Thin Solid Films 544 (2013) 59–63

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Effects of electron blocking and hole trapping of the red guest emitter materials onhybrid white organic light emitting diodes

Lin-Ann Hong a, Hoang-Tuan Vu a, Fuh-Shyang Juang a,⁎, Yun-Jr Lai a, Pei-Hsun Yeh b, Yu-Sheng Tsai a

a National Formosa University, Institute of Electro-Optical and Materials Science, Huwei, Yunlin County, Taiwanb Raystar Optronics, Inc., 5F No. 25, Keya Rd. Daya Township, Taichung County, Taiwan

⁎ Corresponding author. Tel.: +886 5 6315029; fax: +E-mail address: fsjuang@seed.net.tw (F.-S. Juang).

0040-6090/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.tsf.2013.04.128

a b s t r a c t

a r t i c l e i n f o

Available online 2 May 2013

Keywords:High efficiencyHybrid white organic light emitting diodesSpacerHole trappingElectron blocking

Hybrid white organic light emitting diodes (HWOLEDs) with fluorescence and phosphorescence hybridstructures are studied in this work. HWOLEDs were fabricated with blue/red emitting layers: fluorescenthost material doped with sky blue material, and bipolar phosphorescent host emitting material doped withred dopant material. An electron blocking layer is applied that provides hole red guest emitter hole trappingeffects, increases the charge carrier injection quantity into the emitting layers and controls the recombinationzone (RZ) that helps balance the device color. Spacer layers were also inserted to expand the RZ, increase ef-ficiency and reduce energy quenching along with roll-off effects. The resulting high efficiency warm whiteOLED device has the lower highest occupied molecule orbital level red guest material, current efficiency of15.9 cd/A at current density of 20 mA/cm2, and Commission Internationale de L'Eclairage coordinates of(0.34, 0.39).

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Since Tang and Vanslyke developed the first efficient organic lightemitting diodes (OLEDs) based on an organic bilayer structure in 1987[1], many researchers have discovered various methods to increaseOLED efficiency as well as lifetime [2,3], making it adequate for bothdisplay and lighting applications. Hybrid white OLED structuresbased on blue fluorescence and orange phosphorescence host–guestsystems are popular methods that take advantage of both materials:high efficiency of phosphorescence and long lifetime of fluorescence[4]. Previous researches demonstrated that triplets have amuch largerdiffusion length (~10 nm) than singlets (1–3 nm) due to their longerlifetime (1–10 ns for singlets and >100 ns for triplets) [7]. Therefore,the phosphorescence emitter should be located at a distance largerthan the singlet diffusion length but shorter than the triplet diffusionlength to ensure that fluorescent emitter non-radiative triplet exci-tons can be recycled by the phosphorescence emitter [8]. This alsomeans that controlling the recombination zone (RZ) in hybid OLEDis one of the most important methods to increase performance andpurify the device color. As a result, it is necessary to have an insertedspacer between the two emitters to prevent energy transfer from thefluorescence emitter to the phosphorescence emitter in a hybridstructure [4–6].

886 5 6331211.

rights reserved.

This study used 4 nm-thick spacers inserted to optimize the holetrapping and electron blocking effects. Using electron blocking mate-rials with the lowest unoccupied molecule orbital (LUMO) level helpscontrol the RZ, reduce the roll-off effect and increase the charge injec-tion quantity in the emitting layer.

2. Experiment

Hybrid white OLEDs were fabricated on pre-patterned indium tinoxide (ITO) substrates with a sheet resistance of 13.59 ± 5 Ω/square.The substrates were cleaned via acetone, isopropyl alcohol andde-ionized water followed by O2 plasma treatment. All organic andmetal layers were fabricated under 6.65 × 10−4 Pa of pressure. Afterdepositing organic material, the cathode comprising a cesium fluoride(CsF) and an aluminum (Al) was deposited in the metal thermal evap-oration chamber without breaking vacuum. The device active area was5 × 5 mm2, as defined by the cathode deposition shadowmask. The en-ergy level diagrams of all devices fabricated in this study are shown inFig. 1. The layers were processed in the following order: hole injectionlayer Ni-100 (Nichem Fine Technology Co. Ltd.), which has lowerHOMO energy level (5.13 eV), 9,9-bis [4-(N,N-bis-biphenyl-4-yl-amino)phenyl]-9H-fluorene (BPAPF) that has better glass transitiontemperature (164 °C) or electron blacking layer (EBL) and higher holemobility rate (μ ~ 10−2 cm2/Vs) material Di-[4-(N,N-ditolyl-amino-phenyl)cyclohexane (TAPC) with two hole transport layer materials(HTL). For the blue fluorescent emitting layer (EML), EB-502 (e-RayOptoelectronics Technology Co. Ltd.) was used as the sky blue

