Suppressing triplet state extension for highly efficient ambipolar phosphine oxide host materials in blue PHOLEDs

Article abstract of DOI:10.1039/c3cc49020c

A series of phosphine oxide hosts were constructed to investigate the influence of the triplet state extension in hosts on electrophosphorescence, in which DPESPOPhCz with the carbazolyl-localized triplet state endowed its blue-emitting PHOLEDs with favourable performance, including an external quantum efficiency more than 13%. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3cc49020c

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Suppressing triplet state extension for highly
efficient ambipolar phosphine oxide host
materials in blue PHOLEDs†
Cite this: Chem. Commun., 2014,
50, 2670
Received 26th November 2013,
Accepted 17th January 2014
Chunmiao Han,a Liping Zhu,b Fangchao Zhao,b Zhen Zhang,a Jianzhe Wang,a
Zhaopeng Deng,a Hui Xu,*a Jing Li,a Dongge Ma*b and Pengfei Yana
DOI: 10.1039/c3cc49020c
www.rsc.org/chemcomm
A series of phosphine oxide hosts were constructed to investigate the of the excited states of these complicated polynary systems becomes
influence of the triplet state extension in hosts on electrophosphor- significant.21 Recently, we found that the negative effects on T1 excited
escence, in which DPESPOPhCz with the carbazolyl-localized triplet states from surrounding molecules can be suppressed by rationally
state endowed its blue-emitting PHOLEDs with favourable perfor- tuning their locations.22 In this sense, the low T1 energy level of a
mance, including an external quantum efficiency more than 13%.
conventional host CBP is ascribed to its conjugation extension by
diphenylene, which can be suppressed by the two methyl groups in
The organic semiconductors have attracted attention in recent CDBP (Fig. 1a).23 However, according to spin density distributions of
decades owing to their unique optoelectronic characteristics and these two hosts, the diphenylene-involved T1 state of CBP should be
various potential and practical applications.1 After their rapid devel- another determinant. To figure out the influence of the T1 location, a
opment driven by commercial objectives, such as organic light- big challenge is to establish a flexible platform with similar molecular
emitting diodes (OLEDs),2–5 the factors constraining the further components and conjugation and different T1 extension for the
improvement of the material and device performances clearly reflect rational conclusions without other interferences, which is crucial for
the inconsistency between the physical properties,6 which forces further designing highly efficient host materials.
people to obtain an insight into the molecular ground and excited
In this contribution, a series of aryl phosphine oxide (APO) hosts
states and establish a deep understanding of the correlations between named DPExPOPhCzn were designed and prepared as the combina-
the molecular structures and material performances. Both optical tions of hole-transporting carbazolyl and electron-transporting
and electrical properties should be optimized when designing desired diphenylphosphine oxide (DPPO) bridged by diphenylether on the
electroluminescent (EL) materials due to the involvement of electrical basis of a multi-insulating linkage strategy (Fig. 1b). The number and
and energy transfer processes. However, different from electrical
properties of materials constructed with explicit purposes and almost
quantitative molecular design, the influencing factors for optical
properties are more obscure and complicated, which encourage the
continual efforts in clarifying the effect of optical properties of specific
molecular structures on the EL performance.7–8
Recently, the electrical performance of blue phosphorescent OLEDs
(PHOLEDs) has been paid much attention,9,10 requiring both high T1
and strong carrier injecting/transporting ability for host materials
through multifunctionalization.11–20 In this case, the accurate control
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 of Polymer Physics and Chemistry, Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China.
E-mail: mdg1014@ciac.jl.cn
Fig. 1 (a) Potential correspondence between T1 locations of the hosts
containing low T1 segments and their T1 energy levels, exampled by spin
† Electronic supplementary information (ESI) available: Experimental details, cif
file, thermal properties, DFT calculation results, CV curves and EL performance of
the devices. CCDC 973794. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c3cc49020c
density distribution of CBP (left, T1 = 2.65 eV) and CDBP (right, T1
2.99 eV); (b) chemical structures of DPExPOPhCzn.
=
2670 | Chem. Commun., 2014, 50, 2670--2672
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ratio of carbazolyl and DPPO moieties were gradually adjusted to not from 340 to 500 nm with a slight bathochromic shift interval of about
only modulate the electrical activity, but also change the molecular 10 nm. Absorption and emission spectra of DPExPOPhCzn in the PS
geometries. Since DPPO moieties were hardly involved in T1 states, in or PMMA matrix further indicated their similar S1 characteristics
DPExPOPhCzn, two segments with different T1 as carbazolyl (3.0 eV) (Fig. S3, ESI†). Therefore, the conjugation extension was effectively
and diphenylene (2.7 eV) should be considered when investigating the suppressed by the multi-insulating linkage strategy, which was con-
T1 state variation, which just provided a flexible platform to investigate sistent with the density function theory (DFT) simulation results
the correlation between T1 excited state locations in these molecules (Fig. S4, ESI†). Significantly, a sharp reduction of T1 was observed
and the EL performance of their corresponding PHOLEDs without for DPESPOPhCz3 (T1 = 2.84 eV) compared with its mono and di-
interferences from other structural factors.
substituted analogs (T1 = 3.00 eV) due to the 0–0 transitions in their
