Ann Walstrom Norris

An optical comparison of silicone and EVA encapsulants for conventional silicon PV modules: A ray-tracing study

Ray-trace simulation is used to quantify the optical losses of photovoltaic modules containing silicon cells. The simulations show that when the modules encapsulant is silicone rather than ethylene vinyl acetate (EVA), the modules short-circuit current density under the AM1-5g spectrum is 0.7–1.1% higher for screen-printed multi-cSi cells, 0.5–1.2% higher for screen-printed mono-cSi cells, and 1.0–1.6% higher for high-efficiency rear-contact cells, depending on the type of silicone. This increase is primarily due to the transmission of short-wavelength light (≪420 nm) and is therefore greatest when used with low UV-absorbing glass and cells of a high IQE at short wavelength. We also quantify absorption in the glass, EVA and silicone at longer wavelengths and describe the influence of an encapsulants refractive index on escape losses.

Novel silicone materials for LED packaging

Silicone based materials have attracted considerable attention from Light Emitting Diode (LED) manufacturers for use as encapsulants and lenses for many next generation LED device designs. Silicones can function in several roles that include protective lenses, stress relieving encapsulants, mechanical protection and light path materials. The key attributes of silicones that make them attractive materials for high brightness (HB) LEDs include their high transparency in the UV-visible region, controlled refractive index (RI), stable thermo-mechanical properties, and tuneable hardness from soft gels to hard resins. The high current and high operating temperatures of HB-LEDs present a significant materials challenge for traditional organic materials such as epoxies, acrylics and cyclo olefin copolymers (COC) that lack the thermal and molecular stability needed to provide optical clarity and mechanical performance required for next generation devices. In addition, the retention of optical clarity over the lifetime of the device, which involves long term exposure to high flux in the UV-visible wavelength region, is a critical requirement. Silicones have been demonstrated to provide the required stability. This paper will describe recent silicone materials development efforts directed towards providing LED manufacturers with silicone materials solutions for LED device fabrication. Injection molding of novel silicone resin based materials will be discussed as a surmountable challenge for high throughput LED device manufacturing.

Siloxane Materials for Optical Applications

Siloxanes, which can be viewed as hybrids of glass and organic materials, have been used to fabricate polymer waveguides and devices that exploit the large thermo-optical effect of this material. Siloxanes have many unique properties including good thermal stability, chemical resistance, tunable refractive index, tunable mechanical properties and excellent photo-stability. The refractive index of siloxane polymer is composition dependent and generally ranges from 1.4 to 1.54. Introduction of porosity or composition modification can further expand refractive index range to 1.15~1.63. The loss and absorption characteristics for a variety of silicone-based polymers are examined and an example of a UV curable polymer coating illustrates the flexibility of the silicone polymer family to be tailored to meet specific application needs.

High reliability of silicone materials for use as polymer waveguides

Silicones are known for their excellent performance in applications with harsh environmental conditions. They are very well known for their high temperature stability, resistance to moisture and other adverse conditions. This paper will overview key properties of siloxanes that make them attractive materials for numerous photonics device applications with emphasis on polymer waveguides. Both thermal-mechanical and optical properties will be reviewed. Testing of key optical properties of several siloxane materials, both before and after exposure to heat, humidity, and high optical flux will be discussed. Fabrication and processing for production of polymer waveguides, and the resulting polymer device performance will be shown. Finally, the high reliability of siloxane based waveguides is demonstrated by the Telcordia testing of a fully functional, packaged, Variable Optical Attenuator (VOA).

Novel silicone materials for LED packaging and opto-electronics devices

Silicone based materials have attracted considerable attention from Light Emitting Diode (LED) manufacturers. In LEDs, silicones can function in several roles that include optical lenses, stress relieving encapsulants, mechanical protection and light path materials. The key attributes of silicones that make them attractive materials for high brightness (HB) LEDs include their excellent transparency in the UV-visible region, their non-discoloring behavior and their stable thermo-mechanical properties. The first part of this paper/presentation will describe recent silicone materials development efforts directed towards providing LED manufacturers with silicone materials solutions for LED device fabrication. Injection molding of novel silicone resin based materials will be discussed as a viable route for high throughput LED device manufacturing. For other portions of the light spectrum, specifically at telecom wavelengths, the performances of silicone based materials are also verified and this makes them attractive materials for numerous photonics device applications. The second part of this paper/presentation will describe recent demonstrations of siloxane for use as waveguides for datacom and telecom applications. A Variable Optical Attenuator (VOA) utilizing silicone based waveguides (exploiting dn/dT property) and an Optical Backplane built from silicone waveguides and out-of-plane mirrors built on glass and FR-4 substrates are discussed.

