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Novel host materials for blue phosphorescent OLEDs

Peter Strohriegl*a

, Daniel Wagnera, Pamela Schrögel

a, Sebastian T. Hoffmann

b, Anna Köhler

b, Ute

Heinemeyerc, Ingo Münster

c

aMacromolecular Chemistry I, University of Bayreuth, 95440 Bayreuth, Germany

bExperimental

Physics II, University of Bayreuth, 95440 Bayreuth, Germany cBASF SE, Carl-Bosch-Straße 38, 67056 Ludwigshafen, Germany

ABSTRACT

We present two classes of host materials for blue phosphors. The first are carbazole substituted biphenyls 1-9. In these

CBP-type materials the triplets are confined to one half of the molecules by using either twisted biphenyls or by a meta-

linkage of the carbazoles to the biphenyl. We obtained high triplet energies of 2.95-2.98 eV and high glass transition

temperatures in the range of 100-120 °C. OLEDs were fabricated using the host material 6 and the carbene emitter

Ir(dbfmi) with pure blue emission at 450 nm. The devices achieved an external quantum efficiency of 8.7% at 100 cd/m2

and 6.1% at 1000 cd/m2.

MBPTRZ with an electron transporting biscarbazolyltriazine that is separated from the hole transporting carbazole by a

non-conjugated, meta-linked biphenyl unit is an example for a bipolar matrix material. The excellent glass forming

properties and the high Tg of 132 °C ensure morphological stability in OLEDs. The meta-linkage and the additional twist

at the biphenyl unit, which is achieved by two methyl groups in the 2- and 2’-position of the biphenyl in MBPTRZ leads

to a decoupling of the electron accepting and electron donating part and therefore to a high triplet energy of 2.81 eV.

DFT calculations show a clear separation of the electron and hole transporting moieties. A phosphorescent OLED with

MBPTRZ as host and FIrpic as emitter reached a maximum external quantum efficiency of 7.0%, a current efficiency of

16.3 cd/A and a power efficiency of 6.3 lm/W.

Keywords: host material, bipolar, phosphorescence, blue OLED, carbazole , triazine

1. INTRODUCTION

Since the introduction of iridium complexes like tris[2-phenylpyridinato-C2,N]iridium(III) Ir(pyy)3 as phosphorescence

emitters,1 organic light emitting diodes (OLEDs) with internal quantum efficiencies of nearly 100% have been realized

by harvesting both electrogenerated singlet and triplet excitons for emission.2,3

To achieve a highly efficient

phosphorescent OLED, triplet emitters are usually embedded in a suitable host to reduce concentration quenching.4 A

good host material should fulfill the following requirements: i) the triplet energy (ΔE(T1-S0)) must be higher compared to

the emitter to prevent energy back transfer to the host material, ii) suitable energy levels aligned with the neighboring

layers for efficient charge carrier injection to obtain a low driving voltage5 and iii) decent charge carrier transporting

abilities to increase the chance for hole and electron recombination within the emitting layer.6 Especially for blue

phosphorescent emitters the search for an appropriate host material is a challenging task because high triplet energies in

the range from 2.8 to 3.0 eV are required. One commonly used class of host materials are carbazole derivatives,7–10

which have good hole transporting properties and high triplet energies. Electron transporting matrix materials usually

contain electron deficient heterocycles. One promising class of such electron transporting hosts are triazine based

materials.11–14

Bipolar transporting materials are much less described.15–17

In this paper we present two classes of matrix materials for blue phosphors. The first class of materials are biphenyls

substituted with two carbazoles. In these materials the conjugation through the biphenyl unit is reduced by i) twisting of

the biphenyl by two substituents in 2- and 2’-position and ii) substitution of the central biphenyl with two carbazoles in

3- and 3’-position (meta-substitution). The second class are bipolar host material in which a hole conducting carbazole is

linked to an electron transporting triazine moiety by a twisted non-conjugated biphenyl unit.

