Efficiency enhancement of flexible organic light-emitting devices by using antireflection nanopillars

We present an antireflection structure consisted of irregular nanopillars to increase light extraction efficiency of flexible organic light-emitting devices. The nanopillars were made by imprinting the anodized aluminum oxide on polycarbonate substrates. The thermal viscosity effect formed the nanopillars with tapered shapes. Such nanopillars show excellent antireflection properties for a wide range of incident angles and wavelengths. The normal transmittance was improved from 85.5% to 95.9% for 150-nm-height nanopillars. The transmittance was greatly improved from 52.8% to 89.1% at 60° incident angle. With this antireflection structure, the device efficiency was improved 69% as compared to devices with flat substrates. Due to wide-angle antireflection, the image contrast ratio was also significantly improved . ©2011 Optical Society of America OCIS codes: (220.4241) Nanostructure fabrication; (230.3670) Light-emitting devices.


Introduction
Organic light-emitting device (OLED) is one of the most promising technology for flat panel display due to many attractive advantages, such as a great range of colors, high brightness, large viewing angle, and self emission [1].By using plastic substrates, organic layers of OLEDs can be thinner, lighter and more flexible than crystalline layers in commercial displays such as LEDs or LCDs.However, the refractive index difference between air and the plastic substrate not only confines the emissive photons in the substrate but also depresses the image contrast ratio of display panel.For increasing the external quantum efficiency, many methods have been proposed, such as shaped substrates, scattering surface, surface roughening technique, and photonic crystals [2][3][4][5][6][7][8][9][10][11].For increasing image contrast ratio, a circular polarizer film attached to the OLED is commonly used.However, it results in an extra cost and fabrication complexity [12].Some other ways, such as fabrication of absorbing film for ambient light or micro cavity for destructive interference [13][14][15][16][17][18][19][20][21], have also been presented.These extra components reduce the optical intensity.It is often a trade-off between the high contrast ratio and external quantum efficiency.
In recent years, implementing an anti-reflection (AR) layer to solve the trade-off problem had been reported [22].In this method, a single MgF 2 layer was deposited on the glass substrate with a thickness of λ/4 which destructed the interference of reflected light from the substrate.It also minimized the ambient reflection and helped in coupling confined photons our of devices.However, the AR effect is just available for a very limited wavelength range and for an incident angle close to normal direction.These drawbacks can be overcome by using graded index AR layers as mathematically demonstrated by Rayleigh in 1880.In principle, the gradual change of refractive index is achieved with two approaches including multilayered coating and tapered morphology [23][24][25][26].For tapered morphology, the porosity of a layer should be graded in a controlled manner and has a characteristic dimension which is much smaller than the wavelengths of incident light.In this work, we used the nanoimprint method to make such tapered subwavelength structures on flexible polycarbonate substrates.Irregular tapered nanopillars with a graded index layer between polycarbonate (n=1.57) and air (n=1) were successfully made.This antireflection layer not only improved the extraction efficiency by destroying the total internal reflection in the substrate, but also achieved a high image-contrast ratio due to the decrease of reflection from ambient light.

Experimental methods
The device fabrication included two major parts, thermal nanoimprint and conventional polymer LED (PLED) process.The nanoimprint process for the device's substrate is shown in Fig. 1.The imprinted nanostructure was made on the bottom side of polycarbonate (PC) film.The PC film was heated in the steel chamber with both top and bottom hot plates at a temperature higher than the glass transition temperature of polycarbonate.The heated PC film was imprinted by an anodized aluminum oxide (AAO) template to form the antireflection nanopillars.To have an uniform air pressure on the PC film, a thin PET film was placed on the PC film.The air pressure in the chamber was increased from 5 to 25 kg/cm 2 so that the nanopores in the AAO were filled with molten polycarbonate.After several minutes of imprinting, the whole system was cooled down to room temperature and the high chamber pressure was released.Tapered nanopillars were formed on the PC film surface after the removal of the AAO template.The nanopore diameters of the AAO templates used in this study were 100 nm and 200 nm which were much smaller than visible wavelength.The SEM graphs of imprinted results were depicted in the left images of Fig. 2. We applied different air pressures and temperatures in the nanoimprint system.The higher imprinting temperature and air pressure resulted in higher nanopillars.The average height of the nanopillars was observed to be linearly increased with the working pressure.The height increased more rapidly as the imprinting temperature increased.The SEM images cannot indicate the morphology of the nanopillars, an AFM was applied to map shapes of nanopillars.Figures 2(c) show the AFM images of the nanopillars under different fabrication conditions.It can be seen that the nanopillars did not follow the cylindrical profiles of the AAO nanopoles.They formed tapered shape with irregular distribution.We deduced that the tapered nanopillars came from the thermal viscosity of the PC film near the melting temperature.The irregular pattern with tapered shapes make these nanopillars a good antireflection layer.These patterned PC films were used as substrates of PLEDs. Figure 3 shows the device fabrication process.We first evaporated a thin silver film of 20 nm on back side of the imprinted PC film.The silver thin film was acted as a semi-transparent anode.Then the surface treatment of UV-O 3 plasma was applied to clean PC surface and increase the work function.The hole injection layer, Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS), and emission layer, polyfluorene derivative Green B were spun-coated onto this anode sequentially.After organic layer deposition, Cesium Carbonate (Cs 2 CO 3 ) and aluminum (Al) were evaporated by thermal evaporation and acted as the opaque cathode.Figure 3(b) shows the layout of the PLED. Figure 3(c) shows the emission of the flexible green PLED which the CIE coordinate was at (0.29, 0.67).

