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We herein present the results of a study of the direct fabrication of buckled patterns in flexible organic light-emitting devices (FOLEDs) that had a conducting polymer anode on a polyethersulfone substrate. These patterns were produced spontaneously by the thermal deposition of an aluminum cathode on an electroluminescent (EL) composite layer. The polymer used for the anode was modified poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) and the EL layer was composed of a solution-processable small molecular composite including phosphorescent Iridium complex mixed with a poly(vinylcarbazole) host. It is shown that FOLEDs produced with buckled patterns can exhibit a luminance as high as ca. 14,900 cd/m2 with a peak efficiency of 50.5 cd/A. The patterned structure formed by the buckling of the EL layer allows FOLEDs to be produced with a high peak external quantum efficiency of 15% with an increase in light extraction by a factor of ca. 3.1. These results show that spontaneous buckling yields patterned structures that offer considerable promise for the production of high performance, reproducible and reliable FOLEDs.
©2011 Optical Society of America
1. Introduction
Recent research has focused on the development of organic materials and device structures for use in organic light-emitting devices (OLEDs), with the aim of realizing lightweight, flexible, and cost-efficient high-performance display devices and large-area lighting devices [1–4]. In order to achieve this, the scientific developments of greatest interest to researchers are those that simplify the fabrication of the device and improve the device performance and stability. The performance of OLED devices has recently been significantly improved, and some OLEDs that use phosphorescent materials now exhibit an internal quantum efficiency of nearly 100% [3,4]. However, in conventional OLEDs, most of the emitted electroluminescent (EL) light becomes trapped in the glass substrate (substrate mode) and indium tin oxide (ITO)/ organic layers (waveguide modes) by total internal reflection, because of a large difference in refractive index (n) between the air (n air = 1.0) and the layer materials, as is the case for ITO / organic layers (n ITO, organic ~1.7-2) and glass substrates (n glass ~1.5) [5–8]. These trapped modes significantly limit the out-coupling efficiency of the devices. Several research groups have developed techniques to provide increased out-coupling of these trapped modes. Examples include the use of Bragg diffraction gratings [5,6,9,10], low-index grids for the extraction of the waveguide modes in the ITO / organic layer [11], and the use of a scattering medium and micro-lens for extraction of the substrate mode [8,12]. Recently, randomly oriented buckled structures fabricated using an imprinting technique have been introduced to extract the trapped modes [13–16]. By using these techniques, out-coupling efficiencies can be improved in a controlled manner. To date, however, very few studies have been reported on the extraction of the trapped modes in flexible OLEDs (FOLEDs).
ITO, which has been widely used as a transparent anode in FOLEDs, has thus far failed to provide stable anodic properties because of its brittleness and therefore its incompatibility with flexible substrates [17]. Extensive research has therefore been performed on polymer-based electrode materials to replace ITO [18]. Among these, poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) has many benefits, such as a colorless transparency, a high conductivity, an easily variable viscosity, an ease of deposition, and a low surface roughness [19,20]. However, the satisfactory EL performance of FOLEDs, particularly devices with enhanced out-coupling, has not yet been achieved using such polymer anodes. The search therefore continues for a simple and effective method of extracting emitted light over a broad range of wavelengths, in order to produce reproducible ITO-free FOLEDs with improved out-coupling.
We herein report on the characteristics of the spontaneous buckling structure in FOLEDs with PEDOT:PSS anodes on flexible polyethersulfone (PES) substrates, and show that such a buckled pattern effectively extracts the trapped modes inside the fabricated FOLEDs and affords a number of distinct advantages, including a relatively simple and efficient device fabrication technique. The buckled structure produced in the FOLEDs enables us to obtain a high peak external quantum efficiency of 15% combined with an out-coupled emission enhanced by a factor of ca. 3.1. To the best of our knowledge, this is the highest extraction efficiency for an ITO-free FOLED yet reported.
It is known that the compressive stresses caused by the thermal contraction of an underlying polymer film may cause the surface of the film to buckle [21]. Figure 1 shows schematically the structure and fabrication process of a buckled FOLED that has a PEDOT:PSS anode on a flexible PES substrate. A thin EL layer was coated on a PES substrate that had been precoated with a thin PEDOT:PSS layer, and the coated sample was then loaded into a vacuum thermal evaporator. Under vacuum conditions, the thermal evaporation boat heated the Al source metal; the heat also reached the EL layer and caused it to expand. Then the Al layer was deposited on the expanded EL layer. After the evaporation process was complete, the fabricated layers cooled and contracted; waves and/or buckles with random orientations formed on the surface to release the compressive stress, which was a consequence of the large difference between the thermal expansion coefficients of the organic (EL) layer (~10−4 /K) and the Al layer (~10−6 /K). Using this process, the buckled structure may be fabricated directly in the device, without the use of the imprinting method used in previous studies [13–16].
