p-Doped p-phenylenediamine-substituted fluorenes
for organic electroluminescent devices
Zhi Qiang Gao
a,d,*, Ping Fan Xia
b, Pik Kwan Lo
b, Bao Xiu Mi
a,d,1, Hoi Lam Tam
a,c,
Man Shing Wong
a,b,*, Kok Wai Cheah
a,c, Chin H. Chen
ea
Centre for Advanced Luminescence Materials, Hong Kong Baptist University, Kowloon Tong, Hong Kong, SAR China b
Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong, SAR China c
Department of Physics, Hong Kong Baptist University, Kowloon Tong, Hong Kong, SAR China d
Institute of Advanced Materials, Nanjing University of Posts and Telecommunications, Nanjing, China e
Display Institute, Microelectronics and Information Systems Research Center, National Chiao Tung University, Hsinchu, Taiwan
a r t i c l e
i n f o
Article history:
Received 22 November 2008
Received in revised form 19 December 2008 Accepted 24 February 2009
Available online 12 March 2009
PACS: 78 Keywords: Fluorene
p-Phenylenediamine Hole injection material
Organic electroluminescent devices Hole transporting materials
a b s t r a c t
Two novel p-phenylenediamine-substituted fluorenes have been designed and synthe-sized. Their applications as hole injection materials in organic electroluminescent devices were investigated. These materials show a high glass transition temperature and a good hole-transporting ability. It has been demonstrated that the 2,3,5,6-tetrafluoro-7,7,8,8-tet-racyanoquinodimethane (F4-TCNQ) doped p-phenylene-diamine-substituted fluorenes, in which F4-TCNQ acts as p-type dopant, are highly conducting with a good hole-transporting property. The organic light emitting devices (OLEDs) utilizing these F4-TCNQ-doped mate-rials as a hole injection layer were fabricated and investigated. The pure Alq3-based OLED device shows a current efficiency of 5.2 cd/A at the current density of 20 mA/cm2and the operation lifetime is 1500 h with driving voltage increasing only about 0.7 mV/h. The device performance and stability of this hole injection material meet the benchmarks for the commercial requirements for OLED materials.
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
Organic semiconductors have been intensively studied as active materials in various electronic and optoelectronic devices because of the low cost, ability to tune the func-tional/material properties by means of chemical structural modifications, ease of fabrication, and feasibility for flexi-ble devices[1]. In addition, the high absorptivity in the vis-ible range of organic semiconductors offers the possibility
to prepare a thin-film for photovoltaic cells (OPV) and pho-todetectors[2]. Hence, in the past decade, these materials have been intensively studied as alternative active materi-als in various electronic and optoelectronic devices. Re-cently, enormous progresses have been made in tailoring properties of organic semiconductors through chemical structure modification [3]. Organic semiconductors are generally in the form of undoped amorphous state when used as carrier transporters in thin-film optoelectronic de-vices. However, the inherent low charge mobility of organ-ic semorgan-iconductors resulting from the hopping transport in disordered organic thin-films often gives rise to a high operating voltage. For example, the hole mobility in amor-phous organic thin-films is in the range of 103–
105cm2v1s1, which is about 1000 times lower than
that of the amorphous Si thin-film. To improve the
1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.02.025
* Corresponding authors. Address: Department of Chemistry, Centre for Advanced Luminescence Materials, Hong Kong Baptist University, Kow-loon Tong, Hong Kong, SAR China. Tel.: +852 3411 7069; fax: +852 3411 7348.
E-mail address:[email protected](M.S. Wong). 1
Present address: Institute of Advanced Materials, Nanjing University of Posts and Telecommunications, Nanjing, China
Contents lists available atScienceDirect
Organic Electronics
conductivity of organic semiconductors, doping concepts have been adopted from their inorganic counterparts. Sim-ilar to the conventional doping in inorganic semiconduc-tors, the basic concept of doping in organic semiconductor is to add a strong electron-donor or elec-tron-acceptor which either transfer an electron to the low-est unoccupied molecular orbital (LUMO) of a host molecule to produce a free electron (n-type doping) or re-move an electron from the highest occupied molecular orbital (HOMO) of a host molecule to generate a free hole (p-type doping) [4]. In this concept, by matching the LUMO/HOMO of the dopant to the HOMO/LUMO of the or-ganic semiconductor, the Fermi level of the oror-ganic semi-conductor can be successfully shifted towards the transport states, and hence, the organic devices will be beneficial from reducing ohmic losses, lowering carrier injection barriers and increasing the built-in potential of Schottky- or p–n-junctions[5]. The organic optoelectronic device with a hole transporting layer (HTL) doped with p-type dopant such as 2,3,5,6-tetrafluoro-7,7,8,8-tetracya-noquinodimethane (F4-TCNQ), and an electron transport-ing layer (ETL) doped with n-type dopant such as alkali metal atoms are widely reported, and the device structure is known as p–i–n[6]. Currently, p–i–n organic light emit-ting devices (OLED)[7]and p–i–n OPV[8]are two of the hot topics in the organic optoelectronic devices due to the improved ohmic contact and electrical conductivity in the devices. For instance, a very low-operation-voltage multilayered OLED with high efficiency has been achieved by combining a thick F4-TCNQ doped 4,40,400
