Bio-Inspired Flexible Fluoropolymer Film for All-Mode Light Extraction

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Surfaces, Interfaces, and Applications

Bio-Inspired Flexible Fluoropolymer Film for All-Mode Light Extraction Enhancement Renli Liang, Run Hu, Hanling Long, Xinyu Huang, Jiangnan Dai, Linlin Xu, Lei Ye, Tianyou Zhai, Hao-Chung Kuo, and Changqing Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 06 May 2019 Downloaded from http://pubs.acs.org on May 6, 2019

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ACS Applied Materials & Interfaces

Bio-Inspired Flexible Fluoropolymer Film for All-Mode Light Extraction Enhancement Renli Liang1, † , Run Hu2, † , Hanling Long1, † , Xinyu Huang1, Jiangnan Dai*1, Linlin Xu1, Lei Ye1, Tianyou Zhai3, Hao-chung Kuo4 and Changqing Chen1

1 Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Luoyu Road 1037, Wuhan, 430074, China

2 School of Energy and Power Engineering， Huazhong University of Science and Technology, Luoyu Road 1037, Wuhan, 430074, China

3 School of Materials Science and Engineering， Huazhong University of Science and Technology, Luoyu Road 1037, Wuhan, 430074, China

4 Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu 300, Taiwan Corresponding email address: [email protected];

KEYWORDS: Deep ultraviolet light emitting diode, Nanostructures, Flexible Fluoropolymer, Light extraction efficiency, TE/TM mode.

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ABSTRACT Enhancing the light extraction efficiency is a prevalent but vital challenge for most solid-state lighting technologies, especially for deep ultraviolet light-emitting diodes (DUV-LEDs). In this paper, inspired by the microstructure of the butterfly’s eye, we propose and fabricate a flexible fluoropolymer film (FFP Film) to tackle this issue for all-mode, full-wavelength light extraction enhancement for most solid-state lighting technologies compatibly. Experimental results demonstrate that, compared with one mounted with smooth FFP Film, the light output power of DUV-LED is enhanced up to 26.7% by mounting FFP Film with 325-nm radius nanocones at the driving current of 200 mA. Importantly, thanks to the super-flexible feature of FFP Film, it can both cover the top surface and sidewalls of DUV-LED chip, leading to the improvement of transverse electric (TE) and transverse magnetic (TM) mode light extraction by 20.5% and 21.8%, respectively. Finite element analysis (FEA) simulations of electric field distribution of DUV-LEDs with FFP Film reveal the underlying physics. The present strategy is proposed from the view of packaging level, which is cost-effective, largescale manufacturing and compatible with the solid-state lighting technologies.

1.Introduction Over the past two decades, the conventional incandescent and fluorescent lighting industry has been revolutionized to pursue a more energy-efficient, longer-lived, and environment-friendlier solid-state lighting, complying with the increasingly severe energy crisis and the global climate change1-3. In the big family of solid-state lighting,


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the lighting-emitting diodes (LEDs)4-5, flexible organic LEDs (OLEDs)6-7, laser diodes (LDs)8-9, perovskite LEDs10-12 and ultraviolet LEDs (UV-LEDs)13-14 are most significant and promising, which, depending on the material and wavelength, have a broad range of applications in white light illumination, backlighting, vehicle headlamp lighting, water/air purification, spectrometry and medical phototherapy, etc. The main figure of merit for the efficiency of solid-state lighting, in common, is the electronphoton power conversion efficiency, a.k.a. the wall-plug efficiency, which is the product of the external quantum efficiency (EQE) and the electrical efficiency15-16. Reducing the forward voltage and improving the Ohmic contact can benefit to the improvement of the electrical efficiency17-18. However, the EQE is more important, which is the product of the light extraction efficiency (LEE) and the internal quantum efficiency (IQE). Currently, the IQE in the active regions has nearly approached its upper limit for OLEDs (100%)19-20 and blue LEDs (90%)21. While for deep-UV LEDs (DUV-LEDs, 95%) and good stability for almost the whole wavelength from DUV to red spectra, which makes it applicable for most solid-state lighting technologies, especially for the DUV-LEDs, as shown in Figure S1 (Supporting Information). The nano-patterned template are developed to fabricate the



