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This work demonstrates the enhancement of light extraction of polymer light-emitting diodes (PLEDs) by incorporating a 12-fold photonic quasi crystal (PQC) in the device structure. Multi-exposure two-beam interference technique combined with inductively coupled plasma etching was employed to pattern a 12-fold PQC structure on the ITO film on a glass substrate of the diode. The air-hole coverage (AHC) and etching depth dependences of the light emitting performance of the 12-fold PQC patterned PLEDs were investigated. For AHC within the range between 6.4% and 32.3%, a nearly constant enhancement of the luminance efficiency of the PQC PLEDs was observed. On the other hand, the light emitting performance of the PQC PLEDs is very sensitive to the etching depth. The photoluminescence intensity of the PQC PLEDs increases monotonically with the etching depth. In contrast, the electro luminance efficiency shows a non-monotonic dependence on etching depth with a maximum occurring at 55 nm etching depth. The maximum improvement of luminance efficiency of the 12-fold PQC PLEDs reaches nearly 95% compared with an un-patterned PLED at an injection current of 110 mA.
© 2013 Optical Society of America
1. Introduction
Organic/polymer light-emitting diodes (OLEDs/PLEDs) have attracted enormous attention due to their potential applications in flat panel displays and illuminators [1–4]. The advantages of OLEDs/PLEDs are high brightness, fast response time, good color purity, low power consumption, and self-emissive. Furthermore, they can be fabricated on flexible substrates by low cost roll-to-roll or ink-jet printing process [5,6]. A conventional OLED/PLED device structure consists of multi-layer thin films. Starting from the glass substrate, it includes a transparent anode, a hole transport layer, a light emitting layer, and a metallic cathode. The best internal quantum efficiency reported for OLED/PLED device has reached to nearly 100% efficiency [7]. However, the light extraction efficiency is still limited to less than 20% [8]. The poor extraction efficiency results from the confinement of emitted photons inside the device due to total internal reflection at interfaces in the device. About 30% of emitted photons are trapped in the glass substrate and another 50% in the light emitting/anode layers, which are denoted as a glass mode and a waveguide mode, respectively [8].
To improve the low extraction efficiency problem, several methods [8–15] have been proposed in literatures, for example: texturing the substrate [9,10], adding spherically shaped structures or microlens arrays on the backside of the substrate [8,11], and introducing a two-dimensional self-assembled nanoparticle diffraction layer [12] or a photonic crystal (PC) structure [13–15] into a OLED/PLED device. Among them, embedding PC structures into OLED/PLED device configuration is a promising method because the periodic modulation of refractive index converts guided waves to external leaky waves, thus enhances the light extraction efficiency [13,15].
Photonic quasi crystals (PQCs) [16–20], composed of quasi-periodic structures with high degree of rotational symmetry and no translational symmetry, have been reported to possess interesting optical properties [18,19] such as more isotropic photonic bandgap and scattering of light. Recently, it has been demonstrated that PQCs are useful for light extraction of GaN LEDs [21], and sometimes even more effective than triangular PC [22]. Until now, there are still limited reports to demonstrate experimentally that a PQC structure can be used to enhance light extraction efficiency of an OLED/PLED. Additionally, it is important to investigate the influence of the PQC parameters on the performance of an OLED/PLED.
Various techniques have been proposed to fabricate PQCs, for example, electron beam lithography [23], holographic lithography (HL) [19,20,24,25], etc. In particular, HL is a very promising technique to fabricate large area PQCs because it is simple, low cost and feasible for mass production [19,20,24,25]. Multi-beam interference technique is a commonly adopted HL method to fabricate PQCs [19,20,24]. However, it needs precise and complicated optical setup for multi-beam interference. In contrast, multi-exposure two-beam interference technique is more feasible for PQCs fabrication [25] due to its easy arrangement for beam interference and high interference contrast [26].
This work demonstrates the enhancement of light extraction efficiency of PLEDs by incorporating in the device structure a 2D 12-fold PQC produced with multi-exposure two-beam interference technique and inductively coupled plasma (ICP) etching. In addition, the effects of air-hole coverage (AHC) and etching depth of the PQC on the light emitting performance of the PQC PLEDs are investigated.
