Extracting light from organic light-emitting diodes (OLEDs) and improving the angular distribution are essential for their commercial applications in illumination and displays. In this work, hybrid microlens arrays (MLAs) and gratings with periods and depths in the scale of submicron have been designed and incorporated on the lighting surface of OLEDs for simultaneous enhancement of light outcoupling efficiency and angular distribution improvement. It is found that the augmentation of light extraction efficiency is mainly attributed to the MLAs, while the gratings can improve the viewing angle by increasing the angular distribution uniformity. A novel approach was proposed by combining photoresist thermal reflow, soft-lithography and plasma treatments on polydimethylsiloxane (PDMS) surfaces synergistically to realize gratings on the wavy surface of MLAs. It has been proved that with the hybrid MLAs/gratings, the external quantum efficiency (EQE) of the OLED can reach up to 22.8%, which increased by 24% compared to that of bare OLED. Moreover, the OLED with the hybrid MLAs/gratings showed an obvious lateral enhancement at wider viewing angle.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Organic light-emitting diodes (OLEDs) has been regarded as the most promising technology in the display and lighting fields due to their excellent characteristics, such as self-emission, low power consumption, wide viewing angles, fast response, and suitability for flexible devices [1–4]. The luminous efficiency and lighting quality of OLEDs have been greatly improved, with the internal quantum efficiency (IQE) increasing to nearly 100% by optimizing the materials and device structures. However, the external quantum efficiency (EQE) of OLEDs is generally below 30% because of poor light extraction efficiency [5–7]. A large fraction of photons generated by exciton recombination are trapped and lost mainly due to the total reflection at the interface between air and substrate (substrate mode), waveguide mode at the indium–tin-oxide (ITO)/organic interface and surface plasmon-polariton (SPP) mode at the metal cathode surface. Therefore, there is still much room remained for enhancing the light outcoupling efficiency of OLEDs from the perspective of energy saving.
Various strategies categorized as internal and external light extraction methods have been reported to overcome the limited optical outcoupling efficiency. The internal methods mainly minimize the loss of waveguide mode and SPP mode by incorporating a light extraction layer such as corrugated structures [8–11], nano-scattering layers [5,12–14], into the devices. Besides the complex and expensive fabrication processes, this method inevitably affects the electrical properties and surface roughness of OLEDs, even though the leakage current can be limited to a low level [11,15]. The external methods employed microlens arrays (MLAs) and textured surfaces set on the substrate’s outside surface (light-exiting surface) to extract the substrate modes [16–19]. Compared to internal light extraction methods, the external methods are simpler, and the light lost in substrate mode is noticeable because of the large mismatch of refractive index between substrate/air interface.
Lots of researches focusing on external light extraction methods by using directly outcoupling structures on the outside surface of glass have been reported to significantly enhance the light extraction efficiency. Hemispherical lens and MLAs were regarded as common and effective ways to extract glass-confined mode. Meanwhile, dual patterns with simultaneous micro-structures and nano-structures were also found to imparted higher current density and luminance to the white OLED without electrical degradation compared to that with single micro-structures [20–23]. However, micro-structures with isotropic patterns like MLAs has inherent defects, such as angular independence and non-selective wavelength . It has been shown that the light intensity from OLED with isotropic or random micro-structures or nanostructures as light extraction layer varied evidently with view angle. Moreover, the extraction behaviors were also different for light with various wavelength, leading to unstable color property . This will influence the viewing angle and color stability in the OLED display and lighting.
On the other hand, anisotropic grating can extract the light with different wavelengths by manipulating the period and depth [7,26–28]. Furthermore, hybrid micro-/nano-structures with simultaneous isotropic MLAs and anisotropic gratings with nano-scale and submicron scale might have a synergistic effect of enhancing light outcoupling efficiency and improving the angular distribution of light intensity. However, systematic studies on the interplay between the light extraction and angular distribution have not yet been reported, mainly due to the absence of deterministic fabrication methods capable of generating diverse grating on the wavy surface of MLAs.