* [email protected]; http://www.chemie.uni-bayreuth.de/mci; Phone: +49-921 55 3296

Organic Light Emitting Materials and Devices XVII, edited by Franky So, Chihaya Adachi, Proc. of SPIE Vol. 8829, 882906 · © 2013 SPIE · CCC code: 0277-786X/13/$18 · doi: 10.1117/12.2023305

Proc. of SPIE Vol. 8829 882906-1

Downloaded From: http://proceedings.spiedigitallibrary.org/ on 11/06/2013 Terms of Use: http://spiedl.org/terms

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2. CARBAZOLE BASED MATRIX MATERIALS

Due to its high triplet energy of 3 eV, carbazole is an often used building block for the design of host materials. One

commonly used host for phosphorescent emitters is 4,4’-bis(9-carbazolyl)-biphenyl (CBP) (Fig. 1) with a triplet energy

of 2.56 eV.18

For blue emitting phosphors host materials with higher triplet energies are necessary. The confinement of

the conjugated system is the key to such host materials. By changing the substitution pattern from para in CBP to meta

(m-CBP) or ortho (o-CBP) the triplet energy is enlarged to 2.84 eV and 3.00 eV.19

In N,N´-dicarbazolyl-3,5-benzene

(mCP) a triplet energy of approximately 2.90 eV is obtained by exchanging the central biphenyl group in CBP by a

single, meta linked benzene unit.20,21

Another approach to enlarge the triplet energy comprises the introduction of torsion

in the biphenyl unit caused by steric hindrance of two methyl substituents compared to CBP which leads to

4,4’-bis(9-carbazolyl)-2,2’-dimethylbiphenyl (CDBP) (Fig. 1) with a triplet energy of 2.79 eV.22,23

Ma et al. reported a

series of non-conjugated carbazole host materials in which the linker between the carbazoles was varied.10

The loss of

conjugation in these materials lead to high triplet energies.

Here we present a series of CBP-derivatives (Fig. 1), in which the conjugation is limited by twisting of the central

biphenyl linker by two substituents in 2- and 2’-position24

and by substitution of the biphenyl with two carbzoles in

3- and 3’-position (meta-substitution)25

.

CBP

CDBP

twisted

1 2

3 4 5

meta

6 7

8 9

Fig. 1. Top: literature known host materials CBP and CDBP. Central: chemical structures of the matrix materials 1 to 5

based on a twisted biphenyl linker.24 Bottom: chemical structures of the matrix materials 6 to 9 based on a 3,3’- (meta)

substituted biphenyl linker.25



2.1 Thermal properties

The ability of a material to form morphologically stable films is an important requirement for the successfull operation

of OLEDs.26

By using organic glasses as matrix materials the emitter is uniformly diluted in the host and thereby the

effect of concentration quenching is minimized. Additionally, the absence of grain boundaries, which may act as trap

states, makes the use of amorphous materials for OLEDs advantageous.27,28

Ideally, the glass transition temperatue (Tg)

for OLED applications is above 100 °C to prevent crystallization of the amorphous film during operation. In general,

amorphous behavior is achieved by introduction of bulky substituents which hinder the packing of the molecules. The

thermal properites of the CBP-derivatives were investigated by thermogravimetric analysis (TGA) and differential

scanning calorimetry (DSC). TGA measurements show that all materials possess high thermal stabililties with an onset

of weight loss exceeding 310 °C. In Fig. 2 the DSC curves of CBP, the methyl substituted compound 2 and the meta-

linked compound 9 are shown. CBP exhibits a crystalline behavior in the DSC with a melting point at 285 °C and a

crystallization temperature of 183 °C. The melting of 2 occurs at 277 °C. Upon cooling the material solidifies in an

amorphous state. In the second heating a Tg of 121 °C followed by a recrystallization at 200 °C is observed. The meta-

linked compound 9 remains in the amorphous phase after the first melting at 212 °C. In the second heating only the glass

transition at 120 °C is observed.

