metal hybrid cathode for transparent organic light-emitting diodes

Organic Electronics 14 (2013) 2039–2045

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Organic/metal hybrid cathode for transparent organic light-emitting diodes Jin Woo Huh 1, Jaehyun Moon 1, Joo Won Lee, Jonghee Lee, Doo-Hee Cho, Jin-Wook Shin, Jun-Han Han, Joohyun Hwang, Chul Woong Joo, Jeong-Ik Lee ⇑, Hye Yong Chu OLED Research Center, Electronics and Telecommunications Research Institute, Daejeon 305-700, Republic of Korea

a r t i c l e

i n f o

Article history: Received 23 October 2012 Received in revised form 31 March 2013 Accepted 16 April 2013 Available online 9 May 2013 Keywords: Transparent organic light-emitting diodes Organic/metal hybrid cathode Cs-doped electron transport layer (Cs-ETL)/ Ag Microstructure

a b s t r a c t We report a highly transparent organic/metal hybrid cathode of a Cs-doped electron transport layer (Cs-ETL)/Ag for transparent organic light-emitting diode (TOLED) applications. Particular attention is paid to the surface morphology on the Ag film and its influence on the optical transparency and electrical conductivity. With the use of Cs-ETL, a smooth and continuous surface morphology of Ag film was achieved, leading to a high transmittance of 75% in TOLED with a low sheet resistance of 4.5 X/Sq in cathode film. We successfully applied our Cs-ETL/Ag transparent cathode to fabricate highly transparent OLEDs. Our approach suggests a new electrode structure for transparent OLED applications. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction A transparent organic light-emitting diode (TOLED) refers to an OLED type which emits light from the top and bottom directions of the device [1,2]. In the absence of applied voltage, the TOLED is a transparent window, but when voltage is applied to the TOLED, it becomes is a bidirectional light-emitting device. Thus, a TOLED can be used as a light source for innovative window-like lighting luminaire systems: during the daytime, a TOLED can be used as an ordinary window, while after sunset the TOLED can serve as a lighting facility. Because the generated light in a TOLED has to escape the top surface as well as the bottom surface, special attention has to be paid to the top electrode, which is the cathode [3–5]. In conventional bottom-emission OLEDs, the cathode material is usually metallic with a thickness of approximately 200 nm. Hence, the cathode surface, which is in contact with the organic layer, acts as a mirror and reflects ⇑ Corresponding author. Tel.: +82 42 860 1166; fax: +82 42 860 1029. 1

E-mail address: [email protected] (J.-I. Lee). First co-author (equal contribution).

1566-1199/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2013.04.035

the generated light toward direction of the bottom, contributing to the intensity of the bottom emission. On the other hand, in TOLEDs, in addition to the electrical conductivity, the cathode must have optical transparency. Regarding the electrical conductivity in an electrode, metallic materials offer sufficiently low resistance. However, due to their absorption dependency on the thickness, a limitation exists when seeking to obtain both high transmittance and low resistance. To realize practical TOLEDs, the transmittance of the device initially has to be improved. Therefore, a metal cathode is the key part to improve the transmittance of OLED devices [6,7]. Conventionally, LiF/Al/Ag has been used as a transparent cathode [7–9]. But, in this structure, the emission intensity may be lowered due to the absorption loss in the cathode. To make matters worse, with cavity glass seal [10] of the cathode, a TOLED with a LiF/Al/Ag cathode gives transmittance of only 60% at 550 nm because light loss of approximately 10% occurs at the glass cap boundary due to difference in refractive indices. For higher transparency, it is crucial to minimize the absorption and to make the metal film as thin as possible. Hence, the Al layer in a conventional transparent cathode of LiF/Al/Ag

