Fine tuning the purity of blue emission from polydioctylfluorene by end-capping with electron-deficient moieties

Article abstract of DOI:10.1021/ja054777+

We propose a simple way to achieve pure blue emission and improved device efficiency via capping poly(9,9-dioctylfluorene) (PFO) with electron-deficient moieties (EDMs, such as oxadiazole (OXD) and triazole (TAZ)), which can induce a minor amount of long conjugating length species (regarded as β phase) to control extents of energy transfer from amorphous matrix to the β phase. The device efficiency of PFO end-capped with TAZ is higher than that with para-tert-butyl phenyl (TBP) by a factor of 2 (with CsF/Al as cathode), and its electroluminescent spectrum remains stable and with pure blue emission during cyclic operations (C.I.E. color coordinates x = 0.165, y = 0.076, independent of operating voltage and within the limit for pure blue emission x + y < 0.30). The improvement of device efficiency is dependent on the structure of EDM, such as size and planarity. The deep blue emission is originated from the incomplete energy transfer from amorphous matrix to the β phase induced by the end-cappers. Copyright

Full text of DOI:10.1021/ja054777+

Published on Web 09/28/2005
Fine Tuning the Purity of Blue Emission from Polydioctylfluorene by
End-Capping with Electron-Deficient Moieties
Ming-Chin Hung,† Jin-Long Liao,† Show-An Chen,*,† Su-Hua Chen,‡ and An-Chung Su‡
Chemical Engineering Department, National Tsing-Hua UniVersity, Hsinchu 30041, Taiwan, Republic of China, and Institute of
Materials Science and Engineering, National Sun Yat-sen UniVersity, Kaohsiung 804, Taiwan, Republic of China
Received July 16, 2005; E-mail: sachen@che.nthu.edu.tw
Polyfluorenes (PFs) have been regarded as potential blue-emitting
polymers for polymer light emitting diodes (PLED) with high
efficiency due to their high photoluminescent quantum yield
(PLQY)1 as well as good thermal, chemical, and electrochemical
stabilities. However, PLEDs based on PF homopolymers usually
show poor luminance efficiency and additional green emission,
which were attributed to an aggregate,2a excimer,2b or fluorenone
defect.2c Incorporation of bulky comonomer units, such as
anthracene3a and spirobifluorene,3b or of dendron side chains3c on
PF backbones or para-tert-butyl phenyl as end-cappers3d has been
attempted to suppress formation of an aggregate or excimer in order
to obtain stable blue emission (though their C.I.E. color index values
are not available), while the device efficiency has not been
improved. Another effort using triarylamines as end-cappers3e for
PFs with branch ethylhexyl side chains (PF2/6)3f has led to a
suppression of green emission with improved device efficiency,
but the blue emission with high purity was obtained only at a voltage
higher than 5 V. For prevention from fluorenone defect, it has been
Figure 1. The PL and EL spectra of spin-cast films of PFO-end-TBP,
-OXD, and -TAZ: TBP amorphous phase (a) and â phase (b); OXD (c)
and TAZ (d), both with their spectral summations from spectra in (a) and
reported very recently that replacing a C-9 carbon of PF with silicon
gave poly(2,7-dibenzosilole).3g Although its blue emission is stable,
the improvement in device performance is small and the purity of
blue emission is still not saturated (C.I.E. color coordinate x + y
> 0.30 in its electroluminescent (EL) spectrum).3g Among PFs,
poly(9,9-dioctylfluorene) (PFO) has a simple chemical structure
but is rich in morphological features,4a,b depending on processing
conditions, which can affect device performance and emission color.
