Spatially resolved modulus measurements of photovoltaic encapsulation materials using cross-sectional nano-indentation

Abstract

Spatially resolved measurement methods are required to investigate the mechanical properties of polymeric packaging materials in photovoltaic modules due to their multilayered structure and possible heterogeneous degradation during their service life. This work presents a comprehensive evaluation of cross-sectional nano-indentation via the continuous stiffness method and its application to three different transparent backsheets and an encapsulant before and after accelerated aging. It lays out the unique challenges for sample preparation, measurement methodology, and data evaluation posed by these two types of samples, as well as possible limitations stemming from their structure and properties. The main issue for backsheets is the structural compliance close to material interfaces, visible in the measured Young’s modulus as a function of indentation depth. However, the method yields reliable results in the bulk of the materials with a thickness as low as 25 μm, with high spatial resolution stemming from the use of a sharp sphero-conical diamond tip. Localized modulus increases can be measured in different layers, corresponding to embrittlement, cracking, and increased fluorescence. The most significant issues for measuring the encapsulant stem from its comparably much lower modulus. We apply a multi-step approach for sample preparation and use a much larger conical flat-head diamond tip. The measured modulus of the encapsulant decreases after ultraviolet exposure in coupons with one type of backsheet, emphasizing the importance of backsheet stability for the durability of the entire module. These results show that the methodology is a valuable tool for characterizing polymers in PV applications.