In this work, hybrid MLAs/gratings for enhancement of light extraction efficiency and distribution uniformity of OLED were designed using finite-difference time-domain (FDTD). Then, a fabrication method of hybrid MLAs/ gratings was developed using combinatorially photoresist reflow and plasma treatments. As expected, the hybrid MLAs/gratings exhibited EQE and CE of 22.6% and 83.7% respectively, which were 1.24 and 1.24 times compared to a conventional OLED without light outcoupling structure.
2. Experimental details
2.1 Design and optimization of hybrid MLAs/gratings
A commercial FDTD software (FDTD Solutions, Lumerical, Inc.) was used for modeling and simulation. The refractive indices for the various materials were downloaded from the website “refractiveindex.info.”. The perfect matched layer was set as simulation boundary condition except a metal boundary was used on the metal electrode side. An electric dipole with orientations of x, y and z was used as light source and was placed in the emitting layer with emission wavelengths of 460 nm, 520 nm and 700 nm respectively. A far-field monitor was used to obtain the far field electric field (E-field) distribution. In the simulation, the square of the electric field component of the electromagnetic wave is regarded as light intensity, and the electric field distribution around the micro-lens can be regarded as the effect of light on the micro-lens array. In each dipole orientation, the intensity (|E|2) in far-field was obtained by averaging the separate results. The light extraction efficiency of the OLEDs was calculated by integrating electric field intensity of the device. The FDTD region size was x = 39 μm, and y = 39 μm. In this study, the light extraction efficiency enhancement of the OLEDs was obtained by calculating the luminous flux with and without the light extraction structure, and was given by:
2.2 Fabrication of hybrid MLAs/gratings
Figure 1 illustrated the schematic preparation flow of micro-/nano-structures. Firstly, a glass substrate was cleaned in sequence with acetone, alcohol, and deionized water for 15 min, and was then dried with nitrogen. Then, the photoresist (AZ4620) was spin-coated onto the glass substrate with thickness of around 7 μm and baked for 10 min at 100 ℃. After that, a well-designed chromium mask was put on the photoresist and exposed under UV light for 40 s. The sample was developed in AZ400 K solution for 4 min and dried with nitrogen. Subsequently, the sample was heated to a temperature of 125 ℃, and was kept for 6 min for thermal reflowing to form photoresist MLAs.
The MLAs were exposed to trimethylchlorosilane (TMCS, Sigma) vapor for 5 min to facilitate the subsequent release of the soft-lithography molding. Afterward, polydimethylsiloxane (PDMS, Sylgard 184, Dow Corning) pre-polymer solution was prepared with ratio of base polymer to cross-linker of 10:1, and was poured on the MLAs master mold. After curing at 80 ℃ for 1 h, the PDMS stamp was peeled off from the master mold, forming PDMS template with concave arrays. Then the PDMS sheet was stretched by a graduated displacement platform to some extent in one direction, with a length change of 20% from initial 1.5 cm to 1.8 cm. And then it was treated by oxygen plasma using reactive ion etching (RIE) system (RIE-801, Beijing Chuangshiweina Technology Co., LTD.). The experimental conditions used during the plasma treatment were as follows: the 13.56 MHz RF discharge power was 50 to 200 W, the irradiation time was 60 to 120 s, the gas flow rate was 50 sccm, and the pressure was 5 Pa. Periodic wrinkles (gratings) on the concave arrays were obtained after releasing the stretcher. The PDMS mold with gratings was then transferred on UV curable optical adhesive (NOA63) by soft-lithography, forming hybrid MLAs/gratings.
