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We presented enhanced light extraction efficiency of organic light emitting diodes (OLEDs) cells with a nano-sized diffraction grating layer. Various diffraction gratings of different morphologies including linear, cubic, hexagonal and quasiperiodic patterns were fabricated by multiplexing light interference exposure on an azobenzene thin film. The effect of diffraction grating layer on device performances including luminous properties and quantum efficiency was investigated. In contrast to periodic grating patterns, the quasiperiodic structures leading broadband light extraction resulted in improved external quantum efficiency and power efficiency by 73% and 63%, respectively, compared to conventional OLED with flat surface of glass substrate.
© 2016 Optical Society of America
1. Introduction
OLEDs (Organic light emitting devices) have gathered considerable attraction as a next-generation platform of information display and lighting applications because of their good optical properties including relatively high light efficiency, broad color gamut, fast response time, and wide viewing angle [1,2]. Further, OLEDs have excellent potentials in applications such as transparent, flexible, stretchable and rollable devices [3–5]. Therefore, a great deal of theoretical and experimental research has been devoted to the development of high performance OLED devices. For example, Jabbour et al. reported new emitting materials, showing a nearly 100% of internal quantum efficiency (IQE, ηint) by doping the phosphorous materials into a host material [6–8]. However, compared to a considerable progress in the internal quantum efficiency, the external quantum efficiency (EQE, ηext) was limited to under approximately 20% due to low light coupling efficiency (ηcoupling), where their relations are simply determined by the following equation [9].
Usually, it has been reported that about 80% of emitted light is entrapped in conventional bottom emissive OLED structures (glass substrate/indium-tin-oxide (ITO) anode/organic/metal cathode) and thereby, only 20% of emitted light is extracted from the OLED cells. The resulting light loss can be explained by the study of light behavior at the interface between two different media, where the propagation of light depends on the gap of refraction index of two adjacent materials, which may vary relying on the wavelength of the light. More in detail, 50% of light is reflected at the interface between glass substrate and ITO anode, so called total internal reflection, and eventually disappeared [10]. The other 30% of light loss is explained by waveguide mode where the light is propagated along the anode surface and finally diminished. Recently, it is reported that light loss is also induced by surface plasmon polariton mode, arising at the interface between the metal cathode and dielectric organic layer [11].
To address the low light extraction efficiency of OLED devices, there has been much effort to reduce the significant decay of emitted light in the cell by modifying the morphology of interface structure of two different media. For example, Forrest and associates introduced micro-lens array on the surface of a glass substrate, faced to the air, to increase the critical reflection angle of emitted light, and therefore the external quantum efficiency was increased up to 5% [12]. Nakamura's group presented high-refractive index glass scattering layer between ITO anode and glass substrate, thereby the out-coupling efficiency was increased 1.7 times compared to the reference [13]. The aforementioned methods have focused on changing the structural morphology or refractive index of substrate in order to minimize the light loss from total internal reflection between surface of substrate and the air. However, internal light loss between organic layer and electrodes has been challenging to be efficiently improved to date. While various methods have been developed to tackle the internal light loss issue, micro-cavity [14–16], photonic crystal (PC) [17–19], nano-sized scattering layer [20–22] and diffractive grating layer [23–25] are the most intensively researched. Among them, diffractive grating layer is worthy to be exploited further since the optical properties of out-coupled waves can be precisely and easily manipulated, where it would be useful to study the relationship between light-extraction additive layer and OLED performance. To this context, Joo’s group reported OLED devices incorporating a sub-micrometer-scale stripe auxiliary electrode layer and it showed that the stripe-patterned electrode acted as a diffractive grating which increased the forward-directed light by the micro-cavity effect and suppressed light loss by the surface plasmon polaritons. However, it is inevitable that one-dimensional line patterns of grating structure have angular dependency as a function of wavelength. It means that the color gamut of OLED device is altered along the viewing direction which is undesirable for information displays as well as lighting devices. To address this angular selectivity issue of 1-dimensional grating patterns, Koo demonstrated enhanced light extraction by introducing a nano-sized buckling structure between ITO anode and glass substrate [23]. The nanoimprinted buckles has a quasiperiodic wrinkle patterns with broad distribution as well as directional randomness that provided the enhanced light extraction of wide spectral range without any sacrifice of the color gamut along the viewing direction.
In this study, we revisited the nano-scale surface relief grating structure having various morphological surface patterns from one-dimensional, two-dimensional grating to quasiperiodic grating structure to investigate the effect of grating structure on the performance of the OLED device in terms of light extraction efficiency, and color gamut.
2. Experimental details
2.1 Fabrication of nano-sized diffractive gratings light interference lithography
The enhancement of light efficiency of OLEDs with diffractive gratings could be explained in terms of light extraction angle by following equation.
