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We demonstrate that blending of TiO2 nanoparticles into nanoimprint polymer resist yields a composite material with an increased optical refractive index suitable for nanoimprint lithography. Complex refractive indices of n400nm = 1.94-i0.009, n500nm = 1.86-i0.003, and n600nm = 1.83-i0.002 are predicted for composite materials with 30% TiO2 nanoparticles of 35-nm diameter in Amonil UV nanoimprint resist. Layers with concentrations of 1 to 30 volume% of TiO2 nanoparticles blended into the composite resist material are prepared by spin-coating. Good agreement between measured and predicted optical properties is obtained. Using the composite imprint resist periodic linear gratings with periods of 370 nm are fabricated on glass substrates using UV nanoimprint lithography. Subsequently, an organic light emitting diode (OLED) is fabricated on top of the grating. Due to the high-index grating structure, waveguide modes are extracted from the OLED, which are observed in the OLED emission spectrum.
© 2014 Optical Society of America
1. Introduction
Periodic nanostructures (photonic crystal slabs, Bragg gratings) are a promising means for increasing the external efficiency of organic light emitting diodes (OLEDs) [1]. By integration of the nanostructure with the active OLED layer stack, light trapped in waveguide modes may be coupled to radiation modes [2–4]. In [2] one-dimensional (1D) and two-dimensional (2D) gratings with lattice constants between 300 nm and 600 nm are patterned into the indium tin oxide (ITO, n = 2.0) anode layer. In [5] an additional nanostructured Ta2O5 layer (n = 2.1) is added between the glass substrate and a polymer anode as a high-index layer. Both processes require thin-film deposition techniques and a subsequent etching process rendering them costly. In recent years nanoimprinting of organic-inorganic composite materials has been investigated as a promising alternative for obtaining nanostructured high-index layers [6–9]. Organic-inorganic composites combine the good processability of organic materials with the high refractive index of inorganic materials [7]. For example, the addition of nanoparticles of zirconia or titania has been proposed to increase the refractive index of a polymer [10–14]. Here, we investigate the optical properties of a composite material obtained by simple blending of a UV curable resist and titania nanoparticles. The influence of the nanoparticle concentration on the layer refractive index and on the losses due to scattering and absorption are investigated. A periodic grating structure is nanoimprinted into the composite material layer and subsequently, an OLED is fabricated on top. The resulting emission spectrum is characterized.
2. Refractive index modeling
The introduction of nanoparticles into a matrix material allows for the tailoring of the optical properties of the matrix material. Nanoparticles with a refractive index nNP higher than the matrix nM cause an increase of the refractive index. Here, the real part of the refractive index nC of the nanoparticle composite material is modeled following Eq. (1), where ϕNP is the volume fraction of the nanoparticles in the composite material and ϕM = 1-ϕNP is the fraction of matrix material [8].
For modeling the losses in the composite material it needs to be considered that nanoparticles in a matrix cause both absorption and scattering. A beam with an initial irradiance I0 is attenuated upon passing through a composite material with thickness d following Eq. (2) [15].The absorption coefficient α in Eq. (2) is approximated by Eq. (3).Here, αM is the absorption coefficient of the matrix material, ρ is the number density of nanoparticles calculated with the radius a of a nanoparticle as , and σext is the extinction cross section of the nanoparticles. With the light wavelength in vacuum λ0 the absorption coefficient α is related to the imaginary part of the refractive index k by Eq. (4) [15]:The extinction cross section σext is the sum of the absorption cross section σabs and the scattering cross section σsca: σext = σabs + σsca [15]. For small, spherical scattering particles compared to the wavelength of light, the scattering cross section σsca may be calculated using the Rayleigh approximation given in Eq. (5) [16].The exact solution for the scattering cross section of spheres of any size may be calculated from Mie theory [15–17]. Here, Mie simulations were carried out using the computer code MiePlot v4.3.04 by Philip Laven to calculate σsca and σext [17].Figure 1(a) plots the real part and the imaginary part of the refractive index of the matrix material and the nanoparticles. TiO2 nanoparticles with an anatase crystal phase are assumed and the refractive index values are taken from [18]. The UV curable resist Amonil (Amonil MMS4 by AMO GmbH, Aachen, Germany) is employed as the matrix material. In order to obtain the thin-film refractive index of Amonil, a 200-nm layer was spin-coated on a glass substrate and hardened with UV-light as described in the section 3. The transmission spectrum was evaluated. Between 400 nm and 700 nm the spectrum matched the spectrum of a glass substrate. Therefore, the real part of the Amonil refractive index is modeled by the BK7 refractive index in the following. The imaginary part of the Amonil refractive index is calculated from the 200-nm layer extinction coefficient [19]. Figure 1(b) plots the extinction cross sections for the TiO2 nanoparticles in an Amonil matrix for different sizes of nanoparticles. For wavelengths larger than 400 nm Mie calculations agree well with the Rayleigh approximation results. Below 400 nm absorption losses need to be considered additionally, which are only contained in the Mie results. Furthermore, particularly for the 100-nm diameter, the assumption that the wavelength is large compared to the nanoparticle radius is not valid and ripples appear in the scattering cross section with wavelength. Increasing the nanoparticle diameter from 15 nm to 100 nm increases the scattering cross section by five orders of magnitude. Due to the small absorption losses, scattering losses dominate the extinction cross section above a wavelength of 400 nm. Thus, small, non-clustering particles are most advantageous for low-loss, composite materials.
