A solution-processable phosphonate functionalized deep-blue fluorescent emitter for efficient single-layer small molecule organic light-emitting diodes

Article abstract of DOI:10.1039/c2cc34712a

Based on a p-type scaffold, a novel solution-processable phosphonate functionalized deep-blue fluorescent emitter has been designed and synthesized. The corresponding non-doped single-layer SMOLED shows a peak current efficiency of 0.76 cd A^-1 with CIE coordinates of (0.15, 0.09), which is about three orders of magnitude higher than that of the prototype with tert-butyl substituents.

Full text of DOI:10.1039/c2cc34712a

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COMMUNICATION
A solution-processable phosphonate functionalized deep-blue fluorescent
emitter for efficient single-layer small molecule organic light-emitting
diodesw
Bo Chen,ab Junqiao Ding,*a Lixiang Wang,*a Xiabin Jinga and Fosong Wanga
Received 2nd July 2012, Accepted 19th July 2012
DOI: 10.1039/c2cc34712a
Based on a p-type scaffold, a novel solution-processable phos-
phonate functionalized deep-blue fluorescent emitter has been
designed and synthesized. The corresponding non-doped single-
layer SMOLED shows a peak current efficiency of 0.76 cd AÀ1
with CIE coordinates of (0.15, 0.09), which is about three orders
of magnitude higher than that of the prototype with tert-butyl
substituents.
both electron-donor (D) and -acceptor (A) are linked by a
p-conjugated bridge.3 For example, M. R. Bryce et al. recently
reported a D-p-A deep-blue fluorescent emitter containing
carbazole and oxadiazole fragments.3f Its spin-coated single-
layer SMOLED showed a current efficiency of 0.47 cd AÀ1
with Commission Internationale De L’Eclairage (CIE) coordi-
nates of (0.16, 0.07). Despite the effectiveness, this strategy is
far from an easy task, as fine tuning among the donor,
acceptor and p-conjugated bridge is required to avoid the
notable emission shift to longer wavelengths accompanied
by the reduction of the photoluminescence quantum yield
(PLQY) due to the strong intramolecular charge transfer.
In this communication, a simpler approach is provided by
introducing phosphonate groups at the periphery of a p-type
blue-emitting chromophore (II, Fig. 1). Here are several
advantages for this design. Firstly, the decoration of phos-
phonate groups endows the blue emitter with good solubility,
and high quality films can be formed via wet processes.
Furthermore, phosphonate substituents can effectively
improve the electron injection/transporting capability.4
Together with the p-type nature, balanced hole and electron
flux can be expected in the EML, which is beneficial for the
single-layer SMOLEDs. Most importantly, the emission
wavelength and PLQY can remain nearly unchanged, avoiding
the delicate tailoring in the D-p-A structure. With this idea in
mind, a phosphonate functionalized deep-blue fluorescent
emitter, namely TPCA, has been designed by surface group
engineering on TBCA (Fig. 2). The corresponding blue single-
layer SMOLED is prepared by spin-coating, giving a signifi-
cant current efficiency of 0.76 cd AÀ1 with CIE coordinates of
(0.15, 0.09).
In the past two decades, organic light-emitting diodes
(OLEDs) have made great progress towards their potential
applications in flat-panel displays and solid-state lightings.
Compared with the developed red- and green-emitting organic
materials, there are far fewer efficient and stable blue-emitting
ones, which are indispensable for full-color displays as well as
white OLEDs (WOLEDs), and can also serve as hosts for
lower energy dopants to generate other color emissions via
energy transfer.1 Moreover, owing to their ease of fabrication,
low cost and good reproducibility, the solution-processable
single-layer small molecule OLEDs (SMOLEDs) are of parti-
cular interest for the future, where not only the additional hole
and/or electron transporting layers are absent, but also the
emitting layer (EML) is fabricated by wet technologies, such
as spin-coating and ink-jet printing.2 In view of these two
points, therefore, designing solution-processable blue emitters
for efficient single-layer SMOLEDs is highly desirable but
challenging.
To this end, a commonly adopted strategy (I, Fig. 1) involves
the usage of the fluorescent dyes with D–p–A structure, in which
The electrochemical property of TPCA compared with
TBCA was studied via cyclic voltammetry (CV). As presented
in Fig. 3, they both exhibit two oxidation waves, which can be
attributed to the p-doping of triphenylamine and carbazole
moieties, respectively. And the calculated highest occupied
molecular orbital (HOMO) energy level slightly decreases
from À5.21 eV of TBCA to À5.29 eV of TPCA. The small
shift of 0.08 eV indicates that the p-type character of the blue
emitter can be kept almost unaffected after the introduction of
phosphonate groups. During the cathodic sweeping in THF
solution, noticeably, some new reduction peaks appear at
negative potential for TPCA, and the lowest unoccupied
Fig. 1 Design strategies for blue single-layer SMOLEDs.
a State Key Laboratory of Polymer Physics and Chemistry,
Changchun Institute of Applied Chemistry, Chinese Academy of
Sciences, Changchun, P. R. China. E-mail: junqiaod@ciac.jl.cn,
lixiang@ciac.jl.cn; Tel: +86-431-85685653
b Graduate School of Chinese Academy of Sciences, Beijing,
P. R. China
w Electronic supplementary information (ESI) available: Synthesis,
characterization, OLEDs fabrication and characterization. See DOI:
10.1039/c2cc34712a
c
8970 Chem. Commun., 2012, 48, 8970–8972
This journal is The Royal Society of Chemistry 2012

