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MoOx transparent electrode for high-performance OLEDs
Organic Electronics 36 (2016) 61e67
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Design of a MoOx/Au/MoOx transparent electrode for highperformance OLEDs Myeonggi Kim a, 1, Chefwi Lim a, 1, Daekyun Jeong a, Ho-Seok Nam a, Jiyoung Kim b, Jaegab Lee a, * a b
School of Advanced Materials Engineering, Kookmin University, Seoul 136-702, South Korea Materials Science & Engineering and Electrical Engineering, The University of Texas at Dallas, TX, 75080, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 March 2016 Received in revised form 20 May 2016 Accepted 23 May 2016
A MoOx(top)/Au/MoOx(bottom) multilayer was systematically designed for transparent electrodes in green OLEDs in terms of optical transmission and series resistance of the device. The enhancement in optical transmission of MoOx/Au/MoOx (MAM) structures is a result of a series of events, including the optical interference within the multilayers and the interaction of light with surface plasmon polaritons in the metal layer. For the maximum transmission, the optical interference occurring within the multilayers was simulated using a transfer matrix model to determine the optimum thickness of MoOx layers, and then the thickness of the Au interlayer was experimentally optimized for extraordinary optical transmission. In addition, the series resistance added by the top MoOx was characterized to confirm its negligible impact on the performance of the device. The optimum MoOx (40 nm)/Au (10 nm)/MoOx (40 nm) structure showed much higher transmission in the green-red region and lower sheet resistance than indium tin oxide (ITO). We have fabricated MAM-based OLEDs the driving voltage of which was significantly reduced to ~5.5 V at a current density of 20 mA/cm2, and the current efficiency (11.46 Cd/A) was higher than that (10.91 Cd/A) of ITO-based OLEDs, demonstrating that the MAM electrode is a potential replacement for ITO in optical devices. © 2016 Elsevier B.V. All rights reserved.
Keywords: OLEDs Transparent electrode MoO3 OMO electrode
1. Introduction Recently, organic light-emitting diode (OLED) technology has developed rapidly to produce large area and flexible light sources or displays for commercial products. Therefore, transparent conducting oxide (TCO) compatible with flexible substrates needs to be developed as a replacement for indium tin oxide (ITO), which is most commonly used as TCO at present. There are several issues with ITO, such as high processing temperature, low chemical and thermal stability, and its limited mechanical flexibility [1,2], for application in flexible electronics. In recent years, an oxide/metal/oxide (OMO) multilayer structure consisting of a metal sandwiched between the two oxide layers [3e6] has received significant attention because it can be designed to produce highly conductive and transparent multilayer structures [4,7e9,18]. In addition, the OMO structure is compatible
* Corresponding author. E-mail address: [email protected] (J. Lee). 1 Authors contributed equally. http://dx.doi.org/10.1016/j.orgel.2016.05.035 1566-1199/© 2016 Elsevier B.V. All rights reserved.
with the flexible substrate [10e12] and is fabricated by lowtemperature processing. Several oxidesdsuch as ITO [11], WO3 [13,14], ZnO [6], Al-doped ZnO [15e17], MoOx [18e20], Ta2O5 [12], TiO2 [7,13,21], InZnSnOx [10]dhave been investigated combined with metals such as Ag [4,5,7,10,11,13,18,21,23e25] Au [12,19,22] and Cu [15e17] and optimized for maximum performance. However, these show lower transmission than ITO [7]. In this study, we fabricated a MoOx/Au/MoOx (MAM) structure by electron-beam evaporation at room temperature as an anode for green OLEDs and systematically optimized the structure of the triple-layer electrode in terms of transmission and series resistance. Au is chemically stable, and has high plasmonic resonance, large skin depth (~30 nm) around a wavelength of 500 nm [26], high conduction, and adheres to metal oxides. The Au interlayer determines the lateral conductivity, and also brings about an extraordinary optical transmission (EOT) as a result of the coupling of incident light with surface plasmon polaritons (SPP) at the metal-dielectrics interfaces [27e30]. In addition to the thickness of the metal, its surface structure has a marked impact on the electrical and optical properties of the very thin layer; the continuous
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and smooth surface of the metal leads to high conductivity while a perforated or corrugated metallic structure results in the maximum EOT [28,29,31e33]. A MoOx underlayer interacts with Au atoms via Lewis acid-base reaction, which suppresses the agglomeration of Au and thus leads to a continuous and low-resistance film. It also enhances the adhesion of Au on MoOx. In addition, the MoOx has a strong influence on the transmission of the MAM multilayers as a result of optical interference. Therefore, optical interference effects were simulated by the transfer matrix model to optimize the thickness of the top and bottom oxides. Furthermore, since a stoichiometric molybdenum oxide is a wide-bandgap semiconductor, adding a series resistance to the device [34], the thickness dependence of the resistance of the evaporated MoOx was investigated. Regarding the EOT obtained by the Au layer, the optimum thickness of Au was determined experimentally. Finally, MAM-based OLEDs were fabricated with top MoOx and Au layers of different thicknesses to determine the optimum structure of OLEDs with the maximum current efficiency. 