Open Access
Add to BibSonomyAbstract
We report the characterization and analyses of organic light-emitting devices (OLEDs) using microstructured composite transparent electrodes consisting of the high-index ITO (indium tin oxide) micromesh and the low-index conducting polymer PEDOT:PSS [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)], that are fabricated by the facile and convenient microsphere lithography and are useful for enhancing light extraction. The rigorous electromagnetic simulation based on the three-dimensional finite-difference time-domain (FDTD) method was conducted to study optical properties and mechanisms in such devices. It provides a different but consistent viewpoint/insight of how this microstructured electrode enhances optical out-coupling of OLEDs, compared to that provided by ray optics simulation in previous works. Both experimental and simulation studies indicate such a microstructured electrode effectively enhances coupling of internal radiation into the substrate, compared to devices with the typical planar ITO electrode. By combining this internal extraction structure and the external extraction scheme (e.g. by attaching extraction lens) to further extract radiation into the substrate, a rather high external quantum efficiency of 46.8% was achieved with green phosphorescent OLEDs, clearly manifesting its high potential.
© 2016 Optical Society of America
1. Introduction
The advancement of organic light-emitting device (OLED) technologies in recent years has rendered possible their various display and lighting applications [1–4]. Further advancement of OLED technologies and applications imposes increasing requirements on OLED efficiencies. While triplet-harvesting emitting materials have raised internal quantum efficiencies of OLEDs to nearly 100% [5,6], relatively low light out-coupling efficiencies of internally generated photons for typical planar OLEDs remains a critical issue [7,8]. As such, various light extraction structures for OLEDs have been proposed and investigated [9–13]. External out-coupling structures, such as microlens, surface textures, and shaped substrates etc [9,10], that are typically constructed outside the device and are simpler to implement, can effectively out-couple light entering the substrates. Yet to further extract radiation trapped within active layers and to obtain even higher efficiencies, internal light extraction structures inserted between OLED layers and substrates are needed [11–14]. Photonic nanostructures such as photonic crystals or wavelength-scale corrugation/gratings are some representative types of internal light extraction structures for OLEDs [12–14]. Yet, such photonic nanostructures in general are more complicated and expensive to fabricate, making them not so readily applicable for OLED displays or lighting. In addition, these photonic nanostructures very often induce wavelength-, angle-, and structure-dependent optical effects that would significantly distort emission spectra or patterns [12–14].
To overcome such issues, an OLED structure with embedded micrometer-scale low-index grids in the active layers was introduced by Forrest et al. in 2008 [15]. In such an OLED structure, before deposition of active organic layers and top metal electrodes, a low-index material layer (e.g., SiO2) was deposited onto the ITO electrode and was patterned into micrometer-scale grids by conventional photolithography and etching. With such micrometer-scale embedded low-index grids, the waveguided (total-internal-reflection) light within OLED layers can be re-directed and be effectively out-coupled into air or substrates. Furthermore, optical effects induced by such microstructures are less sensitive to wavelengths and device structures. When combining the embedded low-index grid and external extraction structure (e.g., microlens arrays), an EQE of up to 34% was achieved for phosphorescent white OLEDs. However, in such a device structure, emission areas of devices are reduced since the low-index material is insulating. To keep enhancement of optical out-coupling yet without losing device emission areas, in 2010, Yoo et al. reported another OLED structure that had micrometer-scale ITO grids overcoated with the low-index (n~1.5) high-conductivity conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the OLED transparent anode [16]. Such a micropatterned electrode structure and the index difference between PEDOT:PSS and ITO/organic layers again are helpful to redirect waveguided light for out-coupling. Meanwhile, the low-index and high-conductivity PEDOT:PSS can now ensure uniform current spreading and emission over the whole device area. When combining such microstructured electrode and external extraction structure (e.g., microlens arrays), an EQE of up to 2.25% was achieved for green fluorescent OLEDs.
Although the combination of micrometer-scale ITO grids and low-index PEDOT:PSS had been shown to enhance the optical out-coupling of OLEDs, it however still requires rather high-precision photolithography and etching processing [16]. In addtion, the work of Yoo et al. used the Monte Carlo ray-tracing method to perform the simulation of optical properties/mechanisms of such OLEDs [16]. Although dimensions of ITO grids are sufficiently larger than emission wavelengths and thus the geometric optics approach appears intuitive, yet the thicknesses of material layers in the device structure are in general significantly smaller than emission wavelengths, rendering use of the ray-tracing method not so justified and accurate. In this work, on one hand we adopt the facile and convenient microsphere lithography to form the micropatterned ITO meshes for fabricating OLEDs with ITO micromesh/PEDOT:PSS composite electrodes. On the other hand, the more rigorous electromagnetic wave simulation based on the three-dimensional finite-difference time-domain (FDTD) method is conducted to study optical properties and mechanisms in the devices. By combining this internal extraction structure and the external extraction scheme (e.g. by attaching extraction lens), a rather high EQE of 46.8% was achieved with state-of-the-art green phosphorescent OLEDs.