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molecular orbital (LUMO) energy level is determined to be
À2.15 eV. The observed n-doping behaviour originates from
phosphonate, meaning a more efficient electron injection/
transportation for TPCA relative to TBCA.
Fig. 4 depicts the absorption and photoluminescence (PL)
spectra of TPCA as well as TBCA. The optical band gap of
TPCA, estimated from the absorption onset, is the same as
that of TBCA. Moreover, they display the identical emission
maxima with a similar spectral profile in toluene solution.
These results demonstrate that the electronic structure of the
excited state is independent of the attached surface groups.
On going from solutions to solid states, a red shift of 26–34 nm
is observed for both TPCA and TBCA, indicative of the
existence of aggregates. In addition, TPCA in film state
possesses a red shift of 8 nm in comparison to TBCA. This
polarity induced bathochromic shift can be attributed to the
solid-state solvation effect,5 similar to the polar spirofluorene
molecules.6
Fig. 2 Molecular structures of TPCA and TBCA.
It is worth noting that the PLQY of TPCA (0.58) is
comparable to that of TBCA (0.63) (Table 1). This suggests
that phosphonate substitutes nearly do not affect the PLQY,
which is quite different from other polar groups. For instance,
after a quaternary ammonium salt moiety was incorporated
into a chromophore, its PLQY reduced remarkably.5 There-
fore, TPCA is believed to be a promising candidate for high-
efficiency single-layer SMOLEDs.
To verify this potential, the single-layer non-doped device A
has been fabricated with the configuration of ITO/PEDOT:PSS
(40 nm)/TPCA/LiF (1 nm)/Al (100 nm). At the same time, the
control device B with TBCA as the EML is also prepared for
comparison. As shown in the inset of Fig. 5b, the electro-
luminescence (EL) spectrum of TPCA is identical to its PL
counterpart, and no additional emissions corresponding to the
excimer or exciplex appear. The CIE coordinates (0.15, 0.09)
are very close to the National Television System Committee
(NTSC) standard blue CIE coordinates of (0.14, 0.08).6
Additionally, its EL spectra remain nearly identical with varia-
tion of current densities from 2 to 320 mA cmÀ2 (Fig. S13, ESIw),
implying the EL spectral stability.
Fig. 3 The electrochemical spectra of TPCA and TBCA measured in
10À3
M dichloromethane solution (anodic sweeping) and THF
solution (cathodic sweeping), respectively.
Interestingly, different current density–voltage–luminance
characteristics for TPCA and TBCA can be clearly seen in
Fig. 5a. TBCA shows comparable current density with TPCA
but low luminance (o1 cd mÀ2). This is reasonable when
considering the p-type nature of TBCA.7 In device B, hole
injection/transportation is dominant, but electrons can hardly
be injected from the LiF/Al cathode to the EML, leading
to few formed excitons in the EML and thus indiscernible
brightness. In contrast, arising from the interaction between
Fig. 4 The UV-vis absorption spectra of TPCA and TBCA in 10À6 M
dichloromethane solution, and the PL spectra in 10À6 M toluene
solution and film state.
Table 1 The photophysical and electrochemical properties of TPCA and TBCA
max
l
/nm
em
Solutionc
Film
labsa/nm
Egb/eV
FPL (%)
E
e/V
ox
onset
E
f/V
red
onset
HOMOg/eV
LUMOh/eV
d
TPCA
TBCA
250, 270, 288, 356
240, 298, 353
3.00
3.00
404
404
438
430
0.58
0.63
0.49
0.41
2.65
N/A
À5.29
À5.21
À2.13
N/A
a
b
c
Measured in 10À6 M dichloromethane. The optical band gap calculated from the onset of the absorption. Measured in 10À6 M toluene
d
e
solution. The PLQYs were measured in toluene solution, with 9,10-diphenylanthracene in cyclohexane as the reference. The onset of the first
f
oxidation potential measured in dichloromethane solution. The onset of the first reduction potential measured in THF solution. Calculated
g
ox
onset
h
red
from the equation: HOMO = Àe[E
+ 4.8 V]. Calculated from the equation: LUMO = Àe[E
+ 4.8 V].
onset
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 8970–8972 8971

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The authors are grateful to the 973 Project (2009CB623601
and 2009CB930603), National Natural Science Foundation of
China (No. 21174144 and 50803062), and Science Fund for
Creative Research Groups (No. 20921061) for financial support
of this research.
Notes and references
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Fig. 5 The current density–voltage–luminance characteristics (a), and
the current density dependence of current efficiency (b) for TPCA and
TBCA. Inset: the EL spectrum of TPCA at a driving voltage of 10 V.
phosphonate and LiF/Al cathode, efficient electron injection
can be achieved in device A. On the other hand, in comparison
to TBCA, hole flux is not affected for TPCA as discussed
above. As a result, excitons can be generated effectively in the
EML, and the maximum brightness significantly enhances to
754 cd mÀ2. Correspondingly, the current efficiency reaches
0.76 cd AÀ1, which is about three orders of magnitude higher
than that of TBCA.
In summary, a novel solution-processable deep-blue fluor-
escent emitter, TPCA, has been designed and synthesized by
introducing phosphonate groups into a p-type scaffold.
Compared with the prototype with tert-butyl groups, TPCA
exhibits impressively magnified performance. This work,
we think, will provide a promising and simple method to
develop blue emitters suitable for highly efficient single-layer
SMOLEDs.
6 E. Wang, C. Li, Y. Mo, Y. Zhang, G. Ma, W. Shi, J. Peng, W. Yang
and Y. Cao, J. Mater. Chem., 2006, 16, 4133.
7 P. Strohriegl and J. V. Grazulevicius, Adv. Mater., 2002, 14, 1439.
c
8972 Chem. Commun., 2012, 48, 8970–8972
This journal is The Royal Society of Chemistry 2012