High morphology stability and ambipolar transporting host for use in blue phosphorescent single-layer organic light-emitting diodes

(1)

High morphology stability and ambipolar transporting host for

use in blue phosphorescent single-layer organic light-emitting

diodes

Fang-Ming Hsu

a

_{, Liang-Jung Chien}

b

_{, Kuan-Ting Chen}

c

_{, Ya-Ze Li}

c

_{, Shun-Wei Liu}

c,⇑ a

Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30010, Taiwan, Republic of China b

Department of Chemical Engineering, Ming Chi University of Technology, New Taipei City 24301, Taiwan, Republic of China c

Department of Electronic Engineering, Ming Chi University of Technology, New Taipei City 24301, Taiwan, Republic of China

a r t i c l e

i n f o

Article history: Received 1 June 2014

Received in revised form 14 August 2014 Accepted 6 September 2014

Available online 23 September 2014 Keywords:

Single layer

Phosphorescent materials Organic light-emitting diode Ambipolar host

Morphological stability Recombination zone

a b s t r a c t

The authors report a small molecule host of 2,7-bis(diphenylphosphoryl)-9-[4-(N, N-diphenylamino)phenyl]-9-phenylﬂuorene (POAPF) doped with 8 wt% iridium(III)-bis[(4,6-diﬂuorophenyl)pyridinato-N,C20

]picolinate (FIrpic) for use in efficient and single-layer blue phosphorescent organic light-emitting diodes (PHOLEDs) exhibiting a maximum external quantum efficiency of 20.3% at brightness of 100 cd/m2_{. The high performance of} such single layer PHOLEDs is attributed to the POAPF host’s high morphological stability, suitable triplet energy level, and equal charge carrier mobilities of hole and electron to form the broad carrier recombination zone in the emitting layer, thus reducing the triplet-triplet annihilation and resulting in a slight efficiency roll off of 0.5% from the brightness of 1 and 1000 cd/m2_{. This work also systematically investigated the} arrange-ment of the POAPF:FIrpic recombination zone for optimizing the performance of the single layer PHOLED.

1. Introduction

Highly efﬁcient phosphorescent organic light-emitting diodes (PHOLEDs) have been successfully fabricated by solution process using large molecules (dendrimers and/ or polymers) or by vacuum deposition of small electrolu-minescent (EL) molecules[1,2]. Based on these approaches, multilayer devices based on small molecules have been proposed to achieve high device performances through the selection of an appropriate material to tune the carrier transport and/or control the exciton distribution [3–5]. However, using such a multilayer structure with vacuum

thermal deposition has several critical drawbacks,

including complex device structures and high fabrication costs[6]. To the structure of such small-molecule devices can be simpliﬁed through the use of multiple-function materials in single-layer OLED applications.

Recently, many groups have demonstrated that the ﬂuorescent or phosphorescent molecule can be used in vacuum processes for the fabrication of a single layer OLED

with high performance [7–10]. For example, a

boron-containing material has been proposed to demonstrate an efﬁcient yellow-emitting single-layer device [11]. Lai et al. reported an ambipolar transporting host of benzimid-azole/amine-based compounds in an orange/red-emitting phosphorescent single-layer device[12]. Using 3,7-bis[4-(N-carbazolyl)phenyl]benzo[1,2-b:4,5-b0] difuran as the host material, the resulting device exhibited well-balanced charge transport and high hole and electron mobilities of 103_cm2_{/V s, allowing for the fabrication of an efﬁcient}

⇑Corresponding author.

E-mail address:[email protected](S.-W. Liu).

Contents lists available atScienceDirect

Organic Electronics

(2)

single layer device[13]. More recently, Wang et al. demon-strate that the emission properties of a top-emitting sin-gle-layer with p-i-n structure based on ambipolar host of 2,5-bis(2-(9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole are much higher than that for a multilayer device using the same emitting layer (EML)[14]. These results suggest that a host containing equal hole and electron mobilities and a high triplet energy level is required to improve the charge balance and exciton confinement in the single-layer PHOLED[15], especially in application for blue phospho-rescent systems[16,17]. Based on the concept of exciton confinement in EML, our previous work demonstrated the use of a small molecule host containing triphenylamine and diphenylphosphorl-substituted fluorene units in the fabrication of highly efficient blue bilayer PHOLED because their high triplet gap (ETof 2.75 eV) confines the triplet

