Sam Louis Samuels

Optical properties of 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 CPV application. For optical materials in imaging or non-imaging optical systems, detailed knowledge of the wavelength dependent complex index of refraction is important for optical system design and performance. Here we report the index of refraction, optical absorbance and haze results of various polymers of interest for CPV systems. Fluoropolymers such as polyvinylfluoride (Tedlar? PVF Film), which has wide application in crystalline silicon (c-Si) flat plate PV modules, as well as poly(tetrafluoroethylene-co-hexafluoropropylene) (Teflon? FEP Film) and poly(ethylene-co-tetrafluoroethylene) (Tefzel? ETFE Film) have desirable optical and physical properties for optical applications such as CPV. Hydrocarbon polymers such as polyvinylbutyral (PVB) sheet such as DuPont? PV5200, and the ethylene copolymers such as poly(ethylene-co-vinyl acetate) (EVA) such as Elvax? PV1400, poly(ethylene-co-methacrylic acid metal salt) ionomer sheet such as DuPont? PV5300 have applications as encapsulant in c-Si and other flat plate PV applications. These materials have both a wide variety of polymer compositions and also additive packages, which affect their optical properties such as the UV absorption edge. Even materials such as Kapton? polyimide films, which are used behind the PV cell for their electrically insulating properties, have optical requirements, and we characterize these materials also. The detailed optical properties of these materials will be useful for CPV system design of the geometrical optics, optimization of the systems optical throughput, and also provide insights into the systems optical absorption, for example in the UV, where this absorption can impact the radiation durability of the materials.

A field evaluation of the potential for creep in thermoplastic encapsulant materials

There has been recent interest in the use of thermoplastic encapsulant materials in photovoltaic modules to replace chemically crosslinked materials, e.g., ethylene-vinyl acetate. The related motivations include the desire to: reduce lamination time or temperature; use less moisture-permeable materials; use materials with better corrosion characteristics or with improved electrical resistance. However, the use of any thermoplastic material in a high-temperature environment raises safety and performance concerns, as the standardized tests currently do not expose the modules to temperatures in excess of 85°C, though fielded modules may experience temperatures above 100°C. Here we constructed eight pairs of crystalline-silicon modules and eight pairs of glass/encapsulation/glass thin-film mock modules using different encapsulant materials of which only two were designed to chemically crosslink. One module set was exposed outdoors with insulation on the back side in Arizona in the summer, and an identical set was exposed in environmental chambers. High precision creep measurements (±20 μm) and performance measurements indicate that despite many of these polymeric materials being in the melt state during outdoor deployment, very little creep was seen because of their high viscosity, temperature heterogeneity across the modules, and the formation of chemical crosslinks in many of the encapsulants as they aged. In the case of the crystalline silicon modules, the physical restraint of the backsheet reduced the creep further.

Teflon® FEP frontsheets for photovoltaic modules: Improved optics leading to higher module efficiency

Increasing the amount of light that reaches the cell is one approach to improved photovoltaic (PV) module efficiency. In current designs using weathering layers of glass or poly(co- ethylene-tetrafluoroethylene) (Teflon® ETFE), some light is lost due to reflection and scattering due to haze. Teflon® FEP, poly(co- tetrafluoroethylene-hexafluoropropylene), offers both a lower index of refraction and higher transparency than existing front sheet materials for modules, leading to improved efficiency and greater output of current and power. In precise optical measurements, we showed that FEP delivers more light to the underlying PV cells. By modeling the optical transmission data for front sheets of glass, ETFE, and FEP, one can demonstrate an improvement of 1.5–3% in short circuit current output when using FEP, depending on the cell technology. Initial module experiments have confirmed model predictions and demonstrate the power output performance enhancement when using FEP.