Fig. 1 (a) Real and imaginary part of the refractive indices of inorganic TiO2 (anatase) [18] nanoparticles and organic Amonil resist matrix material. (b) Extinction cross sections σext for TiO2-Amonil composite materials for different diameters of the TiO2 nanoparticles calculated from Mie theory (solid lines). Rayleigh approximation for scattering cross sections σRay shown as dashed lines. (c) Calculated real part and (d) imaginary part of refractive indices for organic-inorganic composites with 35-nm diameter nanoparticles (NP) in volume concentrations of 0%, 5%, 10%, 20%, and 30% in the matrix material.
In the following a diameter of 35 nm is assumed for the nanoparticles. The resulting real and imaginary parts of the refractive index as calculated from Eqs. (1), (3), and (4) are plotted in Figs. 1(c) and 1(d) for different volume fractions ϕNP of nanoparticles. ϕNP = 0% corresponds to the pristine Amonil layer. At a wavelength of λ0 = 500 nm an increase of the refractive index from nM = 1.52 to nC_30% = 1.86 at a nanoparticle concentration of ϕNP = 30% is predicted. At λ0 = 400 nm the refractive index is predicted to increase even to nC_30% = 1.94. Figure 1(d) shows that the experimentally measured imaginary part of the refractive index for ϕNP = 0% is limited by measurement noise to a minimum of ~10−3. Composite materials show higher losses due to scattering. For the highest investigated nanoparticle concentration of ϕNP = 30% the imaginary part of the refractive index is predicted as kC_30% = 0.009 at λ0 = 400 nm. The scattering losses are proportional to λ0−4 leading to the lower imaginary part of the refractive index at higher wavelengths. In section 4 calculated optical transmission spectra are shown and compared to experimental results.
3. Experimental
3.1 Fabrication
Organic-inorganic nanocomposite materials were obtained by blending of TiO2 nanoparticles dispersed in xylene (purchased from Sigma Aldrich; specified with size in powder 15 nm and in solution < 100 nm; crystal phase: anatase/rutile mixture ~80:20) into the UV curable resist Amonil (viscosity η = 50 mPas). Solutions with 0%, 1%, 2%, 3%, 5%, 10%, 20%, and 30% TiO2 nanoparticles in volume were prepared. Above 30% the composite layer properties deteriorated. For characterization composite layers were spin-coated on (a) silicon wafers with a 1-µm oxide layer on top (20 x 20 mm2 in size) [Fig. 2(a)] and (b) glass substrates (Selected White Float glass purchased from Praezisions Glas & Optik GmbH; each substrate 25 mm x 25 mm x 1 mm in size) [Fig. 2(b)]. The substrates were cleaned using acetone and isopropanol for 15 minutes each. Subsequently, they were dehydrated on a hot plate at 160°C for 10 minutes. Next the adhesion promoter Amoprime (AMO GmbH, Aachen, Germany) was deposited by spin coating at 3000 rpm for 30 seconds. Finally, the samples were spin coated with the composite material solutions at 3000 rpm for 30 seconds.
Fig. 2 Overview of fabricated samples with organic-inorganic composite layers of TiO2 nanoparticles in an Amonil nanoimprint resist matrix. Composite layers were spin-coated on silicon wafers (a) and glass substrates (b) for characterization of the optical properties. (c) OLEDs were fabricated on top of the composite layer structured with a Bragg grating of period Λ = 370 nm using UV nanoimprint lithography.