2. Experiment
Figure 1(a) shows the design of a PLED embedded with a 2D 12-fold PQC structure. From the top to the bottom are glass substrate, indium tin oxide (ITO) layer patterned with the PQC structure, poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) layer, polyfluorene (PFO) layer, calcium (Ca) layer, and aluminum (Al) layer. To pattern the PQC structure on the ITO layer, multi-exposure two-beam interference technique [26] was employed. A SU-8 negative photoresist (Microlithography Chemical Corp.) layer was first spun-cast on the top of the ITO glass substrate. A He-Cd laser beam with 325 nm wavelength was split into two beams with their mutual angle fixed at 34° to interfere with each other, forming a 1D grating pattern on the SU8 film with a period of 550 nm, close to the emission peak of the active layer shown below. It has been reported that the maximum light extraction efficiency in a PC OLED occurs when the lattice constant of the PC matches the emission wavelength [13]. This is the reason for patterning the PQC structure with 1D interference pattern with the period of 550 nm. Furthermore, it is larger than the cutoff lattice constant, which is estimated to be 200 nm for an OLED [13]. Below the cutoff value, leaky waves remain trapped in the glass substrate. For multi-exposure, the sample was exposed six times with a 30° azimuthal rotation in between exposure steps. Figure 1(b) shows a scanning electron microscopy (SEM) image of the 12-fold SU-8 PQC produced with this method. The inset demonstrates the 12-fold rotational symmetry of the structure: a central air-hole surrounded by 12 outer air-holes. The 12-fold PQC structure was then transferred onto the ITO layer with Cl2-based ICP etching. After dry etching, the residual SU-8 resist were removed with SU-8 remover and acetone. Figure 1(c) shows a SEM image of the PQC pattern on the ITO layer. There are noticeable difference between the SU-8 mask and the transferred pattern: the shapes of air-holes on the ITO layer are elongated and three additional air-holes appeared in between the central hole and 12 outer holes. These resulted from the non-uniformity of the SU8 mask shown in the inset of Fig. 1(b). There is a circular ring around the central air-hole. The SU-8 film in this region is thinner than that of other region. Therefore, additional “holes” appeared in this region as the etching time elongated. The non-uniformity in the thickness of the SU-8 template also leads to etching time dependent shape of air-holes. Figure 1(c) shows that the diffraction pattern contains circles of twelve bright spots around the zero-order diffraction. This indicates that the final PQC structure on the ITO layer exhibits 12-fold rotational symmetry.
Fig. 1 (a) Schematics of a 12-fold PQC PLED. (b) A 12-fold PQC structure fabricated on SU-8 photoresist on the top of an ITO glass. The inset is the zoom-in image of a small area of the SU-8 photoresist to show its 12-fold rotational symmetry. (c) A 12-fold PQC patterned on the ITO after dry etching, and the inset is its diffraction pattern, which possesses a 12-fold rotational symmetry. The depth of the air-holes is about 35 nm.
Standard fabrication processes [2] were employed to fabricate the PLED device used in this investigation: A hole transport layer was obtained by spin-coating PEDOT: PSS solution on the ITO substrate followed by 5 min baking at 180°C on a hot plate. The PFO film, served as the light emitting layer, was obtained by spin-coating PFO toluene polymer solution with 1.5 wt% on top of the PEDOT: PSS thin film. The thicknesses of the PEDOT: PSS layer and the PFO films are 30 nm and 25 nm, respectively. The Ca (50 nm) and Al (100 nm) films were deposited on top of the PFO film with thermal evaporation. They served as the cathode of the device.
To characterize the electroluminescence (EL) of the PQCs PLEDs, the device is powered with a Keithley 2400 power supply and the emitted light is analyzed with a luminance colorimeter (Topcon BM-7) and a fiber optics spectrometer (Ocean S2000). To measure photoluminescence (PL) spectra of device, an s-polarized He-Cd laser emitting at 442 nm wavelength with 2 mm of beam size and 10 mW of excitation power was employed as a pumping source. The same fiber optics spectrometer was used to collect the PL spectra. No damage of the samples was observed during PL measurements.
3. Results and discussion
The AHC dependence of the light emitting performance of PQC PLEDs was first investigated. The air-hole size on the SU8 mask was varied with the dosage of the multi-exposure two-beam interference and the sample was then etched with ICP for the same etching time of 45 s. Figures 2(a) to 2(d) show SEM images of PQC structures patterned on the ITO layer obtained with total exposure dosage of 2.7, 2.8, 2.9 and 3.0 (mJ/cm2), respectively. Since the PQC pattern has no translational symmetry, we define the AHC of a PQC as the ratio of the total area of air-holes to the field of view of the SEM image. From these SEM images, the AHCs of the PQCs from Fig. 2(a) to Fig. 2(d) are determined to be 32.3% ± 2.7%, 24.1% ± 3.2%, 13.3% ± 2.0% and 6.4% ± 1.0%, respectively. The value of AHC remains roughly the same when the sampling position changes on the same sample. Note as displayed in these SEM images, the shapes of air-holes on the ITO layer are varied as AHC changes. Based on AFM measurements, the depth of the air holes in the PQC structures shown in Fig. 2 is about 35 nm.
