Dynamically adaptive characteristics of resonance variation for selectively enhancing electrical performance of organic semiconductors

Article abstract of DOI:10.1002/anie.201304540

Increased resonance: The selective tuning of the optoelectronic properties of organic semiconductors is possible by enantiotropic resonance variation. Using resonance forms of N⁺i￡P-O^- in a series of arylamine-phosphine oxide hybrids afforded low-voltage-driven phosphorescent OLEDs with outstanding performances. Copyright

Full text of DOI:10.1002/anie.201304540

Angewandte
Chemie
DOI: 10.1002/anie.201304540
Organic Optoelectronics
Dynamically Adaptive Characteristics of Resonance Variation for
Selectively Enhancing Electrical Performance of Organic
Semiconductors**
Ye Tao, Jianjian Xiao, Chao Zheng, Zhen Zhang, Mingkuan Yan, Runfeng Chen,* Xinhui Zhou,
Huanhuan Li, Zhongfu An, Zhixiang Wang, Hui Xu,* and Wei Huang*
Controllably and selectively tuning molecular functions
through extensible and universal approaches is always one
of the most important and challenging issues for material
science. However, because of the intrinsic correlations
between the molecular properties, only certain molecular
structural strategies and inter-/intra-molecular effects are
effective for a particular property without negatively influ-
encing other properties, let alone applications, which is one of
the main hindrances for the further development of organic
optoelectronics after it has flourished for decades. Since
optical and/or electronic processes are involved in all of the
organic optoelectronic applications, for example, organic light
emitting diodes (OLEDs),[1] organic photovoltaic cells
(OPVs),[2] organic field-effect transistors (OFETs),[3] bio-/
chemo-/photo-sensors,[4] memories,[5] and lasers,[6] the intrinsic
contradiction between the optical and electronic properties of
organic conjugates gradually stands out, resulting in bottle-
necks in their device performances leaving them far from the
rigorous requirements of commercial applications. The sit-
uation of host development for phosphorescent OLEDs
(PHOLEDs) is one of the most typical embodiments under
the demands of balanced optoelectronic properties for
reducing operating voltage and improving device efficiencies,
taking account of high triplet energy (ET1) for exothermic
energy transfer to the guest, suitable frontier molecular
orbital (FMO) energy levels for charge injection, and optimal
molecular configuration and composition for balanced carrier
transportation.[7] The inherent conflict between optical and
electrical properties originates from the dependence of
optoelectronic properties on molecular structures and fron-
tier orbital distributions and consequently their sensitivity to
modification, leading to solutions mainly focused on the
static-state molecular properties, such as ambipolar charac-
teristics of donor (D)–acceptor (A) structures. Recently, our
group developed several effective strategies, named short-axis
linkage,[8] multi-insulating linkage,[9] and indirect linkage,[10]
for constructing D–A and D-p-A type host materials with
diphenylphospine oxide (DPPO) as the acceptor and realizing
excellent device performance, such as ultralow driving
voltages (less than 3 V for onset) and high and stable
efficiencies.
Nevertheless, the intramolecular interactions, such as
forming low-energy charge transfer (CT) states, in compli-
cated D–A systems should be suppressed by insulating
linkages but at the cost of reducing electrical activity.
Actually, the static-state D–A configuration is inferior in
controlling optoelectronic properties owing to its definite
bipolar structure and limited variation, requiring more
accurate adjustment of the type and ratio of functional
groups for balanced electronic performance. Fortunately, in
À =
our previous work, a special effect of N P O on charge
redistribution by resonance variation was observed as
a potential approach for the adjustment of electrical charac-
teristics.[11] Resonance structures are proposed to describe
delocalized electrons within certain molecules where the
bonding cannot be expressed by one single Lewis formula
following the valence bond theory. These resonance struc-
tures are useful in evaluating the structures of delocalized
[*] Y. Tao,[+] J. J. Xiao,[+] Dr. C. Zheng, M. K. Yan, Dr. R. Chen,
Dr. X. H. Zhou, H. H. Li, Dr. Z. F. An, Z. X. Wang, Dr. H. Xu,
Prof. W. Huang
[+] These authors contributed equally to this work.
