In this work we present a novel optical polymer system based on polyurethane elastomer components, which combines excellent UV transparency with high thermal stability, good hardness, high surface tension and long pot life. The material looks very promising for encapsulation and microlensing applications for chip-on-board (CoB) light-emitting diodes (LED). The extinction coefficient k, refractive index n, and bandgap parameters were derived from transmission and reflection measurements in a wavelength range of 200-890 nm. Thermogravimetry and differential scanning calorimetry were used to provide glass transition and degradation temperatures. The surface tension was determined by means of contact angle measurements. As proof of concept, a commercial InGaN-CoB-LED is used to demonstrate the suitability of the new material for the production of microlenses.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Current requirements and the market volume of light emitting diodes (LEDs) and miniaturized CMOS image sensors have led to an increasing demand for low-cost optics and a notable growth of production capabilities [1–4]. LEDs are regarded as highly efficient light sources that offer a long service life, a wide range of colors and design options. LED chips are generally protected from environmental influences by polymer coatings, where the applied polymer is also used for beam shaping of the emitted light. In addition, the packaging technique should be cost-effective and easy-to-implement. Thus, the optical properties of the polymer coatings are of great importance for the light yield and beam shaping properties.
Conventional optical polymers for encapsulants and lensing of LEDs are epoxy resins [5–8], polymethylmethacrylate (PMMA) [9,10], polycarbonate (PC) [9–12], acrylates , polystyrene (PS)  and cyclic olefin polymer and copolymer (COP/COC) . Polyurethane (PU) materials are also candidates for the encapsulation of LEDs and organic LEDs (OLEDs) in particular due to their excellent properties for lens formation by evaporative polymer droplet deposition and imprint processes [13,14]. Furthermore, silicone [4,15,16] and silicone hybrid materials, like epoxy–silicone resins [17–20] and sol-gel based siloxane [21–24] are widely used. The disadvantages of most conventional polymers are that they are hard and brittle because they have rigid cross-linked networks and decompose under the influence of radiation and high temperature, which leads to chain splitting and discoloration [6–8,10,17]. Silicone resins, on the other hand, have a high thermal stability, UV resistance and are highly transparent in the UV spectral range, but they are relatively soft and have poor adhesion properties [15,16,17]. A further drawback of silicone resins is the refractive index matching of the silicone with respect to the LED material. New material developments to increase lighting efficiency, improve strength and refractive index are achieved by silicone hybrid materials and sol-gel based siloxanes. The combination of materials combines the advantages of conventional polymers such as low cost, easy processing, excellent mechanical properties and good adhesion with the advantages of silicone to improve toughness, internal stresses and thermal stability of encapsulations [17,25]. New research studies have been conducted to improve sealing, insulation resistance, mechanical properties and brightness for conventional optical polymers, silicone and combinations of these materials [25–27]. However, for all materials, except silicone, the transmission is restricted or limited to the visible range.
In this paper, we introduce a novel polymer system that appears promising to overcome this issue. It is based on 2-component polyurethane (PU), which is characterized by easy handling and low manufacturing costs. We show that excellent UV transmission can be achieved with this material. It is well known that aromatic isocyanates are more reactive than aliphatic ones and lead to products with better mechanical properties . Aliphatic isocyanates result in polyurethanes that do not discolor under the influence of light or heat and are highly transparent and thermally stable [29,30]. These media have long pot life, that simplifies the manufacturing process. For LED packaging, the novel material should withstand the soldering processes without impairing the material properties. At the same time, it should meet the requirements of a simple and cost-effective process for the production of a stable encapsulation resin for LEDs with the additional possibility of lens formation. The development of PU according to the above requirements was carried out with the help of various diagnostic methods.
Optical constants in particular play a key role in the selection of suitable optical packaging and lens materials. The targeted application of the novel PU is Chip-on-Board LED micro lens packaging. In this scenario, the refractive index n is essential for (i) refractive index matching with the LED material, (ii) addressing the color dispersion and (iii) optimizing the focal length and the directional characteristics of the polymer micro lenses. The extinction coefficient k is related to absorption and defines the applicability, regarding specific LED emission wavelengths λ. The shortest usable wavelength is defined by the band gap energy Eg, i.e. the transition energy between the highest occupied molecular orbital (HOMO), and the lowest unoccupied molecular orbital (LUMO). For the absorption at photon energies close to the band gap charge trap centers and exciton states play a further role.
Relevant data were derived from optical transmission and reflection (R&T) measurements, using the standard dispersion models Tauc-Lorentz [31,32], Tauc-Lorentz-Urbach  and excitonic Lorentz oscillator [34–36]. The extinction coefficients for photon energies well below Eg, particularly relevant for LED packaging and lensing applications, were determined by R&T measurements of thick PU rods and point by point fitting.