2.3 Fabrication of OLED devices
Highly efficient phosphorescent green OLEDs were fabricated for the evaluation of light extraction, whose structures were schematically depicted in Fig. 2(a). The device consist of ITO/ Dipyrazino [2,3-f:2’,3'-h] quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HAT-CN, 15 nm)/4-methyl-N-[4-[1-[4-(4-methyl-N-(4-methylphenyl)anilino)phenyl]cyclohexyl]phenyl]-N-(4-methylphenyl) aniline (TAPC, 50 nm)/4,4’,4''-Tris(carbazol-9-yl)-triphenylamine (TCTA, 5 nm)/Beryllium bis[2-(2-pyridinyl)phenolate] (Bepp2):8% Acetylacetonatobis (2-phenylpyridine)iridium (Ir(ppy)2acac) (15 nm)/ Beryllium bis[2-(2-pyridinyl)phenolate] (Bepp2, 40 nm)/LiF(1 nm)/Al. The thickness of ITO film (anode) was 180 nm, which was deposited using magnetron sputtering, and was annealed with a high temperature of 200 ℃ to decline the sheet resistance (<10 ohm □-1) and ohmic contact. The ITO was treated by oxygen plasma for 15 min, and then transferred to vacuum deposition system. The HAT-CN as hole injection layer was deposited with a rate of 0.01-0.02 nm/s. Other organic layers were deposited with a rate of 0.1-0.15 nm/s. Lithium fluoride (LiF, 0.01 nm/s, 1 nm) and aluminium (Al, 0.1-0.2 nm/s, 120 nm) were used as electron injection layer and cathode, respectively. Finally, the OLEDs were encapsulated with glass lid.
Field emission scanning electron microscope (FE-SEM, FEI Nova NanoSEM 230, 10 kV acceleration voltage) was used to observe the morphology and cross-section image of hybrid micro-/nano-structures. Furthermore, atomic force microscope (AFM, Bruker) was applied to determine three-dimensional surface morphology of the hybrid micro-/nano-structures. The luminance−current density−voltage (L−J−V) characteristics were measured by a constant current source (keithley 2400) and a spectrophotometer (TOPCON, SR-LEDW). The electroluminescent (EL) spectra and angular distribution were measured using spectrophotometer equipped with a rotatable stage.
3. Results and discussion
3.1 Simulation and optimization of light extraction structures
Two-dimensional (2D) FDTD simulations were conducted to theoretically investigate the optical effects of MLAs and hybrid MLAs/gratings on the light output efficiency for the OLEDs, and to find out the optimal parameters of light extraction structures . Figure 2(a) illustrates the simulation model, where the emitting layer and functional layers of OLED were simplified as a layer with refractive index of 1.78 and the thickness of glass substrate was 2 μm. The optimization process was performed by sweeping the dominant parameters, such as the size of microlens, the size of gratings, the emission wavelength and the dipole orientation.
The effects of MLAs on the light extraction efficiency were first investigated, as shown in the Fig. 2(b) and (c). It was found that the output coupling efficiency was dependent on the diameter of micro-lens in the range from 10 to 20 μm when the height was fixed at 2 μm, while the spacing between the MLAs had little effect on the enhancement. The largest luminance ratio of the devices with MLAs to those without MLAs can be obtained at a diameter of around 12 μm, where the improvement of luminance efficiency was around 30%. The spacing had a slight effect on the light extraction efficiency of OLED, and MLAs with tangent micro-lenses were used in this work. When the height, diameter and pitch of the micro-lenses were 2 μm, 12 μm and 12 μm, respectively, the exit angle of the light was mostly smaller than the critical angle of its total reflection, and the light is more likely to escape into the air. Meanwhile, it was also found that the ratio of height to diameter of MLA will affect the luminance improvement. Within this study, the maximal improvement of the luminance efficiency can reach up to around 50% at a height to diameter ratio of 0.3∼0.4, where the diameter of microlens was 12∼13 μm. It is well known that the light extraction effect can reach a maximum when the shape of the microlens tends to a hemisphere. In the simulation, the surface of microlens was set as a quadric surface. When the height to diameter ratio was between 0.3∼0.4, the curvature was approximately hemisphere. Thus, it showed a better performance when the ratio of height to diameter was between 0.3∼0.4.