[**] This study was supported in part by the National Basic Research
Program of China (2009CB930601), National Natural Science
Foundation of China (21274065, 20804020, 61176020, and
21001065), the Ministry of Education of China (No. IRT1148 and
212039), Natural Science Foundation of Jiangsu Province
(BK2011751), A project funded by the priority academic program
development of Jiangsu higher education institutions, and Sup-
porting Program of New Century Excellent Talents of Ministry of
Education (NCET-12-0706).
Key Laboratory for Organic Electronics and Information Displays
(KLOEID), Institute of Advanced Materials (IAM)
Nanjing University of Posts and Telecommunications
Nanjing 210023 (P.R. China)
E-mail: iamrfchen@njupt.edu.cn
Z. Zhang, Dr. H. Xu
Key Laboratory of Functional Inorganic Material Chemistry
Ministry of Education, Heilongjiang University
74 Xuefu Road, Harbin 150080 (P.R. China)
E-mail: hxu@hlju.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201304540.
Prof. W. Huang
Jiangsu-Singapore Joint Research Center for Organic/Bio- Elec-
tronics & Information Displays and Institute of Advanced Materials
Nanjing University of Technology, Nanjing 211816 (P.R. China)
E-mail: wei-huang@nut.edu.cn
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molecules which have a dynamically variable charge distri-
bution.[12] Therefore, the involvement of distinct resonance
variation establishes the possibility for regulating molecular
electrical properties through new dynamic adaptive processes
in real-time, consequently improving the balanced carrier
injection and transportation.
Herein the use of resonance variations for the dynamic
adaptability of electronic characteristics was systematically
investigated on the basis of a series of D–A molecules, namely
NCzPO, DNCzPO, NDPhPO, and DNDPhPO containing
carbazole or diphenylamine as donor and phenylphosphine
enhanced electron transportation from the polarized carba-
zolyl, accompanied by the preserved hole transportation by
the other neutral carbazoly group which is ascribed to the
À =
dynamic adaptability of N P O for charge transfer by
resonance variation. As a result, DNCzPO endowed its blue
PhOLED with impressively low driving voltages: 2.6 V for
onset, under 3.4 V for 100 cdmÀ2 and under 4.8 V for
1000 cdmÀ2, as well as favorable electroluminescence (EL)
efficiencies, such as a maximum external quantum efficiency
(EQE) of up to 16.5% and power efficiency (PE) > 37 Lm
WÀ1. This work established a solid example for adjusting
optoelectronic process through dynamically adaptive behav-
ior and opened the window for purposefully constructing
resonance-enhanced d–A systems for diverse optoelectronic
applications.
À =
oxide (PPO) as acceptor, on the basis of N P O resonance
(Scheme 1, Figure 1a, and Supporting Information
À =
Scheme S1). N P O was formed through the direct linkage
of arylamine and PPO moieties. The extremely similar
photophysical properties of the mono- and bi-substituted
À
The shortened N P bonds in NCzPO and DNCzPO
=
derivatives owing to the insulating P O groups established
compared with the normal length of 1.76 nm, as indicated by
the feasibility for selectively modifying electrical properties of
the bi-substituted host materials through dynamically adap-
their single-crystal structures, verify the partial double-bond
À
À
+= À
character of N P through the contribution of N P O
resonance structures (Figure 1, Table S1 and Table S2). The
increased contribution of resonance structures to the dynamic
molecular configuration of DNCzPO by its enantiotropic two
tive resonance variations. The greater contribution of enan-
À
+= À
tiotropic N P O resonances to DNCzPO results in a much
À
+= À
N
P O resonance variations is directly evidenced by its
À
shorter N P bond length. We further investigated the
influence of N P O resonance on the charge distribution
À =
on these charged D–A molecules by Natural Bond Orbital
(NBO) calculations (Table S1). With an injected electron, the
negative charge is majorly localized on electron-withdrawing
PPO unit (71.1% for NCzPO and 84.6% for NDPhPO).
