Separation of electrical and optical energy gaps for constructing bipolar organic wide bandgap materials

Article abstract of DOI:10.1039/c2cc17682c

An electrical and optical energy gaps separation strategy is put forward for the design of organic wide bandgap semiconductors. This new principle could achieve optimization of wide bandgap (both high singlet and triplet energies) and favorable carrier injection energy levels simultaneously.

Full text of DOI:10.1039/c2cc17682c

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2012, 48, 3015–3017
www.rsc.org/chemcomm
COMMUNICATION
Separation of electrical and optical energy gaps for constructing bipolar
organic wide bandgap materialsw
Dehua Hu, Fangzhong Shen, He Liu, Ping Lu,* Ying Lv, Dandan Liu and Yuguang Ma*
Received 8th December 2011, Accepted 1st February 2012
DOI: 10.1039/c2cc17682c
An electrical and optical energy gaps separation strategy is put
forward for the design of organic wide bandgap semiconductors.
This new principle could achieve optimization of wide bandgap
(both high singlet and triplet energies) and favorable carrier
injection energy levels simultaneously.
and electron-accepting (pyridine) moieties on the basis of
photophysical, electrochemical measurements and density
functional calculations. The visual illumination as shown in
Fig. 1 expresses our basic strategy of independently adjusting
electrical and optical bandgaps. The HOMO and LUMO levels
of the unsubstituted silane are around 6.2 eV and 2.0 eV,
respectively, resulting in a very difficult injection of carriers.12
While for the substituted silane, the HOMO and LUMO are
mainly located on corresponding electron-donating and electron-
accepting moieties, respectively. This results in a lower electrical
bandgap compared with unsubstituted silane. Meanwhile, the
larger and ‘‘insulating’’ spacer, the tetraphenylsilane moiety,
leads to forbidden electronic transitions between a donor and
an acceptor (from HOMO to LUMO). The allowed electronic
transitions are excited from HOMO to higher energy levels of
LUMO + n. Therefore, the whole molecule would maintain a
wide optical bandgap. This strategy suggests that we could use
different energy levels to control the electrical bandgap and
optical bandgap in one molecule.
Wide bandgap semiconductive materials are usually defined as
the ones that absorb and emit violet or ultraviolet (UV) light
(wavelength shorter than 400 nm). The most important applications
of wide bandgap semiconductive materials are to fabricate UV
light-emitting devices (LEDs),1 laser diodes (LDs)2 and detectors.3
They also show enormous values in high-density information
storage and space exploration.4 Over the past decade, tremendous
progress has been made in the development of inorganic wide
bandgap materials,5 such as ZnO and GaN, which results in the
large scale application of LED lighting.6 By comparison, the
development of organic wide bandgap semiconductive materials
is relatively far behind.7 In general, the organic semiconductors
exhibit very low carrier mobility, which brought about their limited
load-bearing capacity for current.8 This became even more critical
for broadening the bandgap of organic semiconductors because of
additional difficulties of carrier injection from electrodes as a result
of very low highest occupied molecular orbital (HOMO) and/or
very high lowest unoccupied molecular orbital (LUMO) levels.9
To address this problem, most recent molecular designs
focus on bipolar wide bandgap materials by the incorporation
of electron-donating and electron-accepting moieties.10
Although the HOMO and/or LUMO are raised and/or lowered
by this strategy, respectively, it also unavoidably lowers the
bandgap of the material due to decrease of the HOMO–LUMO
bandgap or intramolecular charge-transfer between the electron-
donating and electron-accepting moieties.11 Thus, there remains
a need to develop a novel molecular design strategy to optimize
the optical bandgap and carrier injection simultaneously. In this
communication, we describe here a strategy to achieve this goal
through a substituted silane with electron-donating (carbazole)
State Key Laboratory of Supramolecular Structure and Materials,
Jilin University, 2699 Qianjin Avenue, Changchun, 130012,
P. R. China. E-mail: ygma@jlu.edu.cn, lup@jlu.edu.cn;
Fax: +86 431 5168480; Tel: +86 431 5168480
w Electronic supplementary information (ESI) available: Synthetic
details, photophysical and EL properties of materials. See DOI:
10.1039/c2cc17682c
Fig. 1 A schematic representation of the design principle for bipolar
wide bandgap materials and molecular structures of the wide bandgap
materials.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 3015–3017 3015

View Article Online
Fig. 2 Cyclic voltammogram (a) and the HOMO–LUMO levels
(b) of the wide bandgap materials. The oxidation and reduction
processes were performed in CH2Cl2 and DMF solution, respectively.
