Spectroscopic and electrical characteristics of highly efficient tetraphenylsilane-carbazole organic compound as host material for blue organic light emitting diodes

Organic Electronics 10 (2009) 1372–1377

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

Spectroscopic and electrical characteristics of highly efficient tetraphenylsilane-carbazole organic compound as host material for blue organic light emitting diodes Taiju Tsuboi a,*, Shun-Wei Liu b, Min-Fei Wu b, Chin-Ti Chen b a b

Faculty of Computer Science and Engineering, Kyoto Sangyo University, Kamigamo, Kyoto 603-8555, Japan Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 7 June 2009 Received in revised form 24 July 2009 Accepted 24 July 2009 Available online 4 August 2009 PACS: 33.20.Wr 33.50.Dq 73.61.Ph 78.55.Kz 78.60.Fi 78.66.Qn Keywords: Blue organic light emitting diodes Tetraphenylsilane-carbazole compound Carrier mobility Photoluminescence Fluorescence Phosphorescence

a b s t r a c t Thermal, electrical and spectroscopic properties have been studied for bis(3,5-di(9H-carbazol-9-yl) phenyl)diphenylsilane (SimCP2) which has exhibited high external quantum efficiency of 17.7% and power efficiency of 24.2 lm/W when it is used as host material for iridium bis(4,6-difluorophenypyridinato)picolate (FIrpic) blue emitter. They are compared with 1,3-bis (9-carbazolyl) benzene (mCP) and 3,5-bis (9-carbazolyl) tetraphenylsilane (SimCP) which have been also used as host for blue emitters. SimCP2 exhibits a highest glass transition temperature (148 °C) and is morphologically more stable. The electron and hole mobilities are higher (4.8  104 and 2.7  104 cm2 V1 s1, respectively, at electric field of 9  104 V cm1) than those of mCP and SimCP. The zero-phonon S1 emission band is observed at 344 nm, while the T1 emission band at 412 nm, i.e., this material preserves the characteristics of wide band-gap of 3.56 eV and high T1 triplet energy of 3.01 eV. From the intensity ratio of the T1 emission to the S1 emission, it is suggested that the intersystem crossing rate is smaller for SimCP2 than for mCP and SimCP. From these results, we clarify the reasons why SimCP2 is superior to mCP and SimCP as the host material for blue phosphorescence emitter in organic light emitting diodes. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction Organic light emitting diodes (OLEDs) generate electroluminescence (EL) under application of low bias voltage less than 10 V. Of various OLEDs, blue OLED is important for full color displays with primary RGB OLED emitters. The blue OLED is also important to obtain green and red light emission by combining with color changing media (CCM). The EL with high efficiency has been achieved by using a guest–host system in the emitting layer of OLED * Corresponding author. Tel./fax: +81 75 705 1899. E-mail address: [email protected] (T. Tsuboi). 1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.07.020

device. This system is made by dispersing emitting dopant in host matrix. In blue OLEDs, host materials with large triplet gap energy (>2.8 eV) is requested by the following reasons. One is to avoid triplet excitons of the dopant from quenching by back energy transfer from dopant to host, and the other is to confine the electro-generated triplet excitons in the dopant molecules. Arylamino-containing organic substances such as 4,40 -bis(9-carbazolyl)-2,20 -biphenyl (CBP) have been usually chosen as host materials and are proved working reasonably well for phosphorescent green or red emitters. CBP, however, has a low glass transition temperature (62 °C [1]), resulting in morphological instability [2]. Addi-

