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Emission characteristics of transparent organic light-emitting diodes with molybdenum oxide capping layers
Synthetic Metals 262 (2020) 116335
Contents lists available at ScienceDirect
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Emission characteristics of transparent organic light-emitting diodes with molybdenum oxide capping layers
T
Gunel Huseynovaa, Yong Hyun Kimb, Jae-Hyun Leec, Jonghee Leec,* a
RIPE & 3D Printing, Hanbat National University, 34158, Republic of Korea Department of Display Engineering, Pukyong National University, Busan, 48513, Republic of Korea c Department of Creative Convergence Engineering, Hanbat National University, Daejeon, 34158, Republic of Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: Transparent OLEDs Capping layer Outcoupling efficiency enhancement Bi-Directional emission control Top emission Molybdenum oxide
In this work, we have studied a molybdenum oxide (MoO3) layer as an effective capping layer (CL) for TOLEDs. We varied the thickness of the capping layer in order to investigate its effects on the device performance. Both the top and bottom emission of the devices were found to be easily controlled by changing the thickness of MoO3 layer. Especially, at the CL thickness of 40 nm, the current efficiency shows 2.2 times enhancement compared to reference device which has no capping layer and exhibited the highest transmittance value. The results also revealed that the transmittance and light outcoupling efficiency are significantly dependent on the capping layer thickness and exhibit identical trend in change. In addition, the changes in the emission characteristics were optically simulated which agreed well with the experimental data.
1. Introduction Organic light-emitting diodes (OLEDs) are being hailed as a milestone in the development of the flexible light-emitting technology. These devices are now found in a variety of applications ranging from cell phones, TVs, digital cameras to smart home devices [1,2]. By virtue of industrial promotion and marketing, OLED display and lighting technologies have swiftly pervaded all aspects of consumers’ lives [3]. Since the emergence of a research article from the pioneering scientists at Eastman Kodak in 1987, reporting the world’s first working OLED [4], OLEDs have been continuously evolved and modified in their performance, flexibility, size, and form [3]. Through ongoing research and million-dollar investments by major academic and industrial institutions, huge contribution has been made to OLED technology. Nowadays, OLED displays are widely applied and considered mainstream in the OLED industry. They are embedded in TVs, watches, smartphones, tablets, laptops, personal computers, etc. produced by world’s leading companies such as Samsung, LG, Panasonic, and Sony [3,5,6]. The main motive for growing OLED production includes several factors inevitable to meet the modern consumer needs. Unlike the traditional liquid-crystal displays (LCDs) and inorganic light-emitting diodes (LEDs), OLED display technology does not require any filters or backlight and can deliver sharp and real black color. OLEDs make flexible, rollable, and even foldable screens with the widest viewing angles and highest image quality. These displays offer infinite contrast, ⁎
vivid colors, fast response time, and low power consumption. Every pixel on them is controlled by an individual OLED. They are also much simpler to manufacture and can be made significantly thinner and lighter as well as in smaller and larger sizes compared to LCDs and LEDs [2,6,7]. Besides the well-established OLED displays, OLED panels are becoming viable candidates to replace the conventional incandescent light sources due to their softly glowing diffusive and bright light which is very close to natural light, and due to their design freedom, flexibility, and transparency which pave the way for large area lighting sources able to be integrated into windows, doors, and walls as well as for light fixtures able to be designed as small phosphorescent flowers lighting up a table or as magnificent mesh and chain chandeliers lighting huge buildings from high ceilings. OLED lighting technology is also unique for its color tunability to conform with the surrounding environment. These thin and efficient OLED panels are indeed the only practical large-area and homogenous lighting sources and are already being produced by well-known companies such as LG, Philips, Konica Minolta, etc. [8–12]. Furthermore, optically transparent organic materials enable transparent OLEDs (TOLEDs) which are of great interest for display and lighting technology. TOLEDs permit both bottom and top emission due to their transparency [13,14]. They also allow stacking of multiple OLEDs on each other which is favorable for overall lifetime and color output of the devices. Yet, despite the passionate development
Corresponding author. E-mail address: [email protected] (J. Lee).
https://doi.org/10.1016/j.synthmet.2020.116335 Received 20 December 2019; Received in revised form 6 February 2020; Accepted 20 February 2020 0379-6779/ © 2020 Elsevier B.V. All rights reserved.
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using an optical simulation software.
