Letter pubs.acs.org/NanoLett
Biologically Inspired Organic Light-Emitting Diodes Jae-Jun Kim,† Jaeho Lee,‡ Sung-Pyo Yang,† Ha Gon Kim,§ Hee-Seok Kweon,∥ Seunghyup Yoo,‡ and Ki-Hun Jeong*,† †
Department of Bio and Brain Engineering and KAIST Institute for Optical Science and Technology, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea ‡ Department of Electrical Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea § Muju Agricultural Technology Center, 416 Hanpungnu-ro, Muju-gun, Jeonbuk 568-802, Republic of Korea ∥ Division of Electron Microscopic Research, Korea Basic Science Institute, 169-148 Gwahak-ro, Yuseong-gu, Daejeon 305-806, Republic of Korea S Supporting Information *
ABSTRACT: Many animal species employ highly conspicuous traits as courtship signals for successful mating. Fireflies utilize their bioluminescent light as visual courtship signals. In addition to efficient bioluminescent light emission, the structural components of the firefly lantern also contribute to the enhancement of conspicuous optical signaling. Recently, these firefly lantern ultrastructures have attracted much interest and inspired highly efficient light management approaches. Here we report on the unique optical function of the hierarchical ultrastructures found in a firefly (Pyrocoelia rufa) and their biological inspiration of highly efficient organic light-emitting diode (OLED) applications. The hierarchical structures are comprised of longitudinal nanostructures and asymmetric microstructures, which were successfully replicated using geometry-guided resist reflow, replica molding, and polydimethylsiloxane (PDMS) oxidation. The external quantum efficiency (EQE) of the bioinspired OLEDs was enhanced by up to 61%. The bioinspired OLEDs clearly showed side-enhanced superLambertian emission with a wide-viewing angle. The highly efficient light extraction and wide-angle illumination suggest how the hierarchical structures likely improve the recognition of firefly optical courtship signals over a wide-angle range. At the same time, the biologically inspired designs provide a new paradigm for designing functional optical surfaces for lighting or display applications. KEYWORDS: Biomimetics, firefly light organs, hierarchical structures, photonic structures, organic light-emitting diodes
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photogenic layer also play important roles in efficiently managing the bioluminescent light.14−16 Firefly lantern ultrastructures, such as the cuticular nanostructures or inclined microstructures found on the lantern cuticles of Luciola lateralis Motschulsky and Photuris sp., respectively, have recently been investigated and offer biologically inspired approaches for highly efficient light management.14,15 Their optical functions have been revealed by both numerical analysis and experimental results. For example, the cuticular nanostructures improve light transmission by reducing the mismatch of refractive index between air and chitin,14 while the inclined microstructures increase light extraction by reducing total internal reflection (TIR).15 Found in the dorsal layer, submicron spherical granules with uric acid effectively reflect the bioluminescent light to increase light intensity in the
nimals attract mates with various types of courtship signals including highly conspicuous traits.1 Among courtship signals, visual courtship communication serves as an effective strategy for attracting mates for many insects, birds, fishes, or amphibians.2−4 In particular, fireflies communicate each other by flashing species-specific bioluminescent light in dim or dark environments.5−7 These highly conspicuous light emissions from firefly lanterns are essential for successful sexual communication. In particular, emitting a strong optical signal over a wide-angle range is an effective strategy for improving the successful recognition of courtship signals. In the firefly, the lantern organ is enclosed around the abdomen and consists of a cuticle, a photogenic layer, and a dorsal layer.8,9 These anatomical structures are presumably associated with the efficient transmission of the conspicuous optical signals. It is notable that the photogenic layer of the firefly produces bioluminescent light with the highest quantum yield10 among bioluminescent organs found in ostracods,11 jellyfish,12 or fish.13 The cuticle and the dorsal layer surrounding the © 2016 American Chemical Society
Received: December 18, 2015 Revised: March 2, 2016 Published: March 25, 2016 2994
DOI: 10.1021/acs.nanolett.5b05183 Nano Lett. 2016, 16, 2994−3000
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Figure 1. Biologically inspired hierarchical structures of a firefly light organ. (a) An optical image of a male firefly (Pyrocoelia rufa). (b−g) Scanning electron microscopy (SEM) images of the abdominal segments of a male firefly. (b) Abdominal segments including normal (N) and lantern (L) cuticles. (c) Dense setae on the normal cuticle. (d) Asymmetric microstructures on the lantern cuticle. (e) Enlarged view of the lantern cuticle. Longitudinal nanostructures on the surface of the asymmetric microstructures. (f) Direction of focused ion beam (FIB) section, along the white line. (g) A longitudinal cross-section of the lantern cuticle milled by FIB. (h) Angular transmittances of bioluminescent light through the hierarchical structures on the lantern cuticle calculated by using a finite difference time domain (FDTD) method. The transverse plane is perpendicular, and the sagittal plane is parallel to the longitudinal direction of the nanostructures. The asymmetric microstructures and hierarchical structures improve light extraction compared to a smooth surface, depending on the incident angle of the bioluminescent light. (i, j) Anatomical comparisons between a firefly lantern and a biologically inspired organic light-emitting diode (OLED) panel. (i) A firefly lantern is comprised of dorsal, photogenic, and cuticular layers. The inset shows a schematic illustration of the light path in a firefly lantern. (j) The biologically inspired OLED panel has a similar configuration, with an aluminum layer, organics, and an optical resin layer with hierarchical structures on a glass substrate. Like the cuticular hierarchical structures of a firefly lantern, this biologically inspired configuration enables highly efficient light extraction and wide angle illumination from an OLED panel light.