Fig. 1 Schematic diagram of the fabrication process for a buckled FOLED with a PEDOT:PSS anode on a flexible PES substrate. The buckled EL layer was spontaneously formed by a thermal expansion technique.
2. Experimental methods
A pre-cleaned flexible PES film (Glastic PES, I-components Co., thickness: 0.1 mm, surface roughness: ca. 8 nm) was used as the substrate. PEDOT:PSS (CLEVIOSTM PH 500) was used as a polymeric anode material. We further doped Dimethyl sulfoxide (DMSO) and D-sorbitol into the PEDOT:PSS solution to increase the conductivity (D-sorbitol: 0.625 wt% and DMSO: 7.5 wt%) [22]. The thickness of the spin-coated film of the modified PEDOT:PSS was about ~100 nm with a surface roughness of 5.5 nm and a sheet resistance of 220 Ω/□. Next, we produced a green-light-emitting EL layer via a spin-coating process on the PEDOT:PSS anode using a blended solution [22], which consisted of a hole-transporting material of N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'biphenyl-4,4'-diamine (TPD), an electron transporting material of 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (Bu-PBD), a green-emitting material of phosphorescent tris(2-phenylpyridinato) iridium (Ir(ppy)3), and a hole-transporting host material of poly(vinylcarbazole) (PVK) in mixed solvents of 1,2-dichloroethane and chloroform (mixing weight ratio 3:1). Then, an electron-injecting interfacial layer of cesium fluoride (CsF, 2 nm) and an Al cathode layer (ca. 80 nm) were formed sequentially on the top of the EL layer via thermal deposition at a rate of 0.2 nm/s under a base pressure of 2 × 10−6 Torr. Following the deposition of Al, the coated EL layer was cooled to ambient temperature by keeping the device in the chamber for more than 30 min and venting to atmosphere. Then, the compressive stress induced in the EL layer generated a buckled structure on the FOLED (thermal expansion technique). After fabrication of the devices, the post-production treatment of electrical field annealing [22] was performed in order to produce bright and efficient OLEDs with PEDOT:PSS anodes.
The fabrication and characterization of the device were carried out at room temperature under ambient conditions, without encapsulation. In order to investigate the surface morphologies of the fabricated films, the variation in the surface roughness of the film was monitored using an Atomic Force Microscope (AFM, Nanosurf easyscan2 FlexAFM, Nanosurf AG Switzerland Inc.). During the measurements, a contact mode was used with a cantilever (CONTR-10 point probe-silicon, Nanoworld, Inc.). A Chroma Meter CS-200 (Konica Minolta Sensing, INC.), a spectrometer (Ocean's Optics), and a source meter (Keithley 2400) were used to measure the EL characteristics. An LED measurement system (LCS-100, ShereOptics Inc.) with an integrating sphere were used to measure the emission characteristics.
3. Results and discussion
We began our investigation by assessing the optical properties of the PEDOT:PSS anodes that had been fabricated on PES and glass substrates. Figure 2(a) shows the transmittance spectra of the flexible PES coated with the PEDOT:PSS anode, and reveals the slightly higher transmittance (of about 80% in the visible wavelength region) than that of the glass substrate coated with the PEDOT:PSS anode. The clarity of the thin film interference patterns provides an indication that the PES substrate and the coated PEDOT:PSS anode are reasonably flat and uniform. Figure 2(b) shows a photograph of spin-cast thin films of PEDOT:PSS on PES and glass; the samples were placed on letters to show their transparency.
Fig. 2 (a) Transmittance spectra of PEDOT:PSS anodes on a flexible PES substrate (solid curve) and a glass substrate (dotted curve). (b) Photograph of the PEDOT:PSS layers on PES (left) and glass (right) substrates.
Next, we observed the surface morphology of the OLEDs on both the PEDOT:PSS anode-coated PES (sample) substrate and the glass (reference) substrate, after the formation of an EL layer and an Al cathode. In Fig. 3 , the upper panels show SEM images of the FOLED on the PES substrate. The image of the surface of the Al layer clearly shows a buckled structure having a broad and random distribution of periodicity, with a periodicity centered around wavelengths of ca. 450-500 nm and a buckle depth (amplitude) as low as 30–50 nm. This corrugated structure is clearly formed in both the EL layer and the Al layer, while the PEDOT:PSS layer remains flat and smooth (lower left figure in Fig. 3). In contrast to the buckled structure in the FOLED, the topographic image of the reference OLED with the PEDOT:PSS anode on the glass substrate is fairly smooth, with a root mean square roughness of about 3 nm, as shown in the lower right panel of Fig. 3. The smooth surface of the reference OLED may be caused by there being little or no thermal expansion of the EL layer during the Al deposition process, due to the thermal conductivity of the glass substrate (ca. 0.96 W/m·K) being higher than that (ca. 0.17 W/m·K) of the PES substrate [23], i.e., the glass substrate acts as a good heat sink. It is noteworthy that the FOLEDs with ITO anodes on PES substrates have smooth surfaces without any buckling, which may be due to the high thermal conductivity of the ITO (ca. 5.9 W/m·K) [24], compared to that of PEDOT:PSS (ca. 0.17 W/m·K) [25].
