Multiscale MicroâNano Nested Structures - ACS Publications

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Multiscale Micro-Nano Nested Structures: Engineered Surface Morphology for Efficient Light Escaping in Organic Light-Emitting Diodes Lei Zhou, Xiaoxuan Dong, Yun Zhou, Wenming Su, Xiaolian Chen, Yufu Zhu, and Su Shen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08575 • Publication Date (Web): 17 Nov 2015 Downloaded from http://pubs.acs.org on November 18, 2015

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ACS Applied Materials & Interfaces

Multiscale Micro-Nano Nested Structures: Engineered Surface Morphology for Efficient Light Escaping in Organic Light-Emitting Diodes

Lei Zhou,1,* Xiaoxuan Dong,2 Yun Zhou,2 Wenming Su,3 Xiaolian Chen,3 Yufu Zhu,4 Su Shen,2,*

1

Faculty of Mathematics and Physics, Huaiyin Institute of Technology, Huai‫׳‬an 223003, PR China

2

College of Physics Optoelectronics and Energy, Soochow University, Suzhou, 215006, PR China

3

Printable Electronics Research Center, Suzhou Institute of Nano-Technology and Nan o-Bionics,Chinese Academy of Sciences, Suzhou 215123, PR China

4

Faculty of Mechanical Engineering, Huaiyin Institute of Technology, Huai‫׳‬an 223003, PR China

* Corresponding authors. E-mail addresses: [email protected] [email protected]

ACS Paragon Plus Environment


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ABSTRACT Various micro-to-nanometer scale structures are extremely attractive for light escaping in organic light-emitting diodes. To develop and optimize such structures, an innovative approach was demonstrated for the first time to fabricate multiscale micro-nano nested structures by photolithography with well-designed mask pattern followed by controllable thermal reflow process. The experimental and theoretical characterizations verify that this unique nested structures hold capability of light concentration, noticeable low haze and efficient antireflection. As a proof-of-concept, the incorporation of this pattern onto the glass substrate efficiently facilitates light escaping from device, resulting in current efficiency 1.60 times and external quantum efficiency 1.63 times that of a control flat device, respectively. Moreover, compared to hexagonal arranged microlens array and quasi-random biomimetic moth eye nanostructures, the nested structures proposed here can magically tune the spatial emission profile comply with Lambertian radiation pattern. Hence this novel structure is expected to be of great potential in related ubiquitous optoelectronic applications and provide scientific inspiration to other novel multiscale micro-nanostructures research.

KEYWORDS: photolithography, thermal reflow, Fresnel diffraction, microlens array, moth eye nanostructures, organic light-emitting diodes


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1. INTRODUCTION Organic light-emitting diodes (OLEDs) have been considered as a most promising candidate for next-generation full-color flat-panel displays and solid-state lighting sources owing to their fascinating properties such as self-emission, light weight, fast response, mercury free, wide viewing angle and high color rendering properties.1-4In recent years, significant progress has been made in improving internal quantum efficiency (IQE) to ~100% for energy conversion with the fully use of both singlet and triplet states.5-7 Unfortunately, a large portion of generated photons are unavoidably trapped inside the device, mainly due to a large refractive index mismatch at the different material interface, resulting that the ratio of outcoupled photons is limited to approximately 30% in conventional planar multi-layered architectures.8-10 Accordingly, the low outcoupling efficiency severely limits the potential of OLEDs and calls for novel and efficient light escaping structures. Numerous efforts have been investigated to construct novel device architectures and facilitate the extraction of the confined photons, such as microlens arrays (MLAs),11-12 micro-rod array,13 Bragg grating,14-15

bio-inspired

moth’s

eye

nanostructures

(MENs),16-17

plasmonic

nanocavity,18 random wrinkles,19-20 or nanoislands.21 Specifically, incorporating nanostructures into the devices by forming corrugated structures usually causes a higher leakage currents or electrical distortions due to the high surface roughness.22-23 Conversely, directly employing micro or nano structures onto substrate outside surface can significantly enhance light escaping without affecting electrical properties and



