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Fabrication of tunable [Al2O3:Alucone] thin-film encapsulations for top-emitting organic light-emitting diodes with high performance optical and barrier properties
Organic Electronics 15 (2014) 2546–2552
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Fabrication of tunable [Al2O3:Alucone] thin-film encapsulations for top-emitting organic light-emitting diodes with high performance optical and barrier properties Sun Feng-Bo a,b, Duan Yu a,⇑, Yang Yong-Qiang a, Chen Ping a, Duan Ya-Hui a, Wang Xiao a, Yang Dan a, Xue Kai-wen a a b
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Jilin 130012, China College of Science, Changchun University of Science and Technology, Jilin 130022, China
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Article history: Received 21 May 2014 Received in revised form 1 July 2014 Accepted 2 July 2014 Available online 28 July 2014 Keywords: Top-emitting OLEDs Molecular layer deposition Atomic layer deposition Thin film encapsulation
a b s t r a c t The optical and barrier properties of thin-film encapsulations (TFEs) for top-emitting organic light-emitting diodes (TEOLEDs) were investigated using TFEs fabricated by stacking multiple sets of inorganic–organic layers. The inorganic moisture barrier layers were prepared by atomic layer deposition (ALD) of Al2O3 using trimethylaluminum (TMA) and O3 as precursors and are shown to be efficient barriers against gases and vapors. The organic alucone layers were produced by molecular layer deposition (MLD) using TMA and ethylene glycol as precursors. The [Al2O3:Alucone] ALD/MLD films were used because their adjustable inorganic–organic nanolaminate composition allows for the tuning of the optical properties, thereby enhancing their application potential for the design and fabrication of high performance light out-coupling structures for TEOLEDs. By carefully adjusting the relative thickness ratio of the inorganic–organic encapsulation materials, optimized light extraction was achieved and the films not only maintained their high moisture barrier strength but also showed excellent optical performance. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Organic light-emitting devices (OLEDs) have emerged as a very promising flat-panel display technology because of their high efficiency, fast response time, and wide viewing angle [1–3]. One of the challenging technical issues that hinders further progress in OLED development is device encapsulation. Encapsulation is important for OLEDs to prevent the oxidation of the light-emitting materials and electrodes by blocking the permeation of water vapor and ambient oxygen, and protecting the devices from external shock [4–6]. Typically, UV-curable sealants, cover glass ⇑ Corresponding author. Tel.: +86 0431 85168243 13756531922 (mobile); fax: +86 0431 85168270. E-mail address: [email protected] (D. Yu). http://dx.doi.org/10.1016/j.orgel.2014.07.004 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.
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and desiccants are used to encapsulate bottom-emitting OLEDs. However, top-emitting OLEDs (TEOLEDs) need to be desiccant-free since desiccants are opaque and block light emission from the top transparent electrode. TEOLEDs have been widely studied due to their potential application in active matrix display fabrication [7], and in other lighting and display applications involving non-transparent substrates. In bottom-emitting OLEDs, where light is emitted downward through the substrate, the effective light-emitting area is limited by the opaqueness of the substrates and the thin film transistor (TFT) circuitry. In contrast, in TEOLED structures, light emerge primarily through the top surface away from the substrate and TFT circuitry. TEOLEDs are of immense importance to enable the monolithic integration of OLEDs onto a silicon chip [8,9]. However, the methods used for encapsulation
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of TEOLEDs must exhibit high performance gas barrier and optical transmission properties to be effective in broadening the potential application range for these devices. Several different methods for thin-film encapsulation (TFE) have been previously reported [10,11]. However, two issues remain: (a) barrier layers blocking the permeation of water vapor and ambient oxygen need to be formed using low-temperature processes that are compatible with OLEDs, and (b) an enhanced transmittance at visible wavelengths is crucial to the future commercialization of the devices. Existing deposition methods for preparing dense pinhole-free layers generally operate at elevated temperatures (above 200 °C) or require highly reactive processes such as plasma-enhanced chemical vapor deposition (CVD). Most organic materials, however, exhibit a low fragility and low glass-transition temperatures (below 100 °C). Practical TEOLED encapsulation methods therefore require low temperature deposition processes operating below the glasstransition temperature of the organic materials being used. Atomic layer deposition (ALD) is an attractive deposition method for preparing insulating films for a potential use as gate dielectric capacitors, and other devices [12]. ALD methods enable the deposition of very dense films and are therefore a promising technique for preparing encapsulation layers on top of OLEDs [13]. Thin ALD Al2O3 films were reported to be efficient barrier coatings for certain polymers [14,15]. Furthermore, the low deposition temperature (typically 80 °C) during the ALD process makes it compatible with temperature-sensitive substrates. However, thick layers of inorganic coatings can also result in the accumulation of defects and crystallization, thereby impairing the barrier properties [16]. Molecular layer deposition (MLD) is, in principle, similar to ALD, except that organic molecules are also used as building blocks [17]. The two techniques can be combined (ALD/ MLD) to produce hybrid inorganic–organic films that, due to their enhanced properties, can be more flexibly employed in the development of a broader range of TFE and device applications [18–21], including enhanced performance TEOLEDs. In this article, a hybrid ALD/MLD deposition method is described that has been used to fabricate several TEOLEDs, each with a [Al2O3:Alucone] ALD/MLD TFE on the top lightemitting surface of the device, but with different ratios of inorganic to organic deposition cycles, i.e. 5:1, 6:1 and 7:1, respectively, with an untreated device (without encapsulation) serving as the reference. Trimethylaluminum [TMA:Al(CH3)3] and ethylene glycol [EG:C2H6O2] were used as precursors to deposit aluminum alkoxide [alucone:Al-OCH2CH2OH] hybrid films during TFE preparation. The optical and barrier properties of each device were measured and an extensive analysis was performed, which is described in the following sections of this paper. The results demonstrate that the performance of TEOLEDs with hybrid inorganic–organic TFEs can be optimized by prudent selection of certain design parameters.
