Advanced UV-Fluorescence Image Analysis for Early Detection of PV-Power Degradation

Reliability and durability of photovoltaic modules are a key factor for the development of emerging
PV markets worldwide. Reliability is directly dependent on the chemical and physical stability of the
polymeric encapsulation materials. One method capable of detecting ageing effects of the polymeric
encapsulant directly in the field is ultraviolet Fluorescence (UVF) imaging. This work deals with
advanced imaging analysis of UVF images and the subsequent correlation to electrical parameters of
PV mini-modules, which were exposed to climate-specific, long-term, accelerated aging procedures.
The analyzed data was made available through the project INFINTY, and is currently being processed
within the follow-up project ADVANCE!. For establishing a correlation, a so called UVF area-ratio was
established, resulting from the typical fluorescence patterns of the encapsulant material, which arise due
to stress impact (e.g.: water vapour ingress, elevated temperature, irradiation) and aging processes
(formation of fluorophores). The UVF-ratio (ratio of the area with low UVF in the cell center divided
by the cell area) gives information on the extent of stress-induced changes of the encapsulant (includes
the polymer itself and special constituents e.g. impurities, additives or fragments of a cross linker). With
increasing lifetime or storage time in a climate chamber (accelerated aging), the encapsulant material
ages due to the interaction with climatic stress factors. Besides the architecture of the PV-module e.g.,
permeability of the backsheet and type of encapsulant, the resulting UVF pattern is mainly dependent
on the exposure time and the stress factors applied. In order to exclude material effects, only modules
with identical bill of material were used to develop the advanced UVF image analysis and correlation
to the electrical performance. As the formed UVF – which is mainly triggered by high temperatures and
irradiation - is partially extinguished by oxygen permeating into the encapsulant via the backsheet, very
distinctive UVF patterns were detected leading to detectable fluorescence deviations, which were
transformed into a scalar quantity (ratio). These effects evolve with increasing (accelerated) aging time.
First results of the data analysis already show a clear correlation of electrical parameters and the UVF
area-ratios with increasing aging time. Especially in the context of on-site PV operation and maintenance
approaches this non-invasive UVF image-based approach could provide a fast, on-site and easy-to-applymethod for performance assessment and prediction of derating (presupposed that other degradation effects are not present/can be excluded).