2. Experimental In this study, the triple structures of MoOx (x nm)/Au (y nm)/ MoOx (x nm) with 0 < x < 160 nm and 0 < y < 20 nm were sequentially fabricated at room temperature in an e-beam evaporator, in which the base pressure was 2 106 Torr, and MoOx and Au films were deposited at a rate of 0.3 nm/s and 0.1 nm/s, respectively. The distance between the substrate and the tungsten crucible containing MoO3 source in the e-beam evaporator is 33 cm and the deposition rate was in situ monitored using a quartz crystal microbalance. MoO3 (99.9995% purity) and Au (99.99% purity), used as source materials, were purchased from Taewon Scientific and Sigma Aldrich, respectively. For fabrication of the device, the glass substrate was cleaned in piranha solution (H2SO4:H2O2 ¼ 4:1) for 10 min, rinsed with deionized water and dried under an N2 flow. The triple layer was then coated using a shadow mask to pattern in the e-beam evaporator and then moved to an OLED evaporator (Sunicel plus 100, Sunic system) with a base pressure of 2 107 Torr, in which the organic layers of N,N0 -Bis(naphthalen-1-yl)-N,N0 -bis(phenyl) benzidine (NPB) (20 nm)/tris(8-hydroxyquinoline) aluminum (Alq3): 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7tetramethyl-1H, 5H, 11H-(1)-benzopyropyrano(6,7-8-I,j)quinolizin-11-one (C545T) (36 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (150 nm) were deposited sequentially using a shadow mask. The concentration ratio of Alq3:C545T was controlled at 98:2 wt%. NPB and Alq3 serve as a hole transport layer and an electron transport layer, respectively. Alq3:C545T is the emitter layer and LiF is the electron injection layer. All materials were deposited sequentially at a rate of 1e2 Å/s using thermal evaporation. The overlap between ITO (or MAM) and Al electrodes was 9 mm2, which is the emissive size of the devices. The fabricated OLED structures (Fig. 1) were encapsulated in a glove box. The optical transmittance and electrical conductivity of the MAM were measured using a four-point probe (Changmin, Korea). The current density (J)-luminance (L)-voltage (V) characteristics of the device were measured using an Agilent 4155 C semiconductor parameter analyzer and a silicon photodiode that had been precalibrated using a Minolta chromameter. All tests were carried out in a nitrogen atmosphere and at room temperature. Simulation based on the transfer-matrix model [33] with the optical constants listed in the table, was performed to determine the optimum thickness of MoOx in MAM structures. Ultraviolet photoelectron spectroscopy (UPS, AXIS NOVA, Kratos Analytical Ltd, UK) experiment was performed with He I (21.22 eV) radiation line from a discharge lamp with an energy resolution of 0.1 eV. For the UPS
Fig. 1. The architecture of (a) glass/MoOx/Au/MoOx/NPB/Alq3:C545T/Alq3/LiF/Al and (b) glass/ITO/(2TANA)/NPB/Alq3;C545T/Alq3/LiF/Al based OLEDs, respectively.
measurement, different thicknesses of MoOx films were deposited on Au layer in an evaporator. The samples were placed in petri dishes and then kept in zip-locking plastic bags for 4e5 h before UPS measurement. The relative humidity in the laboratory was ~40% during the air exposure of the samples. 3. Results and discussion 3.1. Effects of the Au interlayer and the top MoOx layer on the total electrical conductivity The total conductivity of a MAM multilayer structure is the sum of the lateral conductivity in the Au interlayer and the vertical conductivity of the top MoOx. Fig. 2 shows the variation in the sheet resistance of Au deposited on MoOx as a function of Au thickness. The critical thickness of Au, below which the sheet resistance of Au increases abruptly due to film discontinuity, on MoOx substrate was ~8 nm. Such a low critical thickness is due to the strong affinity of Au for MoOx, which suppress both the surface diffusion of Au adatoms on the surface and their agglomeration. The sheet resistance of 10 nm Au deposited on 40 nm MoOx was ~10 U/sq, which was further reduced to 7e8 U/sq upon annealing at 150 C for 1 h in air. In addition to the lateral resistance, the vertical resistance through the top MoOx was examined. We fabricated an Au/MoOx/ Au (AMA) structure (inset to Fig. 3) to measure the total resistance of the AMA structure, which is the sum of the vertical resistance of MoOx and the contact resistance between Au and MoOx, as follows:
RT ¼ 2Rc þ Rvt t=A
(1)
where RT is total resistance, Rc is the contact resistance, and Rvt is the resistivity of MoOx, t is the thickness of top MoOx, and A is the area of the top Au electrode (A ¼ 0.00138 cm2). Fig. 3 shows the linear dependence of total resistance RT on the thickness d of the top MoOx layer, which provides the resistivity of MoOx and the contact resistance between MoOx and Au. The slope of the RT-t curve indicates the calculated Rvt of 1.035 103 U-cm [36,37], and the intersection provides the specific contact resistance of 1.56 mUcm2. The fabricated device based on 40 nm MoOx/10 nm Au/40 nm MoOx exhibited a vertical resistance of top MoOx and lateral resistance of the MAM electrode of 0.046 U and 12e20 U, respectively. This indicates that a 40 nm top MoOx layer has a negligible impact on the series resistance of the device. 3.2. Dependence of optical transmittance on MoOx and Au thickness The optical properties of the multilayer films were calculated
M. Kim et al. / Organic Electronics 36 (2016) 61e67
Fig. 2. The variation in sheet resistance of Au deposited on MoOx-coated glass as a function of Au thickness.