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should be removed because it has high absorption in the visible light region. Moreover, for an effective electron injection from the Ag layer, modification of the electron injection layer (EIL) corresponding to LiF should follow. According to previous several reports, in order to enhance electron injection in OLEDs, Cs2CO3 has been used as an ntype dopant [11–13]. Compared to pristine ETL thin films, Cs2CO3 doped organic ETL showed enhanced electron injection and transport properties [11,14,15]. In addition, compared to reactive metals as n-type dopants, the incorporation of Cs2CO3 can effectively eliminate diffusion of metal atoms into the underlying organic layer and consequential nonradiative recombination [11]. In this work, we present an approach which is useful for reducing the detrimental absorption effect without sacrificing the electrical conductance. To be specific, we have replaced the LiF/Al under the metal (Ag) cathode by a Csdoped organic ETL. First, we removed the Al layer because the Al layer is the main cause of absorption loss. Thus, LiF/ Al layer was then replaced by an organic electron transport layer (ETL). However, removing Al layer did not improve the optical transmittance in actual measurement and its optical simulations alone could not explain the observed results of our work. Thus, we investigated the surface microstructure of the cathode film. Thermally deposited organic ETL films can offer surface smoothness and optical transparency. In addition, with Cs2CO3 addition, enhanced electrical properties are expected. Therefore, with an objectivity of obtaining a smooth Ag film surface morphology which is favored in transmittance and conductivity, we have replaced the LiF/Al film by an organic ETL. In our discussion, we emphasize the importance of the microstructure in analyzing and improving both transmittance and electrical conductivity. In this study, we investigated the surface morphologyproperty relationship of the cathode and offered a very important yardstick in designing transparent electrodes. Finally, we demonstrated full functionality of TOLEDs having a cathode structure of Cs-ETL/Ag. 2. Experimental The TOLED has the following stacking sequence: indium tin oxide (ITO) (70 nm)/TAPC (55 nm)/TCTA: Firpic (7%) (5 nm)/DCzPPy:Firpic (10%) (5 nm)/BmPyPB (15 nm)/cathode/TAPC (60 nm). The organic material of each layer is listed in Table 1. In the first part of the experiment, we investigated the thickness effect of the Al layer, which is a part of the cathode, on the transmittance of TOLEDs. The cathode structure

is LiF (1 nm)/Al (X nm)/Ag (15 nm), which is formed on the ETL of BmPyPB (40 nm). The Al thickness was varied, at either 0 or 1.5 nm. Previously, in an effort to enhance the transmittance of TOLEDs, we added a capping layer (CL) of TAPC onto the cathode surface [16]. Using optical simulations, we investigated the transmittance of TOLEDs as a function of the CL thickness. The highest transmittance was obtained at a CL thickness of 60 nm. Based on this result, we fixed the CL thickness at 60 nm in this work. In the second part of the experiment, we replaced the LiF/Al part in the LiF/Al/Ag structure with a Cs-doped ETL (Cs-ETL). Thus, the TOLED cathode has the structure of Cs-ETL (40 nm)/Ag (15 nm). Cs2CO3 was co-evaporated with ETL and the doping level was varied, at 0, 10, 20 or 40%. Using a TOLED device with the Cs-ETL structure, we obtained the Cs doping concentration which yields the highest transmittance. In the final part, we demonstrated a fully functional TOLED with a Cs-ETL/Ag cathode. Its transmittance, EL spectrum, and current density (J)–voltage (V)–luminance (L) characteristics were compared to those of a TOLED with a LiF/Al/Ag cathode. The fabrication processes were described in our previous work [17]. All organic and metallic layers were deposited by thermal evaporation in a high vacuum chamber below 6.7  105 Pa. The fabricated OLEDs were encapsulated with a glass cap. In this course, a glass cap with cavity was glued using a UV curable resin. Also a moisture getter was placed inside the cavity glass. The electroluminescence spectrum was measured using a Minolta CS-1000. The current–voltage (I–V) and luminescence-voltage (L– V) characteristics were measured with a current/voltage source/measure unit (Keithley 238) and a Minolta CS100. Transmittance of the glass-cap encapsulated TOLED was measured using an UV–visible spectrophotometer (U-3501, Hitachi). The surface morphologies of the cathode films were investigated by means of scanning electron microscopy (SEM, Model: Sirion 400, Philips) and atomic force microscopy (AFM, Model: Park System, XE-100). Sheet resistance was measured using four-point probe system ((CMT-series, CHANG MIN CO., Ltd.) The simulations were performed using an OLED optical simulator, SimOLEDÒ [18,19] to optimize the characteristics of transparent OLEDs as a function of the thickness of the TAPC. Briefly, the program has inputs of the refractive index and thickness of every layer. The program employs thin film optics and the dipole emission model to calculate the optical and spectral characteristic of OLEDs. In order to obtain realistic simulation results, we used all of the measured optical constants (n, k) of organic materials, as