DPExPOPhCzn exhibited a similar absorption spectra containing time-resolved phosphorescence spectra (Fig. 2b). Considering the
three bands, corresponding to the n–p* and p–p* transitions from similar T1 value of DPESPOPhCz and DPESPOPhCz2, conjugation
carbazolyl to diphenylether and the p–p* transition of DPPO (Fig. 2a). extension and more functional groups should not be the main reason
In accord with the number of functional groups, the first singlet for this T1 reduction of DPESPOPhCz3. The situation was similar to
energy levels (S1) were slightly decreased from DPESPOPhCz to DPEPOPhCz2, whose T1 was considerably lower than that of DPE-
DPESPOPhCz2 (Table S1, ESI†). However, it was noticed that the POPhCz. Since DPExPOPhCzn were designed with two T1 contribu-
ternary carbazolyl in DPESPOPhCz3 did not lead to further S1 table carbazolyl and diphenylene groups, the T1s of DPESPOPhCz3
reduction. Simultaneously, compared with DPESPOPhCz, the second- and DPEPOPhCz2 almost in the middle of those of carbazolyl and
ary carbazolyl in DPESPOPhCz2 and the secondary DPPO in DPE- diphenylene suggested the simultaneous contribution from these two
POPhCz resulted in a similar S1 decrease of B0.06 eV, which revealed groups to T1 excited states, which was strongly demonstrated by DFT
the slight influence of conjugation extension on S1 owing to the simulation on T1 excited states of DPExPOPhCzn (Fig. 2c). The spin
involvement of insulating linkages such as –O– and PQO. Further- density distributions of T1 states of DPESPOPhCz, DPESPOPhCz2 and
more, the similar S1s of DPESPOPhCz3 and DPEPOPhCz2 indicated DPEPOPhCz are completely localized on carbazolyl, while diphenylene
that the charge transfer (CT) interaction between carbazolyl and DPPO in DPESPOPhCz3 and DPEPOPhCz2 also makes equal contributions.
was effectively suppressed by the meso linkage. Furthermore, all of the In this sense, it seemed that along with the increase of molecular
compounds exhibited UV and deep blue emissions in the same range symmetry and electron-donating effect on diphenylene, the T1 state
extension became facile. Therefore, DPExPOPhCzn established a flexi-
ble platform to investigate the impact of T1 state extension on the host
performance with negligible interference from conjugation extension.
DFT simulation indicated that the increase of phenylcarbazole
groups can reduce the lowest unoccupied molecular orbital (LUMO)
energy level owing to the major contributions from diphenylene
moieties accompanied with the elevation or preservation of the highest
occupied molecular orbital (HOMO) energy level (Fig. S4, ESI†), which
was in accord with the cyclic voltammetrical (CV) results except for the
smaller differences between the experimental HOMO and LUMO data
of DPExPOPhCzn estimated by the onset potentials of their redox peaks
(Fig. S5, ESI†). Considering the predominance of three-dimensional
multifunctionalization in carrier transportation, DPESPOPhCz3 and
DPEPOPhCz2 should be superior in electrical performance compared
with their analogs containing less carbazolyl groups. IV characteristics
of single-carrier transporting devices further indicated the much more
balanced charge transportation in DPESPOPhCzn (n = 2 and 3) and
DPEPOPhCz2 than those of DPExPOPhCz (Fig. S6, ESI†).
To validate the influence of T1 state extension on the EL
performance of the host materials, the blue PHOLEDs with a
conventional device configuration of ITO|MoO3 (10 nm)|NPB
(70 nm)|TCTA (5 nm)|DPExPOPhCzn:FIrpic (15%, 20 nm)|TPBi
(35 nm)|LiF (1 nm)|Al were fabricated, namely PA–PE, respectively.
DPESPOPhCz endowed PA with the lowest driving voltages of 3.1 V
for onset, 4.1 V at 100 cd mÀ2 for display and 5.7 V at 1000 cd mÀ2 for
lighting (Fig. 3a and Table S2, ESI†). Other devices also had similar
turn-on voltages of 3.1 V for PB and PD and 3.3 V for PC and PE,
which was consistent with DFT simulation results. The energy level
scheme of the devices further indicated the analogous direct charge
Fig. 2 (a) UV/vis absorption and fluorescence spectra of DPExPOPhCzn
in CH2Cl2 (10À6 mol LÀ1) at room temperature; (b) time-resolved phos-
phorescence spectra of DPExPOPhCzn in CH2Cl2 at 77 K after a delay of
capture process for the exciton confinement on FIrpic due to the
300 ms; (c) calculated T1 energy levels of DPExPOPhCzn and contours of
the spin density distributions by the DFT method.
similar FMO energy gaps between hosts and guest (Scheme S1, ESI†).
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performance of PC and PE, ascribed to low host–guest energy transfer
efficiencies and worse quenching effects due to increased collisional
probability between T1 exciton and other particles. The latter was
evidenced by a significant decrease in the nonexponential decay time
of FIrpic doped in DPESPOPhCz3 and DPEPOPhCz2 films (8%),
compared with their analogues (Fig. S8, ESI†).
In summary, the influence of the T1 excited state extension in host
materials on their EL performance was investigated through a series of
APO hosts with the same building blocks to get rid of the interferences
from other structural factors. DPESPOPhCz with the carbazolyl-
localized T1 state endowed its blue PHOLEDs with favourable EL
performance, with an E.Q.E. more than 13% and well-controlled
efficiency reduction. It was seen that the T1 state extension remarkably
reduced the EL efficiencies and worsened the efficiency roll-offs due
to the resulting lower T1 energy level and the increased probability of
collisional quenching effects, indicating the significance of the T1 state
location when designing high-energy-gap host materials.
CH, LZ and FZ contributed equally to this work. This project was
financially supported by NSFC (61176020 and 51373050), New Century
Excellent Talents Supporting Program of MOE (NCET-12-0706), Pro-
gram for Innovative Research Team in University (MOE) (IRT-1237),
New Key Project of MOE (212039) and New Century Excellent Talents
Developing Program of Heilongjiang Province (1252-NCET-005).
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