T. Tsuboi et al. / Organic Electronics 10 (2009) 1372–1377

tionally its T1 energy is 2.56 eV (corresponding to 483 nm, which is estimated from the T1 emission [3]). It does not have sufficient high triplet state energy for blue phosphorescent dopants such as iridium bis(4,6-difluorophenypyridinato) picolate (FIrpic) with 2.66 eV T1 energy and iridium bis (4,6-difluorophenylpyridinato-N, C20 ) [5-(2pyridyl)tetrazolate] (FIrN4) with 2.71 eV T1 energy [4]. Recently three fluorescent materials were investigated to check their suitability as host for blue OLEDs. They are 1,3-bis(9-carbazolyl)benzene (mCP), 3,5-bis(9-carbazolyl)tetraphenylsilane (SimCP), and bis(3,5-di(9H-carbazol9-yl) phenyl)diphenylsilane (SimCP2). The molecular structures are shown in Fig. 1. Two 3,5-di(9H-carbazol-9yl) phenyl groups are attached to Si in SimCP2, which is different from SimCP with only one 3,5-di(9H-carbazol-9yl) phenyl group. Of the three materials, we found that SimCP2 host gives external quantum efficiency gext of 17.7% and power efficiency of 24.2 lm/W at 100 cd/m2 for FIrpic blue emitter from OLED device with structure of ITO/PEDOT:PSS(35 nm)/14 wt% FIrpic:host(35 nm)/TPBi(28 nm)/ LiF/Al [5]. This power efficiency is much higher than 10.4 and 5.9 lm/W in the cases of SimCP and mCP hosts, respectively [5]. Here TPBi means 1,3,5-tris(N-phenyl-benzimidazol-2-yl)benzene, which is used as electron transport layer. Several papers have reported the EL efficiency of FIrpic doped in CBP, mCP, and SimCP hosts. For example, 8.9 lm/W (gext = 7.5%) and 7.7 lm/W (gext = 6.1%) were obtained from devices of ITO/CuPc/a-NPD/6%FIrpic:host/ BAlq/LiF/Al with mCP and CBP hosts, respectively [6], 9.3 lm/W (gext = 12.3%) and 12.9 lm/W (gext = 14.4%) from devices of ITO/a-NPD/7%FIrpic:host/TPBi/LiF/Al with mCP and SimCP hosts [7], and 13.3 lm/W (gext = 10.4%) from device of ITO/TNATA/a-NPD/7%FIrpic:mCP/13%FIrpic:mCP/ Bphen/LiF/Al [8]. To our knowledge, there have been no previous reports of EL efficiency higher than 24.2 lm/W and 17.7% which were obtained using FIrpic doped in SimCP2 host. Very high external quantum efficiency of 43.3% was recently reported using FIrpic doped in mCP [9]. However, its device structure is quite different from previous ones, e.g., tandem white OLED structure which has two EL units connected with a charge generation layer and two emitting layers. Green–red emitting (fbi)2Ir(acac) (0.75 wt%) is codoped with FIrpic (6.5 wt%) in mCP host

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to obtain white emission from the emitting layer. Therefore we cannot compare this case with the case of blue emission from FIrpic doped in SimCP2. The present paper attempts to clarify the reason why the SimCP2 is a novel host material for blue emitter. We report not only thermal and electrical properties but also spectroscopic property of SimCP2, because the optical property of SimCP2 is unknown. Comparison with mCP and SimCP is also presented.

2. Experimental method SimCP2 was synthesized as follows. Bis(4-(3,5-dibromophenyl) diphenylsilane) (4.17 g and 6.4 mmol) [10]) and carbazole (5.34 g and 30.7 mmol) were dissolved in dry xylenes (40 mL) containing potassium carbonate (12.72 g and 92.0 mmol) and palladium acetate (0.057 g and 0.25 mmol). After the injection of tri-tert-butylphosphine (0.20 mL and 0.8 mmol), the resulting mixture was stirred for 16 h at refluxing temperature under the protection of nitrogen atmosphere. After cooling to ambient temperature, the reaction was quenched with excess water (100 mL). The mixture was extracted with dichloromethane. The organic phase was washed with water and dried by magnesium sulfate. After the removal of magnesium sulfate, the mixture was evaporated to dryness and the residual was subjected for flash column chromatography (silica gel, dichloromethane/hexanes, 2/8). The product was isolated as a white powder and became a faint yellow glassy solid after sublimation. The glass transition temperature was measured with a differential scanning calorimetry (Perkin–Elmer DSC-6) method. The highest occupied molecular orbital (HOMO) energy was measured with a Riken Keiki AC-2 photoelectron emission spectrometer. The carrier mobility was measured with the time of flight (TOF) method that has been described before [11]. For optical study, thin films of neat SimCP2 were prepared by thermal vacuum deposition. The deposition was performed using a high-vacuum thermal evaporator (ULVAC) at a chamber pressure of 106 Torr. Film thickness of neat SimCP2 was determined by quartz thickness monitor (ULVAC CRTM-6000) and it was 30 nm. Thin films of polystyrene (PS) doped with 3, 5, 10, 15, and 20 wt% Sim-

Fig. 1. Molecular structures of mCP, SimCP, and SimCP2.

