Carbazole-pyrene derivatives for undoped organic light-emitting devices

Article abstract of DOI:10.1016/j.orgel.2010.12.026

Two carbazole-pyrene derivatives, namely 3,6-dipyrenyl-9-(4′-tert- butylphenyl) carbazole (BPyC) and 3,6-dipyrenyl-9-(4′-pyrenylphenyl) carbazole (TPyC), have been designed and synthesized for application in organic light-emitting devices (OLEDs). While the two compounds have similar chemical structures and photoluminescent properties, OLEDs based on them show distinct electroluminescence (EL) spectra. The BPyC-based devices show a single peak saturated blue emission with CIE coordinates of (0.15, 0.18); whereas the TPyC-based devices exhibit two emission peaks at blue and yellow hues with CIE coordinates of (0.22, 0.29). The difference in their EL spectra is attributed to the substitution of the t-butyl unit of BPyC with a pyrenyl group to form TPyC, which effectively increases the electron-donating property and results in exciplex formation at its interface with the electron-accepting TPBI. A high external quantum efficiency of 3.11% is achieved in the TPyC-based devices. Influences of chemical structure and fluorescent quantum yield on the efficiency of exciplex emission are discussed.

Full text of DOI:10.1016/j.orgel.2010.12.026

Organic Electronics 12 (2011) 541–546
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Carbazole–pyrene derivatives for undoped organic light-emitting devices
S.L. Lai a, Q.X. Tong b,c, , M.Y. Chan d, T.W. Ng a, M.F. Lo a, C.C. Ko e, S.T. Lee a, C.S. Lee a,
⇑
⇑
a Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City University of Hong
Kong, Hong Kong Special Administrative Region
b Department of Chemistry, Shantou University, Guangdong 515063, China
c Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, China
d Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong Special Administrative Region
e Department of Biology and Chemistry, City University of Hong Kong, Hong Kong Special Administrative Region
a r t i c l e i n f o
a b s t r a c t
Two carbazole–pyrene derivatives, namely 3,6-dipyrenyl-9-(40-tert-butylphenyl) carbazole
(BPyC) and 3,6-dipyrenyl-9-(40-pyrenylphenyl) carbazole (TPyC), have been designed and
synthesized for application in organic light-emitting devices (OLEDs). While the two com-
pounds have similar chemical structures and photoluminescent properties, OLEDs based
on them show distinct electroluminescence (EL) spectra. The BPyC-based devices show a
single peak saturated blue emission with CIE coordinates of (0.15, 0.18); whereas the
TPyC-based devices exhibit two emission peaks at blue and yellow hues with CIE coordi-
nates of (0.22, 0.29). The difference in their EL spectra is attributed to the substitution of
the t-butyl unit of BPyC with a pyrenyl group to form TPyC, which effectively increases
the electron-donating property and results in exciplex formation at its interface with the
electron-accepting TPBI. A high external quantum efficiency of 3.11% is achieved in the
TPyC-based devices. Influences of chemical structure and fluorescent quantum yield on
the efficiency of exciplex emission are discussed.
Article history:
Received 29 October 2010
Received in revised form 29 December 2010
Accepted 31 December 2010
Available online 13 January 2011
Keywords:
Small-molecule
Organic light-emitting diode
Electroluminescence
Undoped
Carbazole
Pyrene
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
each light-emitting layer (EML). However, this approach
would complicate device configuration as well as fabrica-
Organic light-emitting device (OLED) is emerging as a
leading next generation technology for full-color flat-panel
displays and ambient illumination sources because of its
huge market potential and excellent performance in terms
of brightness, operating voltage and color gamut. Incorpo-
ration of extrinsic dyes into conductive hosts is an effective
way to customize emitting color of OLEDs that is particu-
larly useful for fabricating white OLEDs with two comple-
mentary or three primary colors [1,2]. Electroluminescence
(EL) in terms of Commission Internationale de L’Eclairage
(CIE) coordinates can be fine-tuned and optimized through
precise control of dopant concentration and thickness for
tion process, and inevitably increases the total manufac-
turing cost. In addition, the shortened device lifetime due
to morphological phase separation caused by operation in-
duced heating is another challenge for doped OLEDs.