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thus is suitable for mass production and practical applications.24 Nevertheless, the commonly

reported

micro

structures,

limit

the

further

development

of

high-performance OLEDs due to their inherent drawbacks such as incomplete reflection suppression,25 high haze,26-27 angular dependence28 and image blurring.29-30 Previously investigated nanostructures (e.g., MENs and subwavelength antireflection coating) also have inherent shortcomings such as non-Lambertian scattering,31 weak robustness and durability.32 Currently, bioinspired artificial compound eyes with hierarchical structures have been attracting great attentions because of their unique broadband and omnidierctional antireflection optical properties.33-36 These newly developed fascinating hierarchical structures still could not realize Lambertian scattering and mechanically robust properties as the nanonipple-style structures are still constructed on MLAs surfaces. Owing to these bottlenecks, today enormous investigations are being conducted around the world to find an ideal replacement. In addition, the question whether micro, nano or hierarchical micro- and nanostructures are more beneficial to light out-coupling in optoelectronics remains a subject of debate because only a few studies are focused on optoelectronics with hierarchical structures.37 To address the above-mentioned issues, we demonstrate an innovative approach for the first time to fabricate multiscale micro-nano nested structures featuring micro-lens and micro-triangles array with built-in nano-hole structures (hereafter termed MTNs) by combining the well-designed mask photolithography and controllable thermal reflow process. Moreover, MLAs, MENs and MTNs were


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implemented onto glass substrate via UV-nanoimprinting lithography, respectively. The substrate constructed with MTNs exhibited effective antireflection of 7.1% over entire visible wavelength range, which was 3.5% and 8.7% lower than the bare substrate and substrate coated with MLAs, respectively. Meanwhile, an amazing low haze value (5.6%) was realized for MTNs with the unique, multiscale texture, which was 30.5% lower than MLAs in the entire visible regime. Furthermore, compared to MLAs and MENs, MTNs provided superior light escaping properties and magically tuned the spatial emission pattern with a Lambertian radiation pattern. The resulting current efficiency (CE) and external quantum efficiency (EQE) were 1.60 and 1.63 times that of a conventional OLED without light escaping structures, respectively. Experimental and theoretical characterizations provided valuable insight into the difference in light steering mechanisms among MLAs, MENs and MTNs. We anticipate that our new approach could open up the opportunity to the development of other multiscale hierarchical micro- and nanostructures for ubiquitous optoelectronics with high performance and low manufacturing cost.

2. EXPERIMENTAL DETAILS 2.1. Design and fabrication of MTNs Figure 1a-b schematically illustrates the well-designed mask patterns for MLAs and MTNs fabrication, respectively. The distribution of circles within a mask for MLAs was arranged in a hexagonal closely packed pattern with diameter of 4 µm and fill factor of 0.8, while it was with circle diameter of 4 µm and corresponding built-in



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rectangular with width of 1 µm for MTNs. It is well-known that the edges of mask inevitably generate diffraction efforts induced by Fresnel diffraction.38-39 This point of view highlights a second imaging property: edges diffraction is able to generate multiple images of extended objects and could be predicated by pre-calculation. Therefore, to clearly clarify the diffracted wavefronts distribution on photoresist from the mask patterns, the intensity distributions of Fresnel diffraction images for both mask patterns have been calculated using Fourier-transform method40 and are displayed in Figure 1c, d, respectively (see the details in Experimental Section). It is evident that the intensity distribution of Fresnel diffraction for MLAs on photoresist exhibits perfect morphology, and the edges intensity of Fresnel diffraction are negligible and obviously lower than the threshold cross-link energy required by the photoresist, which would give steep photoresist side wall under exposure. On the contrary, the edges intensity diffracted distribution for mask of MTNs is strongly modulated by the built-in rectangular, resulting in intensity sub-centre for photoresist. Consequently, the desired novel multiscale hierarchical micro- and nanostructures could be achieved with photoresist swelling in developer and thereafter followed by thermal reflow process.


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Figure 1. The mask patterns for MLAs, MTNs fabrication and the corresponding calculated Fresnel diffraction images, respectively. (a) Mask pattern for MLAs with circles diameter of 4 µm and fill factor of 0.8. (b) Mask pattern for MTNs with diameter of 4 µm and rectangular with width of 1 µm. (c), (d) Calculated the normalized intensity distribution of Fresnel diffraction image using Fourier-transform method for the mask of (a) and (b) at an illumination wavelength of 351 nm, respectively.

