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Indication of current-injection lasing from an organic semiconductor
To cite this article: Atula S. D. Sandanayaka et al 2019 Appl. Phys. Express 12 061010
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LETTER
Applied Physics Express 12, 061010 (2019)
https://doi.org/10.7567/1882-0786/ab1b90
Indication of current-injection lasing from an organic semiconductor
Atula S. D. Sandanayaka1,2*, Toshinori Matsushima1,2,3, Fatima Bencheikh1,2, Shinobu Terakawa1,2,
William J. Potscavage, Jr.1,2, Chuanjiang Qin1,2, Takashi Fujihara4,5, Kenichi Goushi1,2,3, Jean-Charles Ribierre1,2, and
Chihaya Adachi1,2,3,4,5*
1
Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan
Japan Science and Technology Agency (JST), ERATO, Adachi Molecular Exciton Engineering Project, Kyushu University, 744 Motooka, Nishi, Fukuoka
819-0395, Japan
3
International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan
4
Innovative Organic Device Laboratory, Institute of Systems, Information Technologies and Nanotechnologies (ISIT), 4-1 Kyudai-shinmachi, Nishi,
Fukuoka 819-0388, Japan
5
Fukuoka i3-Center for Organic Photonics and Electronics Research (i3-OPERA), 5-14 Kyudai-shinmachi, Nishi, Fukuoka 819-0388, Japan
2
*
E-mail: [email protected]; [email protected]
Received April 16, 2019; accepted April 23, 2019; published online May 31, 2019
In this study, we investigate the lasing properties of 4,4′-bis[(N-carbazole)styryl]biphenyl thin films under electrical pumping. The electroluminescent devices incorporate a mixed-order distributed feedback SiO2 grating into an organic light-emitting diode structure and emit blue lasing.
The results provide an indication of lasing by direct injection of current into an organic thin film through selection of a high-gain organic
semiconductor showing clear separation of the lasing wavelength from significant triplet and polaron absorption and design of a proper feedback
structure with low losses at high current densities. This study represents an important advance toward a future organic laser diode technology.
© 2019 The Japan Society of Applied Physics
Supplementary material for this article is available online
T
he properties of optically pumped organic semiconductor lasers (OSLs) have dramatically improved in
the last two decades as a result of major advances in
both the development of high-gain organic semiconductors
and the design of high Q-factor resonators.1–3) Owing to
recent advances in low-threshold distributed feedback (DFB)
OSLs, optical pumping by electrically driven nanosecondpulsed inorganic light-emitting diodes has been demonstrated, providing a route toward a compact and low-cost
visible laser technology.4) However, the ultimate goal is
electrically driven OSL diodes (OSLDs). In addition to
enabling the full integration of organic photonic and optoelectronic circuits, the realization of OSLDs will open novel
applications in spectroscopy, displays, medical devices
(such as retina displays, sensors, and photodynamic therapy
devices), and LIFI telecommunications.
The problems that have prevented the realization of lasing
by the direct electrical excitation of organic semiconductors
are mainly due to optical losses from electrical contacts and
electrical losses induced by triplets and polarons at high
current densities.5) Approaches that have been proposed to
solve these fundamental issues include the use of triplet
quenchers6) to suppress triplet absorption and singlet
quenching by triplet excitons as well as the reduction of
the device active area7) to spatially separate where exciton
formation and exciton radiative decay occur and minimize
the polaron quenching processes. However, even with the
advances that have been made in organic light-emitting
diodes (OLEDs) and optically pumped OSLs,1–3,8–12) a
current-injection OSLD has still not been satisfactorily
demonstrated. Another promising laser architecture is based
on an organic ﬁeld-effect transistor (OFET),13) which shows
great potential for solving electrode and polaron loss problems simultaneously. Much effort has been focused on
improving the performance of light-emitting OFETs by using
single crystals;14,15) introducing multilayer structures to
reduce charge density within an emissive layer,16,17) a splitgate structure for independent control of electron and hole
injections,18) or current-conﬁnement structures for increased
local current density;19) or utilizing microcavity effects
originating from a gate electrode.20)
Previous studies have suggested that current densities over
a few hundred amperes per square centimeter would be
required to achieve lasing from an OSLD if additional losses
associated with the electrical pumping were completely
suppressed.21) One of the most promising molecules for the
realization of an OSLD is 4,4′-bis[(N-carbazole)styryl]biphenyl (BSBCz) [chemical structure in Fig. 1(a)]22) because
of its excellent combination of optical and electrical properties such as a low ampliﬁed spontaneous emission (ASE)
threshold in thin ﬁlms (0.30 μJ cm−2 under 800 ps pulse
photoexcitation)23) and the ability to withstand the injection
of current densities as high as 2.8 kA cm−2 under 5 μs pulse
operation in OLEDs with maximum electroluminescence
(EL) external quantum efﬁciencies (ηEQE) of over 2%.7)
Furthermore, lasing at a high repetition rate of 80 MHz and
under a long-pulse photoexcitation of 30 ms has recently
been demonstrated in optically pumped BSBCz-based DFB
lasers.23) Here, we examine the lasing properties of an
organic semiconductor ﬁlm directly excited by electricity
through the development and characterization of OSLDs
based on a BSBCz thin ﬁlm in an inverted OLED structure
with a mixed-order DFB grating.