Fig. 1. Band diagrams of the device structures used in this study.

Fig. 2. The normalized intensity of device with different carrier trapping phosphores-cence dopant materials at current density of 20 mA/cm2.

60 L.-A. Hong et al. / Thin Solid Films 544 (2013) 59–63

fluorescent dopant and BH-05 (e-Ray Optoelectronics Technology Co.Ltd.) is the host, the triplet energy (ET) of 2.78 eV as a spacer 1. Forthe red phosphorescent EML, osmium(II)bis(3-(trifluoromethyl)-5-(4-tert-butylpyridyl)-1, 2,4triazolate)diphenymethylphosphine (Os)and bis(1-phenylisoquinoline)(acetylacetonate)iridium(III) (Ir(piq)2(acac)) the highest occupied molecule orbital (HOMO) level is 4.8 eVand 5.3 eV in comparisonwith bipolar hostmaterial EPH-31 (e-Ray Op-toelectronics Technology Co. Ltd.) HOMO level difference for hole trap-ping. However EPH-31 can beused as a spacer, because its triplet energyis 2.41 eV, which can inhibit the Os (2.0 eV) triplet exciton transferredto the EPH-31 to reduce the energy loss. Respectively inserted inthe middle of the BH-05/red EML of the spacer 2 and red EML/tris(2,4,6-trimethyl-3-(pyridine-3-yl)phenyl)borane (3TPYMB) as anelectron transporting layer (ETL) of the spacer 3. In the present investi-gation, the fabricated hybrid white OLED was: ITO/Ni-100 (50 nm)/BPAPF or TAPC (15, 25 nm)/BH-15:EB-502 (2%, 8 nm)/BH-05 (2 nm)/EPH-31 (2 nm)/EPH-31:Os or Ir(piq)2(acac) (2%, 26 nm)/EPH-31(2 nm)/3TPYMB (50 nm)/CsF (2 nm)/Al (130 nm). A SpectraScanPR650 and Keithley 2400 were used to measure the luminance–voltage(L–V) and current–voltage (I–V) characteristics, respectively.

3. Results and discussion

Ahybridwhite OLEDwas fabricated usingdifferent phosphorescencedopants Ir(piq)2(acac) and Os following the structure: Ni-100 (50 nm)/BPAPF (15 nm)/BH-15:EB-502 (2%, 10 nm)/EPH-31:Ir(piq)2(acac) orOs(bpftz)2(PPh2Me)2 (2%, 30 nm)/3TPYMB (50 nm)/CsF (2 nm)/Al(130 nm). The energy band diagram is shown in Fig. 1. For the devicewith Ir(piq)2(acac) red dopant (Device A), purified white color wasobtained. Device A had CIE coordinates of (0.32, 0.33) at 20 mA/cm2

and color stability (Δx, Δy) = (0.001, 0.003) from 6 V to 8 V as shownin Fig. 6. However, low current efficiency of 7.5 cd/A was realized at20 mA/cm2 from Device A compared to that of 13.5 cd/A from DeviceB-1. Thepurewhite color and lowefficiency of DeviceA canbe explainedvia band diagram and electroluminescence (EL) spectra. As shown inFig. 1, LUMO and HOMO level of Ir(piq)2(acac) are respectively 3.1 eVand 5.3 eV, which overlaps those of EPH-31(2.37 eV, 5.56 eV) host ma-terial. Meanwhile, LUMO level of red dopant Os is 2.2 eV, which is higherthan 2.37 eV of host material EPH-31. It is considered as a reasonpreventing electrons frommoving to blue EML. Regarding the hole trap-ping effect, Ir(piq)2(acac) has little hole trapping effect because of itshigher HOMO level than Os (4.8 eV). The normalized intensity of these