Moreover, inhomogeneities within the polymer material must be considered, e.g. differences in density or structural defects, which can lead to local refractive index differences, causing light scattering within the polymer bulk material.
The thermal stability of the LED encapsulation material is an important factor in soldering technology and LED operation. The heat generated in an LED device can lead to outgassing of the material and thus affect the performance of the components in the device. Thermogravimetric analysis (TGA) and differential scanning calorimeter (DSC) were performed to assess the thermal stability of the polyurethanes. The surface tension of polymers and housings are essential for the formation of microlenses and determines their radii of curvature. It was derived from contact angle measurements on various substrates based on the harmonic mean approach of Wu [37–39].
2.1 Materials, synthesis and sample preparation
A group of different PU systems, made by resintec GmbH, was investigated, consisting of the resin-based polyester polyol (PO) with different OH functionalities representing the A-component and different hardeners (B-component), as shown in Table 1. The resins used were resPUR-OT-3000 with a polymeric B-component P-MDI based on methylene diphenyl diisocyanate (MDI), resPUR-OT-T24000 and the material resPUR-OT with an oligomeric B-component O-HDI based on hexamethylene diisocyanate (HDI). The polyurethane (PU) is formed by a polyaddition reaction from the two components by a reaction of the hydroxyl groups (OH) and the isocyanate groups which finally form the urethane groups -NH-CO-O- as part of the repeat unit of the main chain. The A-component is a mixture of polyester polyols with reactive hydroxyl (OH) groups that react with isocyanate (NCO) groups to form the PUs. The number of hydroxyl-(OH)-groups depends on the educts of the polyol synthesis. Aromatic methylenediphenyl diisocyanate polymers and aliphatic isocyanate hexamethylene diisocyanate oligomers are used for the B-components to compare different PUs. The two components were mixed and homogenized with a magnetic stirrer for 2 minutes at a temperature of 50 °C. The specifications of the A- and B-components, the mixing ratios and other material parameters are listed in Table 1.
To determine the absorption coefficients over many orders of magnitude by transmission measurements, samples with different thicknesses were prepared. Thin layers of 5-15 nm thickness were produced by spin coating on quartz substrates. After coating, the material was rapidly thermally annealed at 80 °C for the final curing process. Thick rods of 8 mm width were formed in cuvettes whose walls were removed after curing.
2.2 Thermal investigations
Thermal stability is an important factor in LED encapsulation because the heat generation and heat transfer during soldering can reduce the efficiency of the LED through chemical changes in the polymers. The PUs examined here are generally resistant to high temperatures (10 s at 320 °C and 3 min at 260 °C) and should withstand the soldering process. The degradation temperatures Td and glass transition temperatures Tg were measured by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), using Mettler Toledo TG50 MT5 and Netzsch-DSC-204, respectively.
2.3 Surface tension measurements
The surface tension or contact angle of the liquid polymer is important for lens formation during dispensing. Contact angles measurements were performed with a commercial system (Surftens from OEG GmbH). The surface tensions of different modified substrates were derived with models of the interaction of molecules according to Wu , using the methods given in [38,39]. Using two testing liquids of different polarity it is possible to determine polar σ p and nonpolar σ d surface tension of the substrate. In our experiments, water (16-17 MΩcm, σ p = 50.7 dyn/cm, σ d = 22.1 dyn/cm) and methylene iodide (CH2I2, σ p = 6.7 dyn/cm, σ d = 44.1 dyn/cm) were used. If the surface tension of the substrates is known, the surface tension of liquids can be determined numerically [38,39]. For the determination of the surface tension of polyurethane, different substrates are used, for example silicon with the surface tension of σ d = 32 dyn/cm and σ p = 24 dyn/cm, glass with σ d = 35 dyn/cm and σ p = 31 dyn/cm and polytetrafluorethylen (PTFE) modified glass substrate with σ d = 14 dyn/cm and σ p = 2 dyn/cm.
2.4 Optical analysis
The materials were analyzed in terms of refractive index n and extinction coefficient k in a wavelength range of 200 to 890 nm. For this purpose, optical R&T measurements were performed using different setups and samples with varying thicknesses. The thin-film samples were analyzed in wavelength ranges with strong absorption near and below the bandgap wavelength, whereas the thick rods were used in the low absorption range up to 890 nm. The overall behavior in absorption and transmission was obtained by a commercial spectrophotometer (Lambda 1050, Perkin Elmer) without showing evaluable interference effects due to the relatively low spectral resolution and film thickness inhomogeneities within the illuminated area of about 3 × 3 mm2. In order to obtain additional information, mainly on the film thicknesses, to be integrated into our fitting model for accurate k and n determination, thin film interferences were evaluated as well. The interference data were obtained by a self-built reflectometer setup (DeepView), described elsewhere . The setup provides a spectral resolution of ∼ 0.6 nm in a wavelength range of about 400–600 nm and good interference contrast.
