Controlling optoelectronic properties of carbazole-phosphine oxide hosts by short-axis substitution for low-voltage-driving PHOLEDs

Article abstract of DOI:10.1039/c3cc00133d

Preserved high first triplet energy levels and improved electrical properties of two donor-acceptor type carbazole-phosphine oxide hosts were achieved through short-axis substitution to realize efficient PHOLEDs with extremely low driving voltages of 2.6

Full text of DOI:10.1039/c3cc00133d

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Controlling optoelectronic properties of carbazole–
phosphine oxide hosts by short-axis substitution for
low-voltage-driving PHOLEDs†
Weibo Yang,a Zhensong Zhang,b Chunmiao Han,a Zhen Zhang,a Hui Xu,*a
Pengfei Yan,a Yi Zhao*b and Shiyong Liub
Cite this: DOI: 10.1039/c3cc00133d
Received 7th January 2013,
Accepted 19th February 2013
DOI: 10.1039/c3cc00133d
www.rsc.org/chemcomm
Preserved high first triplet energy levels and improved electrical linkage can amplify the molecular polarization. Recently, we
properties of two donor–acceptor type carbazole–phosphine oxide further demonstrated ultra-low-voltage driving PHOLEDs of two
hosts were achieved through short-axis substitution to realize short-axis substituted dibenzothiophene–PO hosts, in which the
efficient PHOLEDs with extremely low driving voltages of 2.6 V negative influence on HOMOs by PO groups was suppressed by the
for onset and o3.2 V at 100 cd mÀ2
.
long-range backing bonding effect between S and P atoms.5d
Nevertheless, the effect of short-axis substitution on optoelectronic
Recently, low-voltage-driving blue phosphorescent organic light- properties of D–A systems is still not clear.
emitting diodes (PHOLEDs) have become attractive due to their
In this contribution, we report the controllable modulation of
potential applications in portable devices.1 This requires the host the optoelectronic properties of two carbazole–phosphine oxide
materials to have both high first triplet energy level (T1) (42.8 eV) hybrids for low-voltage driving electrophosphorescence, namely
and good carrier injecting/transporting ability. One of the main 3,6-di-tert-butyl-1-(diphenylphosphoryl)-9-methyl-9H-carbazole
strategies is constructing ambipolar systems with high T1 through (tBCzMSPO) and 3,6-di-tert-butyl-1,8-bis(diphenylphosphoryl)-
blocking the interplays between donors (D) and acceptors (A) by 9-methyl-9H-carbazole (tBCzMDPO) with the collective name
involving spacers or insulating linkages,2 which either somewhat of tBCzMxPO, in which electron-transporting diphenylphosphine
decrease T1 or weaken carrier injection/transportation. In this case, oxide (DPPO) groups are bonded at the 1,8-position of carbazolyl to
some electrically active insulating linkages, such as phosphine form the local unsymmetrical D–A configuration (Scheme 1 and
oxide (PO), are involved in hosts for improving carrier injection Scheme S1†). Accompanied by the high T1, the hole injecting ability
and transportation while preserving high T1.3 However, it is still a of carbazolyl might be preserved to a great extent for achieving
big challenge to control the optoelectronic properties of the PO the ambipolar characteristics of tBCzMxPO. The investigation
involved D–A type hosts. At the same time as decreasing the energy indicated their T1 of 2.98 eV and suppressed influence of
levels of the LUMOs, the strong electron-withdrawing effect of the DPPO on their HOMOs. As a result, the controllable tuning of
PQO group along the long axis of the D-type chromophores, such optoelectronic properties by short-axis linkage was successfully
as carbazole, also remarkably reduces the energy levels of the realized by remarkably improving electron injecting/transporting
HOMOs. As a result, the first singlet excited energy levels (S1) are ability with reduced influence on other photoelectronic properties.