Fig. 3 UV spectra (a) and PL spectra (b) of the wide bandgap
materials in dichloromethane solution (10À5 M).
and TCzSiPy are still shorter than 400 nm (3.22 eV). From the
analysis above, we can unhesitatingly consider that the optical
transitions and radiations of CzSiPy, DCzSiPy and TCzSiPy
are dominated by their carbazole chromophore, while the
pyridine unit does not have effects on the absorption and emission
spectra. Thus the bandgap of all the materials, calculated from the
lowest absorption peak of the film (Fig. S2 in the ESIw), maintains
high values ranging from 3.58 to 3.62 eV.
The chemical structures of these wide bandgap materials are
also shown in Fig. 1. Detailed synthesis procedures of these
compounds are presented in ESI.w The final molecular structures
were characterized by 1H NMR, 13C NMR, mass spectrometry,
elemental analysis, and corresponded well with their expected
structures.
Cyclic voltammetry (CV) was used to determine the relative
energy levels of these newly synthesized compounds. Because
of the strong electron-donating and electron-accepting abilities
of carbazole and pyridine, the redox properties of the substituted
silanes were obviously changed. This can be seen from Fig. 2a, in
which the reductive onset potential positively shifts from À2.64 V
for silane to À2.23 V for pyridine substituted SiPy, and oxidative
onset potential negatively shifts from 1.71 V for silane to 1.11 V
for carbazole substituted CzSi. While the D–A compound
CzSiPy, exhibiting a combination of the reduction behavior of
SiPy and the oxidation behavior of CzSi, shows the bidirectional
shifts of onset potentials caused by carbazole and pyridine
substituents. On the basis of the onset potentials for reduction
and oxidation, we estimated the LUMO and HOMO energy
levels of these compounds with regard to ferrocene (À4.80 eV
below vacuum). The HOMO/LUMO levels diagram of these
compounds is demonstrated in Fig. 2b. It is evident that all the
D–A silanes have lower LUMO levels and higher HOMO
levels compared with unsubstituted silane, suggesting smaller
carrier-injection barriers from neighboring layers or electrodes
to these materials.
Density functional theory (DFT) calculations at the
B3LYP/6-31G* level are performed to characterize the three-
dimensional geometries, the frontier molecular orbital energy
levels and transitions in order to reveal the origin of special
spectral behaviors. Taking CzSiPy as an example, Fig. 4 shows
the relevant molecular orbitals (MOs) of CzSiPy involved in
the ground-state. The HOMO of CzSiPy is mainly located on
the electron donating carbazole moiety, while the LUMO is
mainly distributed on the electron-accepting pyridine moiety
as anticipated, which are consistent with the CV measurement
results. The geometrical characteristics of CzSiPy depict obviously
that the HOMO and LUMO orbitals are significantly separated
from each other because of the tetrahedral environment of the
central tetraphenylsilane, which plays a role of ‘‘insulating’’ bridge.
Thus this full spatial separation for HOMO and LUMO config-
urations leads to no overlap of the MOs involved with an oscillator
strength of zero, demonstrating the forbidden character of the
transitions between the donor and acceptor (from HOMO to
LUMO). While the allowed singlet transition is found to be from
HOMO to higher energy levels LUMO + 2 in the ground state.
If such D–A substitution induces a great red-shift in spectra
(lowering bandgap), the design strategy may be useless for
wide bandgap materials. Surprisingly, we found from spectral
measurements that unlike a great change in redox properties,
the D–A silanes showed almost the same absorption and PL
spectra as their corresponding carbazolsilanes. As shown in
Fig. 3a, they all exhibit major absorption around 293 nm and
342 nm. And the molar absorption coefficients of the D–A
silanes are close to that of their corresponding carbazole
substituted silanes, showing increase as a function of the
number of carbazole groups (Table S1, ESIw). Longer wavelength
absorptions in the low energy region are not observed for CzSiPy,
DCzSiPy and TCzSiPy, which means that additional transitions,
such as the intramolecular charge-transfer (ICT) from carbazole
to pyridine, do not exist in CzSiPy, DCzSiPy and TCzSiPy. With
the carbazole substituents changing from a carbazole monomer to
a carbazole dimer, and further to a trimer, the PL spectra of these
compounds red-shift (Fig. 3b), which is attributed to the extension
of delocalization arising from the partial electronic coupling of
oligocarbazoles. However, the main emission peaks of TCzSi
Fig. 4 The relevant molecular orbitals density map and the transition
sketch map of CzSiPy calculated at the B3LYP/6-31G* level.