nanocone FFP Film, which is efficient, cost-effective and compatible with industrial manufacturing for uniform and large-area nanocone fabrication. Experimental results reveal that the optimized FFP Films can significantly improve the TE/TM-modes of DUV-LEDs and benefit enhancement of LEE compared with the smooth fluoropolymer film. The electric field distribution mode of DUV-LEDs is also simulated based on the finite element analysis (FEA) to shed light on the underlying physical mechanism. 2.Experiment 2.1. Fabrication Process of FFP Film The detailed experimental description of the nano-patterned template was reported in our previous work.32 After cleaning the nano-patterned template using plasma, the fluoropolymer (S-type, Grade name: CTX-809SP2, produced by CYTOP) is spincoated on the nano-patterned template with the size of 50×50 mm2, and then baked under the temperature of 120 ℃ for 1 hour, obtaining the FFP Film. Subsequently, as the bonding ability of pure fluoropolymer is very poor and can be easily peeled off, the FFP Film is mechanically stripped and cut into small cubes with the size of 3.5×3.5 mm2, preparing to be mounted on the DUV-LEDs. Finally, the FFP Film is mounted both on the top and the sidewalls of DUV-LED chip by GO-based fluoropolymer36 to form sandwich structure under 120°C in a vacuum chamber for 1.0 h. 2.2. Simulation and Measurements Exploring the feasibility to improve the LEE with the FFP Film, the simulated electric and magnetic fields field distribution of DUV-LEDs with the smooth fluoropolymer and the FFP Film are investigated respectively, by using FEA in


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interactive 3D entity-modeling environment. The measured results of UV transmittance of four FFP Film are performed by UV-1800 dual beam UV-visible spectrophotometer manufactured by Shanghai Macylab Instrument Inc, and the light output power of the DUV-LEDs are measured by using the ATA-1000 Photoelectric Analysis System manufactured by Everfine Corporation with a 30-cm-diameter integrating sphere. Nearfield intensity distribution is supported by Gold Medal Analytical & Testing Group. Moreover, the light intensity spatial distribution of these DUV-LEDs are measured by using self-constructed test system with angle resolution bracket, Glan-Taylor prism and spectrometer as shown in the results and discussion session.

3.Results and discussion The microstructure of the blue morpho butterfly’s eye observed by the environmental scanning electron microscope (SEM, Quanta 250, FEI company) is shown in Figure 1a. It is observed that the sub-μm scale nanostructure consists of periodic and close packed ~300-nm-diameter nanocone array, which benefit for the photoreception of the butterfly. To fabricate the similar nanostructure of the butterfly’s eye based on flexible substrate, we first obtain the nano-patterned template. Detailed description of the fabrication process can be referred in the Experimental Section. By controlling the time of wet etching, four 2-inches sapphire nano-patterned template are fabricated with uniform nanoholes with radii of 325 nm, 340 nm, 350 nm, and 360 nm, respectively, and the period is about 1 μm. Top-view SEM image of the nano-patterned template with radius of 325 nm is shown in Figure 2a, and the corresponding depth of



nanohole is measured as ~190 nm by the sectional view SEM image in Figure 2a, which is obtained by precisely cutting the nano-patterned template using focused ion beam (FIB). Meanwhile, it is observed from the atomic force microscope (AFM) in Figure 2a that the morphology of nano-patterned template is uniform and adjustable, by using nano-photolithography and wet-etched technique. Experimental discussions will be referred later. Then, we spin-coat the fluoropolymer solution onto the nano-patterned template, and the FFP Film with uniform nanocones are obtained after baking and stripping, as shown in Figure 2b. Due to the super-flexibility of the fluoropolymer substrate, we use the FFP Film to cover the top and sidewall surfaces of the DUV-LED chip, and the final packaging prototype is displayed in Figure 2b. Corresponding to the four templates with different nanohole sizes, four FFP Film with different nanocone sizes are shown in Figure 1b respectively. Similar to the microstructures of the blue morpho butterfly’ eye, the morphology of FFP Film is largely well-arranged and the nanocone shows the patterns as convex triangle cones due to the anisotropic etching of the sapphire crystal (nano-patterned template). The radii of nanocone of four FFP Film are 325 nm, 340 nm, 350 nm, and 360 nm, respectively. To achieve the higher transmittance, the ratio of the height and the incidence light wavelength is found to be 0.4 or higher, because the light transmittance is related with both the geometry nanostructures and the wavelength of incidence light37-39. And the corresponding heights are about 190 nm, 225 nm, 230 nm, and 240 nm, which satisfy foregoing requirement of high transmittance in deep ultraviolet spectrum. The thickness of the FFP Film depends on spin coating speed and time, and an approximately 4-μm FFP


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Films are fabricated on the condition of 1000 rpm and 100 s, according to the sectional view SEM image in Figure 1c. Such thin a film possesses extraordinary flexibility, which can be easily mounted onto top and side surfaces of the DUV-LEDs in later demonstration.