unchanged or even enlarged. In this sense, the improvement of Efficient blue PHOLEDs based on tBCzMxPO were realized with
electron injection by PO groups is at the cost of weakening the hole driving voltages of only 2.6 V for onset and o3.2 V at 100 cd mÀ2
,
injection of the chromophores, like the situation of carbazole– which were among the best results of carbazole-type high-
phosphine oxide hybrids.4 Our group have reported several high- energy-gap host materials. Significantly, this work establishes
efficiency PO hosts based on short-axis linkage,5 multi-insulating a solid example for controlling photoelectronic properties of
linkage6 or indirect linkage.7 It was shown that the short-axis
a Key Laboratory of Functional Inorganic Material Chemistry, School of Chemistry
and Materials, Heilongjiang University, Ministry of Education, Harbin 150080,
P. R. China. E-mail: hxu@hlju.edu.cn; Fax: +86 451 86608042
b State Key Laboratory on Integrated Optoelectronics, College of Electronics Science
and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012,
P. R. China. E-mail: zhao_yi@jlu.edu.cn
† Electronic supplementary information (ESI) available: Experimental details,
thermal properties, absorption and PL spectra, CV curves and EL performance
Scheme 1 Design strategy and molecular structures of tBCzMxPO.
of the devices. See DOI: 10.1039/c3cc00133d
c
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D–A type hosts by careful management of molecular configura-
tions and intramolecular interplays.
The nature of the modulated optical properties of tBCzMxPO
was investigated by DFT calculation (Fig. 2). The configurations
The temperature of decomposition (Td, at weight loss of 5%) and frontier molecular orbitals (FMOs) of 3,6-substituted
of tBCzMSPO is 323 1C. An extra DPPO in tBCzMDPO results in analogues 3tBCzMSPO and 36CzMDPO, as well as tBCzM, were
a higher Td of 394 1C, which should be ascribed to the reduced also simulated for comparison. The HOMOs and LUMOs of all
molecular volatility (Fig. S1†). The melting point (Tm) of of the compounds are mainly located on carbazolyls. The
tBCzMSPO is 206 1C, accompanied by a rather high temperature methyls are also involved in the HOMOs due to the super-
of glass transition (Tg) of 175 1C, which indicates its improved conjugation effect. The incorporation of DPPOs at 3,6-positions
phase stability. tBCzMDPO has a much higher Tm of 340 1C even in 3tBCzMSPO and 36CzMDPO induces a significant reduction
though no distinct Tg was observed. The high quality of vacuum of LUMO energy levels (0.247 and 0.243 eV, respectively, while
evaporated thin films of tBCzMxPO was demonstrated by AFM in tBCzMxPO the first and second DPPOs result in reductions
and SEM images with the limited roughness of about 0.2 nm of LUMOs of 0.300 and 0.136 eV, respectively. Thus, tBCzMxPO
(Fig. S2†). The remarkably enhanced thermal stability and film have LUMO energy levels comparable with their 3,6-substituted
formability of tBCzMxPO is due to the compact and rigid analogues. However, compared with that of tBCzM, HOMOs of
configuration by short-axis substitution, which is crucial for 3tBCzMSPO and 36CzMDPO are even more remarkably
the improvement of device interfaces and stability.
reduced, by 0.327 and 0.635 eV, respectively. Therefore, the
The optical properties of tBCzMxPO in dilute solutions HOMO–LUMO energy gaps of these 3,6-substituted analogues
(10À6 mol LÀ1 in dichloromethane) are nearly the same. Their are even larger than that of tBCzM. Contrarily, for tBCzMxPO
absorption spectra consist of four bands at about 350 (n - p* the negative effect of DPPO on the HOMO is effectively
transition of carbazolyl), 300 (n - p* transition from carbazolyl suppressed such that the first and second DPPOs only result
to DPPO), 250 (p - p* transition of carbazolyl) and 230 nm in slight reductions of HOMOs of 0.136 and 0.082 eV, respectively.