2. Methods
2.1 Fabrication and characterization
The ITO micromesh was fabricated by microsphere lithography, similar to fabrication of the ITO nanomesh by nanosphere lithography in our previous reports [11,17], except for the use of 3.2 μm polystyrene microspheres (Duke Scientific 5320A) instead of nanospheres. It included forming a close-packed (hexagonal) self-assembly monolayer (SAM) of polystyrene microspheres on the glass substrate, shrinking the sphere size to ~2.8 μm by oxygen plasma etching, depositing the conductive ITO layer (~40 nm thick, with a conductivity of ~1800 S/cm) through the gap of shrunk polystyrene spheres by RF sputtering, and lifting off microspheres to form ITO micromesh. Subsequently, a ~65-nm high-conductivity PEDOT:PSS layer (for lateral conduction, with a conductivity of 900-1000 S/cm) and a ~20-nm low-conductivity PEDOT:PSS layer (for hole injection, with a conductivity of ~0.1 S/cm) was sequentially spin-coated over the ITO micromesh, followed by annealing at 130 °C for 15 minutes after each coating. The preparation of high conductivity and low-conductivity PEDOT:PSS is detailed in previous reports [11,18]. Further deposition of the organic layer stack and the metal electrode onto the ITO micromesh/PEDOT:PSS composite electrode by thermal evaporation completed the whole OLED device, as shown in Fig. 1 (the ITO micromesh device). The OLED stack on top of the composite electrode included NPB (40 nm)/CBP:Ir(ppy)2(acac) 8 wt.% (15 nm)/TPBi (65 nm)/LiF (0.5 nm)/Al (130 nm). NPB (N,N’-Di(1-naphthyl)-N,N’-diphenyl-(1,1’-biphenyl)-4,4’-diamine), CBP [4,4'-bis(carbazol-9-yl)biphenyl] doped with 8wt.% Ir(ppy)2(acac) [bis(2-phenylpyridine)(acetylacetonato) iridium(III)], TPBi (2,2’,2”-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)), LiF, and Al were the hole-transpaort layer, the phosphorescent green emitting layer, the electron transport layer, the electron injection layer, and the cathode, respectively [3,4,7,11,19]. Two reference devices having same organic layer structures except for using the planar ITO anode (~40 nm, the planar ITO device) or the planar ITO/PEDOT:PSS anode (~40 nm ITO + 65 nm high-conductivity PEDOT:PSS + 20 nm low-conductivity PEDOT:PSS, the planar ITO/PEDOT device), were also fabricated and tested for comparison. The active area of the device was 1x1 mm2, as defined by the shadow mask for cathode deposition. More details for preparation and fabrication of ITO meshes, the high-conductivity PEDOT:PSS, ITO mesh/PEDOT:PSS composite electrodes, OLED deposition, and device characterization can be found in our previous report [11]. In addition to standard electroluminescence (EL) characterization of as-fabricated devices, to extract and collect overall radiation coupled into substrates, these devices were also characterized with further attachment of a relatively large hemisphere glass lens (having a diameter of 1 cm) using the index-matching oil [20].
2.2 Optical simulation
In order to rigorously treat aspects of various devices, optical simulation of various devices was conducted by combining the three-dimensional finite-difference time-domain simulation (FDTD, Rsoft 9.0 FullWave, Synopsys Inc.) for near-field behaviors of the active region and the ray-tracing simulation (LightTools 8.2, Synopsys Inc.) for far-field behaviors in the substrate and air (and also for saving the calculation time and computer storage space) [21,22]. The device geometry/structure used for simulation was based on that characterized by microscopy (e.g., AFM, SEM, etc.) of layers. The complex refractive indices of all material layers used for simulation were experimentally determined by spectroscopic ellipsometry (VASE, J. A. Woollam Co.). The FDTD analyses were conducted by locating emitting dipoles of different orientations (i.e., along x, y, z directions; x and y directions are parallel to the substrate surface, and z direction is along the substrate surface normal) and frequencies in the emitting layer over the unit cell of the hexagonal lattice. Considering the emission anisotropy of the emitter Ir(ppy)2(acac) [23], the horizontal to vertical dipole ratio of 0.76:0.24 was used in the FDTD simulations. The far-field behaviors and coupling efficiencies (to the substrate and air) were calculated by taking into account the full photoluminescence (PL) spectrum of the emitting dipoles of all orientations. The configuration parameters for FDTD calculation, such as the size of the computation domain and the number of dipoles per unit cell etc., were determined by convergence of calculation results.