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comprises the following elements (from bottom to top, excluding the bottom glass cover): – The 100 nm-thick Ag anode, – a 5 nm-thick MoO3 layer, – a 35 nm-thick 4,40 ,400 -tris (N-3(3-methylphenyl)-N-phenylamino) triphenylamine (m-MTDATA) thin-film as a hole injection layer, – a 5 nm-thick N,N0 -biphenyl-N,N0 -bis(1-naphenyl)- [1,10 biphenyl]-4,40 -diamine (NPB) as a hole transport layer, – a 30 nm-thick tris-(8-hydroxyquinoline) aluminum (Alq3) with 2% C545T as a light-emitting layer, – a 35 nm Alq3 as an electron transport layer, – and the 0.5 nm-thick LiF capping layer with a 20 nmthick Ag cathode. The active area of the device was 3 3 mm2 and the deposition of the various encapsulation layers was performed as follows. A Lab Nano 9100 ALD system (Ensure Nanotech Inc.) was used to deposit both the Al2O3and the alucone films in a repetitive multiple stacking process until a sufficient number of stacks was grown to achieve a total TFE thickness of 75 nm. For each stack, Al2O3 ALD deposition was performed first, followed by alucone MLD deposition. Three different versions of [Al2O3:Alucone] TFEs were prepared and each version was installed on a different TEOLED. Each version had a different stack structure in terms of the ratio of the inorganic Al2O3 ALD to the organic alucone MLD nanolaminate layers. The ratios applied in this study were 5:1, 6:1 and 7:1, respectively. The 5:1 nanolaminate, for example, was grown using an alternation of 5 cycles of Al2O3 ALD and 1 cycle of alucone MLD. The timing for each nanolaminate layer deposition sequence was segmented into four intervals: t1, t2, t3, and t4, representing the TMA exposure time, the first N2purging time, the O3
2. Material and methods The basic design structure of the TEOLEDs (without encapsulation) used in this study is shown in Fig. 1 and
Fig. 1. Schematic diagram of the layer structure of a TEOLED encapsulated by TFE.
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or EG exposure time, and the second N2 purging time, respectively. For Al2O3 ALD deposition, the timing sequence was 0.02 s, 30 s, 0.1 s, and 10 s, and for alucone MLD deposition, the timing sequence was 0.02 s, 30 s, 0.07 s, and 120 s, respectively. The chamber pressure was 3 102 Pa. The warm-wall reactor was operated at a relatively low temperature of 80 °C. Conventional thermal evaporation was applied during the deposition process at 5 104 Pa without breaking the vacuum, and a nitrogen glove-box system was used at ambient conditions to assist with the encapsulation. Once the encapsulation process for all TEOLEDs was completed, a detailed measurement sequence was initiated. The water contact angles were measured using a Kruss contact angle goniometer [Model DSA30], where the sessile drop of 2–3 lL in volume was dispensed with a micro syringe. The thickness and refractive index of the deposited Al2O3 were determined by a Woolam variableangle spectroscopic ellipsometer. The root-mean-square (RMS) of the films was calculated from atomic force microscopy (AFM, Vecco) surface roughness measurements. The water vapor transmission rate (WVTR) and the TFE microstructure were determined and the results confirmed the effectiveness of the encapsulating films. The calculations demonstrated that increasing the Al2O3:Alucone deposition cycle ratio from 5:1 to 6:1 and then to 7:1 could enhance the light emission output coupling; which was later confirmed by the experimental results. To study the influence of ALD barrier films on integrated OLEDs, the L–I–V characteristics were examined to investigate the electrical behaviors of the TEOLEDs before and after forming the encapsulation structure. The electrical and emission characteristics of the devices were also measured with an Agilent 2920 source meter and a Minolta luminance meter LS-110 in air, and a PR655 spectrometer at room temperature.