-tris(N,N-diphenylamino)-triphenylamine (TDATA) as a HTL with a thin undoped buffer layer[9]. This OLED exhibits a turn-on voltage of 2.5 V at a luminance of 1 cd/m2and an oper-ating voltage of 3.4 V at a luminance of 100 cd/m2for pure
tris-(8-hydroxyquinoline) aluminum (Alq3) device without
dopant emitter. Most recently, a p–i–n white OLED reached a high efficiency of 23.3 lm/W[10]; and a p–i–n red phos-phorescent OLED obtained an external quantum efficiency of 12.4% with extremely high lifetime of about 1 107h
[11]. Among the p-doped hole transporting systems, F4-TCNQ is the-state-of-art dopant, and most of the host materials for p-doping are based on the wide bandgap amorphous hole transporting materials with triphenyl-amine unit, such as 4,40,400
-tris-N-naphthyl-N-phenylami-no-triphenylamine (TNATA), N,N,N0, N0
-tetrakis-(4-methoxyphenyl)benzidine (MeO-TPD) [7a], 4,40,400
-tris(3-methylphenyl-phenyl-amino)triphenylamine (m-MTDA-TA)[7b]. Low bandgap metal phthalocyanines can also be efficiently doped with F4-TCNQ. However, phthalocya-nines are not well suited for OLEDs due to their small en-ergy gap between HOMO and LUMO levels of phthalocyanines, which leads to re-absorption and elec-tron injection from the emitting layer to the HTL [12]. Other type of materials that can be doped with F4-TCNQ are very rarely reported. In the practical point of view, de-vice stability is an important issue that governs the key factor to realize the success of commercialization. In order to achieve the practically targeted stability, organic semi-conductors with high thermal stability are necessary to be developed. Fluorene derivatives have been received considerable attention as potential candidates for the
ac-tive materials in OLED due to their good chemical stability and high luminescence over the past decade [13]. By attaching appropriate functional substituent, fluorene derivatives can act as a HTL [14], or an ETL [15], or a deep-blue emitting material (EM) [16] in OLEDs. To the best of our knowledge, there is no report on the applica-tions of fluorene derivatives as a p-doping host in OLEDs. In this contribution, we have designed and synthesized two new p-phenylenediamine-substituted fluorenes for the hole injection application. It is anticipated that these materials show a high glass transition temperature (Tg)
and a good hole transport ability because of the highly thermally stable nature of fluorene and phenylenediamine moieties as well as the good hole transporting property of phenylenediamine functional group. We have demon-strated that controlled p-type doping system using F4-TCNQ as a dopant can be extended to materials with a flu-orene core which was found to be efficient and stable in application of OLED devices. It has been shown that high conductivities can be achieved through doping with F4-TCNQ in these two derivatives. On the other hand, for good device performance, the ionization potential of the host also plays an important role. The lower the ionization po-tential of the host, the better is the performance. The best Alq3-based OLED device using such a host material doped
with F4-TCNQ as a hole injection layer (HIL) shows an effi-ciency of 5.2 cd/A and operation lifetime of 1500 h at 20 mA/cm2, with the driving voltage increasing about
0.7 mV/h. This material with the best performance can be used to replace the currently employed triphenylamine based HIL materials.
2. Results and discussion 2.1. Synthesis and characterization
Scheme 1shows the synthetic route utilized to prepare p-phenylenediamine-substituted fluorenes, 4a and 4b. 2,7-dibromofluorene derivatives 1a and 1b were prepared according to the literature procedures[16a]. Double ami-nation of 2,7-dibromofluorene derivatives 1a and 1b with two equivalent of diphenylamine in the presence of Pd(OAc)2:2P(o-tolyl)3 as a catalyst afforded the
corre-sponding diamination product 2a and 2b in 72% and 68% yield, respectively. Bromination of diphenylamino end-capped fluorene derivatives 2a and 2b with NBS in chloro-form yielded tetrabromo-substituted intermediates 3a and 3b in excellent yields. Palladium catalyzed amination of 3a and 3b with diphenylamine afforded the desired p-phenyl-enediamine-substituted fluorene derivatives in good yields. All the newly synthesized hole injection materials were fully characterized with1H NMR, 13C NMR,
MALD-TOF HRMS, and elemental analysis and found to be in good agreement with their structures. The detail procedures are shown in experimental section. These fluorene derivatives were purified by the flash column chromatography and de-gassed under vacuum before used for deposition.