Recent studies [3–9] have demonstrated a simple way
to tailor the emitting color via interfacial excited-state
charge-transfer complex (termed exciplex) formation to
control emission peak location and/or intensity by using
different binary combinations of electron-donor/hole-
transporting materials (HTMs) and electron-acceptor/elec-
tron-transporting materials (ETMs), i.e., hmexciplex = ELUMO
(ETM) ꢀ EHOMO (HTM), ignoring the band bending at the
organic/organic interface [10]. In the equation, EHOMO and
ELUMO are the energies of the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbi-
tal (LUMO), respectively. However, only certain combina-
tions of materials were reported to be capable of giving
⇑
Corresponding authors. Address: Department of Chemistry, Shantou
University, Guangdong 515063, China (Q.X. Tong).
E-mail addresses: qxtong@gmail.com (Q.X. Tong), apcslee@cityu.
edu.hk (C.S. Lee).
1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2010.12.026

542
S.L. Lai et al. / Organic Electronics 12 (2011) 541–546
exciplex emission and their external quantum efficiencies
E) are usually low (e.g., m-MTDATA/Alq3 (0.22%) [4], 2-
by the Flash EA 1112 method. Modulated differential scan-
ning calorimetry (MDSC) and thermogravimetric analysis
(TGA) were performed under a nitrogen atmosphere at a
heating rate of 10 °C minꢀ1 on a TA Instruments MDSC
2910 and TGA Q50, respectively. Solution UPL were deter-
mined by using the method of Demas and Crosby [13] with
a degassed aqueous solution of quinine sulfate as a refer-
ence. Optical absorption and photoluminescence (PL) spec-
tra were measured with a Perkin-Elmer Lambda 2S UV/vis
spectrometer and a Perkin-Elmer LS 50B luminescence
spectrometer, respectively. Ultraviolet photoelectron spec-
troscopic (UPS) studies were performed in a VG ESCALAB
220i-XL surface analysis system with an unfiltered He I
(21.22 eV) gas discharge lamp under a base vacuum of
10ꢀ10 Torr.
(g
TNATA/Alq3 (0.30%) [5] and TPD/PBD (1.10%) [6]). Thus,
the appearance of exciplex is often considered as one of
the main reasons responsible for poor device performance.
On the other hand, Palilis et al. recently reported exciplex-
based OLEDs using NPB/PPSPP, and achieved an exception-
ally high gE of 3.40% [7]. Later, our group demonstrated the
TPyPA/BPhen based exciplex emitting devices with gE as
high as 3.93% [8]. It is proposed that matched carrier
mobilities, high photoluminescence quantum yield (UPL),
and suitable electron energy levels of organic materials
are the prerequisites for efficient exciplex-based OLEDs
[8]. These findings provide guidelines for designing organic
materials with desirable physical properties to facilitate
high efficiency exciplex emission.
In addition to triphenylamine [4,5,8], both pyrene [8,11]
and carbazole [9,12] and their derivatives have also been
employed for exciplex emission, owing to their excellent
luminescence characteristics, structural stabilities, and
particularly their strong electron-donating tendencies
when contacting with different electron-acceptors. How-
ever, the influences on how the chemical structures of or-
ganic materials affecting the nature of exciplex emission
have seldom been studied. In this work, two carbazole-
based compounds end-capped with one t-butyl and two
pyrene groups, namely 3,6-dipyrenyl-9-(40-tert-butyl-
phenyl) carbazole (BPyC) and three pyrene groups, namely,
3,6-dipyrenyl-9-(40-pyrenylphenyl) carbazole (TPyC) have
been synthesized. While the two compounds have very
similar structures and photoluminescent spectra, OLEDs
using them as host emitters show distinct EL characteris-
2.2. Fabrication and characterization of OLEDs
Transparent indium tin oxide (ITO) pre-coated and pat-
terned glass slides with a sheet resistance of 30
X
squareꢀ1
were used as anodic substrates. Prior to successive film
deposition, they were thoroughly cleaned with Decon 90,
rinsed in de-ionized water, dried in an oven, and finally
treated in an ultraviolet–ozone cleaner. OLEDs consisting
of a hole-transporting layer (HTL) of N,N0-diphenyl-1,10-
biphenyl-4,40-diamine (NPB), a hole-transporting light-
emitter of BPyC or TPyC, an electron-transporting layer
(ETL) of 1,3,5-tris(N-phenylbenzimidazol-2-yl) benzene
(TPBI) or 4,7-diphenyl-1,10-phenanthroline (BPhen), and
a
LiF/Al cathode. All films were thermally deposited
sequentially without vacuum break in an evaporation
chamber under a base pressure of ꢁ10ꢀ6 Torr. Deposition
rates were monitored with quartz oscillation crystals and
controlled at a rate of 0.1–0.2 nm sꢀ1 for both organic
and metal layers. A shadow mask was used to define the
cathode and to make four 0.1 cm2 devices on each sub-
strate. Current density–voltage–luminance (J–V–L) charac-
teristics, EL spectra, and CIE coordinates were measured
simultaneously with a programmable Keithley model 237
power source and a Photoresearch PR650 spectrometer.
