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Optical Properties of Polymeric Materials for Concentrator Photovoltaic Systems

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Optical Properties of Polymeric Materials for Concentrator Photovoltaic Systems Solar Energy Materials & Solar Cells 95 (2011) 2077–2086 Contents lists available at ScienceDirect Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat Optical properties of polymeric materials for concentrator photovoltaic systems R.H. French a,b,n, J.M. Rodrı´guez-Parada a, M.K. Yang a, R.A. Derryberry a, N.T. Pfeiffenberger a a DuPont Company, Central Research, Exp. Sta., Wilmington, DE 19880, USA b Department of Materials Science & Engineering, 510 White Hall, 2111 Martin Luther King Jr. Drive, Case Western Reserve University, Cleveland, OH 44106-7204, USA article info abstract Article history: As part of our research on materials for concentrator photovoltaics (CPV), we are evaluating the Received 30 July 2010 optical properties and solar radiation durability of a number of polymeric materials with potential Accepted 23 February 2011 in CPV applications. For optical materials in imaging or non-imaging optical systems, detailed Available online 8 May 2011 knowledge of the wavelength-dependent complex index of refraction is important for system Keywords: design and performance, yet optical properties for many polymeric systems are not available in the Fluoropolymer literature. Here we report the index of refraction, optical absorbance, haze, and Urbach edge Ethylene backbone polymer analysis results of various polymers of interest for CPV systems. These values were derived from Polyimide ellipsometry and from using a VUV-VASE and transmission based absorbance spectroscopy on thick Index of refraction film samples. Optical absorbance Fluoropolymers such as poly(tetrafluoroethylene-co-hexafluoropropylene) (Teflons FEP), poly(te- Haze trafluoroethylene-co-perfluoropropyl vinyl ether) (Teflons PFA) and poly(ethylene-co-tetrafluoroethy- lene) (Teflons ETFE Film) have desirable optical and physical properties for optical applications in CPV. Ethylene backbone polymers such as polyvinylbutyral (PVB) sheet (e.g., DuPontTM PV5200), and ethylene co-polymers such as poly(ethylene-co-vinyl acetate) (EVA) (e.g., DuPontTM Elvaxs PV1400), and poly(ethylene-co-methacrylic acid metal salt) ionomer sheet (e.g., DuPontTM 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 Kaptons 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. & 2011 Elsevier B.V. All rights reserved. 1. Introduction systems can be divided into low, medium, and high concentrators, as seen in Fig. 1 [2]. Low concentrators employ 1–10 Â concen- Concentrating photovoltaic (CPV) systems and their optical tration with no tracking of the sun, and cells made of silicon or materials have come from a wide background of similar optical thin film. Medium concentrators employ 12–100 Â concentration reduction technologies. For example, they can be closely related with one- or two-axis tracking of the sun, and cells made of to the field of optical lithography [1], which relies greatly on silicon. High concentrators employ 200–2000 Â concentration reduction optics of liquids for precise nanoscale patterning. This with high precision two-axis tracking of the sun, and multi- carries over to the primary optics (those used with 1 sun junction III–V cells. irradiance) and the secondary optics (those used at multiple sun For optical materials in imaging or non-imaging optical systems, irradiances) in a CPV system. According to Swanson [2], CPV detailed knowledge of the wavelength-dependent complex index of refraction, n^ðÞ¼l nðÞþl ikðÞl , is important for the optical system’s design and performance. The index, n, and the extinction coeffi- n Corresponding author at: Department of Materials Science & Engineering, 510 cient, k (which is directly related to the optical absorbance White Hall, 2111 Martin Luther King Jr. Drive, Case Western Reserve University, ¼ Cleveland, OH 44106-7204, USA. Tel.: þ1 216 368 3655. a 4pk=l), give insights into the system’s optical performance. E-mail address: [email protected] (R.H. French). The index of refraction determines reflectance losses at air/polymer, 0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2011.02.025 2078 R.H. French et al. / Solar Energy Materials & Solar Cells 95 (2011) 2077–2086 High Concentration Low Concentration Clear lens formed in Medium Concentration Light reflected to PV bottom of glazing Linear Fresnel lens Incoming