Fig. 2 SEM images of 12-fold PQC patterns on the ITO layer with the air-hole coverage (AHC) of (a) 32.3% ± 2.7%, (b) 24.1% ± 3.2%, (c) 13.3% ± 2.0%, and (d) 6.4% ± 1.0%.
Figure 3 shows the AHC dependence of the characteristics of PFO PLEDs. Figure 3(a) shows that the I-V curves of the PQC PLEDs are very close to that of the unpatterned PLED. This indicates that the 35 nm deep air-holes in the PQC structure did not significantly alter the electrical properties of the PLED. Inset of Fig. 3(a) shows the EL spectra of the PFO PLEDs with and without the PQC structure under the same injection current. These two spectra are qualitatively the same, except that the PQC PLED produces higher EL intensity. This indicates that the PQC structure mainly improves the luminance intensity of the PLED as shown in Fig. 3(b): all PQC PLEDs exhibit higher luminance than that of the unpatterned PLED over the range of the bias voltage between 10 V to 12 V. Specifically, the maximum luminance of the 12-fold PQC PLEDs with AHCs of 24.1% ± 3.2% and 32.3% ± 2.7% are 23900 and 23400 cd/m2, respectively, which are larger than that of the unpatterned PLED (18800 cd/m2). Figure 3(c) shows the luminance efficiency-current characteristics of the PQC PLEDs. The luminance efficiency is defined as the device area multiplied with the ratio of luminance to injection current. The area of the device is 12 mm2. For current larger than 70 mA, all four PQC PLEDs have higher luminance efficiency than that of the unpatterned PLED. However, the luminance efficiencies of these four PQC PLEDs are nearly the same. This suggests that light emitting performance of the PQC PLEDs does not strongly depend on AHC over the range between 6.4% ± 1.0% and 32.3% ± 2.7%. Additionally, since the shapes of air-holes on the ITO layer of these four devices are varied, as shown in Fig. 2, it indicates that the shape of air-hole is not a crucial factor to influence the light emitting performance of the PQC PLEDs.
Fig. 3 (a) Current-voltage characteristics, (b) luminance-voltage characteristics, and (c) luminance efficiency-current characteristics of the PQC PLEDs with different AHCs: 6.4% ± 1.0% (red circle), 13.3 ± 2.0% (blue upward triangle), 24.1 ± 3.2% (green downward triangle) and 32.3% ± 2.7% (magenta diamond), and an unpatterned PLED (black square). Inset of (a) is the EL spectra of PFO PLEDs embedding with and without a PQC structure.
To investigate the effect of the depth of the air-holes in PQC on the light emitting performance of the PLEDs, we varied the etching time for the ICP dry etching process but fixed the exposure dosage of the SU8 film in the multi-exposure two-beam interference fabrication process. Figures 4(a) and 4(b) show AFM images of two PQC structures obtained with etching time of 35 s and 75 s, respectively. Their average etching depths (Δd) are 20 nm [Fig. 4(a)] and 55 nm [Fig. 4(b)]. Note that increasing etching time not only increased the depth but also the size of air holes, which implies increase in AHC. The etching time dependence of AHC and etching depth of PQC is shown in Fig. 4(c). Note that most of the AHCs shown in Fig. 4(c) are within the range of AHCs shown in Fig. 3, where PQC produces nearly constant enhancement on the light emission of the PLEDs. Therefore, variation in the light emission performance of the PQC PLEDs mainly results from the variation in air hole depth.
Fig. 4 AFM images of 12-fold PQC patterns on the ITO layer with the average depth of (a) 20 nm and (b) 55 nm obtained with etching time of 35 s and 75 s, respectively. The top panels in both (a) and (b) are the line profile scanned along the line segment shown in the image below. (c) The dependence of the AHC and etching depth on etching time.