Fig. 3. Change of the resistance of the top MoOx layer according to MoOx layer thickness.
using the transfer matrix formulation [35], in which multilayer structures with isotropic and homogeneous layers and parallelplane interfaces, and plane waves of light propagating through a stack of layers at normal incidence were assumed. The electric field of light within a single layer can be represented as the superposition of a left- and right-traveling wave (interference effects),
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which can be described by the matrix in terms of the thickness and the complex refractive index of the layer. A stack of layers can be represented as a system matrix, the product of the individual layer matrices, because the equations governing the propagation of the electric field of light are linear and the tangential component of the electric field is continuous. The calculation made at a wavelength of 525 nm and with a real refractive index n of 2.17 and imaginary index k of 0.008 for MoOx and n ¼ 0.605 k ¼ 2.066 for Au and n ¼ 1.5 k ¼ 0 for glass [38], (Glass was considered as infinity thickness) provides optical transmission, the fraction of the electric field intensity of the incident light transmitted through the MAM structure. Fig. 4 shows a contour map for the fraction of the electrical field intensity of the MAM structure as functions of the thicknesses of the top and bottom MoOx layers for a constant thickness of Au (15 nm). Transmission was strongly dependent on the thickness of both the top and bottom MoOx layers. To evaluate the effects of the total thickness of MoOx and the ratio of top to bottom MoOx thickness, we drew the two lines; one was drawn at a constant thickness ratio of 1:1, and the other at a constant total thickness of 80 nm, as shown in the left and right insets to Fig. 4, respectively. The right inset shows the variation in optical transmission with total thickness of MoOx at a top to bottom MoOx thickness ratio of 1:1, revealing transmission maxima at 80e100 nm and ~350 nm. This simulation is qualitatively well fitted to the measured values, but the position (~80 nm) at which maximum transmission was experimentally obtained was slightly different from that (~90 nm) of the simulated maximum value. Here, 80 nm was used as the total thickness of MoOx to examine the effect of thickness ratio on transmission. The left inset shows the dependence of optical transmission on the thickness ratio of top to bottom MoOx at a total MoOx thickness of 80 nm, revealing that maximum transmission occurs at a 1:1 ratio of MoOx. The measured values qualitatively fitted well with those determined in the simulation, confirming that top and bottom MoOx thicknesses contribute almost equally to the enhancement of optical transmission, and maximum transmission was seen at ~40 nm top MoOx/40 nm bottom MoOx. The dependence on Au thickness of a MoOx (40 nm)/Au (x)/ MoOx (40 nm) structure, where x varies from 3 to 20 nm, has been explored. Fig. 5 shows the optical transmittance spectra of MoOx (40 nm)/Au (x)/MoOx (40 nm), as a function of Au thickness and light wavelength. For comparison, the transmittance of ITO (sheet resistance ¼ 10 U/sq, thickness ¼ 150 nm) was measured as a function of light wavelength. Two different dependences of the optical transmission of MAM structure on the wavelength were evident: In the blue wavelength region in which the absorption of light occurs via interband electronic transition, the optical transmission continued to decrease as the Au thickness increases; in the red region in which reflection dominates via interaction of light with free electrons [38,39], the transmission at ~525 nm increased to 90% as the Au thickness increased from 3 to 10 nm, and then decreases with further increases in thickness. In discontinuous Au films, plasmon adsorption dominates in the red region [33] and decreases as the Au particles increase in density and size [29]. As the Au layer becomes continuous, the plasmon absorption diminishes and the EOT increases. This results in maximum optical transmission for an Au layer of ~10 nm thickness. Further increase in thickness decreases light transmission due to an increase in light reflection. The EOT phenomenon has been confirmed by several reports, but the mechanism is unclear [27e29,32]. Therefore, the thickness dependence of EOT must be determined experimentally. 3.3. OLED devices with trilayer transparent electrodes As the MAM structure was optimized to have excellent electrical
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Fig. 4. Plot of contour map for the change in the optical electrical field of the MAM structure with thickness of the top and bottom MoOx layers: Right inset shows the variation in optical transmission according to total MoOx thickness at a top to bottom MoOx thickness ratio of 1:1; left inset shows the dependence of optical transmission on the top to bottom MoOx thickness ratio at a total MoOx thickness of 80 nm.