Where k0 and θ are wavenumber and exit angle of the diffracted lights in free space. Meanwhile, kwg is waveguide vector of the waveguide light and kG is reciprocal grating constant of the nano structures with period d where the value of the integer, m, indicates the scattering order. λ is wavelength of emitted light and neff is effective refractive index between organic material and ITO (the effective refractive index has intermediate value of two faced mediums in the range of the critical angle). The effective refractive index can be tuned by the interfaced nano-sized gratings that induce escaping of waveguide light at the interface and thus, it allows the enhancement of extraction efficiency of the emitted light due to reduced internal reflection. For example, the periodicity of pattern is considered as infinite in case of flat surface of substrate where the diffraction angle varies only by both wavelength of emission light and refractive index of substrate that follows the Snell’s law. However, when the nano-sized gratings at the interface has shorter periodicity than the wavelength of emission light, the light does not recognize the physical interface of ITO and the organic layer and therefore, the difference in refractive index of adjacent media is decreased, resulting in broadening the light extraction angle.
In this study, we fabricated nano-gratings with a periodicity of 416 nm as shown in Figs. 1 and 2, which is slightly higher than the shortest wavelength of green-emitting material used in this study and thus, overall range of emitted light can be affected by nano-gratings. Furthermore, our nano-grating diffraction systems are distributed in a broad range of aspect ratios as well as modulation depth and it covered the various azimuthal directions that fabricated by multi-angle rotational light exposure, which is expected to inhibit the light loss as waveguide mode. Hence, the large and multi-directional in-plane wave vectors of transverse electric modes (called waveguide mode) can be reduced by multi-grating vectors which is inversely proportional to the periodicities of gratings.
Fig. 1 Schematic presentations of fabricating the nano-sized diffractive gratings and OLED devices, (a) optical apparatus of light interference lithography for five different diffractive grating patterns (HW: half-waveplate, PBS: polarizing beam splitter), (b) overall process of OLED devices preparation based on nano-imprinting/thermal evaporation methods and picture of fabricated OLED devices as well as cross-sectional SEM image of active area.
Fig. 2 (a)-(f) 3-dimensional AFM images of diffractive gratings including line, cubic, hexagonal and quasiperiodic patterns, (g) depth profiles of diffractive gratings along the k-vector direction, (h) power spectral density of diffractive gratings representing distribution of grating period, (i)-(n) simulated far field light propagation of diffractive gratings by fast Fourier Transform calculation, (o)-(t) pictures of real working OLED devices with different angular distribution of light emission.
Nano-sized diffractive grating patterns were prepared as reported previously [26]. Briefly explaining, a master mold of grating patterns was fabricated by light interference lithography (LIL) and the grating patterns were transferred onto substrate of OLED cell by nano-imprinting technique. For LIL process, poly (Disperse Red 1 acrylate) as a photosensitive material (PDR1, Sigma-Aldrich) was used without further purification process. To prepare the azobenzene thin film, PDR1 was dissolved in chloroform and the solution was filtrated before being spin-coated. The filtrated solution was spin-coated on pre-cleaned glass substrate and finally, the film with thickness of 320 nm was obtained. The PDR1-coated sample was exposed to two-beam coupled light interference to form various structures of gratings. Due to the unique property of light-induced anisotropic movement of PDR1 induced by photo-isomerization cycles between cis- to trans- conformation, the linear line grating is simply formed upon light exposure where the grating periodicity is precisely determined by both wavelength of light source and incident angle of two beams. For higher light extraction efficiency, according to the previous report [23], higher modulation depth of grating structure is usually more favorable than that of the structure having lower grating height when it has same grating periodicity. Therefore, to produce the nano gratings with multi-azimuthal directions with broad range of both modulation depth and aspect ratio, several types of gratings were prepared with additional multi-step light exposure. For cubic structures, the sample of pre-inscribed line grating patterns was rotated by 90° and then, secondarily exposed to light interference where the additional line gratings were expected to be formed orthogonally to the pre-inscribed grating. Similarly, hexagonal pattern structure was fabricated by rotating the sample by an interval step of 60° upon light exposure. Meanwhile, quasiperiodic gratings having multiple periodicities as well as most broad range of depth modulation can be fabricated by applying narrower interval rotation angle compared to the aforementioned cubic and hexagonal cases as shown in Fig. 1(a).