Significantly, the substantial negative charge, approximately
20%, distributes on the arylamines. This proportion further
increases to up to 33% when two arylamines are involved in
À
+= À
the N P O resonance, as for bi-substituted analogues,
indicating a more stable contribution of the charge-separated
resonance structure N P O to the molecular configura-
tion, which is exactly coincident with the shorter N P bonds
À
+= À
À
found in the single-crystal structures and dramatically modi-
fies the electrical properties of the arylamines favorable for
electron injection and transportation. On the contrary, with
an injected hole, bi-substitution resulted in the remarkable
reduction of the positive charge distribution on PPO, owing to
the preservation of one neutral arylamine group in each
resonance structure with dominant electron-donating ability
Scheme 1. Schematic drawings of dynamically adaptive characteristics
À =
of enantiotropic N P O resonances.
(Scheme 1). These results suggest the potential of enantio-
À
+= À
tropic N P O resonances for dynamically balancing carrier
exchange between adjacent molecules.
[9b]
=
Owing to the insulating characteristic of P O,
the
electronic communication between conjugated groups is
effectively suppressed so that the absorption spectra are
almost unchanged from monosubstituted to bisubstituted
analogues (Figure 2). The electron-withdrawing effect of PPO
results in the remarkable hypochromatic shift about 20 nm for
the emissions of NCzPO compared with free carbazole
(Figure S3 and Table S3). The even larger hypochromatic
shift of DNCzPO further reveals its more electron deficient
carbazole chromophore with the bigger contributions from
À
+= À
N
P O resonance structures. In films, the absorption
Figure 1. a) structural formula of the arylamine–PO hybrids (left:
NCzPO and NDPhPO, right: DNCzPO and DNDPhPO). Single-crystal
structures of b) NCzPO and c) DNCzPO.
spectra are almost unchanged with similar peak wavelengths
and profiles, indicating the reduced intermolecular interac-
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HOMOs are dominated by the electron-donating arylamines,
giving rise to nearly the same HOMOs with the little variation
(experimentally less than 0.07 eV). These HOMOs are lower
than that of carbazole (À5.8 eV)[14] owing to the direct linkage
=
with electron-withdrawing P O. As the opposite effect,
arylamines also induce the elevation of the LUMOs com-
pared with that of common phosphine oxide molecules
(À2.5 eV)[15] even though the LUMOs are mainly located
on PPOs. The separated frontier molecular orbitals (FMOs)
of these D–A molecules not only indicate their ambipolar
+= À
characteristics, but also suggest the great potential of N
P
OÀ resonance owing to the ability of D and A moieties to
stabilize for the positive and negative charges, respectively. It
is indicated that in the static state, the direct linkage of D and
A seems to weaken their relative electrical capability, which is
inconsistent with the actual electrical performance of these
d–A molecules and consequently implies the contribution of
À
+= À
N
P O resonance for dynamic-state charge injection/
transportation. The nominal electron-only and hole-only
devices were further fabricated to verify the dynamic effect
À
+= À
of the N P O resonance on the carrier-transportation
(Experiment Section in the Supporting Information). The
hole-only current density (J) of NDPhPO is much greater
Figure 2. a) Absorption (solid symbols)and fluorescence spectra (open
symbols) of the arylamine-PO hybrids in CH2Cl2 (10À6 molLÀ1); b) J/V
characteristics of the nominal single-carrier transporting devices based
on these arylamine–PO hybrids (solid and open symbols are for
electron-only and hole-only devices, respectively).