two devices is shown in Fig. 2, where the shift of the highest intensitypeak from 475 nm (Device A) to 500 nm (Device B-1) and long wave-length of red Ir(piq)2(acac) dopant can be observed. Comparing DeviceB-1withDevice A, RZ is shifted from theblue emitter to the orange emit-ter due to improved electron injection aswell as hole trapping optimiza-tion, according to the red dopant energy band diagram. Consequently,the Commission Internationale de L'Eclairage (CIE) coordinates of(0.33, 0.39) along with higher efficiency were obtained from DeviceB-1. Based on these results, Os was applied to replace Ir(piq)2(acac) asphosphorescence dopant to achieve high efficiency in this study.

Spacers were inserted in hybrid white OLED structure to studytheir effects on device characteristics. The structures of all tested de-vices are detailed in Fig. 1 and Table 1. Fig. 3 shows the current den-sity of device versus applied voltage, and it is clear that Device B-1without spacer has highest current density compared to all other de-vices having spacer. Therefore, lower driving voltage is observed atDevice B-1 as shown Table 2. Current density decreases for devicewithin spacer inside can be explained by reduction of charge carriers

60

50

40

30

20

10

3 4 5 6 7

10

100

1000

10000

Voltage (V)

Cu

rren

t D

ensi

ty (

mA

/cm

2 )

Lu

min

ance

(cd

/m2 )

Fig. 3. Current density–voltage–luminance characteristics of the devices in this study. Fig. 5. The normalized intensity of devices with different hole transport materials (B-3,C-1) and different TAPC (C-1, C-2) thicknesses at current density of 20 mA/cm2.

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passing through these devices as a result of the charge barrier in-crease. In the transporting process holes and electrons move theEMLs interface and EML/ETL interface are blocked by spacers, as dem-onstrated in previous researches [4,5]. Comparing Device B-2 withB-3, where B-2 had spacer 1 and spacer 2 between two EMLs andB-3 had one more spacer (spacer 3) between EML and ETL, barrierin Device B-3 is higher than that of Device B-2 due to the additionalspacer. As a result, the electrons can hardly move to the EML, causinga sharp current density decline for Device B-3. The driving voltages ofdevice B-1, B-2 and B-3 were 5.7, 5.9 and 6.3 V, respectively.

Device B-2 with spacer 1 and spacer 2 between two EMLs has bet-ter efficiency of 14.1 cd/A than the device without spacers (B-1), asshown in Table 2. The reason is that adding a 4 nm-thick spacer be-tween the two emitters inhibits singlet energy transfer from thehigher-energy emitter to the lower-energy emitter [4]. The spacerprevents energy transfer from the fluorescence emitter to the phos-phorescence emitter and also balances the charge carriers and con-trols the device color [5,6]. Inserting the other spacer between EMLand ETL at Device B-3 increased the efficiency to 15.2 cd/A, reducedthe energy quenching and produced 6% less roll off effect from

20

18

16

14

14

100 20 40 60

Yie

ld (

cd/A

)

Current Density (mA/cm2)

Fig. 4. Current efficiency versus current density of the devices in this study.

5 mA/cm2 to 30 mA/cm2 compared with Device B-2 (Fig. 4). More-over, the LUMO level of EPH-31 (2.37 eV) is lower than that of Osdopant (2.2 eV), and a lower barrier for electrons means that theelectrons can move to the EML easier and faster by jumping directlyfrom 3TPYMB to spacer 3 (EPH-31) and Os dopant in Device B-3.Thus, the RZ at Device B-3 shifts closer to the blue emitter, makingDevice B-3 have CIE coordinates of (0.31, 0.37) that are better thanthose of Device B-1 (0.33, 0.39) and B-2 (0.36, 0.39), as shown inTable 2.