Silicone materials for LED packaging

Silicone based materials have attracted considerable attention from light emitting diode (LED) manufacturers for use as encapsulants and lenses for many high brightness LED (HB LED) devices. Currently silicones function in two key roles in HB LED devices, (1) as protective lenses and (2) stress relieving encapsulants for wire bond protection. The key attributes of silicones that make them attractive as light path materials for high brightness HB LEDs include their high transparency in the UV-visible region, controlled refractive index (RI), stable thermo-mechanical properties, and tuneable modulus from soft gels to hard resins. This paper will describe recent developments in moldable silicone hard resin materials. Progress on cavity moldable and liquid injection moldable (LIM) silicone compositions for discreet components is described. Also, an example of liquid injection overmolding is presented.

An optical comparison of silicone and EVA encapsulants under various spectra

Under the AM1–5g spectrum, the efficiency of a c-Si module can be increased by 0.5–1.5% (relative) by using a silicone encapsulant rather than EVA. This increase is primarily due to photons of wavelengths less than ∼400 nm being transmitted by silicone but blocked by EVA. The highest increase in efficiency arises for cells with a good ‘blue response’ and for silicones of a higher refractive index. In this work we show that when calculated for the AM1–5g spectrum, the optical advantage of silicone over EVA tends to be an underestimate of what can be expected in the field. This is because incident spectra are frequently ‘bluer’ than the AM1–5g spectrum, particularly in summer and on cloudy days. In such cases, a larger fraction of photons have a wavelength less than 400 nm. With ray tracing, we find that the relative advantage of silicone over EVA is increased by an additional 0.3% on a sunny summer day in Phoenix AZ, and by 0.7% on a cloudy summer day in Brownsville TX. Still greater increases are expected for maritime climates and for installations with high albedo.

Silicone polymers for optical films and devices

Silicones are among the most suitable materials for optical telecommunication devices due to their tolerance to high optical flux and their thermo-mechanical and environmental stability; they also have excellent processability. This work focuses on utilizing silicon-based branched resins and linear polymers for optical waveguides and switches where both refractive index and thermo-optic coefficient need to be controlled to the requirements of specific applications. Materials were synthesized with high optical transmission bands between 1.3 and 1.6 μm by varying the amount of aliphatic and aromatic C-H in the material. At the same time, the ratio of methyl to phenyl groups also controls the refractive index in the range of nD = 1.4 ... 1.6 precisely enough that both core and cladding components (Δn < 0.5%) can be obtained. Films of 5 to 20 μm thickness prepared on silicon substrates by spin-coating from solution were evaluated by measuring refractive index, thermo-optic coefficient, optical loss, and film uniformity both before and after exposure to high temperature and humidity. These films can be patterned through a number of techniques to form the required features. The resinous materials show very low birefringence and excellent resistance to heat and moisture.

Improved spectral response of silicone encapsulanted photovoltaic modules

In this work the benefit of using optically superior silicone encapsulant materials over the incumbent ethylene vinyl acetate is demonstrated. Optical characterization of the two materials demonstrates improved transmission in the UV region of the solar spectrum. Single cell mini-modules were prepared using two different manufacturers of screen printed textured monocrystalline Si PV cells. Spectral response measurements demonstrate a 0.74–1.03% relative increase in short circuit current density when silicones are used rather than ethylene vinyl acetate, clearly in the region of improved UV transparency. IV measurements demonstrate a 0.31–1.45% improvement in current for silicone.

Dow Corning photonics: the silicon advantage in automotive photonics

The Automotive Market offers several opportunities for Dow Corning to leverage the power of silicon-based materials. Dow Corning Photonics Solutions has a number of developments that may be attractive for the emergent photonics needs in automobiles, building on 40 years of experience as a leading Automotive supplier with a strong foundation of expertise and an extensive product offering- from encapsulents and highly reliable resins, adhesives, insulating materials and other products, ensuring that the advantage of silicones are already well-embedded in Automotive systems, modules and components. The recent development of LED encapsulants of exceptional clarity and stability has extended the potential for Dow Corning’s strength in Photonics to be deployed “in-car”. Demonstration of board-level and back-plane solutions utilising siloxane waveguide technology offers new opportunities for systems designers to integrate optical components at low cost on diverse substrates. Coupled with work on simple waveguide technology for sensors and data communications applications this suite of materials and technology offerings is very potent in this sector. The harsh environment under hood and the very extreme thermal range that materials must sustain in vehicles due to both their engine and the climate is an applications specification that defines the siloxane advantage. For these passive optics applications the siloxanes very high clarity at the data-communications wavelengths coupled with extraordinary stability offers significant design advantage. The future development of Head-Up-Displays for instrumentation and data display will offer yet more opportunities to the siloxanes in Automotive Photonics.