Figure 2 presents the schematic process to fabricate the MTNs by well-designed mask photolithography. First, the glass was cleaned with acetone, isopropyl alcohol, and distilled water for 10 min each and dried in oven at constant temperature of 100°C. Then, the photoresist (RZJ390PG, SUZHOU RUIHONG CO. LTD.) was spin-coated onto the glass substrate with the thickness of 2 mm and baked for 2 min at 100°C. Thereafter, a thin Chromium mask fabricated by LIGA (Lithography, Electroplating,



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and Molding) technique

41

with thickness of 600 nm was arranged closely on

photoresist (Figure 1a). After exposing for 26 s with a collimated laser beam (351 nm, Diode Pumped Solid-State Laser), the sample was developed in NaOH (0.4%) solution for 5 s and dried in the ambient. After that, the sample was transferred onto a hot plate for baking at 140 °C for 5 min and thus the ubiquitous photoresist hierarchical MTNs with hexagonal closely packed MLAs, nested micro-rectangular and nanohole array were successfully produced. Subsequently, a photocurable liquid Perfluoropolyether (PFPE) precursor solution consisting of PPFP (α, Ω-functionalized dimetha-crylate, 1000 g mol-1) and the photoinitiator (2, 2-diethoxyacetophenone), is poured over the photoresist template, and then a constant pressure of 1.0 bar together with UV illumination at 395 nm of 500 mJ cm-2 is applied onto the PFPE surface for 15 s. As a result, the negative PFPE mold was obtained by demolding off surface-relief photoresist MTNs (Figure 1b). Finally, MTNs topography was inscribed into UV-curable resin drop-casted on glass surface after OLEDs devices fabrication (see the details in Experimental Section) by UV-assisted soft-nanoimprinting lithography under the same illumination intensity and pressure discussed above (Figure 1c and d). Meanwhile, to obtain more evidence of the morphology influence on light escaping, MLAs and MENs are also fabricated and introduced into glass substrate (Figure S1 and S2, Supporting Information).


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Figure 2. Schematic illustration of the fabrication process of MTNs. (a) UV-photolithography with a designed mask. (b) Fabricating PFPE mold from the MTNs formation on photoresist by thermal reflow technique. (c) UV-imprinting the UV-curable resin spin-coated on glass substrate by MTNs-structured PFPE mold. (d) Demolding the MTNs-structured PFPE mold. 2.2. Device fabrication. To realize highly efficient phosphorescent blue OLEDs, the energy level diagram of the organic emitter with the alignment of the vacuum level (Evac) was presented (Figure S3, Supporting Information) for estimating the interface energetics because of the vacuum level continuity is necessary. It is noted that triplet energies of the blue dopant FIrpic (T1 = 2.62 eV) is lower than the host material mCP (T1 = 2.9 eV), implying the appropriate triplet energy level alignment. Additionally, the hole transporting layer of TAPC (T1 = 2.9 eV) and electron transporting layer of



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TmPyPB (T1 = 2.78 eV) have a wider triplet energy level than FIrpic, and thus effectively confine triplet excitons inside the emission layer to generate more photons, which would be liberated by light escaping structures. Here, the doped concentration of FIrpic followed the optimized result from the literature42. For the fabrication of blue OLEDs, ITO-coated glass substrates were ultrasonically cleaned with detergent, acetone, ethanol, and deionized water for 20 min and subsequently dried in an oven. The 60 nm-thick PEDOT:PSS layer was spin-coated onto the ultraviolet-ozone (UVO) treated ITO glass substrate in ambient condition, followed by a 120 °C bake on a hot plate for 20 min before the subsequent organic deposition. Then the substrates were transferred into a high-vacuum chamber with a base pressure of < 2 × 10-6 Torr for film deposition by thermal evaporation with a shadow mask. Finally, the device with structure of ITO glass/PEDOT:PSS (60 nm)/TAPC (45 nm)/mCP:FIrpic (8 wt%, 20 nm)/TmPyPB (35 nm)/LiF (0.5 nm)/Al (100 nm) was fabricated by the film deposition and the effective device area was 0.1 cm2. Blue OLEDs with and without MLAs, MENs and MTNs were fabricated at the same time to ensure consistent results. To inscribe MLAs, MENs and MTNs on glass substrate surface, the PFPE and AAO mold was applied to make conformal coating of positive morphology after drop-casting the UV-curable resin (D10, PhiChem) on the glass surface. The UV-curable resin film was imprinted under a constant pressure 1.5 bar for 15 s with UV illumination at light power intensity of 500 mj cm-2 at a wavelength of 395 nm. 2.3. Properties Characterization. The thickness, refractive index (n), extinction coefficient (k) and film thickness of all the layers were measured using the alpha-SE™