exciton in the emitting layer[18]. In addition, Yin et al. reported efficient white single-layer devices by co-doping blue and orange phosphorescent dyes to the 2,7- bis(diphenylphosphoryl)-9-[4-(N,N-diphenylamino)phenyl]-9-phenylfluorene (POAPF)[19]. Unfortunately, this approach did not systematically study the relationship between the material properties, i.e. carrier mobility and morphology stability, and single-layer device performance. Thus, this work uses electrical and EL characteristics, time-of-flight (TOF) measurement, and atomic-force microscopy (AFM) to address the effects of the POAPF:FIrpic recombination zone on single layer PHOLEDs.

2. Experiments

Organic materials 9,9-bis[4-(N,N-diphenylamino)

pheny]ﬂuorine (BPAF), POAPF,

3-(4-biphenylyl)-4-phenyl-5-(4-tert-butyl-phenyl)-1,2,4-triazole (TAZ), and

phosphorescence dopant of iridium(III)-bis[(4,6-diﬂuoro-phenyl)pyridinato-N,C20

]picolinate (FIrpic) were synthe-sized by reported procedures[18], while blue dopant was purified twice in a home-made vacuum system according to the standard operation procedure in the literature [20]. The substrate was cleaned by ultrasonic bath in suc-cessive solutions of acetone and isoproponal, etched in a dilute solution of sulphuric acid, and finally dried by 5 N nitrogen blow. The single layer PHOLEDs were fabricated on a glass substrate coated with ITO anode (10X/square). In addition, to improve the hole injection efficiency in our proposed device, a 35 nm PEDOT:PSS (CH8000; Clevios) was spin-coated on top of the ITO anode. To allow a perfor-mance comparison, various multilayer devices were also fabricated. The active area of our device was 4 mm2_and

all devices were encapsulated in a glove box.Fig. 1shows the device conﬁgurations, energy levels, and chemical structures. The current density–voltage–luminance (J–V– L) characteristics of the OLED devices were measured using a dc current/voltage source meter (Keithley 2400), while the brightness was monitored with a spectrophotometer (Photo Research PR655). The AFM image of the organic thin ﬁlms on a Si wafer was analyzed by a Park System XE-70 in

(3)

non-contact mode using a NCHR tip cantilever. The work-function of the ITO and the highest occupied molecular orbital (HOMO) level of the organic materials were measured using a photoelectron spectrometer (Riken Keiki AC-2). All measurements were carried out in an air environment.

For the TOF measurement, the POAPF ambipolar mate-rial was deposited at a constant rate of 0.3 nm/s on the

ITO substrate to form a nearly 0.18–0.6

l

m thick ﬁlm,

which was subsequently covered by 25 nm of Ag to form the electrode. The actual thickness of the organic layer was measured by a surface proﬁler (Dektak 150; Veeco). The frequency-doubled output of a 5 ns pulsed dye laser set at 295 nm was chosen to match the peak optical absorption of the organic layer. The laser was aimed at the samples from the semitransparent Ag electrode to gen-erate carriers. The transient photocurrent created by the drift of the photoexcited carriers was monitored by a 2.5 GHz digital oscilloscope (LeCroy, WaveRunner 625Zi). To evaluate POAPF carrier mobility (

l

), the hole or electron transient time was determined from the double-logarith-mic plot of the TOF photocurrent versus the time charac-teristics and followed the Poole–Frenkel relationship to describe the increasing trend of ﬁeld-dependent carrier mobility as the electric ﬁeld strength increased. In addi-tion, the

l

was determined by

l

= D/tTE, where D is the

thickness of the organic layer, tTis the transit time, and E

is the applied electric ﬁeld[21].

