2
much larger than that of AcPI. It can be expected that the
significantly small EST value for AcPI enables efficient up-
conversion from the T1 to S1 states through rapid RISC to
achieve a high electroluminescence (EL) efficiency.
The photoluminescence (PL) properties of AcPI and
AcMI as guest emitters in a solid host matrix were
investigated. To confine the T1 excitons to the emitters, 3,3'-
bis(9-carbazolyl)-1,1'-biphenyl (mCBP; T1 = 2.8 eV) and
4,4'-bis(9-carbazolyl)-1,1'-biphenyl (CBP; T1 = 2.6 eV) were
employed as a host material for AcPI and AcMI, respectively.
As depicted in Figure 2, 6 wt%-AcPI:mCBP and AcMI:CBP
co-deposited films show green and orange PL with an
emission peak at 517 and 559 nm, respectively. Absolute PL
quantum yields (PL) of the 6 wt%-AcPI:mCBP and
AcMI:CBP films were determined to be 50±1% and 42±1%,
respectively, at room temperature (300 K). Next, we studied
the transient PL decay behavior of these doped films using a
streak camera. As reported in previously,3,4 TADF materials
typically show two distinct components of prompt decay
originating from fluorescence, and delayed decay based on
TADF. Obviously, a 6 wt%-AcPI:mCBP film indicated both
a nano-second-scale prompt fluorescence (lifetime of p = 53
ns) and micro-second-scale delayed fluorescence (d = 21 s)
at 300 K (Figure 3). Moreover, the quantum efficiency for
delayed fluorescence (d) appeared to increase with
increasing temperature from 5 K upward. Contributions from
the prompt fluorescence (Φp) and delayed fluorescence (Φd)
to the overall ΦPL were determined to be 11±1% and 38±1%,
respectively. Using the values and lifetimes for the 6 wt%-
AcPI:mCBP film at 300 K, the radiative decay rate constant
of the S1 state (krS), ISC rate constant (kISC), and RISC rate
constant (kRISC) were estimated to be 2.1 × 106 , 1.7 × 107,
and 1.9 × 105 s−1, respectively.3e In contrast, the PL profile of
a 6 wt%-AcMI:CBP film exclusively showed prompt decay
component (i.e., fluorescence) with an emission lifetime of p
= 15 ns even at room temperature, suggesting that AcMI
presented negligible TADF characteristics presumably due to
its relatively large EST (Figure 1).
Figure 3. (a) Temperature dependence of transient PL decay for
a 6 wt%-AcPI:mCBP doped film measured at 5–300 K. (b)
S
Postulated PL decay processes for AcPI; krS and knr are the
radiative and non-radiative decay rate constants of the S1 state,
kISC and kRISC are the ISC (S1→T1) and RISC (T1→S1) rate
T
constants, respectively, and knr is the non-radiative rate constant
of the T1 state.
nm)/Al (100 nm) and ITO/-NPD (35 nm)/ 6wt%-
AcMI:CBP (15 nm)/TPBi (65 nm)/LiF (0.8 nm)/Al (100 nm),
respectively,
where
4,4'-bis[N-(1-naphthyl)-N-
phenyl]biphenyl diamine (-NPD) and 1,3,5-tris(N-
phenylbenzimidazol-2-yl)benzene (TPBi) were used as hole-
transport and electron-transport layers, respectively. Figure 4
depicts the current density–voltage–luminance (J–V–L) and
external EL quantum efficiency (EQE) vs. current density
characteristics of the AcPI- and AcMI-based OLEDs. The
EL emission peaks for the AcPI- and AcMI-based devices
were observed at 530 and 581 nm, respectively, confirming
that EL emission originates from the S1 states of the AcPI
and AcMI emitters. We obtained a high maximum EQE of
11.5% in the AcPI-based green OLED; whereas that for the
non-TADF AcMI-based device was 1.4% in spite of their
similar PL values. In the TADF-based OLEDs, the T1
excitons are directly generated by carrier recombination and
successively converted to the emissive S1 state via efficient
RISC process. Accordingly, the theoretical maximum of
internal EL quantum efficiency (IQE) and EQE can be
expressed by the following equations:3c
Employing AcPI and AcMI as an emitter, two
multilayer OLEDs were fabricated. The device structures
were indium tin oxide (ITO)/-NPD (35 nm)/mCBP (5
nm)/6wt%-AcPI:mCBP (20 nm)/TPBi (65 nm)/LiF (0.8
IQE = S × p + S × d + T × d/ISC
EQE = IQE × out
where S and T denote the singlet-exciton and triplet-
exciton production rates (25% and 75%, respectively), ISC
(~89%) is the ISC efficiency, and out is the light out-
coupling efficiency. We estimate that IQE for the AcPI-
based device is 44%, thus the theoretical maximum EQE
should be approximately 11% assuming that out is 25%. The
experimental maximum EQE of 11.5% obtained for the
AcPI-based OLED is consistent with the theoretical value,
implying that suitable carrier balance and effective exciton
confinement are achieved in the light-emitting layer. The
TADF process indeed enhanced the efficiency of EL
emission since the EQE of the AcPI-based device was more
than three times higher than the theoretical value for
conventional fluorescence emitters with the same PL value.
In summary, we have developed new phthalimide- and
maleimide-based fluorophores, AcPI and AcMI. While
AcMI did not show clear TADF properties, AcPI having
Figure 2. Photoluminescence spectra of 6 wt%-AcPI:mCBP and
AcMI:CBP co-deposited films measured at room temperature.
Jang, Myung Eun