Here, we propose a simple way, with PFO as an example, to
achieve high stability and improved purity in blue emission as well
as improved device efficiency via capping PFO chain ends with
electron-deficient moieties (EDMs, such as oxadiazole (OXD) and
triazole (TAZ) as shown below), which can induce a minor amount
of long conjugating length species (regarded as â phase4b-e) to
control extents of energy transfer from amorphous matrix to the â
phase. It is amazing that the device efficiency of PFO end-capped
with TAZ is higher than that with para-tert-butyl phenyl (TBP) by
a factor of 2 (with CsF/Al as cathode), and its EL spectrum remains
stable and with pure blue emission during cyclic operations (C.I.E.
x ) 0.165, y ) 0.076, independent of operating voltage and within
the limit for pure blue emission x + y < 0.30).
(b) (2: experimental, 0: spectral summation). The picture of emission in
(d) is from PFO-end-TAZ-based device operated at 4 V.
The photoluminescence (PL) spectrum of spin-cast PFO-end-
TBP film is almost totally amorphous (Figures 1a inset, and S1),
characterized with λmax at 423 nm (strong peak, 0-0 band), 447
nm (shoulder, 0-1 band), and 470 nm (weak shoulder, 0-2
band).4b,c Upon exposure of the polymer film to saturated THF vapor
for 15 min and then removing absorbed THF, parts of the chains
have extended the conjugating length (regarded as â phase), as
evidenced by its PL spectrum with well-resolved vibronic transitions
red-shifted by about 0.1 eV at 438 nm (0-0 band), 466 nm (0-1
band), and 498 nm (0-2 band) (Figure 1b inset). For a PFO thin
film, generation of the â phase usually requires additional treatments
after spin-coating, such as cooling and reheating to room temperature4c
or exposure to certain solvent vapors,4b-d or needs to be processed
from its solution in a solvent with low solvent power4e or by addition
of poor solvent.2c These treatments are deleterious to the lifetime
of PLEDs due to resulting uneven surface.4b Here, we find that
end-capping PFO with an electron-deficient moiety, such as OXD
and TAZ, can prompt the formation of â phase along chain ends
(due to its electron-withdrawing capability) after spin-casting
without further treatments, as evidenced by the presence of an
absorption peak at 436 nm4d in addition to the main absorption
peak at 380 nm (Figure 2a and the inset) and an excitation peak at
438 nm in the PL excitation (PLE) spectra, monitored at the
emission of 470 nm in Figure 2b. In addition, their PL spectra,
excited at the absorption of â phase at 436 nm, show two peaks at
467 and 500 nm, which are very close to those of the â phase
(Figure 2b); however, for PFO-end-TBP film prepared from the
same condition, no such characteristic peaks were observed. The
† National Tsing-Hua University.
‡ National Sun Yat-sen University.
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10.1021/ja054777+ CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
and 2500 cd/m2 for amorphous PFO-end-TBP, 1.23 cd/A and 8200
cd/m2 for â phase PFO-end-TBP, 0.56 cd/A and 3400 cd/m2 for
PFO-end-OXD, and 1.67 cd/A and 4000 cd/m2 for PFO-end-TAZ.
The PFO-end-TAZ has the best device performance due to TAZ’s
larger size and nonplanar structure, which could suppress a possible
formation of aggregate at chain ends as supported by the higher
PLQY from the film of PFO-end-TAZ (47%) than that of the other
two (36 and 38% for PFO-end-TBP and -OXD, respectively), and
due to its electron-transporting and hole-blocking characteristics.6
For devices with lower molecular weight PFOs (Mn ∼ 20 000
or ∼50 repeat units versus Mn ∼ 100 000 or ∼250 repeat units),
the performance of devices with OXD- and TAZ-capped PFOs is
no better than that of PFO-end-TBP (Table S2), and their EL spectra
are characteristic of amorphous phase only and unstable with applied
voltage (Figure S3). Since the conjugation length of â phase is
about 30 repeat units or more,4c it is unfavorable for such low MW
PFOs to form â phase after spin-coating.