c
3016 Chem. Commun., 2012, 48, 3015–3017
This journal is The Royal Society of Chemistry 2012

View Article Online
This local transition makes CzSiPy to maintain a high level
of optical bandgap. Similar to CzSiPy, the DCzSiPy and
TCzSiPy showed the same excited state properties investigated
via DFT analysis. This special optical phenomenon provides
these materials with sufficient bandgap.
of the compounds but also avoids the intramolecular charge-
transfer (ICT) from a donor to an acceptor. Consequently, the
optical bandgaps of the compounds are determined by local
excitations of substituents. Therefore, the whole molecule could
maintain a wide optical bandgap. This novel strategy gives us a
hopeful prospect for organic wide bandgap materials design to
achieve optimization of not only wide bandgap (both high singlet
and triplet energies) but also favorable energy levels for charge
injection from neighboring layers or electrodes.
One of the important applications of organic wide bandgap
materials is their use as host materials for phosphorescent dyes
in POLEDs. Because efficient exothermic energy transfer from
the host materials to the dopant phosphorescent dye depends
on whether the triplet-state energy of the host is greater than
that of the dopant, a higher level of triplet energy (ET) for the
host material is required to allow the effective confinement
of triplet excitons on the guest.13 CzSi and CzSiPy show
phosphorescence peaks at 415 and 402 nm, which correspond
to the vibronic 0–0 transition between these two electronic
states by taking the highest-energy peak of phosphorescence as
the transition energy of T1 - S0.14 Thus, the triplet energies
of CzSi and CzSiPy are estimated to be as high as 2.99 and
3.08 eV, respectively. With the same method, the triplet
energies of other compounds are estimated to be all above
2.86 eV, which are higher than the commonly used blue dye
FIrpic (2.65 eV).15 The results show that the effect of the
pyridine unit on the ET is negligible, which is consistent with
the singlet spectral investigation. In this case, the ET is high
enough for these compounds to serve as a decent host.
We are grateful for support from the National Science
Foundation of China (Grant No. 20834006, 21174050).
Notes and references
1 S. Koizumi, K. Watanabe, M. Hasegawa and H. Kanda, Science,
2001, 292, 1899; S. Nakamura, Science, 1998, 281, 956.
2 J. Dai, C. X. Xu and X. W. Sun, Adv. Mater., 2011, 23, 4284;
Y. Zhao, A. Peng, H. Fu, Y. Ma and J. Yao, Adv. Mater., 2008,
20, 1661.
3 H.-W. Lin, S.-Y. Ku, H.-C. Su, C.-W. Huang, Y.-T. Lin,
K.-T. Wong and C.-C. Wu, Adv. Mater., 2005, 17, 2489;
L. Rigutti, M. Tchernycheva, A. D. L. Bugallo, G. Jacopin,
F. H. Julien, L. F. Zagonel, K. March, O. Stephan, M. Kociak
and R. Songmuan, Nano Lett., 2010, 10, 2939.
4 K. Watanabe, T. Taniguchi and H. Kanda, Nat. Mater., 2004,
3, 404; H. Kawazoe, M. Yasukawa, H. Hyodo, M. Kurita,
H. Yanagi and H. Hosono, Nature, 1997, 389, 939.
5 C.-T. Huang, J. Song, W.-F. Lee, Y. Ding, Z. Gao, Y. Hao,
L.-J. Chen and Z. L. Wang, J. Am. Chem. Soc., 2010, 132, 4766;
´
B. M. Wong, F. Leonard, Q. Li and G. T. Wang, Nano Lett., 2011,
We primarily investigated the potential of these compounds
as hosts for POLEDs. For a blue emitter, FIrpic, doped
devices with a configuration of ITO/PEDOT/NPB (80 nm)/
mCP (30 nm)/host : FIrpic (100 : 16, 30 nm)/TPBI (50 nm)/
LiF/Al are fabricated. The device using DCzSiPy as the host
demonstrates the best performance, showing a turn-on voltage
(at 1 cd mÀ2) of 3.4 V and a maximum brightness of 33000 cd mÀ2
at 17 V. The little high turn-on voltage of the devices may be
attributed to the much thicker device film. Its maximum luminous
efficiency is 18.1 cd AÀ1, which is 23% greater than that of a
DCzSi-based device. Normalized electroluminescence (EL)
spectra at a brightness of 100 cd mÀ2 show the emission
exclusively from FIrpic, no emission from the host was
observed, indicating that the triplet energy levels of these
compounds are high enough to prevent the backward energy
transfer from FIrpic. More efficient devices are expected by
further optimization of the device structure.