These results show that by the introduction of additional methyl or triflouromethyl groups at the central biphenyl and/or

the adjacent carbazole moieties the thermal properties are improved. Except for the unsubstituted compounds CBP and

6, which show crystalline behaviour, all materials reveal high glass transition temperatures ranging from 94 °C to

121 °C. The melting, crystallization and glass transition temperatures of all derivatives are summarized in Table 1.

Fig. 2. DSC-curves of carbazole based materials CBP, 2 and 9 at a scan rate of 10 K/min in a nitrogen atmosphere. Shown

are the first and second heating scans (solid lines) as well as the first and second cooling scans (dotted lines).

2.2 Optical properties

One requirement for a host material is a higher triplet energy compared to the emitter to prevent energy back transfer to

the host material. By measuring the phosphorescence at low temperatures the triplet energy is obtained from the first

vibrational transition of the luminescence.

In order to understand how the twisting of the biphenyl unit and the meta-linkage of the carbazole units affect the triplet

energy, we compare the phosphorescence spectra of the para-linked CBP with the twisted para-linked CDBP and the

meta-linked derivative 6 (Fig. 3). We observe a remarkable blue shift in the phosphorescence of CDBP and 6 compared

to CBP. The triplet energies (ΔE(T1-S0)) are shifted towards higher values of 2.95 eV for CDBP and 2.98 eV for 6

compared to 2.58 eV for CBP. The torsion angle of 82° between the two phenyl rings in the biphenyl caused by steric

hindrance of the two methyl substitutions in 2- and 2’-position leads to a loss of conjugation and thus to higher triplet

energies. This proofs that both design concepts lead to high triplet energies by a confinement of the conjugation.

100 150 200 250 30050 100 150 200 250 300 100 150 200

Tg= 121°C

2

heat

flow

, end

o up

CBP10 K/min

temperature [°C] temperature [°C]

Tg= 120°C

temperature [°C]

9



Table 1. Thermal properties of the carbazole substituted biphenyls CBP, CDBP and 1-9.

Tg [°C] Tm [°C] Tcr [°C] Tida [°C]

CBP - 283 205c

365

CDBP 94 - - 310

1 106 - - 310

2 121 277 200d

312

3 100 232 - 310

4 105 210b

- 337

5 119 233b

- 333

6 - 271 191c

315

7 107 270 184d

349

8 108 237 176d

319

9 120 212 - 319

Tg: glass transition temperature, Tm: melting temperature, Tcr: crystallization temperature and Tid: initial decomposition

temperature. a Tid is the temperature at which an initial weight loss was observed in the TGA experiment with a heating rate of 10 K/min

in N2-atmosphere. b Observed only in the first heating scan. c Observed during cooling. d Observed during heating.

Fig. 3. Normalized phosphorescence spectra of CBP, CDBP and 6 measured in 10 wt-% solid solutions in PMMA at 10 K.

2.3 Organic Light-Emitting Diode

Well-known blue emitters like FIrpic and FIr6 show blue emission tailing to the green region with emission maxima at

472 nm (CIE color coordinates x = 0.17, y = 0.34)22

and 458 nm (CIE color coordinates x = 0.16, y = 0.26)29

,

respectively. Emitters with a more saturated blue are beneficial for solid state lighting. Due to the high triplet energies of

the CBP-derivatives 1-9 we have tested the host material 6 with the pure blue carbene emitter Ir(dbfmi) with a

λem = 450 nm and CIE color coordinates of x = 0.16, y = 0.18. The device setup and the materials are shown in Fig. 4. To

improve the hole injection into the emission layer we coevaporated the hole transporting DPBIC into the emission layer

providing a barrier free path for holes. On top of the ITO glass substrate poly(3,4-ethylene dioxythiophene):poly(styrene-



DPBIC

T1.0

0.8

0.6

0.4

0.2

0.0=350 400 450 500 550 600 650

wavelength [nm]

101 102 103

Luminance [cd /m2]

10°

sulfonate) (PEDOT:PSS) was spin coated as hole injection layer followed by 10 nm tris[(3-phenyl-1H-benzimidazol-