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Spectroscopic Ellipsometer (J. A. Woollam Co., Inc). Optical transmission spectra were recorded by a UV/vis/near-IR spectrophotometer (Perkin Elmer Lambda 750) with an integrating sphere. Optical specular transmittance and reflection spectra were directly measured while total transmittance was tested using an integrating sphere by a UV/vis/near-IR spectrophotometer (Perkin Elmer Lambda 750). The corresponding haze values were calculated by the equation: Haze (%) = (total transmittance – specular transmittance) × 100%.27 Surface morphologies were characterized by AFM (Veeco MultiMode V) in tapping mode and SEM (FE-SEM, Quanta 400 FEG). Contact angle was determined by contact angle tester (DataPhysics instruments GmbH). The current density-voltage-luminance (J-V-L) characteristics and EL spectra of

the

devices

were

measured

simultaneously

in

ambient

air

using

a

computer-controlled programmable Keithley model 2400 power source and a PhotoResearch PR655 luminance meter/spectrometer. The angle-dependent emission patterns were measured according to emission angles. 2.4. Optical Simulations. To investigate the Fresnel diffraction distribution of mask, two-dimensional fast Fourier transform (FFT) method was adopted to calculate diffraction integrals by codes generated in house using Matlab software. Plane wave source was arranged normal to the aperture plane and the wavelength was set to 351 nm, where the two-dimensional numerical ray image of the photomasks were generated by Matlab codes programmed according to the designs. To identify the influence of MLAs to the light escaping, three-dimensional Monte Carlo ray tracing method was adopted using TracePro software package by assuming a Lambertian



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point source (1 W, 40000 rays) located within the device. The structure of device was glass substrate (10×10×0.8 mm)/ITO (6×6×0.0001 mm)/multi-organic layers (6×6×0.00016 mm)/LiF/Al bilayer cathode (6×6×0.0.0001 mm). For simplicity, the LiF/Al bilayer cathode was regarded as a perfect reflector and the absorption parameter of glass substrate was set to zero. To obtain the poynting vector S distribution, and the quantitative light extraction ratios as function of viewing angles for the devices with MENs, the finite difference time domain (FDTD) method (RSoft FullWave, RSoft Design Group, Inc) and corresponding codes generated in house by ourselves using Matlab software were employed, where MENs set as the quasi-random in the range of ~10 periods, based on morphologies obtained from AFM measurements. To investigate the light escaping characteristics of the MTNs, the BSDF (Bi-Directional Scattering Function) files containing information about how light was manipulated by the nested nanohole array within MTNs were first simulated by Rsoft FullWave, where the nanohole array was also arranged in a hexagonal packing configuration based on morphologies obtained from SEM measurements. Nonuniform grid was set to model different parts of the device in order to accelerate precise calculation quickly with efficient usage of memory. Subsequently, these BSDF data files were imported into bsdfutility (TracePro software) to calculate the far-field intensity distribution and the corresponding light extraction ratio as function of viewing angles, where the micrometer segments of MTNs (microlens array and the nested micro-rectangular) were created in CAD windows of the TracePro software. The complex optical dielectric function of the Al cathode was fitted using


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Drude-Lorentz

model,

taking

into

account

interband

transitions

and

the

wavelength-dependent n and k values of the other dielectric constants experimentally determined by the ellipsometric measurements.