ventral view.16 These cuticular nanostructures and inclined microstructures have been mimicked in the design of highly
efficient light-emitting diode (LED) lighting. However, the hierarchical photonic structures of the firefly lantern have not 2995
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Figure 2. Hierarchical fabrication methods and microscopic images of asymmetric microstructures and hierarchical structures. (a) Micro and nanofabrication steps, step I. Photolithographic definition of thermoset (SU-8) and thermoplastic resist (AZ9260) with precise alignment, and thermal reflow at a temperature higher than the glass transition temperature of the thermoplastic resin, step II. Oxygen plasma treatment on stretched PDMS mold after PDMS replication, step III. Releasing the oxidized PDMS mold, step IV. Replica molding with UV curable optical resin, and release of the hierarchical structures from the PDMS mold. Optical microscopic images of (b) asymmetric microstructures and (c) hierarchical structures with an inclined angle of 10°. (d) A perspective SEM image of the hierarchical structures. Enlarged SEM images of (e) the asymmetric microstructures and (f) the hierarchical structures. Highly ordered wrinkled nanostructures were well-constructed on the asymmetric microstructures, like the hierarchical structures of a firefly lantern.
calculated for different incident angles from 0 to 60° for every 5°, respectively. The firefly’s unpolarized light was considered by averaging the transverse electric (TE) and transverse magnetic (TM) polarizations. The incident angle varies along the sagittal and transverse planes due to both the asymmetrical shapes of the microstructures and the directionality of the nanostructures. The incident plane is defined in the inset of Figure 1h. The longitudinal nanostructures are perpendicular and parallel to the transverse plane and the sagittal plane, respectively. The FDTD results clearly demonstrate that both the asymmetric microstructures and the hierarchical structures substantially improve light extraction compared to a smooth surface. For the sagittal plane, the average transmittances for a smooth surface, asymmetric microstructures, and hierarchical structures are 58, 60, and 62% for −60 to 60°, respectively. The average transmittances for asymmetric microstructures and hierarchical structures are 10 and 11% above 40°, i.e., the critical angle of TIR for cuticle (n = 1.56), respectively. For the transverse plane, the average transmittances for a smooth surface, asymmetric microstructures, and hierarchical structures are 58, 58, and 60% for 0 to 60°, respectively. The average transmittances of asymmetric microstructures and hierarchical structures are 6 and 8% above the critical angle, respectively.