As illustrated in Fig. 1, the LUMO level of BPAPF is merely 2.38 eVand lower than that of BH-05 (2.27 eV). This means the electrons caneasily pass through BH-05 and jump into BPAPF, causing loss of elec-trons and efficiency decline. Therefore, it is necessary to find a holetransport material that can block electrons so that the electrons areconfined within the emitter, which in turn expands RZ and increasesdevice performance. TAPC, having high triplet energy ET = 2.9 eVand LUMO level of 2.0 eV lower than that of BH-05, was utilizedand optimized as HTL as well as EBL. The device was fabricated by re-placing BPAPF with TAPC as HTL with the following structure: Ni-100

Fig. 6. CIE coordinate of all devices used in this study at current density of 20 mA/cm2.

Table 1Structure parameters of devices in this study (unit: nm).

No Ni-100 HTL BH-05:EB-502

BH-05spacer 1

EPH-31spacer 2

EPH-31: Os

EPH-31spacer 3

3TP

B-1 50 BPAPF15

10 2% 0 0 30 2 % 0 50B-2 8 2 2 28 0B-3 2 2 26 2C-1 TAPC

15C-2 25

Fig. 7. Current versus voltage of hole only structure with different hole transport ma-terial BPAPF and TAPC.

62 L.-A. Hong et al. / Thin Solid Films 544 (2013) 59–63

(50 nm)/TAPC (x nm)/BH-05:EB-502 (2%, 8 nm)/BH-05 (2 nm)/EPH-31 (2 nm)/EPH-31:Os (2%, 26 nm)/EPH-31 (2 nm)/3TPYMB(50 nm)/CsF (2 nm)/Al (130 nm), where TAPC thicknesses at DeviceC-1 and C-2 were 15 nm and 25 nm, respectively.

The relationships of device current density versus applied voltageare shown in Fig. 3. The driving voltage at Device C-1 was 6.6 V com-pared with 6.3 V at Device B-3 (Table 2). The data clearly indicatedthat the current density of Device C-1 with TAPC as HTL is higher thanthat of Device B-3 with BPAPF as HTL at low applied voltage. It isknown that TAPC has very high hole mobility of 1.0 × 10−2 cm2/Vs[9]. Fig. 7 demonstrates TAPC has higher hole mobility than BPAPFusing hole only structure. In the case of Device C-1, more holes wereinjected and then transported into the emitting layer. During operationthe OLED heats itself and the temperature inside the device increases.TAPC has lower glass transition temperature (Tg) of 78 °C than that of164 °C as to BPAPF [10]. Increasing voltage induces higher electron ki-netic energy and severely degrades the TAPC via heat from the operat-ing process. On the other hand, low LUMO level aids electronconfinement inside the emitter, especially at high applied voltagewith high electron kinetic energy. These factors contribute to the resultthat the current density of Device C-1was higher than that of Device B-3at high applied voltage.

L–V characteristics of Devices B-3 and C-1 are also shown inFig. 3. From the data regarding Device C-1 in Table 2, luminance of3200 cd/m2 was achieved at current density of 20 mA/cm2 becausemore holes and electrons recombined in the emitting layer thanksto the expanded RZ. The highest current efficiency was also obtainedfrom Device C-1 for the same reason. The current efficiency at DeviceC-1 was 16.3 cd/A compared with 15.2 cd/A for Device B-3. Althoughthe efficiency of Device C-1 is impressive, it has yellowish color withCIE coordinates of (0.41, 0.41) due to RZ shifting from the blue emit-ter to the red emitter. The shift is caused by the increase in the quan-tity and speed of holes moving to the EMLs since high-hole-mobilityTAPC was applied. The lower intensity at 475 nm and higher intensityat 600 nm of Device C-1 compared with those of Device B-3 furthersupport this explanation.

To increase current density and purify the white color, the HTLthickness was increased to 25 nm at Device C-2. After increasingTAPC thickness, hole injection current was decreased, and charge car-riers were more balanced, resulting operating voltage decreasingfrom 6.6 V (Device C-1) to 5.7 V (Device C-2). Meanwhile, high cur-rent efficiency of 15.9 cd/A and luminance of 3200 cd/m2 at currentdensity of 20 mA/cm2 was achieved at Device C-2. It can be predictedthat charge carriers are more balanced in this device along with less

Table 2Characteristics of devices in this study.