3. Results and discussion

Fig. 2 shows the J–V–L characteristics and external

quantum efficiency (EQE) of various multilayer structures in POAPF based-devices. These devices include the previ-ously reported highly efficient multilayered blue PHOLED (PO-m) with a configuration of ITO/BPAF (30 nm)/POAPF: 8 wt% FIrpic (30 nm)/TAZ (40 nm)/LiF (1 nm)/Al (100 nm)

[18]. BPAF and TAZ were used as the hole-transporting

layer (HTL) and the electron-transporting layer (ETL), respectively, because they have sufficiently high triplet energy gaps to confine the triplet excitons in the POAPF EML. Thus, the device PO-m exhibited the EQE and EL efficiencies reaching as high as 20.6% and 35.4 cd/A, respectively. To increase the efficiency of the blue PHOLED, we simplified the device architecture into a single-layer POAPF device, in an attempt to replicate the performance found in device PO-m. First, the POAPF layer was used as the electron- (PO-b1) or hole- (PO-b2) transporting layer in device PO-m. To provide a basis for comparing the effects of exciton-polaron annihilation and hole injection, we then used the bilayer structure of ITO/PEDOT:PSS (PO-b3: without and PO-b4: with) (35 nm)/POAPF: 8 wt% FIrpic (60 nm)/TAZ (40 nm)/LiF (1 nm)/Al (100 nm) to fab-ricate devices PO-b1, PO-b2, PO-b3, and PO-b4. As shown inFig. 2, device PO-b1 exhibited a high (EQE) of 22.5% at

250 cd/m2 _{because POAPF can be used as an}

electron-transporting and injecting layer in bilayer devices. This indicates that the POAPF can provide the effective material functions of carrier/exciton conﬁnement, electron trans-porting, and electron injection in the ETL, thus improving

the bilayer device performance. In contrast, device PO-m significantly outperformed devices PO-b2 and PO-b3 with the ITO/POAPF interface providing poor injection efficiency due to the large hole injection barriers. In addition, as com-pared to BPAF containing two-triphenylamine-segments, POAPF has only one triphenylamine segment in the mole-cule, which may degrade its hole-transporting ability. As mentioned above, we found that the current density or emission efficiency of device PO-b3 is slightly higher than that of device PO-b2 (seeFig. 2a). As noted by Lee et al., this is potentially because the high carrier mobility FIrpic doped into POAPF can generate a trap state suitable to improving the hole injection efficiency [22]. Recently, many groups have proposed inserting the PEDOT:PSS buf-fer between the ITO and HTL to improve hole injection and reduce exciton quenching near the electrode[23,24]. Thus, the performance of device PO-b4 was similar to that of device PO-m because the PEDOT:PSS possesses a ladder-type (HOMO) level which facilitates hole-injection.

0 2 4 6 8 10 0 20 40 60 80 100

Current Density (mA/cm

2) Voltage (V) PO-m PO-b1 PO-b2 PO-b3 PO-b4

(a)

0 50 100 0 5 10 15 20 25 EQE (%)

Current Density (mA/cm2)

(c)

100 101 102 103 104

_(b)

Brightness ĩ cd/cm 2Ī Voltage (V) 0 2 4 6 8 10

Fig. 2. (a) Current density vs. voltage, (b) brightness vs. voltage, and (c) external quantum efﬁciency vs. current density of various multilayer devices.

(4)

Another possible reason could be that the device’s wide recombination zone signiﬁcantly reduces the exciton quenching effect in PHOLED[25,26].

As shown in the inset ofFig. 3a, five devices were fabri-cated using the single-layer configuration to further con-firm the recombination zone in the POAPF device. These devices were fabricated using the following configuration: ITO/PEDOT:PSS (35 nm)/POAPF (PO-0:x = 0, PO-20:x = 20,

PO-40:x = 40, PO-60:x = 60, and PO-80:x = 80 nm)/

POAPF:8 wt% FIrpic (20 nm)/POAPF(80x nm)/LiF (1 nm)/ Al (100 nm). The total thickness of the single-layer device was under 100 nm. On the other hand, to evaluate the charge balance in the single-layer POAPF device, we also fabricated device PO-s with a 100 nm emitting layer, i.e., ITO/PEDOT:PSS/POAPF:8 wt%FIrpic/LiF/Al. As shown in Fig. 3a, the electrical characteristics of all single-layer devices (PO-x series) were almost identical indicating that altering the HTL or ETL thickness does not induce the extra trap or alter the carrier transporting properties of single-layer POAPF devices. Despite the similar electrical charac-teristics of these single-layer devices, their EL spectra and emission efficiency are quite different from that found in the single-layer devices (seeFig. 3b and c). For example, the devices PO-60 and PO-80 exhibited relatively poor emission efficiency due to charge imbalance as well as the formation of an emission zone near the ITO/POAPF which induced additional exciton quenching. More inter-estingly, the emission profile of device PO-80 (see Fig. 3d) with high electric field is similar to that of device PO-0 (seeFig. 3b). This result suggests that high perfor-mance is dependent on controlling the recombination zone in single-layer POAPF.