The architecture of the OSLDs is schematically represented in Fig. 1(a), and their fabrication/characterization
methods are described in the supplementary data (Fig. S1 is
available online at stacks.iop.org/APEX/12/061010/mmedia,
SD). A sputtered layer of SiO2 on indium tin oxide (ITO)
glass substrates was engraved with electron beam lithography
and reactive ion etching to create mixed-order DFB gratings
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(a)
(b)
(c)
(d)
(e)
Fig. 1. (Color online) (a) Schematic representation of the OSLDs (SiO2 widths of 140 and 70 nm for second- and ﬁrst-order gratings, respectively). (b) Laser
microscope and (c) SEM images at 5000× and 200 000× (inset) magniﬁcation of a DFB grating. (d) Cross-section SEM images of the OSLD. (e) Crosssection EDX images of the OSLD.
with an area of 30 × 90 μm [Fig. 1(b)], and organic layers
and a metallic cathode were vacuum-deposited on the
substrates to complete the devices. We designed the mixedorder DFB gratings to have an alternation of ﬁrst- and
second-order Bragg scattering regions that provide strong
lateral optical feedback and efﬁcient vertical outcoupling
of the laser emission, respectively.16) Grating periods (Λ1
and Λ2) of 140 and 280 nm were chosen for the ﬁrst- and
second-order regions, respectively, based on the Bragg
condition,22,24) mλBragg = 2neffΛm, where m is the order of
diffraction, λBragg is the Bragg wavelength, which was set to
the reported maximum-gain wavelength (477 nm) for
BSBCz, and neff is the effective refractive index of the
structure, which was calculated to be 1.70. The lengths of the
individual ﬁrst- and second-order DFB grating regions were
1.12 and 1.68 μm, respectively, in the ﬁrst set of devices
characterized, hereafter referred to as the OSLDs.
The SEM images in Figs. 1(c) and 1(d) conﬁrm that the
DFB gratings had periods of 140 ± 5 and 280 ± 5 nm with a
grating depth of about 65 ± 5 nm. Complete removal of the
SiO2 layer in the etched areas to expose the ITO is important
for making good electrical contact with the organic layer and
was veriﬁed with EDX analysis (Fig. S1, SD). Crosssectional SEM and EDX images of a complete OSLD are
shown in Figs. 1(d) and 1(e). The surface morphologies of all
layers present a grating structure with a surface modulation
depth of 50–60 nm.
The OSLDs fabricated in this work had an inverted OLED
structure of ITO (100 nm)/20 wt% Cs:BSBCz (60 nm)/
BSBCz (150 nm)/MoO3 (10 nm)/Ag (10 nm)/Al (90 nm)
with the energy levels as shown in Fig. 2(a). Doping the
BSBCz ﬁlm with Cs in the region near the cathode improved
electron injection into the organic layer, and MoO3 was used
as a hole injection layer (Fig. S2, SD). While the most
efﬁcient OLEDs generally use multilayer architectures to
optimize charge balance,25) charges can accumulate at
organic hetero-interfaces at high current densities, which
can be detrimental to device performance and stability.26) The
OSLDs fabricated in this work contained only BSBCz as the
organic semiconductor layer and were speciﬁcally designed
to minimize the number of organic hetero-interfaces.
Reference devices without DFB gratings, hereafter referred
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A. S. D. Sandanayaka et al.