light PV cell Aluminum housing Heat sink Thin film photovoltaic Clear upper glazing and heart sink Air Cooled photovoltaic reciever Fig. 1. Low concentration (left), medium concentration (middle), and high concentration CPV systems (right) [2]. polymer/polymer and polymer/inorganic interfaces, and is impor- shortest wavelength optical absorption edge onset (158 nm or tant for matching or optimizing the numerical aperture (NA) of the 7.85 eV) known for a polymer. We also developed other novel reduction optical system at all points in the optical path. The fluoropolymers, such as –[(CH2CHF)xC(CF3)2CH2]y–, or – extinction coefficient is critical to determine optical losses in the (CH2CF2)x[2,2-bis(trifluoromethyl)-4,5-difluoro-1, 3-dioxole]y–, system, including the passage of ultraviolet (UV) through the with optical absorption edges onsets down to 144 nm (or system. The absorption of UV light by the optical materials can 8.61 eV) [8]. We have also studied amorphous and crystalline also contribute to optical degradation of the materials due to silica [9], other oxides [10,11], nitrides [12], and carbon nano- photodarkening and photobleaching and other types of degradative tubes [13,14], as well as polystyrene [15] and novel backbone processes and reduced performance over time [3]. polymers such as the polysilanes [16]. CPV is typically non-imaging [4]; therefore, cost-effective The optical absorption/cm and haze of various polymeric materials that do not have perfect optical clarity are still of compositions have previously been measured for various fluor- interest. For optics with short throw lengths (or focal lengths), opolymers, including perfluoropolyethers (PFPE) [17], fluorinated haze can help homogenize the light. Haze can also be important polyimides [18], polytetrafluoroethylene (PTFE) [19], and other for optics with long throw lengths, such as primary optics, DuPont fluoropolymers [7,8,20]. While the ASTM standard haze refractive lenses, and reflective mirrors in which specularity is measurement is an object dependent value and varies based on important. The distinctness of image or DOI [5] is the deviation of sample thickness and bulk crystalinity, since all of the evaluations the direction of light propagation from the regular direction via were performed on the same instrument and in the same scattering during transmission or reflection and can be related to orientation, haze can be a useful measurement for rank-ordering haze. DOI is useful for characterization of the visual appearance of the polymer’s clarity that arises from bulk and surface scattering. CPV materials. Prior research on the optical properties of polyimides including In addition to the refractive optical materials, many CPV Kaptons, Upilexs, and polyimide derivatives like TOR and systems utilize mirrors; therefore, materials for multilayer reflec- CP were published by Russellet al. [21]. There have been tors are also of interest. Metallized fluoropolymers for back side also numerous studies on the index of refraction versus wave- mirrors used in CPV or concentrating solar power optics have length for PTFE [22], polyimides [23,24], polyesters [25], and reflectance values above 90% after 3.5–4 years of exposure [6]. acrylics [26]. Their use in concentrated solar power technologies would not be Wallner et al. [27,28] developed a fundamental understanding limited if their specularity could be sufficiently improved, for of the physical relationships between the material structure and example by reducing the haze which can be associated with solar optical properties, as well as the infrared optical properties, scattering and reduced specularity on a reflector [6]. The spec- of transparent polymeric materials. Their studies included low- ularity was determined in both a 25 mrad (milliradian) cone and a and high-temperature resistant polymer films such as polymethyl 7 mrad cone, and for mirror applications which have long optical methacrylate (PMMA), polyethylene terephthalate (PET), polycar- path (or ‘‘throw’’ lengths); the tighter 7 mrad specularity mea- bonate (PC), polyimide (PI), and poly(tetrafluoro-ethylene-co-per- surement is important. CSP tower applications, which typically fluoroalkylvinyl ether) (PFA) [29]. They have also tested the solar have long distances from the reflecting mirror surface to the radiation durability of PC, PMMA and PET [30,31]. power tower absorbing surface, are an example of this long throw For potential CPV applications,
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