3. RESULTS AND DISCUSSION 3.1. Morphologies characterization. The surface morphologies of UV-MLAs coated on glass substrate were characterized by three-dimensional confocal microscopy and scanning electron microscopy (SEM), and displayed in Figure 3a-c. These images clearly reveal that the hexagonal MLAs were successfully transferred onto the UV-resin and exhibit rather smooth surface with diameter of 4 µm, fill factor of 0.8 and aspect ratio of 1. The uniform well-defined morphologies can be attributed to two reasons: one is edges diffracted intensities for mask of MLAs are negligible, which is matched very well with our aforementioned calculated results, another is the extremely low surface energy (< 10 dyn cm-1) of the PFPE mold, which exhibits hydrophobic character with contact angle of 101° (Figure S4, Supporting Information), and is a critical issue in easier demolding process.43 Figure 3d, e present atomic force microscopy (AFM) images of an anodized aluminum oxide (AAO) membrane and the corresponding UV-MENs coated on glass substrate. Tapping-mode AFM studies of these images show that AAO membrane displays a period of ≈ 400 nm, fill factor of ≈ 0.6 and groove depth of ≈ 560 nm, while MENs show a period of ≈ 400 nm, fill factor ≈ 0.6 and groove depth of ≈ 580 nm, respectively. This indicates that topography of UV-MENs is exactly complementary



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for AAO membrane and the UV-resin could also replicate nanostructures effectively as demonstrated in our previous work.44 Note that the groove depth of UV-MENs is ≈ 20 nm smaller than that of AAO membrane master, which is ascribed to slightly surface energy discrepancy between AAO membrane and UV curable resin.

Figure 3. Morphologies characterization of MLAs, MENs and MTNs. (a) The


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three-dimensional confocal microscopy image of the top-view of MLAs. (b) Cross-sectional image of an MLA constructed on glass substrate. (c) The depth profile of (a) (diameter: 4 µm, fill factor: 0.8, aspect ratio: 1). (d) AFM image of AAO template (period: ≈ 400 nm, groove depth: ≈ 500 nm, fill factor: ≈ 0.6). (e) AFM image of MENs constructed on glass substrate. (f) SEM image of MTNs and the inset in right depicts the corresponding zoom in SEM image of MTNs (MLA: diameter of 4 µm, microtriangle: side length of ≈ 1 µm, nanohole: length of ≈ 1 µm , width of ≈ 400 nm, groove depth of ≈ 800 nm).

Figure 3f displays a SEM image of multiscale micro-nano nested structures of MTNs, comprising of hexagnoally arranged MLAs (diameter ≈ 4 µm, fill factor ≈ 0.8, aspect ratio ≈ 1), microtriangle (side length ≈ 1 µm) and nanohole arrays (length ≈ 1 µm , width ≈ 400 nm, groove depth ≈ 800 nm). The uniform ubiquitous morphology clearly reveals that the nested hierarchical structure between the MLAs and built-in microtriangles is initiated from the propagation of photoresist into the intensity sub-centre triggered by edges diffraction during thermal-reflow procedure, which is completely consistent with our simulation results. Notably, MTNs patterns were transferred with high fidelity on UV-resin by soft PFPE mold, indicating that the approach

demonstrated

here

are

fully

compatible

with

high-throughput

low-temperature roll-to-roll large-area manufacturing on flexible substrates. 3.2. Optical properties. Optical properties of various glass substrates without and with MLAs, MENs, and MTNs are compared and shown in Figure 4. As apparently



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displayed in Figure 4a, the use of MLAs, MENs and MTNs on glasses result in higher (≈ 95.8%, ≈ 93.2%, ≈ 94.9%) optical total transmittance than that of a bare glass (≈ 89.7%) in the entire visible regime. Interestingly, the total transmission of MLAs/glass is larger than 100% for some wavelength, which is attributed to that the incident light are concentrated and refracted at MLAs-air interface and totally collected by the integral sphere in the end.45 It is also noted that the incorporation of MTNs leads to an intermediate increase in total transmittance compared to that of bareglass, indicating that MTNs worked as MLAs possess an ability of light concentration, leading to stronger photon collection property compared to that of biomimetic MENs. To determine the amount of light scattering, the transmittance haze, which denotes the percentage of total light transmitted that is scattered, was performed and presented in Figure 4b. Obviously, a lowest haze (2.8%) nearly identical to that of bare glass (2.7%) was achieved for MENs coated on glass, which is ascribed to the subwavelength topography and the efficiently suppressed nonzero diffraction orders of MENs.46 Interestingly and importantly, a noticeable lower haze value (10.6%) was realized for MTNs coated on glass substrate compared to that of MLAs (46.1%), revealing that MTNs could be directly suitable for image display without inducing image blurring, which is a critical element for full-color display and color stability.47 In addition, optical reflection properties were characterized and plotted in Figure 4c. A broadband reduction in average reflectance was clearly observed in multiscale MTNs (7.1%), which is close to that of MENs (5.4%) but remarkably less than that of bareglass (10.5%) and MLAs (15.9%). These experimental proofs distinctly demonstrate that a


