Effect of hole transporting materials in phosphorescent ... - CiteSeerX

Organic Electronics 11 (2010) 427–433

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Organic Electronics journal homepage: www.elsevier.com/locate/orgel

Effect of hole transporting materials in phosphorescent white polymer light-emitting diodes Dong-Hyun Lee a, Yang-Peng Liu a, Kyung-Hee Lee a, Heeyeop Chae a, Sung M. Cho a,b,* a b

School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea Advanced Materials and Process Research Center for IT, Sungkyunkwan University, Suwon 440-746, Republic of Korea

a r t i c l e

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Article history: Received 30 September 2009 Received in revised form 12 November 2009 Accepted 19 November 2009 Available online 26 November 2009 Keywords: Polymer light-emitting diodes Hole transporting material Interlayer TCTA

a b s t r a c t We have investigated the effect of hole transporting materials in phosphorescent white polymer light-emitting diodes. The doping of a hole transporting material TCTA (4,40 ,400 tris(N-carbazolyl)-triphenylamine) resulted in enhanced luminance and efficiency while the doping of other hole transporting materials degraded the device performance. The effect of hole transporting materials on the performance of the device has been explained with the triplet energy level, HOMO (highest occupied molecular orbital)/LUMO (lowest unoccupied molecular orbital) level, and hole mobility of the materials. The doping of TCTA into an emissive layer increased the maximum current efficiency by 43%. Simultaneous utilization of TCTA both as an interlayer and a dopant in an emissive layer has been found to enhance the device performance further. The optimized phosphorescent white polymer light-emitting device showed the current efficiency of 20.5 cd/A with the luminance of 1800 cd/m2 at an operating voltage of 11.5 V. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction White organic light-emitting diodes (WOLEDs) have been studied due to their potential applications in full-color flat panel displays, back-lighting sources for liquid-crystal displays, and solid-state lighting [1–3]. High-efficiency WOLEDs are important since they can save electrical energy and have longer lifetime by reducing heat generation from the devices. In order to achieve highly efficient WOLEDs, multilayer structures have been commonly utilized to realize both the minimized electron/hole injection barriers and the optimized electron–hole balance in the devices. Highly efficient multilayer WOLEDs have been fabricated by vacuum deposition using various low molecular weight organic materials. In contrast, as another type of WOLEDs, white polymer light-emitting diodes (WPLEDs) utilize polymers

* Corresponding author. Address: School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea. Tel.: +82 31 2907251; fax: +82 31 2907272. E-mail address: [email protected] (S.M. Cho). 1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.11.022

instead of low molecular weight organic materials and have much simpler structures, e.g. single emissive layer since the structures can only be fabricated by solution processes which are difficult to avoid the dissolution of the layer underneath. As the result, WPLEDs have usually exhibited much lower efficiencies compared with multilayer WOLEDs with low molecular weight organic materials. In order to achieve white emission from polymer light-emitting diodes (PLEDs), single emissive layer structure with a polymer host such as PVK (poly(N-vinylcarbazole)) doped with two or three phosphorescent dyes has been studied most frequently. A WPLED with two or three iridium-cored phosphorescent dyes doped into PVK:OXD-7 (1,3-bis[(4-tertbutylphenyl)-1,3,4-oxadiazolyl]phenylene) matrix was reported to show the maximum luminous efficiency (LE) of 21.9 cd/A [4]. In the study, OXD-7 was included into the host PVK to facilitate electron transport. More recently, a WPLED with the maximum LE of 36.1 cd/A has been reported [5]. The device utilized a separate water/alcoholsoluble electron transport layer doped with Li2CO3 salt for efficient electron transport and hole blocking. The WPLED

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has also been fabricated with PVK:OXD-7 matrix doped with two phosphorescent dyes. High-efficiency PLEDs reported to date have not utilized a small molecular hole transporting material other than PVK as a hole transporting polymer host. However, most of recent reports on WOLEDs with low molecular weight organic materials have proved the effectiveness of the inclusion of hole transporting TCTA in the device structure [6–10]. Since the hole transporting TCTA has relatively higher triplet energy level (2.8 eV) and lower HOMO level (5.9 eV) than those of most phosphorescent emitters, the inclusion of TCTA into the emissive layer or as an interlayer results in beneficial effect on the device performance [6,7]. In this study, we introduced a hole transporting TCTA into WPLEDs for the first time and investigated the effect of the introduction on the device performance. In order to examine the effect of TCTA in detail, we have compared the results with other hole transporting materials such as TPD (N,N0 -diphenyl-N,N0 -bis(3-methylphenyl)-1,10 -diphenyl-4,40 -diamine) and a-NPD (4,40 -bis[N-(1-naphthyl)-N-phenylamino]biphenyl). We have found the optimal concentration of TCTA in single emissive layer and also showed the effect of TCTA on the device performance when used as an interlayer.