Devices PO-0 and PO-20 exhibited a pure FIrpic emission spectrum because the recombination zone is

completely conﬁned in the EML. Using this conﬁnement strategy in a single-layer device, device PO-s with a wide EML showed high EQE values 20.3% than devices PO-m, mCP-s, and mCP-m. Note that the architectures of devices mCP-s and mCP-m were respectively identical to PO-s and PO-m, and both devices used the conventional host mate-rial N,N0-dicarbazolyl-3,5-benzene (mCP). For comparison,

Table 1 summarizes the EL data (driving voltage, EQE,

power efficiency, current efficiency, and Commission Inter-natonale de L’éclairage coordinate) of all devices. In the measured results, the EL performance of device PO-s (EQE 20.3%) is comparable with that of device PO-m (EQE 20.6%), both of which significantly outperform devices mCP-s and mCP-m (EQE 13%). However, we observed that the performance of device PO-s is slightly lower than that of device PO-b1 because the single-layer device can not completely confine the recombination zone in central region to avoid exciton quenching. Note that the performance of PO-s is still much higher than that of sin-gle-layer green PHOLED based on high carrier mobility and ambipolar transporting 4,40

-N,N0

-dicarbazolylbiphenyl host (EQE 13.3%)[9,27].

We attribute this improved performance of device PO-s to the POAPF host’s ambipolar transporting ability and good thermal or morphological stability to form the broad carrier recombination zone in single-layer PHOLEDs, resulting in the alleviation of undesired exciton quenching [28].Fig. 4shows the TOF result to conﬁrm our hypothesis regarding the transporting property (hole or electron) of POAPF. The POAPF host clearly exhibits rare ambipolar charge transport properties with mobility in the order of 106_cm2_{/V s. However, this result was very consistent}

with our design rule for the synthesis of POAPF[18]. More recently, several studies have successfully used the

0.0 0.2 0.4 0.6 0.8 1.0 PL of POAPF film Inte nsity (a .u .) Wavelength (nm)

(b)

0.0 0.2 0.4 0.6 0.8 1.0 Intens ity (a u) Wavelength (nm) 5 V 7 V 9 V 11 V

(d)

0 5 10 15 20 102 103 104 EQ E (%)

Current Density ĩmA/cm2Ī

(c)

B rightne ss ĩ mA/ c m 2 Ī 350 400 450 500 550 600 650 350 400 450 500 550 600 650 0 50 100 0 2 4 6 8 10 0 20 40 60 80 100 Cu rrent D e nsit y ( mA/cm 2 ) Voltage (V) PO-0 PO-20 PO-40 PO-60 PO-80 PO-s

(a)

POAPF (x nm) POAPF (80 - x nm) -5.6 -2.5 FIrpic doped (20 nm) ITO HOMO LUMO -4.8 -5.3 -2.4 PE D O T :P S S (3 5 n m ) -5.1 Al -4.3 LiF

Fig. 3. (a) Current density vs. voltage, (b) EL spectra, and (c) external quantum efficiency vs. current density of various single-layer devices. (d) The EL spectra of device PO-80 under different applied voltage. Note that the inset figure shows the device configuration of single-layer POAPF devices.