yet been fully investigated or successfully mimicked for engineering applications. Here we report the optical roles and biological inspirations of the hierarchical structures found in firefly lanterns on the abdominal segments of a male firefly (Pyrocoelia rufa) (Figure 1). Unlike the normal (N) cuticle, which has more setae than the lantern cuticle, the lantern (L) cuticle has hierarchical structures, i.e., asymmetric microstructures with longitudinal nanostructures (Figure 1b−e). Extracted from SEM images, the physical dimensions of longitudinal nanostructures are 250 nm in period, 110 nm in height, and 150 nm in width, and the width of the asymmetric microstructures are approximately 10 μm. A cross-section of the lantern cuticle, observed after micromilling with focused ion beam (FIB), clearly indicates that each asymmetric microstructure has an inclined angle of around 5° (Figure 1f−g). The emission spectrum of the male bioluminescent light shows that its center wavelength is around 560 nm (72 nm in bandwidth) (Figure S1). The angular transmittances at λ = 560 nm of a smooth surface, asymmetric microstructures, and hierarchical structures were calculated by using a finite difference time domain (FDTD) method, respectively (Figure 1h). The refractive index of the lantern cuticle was taken as 1.56 for this FDTD analysis.17,18 The optical transmittances were 2996
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Figure 3. Angular transmittances of asymmetric microstructures and hierarchical structures. (a) Total internal reflection (TIR) on the smooth surface (left) and out-coupling on the structured surface (right). Light impinging on the structured surfaces is effectively out- coupled from the top surface above the TIR angle, whereas light on the smooth surface is totally reflected. (b) A schematic illustration of the measurement setup. Angular transmittances of (c) asymmetric microstructures and (d) hierarchical structures obtained by averaging the sagittal and transverse planes. AM, H, and angle values indicate the asymmetric microstructures, hierarchical structures, and the inclined angles of the photonic structures, respectively. Optical transmittance through the structured surfaces is slightly lower than that through the smooth surface for small incident angles but is still out-coupled at the incident angles above 40°. The transmission increases over the critical angle of TIR but decreases under the critical angle as the inclined angle of the microstructures increases.
The hierarchical structures substantially improve light extraction by reducing TIR and Fresnel reflection. The asymmetric microstructures effectively couple out the light trapped above the critical angle, by diffraction and geometrical change. Furthermore, the nanostructures on the asymmetric microstructures increase the light extraction efficiency through optical index matching14 and subwavelength diffraction.19,20 Consequently, the hierarchical structures surrounding the photogenic layer play an important role in the highly efficient extraction of bioluminescent light. From an anatomical perspective, a firefly lantern is comprised of dorsal, photogenic, and cuticular layers (Figure 1i). The photogenic layer emits bioluminescent light in all directions. The emitted light passes through the cuticle either directly or after reflecting off the dorsal layer, which is located under the photogenic layer, and is also known as a dielectric mirror.8,9,21 This natural smart design also inspires highly efficient organic light-emitting diode (OLED) devices. A biologically inspired OLED panel has a similar configuration, with an aluminum layer, organics, and an optical resin layer with hierarchical structures (Figure 1j). Light emitted from the organic emissive layer either passes through the optical resin layer with hierarchical structures directly, or after reflecting off the aluminum layer. This bioinspired configuration apparently contributes to both the light extraction efficiency and angular distribution of OLED devices. This bioinspired OLED configuration successfully mimicked the firefly features by implementing the hierarchical structures on an OLED panel. The device fabrication was done using geometry-guided resist reflow, replica molding, and polydimethylsiloxane (PDMS) oxidation (Figure 2a, Figure S2). A
thermoset resist (SU-8, MicroChem) was initially defined on a 4 in. silicon wafer, and then a thermoplastic resist (AZ9260, AZ Electronic Materials) was precisely defined next to the predefined thermoset microstructures using photolithography with accurate alignment. Both the patterned structures were thermally reflowed on a hot plate at 180 °C for 1 h; the shapes of the thermoplastic microstructures were spontaneously changed along the boundaries of the thermoset microstructures due to the surface energy minimization of the asymmetric microstructures.22 A thin antistiction film of plasma-assisted fluorocarbon was deposited on both of the microstructures to facilitate PDMS replication. The asymmetric microstructures were transferred onto a thin PDMS sheet. The microstructured PDMS sheet was unidirectionally stretched using a custom-made stretching stage and then oxidized with an oxygen plasma treatment.23−26 The prestrain and the oxidation time were precisely controlled from 20 to 50% and from 10 to 40 s, respectively (Figure S3). The dimensions of the nanostructures formed with a prestrain of 40% and an oxidation time of 30 s were similar to the natural ones. Upon releasing the strain from the oxidized PDMS sheet, highly ordered wrinkle nanostructures were formed on the microstructured PDMS sheet. The hierarchical structures were finally transferred onto a UV curable optical resin (Norland optical adhesive 63) on a glass substrate after the PDMS replica molding. The asymmetric microstructures and hierarchical structures, which were fabricated uniformly in large scale, are clearly shown in optical microscopic images and perspective SEM images (Figure 2b−f). The detailed shapes of the asymmetric microstructures and hierarchical structures with 2997
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Figure 4. Optical characteristics of an OLED panel with asymmetric microstructures and with hierarchical structures. Photographs of OLEDs in operation (a) without and (b) with structured surfaces. (c) Maximum external quantum efficiency (EQEmax) of an OLED panel versus luminance from a smooth surface, asymmetric microstructures, and hierarchical structures at different inclined angles (5, 10, 14, and 18°). Compared to a smooth surface, the hierarchical structures exhibit substantial increase in EQE by up to 61%. Furthermore, the hierarchical structures exhibit further improvement in EQE compared to the asymmetric microstructures at the same inclined angles. (d) Angular distributions of an OLED panel with a smooth surface, asymmetric microstructures, and hierarchical structures at different inclined angles (5, 10, 14, and 18°). Both the asymmetric microstructures and hierarchical structures clearly show super-Lambertian distribution with side enhancement. The hierarchical structures also show higher angular intensity than asymmetric microstructures at the same inclined angles over all angles.