and optical properties, its application to OLED devices was examined. First, the devices were fabricated using multilayer anodes with various thickness (10, 20, 30 and 40 nm) of top MoOx to investigate thickness dependence of the device performance. Fig. 6 shows the current density-voltage-luminance (J-V-L) characteristics of the OLED devices as a function of top MoOx layer thickness, and of ITO based OLEDs as a reference. Compared with ITO-based devices, a large shift of the operating voltage to a lower value and higher luminance density in the MAM-based devices was apparent. This was attributed to the higher work function of MoOx than that of 4,40 ,400 -Tris-(N-(naphthylen-2-yl)-N-phenylamino)triphenylamine (2TNATA)-covered ITO [40,41]. It is also noted that the top MoOx layer likely has an influence on the operating voltage; MAM multilayer electrodes with 10 and 20 nm MoOx show a turn-on voltage of ~4.9 V, and those with 30 and 40 nm MoOx layers show an operating voltage of 5.3e5.5 V. Since the top MoOx layer is conductive so as not to increase the series resistance of the device, the work functions of MoOx films of different thicknesses were measured by UPS to assess the effects of MoOx thickness on barrier height. Fig. 7 shows UPS data collected during incremental
formation of MoOx films on Au. The left panel displays the photoemission onset, from which the vacuum level of the surface can be deduced, and the right panel the top of the valence states of each film. All spectra were normalized to the same height for visual clarity. The work function of each film can be directly determined from the photoemission onset via a standard method; i.e., by translating the photoemission onset by the photon energy (He I, 21.22 eV) to obtain the vacuum level position and by comparing the vacuum level position with the Fermi level determined on Au [42]. Au was measured to be 4.69 eV, which increased to 4.97 eV with deposition of 5 nm MoOx on Au. The shift of the work function to a higher value by the incremental deposition of MoOx is likely to saturate at 20 nm, which is ~5.14 eV. The dependence of work function on MoOx thickness is consistent with previous reports [42,43]. The lower work function measured on the sample than the reported value (more than 6.5 eV) of the evaporated MoO3 may be due to the exposure of the sample to air before UPS measurement. Indeed, hole injection proceeds via electron injection from the highest occupied molecular orbital of NPB through the MoOx conduction band [44], and thus nearly constant work function of MoOx
M. Kim et al. / Organic Electronics 36 (2016) 61e67
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Fig. 7. UPS data collected during incremental formation of MoOx films on Au. Left panel displays the photoemission onset, from which the vacuum level of the surface can be deduced, and the right panel the top of the valence states of each film. Fig. 5. The optical transmittance spectra of MoOx (40 nm)/Au (x ¼ 3e20 nm)/MoOx (40 nm) as functions of Au interlayer thickness and light wavelength.
variation in work function or series resistance with thickness, but with the process or experimental variation. In addition to the operating voltage, the thickness of the top MoOx layer has a significant effect on the current efficiency, as shown in Fig. 6(b). Top MoOx layers of 30 and 40 nm thickness showed the highest value of 10e11.05 Cd/A, and 10 nm and 20 nm-thick MoOx layers exhibited a current efficiency of 6e7.03 Cd/A, respectively. This was due to the markedly higher optical transmission of the MAM structure with 30- and 40-nm-thick MoOx layers, compared with those with 10 and 20-nm-thick MoOx layers. Second, we varied the thickness of the Au interlayer from 15 nm to 12 nm and to 10 nm to increase the optical transmission and the resulting performance of the devices. Fig. 8 shows the J-V-L characteristics of OLEDs fabricated on a triple layer consisting of top MoOx (40 nm)/Au (x)/bottom MoOx (40 nm), where x is 10, 12, and 15 nm, respectively. MAM electrodes with Au layers of different thicknesses showed a turn-on voltage in the range of 5.5e5.7 V at a current density of 20 mA/cm2, indicating that an Au thickness of 10e15 nm does not increase the series resistance of the device. However, a clear difference in current efficiency between the devices at different thicknesses was evident; the current efficiency was 11.46 Cd/A for a 10-nm-thick Au interlayer, which is slightly higher than that (10.91 Cd/A) of ITO-based OLEDs. The current efficiency was 10.26 Cd/A and 9.51 Cd/A for 12-nm-thick Au and 15nm-thick Cd/A, respectively. This is attributed to the increased optical transmission of the 10-nm-thick Au layer sandwiched between the two oxide layers, due to the enhancement of EOT by the optimized Au layer.
4. Conclusion
Fig. 6. (a) The current density-voltage-luminance (J-V-L) characteristics and (b) current efficiencies of the OLED devices as a function of top MoOx thickness, compared with those of ITO based OLEDs.
in the thickness range of 20e40 nm yields similar hole injection barrier, leading to similar contact resistance at the MoOx-NPB interface. As a result, the variation in the operation voltage with the thickness of the top MoOx layer may be not associated with the
We systematically optimized the thickness of MoOx and Au layers in MAM multilayer anodes of green OLEDs to enhance optical transmission and low series resistance. A simulation was performed to determine the optimum thickness of MoOx in the MAM structure. The findings indicated that both the total MoOx thickness and the top to bottom MoOx layer thickness ratio are main contributors to optical transmission. In addition to MoOx thickness, a 10-nm-thick Au interlayer showed maximum EOT. Furthermore, the top MoOx layer exhibited low resistance and high work functions and so did not increase the series resistance of the device. The optimum structure comprised 40 nm MoOx/10 nm Au/40 nm MoOx, and showed higher transmission and lower resistance than ITO
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[4]
[5] [6] [7]
[8] [9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18] [19]
[20]
Fig. 8. The J-V-L characteristics of OLEDs fabricated on a triple layer consisting of top MoOx (40 nm)/Au (x)/bottom MoOx (40 nm), where x is 10, 12, and 15 nm, respectively.
[21]
[22]
electrodes. The devices fabricated based on the MAM multilayer electrodes exhibited a low operating voltage and higher current efficiency than ITO-based OLEDs.
[23]
[24]
Acknowledgement [25]
This study was supported by the Leading Foreign Research Institute Recruitment Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2013K1A4A3055679). This study was also supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MSIP) (2015R1A5A7037615).