Table 1 The full name of organics and their functions. Abbreviation

Chemical name

Function

TAPC

1,1-bis[(di-4-tolylamino)phenyl]cyclohexane

TCTA Firpic DCzPPy BmPyPB

4,40 ,400 -tris(N-carbazolyl)-triphenylamine 0 Iridium(III)bis(4,6-difluorophenyl)-pyridinato-N,C2 )picolinate 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine 1,3-bis(3,5-di-pyrid-3-yl-phenyl)benzene

Hole transport layer Capping layer Emitter host Emitter dopant (blue) Emitter host Electron transport layer

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obtained using an ellipsometer (M-2000d, J.A. Woollam Co.). 3. Results and discussion 3.1. Preliminary studies on the Al insert In TOLEDs, a transparent cathode of LiF/Al/Ag structure is frequently used [7–9]. The sheet resistance is expected to decrease further as the thickness of the Al insert increases. In terms of the electrical performance of the device, increasing the thickness of the Al insert can lower the operation voltage condition of the OLED devices. However, in terms of the transmittance, increasing the Al thickness is not desirable. The Beer–Lambert law dictates an exponential decrease in the transmittance upon an increase in the film thickness. Thus, a thicker Al layer has lower transmittance. In this scheme, the transmittance of LiF/Al/Ag is expected to decrease as the thickness of the Al insert increases. Fig. 1 compares the simulated and measured transmittances of the LiF/Al/Ag structure. Here, the thicknesses of the LiF and Ag are fixed at 1 nm and 15 nm. The thickness of Al (tAl) was varied, at 0 nm or 1.5 nm. In the simulations, as expected, the insertion of Al lowers the transmittance in the entire visible wavelength range. However, in the experiments, the insertion of Al does not necessarily lower the transmittance in the entire visible wavelength range. To understand this result, we investigated the surface morphology of the Ag film. In thin-film optical simulations, the optical parameters of the refractive index (n) and extinction coefficient (k) as well as the thin film thickness of the media are considered as the main variables, whereas the actual microstructures of the thin films are not. Fig. 2 shows the two SEM surface images of 15-nm thick Ag films which reside on LiF (Fig. 2a) and on an Al

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layer (Fig. 2b), respectively. In both cases, the Ag films are granular, but the grain sizes of Ag on LiF are much smaller with more rough surface; The average grain size of the Ag film on LiF is 25 nm which is less about a tenth of that of Ag film on Al, and root-mean square (rms) roughness is 15 nm while Ag film on Al has 2 nm. As described earlier, granular and rough Ag films with less than 20 nm in thickness experience light loss due to surface plasmon resonance (SPR) [20–22] as well as grain boundary scattering [23]. It is noticeable that SPR-induced loss increases considerably with rms roughness of the film [20] while the scattering no longer occurs to any significant extent when the size of the grain boundary of the films is below the size of the wavelength of the light being scattered. From this, we expect that more rough granular surface of the Ag film on LiF experiences larger transmittance loss than that of Ag film on Al by SPR-induced absorption rather than scattering. This supports the result showing that the transmittance of LiF/Ag is lower than LiF/Al/Ag, as shown in Fig. 1. The surface morphology in Fig. 2 also explains the changes in the electrical conductance of the Ag films. The initial stage of metallic film growth is governed by the Volmer-Weber mechanism [24]. At the beginning of film growth, separate islands initially form after the deposition of metal. Next, when the number of islands increases, the individual islands start to connect physically. The film becomes continuous only when a certain thickness is reached, which corresponds to the percolation thickness. The percolation thickness is dependent on the material system. In Fig. 2, one can deduce that the percolation thickness of Ag on LiF is much thicker than that of Ag on Al. In the case of Ag on LiF, the Ag granules are not only small (25 nm) but are also rarely connected. Meanwhile, Ag on Al shows much larger granules, which are mainly physically connected.