2. 2 Fabrication of OLED devices with enhanced light extraction efficiency
In order to reduce the light loss of waveguide mode at the interface between the glass substrate and ITO anode, we fabricated diffractive-layer-embedded OLED devices by using the nano-imprinting technique. First, polydimethylsiloxane (PDMS) elastomer (Silgard 184, Dow corning) was prepared by mixing main resin and curing agent with weight ratio of 10:1 and thoroughly mixed with mechanical stirring. The mixed PDMS resin was poured onto the surface of diffractive gratings and then, placed in vacuum oven to remove trapped air bubble. After storing the samples at room temperature for a week, the cured PDMS replica was exfoliated carefully from the PDR1 and fully crosslinked at 70 °C for 30 minutes. To transfer patterns of PDMS replicas to the substrate of OLED, a droplet of UV-curable resin (Norland optical adhesive (NOA) 65, Norland) was dropped on the glass substrate and applied vertical pressure of 1kgf during the period of UV exposure. The NOA is a liquid photopolymer that can be easily cured when it is exposed to UV. The UV light was irradiated to NOA for 12 minutes to be fully cured by photo-crosslinking reaction, followed by carefully peeling off the PDMS replica molds. Then, an ITO layer was deposited on NOA layer by RF sputtering as an anode electrode having a thickness of 150 nm. Afterwards, a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (AI 4083, Clevios, dissolved in de-ionized water at the concentration of 0.1 wt%) was spin-coated on the patterned ITO, and a 50 nm-thick tris-(8-hydroxyquinoline)aluminum (Alq3) emitting material was deposited at a rate of 0.1 nm/sec. To form an electron injection layer and cathode electrode, 1 nm of LiF and 100 nm of aluminum were deposited by thermal evaporation method in a high vacuum chamber under a pressure of 5 × 10−7 Torr. Finally, the fabricated cells were encapsulated in glove box by attaching a glass cover window to prevent exposure to external oxygen and H2O into OLED cells.
3. Results and Discussion
3.1 Characterization of various diffractive grating patterns by AFM
Figures 2(a)-2(f) shows surface morphology of nanoimprinted NOA patterns (hereafter termed as P0, P1, P2, P3, P4, P5 without pattern and with one-dimensional line grating, two-dimensional tetragonal, hexagonal grating, and quasiperiodic gratings, respectively) which are represented as three-dimensional atomic force microscope (AFM) images. All the measurements of AFM were conducted as non-contact mode to avoid undesired scratch of NOA surface. The morphology of the P0 shows a flat surface with a root-mean-square roughness of 0.48 nm that is a negligible level affecting the light extraction efficiency of OLED cells. Also the other AFM images of P1-P5 represent various nano-sized grating patterns from master mold were successfully transferred to NOA layer on the surface of glass substrate of OLED cells. Figure 2(g) shows line profile of depth modulation of each sample across the x-axis whereas the overall depth modulation of gratings was increased as a function of the number of holographic exposure. Especially, the difference in height of modulated grating was significantly increased in case of P5. Meanwhile, the dominant grating period was obtained to be 416 nm in all cases as shown in Fig. 2(h) where the periodicity of grating was calculated by evaluating the power spectral density of AFM data of each sample. The aforementioned grating period is exactly same as the period of light interference pattern of two beam coupled waves as we designed. The P0 shows no distinguished periodicity as expected. P1 and P2 have the same grating period of 416 nm and similar full width at half maximum (FWHM) behavior because two different line-gratings of same period are imposed orthogonal each other in case of the tetragonal grating, P2. On the other hand, P3 shows slightly broader than P2 in terms of FWHM since the line gratings cross in hexagonal-fashion and therefore, a little longer periodicity of 480 nm exists. Analogously, P4 and P5 also show the same grating periods of 416 nm while it is noteworthy that a shoulder peak approximately at 550 nm and another peak at 650 nm were observed with P4 and P5, respectively. From the previous reports [27], it is interpreted that the higher order of quasi-crystal structure can be made by multiplex exposure of holographic irradiation and the another population of grating period at longer position is observed due to the longer space length of crystal lattice.
Figures 2(i)-2(n) shows the simulation results of far field light propagation through diffraction gratings where the calculation was conducted by fast Fourier transformation (FFT). In case of P0 sample, the very strong zero-order transmission of incoming light is shown without any diffracted beams. For P1, the 1st-order as well as higher order diffraction beams are observed along the x-axis since the vertical line gratings acted as a slit against incoming light and eventually, the light was diffracted in horizontal direction. Similarly, in case of tetragonal cubic grating (P2), the incoming light diffracted at the vertical and horizontal direction, respectively, implying that the light coupling efficiency would be improved compared to P1 because the light can be extracted simultaneously not only in vertical direction but also in horizontal direction. It is noteworthy that the additional diffraction beams were observed in diagonal direction in case of P3 because of the triangular manner of repeating lattice structure of hexagonal grating. Finally, dramatic increase of diffraction beams, in terms of the quantity as well as angular distribution of diffraction, was obtained in case of P4 and P5, respectively. The aforementioned simulation results of light diffraction are directly connected to light extraction behavior of working OLED devices fabricated with the corresponding diffraction gratings as shown in Figs. 2(o)-2(t). Unsurprisingly, it is interpreted as the emitting light of OLED devices was propagated by the principle of light diffraction as we designed.
