Thermal Expansion Behavior of a Thermoplastic Polyolefin for Photovoltaic Application Over Hygrothermal Aging

The thermal expansion behavior of polymers is a crucial property for manufacturing photovoltaic (PV) modules. The thermal expansion mismatch between the different module components induces residual stresses in the structure after manufacturing. Some of them are located at the interface between materials, leading to delamination and reliability issues during the PV module lifetime. For tandem applications, the thermal expansion mismatch is also an issue since it leads to the separation between the bottom and top cells. In this article, the thermal expansion behavior of a thermoplastic polyolefin (TPO) encapsulant used in the PV industry is assessed by stereo digital image correlation. This contactless method measures the thermal expansion in the two directions of the polymer thin film. The method is accurate enough to capture transition phases of the material, namely the crystallites fusion and formation. The thermal expansion behavior of the TPO thin film is shown to be anisotropic and dependent on its thermal history. The material contracts when heated, both after manufacturing and after aging; this has not yet been investigated. The aging temperature has an influence on the thermal contraction temperature but does not erase the shrinking behavior. The thermal expansion behavior is explained by a microstructural approach. The microstructure is investigated by differential scanning calorimetry. After crystallites melting, the molecular mobility and residual internal stresses account for the observed shrinking behavior. This behavior may affect the reliability of PV modules through delamination, cells cracks, or separation of the top and bottom cells.