1-yl-2(3H)-ylidene)-1,2-phenylene]-iridium (DPBIC) p-doped with molybdenum(VI) oxide as hole transporting layer. A

10 nm thick layer of DPBIC was deposited as exciton and electron blocking layer. The 20 nm thick emission layer

comprised a mixed matrix system of 75% host material 6, 20% DPBIC and 5% Ir(dbfmi). As exciton and hole blocking

material 2,8-bis(triphenylsilyl)-dibenzofuran (DBFSi) (5 nm) was used. The electron transporting layer consisted of

5 nm 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) followed by 40 nm BCP n-doped with Cs2CO3. The device

was finalized by deposition of 1 nm Cs2CO3 and 100 nm aluminum as cathode.

In Fig. 5 the electroluminescence spectrum and the external quantum efficiency-luminance characteristics of the

OLED-device with 6 as host materials are displayed. The device shows a pure emission of the emitter Ir(dbfmi) with the

CIE-coordinates x = 0.16 and y = 0.18. An external quantum efficiency of 8.7% and a power efficiency of 10.2 lm/W at

100 cd/m2 and 6.1% and 6.0 lm/W at 1000 cd/m

2 were achieved with 6 as host material. The external quantum efficiency

of 6.1% at a brightness of 1000 cd/m2 for the host 6 is comparable to the 6.2% reported by Sasabe et al.

30, who used the

phosphine oxide based host material PO9 for Ir(dbfmi).

Fig. 4. Left: Chemical structures of materials used in the OLED. Right: Energy level diagram of the OLED-device with 6 as

host material; Ionization potentials and electron affinity levels of the different materials are indicated. The dotted lines

represent the levels of the emitter Ir(dbfmi).

Fig. 5. Electroluminescence spectrum (left) and external quantum efficiency-luminance characteristics (right) of the OLED-

device with 6 as host material for Ir(dbfmi).

33



3. TRIAZINE BASED BIPOLAR HOST MATERIALS

By the use of bipolar host materials a balanced charge carrier transport can be achieved in the emitting layer of an

OLED. However, when donor and acceptor moieties form one conjugated molecule the triplet energy may drop

dramatically.31

Therefore it is necessary to break the conjugation between the hole- and the electron transporting moiety.

M. Rothmann et al. used PCTrz as bipolar host material, in which the hole conducting phenoxy-carbazole unit is

separated from the more electron transporting biscarbazolyl-triazine moiety by an ether bond.16

Recently we have

designed the new bipolar host material 3-(carbazol-9'-yl)-6,6’-dimethyl-3’-(4,6-(dicarbazol-9-yl)-1,3,5-triazin-2-yl)-1,1’-

biphenyl MBPTRZ, in which the ether bridge is substituted by a stable meta-linked biphenyl unit.32

The synthesis is

outlined in Scheme 1.

Scheme 1. Synthesis of MBPTRZ. Reagents and conditions: i) 2 eq. Carbazole, n-BuLi, THF, reflux, 6 h, 70%. ii) I2/HIO3,

CHCl3, H2SO4, AcOH, H2O, 80 °C, 6.5 h, 32%. iii) 1 eq. Carbazole, CuI, trans-1,2-diaminocyclohexane, K3PO4, dioxane,

reflux, 20 h, 48%. iv) Bis(pinacolato)diboron, PdCl2(dppf), KOAc, DMSO, 80 °C, 3 h, 85%. v) Pd2dba3, PCy3, K3PO4, H2O,

dioxane, toluene, 90 °C, 20 h, 92%.

3.1 Thermal Properties

The thermal properties of MBPTRZ were investigated by differential scanning calorimetry (DSC) and

thermogravimetric analysis (TGA). The results are summarized in Table 2. MBPTRZ exhibit a high thermal stability in

the TGA experiment. A weight loss of 1% (Td) is observed at a temperature of 475 °C. The glass transition temperature

(Tg) of OLED materials should be above 100 °C to prevent crystallization during operation. The Tg of MBPTRZ is

determined at 154 °C in the second DSC heating scan. In the first heating the melting occurs at 284 °C. Upon cooling the

material solidifies in an amorphous phase.