four different heights (h = 1, 2, 3, 4 μm), a constant width of 10 μm, and a constant gap of 3 μm, are also shown in the enlarged perspective SEM images (Figure S4). Wrinkled nanostructures were formed on the microstructured PDMS surfaces under the same conditions of oxygen plasma treatment and prestrain (150 W RF power, 30 s oxidation time, and 40% prestrain). The inclined angles, measured by using cross-sectional SEM images, are 5, 10, 14, and 18° for different heights of asymmetric microstructures 1, 2, 3, and 4 μm, respectively (Figure S5). The angular transmittances of the structured surfaces with asymmetric microstructures and hierarchical structures were measured by changing the incident angle of a laser beam along the transverse and sagittal planes, due to the structural asymmetry (Figure 3a). The measurement setup was comprised
of a spectrometer, an integrating sphere, a half cylinder, a rotation stage, and a laser (λ = 532 nm) as illustrated in Figure 3b. Optical transmission was measured by using a photospectrometer coupled with an integrating sphere. The incident angle was changed from 0 to 60° by rotating the integrating sphere and the half cylinder. Note that the integrating sphere collects all out-coupled light through the structured surfaces. Both of the photonic structures, i.e., the asymmetric microstructures and hierarchical structures, were separately implemented on the flat side of the half cylinder using a replica molding method. The curved surface of the half cylinder maintains the incident angle of a laser beam on the photonic structures without changing the angle of refraction or the Fresnel reflection, due to normal incidence through the curved 2998
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and bioinspired OLEDs with hierarchical structures (Bi-OLED) exhibited the EQEmax ranging from 31 to 37%, demonstrating a significant enhancement from that of conventional OLEDs with a smooth surface (23%) (Figure 4c, Figure S8, and Table S2). Compared to the EQE of conventional OLEDs, the relative enhancement in the EQE of the OLEDs w/AM and Bi-OLEDs ranged from 35 to 57% and from 43 to 61%, respectively, depending on the inclined angle of the microstructures. All of the bioinspired OLEDs clearly showed higher EQE enhancement than the OLED w/AM at the same inclined angle (Figure S8). Because all of the OLED devices in the comparison showed comparable current density−voltage characteristics (Figure S9), one can conclude that the observed difference in performance mainly results from the bioinspired structures and that the multiscale feature of the hierarchical structures is apparently beneficial for extracting OLED light that would otherwise be confined. In addition, the angle-resolved measurement results indicate that the Bi-OLEDs have side-enhanced superLambertian emission with stable angular electroluminescence (EL) spectra over a wide-viewing angle region (Figure 4d, Figure S8, and Figure S10), as a result of the capability of the quasi-omnidirectional light out-coupling. The experimental results show that the hierarchical structures of a firefly lantern can enhance the optical signals over a wide-angle range, and potentially improve the courtship signal recognition. In conclusion, this work successfully demonstrated the optical role of the hierarchical structures found in the firefly lantern and its biologically inspired replication in highly efficient OLEDs with super-Lambertian emission. The hierarchical structures were successfully mimicked using geometry-guided resist reflow, replica molding, and PDMS oxidation. The experimental results revealed that the hierarchical structures on the lantern cuticle substantially contribute to enhanced light extraction (by up to 61%) and the widening of illumination angles, resulting in highly efficient light sources with side-enhanced super-Lambertian distribution. These features show that the hierarchical structures help deliver strong optical signals over a wide-angle range, which likely enhance the recognition of firefly courtship signals. The observation of this natural phenomena provides a new biologically inspired strategy for efficient light management, and its engineering replication paves a new direction in biologically inspired photonics for advanced display or lighting applications.