surrounding them, their possible exciting applications, and demonstrated prototypes, TOLEDs still face severe challenges related to their light outcoupling efficiency [15]. Enhancement of the light out coupling efficiency of transparent organic light-emitting diodes (TOLEDs) while maintaining their high transparency and electrical performance is the key challenge science faces throughout a growing number of research studies. Investigations show that application of a capping layer enables efficient control on bi-directional emission of TOLEDs [15–25]. Therefore, several approaches which involve nano-structured scattering layers [15], low-index grids or capping layers (CLs) [16–20], high-refractive-index substrates [21], colloidal lithography [22], insertion of thin silver layers between the indium tin oxide (ITO) anode and the hole transporting layer (HTL) [23], etc., have been developed. For instance, Park et al. significantly improved the performance of polymer TOLEDs using aluminum-doped zinc oxide top electrodes modified with a solution-based titanium oxide layer [24]. J.-I. Lee’s group demonstrated a random nano-scattering layer (RSL) as an internal light extracting structure to enhance outcoupling efficiency and stabilize emission spectra of white TOLEDs [25]. In our previous work, we applied organic CLs of different thicknesses to TOLEDs and systematically studied the influence of the CL thickness variation on the emission properties of the devices [17]. Herein, based on our previous results, we introduced a molybdenum oxide (MoO3) layer as a CL instead of the organic one to investigate optical properties of the TOLEDs. It is known that surface plasmons and absorption could be suppressed, in other words, the transmittance could be improved when a material of high dielectric constants (ε) is incorporated on top of Ag metal layer. [26] We found that both top and bottom emission of the devices can be controlled by changing the thickness of MoO3 CL. The results also revealed that the top emission efficiency and the measured transmittance changes show identical trend depending on the thickness variation of the CL. Moreover, our experimental data agreed well with our pre-experimental simulations. These results suggest that MoO3 CL can enable the optimization of TOLEDs performance.
3. Results and discussion MeO-TPD was chosen as the p-type host material due to its high transparency originating from its wide band gap, high hole mobility as well as its low-cost arising from its easy synthesis [26–28]. We doped it with a strong electron acceptor of F4-TCNQ which is a typical dopant for MeO-TPD as the combination of these two small molecules exhibits remarkable performance when used as a hole transport material in p-i-n junction OLEDs. This is mainly due to the reduction of the hole injection barrier between MeO-TPD and the anode which results in a doped transport layer that serves both as an HTL and HIL for the devices [29]. The ETL was formed of the same thickness as the HTL using an n-type material Bphen doped with Cs in order to achieve effective electron injection from the cathode and reach the n-side conductivities comparable to that of the p-side. Thin EBL and HBL of the same thickness and composed of NPD and BAlq, respectively, were deposited to prevent the flow of the electrons and holes from the EML towards the oppositely charged electrodes. An iridium complex Ir(MDQ)2acac was used as a red phosphorescent emitter and doped into NPD at the concentration of 10 wt% to form the EML for red light emission. An ultrathin layer of silver was applied as the metal top electrode due to its high transparency as a transparent cathode is crucial for top-emitting diodes. MoO3 films with different thicknesses ranging from 0 to 150 nm were deposited on top of the cathode to enhance the efficiency of the top emission. In order to analyze the optical effects of the CLs on the light emission from the TOLEDs, their electroluminescence (EL) properties were characterized in the first place. Fig. 1 (b) shows the EL intensity emitted from the top side of the capped and uncapped TOLEDs in the forward direction at a driving current of 1 mA. The devices show one main peak positioned around ∼612 nm and no visible shoulder peaks which is a typical emission profile for NPD-based EMLs doped with red iridium dopants [14,17,30]. As seen, despite the fact that the EL peak wavelength does not shift with the incorporation of the CLs, the spectral intensity of the EL emission is strongly dependent on the CL thickness. The uncapped OLED exhibits an EL spectrum having maximum intensity at ∼612 nm with a broad full width at half-maximum (FWHM) of 75 nm. The addition of a 40-nm thick CL is accompanied by strong enhancement in the spectrum intensity and its FWHM. After the further increase of the CL thickness up to 100 nm, the EL spectrum becomes narrower again with decreased peak intensity lower than that of the uncapped device. This phenomenon can be attributed to the variation of the optical interference occurring at the interfaces formed by the dielectric MoO3 CL with the cathode and air and can be efficiently adjusted by the CL thickness [17,31,32]. The measured transmittance data of the devices as a function of the light wavelength and CL thickness, are summarized in Fig. 2. As seen from Fig. 2 (a), the highest transmittance range of the uncapped device is around ∼40 % and spreads near the ultra-violet (UV) spectral region starting from ∼460 nm down to ∼380 nm. Clearly, it absorbs significantly in the visible portion of the spectrum with maximum transmittance of ∼22 %. It was found that the introduction of a 40 nm-thick MoO3 CL slightly decreases the transmittance near the UV-region whereas the visible light transmittance of the device, on the other hand, becomes twice higher than that of the uncapped OLED in the entire part of the visible spectrum. And although the 100 nm-thick CL increases the near UV-light transmittance intensity by a factor of 4, the visible light transmittance of the diode drops even below the uncapped device values. These results are consistent with the EL data depicted in Fig. 1 (b). Fig. 2 (b) depicts the variation of the transmittance versus the CL thickness. The transmittance increases gradually from ∼22 % to ∼45 % until MoO3 layer thickness reaches 40 nm. The transmittance is reduced below the pristine values by further increasing the CL thickness up to 110 nm after which, it starts to increase again with the increased
2. Materials and methods p-i-n-junction TOLEDs constructed based on N,N′-Bis(naphthalen-1yl)-N,N′-bis(phenyl)benzidine (NPD) host material doped with an iridium-based red phosphorescent emitter Ir(MDQ)2(acac) were composed of the following layers: glass substrate, ITO (anode, 90 nm)/ 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ)doped N,N,N',N'-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD) (HTL, 60 nm)/NPD (electron blocking layer (EBL), 10 nm)/NPD:Ir (MDQ)2acac (emissive layer (EML), 20 nm)/Bis(8-hydroxy-2-methylquinoline)-(4-phenylphenoxy)aluminum (BAlq) (hole blocking layer (HBL), 10 nm)/Cesium (Cs)-doped diphenyl-1,10-phenanthroline (Bphen) (electron transport layer (ETL), 60 nm)/silver (Ag) (cathode, 25 nm)/ MoO3 (CL of different thicknesses) (see Fig. 1 (a)). The transparent devices were built on ITO-coated glass substrates by the deposition of all layers through thermal evaporation in an ultrahigh vacuum chamber at a low base pressure of 10−8 mbar. A deposition speed of MoO3 layer is maintained 1 Å/sec at room temperature. In situ doping of the transport layers was achieved by co-evaporation of the host and dopant materials. The doping concentrations of the HTL and EML host matrices were 4 and 10 wt%, respectively, whereas the ETL was doped with Cs at 1:1 wt ratio. The fabricated transparent OLEDs were electrically and optically characterized at the same time using a Keithley Source Meter and an Instrument Systems GmbH CAS140 spectrometer, respectively, to analyze the current-voltage as well as the luminance patterns of the diodes in the forward direction. A self-calibrated spectro-goniometer was applied to calculate external quantum efficiency (EQE) and luminous efficacy (LE) of the devices using the collected spatial light distribution data of the OLEDs. Optical simulations of the TOLEDs were carried out 2
Synthetic Metals 262 (2020) 116335
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Fig. 1. a) The device structure of the fabricated TOLEDs; b) electroluminescence (EL) spectra of the devices.
