Organic laser Diode

Applied Physics Express
LETTER • OPEN ACCESS
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
View the article online for updates and enhancements.
This content was downloaded from IP address 194.254.173.6 on 04/06/2019 at 15:23

Indication of current-injection lasing from an organic semiconductor
Atula S. D. Sandanayaka
1,2*
, Toshinori Matsushima
1,2,3
, Fatima Bencheikh
1,2
, Shinobu Terakawa
1,2
,
William J. Potscavage, Jr.
1,2
, Chuanjiang Qin
1,2
, Takashi Fujihara
4,5
, Kenichi Goushi
1,2,3
, Jean-Charles Ribierre
1,2
, and
Chihaya Adachi
1,2,3,4,5*
1
Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan
2
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-I
2
CNER), 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 i
3
-Center for Organic Photonics and Electronics Research (i
3
-OPERA), 5-14 Kyudai-shinmachi, Nishi, Fukuoka 819-0388, Japan
*
E-mail: [email protected];[e-mail]
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 electro-
luminescent devices incorporate a mixed-order distributed feedback SiO
2
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
The properties of optically pumped organic semicon-
ductor 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 nanosecond-
pulsed inorganic light-emitting diodes has been demon-
strated, 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 opto-
electronic 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
quenchers
6)
to suppress triplet absorption and singlet
quenching by triplet excitons as well as the reduction of
the device active area
7)
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 pro-
blems 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 split-
gate 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]bi-
phenyl (BSBCz) [chemical structure in Fig. 1(a)]
22)
because
of its excellent combination of optical and electrical proper-
ties such as a low amplified spontaneous emission (ASE)
threshold in thin films (0.30 μJcm
−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 (η
EQE
) 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 repre-
sented 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 SiO
2
on indium tin oxide (ITO)
glass substrates was engraved with electron beam lithography
and reactive ion etching to create mixed-order DFB gratings
Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further
distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
©2019 The Japan Society of Applied Physics061010-1
Applied Physics Express 12, 061010 (2019) LETTER
https://doi.org/10.7567/1882-0786/ab1b90

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 mixed-
order 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 (Λ
1
and Λ
2
) of 140 and 280 nm were chosen for the first- and
second-order regions, respectively, based on the Bragg
condition,
22,24)
mλ
Bragg
=2n
eff
Λ
m
, where mis the order of
diffraction, λ
Bragg
is the Bragg wavelength, which was set to
the reported maximum-gain wavelength (477 nm) for
BSBCz, and n
eff
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
SiO
2
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). Cross-
sectional 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)/MoO
3
(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 MoO
3
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
(a)
(b) (c)
(d) (e)
Fig. 1. (Color online) (a) Schematic representation of the OSLDs (SiO
2
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) Cross-
section EDX images of the OSLD.
©2019 The Japan Society of Applied Physics061010-2
Appl. Phys. Express 12, 061010 (2019) A. S. D. Sandanayaka et al.

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 η
EQE
–Jcharacteristics of an OSLD and OLED under
pulsed conditions (voltage pulse width of 400 ns and repeti-
tion 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 η
EQE
higher
than 2% at low current densities and strong efficiency roll-off
at current densities higher than 1 A cm
−2
, which is presum-
ably 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 η
EQE
was even found to substan-
tially increase above 200 A cm
−2
to reach a maximum value
of 2.9%. The rapid decrease in η
EQE
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-wave-
length 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, sug-
gesting 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
(a) (b)
(c) (d)
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) η
EQE
–Jcharacteristics of the OLEDs and OSLDs under pulse operation (for η
EQE
, the photon intensity
was estimated by a photomultiplier tube).
©2019 The Japan Society of Applied Physics061010-3
Appl. Phys. Express 12, 061010 (2019) A. S. D. Sandanayaka et al.

limited resolution of our spectrometer. Note that the second
apparent transition point in Fig. 3(c) observed around
2kA 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 SiO
2
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 amplifica-
tion 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 demon-
stration 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=λ
peak
2
/FWHM, where λ
peak
is the peak wavelength. For
all of the devices, Lis 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
(a) (b)
(c) (d)
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 photographofan
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.
©2019 The Japan Society of Applied Physics061010-4
Appl. Phys. Express 12, 061010 (2019) A. S. D. Sandanayaka et al.