In conclusion, we report for the first time that a PF homopolymer
with end-capping by EDMs can simultaneously provide deep blue
emission and improve device efficiency. The deep blue emission
is originated from the incomplete energy transfer from the
amorphous matrix to the â phase induced by the end-cappers,
whereas the improvement of device efficiency is dependent on the
structure of EDM, such as size and planarity. Among the three
PFOs, simple device architecture using TAZ as the end-capper has
the highest efficiency (1.67 cd/A) since TAZ possesses larger size,
nonplanar structure, and electron-transporting/hole-blocking char-
acteristics.
Figure 2. The absorption spectra of PFOs with the enlargement in the
inset (a) and the PLE spectra (monitored at 470 nm) and the PL spectra
excited at 436 nm (b).
PL spectra of PFO-end-OXD (Figure 1c inset) and -TAZ (Figure
1d inset), excited at the main absorption of PFO chains (380 nm),
show the bands from â phase at 441 nm (0-0 band), 467 nm (0-1
band), and 500 nm (0-2 band) and an additional weak 0-0 band
from the amorphous matrix at 423 nm. The presence of the
additional emission at 423 nm indicates an incomplete energy
transfer from the amorphous phase to the end-capper induced â
phase, which acts as a self-dopant. Because the energy transfer from
the amorphous matrix to the â phase is efficient (∼3 ps),4d,e even
the amount of energy received is small,5 the incomplete energy
transfer implies that the amount of â phase in PFO-end-OXD and
-TAZ is trace, as also supported by the absence of the characteristic
peak of â phase in wide-angle X-ray diffraction measurement
(Figure S1). The PL spectra of PFO-end-OXD and -TAZ can be
reconstructed by spectral summation of the PL spectra from
amorphous and â phase PFO-end-TBP at the ratios of 33:67 and
71:29, respectively, as shown in the insets of Figure 1c,d. This is
reasonable because the amount of chain-end of the former is 50%
more than that of the latter (Table S1).
The PLED devices ITO/PEDOT/PFOs/CsF/Al are fabricated to
evaluate the stability of the emission spectrum (Figures 1 and S2)
and device efficiency (Table S1). For comparison, the device with
an emission from â phase was fabricated by exposing the film of
PFO-end-TBP to saturated THF vapor for 15 min. Figure 1a,b
shows the EL spectra of PFO-end-TBP from amorphous and â
phases, respectively, which are similar to their corresponding PL
spectra and have C.I.E. color coordinates of (0.176, 0.140) and
(0.164, 0.146), respectively. For the EL spectra of PFO-end-OXD
(Figure 1c) and -TAZ (Figure 1d), they also resemble their
corresponding PL spectra, due to incomplete energy transfer, and
their spectral summations of the EL spectra from the amorphous
and the â phases at the ratios of 18:82 and 68:32, respectively.
Their C.I.E. color coordinates are (0.164, 0.088) and (0.165, 0.076),
which have x + y values (as 0.252 and 0.241) much smaller than
0.30, and thus are bluer than those of the amorphous phase and â
phase both having x + y > 0.30. Such high purity in blue emission
is due to their stronger intensity of blue components and their
weaker long wavelength tail (Figure 1c,d) caused by the bulky
electron-deficient end-capper, which leads to less order alignment
of long conjugating length species and thus weak 0-1 and 0-2
bands as compared to 0-0 band. Fine tunability by this method is
obvious since PFO with TAZ are bluer than that with OXD. Most
importantly, the EL spectra of these two capped PFOs are stable
under increasing voltages (Figure S2). Excellent blue emission from
the PFO-end-TAZ-based device at 4 V taken by a digital camera
is inserted in Figure 1d. The device performance is evaluated by
the maximum current efficiency and brightness, which are 0.74 cd/A
Acknowledgment. We wish to thank the Ministry of Education
through Project 91E-FA04-2-4A and the National Science council
for financial aid.
Supporting Information Available: Instrumental, device fabrica-
tion details, GPC results, XRD results, and the synthesis details of the
monomers and polymers (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
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