efficiency (CE) and the MoO3 CL thickness for the light emission from the top and bottom sides as well as for the total light emission, is reflected in Fig. 4. As seen, the thickness of the capping layer on the top electrode affects significantly the top side luminance behavior, although its effects are almost negligible for both the bottom and total emission. At the CL thickness of 40 nm, the CE enhancement ratio of the top side reaches its maximum value of 2.2 from the pristine value of 1 and its minimum is observed when the CL thickness reaches 100 nm. Unlike the transmittance, the enhancement ratio of the CE starts constantly increasing when the CL thickness is increased above 100 nm. The oscillatory behavior of the CE enhancement ratio is also clearly visible which is consistent with the abovementioned results. On the other hand, the response of the bottom and total CE enhancement to the CL thickness differs substantially from the top side. Here, in both cases, the addition of the CL and its thickness have marginal effects on the CE enhancement ratio which are manifested by slight changes in the ratio fluctuating around the pristine values. It is due to the fact that the top and bottom emission profiles of TOLEDs are different from each other as well as from the Lambertian assumption [17,31]. The CE measured from the top and bottom sides, including the sum derived from both directions and the ratio of the bottom emission to the top emission are plotted against the CL thickness in Fig. 5 (a). The visible similar trend between the bottom emission CE and the CE from both sides implies that the major portion of the total outcoupled light is emitted from the bottom side. As seen, in both cases, after the addition of the CL, the efficiency first decreases until the CL thickness reaches 20 nm and then it increases with the increased CL thickness. However, the TOLED with a 100 nm-thick CL exhibits almost identical CE values with the uncapped diode. The CL thickness further increased to 120 nm,
CL thickness. However, the CL thicknesses above 140 nm result in the reduction of the transmittance once again. This intermittent behavior in the transmittance is also ascribed to the interference effects and the detection of the right trend in its variation is crucial for the achievement of the constructive interference within TOLEDs. Therefore, it should be studied properly to find the optimal CL thickness in order to enhance the total light emission of the devices [31]. The comparison of the current density-voltage (J–V) and luminancevoltage (L–V) characteristics of the fabricated TOLEDs capped with 40 nm- and 100 nm-thick MoO3 layers and the reference device without a CL, is presented in Fig. 3. The identical J–V curves given in Fig. 3 (a) indicate that the incorporation of an additional layer on top of the cathode for capping purposes did not affect the electrical performance of the TOLEDs by any means. However, as the top-side transmittance and EL behavior of the devices were directly affected by the CL and its thickness (see Fig. 1 (b) and 2 (a, b)), the relative tendencies are also observed in the L–V curves depending on the CL thickness. Consistent with the above-discussed optical properties, the luminance of the TOLED having a 40 nm-thick CL is enhanced remarkably compared to the uncapped device. At 4.5 V, the calculated luminance increased from ∼950 cd/m2 for the reference OLED to ∼2200 cd/m2 with the addition of a 40 nm-thick layer of MoO3. Nevertheless, the 100 nm-thick CL degrades the luminance to a lower value of ∼760 cd/m2 in comparison with the reference values. As mentioned above, this difference in the luminance characteristics mainly originates from the variation occurring in the cathode transmittance as well as from the optical interference changes caused by the CLs of different thicknesses and the obtained results are in well agreement with each other. The relation between the enhancement ratio of the current
Fig. 2. Top side transmittance profile of the devices as a function of a) the wavelength and b) MoO3 capping layer (CL) thickness. 3
Synthetic Metals 262 (2020) 116335
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Fig. 3. Top side a) current density-voltage (J-V) and b) luminance-voltage (L-V) characteristics of the devices.