Table 1summarizes the physical properties measured by the corresponding techniques. As determined by differ-ential scanning calorimeter and thermal gravimetric
ana-lyzer with a heating rate of 10 °C min1under N
2, both
flu-orene derivatives possess a high glass transition tempera-ture (Tg), > 125 °C and thermal decomposition
temperature (Tdec), >520 °C, respectively. Such high Tg
val-ues are superior to most of the commonly used hole trans-porting materials (i.e. Tg of 4,40,400
-tris(N-(2-naphthyl)-N-(phenylamino)triphenylamine (2T-NATA) = 110 °C)[17]. It is well-known that the higher the Tgof a material, the
bet-ter is the morphological stability of an organic thin-film. Hence, high morphological stability is anticipated using these materials as a hole transporting material in an OLED application.
Cyclic voltammetry (CV) was carried out in a three-elec-trode cell set-up with 0.1 M of Bu4NPF6as a supporting
electrolyte in CH2Cl2to examine the electrochemical
prop-erties of these molecules. All the potentials reported are referenced to Fc/Fc+standard and the results are tabulated
in Table 1. These fluorene derivatives exhibit a low first reversible one-electron anodic redox wave corresponding to phenylenediamine oxidation with Eoxd1
1=2 at 0.39–0.48 V
and followed by a reversible one-electron and two-elec-tron anodic redox waves with Eoxd21=2 at 0.52–0.58 V and
Eoxd3
1=2 at 0.86–0.89 V, respectively. Such a facile first
electro-chemical oxidation of phenylenediamine moiety suggests a very low first ionization potential of the materials which was confirmed by the estimated HOMO level around 5.1 eV as determined by ultraviolet photoemission spec-troscopy. The low HOMO level make them feasible to be re-moved an electron to generate a free hole upon doped with a strong electron-acceptor. On the other hand, these fluo-rene derivatives also exhibit an irreversible cathodic wave with Ered1
1=2 at 1.00 to 1.27 eV, which corresponds to the
formation of the radical anion on the fluorene/fluorone core. X N N N N N N C Bu Bu Br Br Bu Bu C O C Bu Bu X N N Pd(dba)2, P(t-Bu)3, NBS X N N Br Br Br Br Pd(dba)2, P(t-Bu)3, Br Br O C O X = 4a : X = X = 2a : 2b : 72% t-BuONa, toluene 90 oC CHCl3, r.t. Ph2NH, t-BuONa, toluene 90 oC Ph2NH, 97% 92% 3a 3b 90% or 1b 1a 68% X = 4b : 68%
Scheme 1. Synthesis of p-phenylenediamine-substituted fluorene derivatives 4a and 4b.
Table 1
Summaries of physical measurements of 4a and 4b. Absorption band/nma Eoxd 1=2/V b Ered p /V b HOMO/eVc LUMO eVd Tg/°Ce Tdec/°Cf 4a 309/354/394 0.39, 0.52, 0.86 1.00 5.07 2.16 126 521 4b 314/348/400 0.48, 0.58, 0.89 1.27 5.16 2.34 146 590 a Measured in CHCl 3. b Eoxd
1=2(or Eredp ) vs. SCE estimated by CV method using platinum disc electrode as a working electrode, platinum wire as a counter electrode, and SCE as a reference electrode with an agar salt bridge connecting to the oligomer solution and ferrocene was used as an external standard, E1/2(Fc/Fc+) = 0.50 V vs. SCE).
c
Determined by ultraviolet photoemission spectroscopy using Surface Analyzer model AC-2. d
LUMO = HOMO Optical Bandgap.
e Determined by differential scanning calorimeter with a heating rate of 10 °C min1under N 2. f Determined by thermal gravimetric analyzer with a heating rate of 10 °C min1under N
2.2. Charge-transfer complex of 4 and F4-TCNQ
Due to the strong electron-donating nature of 4 and the strong electron-accepting property of TCNQ, 4 and F4-TCNQ can interact to form a charge-transfer (CT) complex via partial electron transfer upon mixing with which free carriers would be generated as depicted in Fig. 1. There are two possible reaction routes for one-electron transfer between electron-donor 4 and electron-acceptor F4-TCNQ, as shown inFig. 1. Assuming that there is only one route dominates the one-electron transfer process, the reaction between 4 (represented as D) and F4-TCNQ (represented as A) can be written as[18]:
D þ A K Dþ
þ AE1A E1D ð1Þ
where, E1Aand E1Dare the first reduction potential of the
electron-acceptor and the first oxidation potential of the electron-donor, respectively. Thus the equilibrium con-stant (K) for electron transfer can be given:
log K ¼ log½½D þ ½A ½D½A ¼ E1A E1D 0:059 ð2Þ
Using E1Dof 0.39 and 0.48 V for 4a and 4b fromTable 1,
respectively and E1Aof 0.568 V for F4-TCNQ[19], the
reac-tion constants of K4a-F4-TCNQand K4b-F4-TCNQobtained were
103.02and 101.49, respectively. With such large positive
val-ues, the corresponding reactions for one-electron transfer between 4 and F4-TCNQ are likely to occur. The occurrence of such electron transfer reactions are also reflected in the UV–Vis absorption spectra as shown inFig. 2. Upon addition of F4-TCNQ, new bands/peaks at longer wavelengths appear in the solution mixture as compared to the spectra of their pure solutions. In the 4a and 4b absorption spectra, there are no absorption above 700 nm, while the mixture of F4-TCNQ with 4a and 4b show additional strong absorption peaks around 760 and 870 nm, which are attributed to the absorption of anion radical of F4-TCNQ[20]. Concomitantly, the absorption intensity of F4-TCNQ decreases dramatically
X N N N N N N 4 NC CN NC CN F F F F F4-TCNQ + X N N N N N N 4' X N N N N N N 4'' (1) (2) NC CN NC CN F F F F F4-TCNQ +
in the solution mixture. This illustrates that the charge transfer (CT) complex is formed between 4a (and 4b) and F4-TCNQ which also implies that these materials doped with F4-TCNQ would form a good p-type system. In addi-tion, the charge transfer band of F4-TCNQ doped 4a system was about 10 times higher than that of F4-TCNQ doped 4b system, suggesting a more efficient charge transfer between F4-TCNQ and 4a than F4-TCNQ and 4b. This may explain the better hole conduction in the 4a doped with F4-TCNQ thin-film shown in the next section.