The refractive index n and extinction coefficient k are determined using the two-layer structure model shown in Fig. 1 . The parameters n(λ), k(λ) and thickness d were determined with the method of least squares from parameterizable models, where experimental and simulated R&T are compared by
The transmission and reflection are calculated according to
Bond orders and polymer backbone kinks and the variations of the lattice configuration influence the electronic structure of polymers close to the band gap. Analogous to inorganic materials, this absorption can be described by the Urbach model [44,51,52].
In this work, we use the Tauc-Lorentz (TL) model [31,32] by including the Urbach tail  for the interpretation of transitions from the ground state (e.g. occupied π orbitals of the polymers) to the excited state (e.g. unoccupied π* orbitals of the polymers) in accordance with the solids.
Based on this oscillator model, the transitions from the highest occupied molecular orbital HOMO to the lowest unoccupied molecular orbital LUMO, i.e. the bandgap Eg, are determined. By transitions from HOMO to LUMO, electron-hole pairs are generated which build excitonic states. In contrast to inorganic semiconductors, the electrons and holes in organic materials are typically localized on the individual molecules and thus form thermally stable Frenkel excitons due to the strong Coulomb interaction [43–45]. In this work, Lorentz distributions given in [34–36] were used for the exciton transitions.
The scattering parameter ks is determined through R&T measurements by a point by point fit, based on Eq. (1) using simple R&T equations for plane-parallel plates. The fit of ks is performed in the wavelength range of low absorption (k ∼ 0).
2.5 Packaging and lens forming
After mixing and homogenizing the two components, the polyurethane microlenses were manufactured on InGaN-CoB-LEDs with chip dimensions of 250 × 250 µm2 in a ceramic package from EPIGAP Optronic GmbH with dimensions of 2 × 2.5 × 1 mm3 (width × height × depth) using an Eppendorf EDOS-5221 dispenser. The polyurethane was cured in an oven at 120 °C for 2 hours. Prior to the dispensing, the housing surface was modified with polytetrafluoroethylene (PTFE) to further reduce the radius of the microlens to achieve directed light distribution. The light distributions were measured at a wavelength of 525 nm.
3. Results and discussion
3.1 Glass and degradation temperature
First stability tests were carried out by exposing the polymers to practical soldering conditions. Optical and mechanical measurements before and after the soldering process showed no changes for the optimized PU resPUR-OT material for a maximum time of 10 s at 320 °C and 3 min at 260 °C. The other two materials did not withstand the soldering process, which was indicated by yellowing after the temperature treatments (see Table 2).
Figure 2 shows the TGA curves of the PU in the temperature range 30 °C to 600 °C and the 5% weight loss temperature, defined as degradation temperature Td5%. All TGA curves show roughly similar degradation. The DSC curves are presented in Fig. 3 with the glass transition temperatures Tg and degradation temperatures Td.
According to TGA and DSC analysis, the optimized polyurethane resPUR-OT shows the best properties in terms of thermal stability. With a decomposition temperature > 358 °C the material is well suited for encapsulation purposes.
3.2 Surface tension
The microlenses should be produced in a simple and inexpensive dispensing process. Lens formation requires a high surface tension of the material. The surface tension properties are shown in Table 3 and Fig. 4. With a surface tension of 48.4 dynes/cm, the material has excellent droplet formation properties for encapsulation and lens formation. LED chips in ring-shaped or square packages and the modified surface tension of the packages and adapted dosage are determining factors for the radii of curvature of the microlenses, shown in Fig. 4.
3.3 Optical properties
Figure 5 and 6 shows R&T spectra of PU thin films. The overall behavior in a wavelength range of 200-890 nm was obtained by the low-resolution spectrophotometer. Superposed and normalized to these data are high spectral resolution reflection measurements providing interferences in a spectral region of 400-600 nm (Fig. 5), used for the thickness determination in our modelling procedure. The corresponding fit curves are marked red. As shown in Fig. 6, the analyzed resin films exhibit quite different transparencies and different band gap energies of 3.89 eV, 4.77 eV and 5.02 eV for resPUR-OT-3000, resPUR-OT-T24000 and resPUR-OT. The transmission attenuation peaks of resPUR-OT-T24000 at a wavelength of λ = 270 nm and 308 nm in Fig. 6 can be explained by exciton transitions with energies of 4.69 eV and 4.45 eV, whose influence on the dielectric constant reaches far into the 900 nm range (see Fig. 7). This exciton absorption practically prevents the material from being used as encapsulation material for LEDs. The best fit parameter for the resPUR-OT-3000 material leads to two additional interband critical points (CP), which are recognizable by two peaks in the reflection spectrum, as shown in Fig. 5. For resPUR-OT and resPUR-OT-3000 material, the Urbach tail extension was required for parameter setting, which indicates localized states within the band gap. The optical constants n and k determined from the optimized parameters are shown in Fig. 7.