light was primarily emitted through the bottom side. In this case, the CE enhances with the inclusion of the CLs with the thicknesses up to 40 nm. Further increase in the CL thickness reduces the CE down to the pristine values, although it starts increasing slightly with the CLs of 120 nm or larger thicknesses. It should be noted that the variation in the top-emission CE shows a trend opposite to that seen in the variation of the bottom emission CE and the CE from both sides due to the differences in the light emission characteristics between the top and bottom sides of the TOLEDs [17]. The bottom-to-top emission ratio, on the other hand, is significantly affected by the MoO3 CL, following the same variation trend as the CE from the bottom side and the total CE from both sides with the difference that its values fluctuate greatly depending on the CL thickness. Optical simulations of the TOLEDs were also performed to verify the device parameters through the comparison of the experimental results with the simulated data. The results are presented in Fig. 5 (b). The figures clearly show that the range-bound oscillating trend observed in the experimental outcome is in well agreement with the simulated data and the magnitude of the differences between them is minor. Both experiments and simulations show that a 40 nm-thick CL is the optimum condition to enhance the CE of the top emission as it primarily affects only the top direction and has no observed adverse effect on the CE from the bottom emission or the total CE from both sides.
Fig. 4. Comparison of the current efficiency (CE) enhancement of the top and bottom emission of the devices as a function of MoO3 capping layer (CL) thickness.
slightly increases the CE while the CLs of the thicknesses larger than 120 nm start reducing the CE of the devices again. Despite these noticeable changes in the CE, results show that the incorporation of the CL and the variation of its thickness did not alter the profile of the bottom emission and the total emission from both sides essentially as the CE values obtained from the capped devices vary slightly from those of the reference uncapped OLED. It is due to the fact that the impacts of a CL incorporation mainly involve the transmittance and optical structure of the top electrode and affects the light emission from the top direction [17]. However, the CE of the top-side performance is significantly lower than that of the bottom side or both sides in sum due to the fact that
4. Conclusions In summary, we have studied an oxide dielectric MoO3 as an effective CL material for TOLEDs. We investigated the effects of the applied MoO3 CL thickness on the electrical and optical performance of the TOLEDs. It was found that the top emission efficiency can be significantly enhanced through the improvement of the top-side
Fig. 5. a) Experimentally obtained current efficiency (CE) and bottom to top emission ratio values and b) simulated data of the luminous intensity of the devices as a function of MoO3 capping layer (CL) thickness. 4
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transmittance without sacrificing the bottom-side and overall light emission properties of the devices. A 40 nm-thick MoO3 CL is suggested to enable easy control over the top and bottom emission of TOLEDs. The work revealed that the transmittance and light emission performance of the bottom side and both sides in total as well as the overall electrical characteristics of the devices are basically not dependent on the CL thickness and exhibit only minor variations. Also, the simulated and experimentally measured data matched well meaning that the proposed method is a reliable way to enhance the top emission efficiency of TOLEDs and control the bottom-side and overall efficiency simultaneously. In addition, the bottom-to-top emission ratio can also be effectively adjusted by careful optimization of the CL thickness and total device transmittance.
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CRediT authorship contribution statement Gunel Huseynova: Investigation, Writing - original draft. Yong Hyun Kim: Investigation, Data curation, Validation. Jae-Hyun Lee: Data curation, Writing - review & editing. Jonghee Lee: Conceptualization, Writing - review & editing, Supervision, Project administration. Declaration of Competing Interest None. Acknowledgements This research was supported by the research fund of Hanbat National University in 2019. References [1] Y.H. Kim, J. Lee, S. Hofmann, M.C. Gather, L. Müller-Meskamp, K. Leo, Achieving High Efficiency and Improved Stability in ITO–Free Transparent Organic LightEmitting Diodes with Conductive Polymer Electrodes, Adv. Funct. Mater. 23 (2013) 3763. [2] N.T. Kalyani, S.J. Dhoble, Organic light emitting diodes: saving lighting technology – a review, Renewable Sustainable Energy Rev. 16 (2012) 2696. [3] E. Zysman-Colman, Recent advances in materials for organic light-emitting diodes, Beilstein J. Org. Chem. 14 (2018) 1944. [4] H.J. Jang, Lee J.Y. Lee, J. Kwak, D. Lee, ParkJ. –H, B. Lee, Y.Y. Noh, Progress of display performances: AR, VR, QLED, OLED, and TFT, J. Inform. Dis. 20 (2019) 1. [5] A. Salehi, X. Fu, D.-H. Shin, F. So, Recent advances in OLED optical design, Adv. Funct. Mater. 29 (2019) 1808803. [6] C. Sekine, Y. Tsubata, T. Yamada, M. Kitano, S. Doi, Recent progress of high performance polymer OLED and OPV materials for organic printed electronics, Sci. Technol. Adv. Mater. 15 (2014) 034203. [7] H.-W. Chen, J.-H. Lee, B.-Y. Lin, S. Chen, S.-T. Wu, Liquid crystal display and organic light-emitting diode display: present status and future perspectives, Light Sci. Appl. 7 (2018) 17168. [8] S. Reineke, M. Thomschke, B. Lüssem, K. Leo, White organic light-emitting diodes: status and perspective, Rev. Mod. Phys. 85 (2013) 1245. [9] W. Brütting, J. Frischeisen, T.D. Schmidt, B.J. Scholz, C. Mayr, Device efficiency of organic light-emitting diodes: progress by improved light outcoupling, Physica Status Solidi A Appl. Res. 210 (2013) 44.