3. Results and discussion 3.1. Overview of optical performance results As expected, a significantly enhanced TEOLED device performance and stability was achieved by [Al2O3:Alucone] encapsulation compared to the device without encapsulation (untreated device). Fig. 2(a)–(c) shows the theoretical analysis results [Ag(20 nm)/TFE/Air] for the TEOLED transmittance for the [Al2O3:Alucone] films with a cycle ratio of 5:1, 6:1 and 7:1 and an aggregate thickness fixed at 75 nm. The results demonstrate that the transmittance calculated for the 7:1 ratio film was higher than the transmittance calculated for the 6:1 and 5:1 ratio films. As shown in Fig. 2(d), for a TFE thickness above 65 nm, the light out-coupling behavior of the 7:1 ratio film is apparently enhanced compared to the other two films. In other words, increasing the proportion of Al2O3 can increase the Al2O3ALD barrier layer thickness with the same light out-coupling behavior. The optical effects induced by the ALD encapsulation layer can be explained by means of an optical model, which uses a classical transfer matrix method, based on the equivalence
between the photon emission probability of a dipole transition and the power radiated by a classical elementary dipole antenna [22]. The dipole emitter for this model was located at the interface of NPB and Alq3:2%C545T (refer to Fig. 1). It was demonstrated that maximum light emission does not occur at the highest cathode transmittance, but rather, is dependent on the interplay between different interference effects that are governed by the refractive index of the capping layer. This shift of the spectral characteristic as a function of both TFE thickness and refractive index can also be simulated by the optical model. By varying the TFE nanolaminate layer stacking structure, the interplay between the optical interferences can be controlled. 3.2. AFM results and water contact angle measurements To further investigate the [Al2O3:Alucone] films deposited with different deposition cycle ratios, as outlined earlier, the topographic information of the [Al2O3:Alucone] films was studied. The RMS of the obtained films over a scanned area of 500 500 nm2 was calculated from AFM measurements, and is similar to the RMS of the Si substrate. To further investigate the [Al2O3:Alucone] films, we investigated their macroscopic surface behaviors by performing water contact angle analysis. As shown in Table 1, the contact angle observed for the [Al2O3:Alucone] films increases systematically with increasing relative Al2O3 proportion (i.e., absolute thickness), with a consequent improvement in the hydrophobic properties. In addition, the surface roughness may also play an important role on the surface wetting properties. As shown in Table 1, with increasing Al2O3 thickness, the water contact angle increases from 46.0 ± 1.6° to 68.8 ± 2.4°, while the surface energy decreases from 174.5 mN/m to 108.1 mN/m, and the RMS surface roughness also decreases from 0.294 nm to 0.225 nm. Since the surface of alucone film is coarser than the surface of the Al2O3film [23], these results could be evidence for the possible dependence of the contact angle on the surface morphology. However, the change is very small, as shown in the AFM 3D images of Si substrates with [Al2O3:Alucone] films (Fig. 3). 3.3. Investigation of the water vapor transmission rate To evaluate the permeability of the [Al2O3:Alucone] films deposited by the combined ALD/MLD technique as a water diffusion barrier, WVTR measurements were performed using the calcium (Ca) corrosion test method which monitors changes in resistivity resulting in an ohmic behavior. A Ca layer with a thickness and area of 200 nm and 1 1 cm2, respectively, was deposited on a clean glass lined with 100 nm patterned-Al electrodes. The electrical measurements were performed using two electrodes connected by an SMU probe to an Agilent 2920 source meter. Fig. 4 shows the changes in WVTR for a fixed temperature and varying oxide precursor ratios. Notably, the WVTR decreased with increasing Al2O3 thickness. The thicker Al2O3 films exhibited a more stable trend for the conductivity vs. operational time. The WVTRs of the [Al2O3:Alucone] film with deposition cycle ratios of 5:1,
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Fig. 2. Results of the simulation of the transmittance of TFE films for different Al2O3 films thickness: (a) for an alucone layer thickness of 12.5 nm, (b) 10.7 nm and (c) 9.4 nm, respectively. (d) The calculated light out-coupling vs thickness characteristics of [Al2O3:Alucone] films at the EL peak of 540 nm.
