Nano-honeycomb structured transparent electrode for enhanced light extraction from organic light-emitting diodes

Although organic light-emitting diodes (OLEDs) have made a great progress in the past quarter century, their external quantum efficiency (EQE) is still limited to about 30% especially for indium tin oxide (ITO) anode based bottom-emission devices.^1,2 In order to obtain higher EQE in OLEDs, high internal quantum efficiency (IQE), balanced hole–electron recombination, and enhanced light extraction should be achieved.^3,4 Beneficial from the fast development of organic semiconducting materials, 100% IQE has been reached by using phosphorescent and/or thermally activated delayed fluorescent materials.^1,2,5–10 Recently, OLEDs with more than 30% EQE, which is very close to the theoretical limit of the optical model for conventional multilayer bottom-emission OLEDs, were reported by several groups.^3,11–16 Nevertheless, developing device architectures with enhanced light extraction is still necessary for achieving higher EQE of OLEDs. The light extraction efficiency in OLEDs is mainly restricted by the light loss mechanisms including substrate mode, ITO/organic waveguide mode, and surface plasmon-polariton (SPP) mode.^17–22 Many architectures, such as introducing microlens arrays on the substrate back to extract the substrate mode^23–25 and incorporating periodic/aperiodic nano-structures inside the device to extract the ITO/organic waveguide mode and SPP mode,^26–32 have been used in OLEDs. However, the techniques are complicated and not cost effective.

Recently, our group reported an efficient light extraction technique for organic electronics based on the nanosphere lithography.^33,34 All the functional layers in the devices can form a honeycomb structure with a large scale on the planar ITO surface. The honeycomb-structured OLEDs demonstrated over two-fold enhancements in both current efficiency and power efficiency due to the effective scattering of trapped SPP mode. However, interfacial contamination caused by the air exposure during the lithography process of the hole-injecting layer would be difficult to avoid. In addition, a more complicated fabrication procedure than the conventional one has to be applied. In this letter, instead of patterning the hole-injecting layer first, we modified the nanosphere lithography technique to pattern the ITO substrate directly without contaminating the functional layers in the devices and without changing the fabrication processes. As a result, light extraction enhancement greater than 2 fold can still be achievable for both the current efficiency and the power efficiency in the OLEDs.

The ITO-coated glass substrates with the ITO thickness of about 200 nm and with a sheet resistance of 10 Ω/sq were first cleaned with acetone, ethanol, and deionized water for 15 min, followed by a UV-ozone treatment for 15 min. Nano-honeycomb structured ITO substrates were then fabricated based on a lithography technique of monolayer colloidal crystal pattern of polystyrene spheres. Finally, red phosphorescent OLEDs were fabricated on the nano-structured ITO substrates with a conventional thermal evaporation method. MoO₃, 1,3,5-Triazo-2,4,6-triphos-phorine-2,2,4,4,6,6-tetrachloride (TAPC), tris(N-carbazolyl) triphenyl-amine (TCTA), Iridium (III) bis(2-methyldibenzo-[f,h]quinoxaline)(acetylacetonate) (Ir(MDQ)₂(acac)), 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene(TmPyPB) were used to form the hole-injecting layer, the hole-transporting layer, the light-emitting layer as a host, the light-emitting layer as a dopant, and the electron-transporting layer, respectively. The device has a structure of ITO|MoO₃ (10 nm)|TAPC (45 nm)|TCTA: 4 wt. % Ir(MDQ)₂(acac) (15 nm)|TmPyPB (40 nm)|Liq (2.5 nm)|Al. The surface morphology was characterized by field-emission scanning electron microscopy (FE-SEM, Quanta 200 FEG, FEI Co.) and atomic force microscopy (AFM, MultiMode V, Veeco Instruments Inc.). The device characteristics were measured using a programmable Keithley model 2400 power source combining with a PhotoReasearch PR-655 spectrometer.

Figure 1 shows a schematic fabrication process of nano-structured ITO based OLEDs. A monolayer colloidal crystal of polystyrene spheres was first self-assembled on the ITO-coated substrate (Fig. 1(a)). The detail of self-assembling method was discussed elsewhere.^33–36 After drying with a hot plate at 50 °C for 2 h, the tetraethyl orthosilicate (TEOS) solution was spin-coated onto the substrate at a slow speed of 1000 rpm for 9 s and a high speed of 7000 rpm for 60 s. The TEOS solution was made of tetraethyl orthosilicate (98%), hydrochloric acid (HCl, 0.1 M), and ethanol (analytical reagent) with the volume concentration of 1:1:10 through the sol-gel method. The mixture was stirred at room temperature for about 6 h prior to use. TEOS can be easily converted into silicon dioxide through the following hydrolysis reaction

HCl serves as a catalyst for boosting the conversion rate. A composite film of polystyrene spheres embedded inside the SiO₂ film was achieved (Fig. 1(b)) after drying the substrate at 70 °C (2 h, in air). Then, the substrate was transferred into a vacuum chamber (Plasmalab 80plus RIE) for the reactive ion etching (RIE). The polystyrene and SiO₂ were chosen because of the fact that they are sensitive to different gas plasma in the ion-reaction etching process. Ar and CHF₃ gases were used to remove the SiO₂ above the polystyrene spheres (50 W, 4 min). Subsequently, the polystyrene spheres were removed through Ar and O₂ plasma (150 W, 3 min) and a film with ordered nano-bowl arrays (Fig. 1(c)) was obtained. Noticeably, there was a small hole in every nano-bowl because the contact of the polystyrene spheres and ITO layer prevented the immersion of the TEOS sol-gel, which makes it possible to etch the ITO layer through a chemical reaction by utilizing this perforated SiO₂ film as the template. The piranha solution prepared by concentrated sulfuric acid (guarantee reagent) and hydrogen peroxide (30%) with the volume ratio of 7:3 was used for etching. The piranha solution was chosen without specific purpose and other acidic solution (like the aqua regia or the hydrochloric acid) may work in this process as well. The ITO/perforated SiO₂ substrates were immersed into the heated piranha solution (100 °C) for about 150 s. Afterwards, the washed substrates were moved back into the RIE chamber for removing the residual SiO₂ on the ITO glass. Finally, the ITO substrates with the ordered pit arrays were achieved and ready for fabricating nano-structured OLEDs.