Figure 1. (a) A picture of blue morpho butterfly specimen and the SEM image of its eye microstructure. The submicron-scale nanocones are arranged neatly on uneven surfaces like a flexible film; (b) Top-view SEM images of the FFP Film with radii of 325 nm, 340 nm, 350 nm, and 360 nm, respectively. It is observed that FFP Film can maintain a large area of consistency like a butterfly-eye microstructure; (c) the crosssectional SEM image and 5 × 5μm AFM image of FFP Film (R325), and the single nanocone profile.



Figure 2. (a) Top-view and cross-sectional SEM images of the nano-patterned template with radius of 325 nm, and the bottom two pictures show the AFM image and 2-inch nano-patterned template; (b) Fabrication process of FFP Film; the final picture show the schematic diagram of the FFP Film mounted DUV-LED (peak wavelength of 275 nm) fabricated by the eutectic flip-chip bonding on the AlN ceramic substrate; (c) Transmittance of DPSS template with or w/o the FFP Film. The experimental results are shown as solid lines and the simulation results are shown as hollow marks. The inset shows the enhancement factor which is also the effective transmittance for the FFP Film. Transmittance of the FFP Film is a key factor when refer to light extraction enhancement property. However, the FFP Film is extremely thin and hardly to realize self-standing feature so a substrate is required to investigate its transmittance. Therefore, we mounted the FFP Film on double-polished sapphire substrate (DPSS) which is of similar manner to DUV-LED surface. The transmittance of the DPSS with or w/o the R325 FFP Film were measured, respectively. Then the enhancement factor (EF) can be calculated as the ratio of transmittance with and w/o the FFP Film on DPSS, which should also regarded as the effective transmittance for the FFP Film as shown in Figure 2c. The transmittance of the DPSS with FFP Film is improved for the whole spectra


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from 200 to 800 nm, which is favorable for enhancing the LED light extraction. Simulation is also conducted to support the experimental results. In the simulation, the transmittance was numerically calculated by using a three-dimensional finite difference time domain method. The calculation takes into account the frequency dependence of the refractive index and absorption loss of the interface. A 3D model was established based on the experimentally prepared periodic structural unit sizes, and the boundary was set to the Floquet period type. The refractive indices of air and FFP Film were set to 1 and 1.35, respectively. The simulation results shown in Figure 2c exhibit slight deviations from the experimental curves. It shows in the inset of Figure 2c that the transmittance enhancement factor investigated by simulation is also comparable with the experimental result at the designate wavelength of 275 nm, which demonstrate the FFP Film is favourable for light extraction of DUV-LED. This interesting phenomenon should be ascribed to the nanoscale surface of biomimetic FFP Film that effectively decrease the reflectance in a broadband spectrum which can be regarded as a diffraction grating. The relatively high transmittance in the deep-UV light spectrum implies that deep UV-lights can easily escape from the fabricated FFP Film, which may have better performance for the DUV-LEDs. It is attributed to the combined result of the gradual two-dimensional sub-wavelength structure and the continuously tapered morphology on the patterned surface with a superior gradient refractive index profile at the interface40. With FFP Film, the light is therefore manipulated in all azimuthal directions over the entire emission wavelength range. Under this circumstance, light interactions cannot be described well by the Fresnel relations as diffraction and scattering become



