (PVF) from polyvinylidene fluoride (PVDF), and distinguishing true fluoropolymer films from fluorine-containing or fluorine-free coatings.
• Verification of fluorine-free backsheet structures: identification of PA-based, PET-based, or polyolefin-based alternatives intended to replace fluorinated films.
• Coating chemistry confirmation: detection of acrylate, silicone, or inorganic Si–O network signatures in coated outer layers.
• Degradation index measurement: calculation of carbonyl index, hydroxyl index, and C–H related ratios to quantify oxidation and hydrolysis after aging.
• Hydrolysis assessment of the PET core: monitoring of carboxylate formation, chain scission indicators, and crystallinity-related band changes.
• Adhesive and tie-layer identification: verification of the adhesive chemistry bonding adjacent layers, which is frequently involved in delamination failures.
• Foreign matter and contamination analysis: identification of processing residues, silicones, release agents, or environmental deposits on backsheet surfaces.
• Comparative aging studies: spectral comparison of unaged, artificially aged, and field-aged specimens to establish degradation kinetics.

Testing Methods

Attenuated Total Reflectance FTIR (ATR-FTIR)

ATR is the primary method for backsheet surface analysis. The sample is pressed against a diamond or zinc selenide crystal, and the evanescent wave penetrates only 0.5–2 µm into the surface, making the technique essentially non-destructive and ideal for examining outer air-side layers, coatings, and cell-side surfaces without any sample preparation.

Transmission FTIR

Thin films or microtomed sections (typically thinner than 30–50 µm for polymers) are placed directly in the infrared beam. Transmission mode provides the highest spectral quality and is preferred for quantitative peak-ratio analysis of individual separated layers.

FTIR Microspectroscopy and Cross-Sectional Mapping

An FTIR microscope with an ATR objective allows layer-by-layer point analysis and chemical imaging across a polished backsheet cross-section. This approach spatially resolves each layer, measures interfacial chemistry, and maps degradation gradients from the exposed surface into the core.

Spectral Interpretation and Characteristic Absorption Bands

Identification relies on comparison with reference libraries and diagnostic absorption bands, including:

• PET: ester carbonyl stretch near 1713 cm⁻¹, C–O–C stretches near 1240 and 1095 cm⁻¹, aromatic C–H out-of-plane bending near 730 cm⁻¹.
• PVF: CH₂ deformation near 1400 cm⁻¹, C–F related bands near 1065–1070 cm⁻¹, and characteristic absorptions near 890 and 840 cm⁻¹.
• PVDF: strong CF₂ stretching near 1180 and 1070 cm⁻¹, CH₂ deformation near 1400 cm⁻¹, CF₂ rocking near 840 cm⁻¹, and a band near 763 cm⁻¹ associated with the alpha crystalline phase.
• Polyamide (nylon): N–H stretch near 3300 cm⁻¹, amide I near 1630 cm⁻¹, and amide II near 1540 cm⁻¹.
• Polyethylene: C–H stretches near 2916 and 2848 cm⁻¹, CH₂ bending near 1470 cm⁻¹, and rocking doublet near 730/720 cm⁻¹.
• EVA: ester carbonyl near 1735 cm⁻¹, C–O bands near 1240 and 1018 cm⁻¹, and vinyl acetate signatures used to assess encapsulant-backsheet interfaces.
• Silicone or inorganic coatings: broad Si–O–Si absorption spanning roughly 1000–1100 cm⁻¹.

Quantitative Degradation Analysis

Spectra are processed with baseline correction and normalization, then evaluated using band-ratio methods. The carbonyl index (integrated carbonyl absorbance normalized to a stable reference band) tracks oxidation of the PET core and coating binder, while growth of hydroxyl absorbance (3200–3600 cm⁻¹) and carboxylate bands indicates hydrolysis. Changes in the 1400 cm⁻¹ region and fluorine-related bands monitor fluoropolymer degradation. Results from aged samples are compared statistically against unaged controls to produce a quantified degradation profile.

Quality Assurance

Analyses are performed in accredited third-party laboratories following documented procedures consistent with ASTM E1252 infrared practices and the material characterization framework of IEC 62788-2-1, with instrument verification using certified polystyrene film standards and replicate measurements to ensure reproducibility.

Applications

• Incoming material inspection: verifying that delivered backsheets match the qualified construction and contain the specified fluoropolymer or coating layers.
• Supplier and material qualification: confirming layer chemistry of new backsheet designs during certification testing aligned with IEC 61730 and related national standards such as GB/T 31034.
• Production quality control: rapid screening of production batches for coating completeness, contamination, and adhesive correctness.
• Accelerated aging studies: correlating FTIR degradation indices with damp-heat, UV, and thermal-cycling exposure times to rank backsheet durability.
• Field failure forensics: determining whether yellowing, chalking, cracking, delamination, or ground-fault failures originate from PET core hydrolysis, fluoropolymer loss, coating erosion, or adhesive breakdown.
• Warranty and insurance investigations: providing objective chemical evidence of the degradation state and root cause in dispute resolution.
• Research and development: supporting development of fluorine-free backsheets, new coating systems, and co-extruded structures through chemical verification of every iteration.
• Recycling and end-of-life assessment: characterizing polymer composition of decommissioned backsheets to guide material separation and recovery processes.

FAQ

Is infrared spectroscopy destructive to the backsheet sample?

ATR-FTIR is essentially non-destructive because it contacts only the surface and leaves the sample intact. Transmission measurements may require cutting a small coupon or microtoming a thin section, but the material consumed is minimal—typically a few square millimeters.

Can FTIR distinguish PVF from PVDF?

Yes. Although both are fluoropolymers, PVDF shows an intense CF₂ doublet near 1180 and 1070 cm⁻¹ together with a distinctive band near 763 cm⁻¹, while PVF presents a different C–F band pattern near 1065–1070 cm⁻¹ with absorptions near 890 and 840 cm⁻¹. Experienced analysts can reliably differentiate the two, which is critical because the two materials have different weathering behavior.

How does FTIR detect backsheet degradation before visual defects appear?

Chemical changes precede macroscopic damage. As PET hydrolyzes and oxidizes, new carbonyl, carboxylate, and hydroxyl absorptions grow and existing bands shift or broaden. By tracking these spectral changes quantitatively, degradation can be detected hundreds or thousands of exposure hours before yellowing or cracking becomes visible.

How much sample is required and how long does the analysis take?

A sample of a few square millimeters is sufficient. A single ATR spectrum is collected in under a minute, and a complete multi-layer characterization with interpretation is typically completed within a standard laboratory working day.

Can FTIR analyze the backsheet while it is still laminated in a module?

Yes. The inner surface can be measured through the junction-box opening or after cutting a small window, and cross-sections cut from the module edge can be mapped layer by layer using FTIR microscopy, allowing in-situ assessment of modules in service or returned from the field.

Which standards govern backsheet infrared analysis?

There is no single FTIR standard exclusive to backsheets; analysis is performed under general infrared practices such as ASTM E1252, within the polymeric material characterization framework of IEC 62788-2-1, and in support of safety and qualification requirements in IEC 61730 and the IEC 61215 series, as well as relevant national backsheet standards including GB/T 31034.