+
=
than its electron-only J indicating it has the least stable N
À
À
P O resonance as a result of the flexible structure of
diphenylamine. The much improved stability of the enantio-
tropic resonance structures of bisubstituted DNDPhPO
induced the reversed electrical properties so that the electron
transportation became dominant with the electron-only J two
tions, such as aggregation and p–p stacking (Figure S4). The
photoluminescence (PL) spectra of NCzPO and DNCzPO are
similar (Figure 2). However, the remarkably reduced vibra-
tion bands of bisubstituted DNCzPO testifies to its dynam-
ically enantiotropic resonance configurations, which is further
verified by its much lower PL quantum yields (PLQY) than
those of NCzPO which suffers from the predominant non-
radiative transition by long-term preserved excited energy
through stable resonance variations. The spectra of NDPhPO
and DNDPhPO showed a similar tendency. The lowest triplet
state energy levels (T1) of these molecules were more than
2.9 eVaccording to the 0–0 transition bands around 420 nm of
the low-temperature time-resolved phosphorescence spectra
(Figure S5 and Table S3), which are almost equivalent to
those of their chromophores carbazole and diphenylamine[13]
owing to the well-controlled T1 state location at these
chromophore as shown by the spin density distribution
(Figure S6). Therefore, the optical investigation further con-
firmed the more stable enantiotropic resonance configura-
tions of bi-substituted analogues accompanied by the similar
excited energy levels mainly contributed by their chromo-
phores. This situation provided a flexible platform to study
the potential of selectively modulating electrical performance
by resonance variations.
orders of magnitude larger than the hole-only value. Owing to
the planar and rigid structure of carbazole, the N P O
À
+= À
resonance structure of NCzPO is stable enough to support the
electron-dominant electrical characteristic. More importantly,
DNCzPO exhibited the highest electron-only J, accompanied
with the much higher hole-only J compared to NCzPO,
resulting in the most balanced carrier transportation among
these D–A molecules. The stronger electron transporting
ability of the bi-substituted analogues with one more hole-
transporting arylamine group showed the direct evidence for
À
+= À
the contribution of N P O resonance to the dynamic
charge transfer process. Furthermore, the simultaneous
improvement of hole and electron transportation in
DNCzPO demonstrated the outstanding feature of the
enantiotropic resonance variation on dynamic adaptation
for the charge transfer (Scheme 1), which undoubtedly
establishes a new approach for controllable modulation of
optoelectronic properties for organic semiconductors.
To confirm the superiority of resonance-variable materials
in optoelectronic devices, especially their dynamic adaptabil-
ity in electrical processes, the blue-emitting PhOLED devices
of NCzPO, NDPhPO, DNCzPO, and DNDPhPO (hosts)
based on FIrpic (guest), termed devices A–D, respectively,
were fabricated with the configuration of ITO/MoOx (2 nm)/
m-MTDATA:MoOx (15 wt%, 30 nm)/m-MTDATA (10 nm)/
[Ir(ppz)3](10 nm)/Host: FIrpic (10 nm, 10%)/BPhen (20 nm)/
LiF (1 nm)/Al (100 nm) (Scheme S3).[16] Device E based on
the conventional host N,N-dicarbazolyl-3,5-benzene (mCP)
for blue PHOLEDs were also fabricated for comparison. The
turn-on voltages of A--D were similar to that of E, attributed
The static-state charge-capture ability of these molecules
was evaluated according to the energy of their highest
occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) through a combined theoretical
and experimental study using DFT calculations (Figure S7)
and cyclic voltammetry (CV) analysis (Figure S8). The
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maximum efficiencies of 26.0 cdAÀ1, 26.2 LmWÀ1, and
13.5%. The more balanced carrier transportation in
DNCzPO further gave rise to the highest maximum efficien-
cies of C as 32.3 cdAÀ1, 37.2 LmWÀ1, and 16.5%, which were
more than 1.5 fold that of mCP-based E. The efficiencies of C
were also rather stable in that at 100 cdmÀ2 the efficiencies of
C only slightly decreased to 28.9 cdAÀ1, 23.4 LmWÀ1, and
14.7%, while at 1000 cdmÀ2 the efficiencies still remained as
high as 23.4 cdAÀ1, 15.8 LmWÀ1, and 12.2%, respectively. It is
shown that the efficiency stability of these devices is in accord
with the resonance stability of their hosts. Since the optical
properties of these D–A molecules are similar, the superior
EL performance of C should originated from the excellent
dynamically adaptive electrical performance of DNCzPO
provided by its stable enantiotropic resonance variation.