Table 2. Thermal properties of MBPTRZa.

Tg (°C) Tm (°C) Tc (°C) Td (°C)

MBPTRZ 154 284b

147b 475

aTg: glass transition temperature; Tm: melting temperature; Tc: crystallization temperature; Td: weight loss of 1% in N2

atmosphere. bObserved only in the first heating scan.

11

10

ii

+ 10

14

MBPTRZ

13

iii

12

iv

i

v



3.2 Optical Properties

In Fig. 6 the UV/Vis absorption, the fluorescence at room temperature and the phosphorescence spectra at 5 K of

MBPTRZ and N-phenylcarbazole (NPC) in neat films are displayed. The similar absorption spectra of MBPTRZ and

NPC show that the excited states in MBPTRZ have significant contributions from transitions that are localized on the

carbazole moiety.

The phosphorescence spectra of MBPTRZ shows peaks spaced about 160 meV apart at 441 nm (2.81 eV),

468 nm (2.65 eV) and 496 nm (2.50 eV) with relative intensities that are similar to the vibrational peaks in the NPC

phosphorescence spectrum. The comparison of the phosphorescence spectra of NPC and MBPTRZ displays similar

vibrational spacings and intensities, albeit the spectrum for MBPTRZ is somewhat broadened and shifted to red by

140 meV. In contrast to this similarity in the phosphorescence spectra the fluorescence of MBPTRZ and NPC show

clear differences. For NPC the fluorescence is well-structured with narrow vibrational peaks and without any noticeable

Stokes’ shift between the 0-0 peaks of fluorescence and absorption. In contrast, the fluorescence spectrum of MBPTRZ

is broader and shows a pronounced energy shift between the maximum of the fluorescence and either shoulder or peak in

the first absorption bands. In addition, the energy difference between the fluorescence spectra of NPC and MBPTRZ is

much larger than that found for the phosphorescence spectra. Density functional theory (DFT) calculations of the ground

state geometry and the excited states, calculated using Gaussian with the B3LYP hybrid functional, are helpful for

interpretation of the spectra. In MBPTRZ the central triazine ring forms an approximately planar system with the two

adjacent carbazoles and the adjacent phenyl ring (Fig. 6c). This roughly planar half of the molecule is separated from the

third carbazole moiety by a strong torsion of about 90°. Consistent with these strong torsions is a strong localization of

the frontier orbitals. The HOMO is localized onto the carbazole unit, while the LUMO is confined to the triazine and the

adjacent phenyl ring. This implies a spatial separation of the charge transport as the hole transport takes place between

HOMOs and the electron transport between LUMOs. We shall now pay attention to the dominant transitions of the

excited states. The first calculated excited state, S1, involves mainly the transition from the carbazole-based HOMO to a

triazine-based LUMO. In agreement with the strong charge-transfer (CT) character of this state, it carries no oscillator

strength. The next higher excited state, S2, is calculated to be 0.25 eV above S1 and has oscillator strength (0.08). The

dominant transition occurs from the HOMO-2 to the LUMO+1. It has a strong ππ* character involving the π-system that

extends over two carbazoles connected by the central triazine ring.

We associate the CT-type S1 state with the weak tail observed in the absorption spectra. We further consider that the

broad and red-shifted fluorescence results from this S1 state after geometric relaxation in the excited state and thus

concomitant planarization and somewhat improved wavefunction overlap. By virtue of their small wavefunction overlap,

charge transfer states are characterized by a small exchange energy, so that the associated triplet excited state can be

expected to be energetically close. In contrast, the exchange energy associated with ππ* transitions is significantly larger,

typically in the range of 0.7 - 1.0 eV.33

We therefore attribute the phosphorescence of MBPTRZ to a triplet state based

on ππ* transitions such as the HOMO-2 to LUMO+1 transition involved in S2. Such an involvement of carbazole-based

transitions between a π-system extending over two connected carbazoles is in good agreement with the observed

vibrational structure of the phosphorescence spectrum. A qualitative scheme of the excited state order is indicated in

Fig. 6. A similar relative order of states has also been observed for related carbazole derivatives with charge transfer

character.34

Table 3. Optical properties of MBPTRZ.