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large portion of photons incident on the MLAs gets reflected due to an abrupt change in refractive index at the interface. Reflection losses inevitably lead to poor image contrast and poor signal-to-noise ratio, which can deteriorate image quality.48 On the contrary, the significantly reduced reflection for the coating of MTNs on glass substrate resulted in excellent antireflection optical property, which is rather helpful for improving image contrast and suppressing glare.49 The improved optical features of MTNs over the broadband spectrum is attributed to the unique shape of nanohole patterns built in MLAs as opposed to an abrupt change in index in MLAs without the nanohole arrays.50-51 Additionally, the corresponding optical diffusion photographs under laser irradiation of MLAs, MENs and MTNs were tested and presented in Figure 4d-f. A meaningful feature observed from these photographs is that the coating of MLAs on glass substrate resulted in an obvious optical diffusion distribution, while the MENs and MTNs displayed a vivid point source, which clearly indicates that the haze value of MTNs is significantly suppressed by the unique nested nanostructures. This result is in good agreement with the tested optical properties results as discussed above.



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Figure 4. Optical properties and the corresponding diffusion photographs under laser irradiation. (a) Total transmittance characteristics for incident light toward surface of different structures based on glass substrate. Inset denotes the schematic of the measurement procedure. (b) The measured wavelength-dependent haze intensity. (c) Reflection characteristics for incident light toward surface of different structures based on glass substrate. (d), (e), (f) The corresponding optical diffusion photographs under laser irradiation of MLAs, MENs and MTNs, respectively.


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3.3. Performance of blue OLEDs. To confirm the viabitlity of the MTNs, phosphorescent bule-emission OLEDs were fabricated on ITO-glass substrate with flat, MLAs and MENs films for comparison (see the Experimental Section for the details). The current efficiency (CE), external quantum efficiency (EQE) and power efficiency (PE) for these devices are plotted as function of luminance in Figure 5a-c. The CE, EQE and PE values as well as the enhancement ratios relative to that of the control device are summarized in Table 1. It is obvious that the CE, EQE and PE values of blue OLEDs with light escaping structures were remarkably enhanced, and especially the incorporation of MTNs resulted in the largest improvement in efficiency. The blue OLED with MTNs yields a CE of 58.2 cd A-1, an EQE of 25.7%, and a PE of 43.6 lm W-1 (@1000 cd m-2), which are 1.60, 1.63 and 1.68 times that of the reference device (CE = 36.4 cd A-1, EQE =15.8% and PE = 25.9 lm W-1). The maximum CE and EQE of blue OLED with MTNs are increased to 62.3 cd A-1 and 27.1%, respectively. Obviously, the light escaping capability with MTNs is apparently superior to that of MLAs (CE = 57.6 cd A-1, EQE = 24.5% and PE = 42.5 lm W-1) and MENs (CE = 55.3 cd A-1, EQE = 23.5% and PE = 39.2 lm W-1). This feature corresponds very well to the lower reflection loss, higher total transmittance and appropriate haze value observed in Figure 4. Additionally, it is evident that all the devices exhibit almost identical current density-voltage (J-V) characteristics while the luminance of the structured devices are enhanced dramatically at the same driving voltage (Figure S5, Supporting Information), which clearly indicates that introducing out-coupling structures on glass substrate did not change the device electrical properties.



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Figure 5. Performance characteristics of blue OLEDs with and without outcoupling structures. a) Current efficiency-luminance characteristics. b) External quantum efficiency as a function of luminance. c) Power efficiency as a function of luminance. d) EL spectra in the direction normal to the glass substrate at J = 10 mA cm-2.

Table 1. Performance characteristics of blue OLEDs without and with light escaping structure. The current efficiency (PE) and external quantum efficiency (EQE) are compared at their maximum values and at a luminance of 1000 cd m-2. The REQE values are the enhancement ratios relative to that of the reference device at a luminance of 1000 cd m-2.