2. Experimental details For the preparation of WPLEDs, commercial ITO (indium tin oxide) glass substrates with a sheet resistance of 10 X/square were used. The ITO glass was cleaned by sonification in an isopropyl alcohol (IPA) and acetone, and finally rinsed in deionized water. The line pattern of ITO was prepared by conventional photolithography process. After the plasma treatment of ITO surface at 100 W oxygen plasma for 3 min, PEDOT:PSS (poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate); Baytron P AI4083) was spin-coated on the ITO surface to have the thickness of 40 nm and baked at 110 °C for 5 min. Subsequently, single emissive layer was spin-coated with a premixed organic solution to have the thickness of 50 nm. The organic solution was composed of a host PVK, an electron transporting OXD-7, a blue phosphorescent FIrpic (bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium)), a green phosphorescent Ir(mppy)3 (tris(2-(4-toltyl)phenylpyridine)iridium), and a red phosphorescent Ir(piq)2(acac) (bis (1-(phenyl)isoquinoline)iridium acetylacetonate), which were dissolved in chlorobenzene solvent. The composition was pre-optimized and fixed in all experiments with 50:40:9.6:0.2:0.2 for PVK:OXD-7:FIrpic:Ir(mppy)3:Ir(piq)2(acac). For investigating the effect of hole transporting TCTA, TPD and a-NPD on the performance of the devices, some devices were fabricated using an organic solution in which TCTA, TPD, or a-NPD (0–10 wt.% of the amount of PVK) was added additionally to the premixed solution with the fixed composition. After the single emissive layer was formed, the layer was dried at 80 °C for 30 min. Finally, the device was completed by thermal evaporation at a pressure of 2 106 torr of lithium fluoride (LiF) and aluminum (Al), of which the thicknesses are 1 and 100 nm, respectively.

In order to fabricate devices with a TCTA interlayer, a solution containing 0.5 wt.% TCTA in toluene was spin-coated on PEDOT:PSS surface. The interlayer was dried at 180 °C for 30 min on a hot plate prior to the spin coating of single emissive layer. Active area of all fabricated devices was 9 mm2 and all electrical measurements were performed under ambient condition without encapsulation. The current–voltage– luminance (I–V–L) characteristics were measured using a source measure unit (Keithley 2400) and using a luminance meter (Minolta CS100). The electroluminescence (EL) spectra and device efficiencies of the devices were recorded by using Minolta CS1000. 3. Results and discussion In order to investigate the effect of hole transporting materials in single emissive layer PLED, we have tested three different materials of TCTA, TPD, and a-NPD. In the single emissive layer, the compositions of other constituents, PVK, OXD-7, FIrpic, Ir(ppy)3, and Ir(piq)2(acac) are fixed for a meaningful comparison of the hole transporting materials. Hole drift mobility of TCTA, TPD, and a-NPD is known to be 2.0 105, 1.1 103, 8.8 104 cm2 V1s1, respectively [11], which means that the drift speed of holes in TCTA is the lowest when compared with those in TPD and a-NPD at a fixed electric field. By analyzing the I–V– L characteristics and efficiencies of the devices doped with a fixed amount of the materials, we could estimate how hole mobility affects the performance of the devices from the experimental results. As another factor to affect the device performance, the HOMO and LUMO levels of the hole transport materials are shown in Fig. 1a, along with those of other constituents in the device. An important thing

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Fig. 1. (a) Energy level diagram showing HOMO/LUMO levels of constituent materials for the devices and (b) triplet energy levels (T1) of hole transporting materials and phosphorescent emitters.


