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Abstract
To improve the device performances of top-emitting organic light emitting diodes (TEOLEDs), we developed a new inverted TEOLEDs structure with silver (Ag) metal as a semi-transparent top electrode. Especially, we found that the use of relatively thick Ag electrode without using any carrier injection layer is beneficial to realize highly efficient device performances. Also, we could insert very thick overlying hole transport layer (HTL) on the emitting layer (EML) which could be very helpful to suppress the surface plasmon polariton (SPP) coupling if it is applied to the common bottom-emission OLEDs (BEOLEDs). As a result, we could realize noteworthy high current efficiency of approximately ~188.1 cd/A in our new inverted TEOLEDs with 25 nm thick Ag electrode.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The organic light emitting diodes (OLEDs) have received significant attention due to their self-emissive properties, low power consumption, wide viewing angle, color gamut, fast response time, and flexibility [1–9]. Nowadays, most of the active matrix (AM) type OLEDs (e.g., displays for mobile and television application) have been manufactured by applying top-emitting OLEDs (TEOLEDs) structure to realize high-resolution display with low power consumption [10,11]. Those devices typically adopt two metallic electrodes (e.g., a highly reflective bottom electrode and a semi-transparent top electrode) which results in a micro-cavity effect following the Fabry-Pérot resonator model [10–16]. Meanwhile, to improve the device performances of TEOLEDs, it is necessary to control the optical properties of the metallic electrodes (e.g., reflectance, transmittance, absorptivity, etc.) by optimization of the thickness and composition of the electrode materials. And, such optical properties can be further modified or precisely tuned by deposition of additional organic or inorganic capping layer (CL) on (mainly) top electrode [12,17,18]. Indeed, most of AMOLEDs panel makers are applying TEOLEDs structure with their own semi-transparent cathode system covered with organic CL. In this way, the performance of AMOLEDs has improved dramatically, and panel makers are still trying to change the metal electrode itself or its components to improve its properties further while the generally accepted top electrode has been a magnesium:silver (Mg:Ag) system. On the other hand, there have been lots of reports to use the only Ag as a top electrode because it’s suitable as the semi-transparent electrode for TEOELDs because it has excellent optical properties such as high reflectance, higher transmittance, moderately low absorptivity in a visible wavelength region. However, it was very difficult to utilize the Ag metal without using any additional modification as a cathode due to its relatively high work function values (e.g., Ag (Φa) ≈4.3 eV). To solve such problem, many research groups have tried to improve the electron injection by using n-doped electron transport layer (ETL), (e.g., BPhen:Li, BPhen:Cs), multi-layered cathode system (e.g., LiF/Al/Ag), different alloy composition (e.g., Yb:Ag, Ca:Ag) etc [10,14,19,20]. However, the complexity of top electrode applied in TEOLEDs can easily influence the microcavity effects because the change in its composition easily affect the optical parameters of semi-transparent electrode which causes serious color change of the devices. Therefore, the simple metal electrode (e.g., electrode used by only Ag metal) could be best option to control the optical properties of final devices.
So, we focused on the inverted OLEDs structure in this study, because we could use the simple Ag electrode to control the properties of TEOLEDs devices. Indeed, we investigated the device performances for a new inverted TEOLEDs only by controlling the thickness of top electrode (Ag) without any modification of its composition. Especially, we could apply thick overlying layers having multiple junction structures in between cathode and EML to suppress caused charge trapping sites. As a result, we realized highly efficient inverted TEOLEDs showing ~188.1 cd/A of current efficiency without applying additional light extraction technologies.
2. Experimental
2.1 Materials
To realize the highly efficient TEOLEDs with 2nd microcavity structure, we prepared green phosphorescent OLEDs (PHOLEDs) with top-emitting structure. We used indium tin oxide (ITO, 7 nm)/ silver (Ag, 100 nm)/ITO (7 nm) as a reflective cathode (RC); zinc oxide (ZnO)/ poly(ethylenimine) ethoxylated (PEIE) as an electron injection layer (EIL); bis[2-(2-hydroxyphenyl)pyridinato]-beryllium (Bepp2) as a green host (GH) material, bis(2-phenylpyridine) (acetylacetonato)-iridium (III) [Ir(ppy)2(acac)] as a green dopant (GD) for the emitting layer (EML), N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) as a hole transporting layer (HTL) and/or CL, tris(4-carbazoyl-9-ylphenyl) amine (TCTA) as a HTL and/or an exciton blocking layer (EBL) for preventing exciton quenching. Meanwhile, a 1,4,5,8,9,11-hexaazatriphenylene-hexacarbo-nitrile (HATCN) was utilized as a charge generation layer (CGL) and/or electron accepting layer. Besides, Ag electrode as an anode for inverted TEOLEDs. All the materials were purchased from commercial suppliers and used without purification.
