C O M M U N I C A T I O N S
Table 1. Performance of the Devices with the Polymers
turn-on voltagea
(V/100 nm)
max efficiency
(cd/A) (ηmax,%) (V)
max brightness
(cd/m ) (V)
2
polymer
PFO
PFOR01
PFOR1
5.7
6.9
6.9
8.0
6.7
3.3
4.3
4.9
0.049 (0.05) (9 V)
0.037 (0.017) (8V)
0.88 (0.57) (10 V)
1.0 (0.43) (17.5 V)
0.23 (0.16) (15 V)
1.28 (0.74) (5 V)
2.16 (1.32) (9 V)
2.8 (1.59) (7 V)
258 (11 V)
57 (11 V)
1479 (13 V)
508 (23 V)
335 (18 V)
5029 (7 V)
3735 (10 V)
4321 (15 V)
PFOR12
PFOG05R01
Cz100PF
CzPFR08
CzPFR1.3
a Brightness over 0.2 cd/m2
and 690 mA/cm2).3a The incorporation of Cz can significantly
increase the efficiency and lower the turn-on voltage.
In summary, efficient red emission electrophosphorescent PLEDs
are obtained by simultaneous incorporation of Ir complexes and
charge transport moieties on the side chains of polyfluorene. Energy
transfer from an electroplex formed between fluorene main chain
and side-chain carbazole moieties, in addition to that from the PF
main chain, to the red Ir complex can significantly enhance the
device performance. By choosing proper conjugated polymers and
other organometallic complexes, high-efficiency devices for various
emission colors with single polymer/single layer can be expected.
Figure 2. PL and EL spectra from the polymers with Cz. (Inset) Band
diagram, in which the LUMO for btp2Ir(acac) has the energy level for singlet
state at 2.4 and triplet state at 3.1 eV.
chain Cz, that is, an electroplex.8 For the CzPFR with high red Ir
complex content 1.3 mol % in the feed, there is only red emission
from the Ir complex and almost no blue emission from the main
chain in the EL. For that with lower Ir complex content 0.8 mol %
in the feed, all the components of blue, green, and red light
emissions appear, indicating CzPFR is a potential candidate polymer
to provide a high-quality white light for use in full color display
with color filters. The decreased intensity of green light emission
relative to the blue and its disappearance would indicate an
occurrence of energy transfer from the green electroplex to the red
Ir complex.9
Acknowledgment. We are grateful for the financial support
from the National Science Council and from the Ministry of
Education through Project 91E-FA04-2-4A.
Supporting Information Available: Instrumental, device fabrica-
tion details, TOF measurement, GPC results, and the synthesis details
of the monomers and polymers (PDF). This material is available free
The occurrence of charge trapping can be inferred from the J-V
results of the hole-dominating device, hole mobility measurement,
and cyclic voltammetry (CV) study of the red Ir complex, btp2Ir-
(acac). Hole-dominating devices provide a decrease of hole currents
with the increase of the red Ir complex content, and the hole
mobility measurement by time-of-flight (TOF) indicate that with
only 1 mol % of Ir complex the polymer PFOR1 has a hole mobility
(3.1 × 10-7 cm2/V s at 5 × 105 V/cm) 2 orders of magnitude lower
than PFO (7.7 × 10-5 cm2/V s at 5 × 105 V/cm) and PFOR12 has
even lower hole mobility (8.4 × 10-8 cm2/V s at 8 × 105 V/cm).
These indicate the occurrence of hole trapping on the side chain Ir
complex. The CV study of PFO, Cz moiety, and the red Ir complex
gives the band diagram as shown in the inset in Figure 2 (see SI).
As can be seen, the HOMO and LUMO (the triplet state) energy
levels of the red Ir complex lay between those of the main chain,
which permits both hole and electron trapping during the electric
field excitation.
The performances of the bipolar devices are listed in Table 1.
The device with Cz100PF has a maximum external quantum
efficiency (ηmax) higher than that with PFO by a factor of 15. The
device with CzPFR1.3 has the highest efficiency 2.8 cd/A at 7 V
and 65 cd/m2 for red PLED and remains high (1.6 cd/A at 15 V
and 4321 cd/m2). It is the highest in PLED and comparable to that
from the same Ir complex-based OLED (0.98 cd/A at 6800 cd/m2
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