Fig. 1. Simulated and measured transmittances of LiF/Al/Ag. tAl refers to the Al thickness. (Transmittance was measured under glass reference.)

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Fig. 2. Surface SEM images of (a) LiF/Ag (15 nm) and (b) LiF/Al (1.5 nm)/Ag (15 nm).

The surface energy difference can be used for a rough comparison of the percolation thickness of an identical metallic film on a different type of support. If the surface energy of the support is lower than the deposited film, there exists a driving force that exposes the surface of the support, which results in restraining the formation of the percolation network. In this context, a system which has a larger value of dg i=j ! Dci=j is expected to have a thicker percolation thickness. Here, dg i=j ! Dci=j is the surface energy difference between i (deposit) and j (support). The reported surface energies of LiF, Al and Ag are 0.34 J/ m2, 1.1 J/m2, and 1.3 J/m2, respectively [25]. The dg Ag=LiF ! DcAg=LiF and dg Ag=Al ! DcAg=Al values are 0.96 J/ m2 and 0.2 J/m2, respectiv/ely. Thus, the percolation thickness of Ag on LiF is thicker than that of Ag on Al. In our material systems, it is apparent that the Al insert has the effect of thinning the percolation thickness of the deposited Ag. The difference in the percolation thickness can be examined by measuring the sheet resistance. The measured sheet resistance of LiF/Al/Ag was 7 W=sq ! X=sq. The sheet resistance of LiF/Ag was approximately 107 W=sq ! X=sq, which is too high for OLED applications. This reflects the fact that percolation is established in Ag on Al, implying a thinner percolation thickness of Ag on an Al support. To sum up, in a TOLED with a cathode of LiF/Al/Ag, the Al contributes by making the Ag film continuous and facilitates the formation of percolation paths. As a result, compared to the Ag in the LiF case, higher transmittance and lower sheet resistance were achieved. The former is due to the reduced scattering and the latter is due to the high density of the percolation paths. 3.2. Organic film as an Al replacement In this section, we probe the possibility of replacing the Al insert with an organic material. Compared to metallic thin films, organic thin films bear lower optical absorption. For the purpose of eliminating the Al layer, we also removed the LiF as well because LiF is known to work almost exclusively with Al [14,26]. As mentioned before, we choose a Cs-doped ETL (Cs-ETL) as a replacement for Al.

Thus, the new cathode structure is Cs-ETL/Ag (15 nm). In the current work, we focus on achieving not only high optical transmittance but also low cathode sheet resistance. Regarding this task, we investigated both as a function of the Cs concentration, as controlled by adjusting the relative the deposition amounts of the ETL and Cs. The Fig. 3b(1) shows surface images of Ag films on undoped and Cs-doped ETL. With undoped ETL, Ag film on undoped ETL shows granular features with more high density of grain boundaries. As Cs is doped, significant grain growth can be observed. Larger grains and less boundaries are observed in Ag films. Such distinct difference in the surface morphology strongly indicates that the Cs addition is modifying the ETL surface to induce high wettability of Ag on the Cs-ETL. It is thought that Cs addition modifies the surface energy of the ETL to make Ag wetting more favorable. Based on the results of Figs. 1 and 2, this modified surface morphology due to high wettability in Cs-ETL/ Ag gives the result of improvement in transmittance and electrical conductance. Optical and electrical properties of the Ag film were shown in Fig. 3a. The sheet resistance decreases with a relatively steep slope just after Cs-doping. The reduced sheet resistance by Cs-doping is associated with enhanced conduction owing to the increased carrier concentration as well as modified microstructure. But then it saturates to 4.5 X/sq at Cs concentration higher than 10%. Also, the transmittance increases up to 20% Cs and then decreases slightly as the Cs concentration increases. The transmittance is still around 60% in the Cs concentration range of 10–40%. Referring to the SEM images above results, these observations indicate that the Ag surface smoothening effect of Cs addition has a certain upper bound. Fig. 3b(2) shows the nk products from measured n and k of Ag films which reside on undoped and Cs 20% doped ETL. The Cs-doped sample exhibits lower nk product. Because the amount of absorption is proportional to nk, this result indicates a decrease in the transmittance loss due to relatively low absorption in Ag on the Cs-ETL system. However, the measured nk values are only apparent values. The apparent change in nk values should be attributed as a microstructure induced effect. In other words, optics alone