λEAa [nm]

film

ΔE(S0-S1)b

[eV]

λRT

emc [nm]

film

λ5K

emd [nm]

film

ΔE(T1-S0)e

[eV]

MBPTRZ 385 3.22 440 441 2.81

aAbsorption edge measured in neat films at room temperature. bOptical bandgap determined from the onset of the UV/Vis absorption of a neat film. cFluorescence maxima (excitation: 330 nm, 10-5 M cyclohexane solution, room temperature). dWavelength of 0-0 phosphorescence transition measured on film samples (2 wt% host material in PMMA at 5 K). eThe triplet energy was determined from the 0-0 transition of the phosphorescence spectra of neat films at 5 K.



MBPTRZ

Fig. 6. Absorption (solid line), fluorescence (dash-dotted line) and phosphorescence spectra (dotted line) of a) MBPTRZ

and b) N-phenylcarbazole (NPC) in neat films. Absorption and fluorescence (Fl) were taken from thin films at room

temperature, phosphorescence (Ph) (film) was taken at 5K. c) DFT optimized ground state geometries for MBPTRZ, along

with the orbitals having major contributions to the first and second singlet excited state. Also indicated are significant

torsion angles between adjacent units.

3.3 Electrochemical Properties

The electrochemical behavior was investigated by cyclic voltammetry (CV). The HOMO was determined from the half-

wave potential of the first oxidation relative to ferrocene and the LUMO was calculated by adding the optical band gap

to the HOMO. The first oxidation of MBPTRZ is observed at 0.78 V (vs. Fc/Fc+), which can be translated to a HOMO

of 5.58 eV. In Fig. 7 the HOMO and LUMO values of MBPTRZ are compared to the CBP-derivative 8. The HOMOs of

the two materials are 5.58 eV and 5.65 eV and almost identical. Similarly the well known host CBP has a HOMO of

5.63 eV. This shows that the HOMO levels of the two materials are mainly located at the carbazole adjacent to the

biphenyl linkers (Fig. 6c). The LUMO value of MBPTRZ is 2.28 eV and therefore somewhat lower compared to 8. DFT

calculations indicate that the LUMO is located at the electron deficient triazine. To achieve a high triplet energy

decoupling of the electron accepting and electron donating unit in bipolar host materials is important. In MBPTRZ this

is reached by the 90° twist between the two phenyl rings in the biphenyl unit by two methyl groups in the 2- and

2’-position of the biphenyl. The triplet energy of 2.81 eV is high enough for blue emitters.

3.4 Organic Light-Emitting Diodes

To demonstrate the potential of MBPTRZ as host material for blue phosphorescent emitters an OLED was fabricated.

The device setup and the materials are presented in Fig. 8. On top of an indium-tin-oxide (ITO) glass substrate

PEDOT:PSS was spin coated as hole injection layer followed by 10 nm DPBIC p-doped with molybdenum(VI) oxide as

hole transport layer. An additional 10 nm thick layer of DPBIC was used as exciton and electron blocking layer. The

40 nm thick emission layer consisted of MBPTRZ doped with 5% FIrpic. As hole and exciton blocking layer 5 nm

DBFSi were deposited followed by 20 nm cesium carbonate doped BCP as electron transporting layer. The device was

finalized by deposition of 100 nm aluminum as cathode. The OLED device shows a pure emission of FIrpic with a maximum at 473 nm. In Fig. 9 the quantum efficiency-

luminance characteristics and the luminance-voltage-current density characteristics of the device with MBPTRZ as host

are presented. The performance data at a brightness of 1000 cd/m2 are summarized in Table 4. A maximum external

quantum efficiency of 7.0% is achieved at a brightness of 25 cd/m2. At 1000 cd/m

2 the quantum efficiency drops to

5.5%.