Table 1 Surface characteristics and thickness of the [Al2O3:Alucone] films for different Al2O3:Alucone deposition cycle ratios. Al2O3:Alucone deposition cycle ratio Surface RMS (nm) Water contact angle (°) Surface energy (mN/m) Thickness (nm)
5:1 0.294 46.0 ± 1.6 174.5 77.805
6:1 and 7:1 were 3.81 104 g/m2/day, 2.92 104 g/m2/day and 8.68 105 g/m2/day, respectively. These results show that increasing the proportion of Al2O3 in the TFE leads to an improved water-barrier performance. 3.4. Performance of the TEOLEDs integrated with TFE Fig. 5 shows typical plots of the observed normalized luminance versus operating time for OLEDs with various TFEs. All luminance versus operating time measurements were performed non-stop at a DC voltage. The OLEDs were burned in for 100 s to minimize the increase in brightness immediately after turn-on. The lifetimes were measured from an initial luminance of nearly L0 = 850 cd/m2. In this study, we defined the lifetime as the decay time of the luminance to L/L0 = 0.5, i.e. the elapsed time until the instantaneous luminance of the OLEDs only reached 50% of its initial value. As seen in Fig. 5, the untreated device (no encapsulation) degraded more quickly in ambient air,
6:1 0.251 53.3 ± 0.6 155.2 78.205
7:1 0.225 68.8 ± 2.4 108.1 83.567
indicating that the degradation was caused by O2 and H2O permeation. However, a significant degradation was also observed in the nominally O2 and H2O-free environment. This degradation behavior is in agreement with previously reported results on NPB/Alq3 based OLEDs whose Alq3 cations were observed to be unstable fluorescence quenchers [24,25]. The I–V characteristics of encapsulated devices are shown in Fig. 6, revealing only slight differences in the I– V characteristics between the TEOLEDs with TFE and without encapsulation. Hence, the TFE growth temperature of 80 °C and the presence of an active oxidant did not lead to a degradation of the electrical properties of the TEOLEDs. Furthermore, with each successive ALD/MLD cycle, any effect appeared to become severely attenuated. However, all three devices encapsulated by ALD displayed better L–V characteristics, and the luminance of the TFE devices increased compared to the untreated device, similar to the reported effects of enhanced light out-coupling
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Normalized Conductance (1/R)
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Time (min) Fig. 4. Dependence of the normalized conductance vs time of the Ca corrosion tests with [Al2O3:Alucone] films deposited in a controlled environment of 20 °C and 60% RH.
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by dielectric capping layers on TEOLEDs [26]. However, Fig. 7 shows that the electro luminance (EL) spectra exhibit a different functional dependence on the TFE structure. The EL spectra slightly depend on the detailed optical architecture, and the [Al2O3:Alucone] films apparently were not
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Fig. 3. AFM 3D images of Si substrates with (a) an [Al2O3:Alucone] film with a 7:1 deposition cycle ratio; (b) an [Al2O3:Alucone] film with a 6:1 deposition cycle ratio; (c) an [Al2O3:Alucone] film with a 5:1 deposition cycle ratio.
bared device Al2O3:alucone 5:1 Al2O3:alucone 6:1 Al2O3:alucone 7:1
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Fig. 5. Luminance of the TEOLED encapsulated with [Al2O3:Alucone] films and the untreated device, a function of time measured at 25 °C and 80% RH.
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Voltage (v) Fig. 6. Luminance and current density vs voltage characteristics for the TFE of TEOLEDs with [Al2O3:Alucone] films, compared with the untreated device.
affecting the EL spectra characteristics. Fig. 8 shows the measured transmittance of the [Al2O3:Alucone] films. The transmittance typically varies between 48.3% and 51.5%.
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Intensity (a.u.)
fine-tuning the optical thickness of the TFE, the light extraction properties could be improved. Careful tuning of these parameters and, more importantly, the relative thickness of the inorganic–organic materials in the encapsulation layers are important considerations for the optimization of the barrier protection and the transmittance for top-emission organic electronics. Subsequent investigations will focus on the suitability and adjustability of the mechanical properties of ALD/MLD TFE for flexible device fabrication. We believe the tunable TFE deposition method presented in this paper is a promising technology for application in flexible organic device fabrication.
bared device
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Fig. 7. Comparison of EL spectra measured for the TEOLED encapsulated with [Al2O3:Alucone] films and the untreated device at a bias voltage of 8.0 V.
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This work was supported by the Program for International Science and Technology Cooperation (2014DFG12390), the National High Technology Research and Development Program of China (Grant No. 2011AA03A110), the Ministry of Science and Technology of China (Grant Nos. 2010CB327701 and 2013CB834802), the National Natural Science Foundation of China (Grant Nos. 61275024, 61274002, 61275033, 61377206 and 41001302), the Scientific and Technological Developing Scheme of Jilin Province (Grant Nos. 20140101204JC, 20130206020GX, and 20140520071JH), the Scientific and Technological Developing Scheme of Changchun (Grant No. 13GH02), and an Open Fund of the State Key Laboratory on Integrated Optoelectronics No. IOSKL2012KF01. References
500
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Wavelength (nm) Fig. 8. Transmittance of [Al2O3:Alucone] films based on the [Ag (20 nm)/ TFE/Air] structure.
