White Organic Light-Emitting Diode With LinearlyPolarized Emission
Ming-Yi Lin, Hung-Hsin Chen, Ku-Hui Hsu, Yi-Hsiang Huang, Yi-Jiun Chen, Hoang Yan Lin, Yang-Kai Wu,Lon A. Wang, Chung-Chih Wu, and Si-Chen Lee, Fellow, IEEE
Abstract— An organic light emitting diode (OLED) with alinearly polarized white light emission is demonstrated usinga nanograting structure. The aluminum based grating structureis fabricated by laser interference lithography and formed on theback side of the glass substrate of the OLED. The nanogratingstructure functions as a polarizer to select the transverse mag-netic wave in the wavelength range of 400–700 nm. The polariza-tion characteristics are studied experimentally and theoreticallyin detail. The experimental results agree well with the simulationby a rigorous coupled wave analysis. The polarization ratio ofthe polarizer can reach as high as 93.4%.
ORGANIC light emitting diodes (OLEDs) have been thesubjects of intensive studies due to their great potential
for display applications. The outstanding characteristics ofOLEDs for displays include high efficiency, fast response,wide viewing angle, light weight, flexibility, and potentiallylow cost. In addition, over the past decade, the OLED displaytechnology has made rapid progress, and plenty of types ofOLED displays have been demonstrated [1]–[3]. However,in order to use OLEDs as a three-dimensional (3D) displayor a general light source for liquid crystal displays (LCDs),the polarization ratio of the emitted light is very critical.For example, in a 3D display system, the polarization ratioof the emitted light determines the cross talk between twoorthogonally polarized light sources and 3D image quality [4].To achieve higher polarization ratio, several efforts have beenreported [5]–[11]. One of the most common methods is to use
Manuscript received February 3, 2013; revised March 21, 2013; acceptedMay 10, 2013. Date of publication May 17, 2013; date of current versionJune 26, 2013. This work was supported in part by the National Science Coun-cil of the Republic of China, in part by the Center for Emerging Materials andAdvanced Devices, and in part by the Photonic Advanced Research Center ofthe National Taiwan University, under Contract NSC 100-2120-M-002-014-,Contract 10R80908-4, and Contract 10R7b07-4.
H.-H. Chen is with the Graduate Institute of Electronics Engi-neering, National Taiwan University, Taipei 10617, Taiwan (e-mail:[email protected]).
Color versions of one or more of the figures in this letter are availableonline at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2013.2263789
Fig. 1. Scanning electron microscopy (SEM) images of (a) photo-resistpattern formed by laser interference lithography on top of the ARC layer.(b) The patterned photo-resist on top of the glass substrate after RIEprocess, and (c) Al grating with a period of 262 nm and the line width of127 nm. The image inserted in Fig. 1(c) is the corresponding atomic forcemicroscopy (AFM) spectra. (d) The transmission of the nanograting withvarious thicknesses of the Al layer, using a fixed period of 262 nm and afixed line width of 127 nm.
LED as the light source and fabricate the metal grating bye-beam lithography [7]–[9]. Recently, the blue light emittingdiodes grown by metal organic vapor phase epitaxy (MOVPE)[12] had been fabricated with high polarization ratio (∼96%).In this letter, simple laser interference lithography was appliedto form a nanograting structure on the glass substrate, then alinearly polarized white light OLED was fabricated by thesolution process with an evaporated electrode on the back sideof the glass substrate. The nanograting structure selects thelinearly polarized light (TM wave) in the wavelength rangeof 400–700 nm to pass through. The emission spectra of theOLED through the nanograting were measured and matchedusing the rigorous coupled wave analysis (RCWA).
II. EXPERIMENTS
Figure 1(a) shows the scanning electron microscope (SEM)image of the photo-resist pattern formed by laser interfer-ence lithography on an anti-reflective coating (ARC) layer.The ARC (XHRiC-11, Brewer Science) layer was depositedto prevent the reflected light from damaging the photo-resistpattern. A laser source with wavelength of 325 nm was splitinto two pathways and eventually merged and interfered onthe photo-resist and thereby generated the grating pattern.Then the photo-resist pattern and AR-coating were etched bythe reactive ion etching (RIE) process and shown in Fig. 1(b).
Fig. 2. (a) Schematic diagrams of the OLED structures. (b) Schematicillustration of the setup for measuring the polarized spectral intensity of theOLED.
