Optical properties of polymeric materials for concentrator photovoltaic systems

As part of our research on materials for concentrator photovoltaics (CPV), we are evaluating the optical properties and solar radiation durability of a number of polymeric materials with potential in CPV applications. For optical materials in imaging or non-imaging optical systems, detailed knowledge of the wavelength-dependent complex index of refraction is important for system design and performance, yet optical properties for many polymeric systems are not available in the literature. Here we report the index of refraction, optical absorbance, haze, and Urbach edge analysis results of various polymers of interest for CPV systems. These values were derived from ellipsometry and from using a VUV-VASE and transmission based absorbance spectroscopy on thick film samples. Fluoropolymers such as poly(tetrafluoroethylene-co-hexafluoropropylene) (Teflon[super][registered] FEP), poly(tetrafluoroethylene-co-perfluoropropyl vinyl ether) (Teflon[super][registered] PFA) and poly(ethylene-co-tetrafluoroethylene) (Teflon[super][registered] ETFE Film) have desirable optical and physical properties for optical applications in CPV. Ethylene backbone polymers such as polyvinylbutyral (PVB) sheet (e.g., DuPont[super]TM PV5200), and ethylene co-polymers such as poly(ethylene-co-vinyl acetate) (EVA) (e.g., DuPont[super]TM Elvax[super][registered] PV1400), and poly(ethylene-co-methacrylic acid metal salt) ionomer sheet (e.g., DuPont[super]TM PV5300) have applications as encapsulants in crystalline silicon (c-Si) and other flat plate PV applications. These materials are available with both a wide variety of polymer compositions and additive packages which affect their optical properties such as the UV absorption edge. Even materials such as DuPont's Kapton[super][registered] polyimide films, which are used behind the PV cell for their electrically insulating properties, have optical requirements, and we have also characterized these materials. The detailed optical properties of these materials will be useful in the design of the geometrical optics of a CPV system and optimization of the system's optical throughput. This information will also provide insights into the system's optical absorption. This is important, for example in the UV, where this absorption can impact the radiation durability of the materials.