Compared with the simulated data, the experimentally observed transmittances were slight lower but the overall tendency of an increase in transmittance is well consistent with the model. The [Al2O3:Alucone] film with the higher proportion of Al2O3 displayed a higher luminance and an increased light out-coupling. This experimental measurement data is in agreement with calculations from our theoretical optical model, which also demonstrate an improvement of light out-coupling due to the TFE capping layer. 4. Conclusions In summary, we have successfully used a hybrid ALD/ MLD deposition technique at a low temperature of 80 °C to synthesize multiple stacked layers of [Al2O3:Alucone] thin film encapsulations on the top surface of TEOLEDs with different proportions of inorganic vs. organic nanolaminate layers and hence thickness. Ceteris paribus, by increasing the effective thickness of the Al2O3 layers, the films exhibited a reduced surface roughness, a lower WVTR of 8.68 105 g/m2/day, and a longer continuous operating lifetime. At the same time, it was found that, by
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Fabrication of tunable [Al2O3:Alucone] thin-film encapsulations for top-emitting organic light-emitting diodes with high performance optical and barrier properties Sun Feng-Bo a,b, Duan Yu a,⇑, Yang Yong-Qiang a, Chen Ping a, Duan Ya-Hui a, Wang Xiao a, Yang Dan a, Xue Kai-wen a a b
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Jilin 130012, China College of Science, Changchun University of Science and Technology, Jilin 130022, China
a r t i c l e
i n f o
Article history: Received 21 May 2014 Received in revised form 1 July 2014 Accepted 2 July 2014 Available online 28 July 2014 Keywords: Top-emitting OLEDs Molecular layer deposition Atomic layer deposition Thin film encapsulation
a b s t r a c t The optical and barrier properties of thin-film encapsulations (TFEs) for top-emitting organic light-emitting diodes (TEOLEDs) were investigated using TFEs fabricated by stacking multiple sets of inorganic–organic layers. The inorganic moisture barrier layers were prepared by atomic layer deposition (ALD) of Al2O3 using trimethylaluminum (TMA) and O3 as precursors and are shown to be efficient barriers against gases and vapors. The organic alucone layers were produced by molecular layer deposition (MLD) using TMA and ethylene glycol as precursors. The [Al2O3:Alucone] ALD/MLD films were used because their adjustable inorganic–organic nanolaminate composition allows for the tuning of the optical properties, thereby enhancing their application potential for the design and fabrication of high performance light out-coupling structures for TEOLEDs. By carefully adjusting the relative thickness ratio of the inorganic–organic encapsulation materials, optimized light extraction was achieved and the films not only maintained their high moisture barrier strength but also showed excellent optical performance. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Organic light-emitting devices (OLEDs) have emerged as a very promising flat-panel display technology because of their high efficiency, fast response time, and wide viewing angle [1–3]. One of the challenging technical issues that hinders further progress in OLED development is device encapsulation. Encapsulation is important for OLEDs to prevent the oxidation of the light-emitting materials and electrodes by blocking the permeation of water vapor and ambient oxygen, and protecting the devices from external shock [4–6]. Typically, UV-curable sealants, cover glass ⇑ Corresponding author. Tel.: +86 0431 85168243 13756531922 (mobile); fax: +86 0431 85168270. E-mail address: [email protected] (D. Yu). http://dx.doi.org/10.1016/j.orgel.2014.07.004 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.
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and desiccants are used to encapsulate bottom-emitting OLEDs. However, top-emitting OLEDs (TEOLEDs) need to be desiccant-free since desiccants are opaque and block light emission from the top transparent electrode. TEOLEDs have been widely studied due to their potential application in active matrix display fabrication [7], and in other lighting and display applications involving non-transparent substrates. In bottom-emitting OLEDs, where light is emitted downward through the substrate, the effective light-emitting area is limited by the opaqueness of the substrates and the thin film transistor (TFT) circuitry. In contrast, in TEOLED structures, light emerge primarily through the top surface away from the substrate and TFT circuitry. TEOLEDs are of immense importance to enable the monolithic integration of OLEDs onto a silicon chip [8,9]. However, the methods used for encapsulation
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of TEOLEDs must exhibit high performance gas barrier and optical transmission properties to be effective in broadening the potential application range for these devices. Several different methods for thin-film encapsulation (TFE) have been previously reported [10,11]. However, two issues remain: (a) barrier layers blocking the permeation of water vapor and ambient oxygen need to be formed using low-temperature processes that are compatible with OLEDs, and (b) an enhanced transmittance at visible wavelengths is crucial to the future commercialization of the devices. Existing deposition methods for preparing dense pinhole-free layers generally operate at elevated temperatures (above 200 °C) or require highly reactive processes such as plasma-enhanced chemical vapor deposition (CVD). Most organic materials, however, exhibit a low fragility and low glass-transition temperatures (below 100 °C). Practical TEOLED encapsulation methods therefore require low temperature deposition processes operating below the glasstransition temperature of the organic materials being used. Atomic layer deposition (ALD) is an attractive deposition method for preparing insulating films for a potential use as gate dielectric capacitors, and other devices [12]. ALD methods enable the deposition of very dense films and are therefore a promising technique for preparing encapsulation layers on top of OLEDs [13]. Thin ALD Al2O3 films were reported to be efficient barrier coatings for certain polymers [14,15]. Furthermore, the low deposition temperature (typically 80 °C) during the ALD process makes it compatible with temperature-sensitive substrates. However, thick layers of inorganic coatings can also result in the accumulation of defects and crystallization, thereby impairing the barrier properties [16]. Molecular layer deposition (MLD) is, in principle, similar to ALD, except that organic molecules are also used as building blocks [17]. The two techniques can be combined (ALD/ MLD) to produce hybrid inorganic–organic films that, due to their enhanced properties, can be more flexibly employed in the development of a broader range of TFE and device applications [18–21], including enhanced performance TEOLEDs. In this article, a hybrid ALD/MLD deposition method is described that has been used to fabricate several TEOLEDs, each with a [Al2O3:Alucone] ALD/MLD TFE on the top lightemitting surface of the device, but with different ratios of inorganic to organic deposition cycles, i.e. 5:1, 6:1 and 7:1, respectively, with an untreated device (without encapsulation) serving as the reference. Trimethylaluminum [TMA:Al(CH3)3] and ethylene glycol [EG:C2H6O2] were used as precursors to deposit aluminum alkoxide [alucone:Al-OCH2CH2OH] hybrid films during TFE preparation. The optical and barrier properties of each device were measured and an extensive analysis was performed, which is described in the following sections of this paper. The results demonstrate that the performance of TEOLEDs with hybrid inorganic–organic TFEs can be optimized by prudent selection of certain design parameters.