In summary, the selective modulation of electrical proper-
ties by the resonance variation and its dynamically adaptive
characteristics in charge-transfer process are demonstrated by
the bisubstituted carbazole-PO hybrid DNCzPO. On the basis
of preserved optical properties inherited from the chromo-
=
phores by the insulating linkage of P O, the electron trans-
porting ability of DNCzPO is selectively and greatly
À
+= À
enhanced by its N P O resonance. This is accompanied
by the preserved hole transporting ability of the remaining
neutral carbazolyl, which profits from dynamic adaptability of
the enantiotropic resonance variation during the electrical
processes. As a result, DNCzPO provided its FIrpic-based
device with driving voltages as low as 2.6 V for onset, under
3.4 V for 100 cdmÀ2, and under 4.8 V for 1000 cdmÀ2 for
portable applications, and high and stable efficiencies with the
maxima of 32.3 cdAÀ1 for CE, 37.2 LmWÀ1 for PE, and
16.5% for EQE, which are among the best results of low-
voltage-driving blue PHOLEDs reported to date. Benefiting
from the flexibility in manipulating electronic structures and
properties by resonance variation, this work not only
established a new approach for selective modulation of
optoelectronic properties for organic semiconductors in
dynamic processes, but also further stimulates the new
concepts and applications for resonance structured d–A
materials.
Figure 3. a) Current density (J) (open) luminance (solid) voltage
curves of devices A–E; b) Efficiencies–luminance curves of devices A–
E.
to the FMOs of these D–A molecules which are approx-
imately equal to those of mCP (Figure 3 and Table SI4).
However, at 100 and 1000 cdmÀ2, the driving voltages of
devices A, C, and D turned to be about 0.7 and 1.4 V lower
than those of E, which indicated the effect of the resonance-
enhanced carrier transportation compared with mCP. In
accord with the results of single-carrier transporting devices,
DNCzPO endowed C with the lowest driving voltages, less
than 3.4 V and 4.8 V at 100 and 1000 cdmÀ2, which were 0.6
and 0.7 V lower than those of A and should be ascribed to the
dynamically adaptive carrier transportation by its enantio-
tropic resonance structures during device operation. The
driving voltage of less than 4.0 V of C at 200 cdmÀ2 makes its
application in portable displayers driven by low-voltage
power sources feasible.[10a] The small overlap between the
emission of NDPhPO and the metal-to-ligand charge-transfer
(MLCT) absorption band of FIrpic, its low resonance
stability, and the recombination zone close to BPhen with
a low ET1 of 2.6 eV should be the main reasons for the lowest
Received: May 27, 2013
Published online: August 14, 2013
Keywords: donor–acceptor systems ·
.
À
organic light-emitting diodes · phosphine oxides ·
phosphorescence · resonance variation
+
efficiencies of B.[17] Therefore, the stabilized N = P O
À
resonance of DNDPhPO gives its corresponding device D
significantly improved maximum efficiencies of 27.4 cdAÀ1
for current efficiency (CE), 28 LmWÀ1 for power efficiency
(PE), and 14.3% for external quantum efficiency (EQE),
which were better than those of E (20 cdAÀ1, 22.7 LmWÀ1,
and 10.2%). Nevertheless, the sharply reduced efficiencies of
D at high luminance still indicated the worse stability of
charged DNDPhPO arising from its flexible configuration.
NCzPO which has a more stable resonance structure
endowed A with a much reduced efficiency roll-off of 8.1%
for EQE at 100 cdmÀ2, accompanied with the comparable
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