(b)
(a)
(d)
(c)
Fig. 2. (Color online) (a) Energy level diagram of the OSLDs. (b) Photomicrographs of an OSLD and a reference OLED under DC operation at 3.0 V.
(c) Current density–voltage (J–V ) characteristics and (d) ηEQE–J characteristics of the OLEDs and OSLDs under pulse operation (for ηEQE, the photon intensity
was estimated by a photomultiplier tube).
to as the OLEDs, were also fabricated to investigate the
inﬂuence of the gratings on the EL properties.
Figure 2(b) shows optical microscope images of an OSLD
and a reference OLED under DC operation at 3 V. In addition
to the previously described DFB grating, ﬁve other DFB
grating geometries (Table S1 and Fig. S3, SD) were
optimized and characterized in the OSLDs. While EL was
emitted homogeneously from the active area of the reference
OLEDs, more intense emission could be seen from the
second-order DFB grating regions, which were speciﬁcally
designed to promote vertical light outcoupling, in the OSLDs
[Figs. 2(b) and S3 (SD)]. The current density–voltage (J–V )
and ηEQE–J characteristics of an OSLD and OLED under
pulsed conditions (voltage pulse width of 400 ns and repetition rate of 1 kHz) at ambient temperatures are shown in
Figs. 2(c) and 2(d), and the characteristics obtained under DC
conditions are displayed in the SD (Fig. S4). Although some
current ﬂowed through the areas above the grating (∼20%
based on simulations), most ﬂowed through the areas above
the exposed ITO. For simplicity and consistency, the exposed
ITO area was used for the calculation of current density for
all OSLDs, though this may lead to slight overestimations.
The maximum current densities before device breakdown
of the reference OLEDs increased from 6.6 A cm−2 under DC
operation to 5.7 kA cm−2 under pulse operation because of
reduced Joule heating with pulse operation.7,27) Under DC
operation, all of the devices exhibited maximum ηEQE higher
than 2% at low current densities and strong efﬁciency roll-off
at current densities higher than 1 A cm−2, which is presumably due to the thermal degradation of the devices. On the
other hand, efﬁciency roll-off in the OLEDs under pulse
operation [Figs. 2(c), 2(d)] began at current densities higher
than 110 A cm−2, consistent with a previous report.7)
Efﬁciency roll-off was further suppressed in the OSLDs
under pulse operation, and ηEQE was even found to substantially increase above 200 A cm−2 to reach a maximum value
of 2.9%. The rapid decrease in ηEQE above the current density
of 2.2 kA cm−2 is likely due to the thermal degradation of the
device.
While the EL spectra of the OLEDs were similar to the
steady-state photoluminescence (PL) spectrum of a neat
BSBCz ﬁlm (Fig. S4, SD) and did not change as a function
of the current density, the EL spectra from the glass face of
the OSLDs under pulse operation exhibited spectral line
narrowing with increasing current density [Fig. 3(a)]. A
Bragg dip corresponding to the stopband of the DFB grating
was observed at 475 nm for current densities below
650 A cm−2 [Fig. 3(b)]. Lasing occurred at the long-wavelength band edge (480.3 nm) of the Bragg stopband, which
can be explained by the difference in gain at the two band
edges. As the current density increased above this value,
strong spectral line narrowing occurred at 480.3 nm, suggesting the onset of lasing. The intensity of the narrow
emission peak was found to increase faster than that of the
EL emission background, which could be attributed to the
non-linearity associated with stimulated emission.
The output EL intensity height and FWHM of an OSLD
are plotted in Fig. 3(c) as a function of the current. While the
FWHM of the steady-state PL spectrum of a neat BSBCz ﬁlm
is around 35 nm, the FWHM of the OSLD at high current
densities decreased to values lower than 0.2 nm, which is
close to the spectral resolution limit of our spectrometer
(0.17 nm for a wavelength scan range of 57 nm). The FWHM
became saturated above the lasing threshold due to the
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A. S. D. Sandanayaka et al.
(b)
(a)
(d)
(c)
Fig. 3. (Color online) (a) Emission spectra of an OSLD collected in the direction normal to the substrate plane for different injected current densities.