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comprises the following elements (from bottom to top, excluding the bottom glass cover): – The 100 nm-thick Ag anode, – a 5 nm-thick MoO3 layer, – a 35 nm-thick 4,40 ,400 -tris (N-3(3-methylphenyl)-N-phenylamino) triphenylamine (m-MTDATA) thin-film as a hole injection layer, – a 5 nm-thick N,N0 -biphenyl-N,N0 -bis(1-naphenyl)- [1,10 biphenyl]-4,40 -diamine (NPB) as a hole transport layer, – a 30 nm-thick tris-(8-hydroxyquinoline) aluminum (Alq3) with 2% C545T as a light-emitting layer, – a 35 nm Alq3 as an electron transport layer, – and the 0.5 nm-thick LiF capping layer with a 20 nmthick Ag cathode. The active area of the device was 3 3 mm2 and the deposition of the various encapsulation layers was performed as follows. A Lab Nano 9100 ALD system (Ensure Nanotech Inc.) was used to deposit both the Al2O3and the alucone films in a repetitive multiple stacking process until a sufficient number of stacks was grown to achieve a total TFE thickness of 75 nm. For each stack, Al2O3 ALD deposition was performed first, followed by alucone MLD deposition. Three different versions of [Al2O3:Alucone] TFEs were prepared and each version was installed on a different TEOLED. Each version had a different stack structure in terms of the ratio of the inorganic Al2O3 ALD to the organic alucone MLD nanolaminate layers. The ratios applied in this study were 5:1, 6:1 and 7:1, respectively. The 5:1 nanolaminate, for example, was grown using an alternation of 5 cycles of Al2O3 ALD and 1 cycle of alucone MLD. The timing for each nanolaminate layer deposition sequence was segmented into four intervals: t1, t2, t3, and t4, representing the TMA exposure time, the first N2purging time, the O3
2. Material and methods The basic design structure of the TEOLEDs (without encapsulation) used in this study is shown in Fig. 1 and
Fig. 1. Schematic diagram of the layer structure of a TEOLED encapsulated by TFE.
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or EG exposure time, and the second N2 purging time, respectively. For Al2O3 ALD deposition, the timing sequence was 0.02 s, 30 s, 0.1 s, and 10 s, and for alucone MLD deposition, the timing sequence was 0.02 s, 30 s, 0.07 s, and 120 s, respectively. The chamber pressure was 3 102 Pa. The warm-wall reactor was operated at a relatively low temperature of 80 °C. Conventional thermal evaporation was applied during the deposition process at 5 104 Pa without breaking the vacuum, and a nitrogen glove-box system was used at ambient conditions to assist with the encapsulation. Once the encapsulation process for all TEOLEDs was completed, a detailed measurement sequence was initiated. The water contact angles were measured using a Kruss contact angle goniometer [Model DSA30], where the sessile drop of 2–3 lL in volume was dispensed with a micro syringe. The thickness and refractive index of the deposited Al2O3 were determined by a Woolam variableangle spectroscopic ellipsometer. The root-mean-square (RMS) of the films was calculated from atomic force microscopy (AFM, Vecco) surface roughness measurements. The water vapor transmission rate (WVTR) and the TFE microstructure were determined and the results confirmed the effectiveness of the encapsulating films. The calculations demonstrated that increasing the Al2O3:Alucone deposition cycle ratio from 5:1 to 6:1 and then to 7:1 could enhance the light emission output coupling; which was later confirmed by the experimental results. To study the influence of ALD barrier films on integrated OLEDs, the L–I–V characteristics were examined to investigate the electrical behaviors of the TEOLEDs before and after forming the encapsulation structure. The electrical and emission characteristics of the devices were also measured with an Agilent 2920 source meter and a Minolta luminance meter LS-110 in air, and a PR655 spectrometer at room temperature.