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Emission characteristics of transparent organic light-emitting diodes with molybdenum oxide capping layers
T
Gunel Huseynovaa, Yong Hyun Kimb, Jae-Hyun Leec, Jonghee Leec,* a
RIPE & 3D Printing, Hanbat National University, 34158, Republic of Korea Department of Display Engineering, Pukyong National University, Busan, 48513, Republic of Korea c Department of Creative Convergence Engineering, Hanbat National University, Daejeon, 34158, Republic of Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: Transparent OLEDs Capping layer Outcoupling efficiency enhancement Bi-Directional emission control Top emission Molybdenum oxide
In this work, we have studied a molybdenum oxide (MoO3) layer as an effective capping layer (CL) for TOLEDs. We varied the thickness of the capping layer in order to investigate its effects on the device performance. Both the top and bottom emission of the devices were found to be easily controlled by changing the thickness of MoO3 layer. Especially, at the CL thickness of 40 nm, the current efficiency shows 2.2 times enhancement compared to reference device which has no capping layer and exhibited the highest transmittance value. The results also revealed that the transmittance and light outcoupling efficiency are significantly dependent on the capping layer thickness and exhibit identical trend in change. In addition, the changes in the emission characteristics were optically simulated which agreed well with the experimental data.
1. Introduction Organic light-emitting diodes (OLEDs) are being hailed as a milestone in the development of the flexible light-emitting technology. These devices are now found in a variety of applications ranging from cell phones, TVs, digital cameras to smart home devices [1,2]. By virtue of industrial promotion and marketing, OLED display and lighting technologies have swiftly pervaded all aspects of consumers’ lives [3]. Since the emergence of a research article from the pioneering scientists at Eastman Kodak in 1987, reporting the world’s first working OLED [4], OLEDs have been continuously evolved and modified in their performance, flexibility, size, and form [3]. Through ongoing research and million-dollar investments by major academic and industrial institutions, huge contribution has been made to OLED technology. Nowadays, OLED displays are widely applied and considered mainstream in the OLED industry. They are embedded in TVs, watches, smartphones, tablets, laptops, personal computers, etc. produced by world’s leading companies such as Samsung, LG, Panasonic, and Sony [3,5,6]. The main motive for growing OLED production includes several factors inevitable to meet the modern consumer needs. Unlike the traditional liquid-crystal displays (LCDs) and inorganic light-emitting diodes (LEDs), OLED display technology does not require any filters or backlight and can deliver sharp and real black color. OLEDs make flexible, rollable, and even foldable screens with the widest viewing angles and highest image quality. These displays offer infinite contrast, ⁎
vivid colors, fast response time, and low power consumption. Every pixel on them is controlled by an individual OLED. They are also much simpler to manufacture and can be made significantly thinner and lighter as well as in smaller and larger sizes compared to LCDs and LEDs [2,6,7]. Besides the well-established OLED displays, OLED panels are becoming viable candidates to replace the conventional incandescent light sources due to their softly glowing diffusive and bright light which is very close to natural light, and due to their design freedom, flexibility, and transparency which pave the way for large area lighting sources able to be integrated into windows, doors, and walls as well as for light fixtures able to be designed as small phosphorescent flowers lighting up a table or as magnificent mesh and chain chandeliers lighting huge buildings from high ceilings. OLED lighting technology is also unique for its color tunability to conform with the surrounding environment. These thin and efficient OLED panels are indeed the only practical large-area and homogenous lighting sources and are already being produced by well-known companies such as LG, Philips, Konica Minolta, etc. [8–12]. Furthermore, optically transparent organic materials enable transparent OLEDs (TOLEDs) which are of great interest for display and lighting technology. TOLEDs permit both bottom and top emission due to their transparency [13,14]. They also allow stacking of multiple OLEDs on each other which is favorable for overall lifetime and color output of the devices. Yet, despite the passionate development
Corresponding author. E-mail address: [email protected] (J. Lee).
https://doi.org/10.1016/j.synthmet.2020.116335 Received 20 December 2019; Received in revised form 6 February 2020; Accepted 20 February 2020 0379-6779/ © 2020 Elsevier B.V. All rights reserved.
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using an optical simulation software.
surrounding them, their possible exciting applications, and demonstrated prototypes, TOLEDs still face severe challenges related to their light outcoupling efficiency [15]. Enhancement of the light out coupling efficiency of transparent organic light-emitting diodes (TOLEDs) while maintaining their high transparency and electrical performance is the key challenge science faces throughout a growing number of research studies. Investigations show that application of a capping layer enables efficient control on bi-directional emission of TOLEDs [15–25]. Therefore, several approaches which involve nano-structured scattering layers [15], low-index grids or capping layers (CLs) [16–20], high-refractive-index substrates [21], colloidal lithography [22], insertion of thin silver layers between the indium tin oxide (ITO) anode and the hole transporting layer (HTL) [23], etc., have been developed. For instance, Park et al. significantly improved the performance of polymer TOLEDs using aluminum-doped zinc oxide top electrodes modified with a solution-based titanium oxide layer [24]. J.-I. Lee’s group demonstrated a random nano-scattering layer (RSL) as an internal light extracting structure to enhance outcoupling efficiency and stabilize emission spectra of white TOLEDs [25]. In our previous work, we applied organic CLs of different thicknesses to TOLEDs and systematically studied the influence of the CL thickness variation on the emission properties of the devices [17]. Herein, based on our previous results, we introduced a molybdenum oxide (MoO3) layer as a CL instead of the organic one to investigate optical properties of the TOLEDs. It is known that surface plasmons and absorption could be suppressed, in other words, the transmittance could be improved when a material of high dielectric constants (ε) is incorporated on top of Ag metal layer. [26] We found that both top and bottom emission of the devices can be controlled by changing the thickness of MoO3 CL. The results also revealed that the top emission efficiency and the measured transmittance changes show identical trend depending on the thickness variation of the CL. Moreover, our experimental data agreed well with our pre-experimental simulations. These results suggest that MoO3 CL can enable the optimization of TOLEDs performance.