DPB

IC:M

o03

DPB

IC

u, K

u,'8

.:'co

Ul

T -

000

-1z

70F)

'

DB

FSi

BC

P:C

sZC

O3

Fig. 7. Energy diagram showing HOMO and LUMO levels of MBPTRZ and the CBP-derivative 8 for comparison. The

HOMO levels were determined from the half-wave potential of the first oxidation in the cyclic voltammetry experiment. The

LUMO levels were estimated from the HOMO values and the optical bandgaps. The solid lines display the triplet energies

ΔE (T1 - S0).

Fig. 8. Left: Chemical structures of materials used in the OLED. Right: Energy level diagram of the OLED-device with

MBPTRZ as host material; Ionization potentials and electron affinity levels of the different materials are indicated. The

dotted lines represent the levels of the emitter FIrpic.

Table 4. OLED performance at a brightness of 1000 cd/m2 with MBPTRZ as host for the blue emitter FIrpic.

Voltage

[V]

ηC

[cd/A]

ηP

[lm/W]

ηext

[%]

MBPTRZ 11.1 12.9 3.7 5.5

ηC: Current efficiency; ηP: power efficiency; ηext: external quantum efficiency.

ener

gy [

eV]

MBPTRZ 8

2.28

5.58

2.81 2.98

2.16

5.65

ΔE(T

1 -S0 ) [eV

]



8.

ca 6-C

w 4-Erm 2-r

Cf .

o10° 101 102 103

210" 105

Luminance [cd /m ]

105

c7104

Eà 103U

$ 102

c 10'E

10°J10'

8 10 12 14Voltage [V]

1000n

800

600 m/

40073

200

Fig. 9. Quantum efficiency-luminance characteristics (left) and luminance-voltage-current density characteristics (right) for

the blue phosphorescent OLED with MBPTRZ as host material.

4. SUMMARY

In this paper we presented two classes of host materials for blue phosphors. The first class are the carbazole substituted

biphenyls 1-9, in which the triplets are confined to one half of the molecule by either twisting the central biphenyl or by

a meta-linkage of the carbazoles to the biphenyl unit. We obtained high triplet energies between 2.95 eV and 2.98 eV

and high glass transition temperatures in the range of 100 °C to 120 °C. Due to the high triplet energies OLEDs were

fabricated using the host material 6 and the carbene emitter Ir(dbfmi) with pure blue emission at 450 nm. The devices

achieved an external quantum efficiency of 8.7% at 100 cd/m2 and 6.1% at 1000 cd/m

2.

MBPTRZ with an electron transporting biscarbazolyltriazine that is separated from the hole transporting carbazole by a

non-conjugated, meta-linked biphenyl unit serves as example for a bipolar matrix material. The excellent glass forming

properties and the high Tg of 132 °C ensure morphological stability in OLEDs. The meta-linkage and the additional twist

at the biphenyl unit, which is achieved by two methyl groups in the 2- and 2’-position of the biphenyl in MBPTRZ leads

to a decoupling of the electron accepting and electron donating part and therefore to a high triplet energy of 2.81 eV.

DFT calculations demonstrate a clear separation of the electron and hole transporting moieties. A phosphorescent OLED

with MBPTRZ as host and FIrpic as emitter reached a maximum external quantum efficiency of 7.0%, a current

efficiency of 16.3 cd/A and a power efficiency of 6.3 lm/W.

Acknowledgments

We thank I. Bauer for the help with the synthesis of the host materials and C. Lennartz, C. Schildknecht, N. Langer, S.

Watanabe, P, Schrögel and M. Rothmann for helpful discussions. Financial support from the German Science

Foundation (DFG/GRK 1640) and the BMBF (TOPAS 2012) is gratefully acknowledged.

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Proc. of SPIE Vol. 8829 882906-10


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