3. Results and discussion
3.1 Fabricated structures
Figures 2(a)-2(d) show topographic surface images of the fabricated structures, taken by AFM (atomic force microscopy) during sequential deposition of layers onto the ITO micromesh: (a) right after the fabrication of the ITO micromesh, (b) after the PEDOT:PSS coating, (c) after deposition of all OLED organic layers, and (d) after deposition of the metal cathode. Figure 3 shows the surface profiles taken along the x direction (as defined in Fig. 2(a)) for each of Figs. 2(a)-2(d). It is seen that the corrugation height is reduced from ~40 nm to ~20 nm by coating PEDOT:PSS onto the ITO micromesh, and further vacuum deposition of layers was roughly conformal and retained a corrugation height of ~20 nm. It is thus concluded that the PEDOT:PSS coating here partially planarizes the ITO micromesh. Although non-uniform active layer thicknesses, non-uniform current distributions, and stability issues might occur in OLEDs with corrugated structures, yet the partial planarization and smoothing of the corrugated ITO micromesh by solution-coating of PEDOT:PSS and the conformal nature of the vacuum-deposited layers above shall reduce such problems in the present devices.
Fig. 2 AFM topographic surface images taken during sequential deposition of layers onto the ITO micromesh: (a) with the ITO micromesh only, (b) after the PEDOT:PSS coating, (c) after deposition of all OLED organic layers, and (d) after deposition of the metal cathode.
3.2 Device results
Current density-voltage-luminance (J-V-L) characteristics of the ITO micromesh, planar ITO, and planar ITO/PEDOT devices (measured without lens attachment) are depicted in Fig. 4(a). Similar J-V characteristics of these devices indicate similar electrical characteristics and differences in their emission characteristics mainly result from their different optical properties/structures. EQEs and luminous efficiencies of these devices measured without and with attaching extraction lens are depicted in Figs. 4(b) and 4(c). Efficiency perfromances of these devices are also summarized in Table 1. Without lens attachment, the planar ITO and planar ITO/PEDOT devices exhibit EQE and power efficiency of up to (16.9%, 64.6 lm/W) and (17.2%, 62.9 lm/W), respectively, while the ITO micromesh device gives efficiencies of up to (22.2%, 103.5 lm/W). A moderate enhancement in EQE (~1.3X) is obtained in the ITO micromesh device, compared to the planar ITO device. The EQE enhancement for the ITO micromesh device is even more significant when effectively extracting radiation coupled into the substrate by attaching an extraction lens. With attaching the hemisphere lens, all the planar ITO, planar ITO/PEDOT, and ITO micromesh devices exhibit much enhanced EQE and power efficiency of up to (30.6%, 117.3 lm/W), (32.3%, 118.4 lm/W), and (46.8%, 218.5 lm/W), respectively.
Fig. 4 (a) J-V-L characteristics (measured without lens attachment), (b) external quantum efficiencies (measured without and with lens attachment), (c) luminous efficiencies (measured without and with lens attachment) of ITO micromesh, planar ITO, and planar ITO/PEDOT devices.
Table 1. Summary of Efficiencies of OLED Devices.
The EQE enhancement obtained by lens attachment for the ITO micromesh device is 46.8%/22.2% = 2.11X, which is larger than that for the conventional planar ITO device (30.6%/16.9% = 1.8X). In addition, with lens attachment, the EQE gain of the ITO micromesh device relative to the planar ITO device (i.e., 46.8%/30.6% = 1.52X) is also enhanced, compared to that without lens attachment (i.e., 22.2%/16.9% = 1.3X). These observations suggest that the ITO micromesh device configuration does substantially enhance coupling of internal radiation into the substrate (compared to the conventional planar ITO device) and yet a significant portion of such enhanced fluxes inside the substrate is dispatched beyond the critical angle θc~41° of the glass-air interface, considering that the attachment of the large hemisphere lens in general helps to effectively extract radiation initially outside the escape cone in the substrate. To confirm this, the angle-resolved EL characteristics of both lens-attached planar ITO device and ITO micromesh device were measured and are shown in Fig. 5. Figures 5(a) and 5(b) depict measured EL spectra with relative intensities at different viewing angles for the lens-attached planar ITO and ITO micromesh devices, respectively. The EL spectra of the ITO micromesh device exhibit hardly any angle dependence or any sharp spectral feature, and basically retain the intrinsic emission spectrum of the emitter, a particular advantage of such light extraction structure/scheme [15,16]. Figure 5(c) shows the angular dependence of the EL intensity for both lens-attached devices. A significantly enhanced contribution from fluxes beyond the critical angle of the glass-air interface (when integrating fluxes over solid angles) is clearly seen in the ITO micromesh device (compared to conventional planar ITO device). Efficiency enhancement factors calculated from these EL emission patterns, by integration over the full angle range or within θc, are consistent with EQEs measured with or without lens attachment, respectively.