No L (cd/m2) V (V) Y (cd/A) P (lm/W) CIE

@ 20 mA/cm2

B-1 2700 5.7 13.5 7.3 (0.33, 0.39)B-2 2800 5.9 14.1 7.5 (0.36, 0.39)B-3 3000 6.3 15.2 7.8 (0.31, 0.37)C-1 3200 6.6 16.3 8.1 (0.41, 0.41)C-2 3200 5.7 15.9 9.0 (0.34, 0.39)

energy loss due to higher quantity of hole injections. Higher power ef-ficiency of 9 lm/W was also achieved at this device as shown inTable 2. Thicker HTL means holes move slower, shifting RZ to blueemitter as presented by the EL spectra shown in Fig. 5. Lower intensi-ty of red weak peak was observed from Device C-2 comparing withDevice C-1, causing the CIE coordinates to shift from (0.41, 0.41) ofDevice C-1 to (0.34, 0.39) of Device C-2 as shown in Fig. 6.

Based on the experimental results, the optimal hybrid white OLEDdevice exhibits high current efficiency of 15.9 cd/A, luminance of3200 cd/m2 and power efficiency of 9 lm/W with low driving voltageof 5.7 V at current density of 20 mA/cm2.

4. Conclusions

In this research, controlling the RZ location in hybrid white OLEDswas identified as the key factor in achieving high efficiency as well asgood CIE color. By inserting spacer 1 and spacer 2 between the twoemitters and spacer 3 between the red phosphorescence emitterand electron transport layer along with using TAPC that can blockelectrons as HTL, a high efficiency device was obtained with efficiencyof 15.9 cd/A, power efficiency of 9 lm/W and good CIE coordinates of(0.34, 0.39) at current density of 20 mA/cm2. Doping suitable dopantmaterial with overlapped HOMO and LUMO level helped to controlthe RZ and charge carrier injection, resulting in a device with betterCIE (0.32, 0.33).

Acknowledgments

This work is sponsored by the National Science Council of theRepublic of China, projects NSC-99-2221-E-150-044-MY2, NSC-100-2221-E-150-056-, and Science Park University Research CollaborationProgram, projects 98IB02, 101RB04, and advanced technology develop-ment plan of high-tech equipment. Special thanks to Prof. Chi-Yun'sresearch team for the organic orange phosphor light emittingmaterials (Os).

References

[1] C.W. Tang, S.A.V. Slyke, Appl. Phys. Lett. 51 (1987) 913.[2] S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, K. Leo, Nature

459 (2009) 234.[3] G. Zhoua, W.Y. Wong, S. Suo, J. Photochem. Photobiol. 11 (2010) 133.

63L.-A. Hong et al. / Thin Solid Films 544 (2013) 59–63

[4] J.H. Seo, J.S. Park, J.R. Koo, B.M. Seo, K.H. Lee, S.S. Yoon, Y.K. Kim, 2010 IEEE 3rdInternational Nanoelectronics Conference (INEC), Hong Kong, January 3 8, 2010,IEEEXplore, 11169613, 2010, p. 1220.

[5] P. Chen,W.Xie, J. Li, T. Guan, Y. Duan, Y. Zhao, S. Liu, C.Ma, Appl. Phys. Lett. 91 (2007)023505.

[6] T. Zheng, W.C.H. Choy, Thin Solid Films 519 (2010) 872.

[7] W.S. Jeon, T.J. Park, S.Y. Kim, R. Pode, J. Jang, J.H. Kwon, Org. Electron. 10 (2009) 240.[8] G. Schwartz, S. Reineke, T.C. Rosenow, K.Walzer, K. Leo, Adv. Funct.Mater. 19 (2009)

1319.[9] L. Xiao, Z. Chen, B. Qu, J. Luo, S. Kong, Q. Gong, J. Kido, Adv. Mater. 23 (2011) 926.

[10] K.C. Lai, L.A. Hong, C.J. Lai, P.H. Yeh, S.H.Wang, F.S. Juang, Y.S. Tsai, October 11 (2011)15.