The improvement of luminance ratio might be mainly attributed to the extraction of substrate mode by MLAs . However, the focusing characteristic of microlens will lead to angular-dependent luminance of the OLED, which is undesired in the actual application. In this work, we proposed a hybrid MLAs/gratings structure with periodic gratings on the surface of microlens to simultaneously improve the luminance ratio and the angular characteristic of luminance. Furthermore, the luminance ratio and angular characteristic of OLED with different emission wavelength can be optimized by modulating the period and depth of gratings. The light emission behaviors for bare OLED (OLED without light extraction layer), OLED with MLAs and OLED with hybrid MLAs/gratings were investigated using optical modeling. The outcoupling efficiency was obtained by integrating far field electric field intensity . The color maps of electric field distribution of OLEDs with horizontal dipole are illustrated in Fig. 3(a)-(c). It is clear shown that most photons are trapped in the substrate and organic layers due to the substrate mode for bare OLED device. As expected, the substrate mode was disturbed for both OLEDs with MLA and hybrid MLAs/gratings, respectively. The angular dependence of luminescence (dipole with x orientations, i.e., p-polarized emission) was also measured, and the normalized far-field intensities depending on the viewing angle are plotted in Fig. 3(d)-(f). Compared to the bare OLED device, the OLED with MLAs exhibits a strong emission in the vertical direction, which was attributed to the focusing property of microlens. By incorporating gratings on the surface of MLAs, the intensity of light increase significantly at the viewing angle range of around ±40°, which might be attributed to the diffraction and scattering effects. The intensity measurement also indicates that the luminance enhancement is induced mainly by the substrate mode resulting from the spherical interface between substrate and air.
Prior to the fabrication of hybrid MLAs/gratings, the FDTD simulation was also used to find the suitable period and depth of gratings. Figure 3(g)-(i) illustrate the enhancements of outcoupling efficiency normalized to the devices with respect to periodicity and depth of only gratings for emission wavelengths of 460 nm, 520 nm and 700 nm, respectively. It is clear that the enhancement of outcoupling efficiency is dependent on the structure (periodicity and depth) of the gratings. Furthermore, these dependences are different for OLEDs with varying emission wavelength, indicating that the enhancement of outcoupling efficiency for OLEDs with specific emission wavelength can be optimized by the structure of the gratings.
3.2 Fabrication and morphology regulation of hybrid MLAs/gratings
From the numerical analysis results, the grating can be used to regulate the light extraction efficiency of OLEDs with various emission wavelength by manipulating the period and depth, and can also effectively improve the angular distribution of light intensity concurrently. However, the fabrication issue of gratings on the wavy surface of MLAs still remains to be solved. In this work, we proposed a fabrication method of hybrid MLAs/gratings by combining photoresist thermal reflow, soft-lithography and plasma treatments on PDMS surfaces synergistically, as shown in Fig. 1.
The surface morphologies of MLAs and gratings were characterized by FE-SEM, three-dimensional confocal microscopy and AFM, as shown in Fig. 4. It is clearly demonstrated that hexagonal distribution of MLAs formed successfully. In addition, the area ratio (the ratio of the total area of base surface of microlenses to the total substrate area with microlenses.) of MLAs and the profile of individual microlens can be easily regulated by adjusting the processing parameters. Figure 4(a) and (b) illustrate the typical SEM images of MLAs with different area ratios by controlling the size of the optical mask and the wettability of substrate. While the profile of individual microlens (dot line) can be modulated by the thickness of photoresist and the time of development, as shown in Fig. 4(c), where the fittings of an ideal surface to the lens profiles (green solid line) are also presented.
Uniaxial strain was applied by stretching the elastic membrane (PDMS) along its length, then oxygen plasma treatments were performed on the pre-stretched PDMS. After the oxygen plasma treatments, the top surface of PDMS became stiff with smaller thermal expansion. Thus, sinusoidal grooves (gratings) generated due to the mechanical buckling instability. Figure 4(d) shows a typical SEM image of gratings generated on flat PDMS after plasma treatments with oxygen, exhibiting highly ordered gratings with a period of about 500 nm. Figure 4(e) presents the AFM image of the gratings with an area of 5 μm×5 μm and a sectional height profile taken along the direction perpendicular to the grating, from which the amplitude of grating was measured to be around 60 nm. The wrinkles can also be generated on wavy PDMS as shown in Fig. 4(f), where the wrinkles have been transferred successfully to the UV curable adhesive using soft lithography method in order for better application to the subsequent light extraction. It is obviously that although there are some discontinuities in the regions between two adjacent microlenses, the wrinkles on the surface of microlens are uniform, and can be regarded as gratings on the microlenses.