Fig. 5 Measured EL spectra with relative intensities at different viewing angles for the (a) lens-attached planar ITO device and (b) the lens-attached ITO micromesh device. (c) Measured (symbols) and calculated (lines) angular distributions of the EL intensity (spectrally integrated) for devices in (a) and (b). All the intensities shown are normalized to their intensities at 0°.
3.3 Simulation results
To get more insights of how the ITO micromesh device configuration enhances coupling of internally generated light into the substrate and air, optical simulation was conducted. The configurations of the ITO micromesh device for simulation are shown in Fig. 6(a) (cross-section configuration along the x direction) and Fig. 6(b) (three-dimensional representation of the ITO micromesh), in which P represents the period along the x direction (3.2 μm), D the width of the ITO top along the x direction (~1 μm), and W the diameter of ITO mesh holes (openings, ~1 μm). For the simulation of the ITO micromesh OLED, a corrugated light source distribution consisting of a number of dipoles over the unit cell is preferable over a single dipole at a single location, since the radiation characteristics from emitting dipoles exhibit significant dependence on their locations within the microstructure (as to be manifested in the following). In FDTD simulations, only a small portion of the substrate was within the simulation domain (i.e. a few μm in the z direction), since FDTD would be computationally inefficient to deal with a thick substrate. The perfectly matched layer (PML) boundary condition was used so that the substrate could be treated as a semi-infinite boundary with respect to the dipoles. The field distribution at the simulation boundary was then employed as the source of the subsequent ray-tracing simulations within the substrate. Then the entire substrate and the external extraction structure (like attaching extraction lens here) can be simulated properly and be compared with the measurement scheme. For saving the calculation time and for getting deeper insights, radiation characteristics of single dipoles located at different locations over one period of the structure were first calculated and then were summed together to obtain overall emission characteristics of the device (assuming dipoles are non-coherent to each other). As representative examples to illustrate the location dependence of dipole emission characteristics in the ITO micromesh device, in the following results for dipoles located at X1, X2, X3, X4, and X5 in Fig. 6(a) (moving from the center of the ITO top to the center of the mesh hole, i.e., over half of the period along the x direction) in the ITO micromesh device are discussed.
Fig. 6 (a) The configuration of the ITO micromesh device for simulation (cross-section configuration along the x direction). (b) Three-dimensional representation of the ITO micromesh. P represents the period along the x direction (3.2 μm), D the width of the ITO top along the x direction (~1 μm), and W the diameter of ITO mesh holes (openings, ~1 μm).
Figure 7(a) shows the x-z-plane cross sections of the power density flux (i.e., the Poynting vector magnitude) from a single-frequency (at the wavelength 525 nm) y-direction (horizontal) dipole in the conventional planar ITO device, while Figs. 7(b)-7(f) show the x-z-plane cross sections of the power density flux from a single-frequency (at the wavelength 525 nm) y-direction (horizontal) dipole located at X1, X2, X3, X4, and X5 in the ITO micromesh device, respectively. As seen in Fig. 7(a), in the conventional planar ITO device, a substantial amount of radiation is confined and propagates laterally in/near device layers. With the ITO micromesh and layer corrugation structures above, in general the radiation confined in/near device layers is reduced and more radiation is directed toward the substrate (wherever the horizontal dipoles are located, as seen in Figs. 7(b)-7(f)), although the patterns of the power density fluxes in substrates are dependent on dipole locations. As consistent with experiments, in the ITO micromesh device, a significant portion of enhanced radiation coupling to the substrate is into larger angles, that would not be out-coupled to air directly (due to total internal reflection) but could be effectively extracted by external schemes (like attaching extraction lens here). Similarly, Fig. 8(a) shows the x-z-plane cross sections of the power density flux from a z-direction (vertical) dipole in the conventional planar ITO device, while Figs. 8(b)-8(f) show the x-z-plane cross sections of the power density flux from a z-direction (vertical) dipole located at X1, X2, X3, X4, and X5 in the ITO micromesh device, respectively. Although the ITO micromesh and layer corrugation structures also help to couple some of confined radiation into the substrate, yet such effect is weaker than that for horizontal dipoles. This is presumbly because for the z-direction (vertical) dipole in a typical OLED layer structure, most of its radiation is coupled to SPP modes strongly confined at the metal interface (only a small portion is coupled to confined ITO/organic waveguiding modes, as can be seen from the difference between the guided radiation patterns in Figs. 7(a) and 8(a)) and thus are more difficult to extract with the present microstructure [23].