11, 3074.
6 H.-M. Kim, T. W. Kang and K. S. Chung, Adv. Mater., 2003,
15, 567; H. Jia, L. Guo, W. Wang and H. Chen, Adv. Mater., 2009,
21, 4641; Q. Li and G. T. Wang, Nano Lett., 2010, 10, 1554;
O. Lupan, T. Pauporte and B. Viana, Adv. Mater., 2010, 22, 3298.
´
7 A. Chaskar, H.-F. Chen and K.-T. Wong, Adv. Mater., 2011,
23, 3876; Z. Wu, Y. Xiong, J. Zou, L. Wang, J. Liu, Q. Chen,
W. Yang, J. Peng and Y. Cao, Adv. Mater., 2008, 20, 2359; Y. Tao,
Q. Wang, Y. Shang, C. Yang, L. Ao, J. Qin, D. Ma and Z. Shuai,
Chem. Commun., 2009, 77; L.-C. Chi, W.-Y. Hung, H.-C. Chiub
and K.-T. Wong, Chem. Commun., 2009, 3892.
8 F. Gutmann and L. E. Lyons, Organic Semiconductors, Wiley,
New York, 1967; K. C. Kao and W. Hwang, Electrical Transport in
Solids, Pergamon, Oxford, 1981.
9 R. J. Holmes, B. W. D’Andrade, S. R. Forrest, X. Ren, J. Li and
M. E. Thompson, Appl. Phys. Lett., 2003, 83, 3818; S. Gong,
Y. Chen, C. Yang, C. Zhong, J. Qin and D. Ma, Adv. Mater., 2010,
22, 581.
10 Y. Tao, Q. Wang, C. Yang, Q. Wang, Z. Zhang, T. Zou, J. Qin and
D. Ma, Angew. Chem., Int. Ed., 2008, 47, 8104.
In summary, we have developed a new molecular design
strategy for organic wide bandgap materials with a special
molecular structure, using silane to couple electron-donor
and electron-acceptor units. Following this idea, a series of
tetraarylsilane-centered compounds CzSiPy, DCzSiPy and
TCzSiPy are synthesized. A thorough experimental and
theoretical study of these materials leads to the following
conclusions for this molecular design strategy: (i) the
HOMO/LUMO levels of the compounds mainly depend on
the electron-donating and electron-accepting moieties, respectively,
thus we could freely tune the HOMO/LUMO levels of the
materials by incorporating different substituent groups.
(ii) The tetraphenylsilane core not only interrupts the conjugation
11 F. M. Hsu, C. H. Chien, C. F. Shu, C. H. Lai, C. C. Hsieh,
K. W. Wang and P. T. Chou, Adv. Funct. Mater., 2009, 19, 2834;
Z. Q. Gao, M. Luo, X. H. Sun, H. L. Tam, M. S. Wong, B. X. Mi,
P. F. Xia, K. W. Cheah and C. H. Chen, Adv. Mater., 2009,
21, 688; W.-Y. Hung, T.-C. Tsai, S.-Y. Ku, L.-C. Chib and
K.-T. Wong, Phys. Chem. Chem. Phys., 2008, 10, 5822.
12 X. Ren, J. Li, R. J. Homes, P. I. Djurovich, S. R. Forrest and
M. E. Thompson, Chem. Mater., 2004, 16, 4743; J.-J. Lin,
W.-S. Liao, H.-J. Huang, F.-I. Wu and C.-H. Cheng, Adv. Funct.
Mater., 2008, 18, 485.
13 M. Sudhakar, P. I. Djurovich, T. E. H. Esch and M. E. Thompson,
J. Am. Chem. Soc., 2003, 125, 7796.
14 F. M. Hsu, C. H. Chien, P. I. Shih and C. F. Shu, Chem. Mater.,
2009, 21, 1017.
15 R. J. Holmes, S. R. Forrest, Y. J. Tung, R. C. Kwong, J. J. Brown,
S. Garon and M. E. Thompson, Appl. Phys. Lett., 2003, 82, 2422.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 3015–3017 3017