[26] [27]
[28]
[29]
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Design of a MoOx/Au/MoOx transparent electrode for highperformance OLEDs Myeonggi Kim a, 1, Chefwi Lim a, 1, Daekyun Jeong a, Ho-Seok Nam a, Jiyoung Kim b, Jaegab Lee a, * a b
School of Advanced Materials Engineering, Kookmin University, Seoul 136-702, South Korea Materials Science & Engineering and Electrical Engineering, The University of Texas at Dallas, TX, 75080, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 March 2016 Received in revised form 20 May 2016 Accepted 23 May 2016
A MoOx(top)/Au/MoOx(bottom) multilayer was systematically designed for transparent electrodes in green OLEDs in terms of optical transmission and series resistance of the device. The enhancement in optical transmission of MoOx/Au/MoOx (MAM) structures is a result of a series of events, including the optical interference within the multilayers and the interaction of light with surface plasmon polaritons in the metal layer. For the maximum transmission, the optical interference occurring within the multilayers was simulated using a transfer matrix model to determine the optimum thickness of MoOx layers, and then the thickness of the Au interlayer was experimentally optimized for extraordinary optical transmission. In addition, the series resistance added by the top MoOx was characterized to confirm its negligible impact on the performance of the device. The optimum MoOx (40 nm)/Au (10 nm)/MoOx (40 nm) structure showed much higher transmission in the green-red region and lower sheet resistance than indium tin oxide (ITO). We have fabricated MAM-based OLEDs the driving voltage of which was significantly reduced to ~5.5 V at a current density of 20 mA/cm2, and the current efficiency (11.46 Cd/A) was higher than that (10.91 Cd/A) of ITO-based OLEDs, demonstrating that the MAM electrode is a potential replacement for ITO in optical devices. © 2016 Elsevier B.V. All rights reserved.
Keywords: OLEDs Transparent electrode MoO3 OMO electrode
1. Introduction Recently, organic light-emitting diode (OLED) technology has developed rapidly to produce large area and flexible light sources or displays for commercial products. Therefore, transparent conducting oxide (TCO) compatible with flexible substrates needs to be developed as a replacement for indium tin oxide (ITO), which is most commonly used as TCO at present. There are several issues with ITO, such as high processing temperature, low chemical and thermal stability, and its limited mechanical flexibility [1,2], for application in flexible electronics. In recent years, an oxide/metal/oxide (OMO) multilayer structure consisting of a metal sandwiched between the two oxide layers [3e6] has received significant attention because it can be designed to produce highly conductive and transparent multilayer structures [4,7e9,18]. In addition, the OMO structure is compatible
* Corresponding author. E-mail address: [email protected] (J. Lee). 1 Authors contributed equally. http://dx.doi.org/10.1016/j.orgel.2016.05.035 1566-1199/© 2016 Elsevier B.V. All rights reserved.
with the flexible substrate [10e12] and is fabricated by lowtemperature processing. Several oxidesdsuch as ITO [11], WO3 [13,14], ZnO [6], Al-doped ZnO [15e17], MoOx [18e20], Ta2O5 [12], TiO2 [7,13,21], InZnSnOx [10]dhave been investigated combined with metals such as Ag [4,5,7,10,11,13,18,21,23e25] Au [12,19,22] and Cu [15e17] and optimized for maximum performance. However, these show lower transmission than ITO [7]. In this study, we fabricated a MoOx/Au/MoOx (MAM) structure by electron-beam evaporation at room temperature as an anode for green OLEDs and systematically optimized the structure of the triple-layer electrode in terms of transmission and series resistance. Au is chemically stable, and has high plasmonic resonance, large skin depth (~30 nm) around a wavelength of 500 nm [26], high conduction, and adheres to metal oxides. The Au interlayer determines the lateral conductivity, and also brings about an extraordinary optical transmission (EOT) as a result of the coupling of incident light with surface plasmon polaritons (SPP) at the metal-dielectrics interfaces [27e30]. In addition to the thickness of the metal, its surface structure has a marked impact on the electrical and optical properties of the very thin layer; the continuous
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and smooth surface of the metal leads to high conductivity while a perforated or corrugated metallic structure results in the maximum EOT [28,29,31e33]. A MoOx underlayer interacts with Au atoms via Lewis acid-base reaction, which suppresses the agglomeration of Au and thus leads to a continuous and low-resistance film. It also enhances the adhesion of Au on MoOx. In addition, the MoOx has a strong influence on the transmission of the MAM multilayers as a result of optical interference. Therefore, optical interference effects were simulated by the transfer matrix model to optimize the thickness of the top and bottom oxides. Furthermore, since a stoichiometric molybdenum oxide is a wide-bandgap semiconductor, adding a series resistance to the device [34], the thickness dependence of the resistance of the evaporated MoOx was investigated. Regarding the EOT obtained by the Au layer, the optimum thickness of Au was determined experimentally. Finally, MAM-based OLEDs were fabricated with top MoOx and Au layers of different thicknesses to determine the optimum structure of OLEDs with the maximum current efficiency. 