Fig. 7 The x-z-plane cross section of the power density flux (i.e., the Poynting vector magnitude) from a single-frequency (at the wavelength 525 nm) y-direction (horizontal) dipole located (a) in the conventional planar ITO device, (b) at X1 in the ITO micromesh device, (c) at X2 in the ITO micromesh device, (d) at X3 in the ITO micromesh device, (e) at X4 in the ITO micromesh device, (f) at X5 in the ITO micromesh device (moving from the center of the ITO top to the center of the mesh hole, as defined in Fig. 6(a).
Fig. 8 The x-z-plane cross section of the power density flux (i.e., the Poynting vector magnitude) from a single-frequency (at the wavelength 525 nm) z-direction (vertical) dipole located (a) in the conventional planar ITO device, (b) at X1 in the ITO micromesh device, (c) at X2 in the ITO micromesh device, (d) at X3 in the ITO micromesh device, (e) at X4 in the ITO micromesh device, (f) at X5 in the ITO micromesh device (moving from the center of the ITO top to the center of the mesh hole, as defined in Fig. 6(a).
By combining contributions from dipoles of different orientations (assuming a x-, y-, and z-direction dipole ratio of 0.38:0.38:0.24) and different frequencies (considering the full emission spectrum) at each location, Fig. 9 shows the calculated far-fied emission patterns in the substrate for emitters located at X1, X2, X3, X4, and X5 in the ITO micromesh device, compared with that for an emitter in the planar ITO device. As consistent with power density fluxes shown in Figs. 7(b)-7(f)), the emission patterns in the substrate for the ITO micromesh device exhibit the dipole-location dependence, such as the symmetry and the unsymmetry of emission patterns when dipoles are and are not located at the center of the ITO micromesh, respectively (or located and not located at the center of the mesh hole). As consistent with experiments, in the ITO micromesh device, a significant portion of enhanced radiation coupling to the substrate is into larger angles, that would not be out-coupled to air directly (due to total internal reflection) but could be effectively extracted by external schemes (like attaching extraction lens here). By summing emission patterns from emitters at different locations over one period of the structure (such as X1-X5 in Fig. 9 and contributions from the other half of the period) and by taking into account the effect of the extraction lens with the ray-tracing simulation, the calculated angular distributions of the EL intensity for the lens-attached planar ITO device and the lens-attached ITO micromesh device are also shown in Fig. 5(c), which agree well with measured results and confirm the effectiveness of the optical simulation in this work.
Fig. 9 Calculated far-fied emission patterns in the substrate for emitters located at X1, X2, X3, X4, and X5 in the ITO micromesh device, compared with that of an emitter in the planar ITO device. All emission patterns are normalized to their 0° intensity.
By carefully examining both Figs. 7(b)-7(f) and Figs. 8(b)-8(f), one finds that intensities of power density fluxes confined/guided in device layers exhibit a visible drop each time they encounter a structural change/bending in propagation (e.g., from the ITO region to the mesh hole region or vice versa), accompanied by some coupling of such otherwise confined/guided radiation into the substrate. This can be understandable with phenomena often seen in electromagnetic waveguides; when a waveguided radiation propagating in a wavegude sees an abrupt structural change or bending, a significant portion of the radiation might not be smoothly guided into the new structure and go on propagating, and instead be radiated as leaky/escapable waves [24]. This also explains the dipole-location dependence of power density flux patterns in substrates (and in device layers) such as the symmetry and the unsymmetry of power density flux patterns when dipoles are and are not located at the center of the ITO micromesh, respectively (similar for dipoles located and not located at the center of the mesh hole). The analyses here thereby provide a different but consistent viewpoint/insight (based on wave optics) of how the microstructured ITO electrodes (either in the form of ITO grids or with low-index grids on ITO) enhance optical out-coupling of OLEDs, compared to the viewpoint/insight provided by ray optics in previous works (i.e. re-direction of total-internal-reflection light into the escapable cone by the microstructures having index difference etc.) [15,16].