2.3. p-Type doping properties in thin-films
From a chemical point of view, in amorphous thin-films, the p-type doping of a hole transporting molecules D by electron-acceptor molecules A is determined by a mass ac-tion law (In this case, D concentraac-tion is much higher than A concentration):
D þ ^DA D þ ½^DþA
Dþþ ^DA
ð3Þ
Here, D is a random matrix molecule and ^D is the matrix molecule adjacent to the electron-acceptor, which is
sup-posed to give the electron to the acceptor in the initial charge transfer step, this is exactly the reaction shown in
Fig. 1and Eq.(1). Thus, the intermediate state ½^DþA
is a local charge transfer state, where the charges may be bound by different interactions and form a kind of com-plex. The final state is assumed to be unbound, i.e. the ma-trix molecule that carries the positive charge is so far away from the ionized acceptor that it does not feel the Coulomb interaction. In such a situation, the positive charge can move by hopping and the density of Dþcan be associated
with the hole density p. If a very strong electron-acceptor is used and the initial charge transfer (first step of reaction
(3)) is complete, the hole density will be governed by the second step of reaction(3). The problem of the generation of free carriers is not only about the Coulomb attraction be-tween two molecules with opposite charge. It is about the difference in the total energy between the following two states: (a) a matrix with a certain density of charged accep-tors with holes bound to the accepaccep-tors and (b) the same matrix with mobile holes. Here, all kind of structural relax-ation and quantum chemical reorganizrelax-ation processes have to be considered[21]. In our case, it could be thought or assumed that the F4-TCNQ acceptors could react with the host molecules in a great extent or even completely be-cause of the energetically favorable reaction and the small electron-acceptor concentration. Therefore, the free hole density in the doping system shall be only controlled by the dissociation of the intermediate state ½^DþA
. In other words, besides the high tendency for electron transfer from electron-donor to electron-acceptor, another important factor that influences the conductivity of the doped system is the status of the intermediate ½^DþA
: the easier the dis-sociation, the higher conductivity is the doped system.
In order to study the hole transporting properties of 4a and 4b without or with doping, we fabricated hole-only devices with the structure of ITO/4a and 4b with 0% or 1.5% F4-TCNQ (60 nm)/NPB (10 nm)/Al (where NPB = N,N0-bis-(1-naphthyl)-N,N0-dipheny-1,10- biphenyl-4,40
-diamine). N,N0-bis-(1-naphthyl)-N,N0-dipheny-1,10
-biphe-nyl-4,40-diamine). For comparison, the 2T-NATA hole-only
devices with and without F4-TCNQ were fabricated simul-taneously. The I–V measurements were carried out on these devices and the results are shown in Fig. 3. It is clearly indicated that the F4-TCNQ doped films of 4a, 4b and 2T-NATA exhibit significantly higher current as com-pared to their undoped counterparts, demonstrating high conductivity in the doped films. In the F4-TCNQ doped sys-tems, 4a-based film shows a greater improvement in con-ductivity with about 2–3 orders of magnitude increase in hole current with respect to the undoped film. It is impor-tant to note that the current densities of F4-TCNQ doped 4a, 4b, and 2T-NATA hole-only devices at 3 V are 135, 6.6 and 51.8 mA/cm2, respectively. These findings are
consis-tent with the HOMO data determined which indicate that the HOMO level of 4a is lower than that of 4b and 2T-NATA, as shown in Table 1. In addition, the absorption spectra in the doping system indicate that the degree of charge transfer in 4a/F4-TCNQ system is higher than that of 4b/F4-TCNQ system due to the lower HOMO of 4a. Therefore, F4-TCNQ doped 4a film showed higher hole con-ductivity than those of F4-TCNQ doped 4b and 2T-NATA
300 400 500 600 700 800 900
(a)
Absorbance ( a rb ) Wavelength (nm) 4a F4-TCNQ 4a+F4-TCNQ 300 400 500 600 700 800 900(b)
Absor bance ( ar b) Wavelength (nm) 4b 4b+F4-TCNQFig. 2. (a) UV–Vis absorption of 4a (105M), F4-TCNQ (105M) and 4a (105
M) + F4-TCNQ (105
M) in DCM and (b) UV–Vis absorption of 4b (105
M) and 4b (105
M) + F4-TCNQ (105
films. As a result, 4a is anticipated to be a better candidate for the p-type doping host material than 2T-NATA. 2.4. Application in OLED devices
Encapsulated OLED devices with a sheet desiccant in-side a glass cover were fabricated using F4-TCNQ doped
4a and 4b as a HIL. The device structure is ITO/4a or 4b:1.5% F4-TCNQ (150 nm)/NPB (10 nm)/Alq3(60 nm)/LiF
(1 nm)/Al. The performance of the devices was tested and the I–V–B characteristics are shown inTable 2andFig. 4. The operation lifetimes were measured at a constant cur-rent of 20 mA/cm2. The lifetime was recorded after a
10-h burn-in. For comparison, t10-he OLED using 2T-NATA doped with F4-TCNQ as a HIL was also fabricated and tested un-der the same conditions. The results are summarized in Ta-ble 2.