2. Experimental In this study, the triple structures of MoOx (x nm)/Au (y nm)/ MoOx (x nm) with 0 < x < 160 nm and 0 < y < 20 nm were sequentially fabricated at room temperature in an e-beam evaporator, in which the base pressure was 2 106 Torr, and MoOx and Au films were deposited at a rate of 0.3 nm/s and 0.1 nm/s, respectively. The distance between the substrate and the tungsten crucible containing MoO3 source in the e-beam evaporator is 33 cm and the deposition rate was in situ monitored using a quartz crystal microbalance. MoO3 (99.9995% purity) and Au (99.99% purity), used as source materials, were purchased from Taewon Scientific and Sigma Aldrich, respectively. For fabrication of the device, the glass substrate was cleaned in piranha solution (H2SO4:H2O2 ¼ 4:1) for 10 min, rinsed with deionized water and dried under an N2 flow. The triple layer was then coated using a shadow mask to pattern in the e-beam evaporator and then moved to an OLED evaporator (Sunicel plus 100, Sunic system) with a base pressure of 2 107 Torr, in which the organic layers of N,N0 -Bis(naphthalen-1-yl)-N,N0 -bis(phenyl) benzidine (NPB) (20 nm)/tris(8-hydroxyquinoline) aluminum (Alq3): 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7tetramethyl-1H, 5H, 11H-(1)-benzopyropyrano(6,7-8-I,j)quinolizin-11-one (C545T) (36 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (150 nm) were deposited sequentially using a shadow mask. The concentration ratio of Alq3:C545T was controlled at 98:2 wt%. NPB and Alq3 serve as a hole transport layer and an electron transport layer, respectively. Alq3:C545T is the emitter layer and LiF is the electron injection layer. All materials were deposited sequentially at a rate of 1e2 Å/s using thermal evaporation. The overlap between ITO (or MAM) and Al electrodes was 9 mm2, which is the emissive size of the devices. The fabricated OLED structures (Fig. 1) were encapsulated in a glove box. The optical transmittance and electrical conductivity of the MAM were measured using a four-point probe (Changmin, Korea). The current density (J)-luminance (L)-voltage (V) characteristics of the device were measured using an Agilent 4155 C semiconductor parameter analyzer and a silicon photodiode that had been precalibrated using a Minolta chromameter. All tests were carried out in a nitrogen atmosphere and at room temperature. Simulation based on the transfer-matrix model [33] with the optical constants listed in the table, was performed to determine the optimum thickness of MoOx in MAM structures. Ultraviolet photoelectron spectroscopy (UPS, AXIS NOVA, Kratos Analytical Ltd, UK) experiment was performed with He I (21.22 eV) radiation line from a discharge lamp with an energy resolution of 0.1 eV. For the UPS
Fig. 1. The architecture of (a) glass/MoOx/Au/MoOx/NPB/Alq3:C545T/Alq3/LiF/Al and (b) glass/ITO/(2TANA)/NPB/Alq3;C545T/Alq3/LiF/Al based OLEDs, respectively.
measurement, different thicknesses of MoOx films were deposited on Au layer in an evaporator. The samples were placed in petri dishes and then kept in zip-locking plastic bags for 4e5 h before UPS measurement. The relative humidity in the laboratory was ~40% during the air exposure of the samples. 3. Results and discussion 3.1. Effects of the Au interlayer and the top MoOx layer on the total electrical conductivity The total conductivity of a MAM multilayer structure is the sum of the lateral conductivity in the Au interlayer and the vertical conductivity of the top MoOx. Fig. 2 shows the variation in the sheet resistance of Au deposited on MoOx as a function of Au thickness. The critical thickness of Au, below which the sheet resistance of Au increases abruptly due to film discontinuity, on MoOx substrate was ~8 nm. Such a low critical thickness is due to the strong affinity of Au for MoOx, which suppress both the surface diffusion of Au adatoms on the surface and their agglomeration. The sheet resistance of 10 nm Au deposited on 40 nm MoOx was ~10 U/sq, which was further reduced to 7e8 U/sq upon annealing at 150 C for 1 h in air. In addition to the lateral resistance, the vertical resistance through the top MoOx was examined. We fabricated an Au/MoOx/ Au (AMA) structure (inset to Fig. 3) to measure the total resistance of the AMA structure, which is the sum of the vertical resistance of MoOx and the contact resistance between Au and MoOx, as follows:
RT ¼ 2Rc þ Rvt t=A
(1)
where RT is total resistance, Rc is the contact resistance, and Rvt is the resistivity of MoOx, t is the thickness of top MoOx, and A is the area of the top Au electrode (A ¼ 0.00138 cm2). Fig. 3 shows the linear dependence of total resistance RT on the thickness d of the top MoOx layer, which provides the resistivity of MoOx and the contact resistance between MoOx and Au. The slope of the RT-t curve indicates the calculated Rvt of 1.035 103 U-cm [36,37], and the intersection provides the specific contact resistance of 1.56 mUcm2. The fabricated device based on 40 nm MoOx/10 nm Au/40 nm MoOx exhibited a vertical resistance of top MoOx and lateral resistance of the MAM electrode of 0.046 U and 12e20 U, respectively. This indicates that a 40 nm top MoOx layer has a negligible impact on the series resistance of the device. 3.2. Dependence of optical transmittance on MoOx and Au thickness The optical properties of the multilayer films were calculated
M. Kim et al. / Organic Electronics 36 (2016) 61e67
Fig. 2. The variation in sheet resistance of Au deposited on MoOx-coated glass as a function of Au thickness.