prominent for FFP Film with nanoscale periodic structures. As a result, there is an extra resonant pathway exist that could facilitate the broadband light transmittance owing to the smooth transition of refractive index at the interfaces41. Compared to such characteristic achieved by conventional interference-based multiple coatings which rely on destructive interference from multiple reflections, the FFP Film with graded refractive index profile exhibited inherently extraordinary broadband anti-reflectivity. The light output power and the relative electroluminescent (EL) light intensity of bare DUV-LED and the ones with FFP Film measured under the driving current ranging from 50 mA to 400 mA are shown in Figure 3a and 3b. Among the four kinds of nanocone FFP Film, the FFP Film with radius of 325 nm shows the largest light output power and intensity. Meanwhile, the DUV-LED mounted with the smooth FFP Film and bare DUV-LED displays the lowest light output power and intensity due to similar condition of internal total reflection and high transmittance of FFP Film in DUV spectrum. As we mainly focused on the effect of the FFP Film with nanocones in this work, comparison has been carried out between DUV LEDs mounted with smooth and nonacone FFP Film. Under the driving current of 200 mA, the proposed R325 nanocone FFP Film structure could significantly enhance the light output power of DUV-LED by 26.7%, and up to 30.0% at the current of 400 mA, compared with the one with smooth FFP Film. An image of a lighted DUV-LED with nanocone FFP Film is shown in the inset of Figure 3a. Besides, the peak discrepancy among the 6 samples in Fig. 3b is 1 nm at most which is attributed to the nonuniformity of the LED samples from an identical epitaxy wafer rather than the effect of the FFP Film. Moreover, detailed light


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intensity analysis is obtained by using near-field intensity distribution test in 3D and 2D mode (Figures 3c and 3d), and the color distribution represents light output power intensity. Meanwhile, the normalized intensity along X distance and Y distance in 2D mode is presented in Figures 3e and 3f, respectively. The periodicity along Y direction in Figure 3f comes from the electrode. It can be seen from Figures 3c to 3f that the light intensity of DUV-LED with FFP Film is relatively higher benefited from the light extraction enhancement. To further understand the light extraction enhancement of the FFP Film on the top surface and sidewalls of chip respectively, the light intensity spatial distribution test of DUV-LEDs with the FFP Film is performed. Firstly, the DUV-LED chip is fixed on an angle resolution bracket, as shown in the Figure 4a. The angle resolution bracket rotation angle θ corresponds to the angle of light propagation respective to the c-axis of the DUV-LED, and then the emitted light is transmitting through a Glan-Taylor prism (polarization angle parameter φ) and then collected by an optical fiber spectrometer. The spectral curve and optical power can be obtained by such setup, and corresponding to each group (θ, φ). Each of the measured values is based on the spatial relationship between θ and φ. The optical power values are decomposed into TE/TM mode. As shown in the insert, the surface ABCD is the chip plane, the surface ADEF is the polarizer plane. The power corresponding to (θ, φ) measured by the fiber optic spectrometer is P, so the electric field mode oscillating along the transmissive axis AG is expressed in Equation (1):

|𝐸𝐴𝐺| = 𝑎0 𝑃

(1)



|𝐸𝐴𝐻| = 𝑎0 𝑃 𝑐𝑜𝑠𝜑2 ∙ 𝑐𝑜𝑠𝜃2 + 𝑠𝑖𝑛𝜑2 |𝐸𝐺𝐻| = 𝑎0 𝑃𝑐𝑜𝑠𝜑𝑠𝑖𝑛𝜃

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(2)

(3)

where 𝑎0 is a constant, the TE and TM electric field modes corresponding to this spatial position can be calculated by Equations (2) and (3), respectively. And then the angle φ is integrated separately to obtain the TE/TM polarized intensity distribution of the DUV-LED chip in space31. The far-field distributions of the DUV-LEDs with the FFP Film or not are tested at the current of 200 mA. As shown in Figure 4b, the spatial light intensity of DUV-LED with R325 nanocone FFP Film is stronger than that of the one mounted with smooth FFP Film at all viewing angles, which indicates that the nanocone FFP Film indeed improve the light extraction efficiency of DUV-LED. The light extraction enhancement factor is calculated to be 1.27 by integrating the spatial light intensity of the two DUVLEDs. The spatial distributions of TE/TM polarized light intensity of two DUV-LEDs are tested, and the spatial distributions of polarized light intensity in TE mode and TM mode are shown in Figure 4c. It can be seen that the introduction of the nanocone FFP Film has a very significant enhancement effect on both the TE mode and TM mode by about 20.5% and 21.8%, respectively. Such large enhancement indicates the effectiveness of nanocone FFP Film structure, and the reason can be attributed to the enhancement of TE and TM mode light simultaneously.


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Figure 3. (a) Light output power of bare DUV-LED and the ones mounted with smooth and nanocone FFP Film structures as a function of driving current, and the inset presents the image of a lighted DUV-LED with nanocone FFP Film; (b) EL spectra of DUVLEDs at the current of 200 mA, and the insert shows a detail view at the wavelength between 250 nm and 300 nm; Near-field intensity distribution in 3D and 2D views of DUV-LED with smooth (c) and R325-FFP Film (d), and the colour distribution represents light output power intensity; the normalized intensity comparison as a function of X distance (e) and Y distance (f).

