Encapsulation of PV modules using ethylene vinyl acetate copolymer as a pottant: A critical review

The primary purpose of this work is to review the literature about what is and is not known about using ethylene vinyl acetate (EVA0 copolymer as the encapsulant (or pottant) material in photovoltaic (PV) modules. Secondary purposes include elucidating the complexity of the encapsulation problem, providing an overview about encapsulation of PV cells and modules, providing a historical overview of the relevant research and development on EVA, summarizing performance losses reported for PV systems deployed since ca. 1981, and summarizing the general problems of polymer stability in a solar environment. We also provide a critical review of aspects of reported work for cases that we believe are important.Failure modes resolved in the early work to establish reliability of deployed modules and the purposes and properties of pottants, are summarized. Typical performance losses in large field-deployed, large-scale systems ranging from 1% to 10% per year are given quantitatively, and qualitative reports of EVA discoloration are summarized with respect to ultraviolet (UV), world-wide location and site dependence.The general stability of polymers and their desirable bulk properties for solar utilization are given. The stabilization formulation for EVA, its effectiveness, and changes in it during degradation are discussed. The degradation mechanisms for the base resin, e.g., unstabilized Elvax 150^TM, and stabilized EVA are indicated for literature dating to the early 1950s, and the role played by unsaturated chromophores is indicated. The limited number of studies relating discoloration and PV cell efficiency are summarized.Observed degradation of EVA or the unstabilized base resin in the laboratory and examples used to measure the degradation are summarized in sections entitled: (1) thermally-induced degradation; (2) photodegradation and photothermal degradation of EVA in different temperature regimes; (3) photobleaching and photodegradation of the UV absorber and cross-linking agent; (4) acetic acid and metal and metal-oxide catalyzed oxidative degradation; and (5) discolaration and PV cell efficiency losses.Processing effects/influences on EVA stability are discussed in sections entitled: (1) EVA raw materials and extruded, uncured films; (2) thermal encapsulation processes; (3) effects of lamination, curing, and curing peroxide on gel content and chromophores formed; and (4) incomplete shielding of curing-generated chromophores. A summary is given for the limited number of accelerated lifetime testing efforts and examples of erroneous service lifetime predictions for EVA are discussed. The known factors that effect the discoloration rate of several EVA formulations are discussed in which the reduction in rate by using UV-absorbing superstrates is a prime example. A summary is given of what is and is not known about EVA degradation mechanisms, degradation from exposures in field-deployed modeules and/or laboratory testing, and factors that contribute to EVA stability or degradation. Finally, conclusions about using Elvax 150 in EVA formulations are summarized, and future prospects for developing the next-generation pottant for encapsulating PV modules are discussed.     

Durability issues and service lifetime prediction of electrochromic windows for buildings applications

In general, the purposes of this paper are to elucidate the crucial importance of durability and service lifetime prediction (SLP) for electrochromic windows (ECWs) and to present an outline for developing a SLP methodology for ECWs. The specific objectives are (a) to illustrate the generic nature of SLP for several types of solar energy conversion or energy conservation devices, (b) to summarize the major durability issues associated with ECWs, (c) to justify using SLP in the triad of cost, performance, and durability rather than just durability, (d) to define and explain the seven major elements that constitute a generic SLP methodology, (e) to provide background for implementing the SLP methodology for ECWs, including the complexity of the potential degradation mechanisms, and (f) to provide an outline of studies using ECWs for improving the durability of ECW materials and predicting a service lifetime for ECWs using the SLP methodology outlined in objective (d). Our major conclusions are that substantial R&Dis necessary to understand the factors that limit ECW durability, and that it is possible to predict the service lifetime of ECWs.     

Optical constants of cupric oxide from 450 to 800 mμ

The wavelength dependence of the optical constants of cupric oxide films was determined over the range 450 to 800 mω from the transmittance measured on 12 films from 350 to 1800 Å thick. The film thickness was determined by the use of a quartz crystal oscillator. Computerized techniques were used to obtain the transmittance data, carry out the analysis, and determine the calibration constant of the oscillator. It was assumed that the optical constants are independent of film thickness within this thickness range. The index of refraction, n, was found to increase from 2.46 at 450 mω to an approximately constant value (2.60–2.65) between 600 and 800 mω. The extinction constant, k, decreased monotonically from 0.744 at 450 mω to 0.132 at 800 mω.     

Growth of α-Al2O3 Within a Transition Alumina Matrix

The growth of α-Al₂O₃ from a planar specimen of thermally grown γ-alumina on a molybdenum transmission electron microscope grid was studied. The α-Al₂O₃ grows into the transition alumina matrix and then thickens via a ledge growth mechanism. Faceted Mo crystallites cause pinning of α-Al₂O₃ ledges and are larger on α-Al₂O₃ than on the transition alumina matrix.     

Penetration of Deposited Ag and Cu Overlayers Through Alkanethiol Self-Assembled Monolayers on Gold

The purpose of our research is to study the reactions, interactions, or penetration between vacuum deposited metals (M) and the organic functional end groups (OFGs) of self-assembled monolayers (SAMs) on Au films under controlled conditions. Metal/SAM/Au systems are models for understanding bonding at M/organic interfaces and the concomitant adhesion between the different materials. In broad terms, the M/OFGs form interacting interfaces (e.g., Cr/COOH or Cu/COOH) in which the deposit resides on top of the OFGs or weakly interacting interfaces through which the overlayer penetrates and resides at the SAM/gold interface. We present a review of XPS results from weakly interacting systems (Cu/CH₂OH, CU/CN, Ag/CH₃, Ag/COOH) and discuss in more depth the time-temperature dependence of the disappearance of the metal from the M/SAM interface following deposition using ISS on the Ag/CH₃ and Ag/COOH systems. XPS and ISS were used to characterize five alkanethiols terminated with CH₃, COOH(C—11 and C—16 chain lengths), CN and CH₂OH before and after depositing up to 1.0 nm Ag or Cu at ca. 10^-7 torr. XPS spectra indicate that no strong interaction occurs between the deposited Ag and the COOH organic functional group, although a stronger interaction is evident on a C—16 COOH, and a unidentate is formed for Cu on the CH₂OH. An interaction between Cu and CN is evident, and penetration into the SAM occurs to a greater extent than for Cu on CH₂OH. The Ag interaction with CH₃ is weak. ISS compositional depth profiles for Ag on COOH and CH₃, taken from 113 to 293 K, indicate that Ag remains on the surface of the C—11 COOH for up to 1 h after deposition, whereas Ag penetrates CH₃ in less than 5 min at 295 K. The time for Ag to penetrate into a C-16 COOH is several times longer than for the C—11 COOH and depends on the SAM temperature.     

Growth of γ-alumina on crystallographically distinct aluminium substrates

The growth of-alumina on crystallographically distinct aluminium substrates has been studied using transmission electron microscopy and diffraction and Rutherford backscattering. Oxides grown thermally on single-crystal substrates showed a preferred epitactic orientation relationship with the substrate, while oxides grown by the same technique on polycrystalline substrates did not exhibit a preferred orientation relationship. The grain size of the oxide was found to be at least one order of magnitude smaller than the initial grain size of the polycrystalline aluminium substrate.