T. Tsuboi et al. / Organic Electronics 10 (2009) 1372–1377

CP2 were prepared by spin-coating a dichloromethane solution onto quartz substrates at ambient temperature. Optical absorption spectra were measured with a Shimadzu UV-3100PC spectrophotometer. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra were measured at various temperatures between 10 and 300 K with a Spex Fluorolog-3 fluorophotometer. The excitation source was a 450 W Xe-lamp. Filters were used to avoid the half and second harmonics of the excitation light.

2000

Counting Rate (cps)

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non-annealed o 70 C o 120 C o 150 C

1500

1000

500

3. Experimental results and discussion 3.1. Glass morphology, HOMO energy, and carrier mobility Fig. 2 shows the DSC thermogram of neat SimCP2 film. The glass transition temperature (Tg) was estimated to be 148 °C. It is higher than Tg of mCP and SimCP, which are 55 and 101 °C, respectively [11]. The heat exchange energy related to this phase transition increases on going from mCP to SimCP to SimCP2, i.e., from 0.9 to 1.1 to 1.9 kJ/ mol. Although it was not detected for SimCP2 by DSC, the heat exchange energy at the melting temperature also increases on going from mCP to SimCP, i.e., from 33.9 to 82.6 kJ/mol, which are all significantly larger than the heat exchange for the glass phase transition. We have put the sample (SimCP2) under the microscope and heated the sample up to 180 °C. We saw no phase transition (solid to liquid) happened to the sample. From these results, it is concluded that the endothermic signal around 148 °C is due to glass transition and not due to phase transition of melting. Fig. 3 shows the photoelectron spectra of the neat SimCP2 film. The HOMO energy is estimated as 6.12 eV, which are close to the 6.15 and 6.10 eV HOMO energies of mCP and SimCP, respectively [4]. The photoelectron spectra were also measured at room temperature after the neat film was heated at 70 °C, 120 °C, and 150 °C (Fig. 3), but no change was observed among them. The neat film, which was kept in open air at room temperature after annealing at 150 °C, shows morphological stability, i.e., neither aggregation nor crystallization was

22.0

heat flow

21.5

21.0

20.5

20.0 50

100

150

200

250

300

temperature (ºC) Fig. 2. DSC (differential scanning calorimetry) thermogram of SimCP2 which was obtained during heating of the neat SimCP2 film from 40 °C to 350 °C.

0 5.0

5.2

5.4

5.6

5.8

6.0

6.2

6.4

Energy(eV) Fig. 3. The HOMO energy of SimCP2 obtained from the photoelectron emission spectroscopy for neat non-annealed SimCP2 film and neat SimCP2 films annealed at 70 °C, 120 °C and 150 °C.