By combining contributions from dipoles of different orientations (assuming a x-, y-, and z-direction dipole ratio of 0.38:0.38:0.24) and different frequencies (considering the full emission spectrum) and by taking into account the effect of the extraction lens with the ray-tracing simulation, Table 2 shows the calculated coupling efficiencies of internally generated emission into air with the extraction lens for emitters in the ITO planar device and for emitters at different locations (X1 to X5) of the ITO micromesh device. These coupling efficiencies correspond to coupling efficiencies of internally generated emission into the substrate. As can be expected from results of Fig. 7, Fig. 8 and Fig. 9, with the ITO micromesh and layer corrugation structures, the coupling efficiency into the substrate is generally enhanced (54.1%-60.2% vs. 43.8%) no matter where the emitters are located. The enhancement is larger for emitters located on top of the ITO region (i.e., X1, X2, X3) than for emitters located in the mesh hole (no-ITO) region (i.e., X4, X5). Such difference may be associated with the difference in characteristics of waveguided modes induced in the region with ITO or in the region without ITO (i.e., mesh hole region). Figure 10 shows the calculated spatial distributions (along the z axis) of the field intensities (transverse electric field Ey) of the waveguided modes (transverse electric modes) for the planar glass/ITO/PEDOT:PSS/organic layers/metal structure (top panel) and the planar glass/PEDOT:PSS/organic layers/metal structure (bottom panel), at the wavelength of 525 nm. In the region with ITO, the waveguided radiation is distributed from the ITO layer to the OLED organic layers (due to their high refractive indices), while in the region without ITO (i.e., the region with only the low-index PEDOT:PSS electrode), the waveguided radiation is confined more in the OLED organic layers. The waveguided modes in the region with ITO and their stronger coupling into the substrate can be understood with the coupled mode theory [24]. The ITO structure neighboring the OLED organic waveguide forms a proximity waveguide and the interaction between the OLED organic and ITO waveguides helps to couple the waveguided radiation from the OLED organic waveguide into the ITO waveguide. Yet, due to discontinuity in the ITO waveguide, waveguided radiation in ITO sees stronger re-radiation into leaky/escapable waves (i.e., coupling into the substrate). On the other hand, for waveguided radiation induced by dipoles in the mesh hole (no ITO) region, although it also sees waveguide bending/change, yet the OLED organic waveguide is still continuous and thus the effect of re-radiation/out-coupling is weaker. The analyses here (based on wave optics) again provide different viewpoints and insights of how the microstructured ITO electrodes enhance optical out-coupling of OLEDs. It can take into account of the effects of different dipole orientations and positions and provide a comprehensive physical picture of out-coupling enhancement in microstructured OLEDs, that geometric optics has no way to provide.
Table 2. Calculated coupling efficiencies of internally generated radiation into air (with extraction lens) for emitters in the planar ITO device and for emitters at different locations in the ITO micromesh device.
Fig. 10 The spatial distributions (along the z axis) of the field intensities (transverse electric field Ey) of the waveguided modes (transverse electric modes) for the planar glass/ITO/PEDOT:PSS/organic layers/metal structure (top panel) and the planar glass/PEDOT:PSS/organic layers/metal structure (bottom panel), at the wavelength of 525 nm.
The measured EQE (46.8%) of the lens-atteched ITO micromesh device is lower than what expected from calculation (54.1%-60.2%). This discrepancy may be associated with the deviation of the internal quantum efficiency of the green phosphorescent OLED in this work from ideal 100%, since higher EQEs had been reported for similar planar green phosphorescent OLEDs and the optical out-coupling efficiency for the planar ITO device is calculated to be ~21%, higher than the experimentally obtained EQE of 16.9% here [25]. As an additional note, both the calculated optical coupling efficiencies into substrates and the experimental EQEs for the ITO micromesh devices currently obtained here are still lower than those for ITO nanomesh devices previously reported [11], presumbly due to weaker out-coupling of SPP modes and even waveguided modes into substrates in the current micromesh structure than in the nanomesh structure [11]. Nevertheless, the out-coupling enhancement gained is still attractive, and further optimization of the ITO micromesh structures (e.g., the ITO thickness, the duty ratio, OLED stacks etc.) may raise the efficiencies and make them closer to those of ITO nanomesh devices. In addition, the ITO micromesh devices in general can be more easily fabricated by the more readily available and more cost-effective microfabrication techniques and thus render them still attractive options for OLED light out-coupling.