To estimate the grating thickness effect, we simulated thetransmission of the sample which varies with different thick-nesses of Al as shown in Fig. 1(d). The transmission wouldbe enhanced at the wavelength of 600 nm as we increased thethickness of the Al layer from 0 nm to 150 nm. However, thethickness of the photoresist limited the maximum thickness ofthe Al layer. It was hard to lift off the metal when the thicknessof metal was above 120 nm. Therefore, an aluminum layerwith thickness of 100 nm was evaporated and lifted-off on thesample and finally the metallic nanograting structure with aperiod of 262 nm and the line width of 127 nm on the glasssubstrate is completed as shown in Fig. 1(c).
The transmission spectrum of the grating on the glasssubstrate was measured by a UV/Visible spectrophotometer(SHIMADZU UV-1650P). Then, the OLED was deposited onthe flat side of the glass to avoid causing cracks in the devicesby the grating. The layer structure of the OLED consisted ofglass substrate/ indium-tin-oxide (ITO) (70 nm)/PEDOT:PSS(∼40 nm)/ polyfluorene (PF) (∼200 nm) /LiF (0.5 nm)/Al(150 nm) as shown in Fig. 2(a). The ITO layer was depositedon the planar surface of the glass by sputtering and fol-lowed by spin coating the poly(3,4-ethylenedioxythiophene)–polystyrenesulfonic acid (PEDOT:PSS) layer and annealing at120 °C for 10 minutes to dry. Then, the PF layer was spin coatand annealed at 120 °C for 20 minutes. The LiF layer and Allayer were deposited by evaporation. Fig. 2(b) displays thesetup for measuring the polarization and the spectral intensityof the OLEDs. The gratings are along x axis, the angle �is defined to be the angle between the polarizer axis and theperpendicular direction to the grating (y axis).
III. RESULTS AND DISCUSSION
The black line shown in Fig. 3(a) is the measured transmis-sion spectrum of the nanograting structure on the glass sub-strate without the polarizer. The red line is the simulation resultby using rigorous coupled wave analysis (RCWA). The diparound 410 nm is caused by Wood’s anomaly which is relatedto the grating period and the effective refraction index of thegrating [13]. The peak around 630 nm is due to the lamp, andcan be ignored. The rest are fitted quite well. Fig. 3(b) displaysthe transmission spectra of the nanograting structure through
Fig. 3. (a) The experimental and simulation results of transmission spectra ofthe nanograting structure on the glass substrate. (b) The transmission spectraof the nanograting structure with a rotating linear polarizer. The image insertedin Fig. 3(a) is the top view of the nanograting structure.
Fig. 4. (a) The EL spectra of the OLEDs with and without annealing process.(b) The spectral intensity of OLED as the polarizer rotates at an angle �.
the polarizer at an angle � defined above. The transmissiondecreases as the polarizer rotates from � = 0 to 90 degrees inthe wavelength range of 400–700 nm, revealing that the lighttransmitted through the grating is linearly polarized in a broadwavelength band. The polarization ratio of the transmitted lightcan be calculated by integrating the area below the red (ETM)and blue lines (ETE). The polarization ratio is defined as(ETM−ETE)/ (ETM+ETE) [14].
Figure 4(a) shows the EL spectrum of the OLEDs with andwithout the annealing process. The device without annealingemits strongly in the blue region which has been observedin many reports [16], [17]. To achieve the white emissionspectrum, the PF layer of the device was annealed at 120 oC for20 minutes. The white light emission is due to the aggregationof PDOFO [17]. The white light spectra composed of 432 and490 nm emission peaks, agree with previous reports [16], [18].Fig. 4(b) displays the spectral emission intensity of the OLEDat various polarizer angles. The intensity also decreases as �increases from 0 to 90 degrees. The polarization ratio of thewhite OLED can achieve 93.4%.
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IV. CONCLUSION
In conclusion, the Al grating structure with a periodof 262 nm and the line width of 127 nm was fabricatedsuccessfully by using simple laser interference lithography.When applying this technology to OLEDs, we successfullyfabricated a white light OLED with linearly polarized emissionin the wavelength range of 400–700 nm. The polarization ratiowas as high as 93.4%.
This device provides a promising candidate for the basicemission unit of future 3D displays.
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