(5)

concept of balancing the holes and electrons in the EML to

improve the performance of single-layer devices

[29,30,10]. In addition, the TOF transient photocurrents

for POAPF holes and electrons (insets to Fig. 4a and b) exhibited highly dispersive proﬁles due to the presence of a large local site-energy in the host which induced the board charge carrier distributions[31]. This phenomenon is in agreement with previous assumption that the single-layer POAPF device features a broad carrier

recom-bination zone. In order to further study the issue of the EML property, we expect that the good thermal stability of POAPF can provide stable injection and transporting properties in the EML, which suggests the lifetime of the POAPF-based device is longer than that of its mCP-based counterpart. Thus, it is well known that the carrier mobil-ity of an organic semiconducting layer can be effectively improved by using a thermal annealing[32,33]. Therefore, we observed that the thermal effect improves the carrier mobility of POAPF thin ﬁlm to form a good molecule-pack-ing conﬁguration and induce a smoother morphology in POAPF, resulting in increased carrier mobility in the POAPF host as shown in Fig. 5. Although the POAPF exhibits a good thermal stability and ambipolar transporting prop-erty, the hole or electron carrier mobility of POAPF is still lower than that of common HTL or ETL[34,35]. Thus it is reasonably explained that we have observed that device PO-s or PO-x showed high driving voltage as compared with device PO-m.

Fig. 6shows the AFM images of POAPF and mCP hosts

with thermal annealing at 100 °C for 12 h. The surface proﬁle of the root mean square (rms) image is measured to be 0.38 nm for POAPF. Most importantly, the POAPF thin ﬁlm exhibits a very smooth and stable surface morphology as compared with the mCP host (seeFig. 6). In fact, the formation of a highly smooth host is a key factor in

350 400 450 500 550 600 10-8 10-7 10-6 10-5 0 20 40 60 80 0.00 0.02 0.04 0.06 0.08 0.10 0.12 1 10 100 10-4 10-3 10-2 µh (cm 2 /Vs) E1/2_(V/cm)1/2

(a)

P hotovo ltag e (V ) Time (µs) D = 0.6 µm E = 1.67×105_V/cm tT = 93 µs P ho tovol ta ge (V ) Time (µs) t_T 700 800 900 1000 1100 10-8 10-7 10-6 0 20 40 60 80 0.0 0.1 0.2 0.3 0.4 0.5 0.6 1 10 100 10-4 10-3 10-2 10-1 µe (cm 2 /Vs) E1/2_(V/cm)1/2

(b)

P ho to vol ta ge (V ) Time (µs) D = 0.18 µm E = 5.56 105 V/cm t_T = 32 µs P hotovo ltag e (V ) Time (µs) tT ×

Fig. 4. (a) Hole and (b) electron mobilities plotted with respect to square root of applied electric ﬁeld. Insets: double-linear and double-logarithmic plots of TOF transient proﬁles.

Table 1

Electroluminescence data for the devices. Device PO-m PO-b1 PO-b2d

PO-b3d

PO-b4 PO-0 PO-20 PO-40 PO-60 PO-80 PO-s mCP-sd mCP-m Va (V) 2.5/4.2 2.5/4.0 6.0/– 5.0/– 3.0/4.8 3.0/5.6 3.0/5.4 3.0/5.6 3.0/6.4 3.0/7.4 3.0/5.0 10.8/– 3.8/6.6 EQEb (%) 20.6/18.8 22.0/21.8 –/– –/– 20.8/20.2 9.6/8.2 11.1/9.0 7.8/6.1 4.6/3.2 3.3/1.8 20.3/20.2 –/– 13.2/11.2 PEb_{(lm/W) 36.7/24.3 35.2/35.1 –/–} _–/– _{24.7/22.8 15.6/9.5} _16.5/9.8 _11.8/6.5 _8.2/3.1 _5.9/1.6 _{26.5/26.2 –/–} _14.9/9.6 LEb_(cd/A) _{35.4/32.3 44.8/44.7 –/–} _–/– _{35.8/34.8 19.8/16.9 21.0/17.0 15.0/11.7 9.1/6.2} _6.6/3.6 _{42.2/42.1 –/–} _23.7/20.1 CIEc_{(x, y)} _{0.13,0.27 0.14,0.34 0.14,0.34 0.14,0.34 0.11,0.28 0.15, 0.33 0.12,0.31 0.14, 0.31 0.16,0.30 0.18,0.28 0.15,0.35 –} _0.12,0.29 a

Measured operating voltages, presented in the order of the values at 1 and 1000 cd/m2 . b

Measured values, presented in the order of the maximum value and the value at 1000 cd/m2 . c

Measured at 7 V. d

The devices showed the very poor electroluminescence property. (PO-b2:11 cd/m2

at a driving voltage of 8.5 V; PO-b3:14 cd/m2

at a driving voltage of 7.0 V; mCP-s:12 cd/m2 at a driving voltage of 12 V.) 300 400 500 600 700 800 900 1000 1100 10-7 10-6 10-5 10-4 Thermal annealing Hole (w/o) Hole (w) Electron (w/o) Electron (w) µ (cm 2 /Vs) E1/2 (V/cm)1/2

Fig. 5. Carrier mobilities plotted with respect to E1/2

without (w/o) and with (w) thermal annealing.