Due to the doping of HIL, a high luminance is achieved at low voltage which results in a high power efficiency even though a comparatively thick HTL (160 nm in thick-ness) is used. For the 4a-based device, the turn-on voltage, defined as the bias at a brightness of 1 cd/m2, is only 2.7 V.
At 20 mA/cm2, the device driving voltage is 6.0 V and
cur-rent efficiency is 5.2 cd/A as compared to the commonly used 2T-NATA doped with F4-TCNQ as a HIL, the driving voltage is 6.7 V with a current efficiency of 4.5 cd/A at the same current density. These results indicate that 4a is a superior material for the HIL in OLED. For 4b-based de-vice, it shows a slightly higher driving voltage which agrees with its relatively inferior hole transporting proper-ties in F4-TCNQ doped thin-film and a shorter lifetime.
Fig. 5shows the stability curve of 4a-based device to its half brightness. As shown in the figure that such an OLED device has an operation lifetime of 1500 h at 20 mA/cm2,
and the driving voltage increasing is about 0.7 mV/h. This result demonstrates that our material 4a is a very useful and stable p-type dopant host for the use of OLEDs and has a great potential for practical applications.
3. Experimental 3.1. Synthesis
2a. A mixture of 2,7-dibromo-9,9-bis(n-butyl)fluorene, 1a (4.36 g, 10.0 mmol), diphenylamine (3.72 g, 22.0 mmol), palladium(II) acetate (0.11 g, 0.5 mmol), tri-(o-tolyl)phos-phine (0.30 g, 1.0 mmol) and sodium tert-butoxide (2.88 g, 30.0 mmol) in dry toluene (50 mL) was stirred un-der a nitrogen atmosphere at 110 °C for 24 h. After cooling to room temperature, the reaction mixture was poured into a saturated aqueous solution of ammonium chloride and extracted with dichloromethane (3 60 mL). The combined organic extract was washed with water and dried over anhydrous Na2SO4. Evaporation of volatiles gave
a brown solid, which was separated by silica gel column chromatography using petroleum ether/dichloromethane as a gradient eluent affording 2a in 72% yield (4.4 g) as a white solid. 1H NMR (400 MHz, CDCl 3, d) 7.47 (d, J = 8.0 Hz, 2H), 7.22–7.25 (m, 8H), 7.07–7.13 (m, 10H), 6.97– 7.01 (m, 6H), 1.73–1.77 (m, 4H), 1.02–1.10 (m, 4H), 0.72 (t, J = 7.6 Hz, 6H), 0.64–0.67 (m, 4H).13C NMR (100 MHz, CDCl3, d) 152.0, 148.0, 146.4, 136.2, 129.1, 123.7, 123.6, 122.3, 119.7, 119.4, 54.9, 39.7, 26.1, 22.9, 13.9. MS (MAL-DI-TOF) m/z 612.8 (M+).
2b. The synthetic procedure for 2a was followed using 2,7-dibromofluorenone, 1b (3.38 g, 10.0 mmol), Ph2NH
(3.72 g, 22.0 mmol), t-BuONa (2.88 g, 30.0 mmol), Pd(OAc)2
1 2 3 4 5 6 7 8 1E-4 1E-3 0.01 0.1 1 10 100 (a) Curre nt De nsity (mA/cm 2 ) Voltage (V) I-V of 4a I-V of 4a+1.5% F4-TCNQ 1 2 3 4 5 6 7 8 1E-4 1E-3 0.01 0.1 1 10 100 (b) Cu rrent Den s ity (mA/cm 2) Voltage (V) I-V of 4b I-V of 4b+1.5% F4-TCNQ 0 1 2 3 4 5 6 7 8 1E-3 0.01 0.1 1 10 100 (c) Cur rent Densit y ( mA/ cm 2) Voltage (V) I-V of 2T-NATA I-V of 2T-NATA+1.5% F4-TCNQ
Fig. 3. I–V characteristics of (a) 4a, (b) 4b, and (c) 2T-NATA hole-only devices with and without F4-TCNQ.