All measurements were carried out at room temperature
under ambient atmosphere without any encapsulation.
tics. Particularly, the BPyC-based devices (gE = 2.21%) show
only a single blue emission peak while the TPyC-based de-
vices show two emission peaks with higher maximum gE
of 3.11%. Reason for these differences is attributed to the
higher degree of electron-donating property with increas-
ing number of pyrene groups. This results in the formation
of the yellow exciplex emission which shifts the EL spec-
trum from saturated blue (BPyC) to near white (TPyC). It
is worth noting that both carbazole derivatives have UPL
of about 95%, which are much higher than that of our pre-
viously reported triphenylamine TPyPA (UPL = 80%) [8].
This results in a higher maximum
vices, as compared to that of a similarly structured TPyPA-
based OLED ( E = 2.83%). Based on these results the influ-
gE of the TPyC-based de-
g
3. Results and discussion
ences of chemical structures and UPL on the efficiency of
exciplex emission are discussed.
3.1. Material syntheses
Molecular structures of BPyC and TPyC and their syn-
thetic routes are shown in Scheme 1. Incorporation of the
9-phenylcarbazole group is expected to enhance hole-
transporting properties of the two compounds. BPyC and
TPyC can be easily prepared with high yields using typical
Suzuki coupling reactions from 3,6-dibromo-9-(4-tert-
butylphenyl)-9H-carbazole [14], 3,6-dibromo-9-(4-bromo-
phenyl)-9H-carbazole [15], and commercial starting mate-
rial pyrene-1-boronic acid, respectively, according to
reported procedures [16]. Molecular structures of the final
compounds were confirmed by 1H nuclear magnetic reso-
nance, mass spectrometry, and elemental analysis.
2. Experimental
2.1. Materials and instruments
All reagents and solvents for the synthesis of BPyC and
TPyC were purchased from either Sigma–Aldrich or Acros
Organics and used as received without further purification.
1H NMR spectra were recorded with the use of a Bruker
DPX-400 spectrometer (400 MHz). Mass spectrometry
was performed on a PE SCIEX LC/MS spectrometer. Ele-
mental analyses were carried out on a Vario EL Elementar

S.L. Lai et al. / Organic Electronics 12 (2011) 541–546
543
Scheme 1. Synthetic routes and molecular structures of 3,6-dipyrenyl-9-(40-tert-butylphenyl) carbazole (BPyC) and 3,6-dipyrenyl-9-(40-pyrenylphenyl)
carbazole (TPyC).
3.2. Chemical and photophysical properties
and TPyC, respectively, while the PL spectra show intense
blue emissions peaked at 443 nm (BPyC) and 465 nm
(TPyC) under excitation at 385 nm. It is well recognized
that an increase in conjugation length would generally re-
sult in red-shifts in both kabs and PL. Indeed, the addition of
a pyrene arm (from BPyC to TPyC) does red-shift the
absorption and PL peaks by respectively 11 and 22 nm.
From the PL quantum yield measurement, UPL of BPyC
and TPyC in solution are 94.8% and 95.0%, respectively.
These high UPL values may be attributed to large Stoke
shifts (difference between positions of band maxima of
the absorption and emission spectra) in both organic mate-
rials that can effectively avoid re-absorption of the emitted
photons during the PL process.
The rigid molecular structures in the two carbazole
derivatives are beneficial to thermal stability, as mani-
fested by their high decomposition temperatures (Td) of
496 and 516 °C, and melting temperatures (Tm) of 293
and 245 °C for BPyC and TPyC, respectively (Td corresponds
to 5% weight loss in the TGA measurements whereas Tm
was determined by MDSC) although no obvious glass tran-
sitions were observed.