Results and discussion
To check AR properties of the tapered nanopillar structures, we measured the reflection spectra of PC films with and without nanostructures.Figure 4(a) shows the spectra.The dimension of nanopillars was 200 nm and the average height was 153.81 nm.The referenced light was incident on the sample at the normal direction, and the reflected light was measured by an integral sphere.The reflectance increased with the wavelength.Nevertheless, the reflectance was smaller than 5% in 450nm -800nm wavelength range.The average reflectance of visible wavelength was effectively depressed from 10.2% to 4.0%.The AR structure also increased the optical transmission.Figure 4(b) shows the measured transmittance for different incident angles.The average transmittance was increased from 85.5% to 95.9% for 0° incidence.From 0° to 50° incident angles, there was a flat transmission with efficiencies higher than 90%.As compared with the unpatterned substrate, the transmission was greatly improved from 52.8% to 89.1% for 60° incidence.It is noted that conventional AR coatings are limited for a small wavelength range and for an incident angle close to normal direction.By classical ray optics, it is predicted that more than 100% photons is confined in the substrates of light emitting devices due to total internal reflection [27].The improvement of transmittance at large incident angle is more important for reducing total internal reflection and enhancing the external quantum efficiency.Therefore, the AR layer consisted of tapered nanopillars is better than conventional AR coating.Figure 5 (a) shows the measured efficiency enhancement of PLEDs with tapered nanopillars under different fabrication conditions.The light enhancement ratio was found linearly increased with of the average height of the nanopillars.For 100 nm and 200 nm diameters of the AAO template, the light extraction enhancement was almost the same.It indicates that the enhancement was not due to the scattering of the nanopillars.The enhancement was indeed attributed to the AR property of the tapered nanopillars.The higher nanopillars resulted in a smoother change of refractive index profile at the interface.A higher nanopillar has a better AR property and a larger improvement of light extraction efficiency.The enhancement ratio of 69% was achieved when the height of the imprinted nanopillars was 153.8 nm.It is noted that the emission light did not show any critical spectrum shift.The peak wavelength was the same at 528 nm as shown in the Fig. 6(b).The nanopillars have a broadband antireflection behavior as indicated in Figs. 5. Hence, the transmission spectra is not modified by the these nanostructures.There is no distortion in the emitting color.The irregular tapered nanopillars on to device not only helps in extracting light at large incident angles but also improving the image contrast.To check the contrast ratio, we analyzed the images with and without the nanopillar structures.Figure 6 shows the picture took by a CCD camera under bright light illumination.The light source was set at the top front of the paper as depicted.Obviously, the region covered with nanopillars surface was clearer and sharper than the flat surface.The nanopillars were acted as an AR layer.We used a Matlab program to analyze the image contrasts for the patterned PC film and flat PC film.From the table inside Fig. 6, we can observe brighter white part (with AR surface: 153.4 a.u.; flat surface: 141.0 a.u.) and darker black part (with AR surface: 15.6 a.u.; flat surface: 27.2 a.u.) in the region covered with AR nanopillars.The ANSI contrast ratio (intensity of white side divided by black side) was 9.81 for the pattern film and 5.18 for the flat film.By depressing the ambient reflection, the contrast ratio was greatly improved.

Conclusions
We demonstrated a simple and inexpensive method to depress the ambient reflection and improve the light extraction for flexible PLED device.With the nanoimprinted irregular and tapered nanopillars on the PC film, a high-performance AR layer was made.The AR layer simultaneously increased the image contrast ratio and light extraction efficiency.In our experiment, the ANSI contrast ratio was improved ~89% and the device efficiency was increased up to ~70% as compared to the flat substrate.

Fig. 3 .
Fig. 3. (a) Thermal nanoimprint process with AAO template, (b) the device structure, and (c) the SEM graphs of AAO template and patterned PC thin film, and the picture of the flexible device.

Fig. 5 .
Fig. 5. Enhancement ratio (a) and emission spectra (b) of devices with the antireflection substrate (dimension: 200 nm, average height: 153.81 nm) compared to the flat substrate

Fig. 6 .
Fig. 6.Picture of a paper covered with the PC film under bright light source and the diagram of the experimental setup