For nanoimprint lithography we replicated working stamps from glass master stamps with, respectively, a 370-nm and a 460-nm Bragg grating on the surface fabricated by laser interference lithography [2]. Working stamps were prepared in polydimethyl-siloxane (PDMS) [20]. The PDMS precursor (Sylgard 184 from Dow Corning) was blended with a curing agent in the ratio 8:1 in a tube with stirring device (IKA ST-20) for 20 minutes. Afterwards, the mixture was cast onto the glass stamp, bubbles were removed by vacuum treatment, and the stamps were cured in an oven at 130°C for 20 minutes. For pattern replication the PDMS working stamp was placed on the organic-inorganic composite layer. The resist was exposed using a 2 J/cm2 dose of a UV light source (Beltron GmbH, Rödemark, Germany).
OLEDs were fabricated on top of nanostructured layers with a period of 370 nm [Fig. 2(c)]. First, a 70-nm layer of the anode polymer (PH500, purchased from H.C.Starck) was deposited using spin-coating. Then, an 80-nm light emitting layer of PDY-132 (SuperYellow, purchased from Merck OLED Materials GmbH) was spin-coated onto the anode polymer. Finally, a cathode of 1 nm LiF and 200 nm Al was deposited using thermal evaporation. A mask defined an OLED active area of 5 mm x 5 mm. All fabricated OLEDs were encapsulated using an epoxy resin adhesive and a glass cover.
3.2 Characterization
The organic-inorganic layers fabricated on top of silicon wafers [Fig. 2(a)] were characterized using a reflectometer (Sentech Instruments GmbH) in order to derive the layer thickness. For comparison, the layer thickness was measured using a profilometer (Ambios Technology XP2). The transmission spectra of the organic-inorganic layers fabricated on top of glass substrates [Fig. 2(b)] were measured using a UV-Vis spectrometer (Perkin Elmer, lambda 900) in the wavelength range of 300 nm to 700 nm.
The nanopatterned organic-inorganic layers were characterized using Atomic Force Microscopy (AFM, Park Scientific Instruments). The current-voltage (I-V) curves of the finished OLEDs were recorded at ambient conditions using a source-measurement unit (SMU, Keithley). Angle-resolved electroluminescence (EL) measurements were performed using a goniometer setup for sample rotation and bare fiber light collection at 50 mm.
4. Results and discussion
Table 1 gives the layer thickness obtained for TiO2 nanoparticle/Amonil composite layers spin-coated on silicon substrates obtained with profilometer and reflectometer measurements as well as calculated refractive index values at λ0 = 632 nm. The layer thickness increases with the nanoparticle concentration for constant spin-coating parameters showing an increase in the viscosity. The agreement between the profilometer and the reflectometer measurements is within the tolerance range of the profilometer of 5% to 10% deviation in the nanometer range.
Table 1. Thickness of organic-inorganic composite layers composed of TiO2 nanoparticles in Amonil nanoimprint resist on a silicon substrate measured using a profilometer as well as a reflectometer. All layers were fabricated by spin-coating at 3000 rpm for 30 seconds. Calculated refractive index values are given in the last column.
For a comparison between modeled values of the refractive index and experimental values, the transmission spectra of the nanocomposite layers on glass substrates are considered. Figure 3 plots the transmission spectra calculated with the complex refractive index values given in Figs. 1(c) and 1(d) and the experimental results. Good agreement is obtained between measured and simulated values. In the transparent region above 450 nm thin-film interference fringes are observed. Below 450 nm scattering and absorption losses increase. The refractive index model derived in section 2 describes the absorption edge as well as scattering losses correctly. In the model, an identical 35-nm nanoparticle diameter was assumed for all concentration values. This is an approximation as the employed titania nanoparticles exhibit a size distribution and clustering effects could not be quantified. The layer thickness was adjusted to obtain the experimental minima and maxima of the interference fringes. The resulting layer thicknesses are in good agreement with the values given in Table 1 for the composite layers on silicon substrates.
Fig. 3 Transmission spectra for organic-inorganic composite layers on glass substrates with varying volume percentage of TiO2 nanoparticles in an Amonil matrix. Symbols represent measurement data. Solid lines are calculated transmission curves for 35-nm diameter TiO2 nanoparticles with the optical properties given in Fig. 1. The layer thicknesses d assumed in the calculation are given in the legend.