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Fig. 3. (a) Transmittance and sheet resistance of the Cs-ETL/Ag(15 nm) film as a function of Cs concentration (Transmittance was measured under air reference and evaluated by averaging in the wavelength range of 400–700 nm.). (b-1) Surface SEM images of Cs-ETL/Ag films depending on Cs concentration. (b-2) nk product and (b-3) transmittance of two Cs-ETL/Ag films, both undoped or Cs 20% doped, in the visible wavelength range.

can only explain the transmittance change in terms of nk, which is misleading without the description of the microstructure. From the result of Fig. 3a, we chose a Cs concentration of 20%, which results in a highest transmittance and low sheet resistance of 63% and 4.5 X/sq, respectively. Regarding the quality of the transparent conductive films, one may use a figure of merit, which is defined as r/a [27]. Here r and a are the electrical conductivity and the visible absorption coefficient, respectively. The r/a is obtained using the relations of {Rs ln(T + R)}1, in which Rs is the sheet resistance in ohms per square, T and R is the total visible transmittance and reflectance, respectively. As it shows, a larger value of r/a indicates better performance of the transparent electrodes. The r/a values of our Cs-undoped and – doped(20%) ETL/Ag films are 0.7 and 1.2, respectively, which show Cs-doped (20%) ETL/Ag films have better opto-electrical performance than that of undoped ETL/Ag films and is comparable to that of film of doped-ZnO. Fig. 4a shows the surface roughness of the Ag films on a Cs (20%)-ETL and on LiF/Al according to SEM results as shown in Fig. 4b. As mentioned previously, the SEM images show that the Ag film on the Cs-ETL is almost featureless, whereas the morphology of the Ag film on Al is granular. The AFM scan diagrams revealed that the peak-to-peak roughness of Ag films on the Cs-ETL does not exceed 3 nm, while that on LiF/Al exceed 11 nm. The surface morphology of Ag on the Cs-ETL is not only smoother but also more continuous. Such a feature indicates the high wettability of Ag on the Cs-ETL, which leads to an improvement in the transmittance due to both less scattering- and less SPR-induced light loss as well as lower sheet resistance due to the dense percolation paths in the Ag film on the Cs-ETL, as mentioned above. The rms roughness values of Ag films on a Cs-ETL and on LiF/Al were found to be

0.6 nm and 1.9 nm, while the sheet resistances were 4.5 X/Sq and 7 X/Sq, respectively. 3.3. TOLED with a Cs-ETL/Ag cathode Fig. 5 demonstrates a TOLED bearing our new cathode structure of Cs-ETL/Ag. The figure shows the transmittance (Fig. 5a), EL spectra (Fig. 5b) and the J–V–L relationship (Fig. 5c and d) of TOLEDs with an Ag cathode on the Cs(20%)-ETL and LiF/Al structures. Fig. 5a shows that the transmittance of the TOLED with the Cs-ETL/Ag cathode is higher than that of the TOLED with the LiF/Al/Ag cathode without a change in the overall transmittance curve shapes throughout the visible wavelength range. The Cs-ETL/Ag cathode led to an improvement of up to 20% in the visible wavelength range. Transmittance of over 70% was achieved at 550 nm by means of conventional glass encapsulation. The inset of Fig. 5a shows an actual image of our TOLED with the Cs-ETL/Ag cathode. The institutional logo is clearly observable. Fig. 5b shows that replacing LiF/Al with a Cs-ETL does not induce any change in the EL spectra of the bottom and top side emissions. These results imply that replacing LiF/Al with an organic ETL introduces an enhancement of the transmittance without any significant change in the internal optics of the TOLED. Fig. 5c and d compare the J–V and L–V characteristics of two TOLEDs with different cathode structures. In the J–V plot, the two samples exhibit current levels that are nearly identical to the applied voltage. The L–V plot shows that the TOLED with the Cs-ETL/Ag structure shows an improvement in the luminance of the top emission. This improvement in the top side is thought to have its origin in the higher transmittance in the top cathode of the CsETL/Ag structure, as shown in Fig. 5a. Similar J–V characteristics or better L–V characteristics imply that the OLED