(p - p* transition of DPPO) (Fig. 1 and Table S1†). The larger As a result, their HOMO–LUMO energy gaps are successfully
absorption cross section of tBCzMDPO induces its larger decreased by 0.164 and 0.218 eV, respectively. This is in accord
extinction coefficient, especially for the DPPO-involved transi- with the experimental data measured by cyclic voltammetry that
tions. The fluorescent (FL) emissions of tBCzMxPO in solution the HOMO energy levels were 5.85, 5.99 and 6.13 eV for tBCzM,
at room temperature are also similar, with peaks at 390 nm, tBCzMSPO and tBCzMDPO, respectively (Fig. S7†). The J–V
accompanied by almost equal lifetimes of B5 ns (Fig. S3†). characteristics of the nominal single-carrier transporting devices
Nevertheless, S1 of tBCzMDPO is 3.26 eV, estimated according based on tBCzMSPO indicated that its hole-only current density
to the absorption edge, which is about 0.1 and 0.18 eV smaller (J) was much larger than its electron-only J, which revealed the
than those of tBCzMSPO and 3,6-di-tert-butyl-9-methyl- suppressed negative influence of short-axis substituted DPPO on
carbazole (tBCzM) (Fig. S4†), respectively. This indicates the hole transportation of carbazolyl (Fig. S8†). However, the hole-
accumulated polarization by short-axis substituted DPPOs. The only J and electron-only J of tBCzMDPO are comparable, which
similar emissions of tBCzMxPO in solvents with different indicated the remarkable contribution of DPPO to electron
polarities further reveal the suppressed intramolecular charge transportation and the limited embedding effect of DPPOs on
transfer by short-axis linkage (Fig. S6†). T1s of tBCzMxPO are carbazolyl. Therefore, the electron injecting/transporting ability
estimated according to 0–0 transitions of their time-resolved of tBCzMxPO is dramatically improved with controllable influence
phosphorescent (PH) spectra at 77 K in CH2Cl2 glasses, which on their hole injecting/transporting ability, which contrasts sharply
are 2.98 eV, the same as that of tBCzM. Thus, preserving T1 and with their long-axis substituted analogues.4
polarizing molecules are simultaneously realized in tBCzMxPO
The high T1 and modified electrical properties of tBCzMxPO
by short-axis substitution. The energy gaps between S1 and T1 encouraged us to investigate their performance as hosts in
(DEST) of tBCzMSPO and tBCzMDPO are only 0.38 and 0.28 eV, blue-emitting PHOLEDs BA–BD with configuration of ITO|MoOx
respectively, which are among the smallest results reported so
far and imply the improved carrier injecting ability.
Fig. 2 DFT calculation results of tBCzMxPO, tBCzM and 3,6-substituted
Fig. 1 Absorption and emission spectra of tBCzMxPO in dilute CH2Cl2.
analogues.
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low driving voltages of 2.6 V for onset, o3.0 V at 100 cmÀ2 and
o3.4 V at 1000 cd mÀ2 were also realized as well as high
efficiencies (Table S2†).
In summary, this work successfully demonstrated the
controllable modulation of optoelectronic properties of two
D–A type hosts tBCzMSPO and tBCzMDPO on the basis of
short-axis linkage. The modified electrical performance and
preserved T1 of tBCzMSPO endowed its efficient PHOLEDs with
extremely low driving voltages of 2.6 V for onset and about 3.0 V
at 100 cmÀ2 for portable display.
W.Y., Z.S.Z. and C.H. contributed equally to this work. This
project was financially supported by the National Key Basic
Research and Development Program of China (2010CB327701),
NSFC (50903028, 61176020 and 61275033), New Century
Talents Supporting Program of MOE, Key Project of MOE
(212039), New Century Talents Developing Program of Heilongjiang
Province (1252-NCET-005), Education Bureau of Heilongjiang
Province (10td03), and the Supporting Program of Highlevel
Talents of HLJU (2010hdtd08).
Notes and references
Fig. 3 B–J–V curves (a) and efficiency curves (b) of blue-emitting PHOLEDs based
on tBCzMxPO.
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(2 nm)|m-MTDATA:MoOx (15%, 30 nm)|m-MTDATA (10 nm)|Ir(ppz)3
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This journal is The Royal Society of Chemistry 2013
Chem. Commun.