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Chemistry—A European Journal
doi.org/10.1002/chem.202005360
However, the external quantum efficiencies (EQE) decreased
because the NIR emitters with a narrow energy gap tend to
act as a carrier trap (Figure 6c). A decrease of the shoulder
emission was also observed due to the concentration quench-
ing when the concentrations of TPA-DCPP were increased (Fig-
ure 6b). The driving voltage in the current density–voltage (J–
V) curves were decreased with increasing the TPA-DCPP con-
tensity, a stimulated emission was observed at ca. 770 nm to-
gether with a decrease of FWHM (Figure 7c). The ASE thresh-
À2
old energy of ca. 80 mJcm was determined from a change in
the slope of emission as a function of input intensity (Fig-
ure 7b). According to a summary of materials and their ASE
and lasing thresholds, which was reported by Kuehne and
[
40]
Gather and replotted as a function of wavelength by us (Fig-
[
38]
[41]
centrations as observed in the previous report (Figure 6d),
ure S39), DAD-1 is positioned in the NIR region with a suffi-
whereas DAD-1 concentration had a minor effect on the J–V
characteristics (Figure S37). The best EQE exclusive of emission
from TPA-DCPP was 3.7% for the device containing 0.5 wt%
DAD-1 and 20 wt% TPA-DCPP. This value is comparable to
those for the state-of-the-art TAF NIR-OLEDs despite the so-
lution-based device. The ideal EQE estimated from the PL
ciently low ASE threshold, ensuring that PPABs are promising
organic NIR laser materials.
[38]
quantum yield (F ) and light out-coupling efficiency based on
PL
the optical simulation was 7.1% (Figure S38). The ratio of the
EQEs to the ideal ones decreased with increasing the DAD-1
concentrations from approximately 0.5 for 0.5 wt% to 0.35 for
1
.0 wt% and 0.28 for 1.5 wt% (Table S6). Considering similar
career balances for all fabricated devices as anticipated from
the above-mentioned small dependence of the J–V characteris-
tics on the DAD-1 concentrations, this result implies enhanced
direct recombination on DAD-1 molecules and attenuated
contribution of the TAF mechanism in the high doping con-
centrations of DAD-1. Therefore, further improvement of the
EQE may be possible by optimizing the TAF system in future
work.
Figure 7. (a) Stimulated emission cross-section (sem) spectrum of DAD-1 in a
doped CBP film. (b) Output PL intensity (blue filled circles) and FWHM (black
squares) from the edge of DAD-1-doped CBP film as a function of excitation
energy. (c) PL spectra of DAD-1-doped CBP film excited at different energies
above and below the ASE threshold.
Despite the increasing attention to solid-state lasers with
successful electrical pumping and continuous wave operations
in recent years, the number of organic NIR laser materials is
[40,41]
still limited (Figure S39).
Therefore, the development of
new NIR chromophores that enable light amplification is
highly valuable.
The ASE threshold energy is inversely proportional to the
Conclusions
[42]
stimulated emission cross-section (sem), which is related to
Einstein’s B coefficient in Equation (1):
The donor–acceptor–donor (D–A–D) architecture was success-
fully applied to shift emissions of the pyrrolopyrrole aza-
BODIPY (PPAB)-based chromophore DAD-1 to the new fluores-
cence brightness range in the NIR region. The spacer units and
heteroaromatic ring units played an important role in the con-
trol of D–A interactions to exhibit the intense emission due to
the hybridized locally excited and charge transfer (HLCT) state
in the case of DAD-1 and the solvatochromic responses in
emission due to the contribution of the CT state in the case of
DAD-2 and DAD-3. The HLCT state of DAD-1 is ideal for red-
shifts of the emission to the NIR region without sacrificing the
fluorescence brightness. Owing to the high fluorescence quan-
4
hnnðnÞ
l E ðlÞ
f
semðlÞ ¼
BE ðnÞ ¼
ð1Þ
f
2
cF
8pn ðlÞct
Fl
Fl
where l is the wavelength, E ðlÞ and E ðnÞ are the distribution
f
f
of fluorescence quantum yields in wavelength and frequency,
respectively, nðlÞ and nðnÞ are the refractive indices of the
active gain layer, h is Plank constant, n is the frequency, c is
the speed of light, and tFl is the fluorescence lifetime. This
equation specifies that low FFl and long tFl typical for most
NIR emitters with strong CT natures make light amplification
difficult. In contrast, because of the high FFl and moderately
tum yield (F ), a solution-processed NIR-OLED of DAD-1 using
Fl
short t of DAD-1, a low ASE threshold was expected. In addi-
tion, the negligible excited-state absorption at the emission
wavelength is of great advantage.
the thermally activated delayed fluorescence (TADF)-assisted
fluorescence (TAF) system exhibited a high external quantum
efficiency (EQE) of 3.7% at an electroluminescence (EL) maxi-
Fl
The ASE properties of DAD-1 were investigated using a CBP
film doped with 1 wt% DAD-1 by optically pumping with a ni-
trogen laser at 337 nm (Figure 7 and Table S7). The sem of the
mum of ca. 760 nm. In addition, the high F and moderately
Fl
short fluorescence lifetime (t ) of DAD-1 were also of benefit
Fl
in achieving the low amplified spontaneous emission (ASE)
À16
À2
doped film was estimated to be 1.510 at 760 nm, which is
threshold of ca. 80 mJcm . Overall, these results ensure high
as high as those of organic laser materials, such as bis(N-carba-
potential of PPAB as a NIR emitter in optoelectronic applica-
tions. Furthermore, considering the higher fluorescence bright-
[43]
zolyl)styryl biphenyl (BSBCz). Upon increasing the input in-
Chem. Eur. J. 2021, 27, 5259 – 5267
5266
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