(b) Emission spectra near the lasing threshold (the dashed line indicates the Bragg dip). (c) Output intensity and FWHM as a function of the current [to plot the
FWHM as a function of current density, we used only narrow emission without taking into account broad emission as shown in Figs. 3(a) and S5b; to plot EL
intensity as a function of current density, we used both peak and broad emission]. (d) Output power as a function of the current. The inset is a photograph of an
OSLD under pulse operation at 50 V. The emitted laser light from the OSLDs was collected with an optical ﬁber connected to a multichannel spectrometer,
PMA50. Considering the degradation issues of the devices, note that the experimental data of Figs. 3(c) and 3(d) were measured in different devices. The data
in Fig. 3(a) were obtained using a spectrometer with a resolution of 0.2 nm while the results in Fig. 3(b) were measured using a spectrometer with a resolution
of 2 nm.
limited resolution of our spectrometer. Note that the second
apparent transition point in Fig. 3(c) observed around
2 kA cm−2 is presumably due to device instability at high
current density. The slope efﬁciency of the output intensity
abruptly changed with increasing current and can be used to
determine a threshold of 600 A cm−2 (8.1 mA). Above
3.5 kA cm−2, we observed signiﬁcant emission of broad EL
in addition to the lasing emission. This might have been
induced by the partial breakdown of the SiO2 grating due to
the extremely high voltage and current. The output power
as a function of the current density is plotted in Fig. 3(d).
The maximum output power measured with a power meter
placed in front of an OSLD at a distance of 3 cm from the
ITO glass substrate [Fig. 3(d)] was 0.50 mW at 3.3 kA cm−2.
Here, we note that the slope efﬁciencies in Figs. 3(c) and 3(d)
are appreciably different. Since the fabrication batches were
different in these devices, this may have come from a
difference in quality of the DFB structures. In a future
study, we would like to clarify this ﬂuctuation issue. These
observed EL properties strongly suggest that light ampliﬁcation occurs at high current densities and indicate that
electrically driven lasing is achieved above a current density
threshold.
Polarization of the emitted light was characterized below
and above the threshold to provide a further indication that
this was lasing.28) As shown in Fig. S6, the output beam of an
OSLD above the threshold was strongly linearly polarized
along the grating pattern, which is expected for laser
emission from a one-dimensional DFB. Coherence and clear
observation of an output beam above the threshold are other
central facets of lasing and must be carefully examined to
support a claim of lasing. As shown in Figs. S6(b)–S6(c), the
projection of the output beam of an OSLD on a screen
resulted in a fan-shaped pattern only above the threshold, as
expected for a one-dimensional DFB. Regarding the demonstration of the coherence of the output beam, it is necessary to
include proof of temporal and spatial coherence. To gain
insight into the temporal coherence of the OSLD output,
we estimated coherence lengths (L) from the equation
L = λpeak2/FWHM, where λpeak is the peak wavelength. For
all of the devices, L is 1.1–1.3 mm, which is fully consistent
with lasing. For comparison, it is worth noting that the
coherence length of a 405 nm gallium nitride diode is around
150 μm.
To characterize the spatial coherence and the spatial
proﬁles of the output beam of the organic laser diode under
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A. S. D. Sandanayaka et al.
electrical pumping, we developed the setup described in the
SD [Fig. S7(a)]. One CCD camera was used to characterize
the evolution of the beam in the far-ﬁeld for two different
distances between the device and the camera. Another CCD
camera was used to examine the near-ﬁeld pattern of the
output beam. Our results about coherence are reported in the
SD (Figs. S7–S8). The far-ﬁeld patterns indicate the existence of a well-deﬁned beam, which was formed only above
the lasing threshold. These far-ﬁeld beam cross sections are
consistent with those of an OSL under optical excitation. In
addition, zero-order diffracted emission could be observed
above the threshold but was not present below the threshold
because of the absence of spatial coherence. This can be seen
as one proof of spatial coherence. However, the fact that we
did not observe other diffracted orders suggests that the
emission from our devices contained coherent laser emission
and some incoherent EL emission. Looking at the near-ﬁeld
patterns of the output beam, we can see some fringes only
above the threshold. This behavior is identical to that we
observed in an optically pumped organic laser above the
threshold and is another indication of current-injection lasing
from OSLDs.