Intriguingly, although the current ITO micormesh/PEDOT:PSS composite electrode is effective to enhance out-coupling of OLED internal emission, it hardly induces haze. Figure 11 depicts the transmittance spectra collected only along the normal direction and collected over all angles by an integrating sphere, for both the planar ITO electrode and the ITO micromesh/PEDOT:PSS composite electrode. The ITO micromesh/PEDOT:PSS composite electrode indeed exhibits a higher overall transmittance than the planar ITO electrode. Importantly, it indicates a low haze of only up to 1.1% at the wavelength 525 nm for the ITO micromesh/PEDOT:PSS composite electrode. Such low-haze chatateristics suggest that the current ITO micromesh/PEDOT:PSS composite electrode and OLED structure hold promise not only for lighting applications but also for display applications.
Fig. 11 The transmittance spectra collected only along the normal direction and collected over all angles by an integrating sphere, for both the planar ITO electrode and the ITO micromesh/PEDOT:PSS composite electrode.
4. Conclusion
In summary, the facile and convenient microsphere lithography was adopted to form micropatterned ITO meshes for fabricating efficiency-enhanced OLEDs with microstructured composite electrodes consisting of the high-index ITO micromesh and the low-index conducting polymer PEDOT:PSS. The rigorous electromagnetic wave simulation based on the three-dimensional finite-difference time-domain (FDTD) method is conducted to study optical properties and mechanisms in such devices. It provides a different but consistent viewpoint/insight of how this microstructured electrode enhances optical out-coupling of OLEDs, compared to that provided by ray optics simulation in previous works. Both experimental and simulation studies indicate such a microstructured electrode effectively enhances coupling of internal radiation into the substrate, compared to devices with the typical planar ITO electrode. By combining this internal extraction structure and the external extraction scheme (e.g. by attaching extraction lens) to further extract radiation into the substrate, a rather high EQE of 46.8% was achieved with state-of-the-art green phosphorescent OLEDs, clearly manifesting the high potential of such OLED structures for high-efficiency OLEDs.
Acknowledgments
The authors gratefully acknowledge the financial support from Ministry of Science and Technology of Taiwan.
References and links
1. S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, and K. Leo, “White organic light-emitting diodes with fluorescent tube efficiency,” Nature 459(7244), 234–238 (2009). [CrossRef] [PubMed]
2. B. W. D’Andrade and S. R. Forrest, “White organic light-emitting devices for solid-state lighting,” Adv. Mater. 16(18), 61–65 (2004). [CrossRef]
3. Y.-J. Lu, C.-H. Chang, C.-L. Lin, C.-C. Wu, H.-L. Hsu, L.-J. Chen, Y.-T. Lin, and R. Nishikawa, “Achieving three-peak white organic light-emitting devices using wavelength-selective mirror electrodes,” Appl. Phys. Lett. 92(12), 123303 (2008). [CrossRef]
4. C.-H. Chang, H.-C. Cheng, Y.-J. Lu, K.-C. Tien, H.-W. Lin, C.-L. Lin, C.-J. Yang, and C.-C. Wu, “Enhancing color gamut of white OLED displays by using microcavity green pixels,” Org. Electron. 11(2), 247–254 (2010). [CrossRef]
5. H. Sasabe and J. Kido, “Recent progress in phosphorescent organic light-emitting devices,” Eur. J. Org. Chem. 2013(34), 7653–7663 (2013). [CrossRef]
6. H. Uoyama, K. Goushi, K. Shizu, H. Nomura, and C. Adachi, “Highly efficient organic light-emitting diodes from delayed fluorescence,” Nature 492(7428), 234–238 (2012). [CrossRef] [PubMed]
7. C.-L. Lin, T.-Y. Cho, C.-H. Chang, and C.-C. Wu, “Enhancing light outcoupling of organic light-emitting devices by locating emitters around the second antinode of the reflective metal electrode,” Appl. Phys. Lett. 88(8), 081114 (2006). [CrossRef]
8. K. Neyts, “Microcavity effects and the outcoupling of light in displays and lighting applications based on thin emitting films,” Appl. Surf. Sci. 244(1), 517–523 (2005). [CrossRef]