Figure 3(a), 3(b) show distribution profile and averaged value of modulation depth of diffractive gratings, respectively. In Fig. 3(a), the P0 with flat surface shows no meaningful distribution peak while P1 shows the most intensely positioned peak at 80 nm, which coincides with the modulation depth of diffractive grating of P1 as shown in Fig. 2(g). It is interpreted that one-dimensional linear grating was fabricated in case of P1 and thus, resulted in concentrated distribution peak matched its modulation depth. The diffractive gratings of P2 to P5, however, show a much broader shape of distribution peak and constantly increased modulation depth range compared to P1. This is because that the modulation depth and morphology of pre-formed grating patterns were influenced by the following light exposure. The increasing behavior of modulation depth is more apparently shown at Fig. 3(b). At the quasiperiodic diffractive grating of P5, where the most highly and randomly modulated gratings are shown, the distribution peak positioned along a wide range of modulation depth with triangular shape.
Fig. 3 (a) Distribution of modulation depth of diffraction gratings, (b) averaged modulation depth of P0-P5 gratings.
3.2 OLEDs device performance
To evaluate the effect of nano-grating structures on OLEDs performance, the green-light visible emissive OLED devices were fabricated as described in the aforementioned experimental section and their J-V-L (current density-voltage-luminance) characteristics as well as electroluminescence spectra of OLED devices were measured simultaneously by using a source meter (Keithley 2400) and a spectroradiometer (Minolta CS-2000). The light emission at the surface normal from OLED devices were only considered. Figure 4(a) represents the J-V characteristics of the OLED devices with and without nano-sized diffractive grating structures. In order to minimize the morphological effect on electrical properties, it is required to make similar electrical contact conditions of each layer in OLED cells. To do this, the flatten HTL layer was prepared by spin coating the PEDOT:PSS hole transport material on nano-structured surface of ITO anode. The turn-on voltages of all devices were measured nearly at 4 V and the current density as a function of applied voltage also showed similar growth behaviors with a negligible leakage current. It means that the performance of OLED cells could be directly compared in terms of light extraction efficiency without considering the effect of electrical properties on OLEDs performance.
Fig. 4 Device performances of OLED cells with and without diffraction gratings (a) voltage-current density plots, (b) power efficiency graph as a function of current density, (c) external quantum efficiency (EQE), (d) electroluminescence spectra for various OLED cells.
In Fig. 4(b), the power efficiencies of OLED devices with respect to applied current density are shown. It shows rapid increase in power efficiency by current density of 30 mA/cm2 and then, gradually decreased as current density increased. Moreover, the relative maximum of power efficiency was improved as the diffractive grating order increased from P0 to P5. Finally, P5 showed the significant increase of power efficiency up to 63% compared to the flat reference device of P0, verifying the effect of the nano-gratings on the enhancement of light extraction. We believe that the randomly developed gratings diffracted emitting light of waveguide mode in multiple azimuthal directions and thus, the highly improved light extraction was obtained. Figure 4(c) presents the property of EQE as a function of luminance of OLED cells. It showed that the higher EQE was obtained at the higher intensity of luminance as the order of diffraction grating increased from P0 to P5. The quasiperiodic nanostructure of P5 resulted in an improved gain of 73% in terms of EQE as compared to the flat device of P0. To investigate the effect of diffraction gratings on light extraction efficiency of OLED cells, the electroluminescence (EL) spectra of OLED cells were measured using a spectroradiometer (CS-2000, Minolta). It reveals that the light emission at 530 nm, corresponding to emitting material of Alq3 used in this study, was significantly enhanced in case of quasiperiodic diffraction grating incorporated OLED cell, P5. Furthermore, the emission light at wavelength near 630 nm is also extracted, resulting in enhanced light extraction efficiency of OLED cells.
4. Conclusions
In summary, we presented light extraction enhanced OLED cells with nano-sized diffraction gratings. The diffraction gratings were fabricated by multiplexing light interference exposure at azobenzene thin film, resulting in quasiperiodic structure useful for broadband light extraction. The device performances including power efficiency and external quantum efficiency were significantly improved by out-coupling the waveguided mode between glass substrate and ITO electrode.
5. Funding
KITECH ((Korea Institute of Industrial Technology) (EO160005, ‘Solution-based micro/nano process technology for smart electric bandage’)); MOTIE (Ministry of Trade, Industry & Energy (project number 10051918)) and KDRC (Korea Display Research Consortium) support program for the development of future devices technology for display industry.
Acknowledgments
We would like to thank Dr. Kang and CAPE members from KITECH for their OLED device manufacture equipment.
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