Optical absorption and PL spectra of BPyC and TPyC vac-
uum-deposited solid thin-films on quartz plates are de-
picted in Fig. 1. The longest wavelength absorption
maxima (kabs, max) appear at 358 and 369 nm for BPyC

544
S.L. Lai et al. / Organic Electronics 12 (2011) 541–546
Fig. 1. Optical absorption (Abs) and photoluminescence (PL) spectra of
BPyC and TPyC solid thin-films on quartz substrates. Inset: PL spectra of
10% TPBI:TPyC, TPBI, and TPyC thin-films, respectively.
Fig. 2. Electroluminescence spectra of the BPyC-based OLEDs without
(lower) and with (upper) NPB viewed in the normal direction at the
luminance of 10, 100, and 1000 cd mꢀ2, respectively.
UPS was employed to measure the EHOMO values of the
carbazole derivatives, in which they are found to be 5.57
and 5.67 eV for BPyC and TPyC, respectively. By subtracting
from the optical band gaps determined from long wave-
length onsets of the absorption bands, ELUMO are estimated
to be 2.58 eV (for BPyC) and 2.71 eV (for TPyC). Table 1
summarizes the thermal and photophysical data of BPyC
and TPyC.
3.3. Device characteristics
Devices with the structures of ITO/EML (100 nm)/TPBI
(20 nm)/LiF (0.5 nm)/Al (100 nm) (w/o NPB) and ITO/NPB
(70 nm)/EML (30 nm)/TPBI (20 nm)/LiF (0.5 nm)/Al
(100 nm) (w/NPB) were fabricated to evaluate the proper-
ties of BPyC or TPyC as hole-transporting EML. Fig. 2 de-
picts EL spectra of the BPyC-based OLEDs with (upper)
and without (lower) the NPB layer viewed in the normal
direction at the luminance of 10, 100, and 1000 cd mꢀ2
.
Both devices exhibit similar pure blue emissions with mild
change in CIE coordinates of (0.17, 0.25) (Device w/o NPB)
and (0.15, 0.18) (Device w/NPB) over a wide range of lumi-
nance from 10 to 1000 cd mꢀ2. This suggests that the hole
and electron recombination is well confined within the
emissive BPyC layer. In addition, these undoped blue-emit-
ting devices show high efficiencies (as shown in Fig. 3) and
low operating voltages (as shown in Fig. 4) in which cur-
rent and power efficiencies (gC and gP), and gE and operat-
ing voltage at 20 mA cmꢀ2 are 4.3 cd Aꢀ1, 4.5 lm Wꢀ1
2.49%, and 3.7 V for device without NPB, and are 2.9 cd Aꢀ1
,
Fig. 3. (a) Current efficiency (
gC), external quantum efficiency (gE) and (b)
power efficiency ( P) as a function of current density. Device configura-
g
,
tions: ITO/BPyC (100 nm)/TPBI (20 nm)/LiF (0.5 nm)/Al (100 nm); ITO/
NPB (70 nm)/BPyC (30 nm)/TPBI (20 nm)/LiF (0.5 nm)/Al (100 nm).
2.7 lm Wꢀ1, 2.21%, and 3.5 V for device with NPB. After
Table 1
Thermal and photophysical properties of BPyC and TPyC. Td and Tm were determined by thermogravimetric analysis (TGA) and modulated differential scanning
calorimetry (MDSC) measurements at a same heating rate of 10 °C minꢀ1 under a nitrogen atmosphere whereas Td corresponds to 5% weight loss. kabs and kPL
were measured with solid thin-film. Eopt was estimated from the long wavelength onset of the absorption band. EHOMO was defined by ultraviolet photoelectron
spectroscopy (UPS) while ELUMO was estimated by ELUMO = EHOMO ꢀ Eopt. UPL was measured in solution.
Compound
Tm/Td (°C)
kabs, max (nm)
kPL (nm)
Stokes shift (nm)
kabs, onset (nm)/Eopt (eV)
EHOMO/ELUMO (eV)
UPL (%)
BPyC
TPyC
293/496
245/516
358
369
443
465
85
96
415/2.99
419/2.96
5.57/2.58
5.67/2.71
94.8
95.0

S.L. Lai et al. / Organic Electronics 12 (2011) 541–546
545
whereas the red-shifted band centered at 558 nm appears
to be arising from the exciplex emission formed at the
TPyC/TPBI interface. This can be confirmed with the PL
spectrum of the TPBI:TPyC blended film as shown in the in-
set of Fig. 1. It is considered that coupling between the
strong electron-donating TPyC and the strong electron-
accepting TPBI results in exciplex emission at their inter-
face. Combining with the intrinsic blue emission at
475 nm, near white light emission with CIE coordinates
of (0.21, 0.26) and (0.22, 0.29) were obtained for the de-
vices without and with the NPB layer, respectively. In fact,
the CIE coordinates change even closer to white emission
of (0.24, 0.30) and (0.25, 0.33) when the ETL (i.e., TPBI) is
replaced by BPhen (as shown in the two top-most spectra
of Fig. 5).