2.2 Device fabrication and measurements
The glass substrate with patterned ITO/Ag/ITO with an active area of 4 mm2 was formed by a photolithography process. The substrate was cleaned with sonication in acetone and isopropyl alcohol, rinsed in deionized water, and treated in UV-ozone to eliminate all the remained organic impurities from the previous processes. All organic materials were deposited by vacuum evaporation under a pressure of ∼5 × 10−7 Torr and deposition rate of organic layers was about 0.5 Å s−1. Ag for electrode was deposited at rates of 0.2 Å s−1 under around 10−6 Torr, respectively. The transmittance and reflectance of Ag electrode were obtained by ultraviolet-visible (UV-vis) spectroscopy (Scinco, S-4100) and (Agilent, Cary 5000), respectively. And the current density–voltage (J–V) and luminance–voltage (L–V) data of the OLEDs were measured by a Keithley 2635A and Minolta CS-100A, respectively. The electroluminescence (EL) spectra and the Commission Internationale de L’Eclairage (CIE) coordinates were obtained using a Minolta CS-2000A spectroradiometer. The power efficiencies were calculated by integrating angular and spectrally resolved emissions. The optical simulation of TEOLEDs was used for SimOLED and SETFOS 4.3 (Fluxim AG). The morphology of thin film was investigated using on the field emission scanning electron microscopy (FE-SEM, HITACHI, S-4700) and an atomic force microscopy (AFM, Park Systems XE-100).
3. Results and discussion
3.1 Analysis of Ag electrode
In general, light intensity of microcavity devices can be strongly affected by two important optical parameters as following equations [13–16]:
where and represent the reflectance at interface of top and bottom electrodes in TEOLEDs and is the transmittance of the top electrode. A is the distance between dipole of an emitter and a reflective electrode. and are total phase difference by Fabry-Pérot and two-beam interference, respectively. and are phase changes which occur upon reflectance at the interfaces of top electrode-organic and bottom electrode-organic, respectively. And and are refractive index and thickness of i-th organic layer, respectively. In addition, is the internal observation angle from the surface normal of the microcavity inside the i-th organic layer and is the angle of the light propagation in the emitting layer. is the intensity of original emission spectrum in the free space. From this equation, we can understand that the extracted light emission strongly depends on the reflectance and transmittance of the used electrodes. In other words, it is very important that the electrode conditions should be optimized with appropriate optical characteristics (e.g., reflectance and transmittance) to maximize the device performance as well as microcavtiy effect of TEOLEDs.Meanwhile, the conventional TEOLEDs with a top cathode have focused on Mg:Ag (10:1 = wt.%/wt.%) as a semi-transparent electrode [10,16,21]. However, use of a Mg:Ag electrode can easily lead to a performance degradation caused by low transmittance and high sheet resistance as reported previously. Besides, two component cannot be dispersed perfectly to form a very homogeneous mixture of the same proportions in all parts [21]. So, we thought it would be beneficial if we could use a single composition of the metal (e.g. Ag only) in our devices.
Before we optimize an Ag metal condition as a top semi-transparent electrode, the optical simulation was performed to predict the correlation between the thickness of Ag and the intensity of light. Meanwhile, we additionally deposited CL (60 nm) to modify the optical properties of Ag electrode for better performances as reported before [12]. We could predict the change of radiance according to the thickness of Ag through optical simulation as shown in Fig. 1. As a result, we could estimate that we could extract maximum radiance of ~18.1 x 102 W/sr·m2 [at 510 nm, the peak wavelength of the Ir(ppy)2(acac)] when we use up to 25 nm of Ag as shown in Fig. 1. It means that there might be an optimum condition of optical properties (e.g., reflectance, transmittance, and sheet resistance) at the thickness of 25 nm.
To investigate the optical properties of Ag metal, we prepared the following samples having the thickness of 15, 20, and 25 nm which are pointed out also in the Fig. 1: (Sample A, B, C w/o CL and Sample D, E, F w/ CL)
Sample A: HATCN (7 nm) / Ag (15 nm)
Sample B: HATCN (7 nm) / Ag (20 nm)
Sample C: HATCN (7 nm) / Ag (25 nm)
Sample D: HATCN (7 nm)/ Ag (15 nm) / CL (60 nm)
Sample E: HATCN (7 nm) / Ag (20 nm) / CL (60 nm)
Sample F: HATCN (7 nm) / Ag (25 nm) / CL (60 nm)
Here, we fabricated only 15, 20, and 25 nm thickness of Ag as previous simulation results. Figure 2 and Fig. 3 show transmittance and reflectance data of various samples fabricated as aforementioned. As we expected, the transmittance values at 550 nm decreased when the thickness varies thinner (e.g., 57.9, 44.2, and 29.2% for Sample A, B, and C, respectively, see also Fig. 2(a) and Table 1.