Additionally, the periodicity and amplitude of the wrinkling shape gratings can be easily tuned by varying the deformation amount of PDMS and discharge power and reaction time of RIE treatment [11,32]. In the proper deformation range, the nanometer grating with wavelength selectivity and haze can be obtained by combining stretching with RIE and adjusting the process parameters, providing flexibility for the modulation of outcoupling efficiency and light distribution, which is good for more uniform lighting [13,21]. AFM measurements on different randomly chosen positions for each sample were carried out to locally probe the periodicity and amplitude of gratings. The experimental repeatability was monitored by measuring the periodicity and depth for the gratings generated in multiple batches with the same recipe, showing a small deviation of the dominant periodicity and the average depth.
As shown in Fig. 5, it is possible to tune the periodicity and amplitude by varying the plasma treatment power and processing duration. Specifically, when the RIE treatment time is kept constant at t=60 s, the period increases from 450 ± 100 nm to 700 ± 100 nm and the amplitude grows from 20 ± 5 nm to 130 ± 10 nm as the plasma treatment power increases from 50 to 250 W, as shown in Fig. 5(a) and (b). Besides, the average period and depth approximately linearly increases from 400 ± 50 nm and 20 ± 5 nm to 1800 ± 100 nm and 250 ± 20 nm, as plasma treatment duration is prolonged from 60 s to 180 s. Figure 5(c) and (f) also present the according AFM images of grating patterns. According to a buckling theory, the period (P) and amplitude (A) of the grating can be illuminated qualitatively according to the following equations [33,34]:
3.3 Effects of hybrid MLAs/gratings on the light extraction of OLEDs
To demonstrate the effects of the hybrid MLAs/gratings on the light extraction of the OLED, various properties of OLEDs with MLAs and hybrid MLAs/gratings as light extraction layers, as well as bare OLEDs were investigated as shown in Fig. 6, where the light extraction layers were attached to glass substrate of OLEDs with refractive index match liquid. The current density (J)–voltage (V) and luminance (L)–voltage(V) curves were plotted in Fig. 6(a). It is found that the J-V properties of devices are almost identical for the three devices, and the subtle difference in the L-V curves can be attributed to the variation of the out-coupling light. These results indicate that the introduction of MLAs and hybrid MLAs/gratings onto the OLED devices has little effect on the emission characteristics. Figure 6(b) illustrates the according normalized electroluminescence (EL) spectra of the OLEDs, where the profile of the emission spectrum and peak positions are found to be hardly influenced by the incorporation of external light extraction layers. The inset in Fig. 6(b) shows a photograph of OLED with MLAs, showing bright and uniform lighting. The external quantum efficiency (EQE), current efficiency (CE) and luminance efficiency (LE) were illustrated in Fig. 6(c)-(e), and the data were collected at the normal incidence. It is obviously that the EQE, CE and LE of the OLEDs with MLAs and hybrid MLAs/gratings (with grating’s depth of around 60 nm and period of around 450 nm) are improved compared to that with a flat structure (bare OLED). Furthermore, the EQE, CE and LE of the OLED with MLAs is higher than that with hybrid MLAs/gratings. The OLEDs with MLAs and hybrid MLAs/gratings show an EQE of 26.5% and 22.7%, a CE of 97.1 cd/A and 83.7 cd/A at a luminance of 1000 cd/m2, respectively. While the according EQE and CE of bare OLED are 18.4% and 67.3 cd/A. The largest EQE enhancement can be obtained with MLAs at a voltage of 3.6 V, which is 1.44 times of bare OLED. For hybrid MLAs/gratings, the enhancement also can reach up to 1.24 times compared to flat OLED.