Fig. 4. Current density and luminance characteristics as a function of
voltage in various devices. Device configurations: ITO/TPyC (100 nm)/
TPBI (20 nm)/LiF (0.5 nm)/Al (100 nm); ITO/NPB (70 nm)/TPyC (30 nm)/
TPBI (20 nm)/LiF (0.5 nm)/Al (100 nm); ITO/BPyC (100 nm)/TPBI (20 nm)/
LiF (0.5 nm)/Al (100 nm); ITO/NPB (70 nm)/BPyC (30 nm)/TPBI (20 nm)/
LiF (0.5 nm)/Al (100 nm).
Fig. 6 demonstrates efficiencies of the TPyC-based de-
vices as a function of current density. With the insertion
of NPB layer, gC
6.4 cd Aꢀ1, 5.3 lm Wꢀ1, and 3.11%, respectively, much high-
er than those of device without the NPB layer (3.8 cd Aꢀ1
3.3 lm Wꢀ1
and 2.01%). More importantly, the gE of
, gP, and gE dramatically increase to
,
insertion of NPB at the ITO/BPyC interface, the EHOMO of
NPB can function as a stepping-stone for hole-injection,
similar to the case of ITO/m-MTDATA/NPB [17]. This
improvement for hole-injection definitely reduces the
driving voltage. It is believed that the amounts of hole
and electron are balanced inside the device w/o NPB. How-
ever, the excess of holes in the device w/NPB eventually re-
duces the device efficiency. Nevertheless, these relatively
high device efficiencies and low driving voltage are compa-
rable or even better than current state-of-the-art undoped
emitters with saturated blue fluorescence [18–20].
,
TPyC-based device is considerably higher than that of our
previously reported triphenylamine TPyPA-based device
(gE = 2.83%) with the same device configuration. One possi-
ble explanation for this difference is that the UPL of TPyC
(95%) is much higher than that of TPyPA (80%). It should
be noted that as UPL of both TPyC and TPyPA materials
are very high in solution, various solid-state quenching ef-
fects could be much more important than the yields in
solution. The exact solid-state quenching effects in both
Unexpectedly, the extra pyrene arm in TPyC leads to
large difference in EL properties. Fig. 5 depicts EL spectra
of the TPyC-based OLEDs with (middle) and without (low-
er) NPB viewed in the normal direction at the luminance of
10, 100, and 1000 cd mꢀ2. In contrast with the PL spectrum
of neat thin-film, both devices demonstrate two broad
emission peaks at ca. 475 and 558 nm. The blue emission
peak at 475 nm is contributed from the TPyC emission,
Fig. 5. Electroluminescence spectra of the TPyC-based OLEDs without
(lower) and with (middle) NPB viewed in the normal direction at the
luminance of 10, 100, and 1000 cd mꢀ2, respectively. Electroluminescence
spectra for devices with the structures of ITO/TPyC/BPhen/LiF/Al and ITO/
NPB/TPyC/BPhen/LiF/Al (upper) are also inserted.
Fig. 6. (a) Current efficiency (
gC), external quantum efficiency (gE) and (b)
power efficiency ( P) as a function of current density. Device configura-
g
tions: ITO/TPyC (100 nm)/TPBI (20 nm)/LiF (0.5 nm)/Al (100 nm); ITO/
NPB (70 nm)/TPyC (30 nm)/TPBI (20 nm)/LiF (0.5 nm)/Al (100 nm).

546
S.L. Lai et al. / Organic Electronics 12 (2011) 541–546
Table 2
Electroluminescence performance of BPyC and TPyC-based devices. Device configurations: ITO/BPyC (100 nm)/TPBI (20 nm)/LiF (0.5 nm)/Al (100 nm); ITO/NPB
(70 nm)/BPyC (30 nm)/TPBI (20 nm)/LiF (0.5 nm)/Al (100 nm); ITO/TPyC (100 nm)/TPBI (20 nm)/LiF (0.5 nm)/Al (100 nm); ITO/NPB (70 nm)/TPyC (30 nm)/TPBI
(20 nm)/LiF (0.5 nm)/Al (100 nm).