(6)

determining the injection efﬁciency at the anode/organic or cathode/organic interface. Our experiment results dem-onstrate that the single-layer device PO-s has potential to serve as an alternative to the traditional multilayer archi-tecture in PHOLEDs.

4. Conclusion

We have demonstrated a highly efficient single-layer blue PHOLED based on the POAPF host doped with 8 wt% FIrpic fabricated via a step-by-step method to simplify the conventional multilayer structures. The single-layer PO-s device with an EQE of 20.3% exhibited EL perfor-mance similar to that of the multilayer PO-m device. In addition, the ambipolar transporting ability and stable morphology properties of the POAPF may significantly improve the charge balance and reduce undesired exciton quenching, thus significantly improving the performance of single-layer devices.

Acknowledgments

The authors acknowledge the ﬁnancial support from the National Science Council (Grant Nos. NSC 102-3113-E-001-001, NSC 102-2627-E-002-002, NSC 011-142, NSC 102-2511-S-131-002, NSC

102-2221-E-131-030-MY2, NSC 102-2221-E-131-026-MY2, MOST

103-2627-E-002-002, and MOST 103-2622-E-131-007-CC3). In addition, the author (Dr. F.M. Hsu) would like to give special thanks to Prof. Ching-Fong Shu (NCTU) for sup-plied the POAPF materials in this work.

References

[1]C. Adachi, M.A. Baldo, M.E. Thompson, S.R. Forrest, J. Appl. Phys. 90

(2001) 5048.

[2]S.-J. Su, D. Tanaka, Y.-J. Li, H. Sasabe, T. Takeda, J. Kido, Org. Lett. 10

(2008) 941–944.

[3]H. Sasabe, K. Minamoto, Y.-J. Pu, M. Hirasawa, J. Kido, Org. Electron.

13 (2012) 2615–2619.

[4]S.E. Jang, K.S. Yook, J.Y. Lee, Org. Electron. 11 (2010) 1154–1157.

[5]N. Chopra, J.S. Swensen, E. Polikarpov, L. Cosimbescu, F. So, A.B.

Padmaperuma, Appl. Phys. Lett. 97 (2010) 033304.

[6]H.B. Wu, J.H. Zou, F. Liu, L. Wang, A. Mikhailovsky, G.C. Bazan, W.

Yang, Y. Cao, Adv. Mater. 20 (2008) 696–702.

[7]H.-H. Chang, W.-S. Tsai, C.-P. Chang, N-P. Chen, K.-T. Wong, W.-Y.

Hung, S.-W. Chen, Org. Electron. 12 (2011) 2025–2032.

[8]S.-W. Liu, Y.-T. Chang, C -C. Lee, C.-H. Yuan, L.-A. Liu, Y.-S. Chen, C.-F.

Lin, C.-I. Wu, C.-T. Chen, Jpn. J. Appl. Phys. Lett. 52 (2013) 012101.

[9]Z.M. Hudson, Z. Wang, M.G. Helander, Z.H. Lu, S. Wang, Adv. Mater.

24 (2012) 2922–2928.

[10] Y. Liu, L.-S. Cui, M.-F. Xu, X.-B. Shi, D.-Y. Zhou, Z.-K. Wang, Z.-Q. Jiang,

L.S. Liao, J. Mater. Chem. C 2 (2014) 2488–2495.

[11]H. Zhang, C. Huo, J. Zhang, P. Zhang, W. Tian, Y. Wang, Chem.

Commun. (2006) 281–283.

[12]M.-Y. Lai, C.-H. Chen, W.-S. Huang, J.-T. Lin, T.-H. Ke, L.-Y. Chen, M.-H.