Fig. 3. Change of the resistance of the top MoOx layer according to MoOx layer thickness.
using the transfer matrix formulation [35], in which multilayer structures with isotropic and homogeneous layers and parallelplane interfaces, and plane waves of light propagating through a stack of layers at normal incidence were assumed. The electric field of light within a single layer can be represented as the superposition of a left- and right-traveling wave (interference effects),
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which can be described by the matrix in terms of the thickness and the complex refractive index of the layer. A stack of layers can be represented as a system matrix, the product of the individual layer matrices, because the equations governing the propagation of the electric field of light are linear and the tangential component of the electric field is continuous. The calculation made at a wavelength of 525 nm and with a real refractive index n of 2.17 and imaginary index k of 0.008 for MoOx and n ¼ 0.605 k ¼ 2.066 for Au and n ¼ 1.5 k ¼ 0 for glass [38], (Glass was considered as infinity thickness) provides optical transmission, the fraction of the electric field intensity of the incident light transmitted through the MAM structure. Fig. 4 shows a contour map for the fraction of the electrical field intensity of the MAM structure as functions of the thicknesses of the top and bottom MoOx layers for a constant thickness of Au (15 nm). Transmission was strongly dependent on the thickness of both the top and bottom MoOx layers. To evaluate the effects of the total thickness of MoOx and the ratio of top to bottom MoOx thickness, we drew the two lines; one was drawn at a constant thickness ratio of 1:1, and the other at a constant total thickness of 80 nm, as shown in the left and right insets to Fig. 4, respectively. The right inset shows the variation in optical transmission with total thickness of MoOx at a top to bottom MoOx thickness ratio of 1:1, revealing transmission maxima at 80e100 nm and ~350 nm. This simulation is qualitatively well fitted to the measured values, but the position (~80 nm) at which maximum transmission was experimentally obtained was slightly different from that (~90 nm) of the simulated maximum value. Here, 80 nm was used as the total thickness of MoOx to examine the effect of thickness ratio on transmission. The left inset shows the dependence of optical transmission on the thickness ratio of top to bottom MoOx at a total MoOx thickness of 80 nm, revealing that maximum transmission occurs at a 1:1 ratio of MoOx. The measured values qualitatively fitted well with those determined in the simulation, confirming that top and bottom MoOx thicknesses contribute almost equally to the enhancement of optical transmission, and maximum transmission was seen at ~40 nm top MoOx/40 nm bottom MoOx. The dependence on Au thickness of a MoOx (40 nm)/Au (x)/ MoOx (40 nm) structure, where x varies from 3 to 20 nm, has been explored. Fig. 5 shows the optical transmittance spectra of MoOx (40 nm)/Au (x)/MoOx (40 nm), as a function of Au thickness and light wavelength. For comparison, the transmittance of ITO (sheet resistance ¼ 10 U/sq, thickness ¼ 150 nm) was measured as a function of light wavelength. Two different dependences of the optical transmission of MAM structure on the wavelength were evident: In the blue wavelength region in which the absorption of light occurs via interband electronic transition, the optical transmission continued to decrease as the Au thickness increases; in the red region in which reflection dominates via interaction of light with free electrons [38,39], the transmission at ~525 nm increased to 90% as the Au thickness increased from 3 to 10 nm, and then decreases with further increases in thickness. In discontinuous Au films, plasmon adsorption dominates in the red region [33] and decreases as the Au particles increase in density and size [29]. As the Au layer becomes continuous, the plasmon absorption diminishes and the EOT increases. This results in maximum optical transmission for an Au layer of ~10 nm thickness. Further increase in thickness decreases light transmission due to an increase in light reflection. The EOT phenomenon has been confirmed by several reports, but the mechanism is unclear [27e29,32]. Therefore, the thickness dependence of EOT must be determined experimentally. 3.3. OLED devices with trilayer transparent electrodes As the MAM structure was optimized to have excellent electrical
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Fig. 4. Plot of contour map for the change in the optical electrical field of the MAM structure with thickness of the top and bottom MoOx layers: Right inset shows the variation in optical transmission according to total MoOx thickness at a top to bottom MoOx thickness ratio of 1:1; left inset shows the dependence of optical transmission on the top to bottom MoOx thickness ratio at a total MoOx thickness of 80 nm.