observed after 60 days when we look the film by eyes. In cases of neat mCP and SimCP films, quick morphology degradation was found after annealing at 70 °C [11]. This indicates that the SimCP2 film is morphologically more stable and more robust to water and oxygen molecules than mCP and SimCP. This is consistent with Fig. 3 which shows that no change occurs in the HOMO energy value between the SimCP2 films unheated and heated at 150 °C. From this result we confirm that high glass transition point is necessary to obtain morphologically stable and uniform amorphous thin films. Fig. 4 shows example of the TOF curve for electron and hole carriers of neat SimCP2 film with thickness of 220 nm, respectively. The carrier mobility l is estimated using equation lE = L/tT, where E is electric field, L is film thickness, and tT is transit time which is determined from the log–log plotted curve as shown in inset of Fig. 4. Carrier mobility of electron and hole is plotted against electric field in Fig. 5, and compared with those of mCP and SimCP. Like the cases of various organic semiconductors [12,13], the electron and hole mobility of SimCP2, mCP and SimCP follows the linear relationship with applied electric field (Poole–Frenkel model) when the mobility is log plotted against square root of electric field. The hole and electron mobility in SimCP2 is found to be 4.8  104 and 2.7  104 cm2 V1 s1, respectively, at electric field of 9  104 V cm1. This indicates that SimCP2 has a balanced bipolarity although the hole mobility is a little higher than the electron mobility. The mobility is higher for SimCP2 than for mCP and much higher than for SimCP. High charge carrier mobility of the material is one of the factors that can reduce the driving voltage of OLED and hence increase the power efficiency (lm/W) of the device. Based on the Poole–Frenkel model, the mobility at zerofield (called zero-field mobility) is obtained by extraction. Table 1 summarizes the zero-field electron and hole mobility for SimCP2, mCP, and SimCP. For comparison, the mobility is also shown for neat films of CBP [14,15] and N, N0 -bis(1-naphtyl)-N, N0 -diphenyl-1,10 -biphenyl-4,40 -diamine (a-NPD) [16] in Table 1. The zero-field mobility of SimCP2 is lower than those of CBP, while it is higher than those of NPB and Alq3.

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(a)

tT

-2

10 photocurrent (arb.units)

Table 1 Zero-field mobilities, loh and loe for holes and electrons, respectively, in neat SimCP2, mCP, SimCP, CBP, and a-NPD films.

0.04 0.03

SimCP2 mCP SimCP CBP a-NPD

SimCP2 (Electron) 1/2 1/2 E = 311 (V/cm) -6 tT=2*10 s

-3

10

0.02 -4

10 0.01

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1

time (µs)

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4

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8

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(b)

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0.020 -3

10 0.015

SimCP2 (hole) 1/2 1/2 E = 282 (V/cm) -6 tT=1.72*10 s

-4

0.010

10

absorption (arb. units)

0

time (µs)

photocurrent (arb.units)

2.4  104 1.2  104 5.8  105 3.8  104 [14,15] 1.8  104 [16]

1.3  104 3.4  105 5.0  105 4.4  104 [14,15] 1.0  105 [16]

6

-2

0.005

1

2

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240

280 320 360 wavelength (nm)

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6

2: absorption, neat film 3. PL, CH2Cl2 solution

1

1

0.000 0

SimCP2 at 296 K 1: absorption, CH2Cl2 solution

2

5

200

-2

loe (cm2 V1 s1)

10

0.00

0.025

loh (cm2 V1 s1)

PL intensity (arb. units)

0.05

400

Fig. 6. Absorption and PL spectra of SimCP2 in CH2Cl2 and absorption spectrum of neat SimCP2 film. Absorption spectrum is cut at k < 240 nm for the case of solution because of strong absorption by the solvent .

time (µs) Fig. 4. Example of transient time of flight curves for electrons (a) and holes (b) of SimCP2 measured at electric fields 311 and 282 V/cm, respectively. Inset shows the log–log plotted curve.

: (h) Hole carrier mobility : (e) Electron carrier mobility SimCP2

2

Carrier mobility (cm /Vs)

-3

10

mCP

(h) (e)

(h)

SimCP (h)

(e)

(e)

311, 292, 285, and about 255 nm for the solution. On the other hand, absorption peaks are observed at 342, 328, 313, 296, 290, 240, 227, and 207 nm in the neat film. Two absorption spectral line shapes are quite similar to each other. The emission spectrum consists of three vibronic peaks at 349, 363 and 378 nm where their intensities decrease with decreasing photon energy. This emission is attributed to the S1 emission, i.e., emission due to electronic transition from the lowest singlet state S1 to the ground state 1S0. Fig. 7 shows the absorption spectra of SimCP2 in PS films doped with 5 and 15 wt% SimCP2 and in neat SimCP2 film. The three spectra are quite similar to each other. This

RT

0.25 -4

1 2 3

10

200

300

400

500 1/2

Electric field

600

700

800

Fig. 5. Plot of electron and hole mobilities of SimCP2 against the electric field, compared with those of mCP and SimCP.