Figure 2(a) presents the SEM surface morphology of the perforated SiO₂ film on the ITO-coated glass substrate. As the gaps among the polystyrene spheres were filled by the TEOS sol-gel, the SiO₂ film possesses ordered nano-bowl arrays which were occupied by the spheres. Since the contact between polystyrene spheres and ITO prevented the immersion of the TEOS sol-gel, there was a small aperture at the bottom of each bowl. After immersing, the substrate into the piranha solution, ITO was etched in isotropy through the ordered apertures by the acid leaving a nano-honeycomb structured ITO layer (Figs. 2(b) and 2(d)). The depth and diameter of the pits in the ITO layer can be adjusted by the etching time, while the distance between the adjacent dip centers is determined by the period of the initial monolayer colloidal crystal pattern (or the size of the polystyrene spheres). Deeper pits would generate stronger diffraction effect of the nano-structure but higher probability of short-circuit in the OLEDs. There is a trade-off between the diffraction effect and the short-circuit issue. The optimized dip depth is in the range of 40–60 nm in the present study. Figures 2(c) and 2(e) give the morphology of the Al cathode and the cross-section view of an accomplished OLED. Thermal evaporation method makes it very easy to repeat the nano-structure in all deposited layers. The layer by layer nano-structure would result in efficient light extraction from the whole OLED device. Figure 2(d) shows the 3D AFM image of the nano-structured ITO with a scan area of 10 × 10 μm². The actual size of the nano-structured ITO substrate we fabricated was 32 × 32 mm². And larger size over 4 in. wafer scale can be also realized easily. The inset of Fig. 2(d) is the corresponding reciprocal space patterns after the fast Fourier transform of the AFM image. A bright circle which is composed of many bright reciprocal-lattice points is observed. It indicates that there were several domains in this nano-structure. The domains have the same magnitude of basis vector or periodicity, but different crystal orientations. Since the crystal orientations of the domains are randomly distributed without preferred orientations in the whole structure, it is possible to realize a homogeneous light extraction from the loss modes at all azimuthal angles.

To demonstrate how the nano-structure works in OLEDs for the light extraction, the phosphorescent OLEDs based on a red emitter Ir(MDQ)₂(acac) were fabricated. Figure 3 depicts the average performance of OLEDs deposited on the nano-structured ITO. The device performance of a planar OLED with the same device structure as that of the nano-structured one is also presented for comparison. The current density versus voltage curves are plotted in Fig. 3(a). The nano-structured OLED shows much better charge transport capability than the planar one. It is attributed to the enhanced local electrical field caused by curved surfaces in the nano-structured OLEDs.^27,32 OLEDs based on the nano-honeycomb structured ITO with a 360 nm periodicity has a scattering SPP peak at about 625 nm and two scattering waveguide peaks at around 475 and 570 nm according to our previous work.³³ And the scattering of the waveguide mode is less effective compared with the scattering of the SPP mode, because the refractive-index contrast at the ITO/organic interface is very small (only about 0.1). Therefore, the extraction of trapped SPP mode is dominated in this work and red phosphorescent dopant of Ir(MDQ)₂(acac) was chosen as the emitter since its emissive peak is close to the SPP resonant peak. This was also verified from the electroluminescence (EL) spectra in which the maximum monochrome enhancement locates at around 600 nm (Fig. 3(b)). High enhancements of current efficiency and power efficiency with more than two folds were achieved as well. The maximum current efficiency is increased from 30.65 to 62.3 cd/A and the maximum power efficiency is improved from 27.92 to 60.57 lm/W. Figure 3(d) presents the angular distribution of the radiative intensity from planar and nano-structured OLEDs. The nano-structure made the radiation focusing in the small emissive angle region compared to the standard Lambertian distribution.

In summary, we have demonstrated a universal nano-sphere lithography method to fabricate the nano-structures on the ITO substrate. The perforated SiO₂ film made from the monolayer colloidal crystal of polystyrene spheres and the tetraethyl orthosilicate sol-gel was used as the template. Ordered pits on the ITO layer surface were realized after chemical etching. The proposed method can be utilized to form large-area nano-structured ITO electrode with low cost. More than two folds' enhancement in both current efficiency and power efficiency were achieved in the red phosphorescent OLEDs which were deposited on the nano-structured ITO substrate. Higher enhancements could be achieved through fabricating the glass/ITO interface into the nano-honeycomb structure as it has larger refractive-index contrast of about 0.3 which benefits for the waveguide scattering. As nano-structures like gratings^37,38 and inverted moth-eye structure³⁹ were used to improve the performance of the organic solar cells as well, our proposed method for fabricating nano-honeycomb structured transparent electrode would also be very promising to manipulate light in the other optoelectronic devices.

We acknowledge financial support from the Natural Science Foundation of China (Nos. 61177016 and 61307036) and from the Natural Science Foundation of Jiangsu Province (No. BK20130288). This project is also funded by the Collaborative Innovation Center of Suzhou Nano Science and Technology and by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).