we can notice from the figure is the fact that the emission from FIrpic could be hindered upon the inclusion of TPD and a-NPD since the materials should act as competitive hole traps due to their higher HOMO levels than that of FIrpic. We think that it should be the reason why the previous studies reporting single layer PLEDs doped with FIrpic [4,5] have not utilized the hole transport materials other than the hole transporting host PVK in their devices. In single layer PLEDs doped with Ir(mppy)3 only, however, the effective utilization of TPD and a-NPD has been reported in the literature [12,13]. Since HOMO levels of TPD and a-NPD are lower than that of Ir(mppy)3, TPD and a-NPD do not act as efficient hole traps preventing the formation of excitons at the phosphorescent emitter. Moreover, higher hole mobility of TPD and a-NPD than that of the host PVK can enhance the performance of the devices upon the doping

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of the materials. Even though the hole mobility and HOMO/LUMO levels of the hole transporting materials can affect the device performance, the most important factor affecting the performance of the devices upon the utilization of hole transporting materials should be the triplet energy of the materials. The utilization of TPD and a-NPD has been examined with multilayer phosphorescent OLEDs doped with FIrpic or Ir(mppy)3 in the literature [14,15]. They found that exciton energy could transfer from FIrpic or Ir(mppy)3 to TPD and a-NPD since the triplet energies of the hole transporting materials are lower than those of FIrpic or Ir(mppy)3. The triplet energies of hole transporting TCTA, TPD, a-NPD are 2.8 [7], 2.34 [14], 2.29 eV [14], respectively and the energies of phosphorescent FIrpic, Ir(mppy)3, and Ir(piq)2(acac) emitters are 2.7 [14], 2.4 [6], 2.0 eV [16], respectively. As shown in Fig. 1b, it is expected

(a)

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Fig. 2. (a) Current–voltage–luminance (I–V–L) curves of WPLEDs with different hole transporting materials and (b) current efficiency characteristics of the devices.

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that low triplet energies of TPD and a-NPD should provide non-radiative paths for exciton quenching and reduce the phosphorescent light emission from FIrpic and Ir(mppy)3. Since the triplet energy of TCTA is higher than those of three different phosphorescent emitters, however, TCTA is not expected to cause the exciton quenching. In addition, the lowest HOMO level of TCTA compared with those of other constituents could enhance the performance of the devices. As expected, we indeed have found the reduced efficiency upon the doping of TPD or a-NPD into emissive layer as shown in Fig. 2a and b. On the other hand, the doping of TCTA into the emissive layer was found to increase the highest current efficiency by 43% along with the decrease in the voltage at the highest current efficiency as shown in Fig. 2b. The result indicates that TCTA confines the triplet excitons within the emissive layer effectively

while TPD or a-NPD acts as centers for the excitons to be transferred readily. As shown in Fig. 2b, the devices singly doped with TPD or a-NPD exhibited the lower efficiencies compared with that of undoped device. Specially, the doping of a-NPD lowers the efficiency most significantly. We think that it is mainly because the triplet energy is the lowest for a-NPD. However, the doping of TCTA enhances both luminance and efficiency of the device due to the enhanced exciton confinement. The co-doping of TCTA with TPD or a-NPD results in the increases in the efficiency compared with the singly doped cases but the resulting efficiency still does not exceed that of undoped device. It is worth to mention that the voltages at the highest efficiencies are in order of TPD, a-NPD, and TCTA which is the same as in descending order for the hole mobilities of the three compounds. Since the hole mobility of PVK is 2.5 106 cm2 V1s1 [17] which is the lowest compared with those of TCTA,

(a)

(b)

Fig. 3. (a) Current–voltage–luminance (I–V–L) curves of WPLEDs with different TCTA concentrations and (b) current efficiency characteristics of the devices.