3. Results and discussion MeO-TPD was chosen as the p-type host material due to its high transparency originating from its wide band gap, high hole mobility as well as its low-cost arising from its easy synthesis [26–28]. We doped it with a strong electron acceptor of F4-TCNQ which is a typical dopant for MeO-TPD as the combination of these two small molecules exhibits remarkable performance when used as a hole transport material in p-i-n junction OLEDs. This is mainly due to the reduction of the hole injection barrier between MeO-TPD and the anode which results in a doped transport layer that serves both as an HTL and HIL for the devices [29]. The ETL was formed of the same thickness as the HTL using an n-type material Bphen doped with Cs in order to achieve effective electron injection from the cathode and reach the n-side conductivities comparable to that of the p-side. Thin EBL and HBL of the same thickness and composed of NPD and BAlq, respectively, were deposited to prevent the flow of the electrons and holes from the EML towards the oppositely charged electrodes. An iridium complex Ir(MDQ)2acac was used as a red phosphorescent emitter and doped into NPD at the concentration of 10 wt% to form the EML for red light emission. An ultrathin layer of silver was applied as the metal top electrode due to its high transparency as a transparent cathode is crucial for top-emitting diodes. MoO3 films with different thicknesses ranging from 0 to 150 nm were deposited on top of the cathode to enhance the efficiency of the top emission. In order to analyze the optical effects of the CLs on the light emission from the TOLEDs, their electroluminescence (EL) properties were characterized in the first place. Fig. 1 (b) shows the EL intensity emitted from the top side of the capped and uncapped TOLEDs in the forward direction at a driving current of 1 mA. The devices show one main peak positioned around ∼612 nm and no visible shoulder peaks which is a typical emission profile for NPD-based EMLs doped with red iridium dopants [14,17,30]. As seen, despite the fact that the EL peak wavelength does not shift with the incorporation of the CLs, the spectral intensity of the EL emission is strongly dependent on the CL thickness. The uncapped OLED exhibits an EL spectrum having maximum intensity at ∼612 nm with a broad full width at half-maximum (FWHM) of 75 nm. The addition of a 40-nm thick CL is accompanied by strong enhancement in the spectrum intensity and its FWHM. After the further increase of the CL thickness up to 100 nm, the EL spectrum becomes narrower again with decreased peak intensity lower than that of the uncapped device. This phenomenon can be attributed to the variation of the optical interference occurring at the interfaces formed by the dielectric MoO3 CL with the cathode and air and can be efficiently adjusted by the CL thickness [17,31,32]. The measured transmittance data of the devices as a function of the light wavelength and CL thickness, are summarized in Fig. 2. As seen from Fig. 2 (a), the highest transmittance range of the uncapped device is around ∼40 % and spreads near the ultra-violet (UV) spectral region starting from ∼460 nm down to ∼380 nm. Clearly, it absorbs significantly in the visible portion of the spectrum with maximum transmittance of ∼22 %. It was found that the introduction of a 40 nm-thick MoO3 CL slightly decreases the transmittance near the UV-region whereas the visible light transmittance of the device, on the other hand, becomes twice higher than that of the uncapped OLED in the entire part of the visible spectrum. And although the 100 nm-thick CL increases the near UV-light transmittance intensity by a factor of 4, the visible light transmittance of the diode drops even below the uncapped device values. These results are consistent with the EL data depicted in Fig. 1 (b). Fig. 2 (b) depicts the variation of the transmittance versus the CL thickness. The transmittance increases gradually from ∼22 % to ∼45 % until MoO3 layer thickness reaches 40 nm. The transmittance is reduced below the pristine values by further increasing the CL thickness up to 110 nm after which, it starts to increase again with the increased
2. Materials and methods p-i-n-junction TOLEDs constructed based on N,N′-Bis(naphthalen-1yl)-N,N′-bis(phenyl)benzidine (NPD) host material doped with an iridium-based red phosphorescent emitter Ir(MDQ)2(acac) were composed of the following layers: glass substrate, ITO (anode, 90 nm)/ 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ)doped N,N,N',N'-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD) (HTL, 60 nm)/NPD (electron blocking layer (EBL), 10 nm)/NPD:Ir (MDQ)2acac (emissive layer (EML), 20 nm)/Bis(8-hydroxy-2-methylquinoline)-(4-phenylphenoxy)aluminum (BAlq) (hole blocking layer (HBL), 10 nm)/Cesium (Cs)-doped diphenyl-1,10-phenanthroline (Bphen) (electron transport layer (ETL), 60 nm)/silver (Ag) (cathode, 25 nm)/ MoO3 (CL of different thicknesses) (see Fig. 1 (a)). The transparent devices were built on ITO-coated glass substrates by the deposition of all layers through thermal evaporation in an ultrahigh vacuum chamber at a low base pressure of 10−8 mbar. A deposition speed of MoO3 layer is maintained 1 Å/sec at room temperature. In situ doping of the transport layers was achieved by co-evaporation of the host and dopant materials. The doping concentrations of the HTL and EML host matrices were 4 and 10 wt%, respectively, whereas the ETL was doped with Cs at 1:1 wt ratio. The fabricated transparent OLEDs were electrically and optically characterized at the same time using a Keithley Source Meter and an Instrument Systems GmbH CAS140 spectrometer, respectively, to analyze the current-voltage as well as the luminance patterns of the diodes in the forward direction. A self-calibrated spectro-goniometer was applied to calculate external quantum efficiency (EQE) and luminous efficacy (LE) of the devices using the collected spatial light distribution data of the OLEDs. Optical simulations of the TOLEDs were carried out 2
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Fig. 1. a) The device structure of the fabricated TOLEDs; b) electroluminescence (EL) spectra of the devices.