3

0.20

1/2

(V/cm)

optical density

100

0.15

5wt% SimCP2 in PS 15wt% SimCP2 in PS neat SimCP2

x0.5 2

0.10

3.2. Spectroscopic properties 0.05

Fig. 6 shows absorption and emission spectra of SimCP2 diluted in dichloromethane (CH2Cl2) solution and absorption spectrum of neat SimCP2 film at room temperature. Similar absorption spectrum was also obtained from SimCP2 diluted in chloroform (CHCl3) solution. Absorption peaks due to SimCP2 molecule are observed at 339, 325,

1

240

280

320

360

400

wavelength (nm) Fig. 7. Absorption spectra of SimCP2 which are measured using PS films with 5 and 15 wt% SimCP2 and using neat SimCP2 film at 296 K.

indicates that no aggregation is formed with increasing concentration of the dopant. We observe the absorption edge (i.e., onset) at about 348 nm (3.56 eV). The lowest unoccupied molecular orbital (LUMO) energy is approximated by the difference of the HOMO energy and the absorption edge of the UV absorption spectra [17–19]. Therefore, from the HOMO energy of 6.12 eV, the LUMO energy of SimCP2 is determined to be 2.56 eV. Fig. 8 shows the PL and PLE spectra for PS films doped with 3, 15, and 20 wt% SimCP2 at 10 K. The PLE spectra for 370 nm S1 emission (curves 4 and 5) consist of peaks at 338.1, 324.1, 311.0, and 293.0 nm, which are quite similar to the absorption spectra of Figs. 6 and 7. The PLE bands at 338.2, 325.1, and 311 nm are attributable to the S1 state. Intense emission peaks are observed at 343.8, 360.1, and 378 nm for the 3 wt% film, and weak PL bands are observed at 412.2, 433, and 441.8 nm. Like the case of mCP and SimCP [4,11], the latter bands are attributed to the T1 emission. Although the T1 emission is enhanced with decreasing temperature, it is too weak to observe clearly for the 20 wt% doped film even at 10 K. It is noted that the T1 emission intensity decreases with increasing concentration and not observed in neat film (see Fig. 9). Comparison of the PL spectra among SimCP2, mCP, and SimCP is presented in Fig. 10, where the spectra are normalized at the most intense peak height of the zerophonon band of the S1 emission. The spectrum of SimCP2 is quite similar to those of mCP and SimCP. Taking into account that SimCP and SimCP2 consist of one and two mCP molecules, respectively, this indicates that the S1 and T1 electronic states of mCP, SimCP, and SimCP2 are determined predominantly by mCP molecule. Difference, however, is found among the three materials, e.g., the vibronic structure in the S1 emission. The intensity ratio of the one-phonon vibronic band to the most intense zero-phonon band is 0.53, 0.76, and 0.90 for SimCP2, mCP, and SimCP, respectively. The Huang–Rhys parameter (or called Huang–Rhys factor) S is approximately estimated from this intensity ratio [20,21], i.e., S = 0.53 for SimCP2.

1.2

10K

PL intensity (normalized)

PLE 1 2 3 4 5

PL

1.0

5 3

0.8

4

0.6

3wt% PL 15wt% PL 20wt% PL 3wt% PLE 20wt% PLE

2 1

0.4 0.2 0.0 280

320

360

400

440

wavelength (nm) Fig. 8. PL spectra of SimCP2 which are measured using PS films with 3, 15, and 20 wt% SimCP2 excited at 305 nm at 10 K, and PLE spectra of SimCP2 for 370 nm emission which are measured using PS films with 3 and 20 wt% SimCP2 at 10 K.

intensity ratio of T1 emission to S1 emission

T. Tsuboi et al. / Organic Electronics 10 (2009) 1372–1377

0.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 0

20

40

60

80

100

concentration of SimCP2 in PS (wt%) Fig. 9. Intensity ratio of the 412 nm T1 emission to the 344 nm S1 emission at 12 K plotted against concentration of SimCP2 doped in polystyrene.