3. Results and discussion 3.1. Overview of optical performance results As expected, a significantly enhanced TEOLED device performance and stability was achieved by [Al2O3:Alucone] encapsulation compared to the device without encapsulation (untreated device). Fig. 2(a)–(c) shows the theoretical analysis results [Ag(20 nm)/TFE/Air] for the TEOLED transmittance for the [Al2O3:Alucone] films with a cycle ratio of 5:1, 6:1 and 7:1 and an aggregate thickness fixed at 75 nm. The results demonstrate that the transmittance calculated for the 7:1 ratio film was higher than the transmittance calculated for the 6:1 and 5:1 ratio films. As shown in Fig. 2(d), for a TFE thickness above 65 nm, the light out-coupling behavior of the 7:1 ratio film is apparently enhanced compared to the other two films. In other words, increasing the proportion of Al2O3 can increase the Al2O3ALD barrier layer thickness with the same light out-coupling behavior. The optical effects induced by the ALD encapsulation layer can be explained by means of an optical model, which uses a classical transfer matrix method, based on the equivalence
between the photon emission probability of a dipole transition and the power radiated by a classical elementary dipole antenna [22]. The dipole emitter for this model was located at the interface of NPB and Alq3:2%C545T (refer to Fig. 1). It was demonstrated that maximum light emission does not occur at the highest cathode transmittance, but rather, is dependent on the interplay between different interference effects that are governed by the refractive index of the capping layer. This shift of the spectral characteristic as a function of both TFE thickness and refractive index can also be simulated by the optical model. By varying the TFE nanolaminate layer stacking structure, the interplay between the optical interferences can be controlled. 3.2. AFM results and water contact angle measurements To further investigate the [Al2O3:Alucone] films deposited with different deposition cycle ratios, as outlined earlier, the topographic information of the [Al2O3:Alucone] films was studied. The RMS of the obtained films over a scanned area of 500 500 nm2 was calculated from AFM measurements, and is similar to the RMS of the Si substrate. To further investigate the [Al2O3:Alucone] films, we investigated their macroscopic surface behaviors by performing water contact angle analysis. As shown in Table 1, the contact angle observed for the [Al2O3:Alucone] films increases systematically with increasing relative Al2O3 proportion (i.e., absolute thickness), with a consequent improvement in the hydrophobic properties. In addition, the surface roughness may also play an important role on the surface wetting properties. As shown in Table 1, with increasing Al2O3 thickness, the water contact angle increases from 46.0 ± 1.6° to 68.8 ± 2.4°, while the surface energy decreases from 174.5 mN/m to 108.1 mN/m, and the RMS surface roughness also decreases from 0.294 nm to 0.225 nm. Since the surface of alucone film is coarser than the surface of the Al2O3film [23], these results could be evidence for the possible dependence of the contact angle on the surface morphology. However, the change is very small, as shown in the AFM 3D images of Si substrates with [Al2O3:Alucone] films (Fig. 3). 3.3. Investigation of the water vapor transmission rate To evaluate the permeability of the [Al2O3:Alucone] films deposited by the combined ALD/MLD technique as a water diffusion barrier, WVTR measurements were performed using the calcium (Ca) corrosion test method which monitors changes in resistivity resulting in an ohmic behavior. A Ca layer with a thickness and area of 200 nm and 1 1 cm2, respectively, was deposited on a clean glass lined with 100 nm patterned-Al electrodes. The electrical measurements were performed using two electrodes connected by an SMU probe to an Agilent 2920 source meter. Fig. 4 shows the changes in WVTR for a fixed temperature and varying oxide precursor ratios. Notably, the WVTR decreased with increasing Al2O3 thickness. The thicker Al2O3 films exhibited a more stable trend for the conductivity vs. operational time. The WVTRs of the [Al2O3:Alucone] film with deposition cycle ratios of 5:1,
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Fig. 2. Results of the simulation of the transmittance of TFE films for different Al2O3 films thickness: (a) for an alucone layer thickness of 12.5 nm, (b) 10.7 nm and (c) 9.4 nm, respectively. (d) The calculated light out-coupling vs thickness characteristics of [Al2O3:Alucone] films at the EL peak of 540 nm.