(0.11 g, 0.5 mmol), P(o-tolyl)3, (0.30 g, 1.0 mmol) and dry
toluene (50 mL). The product was purified by silica gel col-umn chromatography using petroleum ether/dichloro-methane as a gradient eluent affording 3.48 g (68%) of 2b as a dark red solid.1H NMR (400 MHz, CDCl
3, d) 7.24 (d,
J = 2.0 Hz, 2H), 7.14–7.19 (m, 10H), 6.94–7.04 (m, 14H).
13C NMR (100 MHz, CDCl
3, d) 193.3, 148.1, 147.0, 137.9,
135.7, 129.4, 128.5, 124.5, 123.4, 120.3, 119.3. HRMS (MALDI-TOF) Calc. for C37H26N2O: 514.2040. Found:
514.2041.
3a. A mixture of 2a (4.28 g, 7.0 mmol) and NBS (5.0 g, 28.0 mmol) in 50 mL of CHCl3were stirred at room
tem-perature for 4 h. Evaporation of solvent under vacuum
re-sulted in a yellow solid that was adsorbed on silica gel and purified by flash column chromatography using dichloromethane/petroleum ether mixture as eluent affording 3a in 97% yield (6.30 g) as a white solid. 1H NMR (400 MHz, CDCl3, d) 7.49 (d, J = 8.0 Hz, 2H), 7.34 (d, J = 8.8 Hz, 8H), 7.02 (d, J = 2.0 Hz, 2H), 6.97 (m, 10H), 1.75–1.79 (m, 4H), 1.02–1.10 (m, 4H), 0.72 (t, J = 7.2 Hz, 6H), 0.61–0.69 (m, 4H). 13C NMR (100 MHz, CDCl 3, d) 152.3, 146.6, 145.6, 136.6, 132.2, 125.1, 123.6, 120.1, 119.3, 115.2, 55.0, 39.6, 26.1, 22.9, 14.0. MS (MALDI-TOF) m/z 928.1 (M+).
3b. The above procedure for 3a was followed using 2b (3.4 g, 6.6 mmol) and NBS (4.71 g, 26.5 mmol). The crude product was purified by silica gel flash column chromatog-raphy using dichloromethane/petroleum ether mixture as eluent affording 3b in 92% yield (5.04 g) as a dark purple solid.1H NMR (400 MHz, CDCl3, d) 7.37 (d, J = 8.8 Hz, 8H), 7.28–7.30 (m, 4H), 7.11 (dd, J = 2.0, 8.0 Hz, 4H), 6.94 (d, J = 8.8 Hz, 8H).13C NMR (100 MHz, CDCl 3, d) 192.7, 147.4, 145.6, 138.6, 135.8, 132.6, 129.0, 125.8, 120.8, 119.6, 116.4. MS (MALDI-TOF) m/z 829.9 (M+).
4a. A dried 250 mL two-necked flask containing the solution of 3a (4.64 g, 5.0 mmol), Ph2NH (4.06 g,
24.0 mmol), t-BuONa (2.88 g, 30.0 mmol), Pd(dba)2
(116 mg, 0.2 mmol), tri-tert-butylphosphine (40 mg, 0.2 mmol) and 70 mL of dry toluene was heated at 90 °C overnight with good stirring under N2. After cooling to
room temperature, the purple-blue mixture was poured into a saturated aqueous solution of ammonium chloride and extracted twice with toluene. The combined organic layers were washed with brine, dried over anhydrous mag-nesium sulfate and evaporated under vacuum. The residue was rapidly eluted through silica gel column using toluene/ hexane (4:1) to give crude product, which was further purified by crystallization from EtOAc/EtOH affording 5.44 g (85% yield) of 4a as a light-yellow solid. 1H NMR (400 MHz, C6D6, d) 7.35–7.38 (m, 4H), 7.21 (dd, J = 1.6, 8.0 Hz, 2H), 7.12–7.16 (m, 24H), 7.00–7.08 (m, 24H), 6.83 (t, J = 7.6 Hz, 8H), 1.70–1.73 (m, 4H), 0.94–1.07 (m, 4H), 0.84–0.91 (m, 4H), 0.63 (t, J = 7.6 Hz, 6H) 13C NMR (100 MHz, C6D6, d) 152.5, 148.4, 147.1, 143.7, 143.0, 136.5, 129.5, 125.9, 125.0, 124.2, 123.7, 122.7, 120.4, 119.4, 55.2, 40.0, 26.5, 23.2, 14.1. HRMS (MALDI-TOF) calc. for C93H80N6: 1281.6471. Found: 1281.6514, Anal. Calc. for
C93H80N6: C 87.15, H 6.29, N 6.56. Found: C 87.30, H 6.19, N
6.40.