The angular EL spectra and the distribution of the luminance for devices are also investigate. It is obvious that the incorporation of MLAs will lead to an enhancement at normal direction. However, the luminance decays with increasing the angle of inclination. The angular characteristics were shown in Fig. 6(f). Compared with bare OLED, the luminance of OLED with MLAs/grating were improved by 0.7%, 3.4%, 3.3%, 4.3%, 5.2%, 5.0%, 12.9% and 28.0% in the 0-70° direction of the offset main line (90° position), respectively. The results are consistent with the simulation results shown in Fig. 3, where the increase in the front brightness of the microstructure is due to the focusing characteristics of the microlens, while the luminance becomes more uniform after adding the gratings on the MLAs. The results demonstrated that the gratings play an important role in the scattering and light distribution, which is consistent with simulation results. However, from a specific perspective, the results are somewhat different. This is because the light source shown in the simulation uses a dipole in the x direction, which is different from the actual light-emitting source of OLED devices. However, it is helpful for us to explain the mechanism of micro-nano structure. The angular EL spectra of the bare OLED and OLED with MLAs/grating structure as a function of angle (θ) are presented in Fig. 6(g) and Fig. 6(h), respectively. For the bare OLED, as the viewing angle (θ) increases from 0 ° to 60 °, the main emission peak shifts correspondingly from 518 nm to 526 nm, which might be related to the slight microcavity effect of the device . In the case of OLED with MLAs/grating, the main emission peak shifts change from 518 nm to 521 nm, showing smaller red-shift, indicating that the performance of the prepared OLED device is relatively stable, and the MLAs/grating composite structure can stabilize the angular EL spectrum of OLED, thus improving the quality of the light source. Furthermore, the CIE coordinates as a function of angle were descripted in Fig. 6(i). The standard deviations of CIE x and CIE y coordinates of bare OLED are 0.0054 and 0.0068, respectively. In the case of OLED with MLAs/grating, the corresponding values are as small as 0.0014 and 0.0014, respectively. These results indicated that MLAs/grating composite structure can provide more stable and uniform emission over a wide viewing angle. It should be pointed out that the microcavity effect was not consider in both the simulation and experiments, since ITO with high transmittance was used as anode in the OLED devices.
From the above analysis, it is clearly indicated that the hybrid MLAs/gratings can be used for simultaneous light extraction and angular distribution improvement. Although the EQE in the normal direction was reduced by add grating on top of the MLAs, the EQE in the directions larger than 60° would be increased. In addition, the extraction efficiency enhancement generated by gratings is also relied on the wavelength of light as well as the morphology of gratings (depth and period). It was also found that with the same grating parameters, the EQE of blue OLEDs wasn’t reduced after adding grating on the top of MLA (not shown). The hybrid MLAs/gratings could bring superior scattering characteristic closer to the Lambertian distribution, leading to better angular uniformity and wider viewing angle. Nevertheless, the light extraction structures will inevitably increase the haze, subsequently cause possible light diverging and cross-talk in display application, which should be solved using other techniques, e.g., black matrix.
In summary, we have demonstrated that hybrid microlens arrays (MLAs) and gratings with periods and depths in the scale of submicron can be used for the lighting surface of OLEDs for the enhancement of light outcoupling efficiency. Furthermore, we proposed a novel and simple approach to realize gratings in the submicron scale on the wavy surface of MLAs by incorporating thermal reflow, soft-lithography and reaction ion etch. It is found that the augmentation of light extraction efficiency is mainly attributed to the MLAs, while the gratings can improve the viewing angle by increasing the angular distribution uniformity. The OLED with hybrid MLAs/gratings exhibited higher current and power efficiencies compared to that of the reference. Specifically, both the CE and EQE increased by 24% compared to that of a bare OLED without changing the electrical characteristics and EL spectral. Additionally, the OLED with hybrid MLAs/gratings clearly showed a higher luminance at wide viewing angle. The experimental and theoretical results proved that the enhancement of light outcoupling efficiency is attributed to the outcoupling of the substrate mode and the scattering effect of gratings, indicating that hybrid MLAs/gratings are attractive for enhancing the out-coupling efficiency of OLEDs.
Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China (2020ZZ111, 2020ZZ113, 2021ZZ130); National Natural Science Foundation of China (No. 61775038).
The authors declare no conflicts of interest.
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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