9. Y. Sun and S. R. Forrest, “Organic light emitting devices with enhanced outcoupling via microlenses fabricated by imprint lithography,” J. Appl. Phys. 100(7), 073106 (2006). [CrossRef]
10. B. W. D’Andrade and J. J. Brown, “Organic light-emitting device luminaire for illumination applications,” Appl. Phys. Lett. 88(19), 192908 (2006). [CrossRef]
11. C.-Y. Chen, W.-K. Lee, Y.-J. Chen, C.-Y. Lu, H. Y. Lin, and C.-C. Wu, “Enhancing optical out-coupling of organic light-emitting devices with nanostructured composite electrodes consisting of indium tin oxide nanomesh and conducting polymer,” Adv. Mater. 27(33), 4883–4888 (2015). [CrossRef] [PubMed]
12. Y. R. Do, Y. C. Kim, Y. W. Song, C. O. Cho, H. Jeon, Y. J. Lee, S. H. Kim, and Y. H. Lee, “Enhanced light extraction from organic light-emitting diodes with 2D SiO2/SiNx photonic crystals,” Adv. Mater. 15(14), 1214–1218 (2003). [CrossRef]
13. J. Feng, T. Okamoto, and S. Kawata, “Highly directional emission via coupled surface-plasmon tunneling from electroluminescence in organic light-emitting devices,” Appl. Phys. Lett. 87(24), 241109 (2005). [CrossRef]
14. S. M. Jeong, F. Araoka, Y. Machida, Y. Takanishi, K. Ishikawa, H. Takezoe, S. Nishimura, and G. Suzaki, “Enhancement of light extraction from organic light-emitting diodes with two-dimensional hexagonally nanoimprinted periodic structures using sequential surface relief grating,” Jpn. J. Appl. Phys. 47(66R), 4566–4571 (2008). [CrossRef]
15. Y. Sun and S. R. Forrest, “Enhanced light out-coupling of organic light-emitting devices using embedded low-index grids,” Nat. Photonics 2(8), 483–487 (2008). [CrossRef]
16. T.-W. Koh, J.-M. Choi, S. Lee, and S. Yoo, “Optical outcoupling enhancement in organic light-emitting diodes: highly conductive polymer as a low-index layer on microstructured ITO electrodes,” Adv. Mater. 22(16), 1849–1853 (2010). [CrossRef] [PubMed]
17. Y.-H. Jhang, Y.-T. Tsai, C.-H. Tsai, S.-Y. Hsu, T.-W. Huang, C.-Y. Lu, M.-C. Chen, Y.-F. Chen, and C.-C. Wu, “Nanostructured platinum counter electrodes by self-assembled nanospheres for dye-sensitized solar cells,” Org. Electron. 13(10), 1865–1872 (2012). [CrossRef]
18. J.-G. Chen, H.-Y. Wei, and K.-C. Ho, “Using modified poly(3,4-ethylene dioxythiophene): Poly(styrene sulfonate) film as a counter electrode in dye-sensitized solar cells,” Sol. Energy Mater. Sol. Cells 91(15–16), 1472–1477 (2007). [CrossRef]
19. Y.-L. Wu, C.-Y. Chen, Y.-H. Huang, Y.-J. Lu, C.-H. Chou, and C.-C. Wu, “Highly efficient tandem organic light-emitting devices utilizing the connecting structure based on n-doped electron-transport layer/HATCN/hole-transport layer,” Appl. Opt. 53(22), E1–E6 (2014). [CrossRef] [PubMed]
20. C. F. Madigan, M.-H. Lu, and J. C. Sturm, “Improvement of output coupling efficiency of organic light-emitting diodes by backside substrate modification,” Appl. Phys. Lett. 76(13), 1650–1652 (2000). [CrossRef]
21. K.-D. Chang, C.-Y. Li, J.-W. Pan, and K.-Y. Cheng, “A hybrid simulated method for analyzing the optical efficiency of a head-mounted display with a quasi-crystal OLED panel,” Opt. Express 22(S2Suppl 2), A567–A576 (2014). [CrossRef] [PubMed]
22. M. Bahl, G.-R. Zhou, E. Heller, W. Cassarly, M. Jiang, R. Scarmozzino, and G. G. Gregory, “Optical simulations of organic light-emitting diodes through a combination of rigorous electromagnetic solvers and Monte Carlo ray-tracing methods,” Proc. SPIE 2014, 919009 (2014).
23. W. Brütting, J. Frischeisen, T. D. Schmidt, B. J. Scholz, and C. Mayr, “Device efficiency of organic light-emitting diodes: Progress by improved light outcoupling,” Phys. Status Solidi., A Appl. Mater. Sci. 210(1), 44–65 (2013). [CrossRef]
24. R. G. Hunsperger, Integrated Optics: Theory and Technology (Springer-Verlag, 2009).
25. Y.-S. Park, S. Lee, K.-H. Kim, S.-Y. Kim, J.-H. Lee, and J.-J. Kim, “Exciplex-forming co-host for organic light-emitting diodes with ultimate efficiency,” Adv. Funct. Mater. 23(39), 4914–4920 (2013). [CrossRef]