4b. The general amination procedure was followed using 3b (2.1 g, 2.53 mmol), Ph2NH (2.06 g, 12.2 mmol),
2 4 6 8 10 0 50 100 150 200 1 10 100 1000 10000 Current De nsity (mA/ cm 2) Driving Voltage (V) 4a-based Device 4b-based Device Lumi nance ( c d/ m 2)
Fig. 4. I–V–B characteristic of 4a- and 4b-based OLED devices. Table 2
Summaries of OLED Performance. Devicea
HIL Turn On/Vb
V20/V CE20/cd/Ac Lifetime20/hd
I 2T-NATA 2.7 6.7 4.5 1600
II 4a 2.7 6.0 5.2 1500
III 4b 3.1 7.6 3.4 1100
a Device I: ITO/2T-NATA:1.5%F4-TCNQ(150 nm)/NPB(10 nm)/Alq
3(60 nm)/LiF(1 nm)/Al. Device II: ITO/4a:1.5%F4-TCNQ(150 nm)/NPB(10 nm)/ Alq3(60 nm)/LiF(1 nm)/Al. Device III: ITO/4b:1.5%F4-TCNQ(150 nm)/NPB(10 nm)/Alq3(60 nm)/LiF(1 nm)/Al.
b
Turn on is the voltage to show 1 cd/m2
luminance. c
CE20refers to the current efficiency under 20 mA/cm. d
Lifetime20presents the lifetime at constant driven-current of 20 mA cm.
0 500 1000 1500 0 200 400 600 800 0 2 4 6 8 10 Luminance (cd/m 2) Operation Lifetime (hrs) V o lt age ( V )
Pd(dba)2 (58.7 mg, 0.1 mmol), P(t-Bu)3 (20.2 mg,
0.1 mmol),t-BuONa (1.46 g, 15.2 mmol) and toluene. The
crude product was purified by silica gel flash column chro-matography (toluene/hexane) and then by crystallization from THF/CH3OH affording 2.03 g (68% yield) of 4b as a
red solid. 1H NMR (400 MHz, C 6D6, d) 7.68 (d, J = 2.0 Hz, 2H), 7.09–7.13 (m, 18H), 7.02–7.06 (m, 16H), 6.93–6.98 (m, 16H), 6.89 (d, J = 8.0 Hz, 2H), 6.83 (t, J = 7.2 Hz, 8H). 13C NMR (100 MHz, C 6D6, d) 192.7, 148.6, 148.2, 144.0, 142.2, 137.9, 136.4, 129.6, 127.6, 125.9, 125.5, 124.5, 123.0, 120.6, 118.6. HRMS (MALDI-TOF) calc. for C85H62N6O: 1182.4980; Found: 1182.4988. Anal. Calc. for
C85H62N6O: C 86.27, H 5.28, N 7.10; found: C 86.14, H
5.17, N 7.02.
3.2. Device fabrication
OLED and hole-only devices were fabricated using a vacuum thermal evaporation chamber with a base pres-sure of 1 106Torr. All the materials were deposited in
one pump-down. Two shadow masks were used to define the deposition areas for organic and metal cathode, respec-tively. The I–V characteristics of hole-only devices were measured with a computer-controlled DC power supply at room temperature in open air. OLED devices were encapsulated with sheet desiccant under dry N2
atmo-sphere before measurements. The luminance of OLEDs was measured with PR650 spectrophotometer and a KETH-LEY 236 source meter. The lifetime of OLED device was re-corded by a homemade lifetime measurement system at constant current mode. The ionization potential (or HOMO) of a thin-film was measured by ultraviolet photo-emission spectroscopy using Surface Analyzer model AC-2. 4. Conclusions
In summary, we have designed and synthesized two new p-phenylenediamine-substituted fluorenes, 4a and 4b which possess a high glass transition temperature and a good hole transport ability. The p-type doping properties of these materials were studied using F4-TCNQ as a p-type dopant in hole-only device. Furthermore, the mechanism in the doping system was discussed and some insights were highlighted. The OLED device with a HIL utilizing 4a/4b doped with F4-TCNQ were fabricated and character-ized. The pure Alq3-based device using 4a doped with
F4-TCNQ as a HIL shows an operation lifetime of 1500 h at 20 mA/cm2, and the driving voltage increasing about
0.7 mV/h. These results suggest that 4a is a useful and sta-ble hole injection material for OLEDs which shows a great potential for practical applications.
Acknowledgements
This work is supported by Guangdong-HK ITF (GHP/ 057/05), Research Grants Council of Hong Kong, Earmarked
Research Grant HKBU 202507, Nanjing University of Posts and Telecommunications Grant NY207162 and Natural Sci-ence Foundation of Jiangsu High Education under Grant 08KJB430011.
References
[1] C.D. Dimitrakopoulos, P.R.L. Malenfant, Adv. Mater. 14 (2002) 99. [2] (a) P. Peumans, A. Yakimov, S.R. Forrest, J. Appl. Phys. 93 (2003)
3693;
(b) S. Günes, H. Neugebauer, N.S. Saricifitci, Chem. Rev. 107 (2007) 1324.