efficiency (CE) and the MoO3 CL thickness for the light emission from the top and bottom sides as well as for the total light emission, is reflected in Fig. 4. As seen, the thickness of the capping layer on the top electrode affects significantly the top side luminance behavior, although its effects are almost negligible for both the bottom and total emission. At the CL thickness of 40 nm, the CE enhancement ratio of the top side reaches its maximum value of 2.2 from the pristine value of 1 and its minimum is observed when the CL thickness reaches 100 nm. Unlike the transmittance, the enhancement ratio of the CE starts constantly increasing when the CL thickness is increased above 100 nm. The oscillatory behavior of the CE enhancement ratio is also clearly visible which is consistent with the abovementioned results. On the other hand, the response of the bottom and total CE enhancement to the CL thickness differs substantially from the top side. Here, in both cases, the addition of the CL and its thickness have marginal effects on the CE enhancement ratio which are manifested by slight changes in the ratio fluctuating around the pristine values. It is due to the fact that the top and bottom emission profiles of TOLEDs are different from each other as well as from the Lambertian assumption [17,31]. The CE measured from the top and bottom sides, including the sum derived from both directions and the ratio of the bottom emission to the top emission are plotted against the CL thickness in Fig. 5 (a). The visible similar trend between the bottom emission CE and the CE from both sides implies that the major portion of the total outcoupled light is emitted from the bottom side. As seen, in both cases, after the addition of the CL, the efficiency first decreases until the CL thickness reaches 20 nm and then it increases with the increased CL thickness. However, the TOLED with a 100 nm-thick CL exhibits almost identical CE values with the uncapped diode. The CL thickness further increased to 120 nm,
CL thickness. However, the CL thicknesses above 140 nm result in the reduction of the transmittance once again. This intermittent behavior in the transmittance is also ascribed to the interference effects and the detection of the right trend in its variation is crucial for the achievement of the constructive interference within TOLEDs. Therefore, it should be studied properly to find the optimal CL thickness in order to enhance the total light emission of the devices [31]. The comparison of the current density-voltage (J–V) and luminancevoltage (L–V) characteristics of the fabricated TOLEDs capped with 40 nm- and 100 nm-thick MoO3 layers and the reference device without a CL, is presented in Fig. 3. The identical J–V curves given in Fig. 3 (a) indicate that the incorporation of an additional layer on top of the cathode for capping purposes did not affect the electrical performance of the TOLEDs by any means. However, as the top-side transmittance and EL behavior of the devices were directly affected by the CL and its thickness (see Fig. 1 (b) and 2 (a, b)), the relative tendencies are also observed in the L–V curves depending on the CL thickness. Consistent with the above-discussed optical properties, the luminance of the TOLED having a 40 nm-thick CL is enhanced remarkably compared to the uncapped device. At 4.5 V, the calculated luminance increased from ∼950 cd/m2 for the reference OLED to ∼2200 cd/m2 with the addition of a 40 nm-thick layer of MoO3. Nevertheless, the 100 nm-thick CL degrades the luminance to a lower value of ∼760 cd/m2 in comparison with the reference values. As mentioned above, this difference in the luminance characteristics mainly originates from the variation occurring in the cathode transmittance as well as from the optical interference changes caused by the CLs of different thicknesses and the obtained results are in well agreement with each other. The relation between the enhancement ratio of the current
Fig. 2. Top side transmittance profile of the devices as a function of a) the wavelength and b) MoO3 capping layer (CL) thickness. 3
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Fig. 3. Top side a) current density-voltage (J-V) and b) luminance-voltage (L-V) characteristics of the devices.