Table 1. Electrode characteristics of the different Ag thickness
Those values were all significantly improved to 77.3, 68.5, and 52.9% for Sample D, E, and F, respectively, as shown in Fig. 2(b). Mainly, those transmittance values were enhanced in the whole visible wavelength region (e.g., 450 ~700 nm) after deposition of the CL as well-known principles [12].
In contrast, the reflectance values increased in the order of Sample A (Sample D), Sample B (Sample E), and Sample C (Sample F) at 550 nm for without CL (and with CL), respectively, as shown in Fig. 3. As a result, the reflectance of Ag increased as thickness increases while transmittance decreased as a thickness of Ag increases regardless of CL, which means that reflectance and transmittance are in a trade-off relationship. The absorbance values of each sample are shown in Fig. 4.
Anyhow, the transmittance and reflectance of 25nm thick Ag were 34.1% (52.2%) and 63.0% (42.1%) for Sample C (Sample F) at 510 nm, respectively. On the other hand, since the conductivity of Ag is a very important characteristic for the application of the display, we investigated the conductivity and sheet resistance characteristics. Indeed we performed 4-probe measurement and calculated by using van der Pauw method [22]. As a result, the conductivity was 0.6 x 106, 3.7 x 106, and 4.6 x 106 S/m for Sample A, B, and C, respectively. Meanwhile, the sheet resistance was 10.7, 1.4, and 0.9 Ω/□ for Sample A, B, and C, respectively. Very importantly, we found that the sheet resistance can be significantly diminished if we used relatively thick Ag (25 nm) electrode which could be applied even in the large area display applications as shown in Fig. 5 and Table 1.
3.2 Optimal simulation for highly efficient inverted TEOLEDs
To realize the highly efficient inverted TEOLEDs having a thick overlying layer (HTL), we performed an optical simulation with the software, SimOLED and SETFOS 4.3 for Fig. 6(a) and Fig. 6(b), respectively, where refractive index (n) and extinction coefficient (k) values of the organic materials used were in the ranges of 1.7-2.0 and 0-0.15, respectively. The detailed device configuration for this study was as follows: [RC / ETL (25 nm) / Bepp2:Ir(ppy)2(acac) (3%, 20 nm) / TCTA (15 nm) / HTL (X nm) / Ag (25 nm)/ CL (60 nm)]. Based on previous results, the Ag thickness fixed at 25nm. From this simulation, we found that it could extract maximum light emission at the certain thickness condition of about 25-50 (1st microcavity) or 160-175 nm (2nd microcavity) for HTL, as shown in Fig. 6(a). Among these conditions, we chose a relatively thick HTL about 174 nm to elongate the distance between metallic electrode and EML, and the total thickness of the 1st microcavity is too thin to prevent negative effects related to manufacturing issues [10]. Also, we found that the portion of SPP coupling mode is not changed that serious if we apply a metallic electrode (Ag) as an anode, as shown in Fig. 6(b). Meanwhile, the inverted structure adopting transparent electrode (e.g., TCO: transparent conducting oxide) showed a significant level of SPP coupling as reported by Kim et al [23,24]. However, we thought this structure is still beneficial to get rid of all the possible metal diffusion from the top electrode rather than the suppression of SPP coupling. Based on these results, we designed inverted TEOELDs with different thickness of Ag electrode as an anode.
Fig. 6 (a) Simulated radiance contour plots of the microcavity OLED with green emitter as functions of the HTL versus ETL thickness. The solid red box designates the optimal thickness range of HTL/ETL to satisfy the 2nd microcavity condition; (b) optical simulation result according to the thickness of HTL for inverted TEOLEDs.