[3] (a) T.P.I. Saragi, T. Spehr, A. Siebert, T. Fuhrmann-Lieker, J. Salbeck, Chem. Rev. 17 (2007) 1011;
(b) A. Gadisa, W. Mammo, L.M. Andersson, S. Admassie, F. Zhang, M.R. Andersson, O. Inganas, Adv. Func. Mater. 17 (2007) 3836. [4] (a) C.K. Chan, E.-G. Kim, J.-L. Bredas, A. Kahn, Adv. Funct. Mater. 16
(2006) 831;
(b) Z.Q. Gao, B.X. Mi, G.Z. Xu, Y.Q. Wan, M.L. Gong, K.W. Cheah, C.H. Chen, Chem. Commun. (2008) 117.
[5] M. Pfeiffer, K. Leo, X. Zhou, J.S. Huang, M. Hofmann, A. Werner, J. Blochwitz-Nimoth, Org. Electron. 4 (2003) 89.
[6] K. Walzer, B. Maenning, M. Pfeiffer, K. Leo, Chem. Rev. 107 (2007) 1233.
[7] (a) J.S. Huang, M. Pfeiffer, A. Werner, J. Blochwitz, K. Leo, S. Liu, Appl. Phys. Lett. 80 (2002) 139;
(b) G. He, M. Pfeiffer, K. Leo, Appl. Phys. Lett. 85 (2004) 3911. [8] B. Maennig, J. Drechsel, D. Gebeyehu, P. Simon, F. Kozlowski, A.
Werner, F. Li, S. Grundmann, S. Sonntag, M. Koch, K. Leo, M. Pfeiffer, H. Hoppe, D. Meissner, N.S. Sariciftci, I. Riedel, V. Dyakonov, J. Parisi, Appl. Phys. A 79 (2004) 1.
[9] X. Zhou, M. Pfeiffer, J. Blochwitz, A. Werner, A. Nollau, T. Fritz, K. Leo, Appl. Phys. Lett. 78 (2001) 410.
[10] G. Schwartz, M. Pfeiffer, S. Reineke, K. Walzer, K. Leo, Adv. Mater. 19 (2007) 3672.
[11] R. Meerheim, K. Walzer, M. Pfeiffer, K. Keo, Appl. Phys. Lett. 89 (2006) 061111.
[12] X. Zhou, J. Blochwitz, M. Pfeiffer, A. Nollau, T. Fritz, K. Leo, Adv. Funct. Mater. 11 (2001) 310.
[13] P. Chen, G. Yang, T. Liu, T. Li, M. Wang, W. Huang, Polym. Int. 55 (2006) 473.
[14] (a) K.-F. Shao, Y.-F. Li, L.-M. Yang, X.-J. Xu, G. Yu, Y.-Q. Liu, Chem. Lett. 34 (2005) 1604;
(b) Z.H. Li, M.S. Wong, H. Fukutani, Y. Tao, Chem. Mater. 17 (2005) 5032.
[15] (a) L. Oldridge, M. Kastler, K. Müllen, Chem. Commun. (2006) 885;
(b) Z.H. Li, M.S. Wong, Y. Tao, H. Fukutani, Org. Lett. 9 (2007) 3659.
[16] (a) Z.H. Li, M.S. Wong, Y. Tao, J.P. Lu, Chem. Eur. J. 11 (2005) 3285;
(b) Z.Q. Gao, Z.H. Li, P.F. Xia, M.S. Wong, K.W. Cheah, C.H. Chen, Adv. Funct. Mater. 17 (2007) 3194;
(c) T.-C. Chao, Y.-T. Lin, C.-Y. Yang, T.S. Hung, H.-C. Chou, C.-C. Wu, K.-T. Wong, Adv. Mater. 17 (2005) 992;
(d) P.A. Levermore, R. Xia, W. Lai, X.H. Wang, W. Huang, D.D.C. Bradley, J. Phys. D: Appl. Phys. 40 (2007) 1896.
[17] Y. Shirota, H. Kageyama, Chem. Rev. 107 (2007) 953. [18] R.C. Wheland, J. Am. Chem. Soc. 98 (1976) 3926.
[19] Notes: At Ref[4b], the first reduction potential of F4-TCNQ was measured as 0.61 V vs. Ag/AgCl electrode in saturated KCl solution, where the potential of reference electrode itself is 0.200 V. According to the SCE potential of 0.242, the first reduction potential of F4-TCNQ vs. SCE is calculated as: 0.61 + 0.200 0.242 = 0.568 V.
[20] L.R. Melby, R.J. Harder, W.R. Hertlbr, W. Mahler, R.E. Benson, W.E. Mochel, J. Am. Chem. Soc. 84 (1962) 3374.
[21] B. Kramer (Ed.), Advances in Solid State Physics, vol. 39, Vieweg, Braunchweig, Wiesbaden, 1999, pp. 77–90.