light was primarily emitted through the bottom side. In this case, the CE enhances with the inclusion of the CLs with the thicknesses up to 40 nm. Further increase in the CL thickness reduces the CE down to the pristine values, although it starts increasing slightly with the CLs of 120 nm or larger thicknesses. It should be noted that the variation in the top-emission CE shows a trend opposite to that seen in the variation of the bottom emission CE and the CE from both sides due to the differences in the light emission characteristics between the top and bottom sides of the TOLEDs [17]. The bottom-to-top emission ratio, on the other hand, is significantly affected by the MoO3 CL, following the same variation trend as the CE from the bottom side and the total CE from both sides with the difference that its values fluctuate greatly depending on the CL thickness. Optical simulations of the TOLEDs were also performed to verify the device parameters through the comparison of the experimental results with the simulated data. The results are presented in Fig. 5 (b). The figures clearly show that the range-bound oscillating trend observed in the experimental outcome is in well agreement with the simulated data and the magnitude of the differences between them is minor. Both experiments and simulations show that a 40 nm-thick CL is the optimum condition to enhance the CE of the top emission as it primarily affects only the top direction and has no observed adverse effect on the CE from the bottom emission or the total CE from both sides.
Fig. 4. Comparison of the current efficiency (CE) enhancement of the top and bottom emission of the devices as a function of MoO3 capping layer (CL) thickness.
slightly increases the CE while the CLs of the thicknesses larger than 120 nm start reducing the CE of the devices again. Despite these noticeable changes in the CE, results show that the incorporation of the CL and the variation of its thickness did not alter the profile of the bottom emission and the total emission from both sides essentially as the CE values obtained from the capped devices vary slightly from those of the reference uncapped OLED. It is due to the fact that the impacts of a CL incorporation mainly involve the transmittance and optical structure of the top electrode and affects the light emission from the top direction [17]. However, the CE of the top-side performance is significantly lower than that of the bottom side or both sides in sum due to the fact that
4. Conclusions In summary, we have studied an oxide dielectric MoO3 as an effective CL material for TOLEDs. We investigated the effects of the applied MoO3 CL thickness on the electrical and optical performance of the TOLEDs. It was found that the top emission efficiency can be significantly enhanced through the improvement of the top-side
Fig. 5. a) Experimentally obtained current efficiency (CE) and bottom to top emission ratio values and b) simulated data of the luminous intensity of the devices as a function of MoO3 capping layer (CL) thickness. 4
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transmittance without sacrificing the bottom-side and overall light emission properties of the devices. A 40 nm-thick MoO3 CL is suggested to enable easy control over the top and bottom emission of TOLEDs. The work revealed that the transmittance and light emission performance of the bottom side and both sides in total as well as the overall electrical characteristics of the devices are basically not dependent on the CL thickness and exhibit only minor variations. Also, the simulated and experimentally measured data matched well meaning that the proposed method is a reliable way to enhance the top emission efficiency of TOLEDs and control the bottom-side and overall efficiency simultaneously. In addition, the bottom-to-top emission ratio can also be effectively adjusted by careful optimization of the CL thickness and total device transmittance.
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CRediT authorship contribution statement Gunel Huseynova: Investigation, Writing - original draft. Yong Hyun Kim: Investigation, Data curation, Validation. Jae-Hyun Lee: Data curation, Writing - review & editing. Jonghee Lee: Conceptualization, Writing - review & editing, Supervision, Project administration. Declaration of Competing Interest None. Acknowledgements This research was supported by the research fund of Hanbat National University in 2019. References [1] Y.H. Kim, J. Lee, S. Hofmann, M.C. Gather, L. Müller-Meskamp, K. Leo, Achieving High Efficiency and Improved Stability in ITO–Free Transparent Organic LightEmitting Diodes with Conductive Polymer Electrodes, Adv. Funct. Mater. 23 (2013) 3763. [2] N.T. Kalyani, S.J. Dhoble, Organic light emitting diodes: saving lighting technology – a review, Renewable Sustainable Energy Rev. 16 (2012) 2696. [3] E. Zysman-Colman, Recent advances in materials for organic light-emitting diodes, Beilstein J. Org. Chem. 14 (2018) 1944. [4] H.J. Jang, Lee J.Y. Lee, J. Kwak, D. Lee, ParkJ. –H, B. Lee, Y.Y. Noh, Progress of display performances: AR, VR, QLED, OLED, and TFT, J. Inform. Dis. 20 (2019) 1. [5] A. Salehi, X. Fu, D.-H. Shin, F. So, Recent advances in OLED optical design, Adv. Funct. Mater. 29 (2019) 1808803. [6] C. Sekine, Y. Tsubata, T. Yamada, M. Kitano, S. Doi, Recent progress of high performance polymer OLED and OPV materials for organic printed electronics, Sci. Technol. Adv. Mater. 15 (2014) 034203. [7] H.-W. Chen, J.-H. Lee, B.-Y. Lin, S. Chen, S.-T. Wu, Liquid crystal display and organic light-emitting diode display: present status and future perspectives, Light Sci. Appl. 7 (2018) 17168. [8] S. Reineke, M. Thomschke, B. Lüssem, K. Leo, White organic light-emitting diodes: status and perspective, Rev. Mod. Phys. 85 (2013) 1245. [9] W. Brütting, J. Frischeisen, T.D. Schmidt, B.J. Scholz, C. Mayr, Device efficiency of organic light-emitting diodes: progress by improved light outcoupling, Physica Status Solidi A Appl. Res. 210 (2013) 44.
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