Tsai, C.-C. Wu, Angew. Chem. Int. Ed. 47 (2008) 581–585.

[13]H. Tsuji, C. Mitsui, Y. Sato, E. Nakamura, Adv. Mater. 21 (2009) 3776–

3779.

[14]Q. Wang, Y. Tao, X. Qiao, J. Chen, D. Ma, C. Yang, J. Qin, Adv. Funct.

Mater. 21 (2011) 1681–1686.

[15]J. Huang, G. Li, E. Wu, Q. Xu, Y. Yang, Adv. Mater. 18 (2006) 114–117.

[16]W.-Y. Hung, Z.-W. Chen, H.-W. You, F.-C. Fan, H.-F. Chen, K.-T. Wong,

Org. Electron. 12 (2011) 575–581.

[17]Y. Chen, J. Chen, Y. Zhao, D. Ma, Appl. Phys. Lett. 100 (2012) 213301.

[18]F.-M. Hsu, C.-H. Chien, C.-F. Shu, C.-H. Lai, C.-C. Hsieh, K.-W. Wang,

P.-T. Chou, Adv. Func. Mater. 19 (2009) 2834–2843.

[19]Y. Yin, X. Piao, Y. Li, Y. Wang, J. Liu, K. Xu, W. Xie, Appl. Phys. Lett. 101

(2012) 063306.

[20] C.-C. Chi, C.-L. Chiang, S.-W. Liu, H. Yueh, C.-T. Chen, C.-T. Chen, J.

Mater. Chem. 19 (2009) 5561–5571.

[21]C.-H. Hsiao, S.-W. Liu, C.-T. Chen, J.-H. Lee, Org. Electron. 11 (2010)

1500–1506.

[22]Y. Li, K. Xu, X. Wen, L. Zhang, Y. Yin, S. Liu, X. Piao, W. Xie, Org.

Electron. 14 (2013) 1946–1951.

[23]P.A. Lane, G.P. Kushto, Z.H. Kafaﬁ, Appl. Phys. Lett. 90 (2007) 023511.

[24]M.A. Baldo, R.J. Holmes, S.R. Forrest, Phys. Rev. B 66 (2002) 035321.

[25]C. Adachi, M.A. Baldo, S.R. Forrest, M.E. Thompson, Appl. Phys. Lett.

77 (2000) 904–906.

[26]N.C. Erickson, R.J. Holmes, Adv. Funct. Mater. 23 (2013) 5190–5198.

[27]J.-W. Kang, S.-H. Lee, H.-D. Park, W.-I. Jeong, K.-M. Yoo, Y.-S. Park, J.-J.

Kim, Appl. Phys. Lett. 90 (2007) 223508.

[28]F.-M. Hsu, C.-H. Chien, Y.-J. Hsieh, C.-H. Wu, C.-F. Shu, S.-W. Liu, C.-T.

Chen, J. Mater. Chem. 19 (2009) 8002–8008.

[29]C.-C. Lee, C.-H. Yuan, S.-W. Liu, L.-A. Liu, Y.-S. Chen, J. Display

Technol. 7 (2011) 636–639.

[30] S.-W. Liu, C.-H. Yuan, S.-J. Yeh, M.-F. Wu, C.-T. Chen, C.-C. Lee, J. Soc.

Inf. Display 19 (2011) 346.

[31]Y. Shirota, H. Kageyama, Chem. Rev. 107 (2007) 953–1010.

[32]B.J. Chen, X.W. Sun, T.K.S. Wong, X. Hu, A. Uddin, Appl. Phys. Lett. 87

(2005) 063505.

[33]C.Y. Kwong, A.B. Djurišic´, V.A.L. Roy, P.T. Lai, W.K. Chan, Thin Solid

Films 458 (2004) 281–286.

[34]K.R. Justin Thomas, M. Velusamy, J.T. Lin, C.H. Chien, Y.-T. Tao, Y.S.

Wen, Y.-H. Hu, P.-T. Chou, Inorg. Chem. 44 (2005) 5677–5685.

[35]H.-F. Chen, S.-J. Yang, Z.-H. Tsai, W.-Y. Hung, T.-C. Wang, K.-T. Wong,

J. Mater. Chem. 19 (2009) 8112–8118.