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Fig. 4. (a) The AFM line-scan across the Ag films on Cs(20%)-ETL and LiF/Al (The deep points in the AFM scan diagram LiF/Al/Ag represent the open granular border regions.). (b) The Ag film morphology on (a) Cs(20%)-ETL and LiF/Al.

Fig. 5. (a) The transmittances, (b) EL spectra and (c) J–V (d) L–V characteristics of TOLEDs with Cs(20%)-ETL/Ag or LiF/Al/Ag cathodes. (Transmittance was measured under air reference. Inset in (a) is the photograph of the institutional logo through the TOLED device.)

structure used in the TOLED with the LiF/Al/Ag cathode is applicable to a TOLED with a Cs-ETL/Ag cathode, which also implies that the Cs-ETL/Ag cathode structure is feasi-

ble for use in highly efficient TOLEDs. The technical importance of Figs. 4 and 5 is that the replacement of Al with a Cs-ETL makes the Ag surface smoother and more

J.W. Huh et al. / Organic Electronics 14 (2013) 2039–2045

continuous, which then improves the optical transmittance and electrical characteristics of the TOLED. From these results, we found that an improvement of the optical and electrical properties can be achieved by applying the Cs-ETL/Ag cathode structure. This indicates that our organic/metal hybrid cathode structure is a potential candidate as a cathode in efficient transparent OLEDs. 4. Conclusion Designing a highly transparent cathode is of prime importance in transparent OLED applications. For this reason, we designed a new organic/metal hybrid cathode with a Cs-ETL/Ag layer to obtain highly transparent efficient TOLEDs. In a conventional LiF/Al/Ag transparent cathode, the Al induces the formation of highly granular Ag films, which can potentially deteriorate the optical transmittance and sheet resistance. By replacing the LiF/Al with an organic ETL layer, it is possible to obtain a smooth continuous Ag surface morphology, which plays a significant part in reducing the light loss due to scattering and SPR, and enhancing the transmission. By doping Cs into the ETL, compared to LiF/Al/Ag cathode TOLEDs, better L–V characteristics are obtained. Also, identical EL spectra are obtained. In terms of TOLED processing, our cathode design is highly attractive because it obviates the need to modify the OLED stack structure. Our approach for an organic/metal transparent electrode may form the basis of highly efficient TOLED applications. Acknowledgements This work was supported by IT R&D program of MOTIE/ KEIT, Rep. of Korea (KI002068, Development of Eco-Emotional OLED Flat-Panel Lighting) and ICT R&D program 2013 of MSIP (Ministry of science ICT & Future Panning) (10041416, Development of Light and Space Adaptable Display). References [1] G. Gu, V. Bulovic, P.E. Burrows, S.R. Forrest, Transparent organic light emitting devices, Appl. Phys. Lett. 68 (1996) 2606–2608. [2] X. Zhou, M. Pfeiffer, J.S. Huang, J. Blochwitz-Nimoth, d.S. Qin, A. Werner, J. dreshsel, B. Maennig, K. Leo, Low-voltage inverted transparent vacuum deposited organic light-emitting diodes using electrical doping,, Appl. Phys. Lett. 81 (2002) 922–924. [3] H. Riel, S. Karg, T. Beierlein, B. Ruhstaller, W. Rieb, Phosphorescent top-emitting organic light emitting devices with improved light outcoupling, Appl. Phys. Lett. 82 (2003) 466–468.

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