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EQE

CE

EQE

PE

REQE

[1000 cd m-2]

[1000 cd m-2]

[max]

[1000 cd m-2]

[%]

[cd A-1]

[%]

[lm W-1]

Ref

15.8

36.4

17.2

25.9

1

MLAs

24.5

57.6

26.5

42.5

1.55

MENs

23.5

55.3

25.6

39.2

1.48

MTNs

25.7

58.2

27.1

43.6

1.63

Device structures

To investigate the influence of MTNs on light escaping, the EL spectra of devices with and without light escaping structures were measured and plotted in Figure 5d. Additionally, the normalized EL spectra of devices with MTNs displays a nearly identical spectral shape to that of the control device (Figure S6, Supporting Information). It is clearly shown that the devices with light escaping structures exhibit wavelength-independent enhancement in efficiency. Especially, there is no shift and distortion in EL spectra for the devices with MTNs and MENs, implying no periodic grating diffraction effects with the MENs and MTNs structures. We surmise that this wavelength-independent enhancement over all emission wavelengths is attributed to the quasirandom distribution of the built-in nanoholes array and bionic parabolic nanocone array for MTNs and MENs, respectively.52 The angular dependences of the EL intensities for devices are presented in Figure 6a. Compared with the ideal Lambertian emission pattern of the control device, it is apparent that the incorporation of MLAs and MENs cause a stronger top and side



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emission compared to that of the control device, respectively. These features are directly attributed to the light concentrate property of MLAs and ominidirectional property of MENs, respectively.34 Particularly, it is noteworthy that the use of MTNs displays perfect Lambertian emission pattern just like that of the control device, which is mainly due to redirected emitted light over all azimuthal direction by the surfaces of hierarchical multiscale micro-nano nested structures. This magical characteristic is fully compatible with flat-panel displays as well as perfect light in-coupling in a thin semiconductor slab,53 suggesting potential applications in solar cells, LEDs, photodetectors, image sensors and numerous other micro-nano optoelectronics.

Figure 6. Angular dependence of EL performance for blue OLEDs with and without outcoupling structures. (a) Normalized angular dependence of light intensity. The dashed dot line represents the ideal Lambertian emission pattern as a guide to the eye. (b) Commission Internationale de I′ Eclairage 1931 color coordinates, arrows indicate the shift the color coordinate from 0° to 75° in 15° steps.

The color stability of the different devices in Commission Internationale de


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I′ Eclairage (CIE) 1931 coordinates (X, Y) is summarized in Figure 6b. The changes of CIE coordinates (∆x, ∆y) from 0° to 75° are (0.002, 0.031), (0.002, 0.028), (0.001, 0.020) and (0.007, 0.045) for the devices with MLAs, MENs, MTNs and the control device, respectively. It is apparent that the control device exhibits a very prominent shift of CIE coordinates, resulting in poor color stability. On the contrary, the CIE color coordinates is far more stable with viewing angle for the device with MTNs than all of the others. The improved color stability reflects in turn the viewing-angle independent emission properties. Additionally, it is clearly shown that the color coordinates changed anticlockwise for the reference and the device with MENs, but clockwise for the devices with the MLAs and MTNs, which is mainly ascribed to the MLAs and MTNs can mix and integrate the emission spectra54 while MENs only extracted trapped photos omindirectionally.16



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Figure 7. Optical simulations for blue OLEDs with and without outcoupling structures. Illuminance maps of the substrate output surface for flat device (a) and device with MLAs (b) calculated by 3D ray tracing method, respectively. Calculated poynting vector S (energy flow) distribution excited by a plane wave (472 nm) located at the semi-infinite glass substrates for flat device (c) and device with MENs (d) Far-field intensity distribution for flat device (e) and device with MTNs (f) simulated by


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Finite-difference time domain method, respectively.