and optical properties, its application to OLED devices was examined. First, the devices were fabricated using multilayer anodes with various thickness (10, 20, 30 and 40 nm) of top MoOx to investigate thickness dependence of the device performance. Fig. 6 shows the current density-voltage-luminance (J-V-L) characteristics of the OLED devices as a function of top MoOx layer thickness, and of ITO based OLEDs as a reference. Compared with ITO-based devices, a large shift of the operating voltage to a lower value and higher luminance density in the MAM-based devices was apparent. This was attributed to the higher work function of MoOx than that of 4,40 ,400 -Tris-(N-(naphthylen-2-yl)-N-phenylamino)triphenylamine (2TNATA)-covered ITO [40,41]. It is also noted that the top MoOx layer likely has an influence on the operating voltage; MAM multilayer electrodes with 10 and 20 nm MoOx show a turn-on voltage of ~4.9 V, and those with 30 and 40 nm MoOx layers show an operating voltage of 5.3e5.5 V. Since the top MoOx layer is conductive so as not to increase the series resistance of the device, the work functions of MoOx films of different thicknesses were measured by UPS to assess the effects of MoOx thickness on barrier height. Fig. 7 shows UPS data collected during incremental
formation of MoOx films on Au. The left panel displays the photoemission onset, from which the vacuum level of the surface can be deduced, and the right panel the top of the valence states of each film. All spectra were normalized to the same height for visual clarity. The work function of each film can be directly determined from the photoemission onset via a standard method; i.e., by translating the photoemission onset by the photon energy (He I, 21.22 eV) to obtain the vacuum level position and by comparing the vacuum level position with the Fermi level determined on Au [42]. Au was measured to be 4.69 eV, which increased to 4.97 eV with deposition of 5 nm MoOx on Au. The shift of the work function to a higher value by the incremental deposition of MoOx is likely to saturate at 20 nm, which is ~5.14 eV. The dependence of work function on MoOx thickness is consistent with previous reports [42,43]. The lower work function measured on the sample than the reported value (more than 6.5 eV) of the evaporated MoO3 may be due to the exposure of the sample to air before UPS measurement. Indeed, hole injection proceeds via electron injection from the highest occupied molecular orbital of NPB through the MoOx conduction band [44], and thus nearly constant work function of MoOx
M. Kim et al. / Organic Electronics 36 (2016) 61e67
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Fig. 7. UPS data collected during incremental formation of MoOx films on Au. Left panel displays the photoemission onset, from which the vacuum level of the surface can be deduced, and the right panel the top of the valence states of each film. Fig. 5. The optical transmittance spectra of MoOx (40 nm)/Au (x ¼ 3e20 nm)/MoOx (40 nm) as functions of Au interlayer thickness and light wavelength.
variation in work function or series resistance with thickness, but with the process or experimental variation. In addition to the operating voltage, the thickness of the top MoOx layer has a significant effect on the current efficiency, as shown in Fig. 6(b). Top MoOx layers of 30 and 40 nm thickness showed the highest value of 10e11.05 Cd/A, and 10 nm and 20 nm-thick MoOx layers exhibited a current efficiency of 6e7.03 Cd/A, respectively. This was due to the markedly higher optical transmission of the MAM structure with 30- and 40-nm-thick MoOx layers, compared with those with 10 and 20-nm-thick MoOx layers. Second, we varied the thickness of the Au interlayer from 15 nm to 12 nm and to 10 nm to increase the optical transmission and the resulting performance of the devices. Fig. 8 shows the J-V-L characteristics of OLEDs fabricated on a triple layer consisting of top MoOx (40 nm)/Au (x)/bottom MoOx (40 nm), where x is 10, 12, and 15 nm, respectively. MAM electrodes with Au layers of different thicknesses showed a turn-on voltage in the range of 5.5e5.7 V at a current density of 20 mA/cm2, indicating that an Au thickness of 10e15 nm does not increase the series resistance of the device. However, a clear difference in current efficiency between the devices at different thicknesses was evident; the current efficiency was 11.46 Cd/A for a 10-nm-thick Au interlayer, which is slightly higher than that (10.91 Cd/A) of ITO-based OLEDs. The current efficiency was 10.26 Cd/A and 9.51 Cd/A for 12-nm-thick Au and 15nm-thick Cd/A, respectively. This is attributed to the increased optical transmission of the 10-nm-thick Au layer sandwiched between the two oxide layers, due to the enhancement of EOT by the optimized Au layer.
4. Conclusion
Fig. 6. (a) The current density-voltage-luminance (J-V-L) characteristics and (b) current efficiencies of the OLED devices as a function of top MoOx thickness, compared with those of ITO based OLEDs.
in the thickness range of 20e40 nm yields similar hole injection barrier, leading to similar contact resistance at the MoOx-NPB interface. As a result, the variation in the operation voltage with the thickness of the top MoOx layer may be not associated with the
We systematically optimized the thickness of MoOx and Au layers in MAM multilayer anodes of green OLEDs to enhance optical transmission and low series resistance. A simulation was performed to determine the optimum thickness of MoOx in the MAM structure. The findings indicated that both the total MoOx thickness and the top to bottom MoOx layer thickness ratio are main contributors to optical transmission. In addition to MoOx thickness, a 10-nm-thick Au interlayer showed maximum EOT. Furthermore, the top MoOx layer exhibited low resistance and high work functions and so did not increase the series resistance of the device. The optimum structure comprised 40 nm MoOx/10 nm Au/40 nm MoOx, and showed higher transmission and lower resistance than ITO
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[4]
[5] [6] [7]
[8] [9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18] [19]
[20]
Fig. 8. The J-V-L characteristics of OLEDs fabricated on a triple layer consisting of top MoOx (40 nm)/Au (x)/bottom MoOx (40 nm), where x is 10, 12, and 15 nm, respectively.
[21]
[22]
electrodes. The devices fabricated based on the MAM multilayer electrodes exhibited a low operating voltage and higher current efficiency than ITO-based OLEDs.
[23]
[24]
Acknowledgement [25]
This study was supported by the Leading Foreign Research Institute Recruitment Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2013K1A4A3055679). This study was also supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MSIP) (2015R1A5A7037615).
[26] [27]
[28]
[29]
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