Device
Vturn-on (V)
V at 20 mA cmꢀ2 (V)
gC (cd Aꢀ1
)
gP (lm Wꢀ1
)
gE (%)
CIE (x, y)
BPyC
NPB/BPyC
TPyC
2.7
2.7
2.9
2.8
3.65
3.48
4.58
4.34
4.25
2.94
3.76
6.42
4.45
2.72
3.34
5.34
2.49
2.21
2.01
3.11
0.17, 0.25
0.15, 0.18
0.21, 0.26
0.22, 0.29
NPB/TPyC
thin-films are now under investigation. Nevertheless, the
discrepancy of UPL on the device performance is in consis-
tent with our previous study [8] and underscores the
importance of high UPL in organic materials for achieving
highly efficient exciplex-based OLEDs. Characteristics of
all BPyC- and TPyC-based devices are summarized in
Table 2.
exciplex emission. By employing the carbazole derivatives
as emitters in contact with the electron-transporting TPBI,
both devices exhibit distinct EL properties. Particularly, the
TPyC-based devices display two emission peaks with CIE
coordinates of (0.22, 0.29); while the BPyC-based devices
show a single blue emission peak with CIE coordinates of
(0.15, 0.18). More importantly, a relatively high gE of
3.11% can be obtained for the TPyC-based device. This dis-
crepancy on the device performance is attributed to the
substitution of t-butyl unit of BPyC with a pyrenyl group,
resulting in an increase on the degree of electron-donating
property and hence the presence of exciplex emission at
the HTL/ETL heterojunction.
It is interesting to note that, in our previous study, the
use of a better ETL (i.e., BPhen) would effectively increase
the intensity of exciplex emission and, in turns, results in
a better balanced white emission. In particular, the CIE
coordinates change from (0.18, 0.25) for devices with TPBI
as ETL to (0.31, 0.35) for the BPhen-based TPyPA device. In
addition, the replacement of BPhen results in a high gE of
3.93%, much higher than that of the TPBI-based device
(2.83%). These enhancements have been attributed to the
different carrier mobility of BPhen and TPBI. Indeed, the
intensity of exciplex emission is determined by the charge
accumulation densities, which in turns depends on the rel-
ative hole mobility and electron mobility in HTL and ETL. In
particular, the use of ETL with higher electron mobility
would lead to higher exciplex emission intensity. As the
electron mobility in TPBI is two orders of magnitude lower
than that in BPhen, the relatively small electron mobility in
TPBI would inevitably reduce the number of electron–hole
pairs formed for exciplex emission, and in turns leads to a
weak exciplex emission. However, it is not the case in our
present study, in which there is only a mild shift on the CIE
coordinates from (0.22, 0.29) to (0.25, 0.33) and a small in-
Acknowledgments
This work was supported by the Research Grants Coun-
cil of the Hong Kong Special Administrative Region, China
(Project No. CityU 101508). Q.X. Tong thanks his financial
supports from the National Science Foundation of China
(No. 91027041) and the Key Laboratory of Photochemical
Conversion and Optoelectronic Materials, TIPC, CAS.
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crease on gE from 3.11% to 3.16% when the ETL is replaced
by BPhen. Such minor device performance improvement
for the TPyC-based devices may be ascribed by the lower
mobility in TPyC than that in TPyPA. Particularly, it is ex-
pected that the hole mobility of TPyC is much lower than
that of TPyPA, and thus the use of better electron-trans-
porting materials, e.g., BPhen, would not result in a dra-
matic increase on the interfacial charge accumulation
density at the HTL/ETL interface. It is also the reason for
the larger driving voltage for the TPyC-based devices. For
instance, to generate a current density of 20 mA cmꢀ2
,
the driving voltages of the TPyC-based device with TPBI
or BPhen as ETL are 4.34 and 3.76 V, respectively, while
those of the TPyPA-based device with TPBI or BPhen as
ETL are 3.60 and 3.31 V, respectively [8].
4. Conclusions
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Chen, C.H. Cheng, Adv. Funct. Mater. 18 (2008) 67.
We have designed and synthesized two carbazole–pyr-
ene based fluorophores, namely BPyC and TPyC, to investi-
gate how chemical structures of organic materials affecting