Fig. 3 SEM analyses of buckling patterns. Upper panels: topographic images of buckled structures formed in the FOLED. Lower panels: cross-sectional views of the FOLED with buckling on PES substrate (left) and the reference OLED without buckling on glass substrate (right).
The next part of our investigation concerned the performance of the fabricated OLEDs. Figure 4(a) shows a photograph of the sample FOLED device operating under a bias voltage of 10 V. The figure clearly shows that there is fairly bright EL emission from the active area in the buckled FOLED. The performance of the FOLED devices with buckled surfaces was measured and analyzed. The results show that the FOLEDs on PES substrates depend on the thickness of the EL layer for their brightness to the same extent as the OLEDs on glass substrates. Thicknesses of the EL layer that performed well when used on glass substrates also performed relatively well with the PES substrate. Moreover, when comparing the device performances for a given thickness, we observed that the FOLEDs fabricated on the PES substrate were consistently superior to the OLEDs fabricated on the glass substrate. Figure 4(b) shows representative examples of the current density-voltage (J-V) and the luminescence-voltage (L-V) characteristics of OLEDs having a 160 nm EL layer on a 100 nm PEDOT:PSS anode. As shown in the figure, the J-V curves show the excellent diode-like behavior of both the sample FOLED with buckles and the reference OLED without buckles and thereby indicate the good coverage of the EL layers. Notably, the characteristics of the sample FOLED exhibit a more pronounced diodic slope behavior than that of the reference OLED. This larger J in the corrugated sample device mainly results from a stronger electric field because of the partially reduced organic layer thickness in the intermediate region between the peak and valley of the sinusoidally patterned gratings [26]. The L-V characteristics clearly show that the buckled FOLED was characterized by relatively low turn-on voltages (ca. 3 V) and bright EL emission with excellent performance; operating voltages of about 7.0 V and higher luminance than that obtained for the reference OLED on glass. We then deduced the efficiencies of the studied OLEDs (Fig. 4(c)). For the sample FOLED, a current efficiency (ηC,) of 50.5 cd/A was reached at a luminance of 100 cd/m2, ηC, was maintained at 47.6 cd/A up to 1,000 cd/m2 and ηC, remained above 30.0 cd/A at luminance of up to 14,900 cd/m2. We also estimated the power efficiency ηP, which increased, reached a maximum value of 32.0 lm/W (at 10 cd/m2), and then slowly decreased, with increasing bias voltage. In contrast, for the reference device without buckling, the peak current and peak power efficiencies were only 20.8 cd/A and 10.9 lm/W, respectively.
Fig. 4 (a) Photograph of an operating FOLED with a buckled surface on a PES substrate at 10 V. (b) J-V (red) and L-V (blue) characteristics and (c) current efficiency (red) and power efficiency (blue) for the sample FOLED (solid curves) and the reference OLED (dotted curves).
In order to understand the improved efficiency of the sample FOLED, we observed the light-emitting characteristics of the devices, as shown in Fig. 5 . Figure 5(a) shows the normalized angular dependences of the EL intensity from the sample FOLED and the reference OLED. The figure clearly indicates that the devices with and without buckling show Lambertian emission patterns with a maximum intensity in the normal direction. Because the buckling exhibits a random orientation and broad periodicity, the out-coupled emission becomes concentrated in the normal direction, resulting in a Lambertian emission pattern. Next, we investigated the EL spectra obtained from these devices at the surface normal direction. Figure 5(b) shows that the shape of the EL emission spectrum of the sample FOLED in the normal direction is very similar to that of the reference device.
Fig. 5 (a) Normalized viewing angle dependence of EL intensity for the sample FOLED (circles) and the reference OLED (squares). The dotted line represents the Lambertian emission pattern. (b) Normalized EL spectra measured at the surface normal direction for the sample FOLED (blue) and reference OLED (red).