Figure 4. (a) Light intensity spatial distribution measurement system with angle resolution bracket, Glan-Taylor prism and spectrometer, and the insert presents orientation and size relation of TE/TM mode light; (b) Measured far-field emission



pattern of DUV-LEDs with FFP Film of R325 nm and smooth films at the current of 200 mA, and the insert presents a schematic illustration of light transmission in DUVLEDs with proposed structures; (c) Full spatial TE/TM mode light intensity distributions of DUV-LEDs with FFP Film and smooth films. In addition, the simulated TE/TM-mode near-field electric field distributions of DUV-LEDs with the smooth fluoropolymer and the FFP Film (R=325 nm) are analysed via three-dimensional FEA method respectively, as shown in Figure 5, and the colour scale bar represents the normalized electric field intensity. Four cases with zero incidence, small angle incidence, total reflection angle incidence and large angle incidence, are displayed. At a wavelength of 275 nm, for both TE and TM mode, the angle of total reflection of smooth interface is calculated as 48°, thus the electric field of smooth interface occurred strong TIR and most emission is trapped within the film when the angle of incidence (θs) is larger than 48°. The electric field distribution of smooth interface is in accordance with the feature of evanescent wave. In contrast, for the interface with FFP Film with θs > 48°, there still exists strong electric field distribution in ambient air though some light is reflected back into film, as shown in Figure 5a and 6c. For detailed analysis, the 3D intensity distributions at the incident angle of 70° are shown in Figure 5b and 5d, respectively, in which the red arrow represents electric field direction and intensity. It is clearly shown that the TE/TM electric field of smooth interface in the ambient air is zero but there existed electric field distribution in air ambient for the interface with FFP Film due to the nanostructure. The FFP Film can effectively increase the out-coupling probability of the emitted light according to the intensity field distribution with propagation at the FFP Film/air interface, which is chiefly attributed to the gradual transition in refractive index of the


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nanocone arrays on the surface of DUV-LED chip for both TE- and TM- mode light as shown in Figure 4c. These factors enable the emitted light towards the surface normal with higher out-coupling efficiency.

Figure 5. Normalized electrical field intensity distribution close to the smooth or R325-FFP Film surface of (a) TE-polarized and (c) TM-polarized at different incident angle (θs); (b) and (d) display the 3D- electric field distribution of TE and TM at the incident angle of 70°, respectively. The red arrow represents electric field direction and intensity.

4. Conclusion In this paper, inspired by the microstructure of the butterfly’s eye, we propose and



fabricate a flexible fluoropolymer film (FFP Film) for all-mode, full-wavelength light extraction enhancement for most solid-state lighting technologies compatibly, especially for the deep ultraviolet light-emitting diodes (DUV-LEDs). This is the first time to achieve a large area uniform nanostructure on the sidewall of the light-emitting device. Experimental results demonstrate that, compared with smooth FFP Film mounted structure, the light output power of DUV-LED is enhanced up to 26.7% by mounting FFP Film with radius of 325 nm at the driving current of 200 mA. Importantly, thanks to the flexible feature of FFP Film, it can both cover the top surface and side walls of DUV-LED chip, leading to the obvious improvement of TE and TM mode by 20.5% and 21.8%, respectively. Finite element analysis (FEA) simulations of electric field distribution of DUV-LEDs with the smooth or nanocone FFP Film validate the mechanism. The present strategy is proposed from the view of packaging level, which is cost-effective, large-scale manufacturing, and compatible with the most solid-state lighting technologies, and it can benefit the all-mode full-wavelength light extraction enhancement with promising applications.

SUPPORTING INFORMATION Transmittance test and molecular structural formula of S-type fluoropolymer.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]; ORCID


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Jiangnan Dai: 0000-0001-9805-8726 Author Contributions †R. L., R. H. and H. L. contributed equally to this work. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is supported by Key Project of Chinese National Development Programs (Grant No. 2016YFB0400901), the Key Laboratory of infrared imaging materials and detectors, Shanghai Institute of Technical Physics, Chinese Academy of Sciences (Grant No. IIMDKFJJ-17-09), the National Natural Science Foundation of China (Grant No. 61675079, 11574166, 61377034, 61774065, 51606074), and the Director Fund of WNLO. REFERENCES 1.

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