To gain a deep insight into the optical steering mechanisms of MLAs, MENs and MTNs structures in OLEDs, optical modeling calculations using three dimensional Monte Carlo ray tracing method and finite difference time domain (FDTD) are dedicatedly investigated (see Experimental Section for the detailed description of the optical modeling). Figure 7a-b compares the far-field illuminance maps of the substrate output surface for the devices with and without MLAs. It is clear that the maximum illuminance value is 14, 500 lux for the OLEDs with MLAs, which is much higher than that of the control device (9, 500 lux). This comparison demonstrates the capability of the MLAs to function as efficient integrating element and a diffuser, that is, the originally trapped photons at higher angles (larger than the critical angle of glass substrate) can escape to air efficiently. To elucidate the physical origin of light escaping for MENs, the Poynting vector S (energy flow) distributions for glass substrate without and with MENs were simulated and plotted in Figure 7c-d. It is noted that the energy flows traveling normally and directly into air from flat glass substrate with MENs were distinctly squeezed and funneled into air apertures, leading to more efficient escape of the originally trapped photons than that from the flat bare glass.55 In addition, the theoretical quantitative calculation of light extraction ratios clearly reveals that the device with MENs could extract more trapped energy at very high incident angles (Figure S7, Supporting Information), which is consistent with the experimentally measured angular emission as shown in Figure 6a. This capability is



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mainly attributed to broadband and quasi-omnidirectional anti-reflection capability of MENs.56 Particularly, to understand the effects observed, a rigorous dipole model was performed to calculate and analyze the light escaping performance of the device with MTNs in Figure 7e-f. Furthermore, to provide a quantitative understanding of the enhanced light escaping with respect to the change of viewing angles, the corresponding light escaping ratios were calculated for the devices without and with MTNs, respectively (Figure S7, Supporting Information). It can be observed that the emission photo flux profile of the device with MTNs does not show any special polar and azimuthal angle dependence, which is similar to that of the ideal Lambertian emission pattern for the control devices, which matchs well with the tested angular dependence as shown in Figure 6a, confirming that the proposed novel MTNs are a promising option as an advanced photon steering scheme in OLEDs with high efficiency and superior color stability.

4. CONCLUSION In conclusion, we have demonstrated an innovative approach for the first time to fabricate multiscale micro-nano nested structures (namely, MTNs) by controllable edges-diffraction distributions induced by the well-designed mask photolithography. Meanwhile, MTNs were implemented into glass substrate with great pattern fidelity via UV-nanoimprinting lithography in the quest toward the efficient light escaping for OLEDs. The unique advantages of the MTNs discussed here are their capabilities to 1) function as light concentrator like MLAs, resulting in strong photons collection


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property; 2) achieve a noticeable lower haze, leading to be directly suitable for image display without inducing image blurring; 3) efficiently suppress reflection on the hierarchical surface for a significant enhancement of the image contrast. Moreover, compared to hexagonal arranged MLAs and quasi-random MENs, the MTNs can magically tune the spatial light emission pattern to Lambertian radiation profile, hence resulting in potential applications in flat-panel displays as well as perfect light in-coupling in a thin semiconductor slab. The CE and EQE are 1.60 and 1.63 times that of a control flat device, respectively. It confirms that the proposed novel MTNs are a promising option as an advanced photon steering scheme in OLEDs with high efficiency and superior color stability. Thereby, our new approach is also expected to help realize ubiquitous optoelectonic applications and provide scientific inspiration to other novel mutiscale micro-nanostructures research.

ASSOCIATED CONTENT Supporting Information Basic information about fabrication procedure of concave MLAs PFPE mold, fabrication procedure of UV-MENs on glass substrate, energy level diagram of the organic emitter with the alignment of the vacuum level, contact angle measurements of flat PFPE, Current density-voltage characteristics, luminance as a function of voltage, normalized EL spectra in the direction normal to the glass substrate at J = 10 mA cm-2 for devices with and without MTNs, simulated light escaping ratios as function of viewing angles for the devices with and without light escaping structures based on



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rigorous dipole model using FDTD method. This information is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions L. Zhou and X. X. Dong contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We gratefully acknowledge the optical characterizations support from Prof. J. X. Tang group from Institute of Functional Nano & soft Materials (FUNSOM) in Soochow university. This work was supported by the Priority Academic Program Development of

Jiangsu

Higher

Education

Institutions

(PAPD),

by

the

National Science Foundation (NSFC) (60907010, 11004218), by the Natural Science Foundation of Jiangsu Province (BK2012631, BK20140357), by the Zhejiang Key Discipline of Instrument Science and technology and by the Open Fund of the State


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Key Laboratory of Luminescent Materials and Devices (South China University of Technology, 2012-skllmd-05).

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ToC figure


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