Table 1 Surface characteristics and thickness of the [Al2O3:Alucone] films for different Al2O3:Alucone deposition cycle ratios. Al2O3:Alucone deposition cycle ratio Surface RMS (nm) Water contact angle (°) Surface energy (mN/m) Thickness (nm)
5:1 0.294 46.0 ± 1.6 174.5 77.805
6:1 and 7:1 were 3.81 104 g/m2/day, 2.92 104 g/m2/day and 8.68 105 g/m2/day, respectively. These results show that increasing the proportion of Al2O3 in the TFE leads to an improved water-barrier performance. 3.4. Performance of the TEOLEDs integrated with TFE Fig. 5 shows typical plots of the observed normalized luminance versus operating time for OLEDs with various TFEs. All luminance versus operating time measurements were performed non-stop at a DC voltage. The OLEDs were burned in for 100 s to minimize the increase in brightness immediately after turn-on. The lifetimes were measured from an initial luminance of nearly L0 = 850 cd/m2. In this study, we defined the lifetime as the decay time of the luminance to L/L0 = 0.5, i.e. the elapsed time until the instantaneous luminance of the OLEDs only reached 50% of its initial value. As seen in Fig. 5, the untreated device (no encapsulation) degraded more quickly in ambient air,
6:1 0.251 53.3 ± 0.6 155.2 78.205
7:1 0.225 68.8 ± 2.4 108.1 83.567
indicating that the degradation was caused by O2 and H2O permeation. However, a significant degradation was also observed in the nominally O2 and H2O-free environment. This degradation behavior is in agreement with previously reported results on NPB/Alq3 based OLEDs whose Alq3 cations were observed to be unstable fluorescence quenchers [24,25]. The I–V characteristics of encapsulated devices are shown in Fig. 6, revealing only slight differences in the I– V characteristics between the TEOLEDs with TFE and without encapsulation. Hence, the TFE growth temperature of 80 °C and the presence of an active oxidant did not lead to a degradation of the electrical properties of the TEOLEDs. Furthermore, with each successive ALD/MLD cycle, any effect appeared to become severely attenuated. However, all three devices encapsulated by ALD displayed better L–V characteristics, and the luminance of the TFE devices increased compared to the untreated device, similar to the reported effects of enhanced light out-coupling
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Normalized Conductance (1/R)
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Time (min) Fig. 4. Dependence of the normalized conductance vs time of the Ca corrosion tests with [Al2O3:Alucone] films deposited in a controlled environment of 20 °C and 60% RH.
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by dielectric capping layers on TEOLEDs [26]. However, Fig. 7 shows that the electro luminance (EL) spectra exhibit a different functional dependence on the TFE structure. The EL spectra slightly depend on the detailed optical architecture, and the [Al2O3:Alucone] films apparently were not
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Fig. 3. AFM 3D images of Si substrates with (a) an [Al2O3:Alucone] film with a 7:1 deposition cycle ratio; (b) an [Al2O3:Alucone] film with a 6:1 deposition cycle ratio; (c) an [Al2O3:Alucone] film with a 5:1 deposition cycle ratio.
bared device Al2O3:alucone 5:1 Al2O3:alucone 6:1 Al2O3:alucone 7:1
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Fig. 5. Luminance of the TEOLED encapsulated with [Al2O3:Alucone] films and the untreated device, a function of time measured at 25 °C and 80% RH.
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Voltage (v) Fig. 6. Luminance and current density vs voltage characteristics for the TFE of TEOLEDs with [Al2O3:Alucone] films, compared with the untreated device.
affecting the EL spectra characteristics. Fig. 8 shows the measured transmittance of the [Al2O3:Alucone] films. The transmittance typically varies between 48.3% and 51.5%.
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Intensity (a.u.)
fine-tuning the optical thickness of the TFE, the light extraction properties could be improved. Careful tuning of these parameters and, more importantly, the relative thickness of the inorganic–organic materials in the encapsulation layers are important considerations for the optimization of the barrier protection and the transmittance for top-emission organic electronics. Subsequent investigations will focus on the suitability and adjustability of the mechanical properties of ALD/MLD TFE for flexible device fabrication. We believe the tunable TFE deposition method presented in this paper is a promising technology for application in flexible organic device fabrication.
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Fig. 7. Comparison of EL spectra measured for the TEOLED encapsulated with [Al2O3:Alucone] films and the untreated device at a bias voltage of 8.0 V.
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This work was supported by the Program for International Science and Technology Cooperation (2014DFG12390), the National High Technology Research and Development Program of China (Grant No. 2011AA03A110), the Ministry of Science and Technology of China (Grant Nos. 2010CB327701 and 2013CB834802), the National Natural Science Foundation of China (Grant Nos. 61275024, 61274002, 61275033, 61377206 and 41001302), the Scientific and Technological Developing Scheme of Jilin Province (Grant Nos. 20140101204JC, 20130206020GX, and 20140520071JH), the Scientific and Technological Developing Scheme of Changchun (Grant No. 13GH02), and an Open Fund of the State Key Laboratory on Integrated Optoelectronics No. IOSKL2012KF01. References
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Wavelength (nm) Fig. 8. Transmittance of [Al2O3:Alucone] films based on the [Ag (20 nm)/ TFE/Air] structure.
Compared with the simulated data, the experimentally observed transmittances were slight lower but the overall tendency of an increase in transmittance is well consistent with the model. The [Al2O3:Alucone] film with the higher proportion of Al2O3 displayed a higher luminance and an increased light out-coupling. This experimental measurement data is in agreement with calculations from our theoretical optical model, which also demonstrate an improvement of light out-coupling due to the TFE capping layer. 4. Conclusions In summary, we have successfully used a hybrid ALD/ MLD deposition technique at a low temperature of 80 °C to synthesize multiple stacked layers of [Al2O3:Alucone] thin film encapsulations on the top surface of TEOLEDs with different proportions of inorganic vs. organic nanolaminate layers and hence thickness. Ceteris paribus, by increasing the effective thickness of the Al2O3 layers, the films exhibited a reduced surface roughness, a lower WVTR of 8.68 105 g/m2/day, and a longer continuous operating lifetime. At the same time, it was found that, by
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