12 K

1.0

PL intensity (normalized)

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1 2 3

0.8

3 wt% SimCP2 3 wt% mCP 3 wt% SimCP

0.6

3

0.4

2 0.2

1

0.0 340

360

380

400

420

440

460

480

wavelength (nm) Fig. 10. PL spectra of mCP, SimCP and SimCP2 at 12 K, which were obtained from the PS films doped with 3 wt% mCP, SimCP and SimCP2, respectively.

The Huang–Rhys factor becomes large with increasing the distance between the equilibrium position Q(S1)0 of the S1 electronic excited state and the equilibrium position Q(S0)0 of the S0 ground state. The distance becomes large when the electron–phonon coupling in the excited state becomes largely different from the coupling in the ground state [21]. Therefore Fig. 10 suggests that the electron– phonon coupling difference between the S1 and S0 states is smaller in SimCP2 than in mCP and SimCP. Another difference of the PL spectra among SimCP2, mCP, and SimCP is the intensity ratio of the T1 emission to the S1 emission. The ratio is approximately estimated from the ratio of intense zero-phonon band intensity of the T1 emission to that of the S1 emission, which is 0.08, 0.12, and 0.35 for SimCP2, mCP, and SimCP, respectively. The ratio reflects strength of spin–orbit coupling. Therefore Fig. 10 indicates that the spin–orbit coupling is weaker for SimCP2 than for mCP and SimCP. The weak spin–orbit coupling reduces the rate of intersystem crossing (ISC) from the S1 state to the triplet states. In the case of FIrpic doped in hosts with high gap energy such as SimCP2, mCP, and

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SimCP, the Förster energy transfer (ET) occurs from the excited singlet state of the host to the singlet state of FIrpic dopant. If the ISC rate is low, we expect high energy transfer to the dopant from competition between two relaxation processes of ISC and ET. Therefore it is suggested that SimCP2 host with lower ISC rate gives higher energy transfer rate to the dopant than mCP and SimCP hosts, resulting in higher external quantum efficiency and higher power efficiency for SimCP2 than for mCP and SimCP. This is consistent with the results obtained from FIrpic-based OLED devices that are mentioned in Section 1.

Acknowledgements

4. Summary

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SimCP2. presents the most intense zero-phonon S1 emission band at 344 nm and the T1 emission band at 412 nm, which gives 3.01 eV as the T1 energy. The HOMO and LUMO energies are determined to be 6.12 and 2.56 eV, respectively. High gap energy is also observed for mCP and SimCP. SimCP2, however, is morphologically more stable than mCP and SimCP because of Tg of 148 °C which is much higher than mCP and SimCP, and additionally SimCP2 has higher mobility for both electron and hole (4.8  104 and 2.7  104 cm2 V1 s1, respectively, at electric field of 9  104 V cm1) than mCP and SimCP. The zero-field mobility is also higher for SimCP2 than for mCP and SimCP. The intersystem crossing rate is smaller for SimCP2 than for mCP and SimCP, leading to higher rate of energy transfer to FIrpic dopant from SimCP2 host than from mCP and SimCP hosts. From these thermal, electrical, and optical properties, we understand that SimCP2 is superior to mCP and SimCP as the host material for blue phosphorescence emitter in OLEDs. The PL and absorption spectra of SimCP2 are not so different from those of mCP and SimCP because they are determined predominantly by the common mCP moiety. However, the glass transition point and morphological stability are quite different. Unlike the case of SimCP, two mCP moieties locate around Si symmetrically in SimCP2. It is suggested that such a molecular structure gives rise to very stable morphology for SimCP2 because molecule with asymmetric structure such as SimCP leads to form aggregation at lower temperature.

This work was partly supported by the Grant-in-Aid for the Scientific Research from the Japan Society for Science Promotion (Project No. 19560018) and the City Area Program 2008 (Mutsu-Ogawara-Hachinohe Area) from the Ministry of Education, Culture, Sports, Science and Technology in Japan. This work was also supported by Academia Sinica and the National Science Council of Taiwan. References