TPD, and a-NPD, the voltage at the highest efficiency was found the highest for the undoped device. This result suggests that the addition of hole transport material in single layer PLEDs affect the operating voltages giving the highest efficiency of the devices due to their different hole mobility. Another thing to be noticed from Fig. 2a is that the current density is always higher whenever TCTA exists in the emissive layer. We attribute it to the lowest HOMO levels of TCTA close to that of PVK. In order to optimize the concentration of TCTA, we have varied the TCTA concentration in single layer WPLEDs and

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measured the performance of the devices as shown in Fig. 3a and b. The highest efficiency was obtained at TCTA concentration of 7 wt.%. As can be seen from the figure, however, the highest efficiency of the devices with a TCTA concentration range from 5 to 7 wt.% was found close to be around 20 cd/A at a voltage around 12 V. The voltage giving the highest efficiency was found to decrease as the TCTA concentration increases. We attribute it to the fact that the hole mobility of TCTA is higher than that of PVK. After optimization, the voltage showing the highest efficiency was reduced from 13 to 12.3 V even though the

(a)

(b)

Fig. 4. (a) Electroluminescence spectra of WPLEDs with the 7 wt.% doping of TCTA and the corresponding color coordinates at different applied voltages and (b) electroluminescent spectra of WPLEDs of four different structures.

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luminance and current efficiency were almost the same as shown in Figs. 2b and 3b. We have shown the EL spectra of the TCTA-doped single emissive-layer device in Fig. 4a. As the driving voltage increases, the spectra and color coordinate were not found changed significantly. The color coordinate was (0.30, 0.50) which corresponded to greenish white emission. When TCTA is separated as an interlayer instead of the doping into the emissive layer as a hole transporting constituent, the role of TCTA can be examined more clearly. Since HOMO levels of TCTA and PVK are very close, the hole injecting property to the emissive layer should be almost the same. We expect that the difference in hole mobility between TCTA and PVK can give different device property. More importantly, the triplet exciton confinement given by TCTA interlayer enhances the device efficiency more pronouncedly than that by TCTA addition in the emissive layer. In the literature [6], TCTA interlayer has been introduced by

a wet process to enhance the device efficiency greatly. The same experimental procedure was followed in this study. We have measured the current efficiency to compare the performance between devices of different structures. In order to compare the device performance with the current efficiency, the emission spectra should be the same for all the devices in the comparison. We have fabricated four different kinds of devices which are the followings: (1) conventional single emissive-layer device without TCTA doping; (2) single emissive-layer device (without TCTA) with a TCTA interlayer; (3) TCTA-doped single emissivelayer device; (4) TCTA-doped emissive-layer device with a TCTA interlayer. We have measured the emission spectra for the devices and shown the data in Fig. 4b. As shown in the figure, the emission spectra were found almost the same for the four different devices. As shown in Fig. 5a and b, the introduction of TCTA in the devices both as an interlayer and a co-dopant showed

(a)

(b)

Fig. 5. (a) Current–voltage–luminance (I–V–L) curves of WPLEDs with a TCTA interlayer and (b) current efficiency characteristics of the devices.


the increase in the current efficiency of the devices. Relative to the current efficiency of the single emissive-layer device (without TCTA) without the interlayer, the utilization of TCTA as a co-dopant into the single emissive layer showed the higher increase in the current efficiency compared with the utilization of TCTA as an interlayer. Simultaneous utilization of TCTA both as an interlayer and as a dopant in the emissive layer can enhance the device performance further. The simultaneous utilization of TCTA is found to maximize the current efficiency. It should be mentioned here that the utilization of TCTA interlayer has not only increased the current efficiency but also increased the luminance significantly as shown in Fig. 5a. The optimized phosphorescent white polymer light-emitting device with TCTA showed the current efficiency of 20.5 cd/A with the luminance of 1800 cd/m2 at an operating voltage of 11.5 V. The performance of WPLEDs in this study is comparable to the performance of previously reported WPLEDs [4,5] which showed the best efficiency to date. 4. Conclusions We have investigated the effect of hole transporting materials in phosphorescent white polymer light-emitting diodes. The doping of a hole transporting material TCTA resulted in the enhanced luminance and efficiency while the doping of other hole transporting materials degraded the device performance. We have revealed that the doping of hole transporting TPD and a-NPD into an emissive layer gave a harmful effect on the device performance due to their low triplet energies compared with those of phosphorescent emitters. On the other hand, the utilization of TCTA as an interlayer or as a dopant in an emissive layer enhanced the luminance and current efficiency of the devices. The results were explained with the triplet energy level, HOMO/LUMO level, and hole mobility of the materials. The optimized phosphorescent white polymer lightemitting device showed the current efficiency of 20.5 cd/ A with the luminance of 1800 cd/m2 at an operating voltage of 11.5 V. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2009-0083540). This work was also

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