Scattering was investigated by transmission experiments of the thick PU rods using the spectrophotometer. To quantify the scattering effect, a scattering parameter ks was defined, so that the effective absorption coefficient is given by αe = α + αs = 4π (k + ks) / λ, where k is the extinction coefficient, α the absorption coefficient and αs describes the scattering losses. This approach is obvious, since in transmission experiments scattering acts similar to absorption.
The scattering effect could be made visible to the eye with a green laser beam which was guided through the 8 mm thick PU rods. The photos shown in Fig. 8(a) - (c), taken at right angles to the beam axis, show fluorescence as well as strong and weak scattering for the three different polymers, which is in qualitative agreement with the transmission measurements (Fig. 9).
With this approach, we quantified the scattering part besides the absorbing part. The separation of α and αs is particularly important in order to estimate the heat transfer to the material which is only determined by α. Especially with high power LED even small α can lead to overheating and degradation of the packaging material, where αs does not contribute to energy absorption. The scattering parameter ks, was derived from the fitting procedure, comparing calculated and measured transmission of the PU rods (Fig. 9). The results are shown in Table 4. We have found that ks depends only weakly on the wavelength and in transmission experiments can be described by Lambert's law in a fairly good approximation. Since αs = 4πks/λ, the corresponding contribution to the effective absorption αe is wavelength dependent. We assume that scattering is mainly caused by a difference in the refractive indices induced by variations in composition, density differences or structural defects within the polymers. Taking into account the scattering theory of Filinski , the application of the Lambert approach leads to changes in the refractive indices in the range from 10−4 to 10−3.
In accordance with [29,30], our results show that highly transparent PU can be produced primarily with the B-component HDI. The non-disperse low opacity and fluorescence of resPUR-OT-3000 are caused by the B-component MDI. Furthermore, the conversion of the polyester polyol to a lower OH content compared to the optimized resPUR-OT material leads to the formation of excitons. The optimized polyester with an average OH functionality per molecule of 2 - 3, which corresponds to an OH content expressed by approx. 130 mg KOH/g, was mixed with HDI at a ratio of 100:60. The resulting product (resPUR-OT) was clear, hard-elastic and UV-transparent. The transmission properties of the commonly used optical plastics  compared to resPUR-OT are shown in Fig. 10 for a layer thickness of d = 3.174 mm. The transmission of PU was calculated using n and k from Fig. 7(a) and (b). With a band gap energy of 5.02 eV the material exhibits excellent transparency in the UV and VIS range and has a moderate Abbe number νd = (nd - 1) / (nF - nC)) of 38 (see Table 4). For thin layers, this material can be used up to a wavelength of 300 nm.
3.4 CoB LED lens processing and light distribution
Due to its long pot life, the optimized 2-component polyurethane resPUR-OT proved to be easy to be processed. It showed good lens forming properties, as demonstrated in Fig. 11(a) - (c) on a commercial InGaN-CoB LED in a ceramic package with dimensions 2 × 2.5 × 1 mm3 (width x height x depth). The size of the diode was 250 × 250 µm2. Before dispensing, the outer surface of the ceramic housing was coated with PTFE to reduce the surface tension and thus achieve small lens radii of about 1 mm, as determined from contact angle measurements, described in chapter 2.3. The curing temperatures are given in Table 1.
This work confirms that highly transparent polyurethane can be produced from aliphatic isocyanates and polyester polyols by optimizing the mixing ratios. It is demonstrated that the optical properties and the band structure parameter of polyurethanes can be drastically changed by different material components. The method presented here turned out to be a useful approach for the development of optical materials. The optimization of polyurethane elastomer components enables the development of new ingredients and the realization of high-performance optical components. The polyurethane resPUR-OT, developed in this work, has a band gap energy of 5.02 eV and thus excellent transparency in the UV and VIS range. Furthermore, it is characterized by high thermal stability, good hardness and a high surface tension. It offers a simple and cost-effective dispensing technique for the production of micro lenses and enables the manufacture of high-power LEDs with customized beam shaping properties.
Bundesministerium für Wirtschaft und Energie (16KN074923).
The authors declare that there are no conflicts of interest related to this article.
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