Next, we deduced the external quantum efficiencies (EQEs) of the devices from these results assuming a perfectly diffusive EL emission surface [27], and the results are shown in Fig. 6(a) . The figure shows that the EQE of the unbuckled reference device decreases monotonically from 5.2% as the input power density increases, while the EQE of the sample FOLED with a buckled surface reaches a peak value of 15% and ηEXT is always ca. 3.1 times higher than that of the reference device. For another comparison, the EQE (15%) of our sample FOLED on flexible PES is higher than those (8-12%) of the conventional hetero-structured phosphorescent OLEDs on glass substrates with well-optimized ITO anodes, reported in Refs. 28 and 29. We attribute the greatly increased EQE of the sample FOLED to superior out-coupling efficiencies brought about by simultaneous extraction processes, namely i) the effective extraction of the waveguide modes [13] from the EL layer due to the buckled structure, ii) the suppression of the waveguide modes in the anode layer due to the low refractive index of the PEDOT:PSS (n PEDOT:PSS ~1.42 @ 550 nm) [30], and iii) the additional extraction of the substrate modes through multiple reflections with the randomly bulked cathode layer due to the thinner (0.1 mm) PES substrate used in the sample FOLED than the glass (0.7 mm) substrate used in the reference OLED [15]. In order to confirm this enhanced out-coupling efficiency, we also investigated the increases in the ratio of EL spectral outputs from the devices at a fixed input power (222 mW/cm2) by measuring radiant power using an integrated sphere (Fig. 6(b)). The figure shows that the peak ratio of the radiant power from the sample FOLED to that from the reference device is ca. 3.4:1 near 548 nm, and the average ratio of the radiant power over the whole EL spectral range is nearly 3.2:1, confirming that the buckled structure provides a great improvement to the simultaneous light extraction from the FOLED. This improvement by more than a factor of 3 is even higher than the highest reported increase (by a factor of about 2.2) of light extraction [13].
Fig. 6 (a) EQE as a function of input power for the sample FOLED (blue) and reference OLED (red). (b) Spectral ratio of output radiant power measured by using an integrated sphere for the sample FOLED to that for the reference OLED at a fixed input power (222 mW/cm2). Inset shows the total EL spectral outputs of the radiant power from the devices by using the integrated sphere.
In order to provide a better comparison of the fabricated OLEDs, we have also deduced the enhancement ratios of the current efficiency and power efficiency at several output luminances. Figure 7 shows the current efficiency (Fig. 7(a)) and power efficiency (Fig. 7(b)) of the sample FOLED and reference OLED as functions of output luminance. From the figures we can deduce the enhancement ratios of the current and power efficiencies at several luminances. For example, the enhancement ratio of current efficiency is about 2.7 at 10 cd/m2, 3.3 at 100 cd/m2, and 3.5 at 740 cd/m2, while the enhancement ratio of the power efficiency is about 3.9 at 10 cd/m2, 5.8 at 100 cd/m2, and 7.1 at 740 cd/m2. From these results, it is demonstrated that the spontaneously formed buckling in the FOLEDs offers considerable promise for the production of reproducible and reliable, high-performance FOLEDs.
Fig. 7 The current efficiencies (a) and power efficiencies (b) of the sample FOLED and reference OLED as functions of output luminance.
Finally, we comment on the formation of a buckled surface in the FOLEDs. It was found that the radius and/or the curvature of the buckled profile can be changed by controlling the concentration ratio of small molecules to PVK polymer in the EL composite layer. This finding may imply that the temperature induced phase separation of small molecules from the PVK matrix during the thermal process of Al deposition may also be a cause of surface buckling, due to variations in surface tension in the EL layer. This behavior has a similar origin to the Bernard-Marangoni patterns [31] formed at the free surface of films, where variations of the surface tension are caused by the temperature gradient induced within the layer. Thus, the precise control of the phase separation in the EL composite layer may also provide a useful means of further optimization of the buckled patterns.
4. Conclusions
In summary, we have described a buckling structure formed spontaneously capable of extracting light from ITO-free FOLEDs that have modified PEDOT:PSS anodes. ITO-free FOLEDs with buckled patterns were successfully fabricated using a thermal expansion technique, and showed luminance as high as 14,900 cd/m2 with a peak efficiency of 50.5 cd/A. It was also demonstrated that buckled FOLEDs prepared with PEDOT:PSS anodes provide an increase in light extraction by a factor of more than 3, without introducing serious spectral changes and directionality. This novel method for the fabrication of FOLEDs with spontaneously formed buckled surfaces will provide a solid basis for the fabrication of cost-efficient, large-area, and high-performance flexible displays and broad spectrum lighting.
Acknowledgments
The present Research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology, Republic of Korea (2011-0005471). This research was also supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (20100029416), by the Converging Research Center Program through the Ministry of Education, Science and Technology (2011K000613).
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