Figure 4 shows AFM images of Bragg gratings nanoimprinted into a pristine Amonil layer and a composite layer with 30% TiO2 nanoparticles. The grating structure is clearly visible in both images. No grating deterioration is observed for the nanoimprinted composite material layer. To demonstrate the influence of the refractive index tailoring on OLED waveguide mode extraction, OLEDs were fabricated on top of layers of pristine Amonil, Amonil with 3% TiO2 nanoparticles, and Amonil with 30% TiO2 nanoparticles. For each of the three layer compositions a nanostructured device with a Bragg grating of period Λ = 370 nm and a reference device without a nanostructure were characterized. Figure 5(a) shows the I-V characteristics of the OLEDs. All devices have a similar turn-on voltage. In Fig. 5(b) a photograph a device is presented.Figure 6 shows the electroluminescence spectra as a function of angle for the six devices. The unstructured reference devices exhibit –as expected– no waveguide mode extraction features. The nanostructured devices with pristine Amonil layers and layers loaded with 3% TiO2 nanoparticles also show no waveguide mode extraction features. This may be attributed to the small refractive index difference between the Amonil layer and the following anode polymer layer. For the nanostructured device with a composite Amonil-30% TiO2 nanoparticles layer in Fig. 6(f), on the other hand, the waveguide mode extraction feature is clearly visible. The mode's effective refractive index neff for the OLED waveguide stack may be derived from the first order Bragg condition as sin(θ) = neff - λ0/Λ. Taking the out-coupling peak's vacuum wavelength of λ0 = 593.2 nm at θ = 0° and the grating periodicity of Λ = 370 nm this results in neff = 1.603.
Fig. 4 AFM images of imprinted Bragg grating, Λ = 460 nm, for (a) pristine Amonil and (b) Amonil with 30% TiO2 nanoparticles.
Fig. 5 (a) I-V curves of OLEDs fabricated on pristine Amonil, Amonil with 3% TiO2 nanoparticles, and Amonil with 30% TiO2 nanoparticles. For each of the three types, a reference device without a nanostructure and a nanostructured device with a Bragg grating of period Λ = 370 nm are characterized. (b) Photograph of OLED.
Fig. 6 Angle- and wavelength-resolved electroluminescence intensity perpendicular to the grating grooves for OLED devices with pristine Amonil, Amonil with 3% TiO2 nanoparticles, and Amonil with 30% TiO2 nanoparticles. The reference devices are unstructured and the nanostructured devices have a Bragg grating of period Λ = 370 nm. The spectrum at each angle is normalized to unity.
5. Conclusion
We demonstrated that the refractive index of Amonil nanoimprint resist may be tailored between n500nm = 1.52 and n500nm = 1.86 at a wavelength of λ0 = 500 nm by simple blending of up to 30 vol% TiO2 nanoparticles into the resist. For nanoparticles with an effective diameter of 35 nm the imaginary part of the refractive index is k500nm = 0.003 for the maximum investigated concentration of 30 vol% of TiO2 nanoparticles. These values were derived from a model for calculating the wavelength-dependent real and imaginary part of the refractive index including scattering and absorption losses. This model was verified by optical transmission experiments. The composite resist layer was structured successfully with a Bragg grating of period Λ = 370 nm. OLEDs were fabricated on top of unstructured and nanostructured layers of pristine Amonil, Amonil with 3% TiO2 nanoparticles, and Amonil with 30% TiO2 nanoparticles. OLEDs with unstructured composite layers show no waveguide mode extraction due to the absence of a grating. For nanostructured pristine Amonil and Amonil with 3% TiO2 nanoparticles no waveguide mode extraction is obtained due to the small refractive index difference between the nanostructured layer and the subsequent anode material. Increasing the refractive index of the nanostructured composite Amonil layer with 30% TiO2 nanoparticles, distinct waveguide mode extraction is observed in the electroluminescence emission characteristic. This demonstrates the usefulness of refractive index tailoring of nanoimprint resist layers with TiO2 nanoparticles for waveguide mode extraction.
Acknowledgments
We acknowledge support by the Bundesministerium für Bildung und Forschung (BMBF) within the project NanoFutur under Project No. 03X5514.
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