Figure 5(a) shows the luminance efficiency-current characteristics of PQC PLEDs with different etching depth (Δd = 20 nm, 35 nm, 55 nm, 80 nm and 105 nm), and that of an unpatterned PLED. The efficiency of the PLEDs with 20 nm and 35 nm etching depths first increases with current and reaches saturation after 30 mA. The PQC PLEDs with 55 nm etching depth produces highest efficiency ~4 cd/A at around 20 mA current, then reaches a plateau ~3.5 cd/A between 30 and 60 mA, and then slowly decrease to 2 cd/A at 140 mA. The efficiency of the PQC PLEDs with 80 nm and 105 nm etching depths reaches saturation at a larger current of 100 mA. It is obvious that the luminance efficiency of PLEDs was improved with PQC structures except the PQC with Δd = 105 nm, whose efficiency at saturation is lower than that of an unpatterned PLED. For fair comparison, we plot the efficiency improvement of the PLEDs at injection current around 110 mA as a function of etching depth in Fig. 5(b). The efficiency improvement is defined as ((luminance of PQC PLED/ luminance of unpatterned PLED)-1) X100%. The efficiency improvement increases with the etching depth, reaching a maximum at 55 nm etching depth, and then decreases with etching depth. The optimum efficiency improvement can reach to 95%, and its luminance efficiency is about 2.5 (cd/A). To probe the influence of PQC structure on the light extraction efficiency, we further measured PL peak intensities from these PLEDs. PL spectra of the PFO PLEDs are very close to their EL spectra except a minor shift in the peak position. The peak position of PL spectra is at 532 nm. Figure 5(b) also displays the PL peak intensity enhancement of the PQC PLEDs (black curve, right axis) as a function of etching depth. The PL peak intensity enhancement is defined as: ((PL peak intensity of PQC PLED/ PL peak intensity of unpatterned PLED)-1) X100%. The PL peak intensity enhancement increases monotonically with etching depth, up to etching depth of 105 nm, at which the PL peak intensity enhancement is about 75%, much higher than that of 55 nm. The PL measurement results reveal that the PQC structure is effective in improving the light extraction from the PLEDs, and the light extraction efficiency should increase monotonically with etching depth. The discrepancy between the EL results and PL results can be explained by the I-V curves shown in Fig. 5(c). Compared with the I-V curve of an unpatterned PLED and those of the PQC PLED with 55 nm and 105 nm etching depths, the I-V curve of the PQC PLED with 105 nm shows a significant increase in the threshold voltage. It is plausible that when etching depth approaches to the thickness (110 nm) of the ITO layer substantial reduction in the conductance of the ITO layer occurred. Therefore, the electronic properties of the PQC PLED are significantly affected by the patterning, leading to lower injection current at the same bias voltage. The competition between the enhancement of the light extraction by the PQC structure and the penalty in the electronic properties of the device due to modification of the ITO layer leads to the non-monotonic dependence of the EL extraction on the etching depth.
Fig. 5 (a) Luminance efficiency-current characteristics of different types of PFO PLEDs, including an unpatterned PLED and the PQC PLEDs with different etching depths. (b) Luminance efficiency improvement (left axis) and PL (right axis) enhancement as a function of the etching depth of the PQC PLEDs. (c) Enlargement of current-voltage characteristics curves of unpatterned PLED (black curve) and the PQC PLEDs with the etching depths of 55nm and 105nm (different color curves). Inset is the full I-V curves of (c).
4. Conclusions
This work demonstrates the enhancement of EL light extraction from a PLED by patterning its ITO layer with a 12-fold PQC texture via a simple and low cost fabrication method: the combination of multi exposure of two-beam interference and ICP etching techniques. The enhancement of the EL light extraction is weakly affected by the air-hole coverage between 6.4% and 32.3% but strongly depends on the etching depth of the PQC. The non-monotonic dependence of the extraction efficiency results from the combined effects of the modification of optical properties of the PLED and the electric properties of the ITO layer due to patterning of PQC. The EL measurement result demonstrates that the maximum enhancement of EL light extraction efficiency of the 12-fold PQC PLED occurred at etching depth about 55 nm, with the improvement of luminance efficiency about 95% compared with an unpatterned PLED at an injection current of 110 mA.
Acknowledgments
The authors gratefully acknowledge financial support from the National Science Council, Taiwan, under grant Nos. NSC 101-2112-M194-009-MY3 and NSC 99-2112-M-194-008-MY3. The authors also acknowledge the financial support from Mechanical and System Research Laboratories, Industrial Technology Research Institute, Taiwan. J. H. Lin acknowledges the support of postdoctoral fellowship from National Science Council, Taiwan.
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