side. Consequently, the incident angle was controlled above the critical angle of TIR. The angular transmittances were measured using a smooth surface and the photonic structures with four different inclined angles, of 5, 10, 14, and 18°, under a constant width of 10 μm and a constant gap of 3 μm (Figure 3c,d and Figure S6). The laser beam was impinged along the sagittal plane from two different directions, as illustrated in the inset of Figure S6a,c due to the structural asymmetry. The angular transmittances for the transverse plane were measured by rotating the laser beam in one direction due to the structural symmetry (Figure S6b,d). The photonic structures showed asymmetric and symmetric angular transmittances as the incident angles were varied along the sagittal and transverse planes, respectively (Figure S6). The angular transmittances of the photonic structures, obtained by averaging the sagittal and transverse planes, clearly exhibit asymmetric angular transmittances (Figure 3c,d). This asymmetric distribution indicates that the trapped light can be coupled out by the geometrical change. Depending on the incident angle, the structured surface shows lower transmittance than the smooth surface at the incident angles below 40°, i.e., the critical angle of TIR for UV curable optical resin (n = 1.56), but apparently higher light extraction at the incident angles above 40°. As the inclined angle of the microstructures increases, the optical transmittance also increases at the incident angles above the TIR angle but decreases below the TIR angle. The amount of transmission for the transverse and sagittal planes were averaged to calculate the out-coupling efficiency. The average transmittances for a smooth surface and the structured surfaces are summarized in Table S1. Although the total average transmittances have similar values for all surfaces, the average values above the TIR angle, i.e., 40−60°, significantly increased compared to a smooth surface. The asymmetric microstructures and hierarchical structures considerably enhanced light extraction by reducing TIR. The angular transmittances of the hierarchical structures, except an inclined angle of 18° above the critical angle, were higher than that of the asymmetric microstructures due to the subwavelength diffraction of the nanostructures on the asymmetric microstructures. The structured surfaces with asymmetric microstructures and hierarchical structures were finally mounted onto prepared green phosphorescent OLEDs by adding an index-matching fluid (F-IMF-105, Newport) (Figure S7, see more details for OLED fabrication in Supporting Information). The OLED devices were based on green phosphorescent emitters of bis(2phenylpyridine)iridium(III)-acetylacetonate (Ir(ppy)2acac) configured in an exciplex system based on a co-host of 4,4′Bis(N-carbazolyl)-1,1′-biphenyl (CBP) and bis-4,6-(3,5-di-3pyridylphenyl)-2-methylpyrimidine (B3PYMPM) for energyefficient performance.27,28 The OLED devices were evaluated in a nitrogen filled glove box (Korea Kiyon) using a sourcemeasure unit (Keithley 2400) and a calibrated spectral response Si photodiode (FDS-100-CAL, Thorlabs) and a fiber-optic spectrometer (EPP2000, StellarNet, Inc.), both of which were held on a motorized goniometric arm for full angle-resolved characterizations. Photographs of the OLEDs in operation with and without the photonic structures (at 1 mA/cm2 for both cases) indicate that the trapped light of the OLEDs is efficiently extracted from a glass substrate by the photonic structures (Figure 4a,b). The OLEDs with the asymmetric microstructures (OLED w/AM)
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b05183. Materials and methods, Figures S1−S10, Table S1−S2 (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address
J.-J.K.: Department of Electrical and Computer Engineering, University of Wisconsin-Madison, 1415 Engineering Drive, Madison, Wisconsin 53706, United States. 2999
DOI: 10.1021/acs.nanolett.5b05183 Nano Lett. 2016, 16, 2994−3000
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Nano Letters Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant (No. 2014022751, No. 2015036205), Center for Integrated Smart Sensors funded by the Ministry of Science, ICT & Future Planning as Global Frontier Project (CISS-2011-0031866), Ministry of Trade, Industry, and Energy (MOTIE 10041120), and Korea Health Technology R&D Project through the Korea Health Industry Development Institute (KHIDI), funded by the Ministry of Health & Welfare, Republic of Korea (HI13C2181).
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REFERENCES
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