3.4 Device characteristics for Inverted TEOLEDs
To confirm the the effect on the optical properties Ag acting as an anode, we fabricated the inverted TEOLEDs, when we applied different thickness of Ag electrode as follows (Devices A–C, see also Fig. 7):
Device A: RC / ZnO (15 nm) / PEIE (10 nm) / Bepp2: Ir(ppy)2(acac) (3%, 20 nm) / TCTA (15 nm) / NPB (60 nm) /HATCN (7 nm) / NPB(100 nm) / HATCN(7 nm) / Ag (15 nm) / CL (60 nm)
Device B: RC / ZnO (15 nm) / PEIE (10 nm) / Bepp2: Ir(ppy)2(acac) (3%, 20 nm) / TCTA (15 nm) / NPB (60 nm) / HATCN (7 nm) / NPB(100 nm) / HATCN (7 nm) / Ag (20 nm) / CL (60 nm)
Device C: RC / ZnO (15 nm) / PEIE (10 nm) / Bepp2: Ir(ppy)2(acac) (3%, 20 nm) / TCTA (15 nm) / NPB (60 nm) / HATCN (7 nm) / NPB(100 nm) / HATCN(7 nm) / Ag (25 nm) / CL (60 nm)
Here, we applied an inverted structure using ZnO/PEIE on the underlying electrode. In addition, we applied p-n-p-n multiple junctions to improve the overall charge balance originating from the thick HTL at the central EML, aforementioned [25,26].
Figure 8(a) shows the J–V–L characteristics of the devices fabricated in this study. The turn-on voltage (Von) to reach 1cd/m2 were recorded 4.0, 3.5, and 3.5 V for Device A, B, and C, respectively. Meanwhile, the operation voltage (Vop) to reach 1,000 cd/m2 were 6.0, 5.8, and 5.7 V for Device A, B, and C, respectively. (see also Table 2). At a given constant luminance of 1000 cd/m2, the current and power efficiencies were 91.8 cd/A and 48.1 lm/W for Device A, 139.7 cd/A and 75.4 lm/W for Device B, 155.8 cd/A and 85.9 lm/W for Device C, respectively. And the maximum current and power efficiencies were 123.7cd/A and 86.3 lm/W for Device A, 152.1 cd/A and 106.4 lm/W for Device C, 188.1 cd/A and 131.3 lm/W for Device C, respectively, as shown in Fig. 8(b) and Table 2. With this result, we found that the fabricated TEOLEDs with thick overlying layer could relatively realize a good charge balance, which is beneficial to improve the device stability originating from the metal diffusion. Those results are well consistent with our previous simulation results as shown in Fig. 1.
Table 2. Device characteristics of the inverted TEOLEDs
To understand the reason of the performance change upon variation of Ag thickness, we proceeded FE-SEM and AFM of the samples and obtained the surface morphology as shown in Fig. 9 and Fig. 10, respectively. From this approaches, we found that the 15-nm-thick Ag film contains numbers of dark regions at SEM photographic image, plausibly due to a grain boundary, a crack, or nano-voids, which may affect the device performances related to resistivity, as aforementioned. In other words, the deposition of very thin Ag layer on CL may result in an insufficient island-like growth of Ag as shown in Fig. 9(a).
Fig. 9 SEM images of Ag films with (a) 15 nm and (b) 25 nm thick condition (Scale bar: 0.5 μm). The yellow circles in (a) indicate the defect site such as crack, voids, etc.
While, the 25 nm-thick Ag film showed relatively uniform Ag film with much denser island-like morphology due to a much better step coverage which also cause an order of magnitude reduced sheet resistance as well as much improved conductivity, as previously commented (see also Fig. 9(b) and Fig. 10) [27–29]. Besides, we could also confirm that the surface morphology of Ag film seems to be much denser if the thickness of Ag film increases as shown in the AFM images of Fig. 10.
Figure 11 shows the normalized EL spectra for inverted TEOELDs in this study. Very interestingly, we observed relatively narrow EL spectra with full width half maximum (FWHM) about 34, 30, and 24 nm, for Device A, B, and C, respectively.(see also Table 2.)
Especially, we found that the 25 nm thick Ag electrode exhibited significantly narrow EL emission intensity due to the strongest microcavity effect as compared with others [10–16,30].
Surprisingly, we could obtain efficient TEOLEDs without using any reactive electrode. Instead of, we found that Ag electrode can efficiently act as an anode without any modification.
4. Conclusions
In this study, we demonstrated the device performances depending on the different thickness of Ag electrode. From this results, we found that the simple metal electrode (only used Ag electrode) could effectively act as a semi-transparent top electrode of inverted TEOLEDs, and without any hole injection layers, we obtained relatively good charge balance by using multiple p-n junctions. Consequently, the Device C (with 25 nm thick Ag condition) showed the outstanding performances resultants in their relatively superior optical properties (e.g., high reflectance, good interface conditions, and conductivity) as compared with than others (Device A and B). From this result, we confirm that the usage of the only Ag electrode could utilize the highly efficient inverted TEOELDs, which also could help to resolve the electrode issues for TEOLEDs.
Funding
Ministry of Trade, Industry and Energy (MOTIE) (10079974).
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