Several phenomena that have been misinterpreted as lasing
in the past must be ruled out as the cause of the observed
behavior.28) The emission from our OSLDs was detected in
the direction normal to the substrate plane and showed clear
threshold behavior, so line narrowing arising from the edge
emission of waveguide modes without laser ampliﬁcation can
be dismissed.29) ASE can appear similar to lasing, but the
FWHM of our OSLDs (<0.2 nm) is much narrower than the
typical ASE linewidth of an organic thin ﬁlm (a few
nanometers) and is consistent with the typical FWHM of
optically pumped organic DFB lasers (<1 nm).1) A very
narrow emission spectrum obtained by inadvertently exciting
an atomic transition in ITO has also been mistaken for
emission from an organic layer.30) However, the emission
peak wavelength of the OSLD in Fig. 3(a) is 480.3 nm and
cannot be attributed to emission from ITO, which has atomic
spectral lines at 410.3, 451.3, and 468.5 nm.31)
If this is lasing from a DFB structure, then the emission of
the OSLD should be characteristic of the resonator and the
output should be very sensitive to any modiﬁcations of the
laser cavity. Thus, OSLDs with different DFB geometries,
labeled OSLD-1 through OSLD-7 and OSL (Table S1, SD),
were fabricated and characterized (Fig. S3, SD). The emission peaks were nearly the same for OSL, OSLD-1, OSLD-2,
and OSLD-3 (480.3 nm, 479.6 nm, 480.5 nm, and 478.5 nm,
respectively), which had the same DFB grating periods.
Furthermore, OSLD-1, OSLD-2, and OSLD-3 all had low
minimum FWHMs (0.20 nm, 0.20 nm, and 0.21 nm, respectively) and clear thresholds (1.2 kA cm−2, 0.8 kA cm−2, and
1.1 kA cm−2, respectively). On the other hand, OSLD-4 and
OSLD-5, which had different DFB grating periods, exhibited
an emission peak at 459.0 nm with an FWHM of 0.25 nm
and a threshold of 1.2 kA cm−2 (OSLD-4) and at 501.7 nm
with an FWHM of 0.38 nm and a threshold of 1.4 kA cm−2
(OSLD-5). These results clearly demonstrate that the emission peak wavelength and the threshold are controlled by
the DFB resonator architecture. The resonant wavelengths,
the Q-factors and the conﬁnement factors were calculated
for the different OSLDs (Table S1). The conﬁnement factor
was similar for all the geometries. The calculated resonant
wavelengths agree well with the experimental lasing wavelengths. OSLD-1, OSLD-3 and OSL showed higher Q-factors
than the other OSLDs (Table S1). The calculated Q-factors for
OSL and the OSLDs are to some extent consistent with the
experimentally estimated Q-factor (Fig. S5). In further work,
we will carefully examine the inﬂuence of the DFB structure on
OSLD performance and expect to see a clear trend in the
relationship between the threshold current density and Q-factor.
Overall, as supported by the additional data in Figs. S9–
S11, SD, laser emission under current injection was observed
in this study due to (1) the clear separation of the lasing
wavelength from signiﬁcant absorption due to charge carriers
and triplet excitons; (2) the reduction of optical losses due to
electrical contacts because of the DFB architecture; and (3)
the low lasing threshold of the BSBCz ﬁlms allowing us to
observe current-injection lasing before device degradation
took place.
In conclusion, this study provides an indication that lasing
from a current-injection organic semiconductor is possible
through the proper design and choice of resonator and
organic semiconductor to suppress losses and enhance
coupling. The narrow emission demonstrated here has been
reproduced in multiple devices, and clear thresholds in both
input–output characteristics and strong polarization were
observed, thereby excluding other phenomena that could be
mistaken for lasing. The low losses in BSBCz are integral to
enabling lasing, so the development of strategies to design
laser molecules with similar or improved properties is an
important next step. This report opens up opportunities in
organic photonics and serves as a basis for the future
development of an organic semiconductor laser diode technology that is simple, cheap, and tunable and can enable
fully and directly integrated organic-based optoelectronic
platforms.
Acknowledgments This work was supported by the Japan Science and
Technology Agency (JST), ERATO, Adachi Molecular Exciton Engineering
Project, under JST ERATO Grant Number JPMJER1305 and by the International
Institute for Carbon-Neutral Energy Research (WPI-I2CNER) sponsored by the
Ministry of Education, Culture, Sports, Science and Technology. We thank Prof.
Y. Oki for his advice and Dr. K. Yoshida and Dr. M. Inoue for their technical
assistance.
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Chihaya Adachi
https://orcid.org/0000-0001-6117-9604
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