Download Experimental Optimization of the Optical and Electrical

Experimental Optimization of the Optical and Electrical Performances of an
OLED heterostructure in a Multilayer Microcavity
A. Coens1, M. Chakaroun1, A. P. A. Fischer1,2, M. W. Lee1, A. Boudrioua1, and B. Geffroy3
1) Laboratoire de Physique des Lasers CNRS UMR 7538 , Universite Paris 13, 93430 Villetaneuse, France
2) Centrale de Proximité en Nanotechnologies de Paris Nord, Université Paris 13, 93430 Villetaneuse, France
3) Ecole Polytechnique, Organic Electronic Team LPICM, Ecole Polytechnique, 91128 PALAISEAU, France
Abstract:
In the context of coherent emission in organic light emitting diode, we investigate experimentally
the optical and electrical conditions for the realization of high quality factor high current density
microcavity OLED, with a focus on the optimal design of the OLED heterostructure. On a
quater-wavelength TiO2/SiO2 multilayer mirror, a half wavelength OLED heterostructure is
deposited and ended with a semitransparent cathode. The emission wavelength, the line-width
narrowing and the current density are studied as a function of two key parameters, the heterostructure optical thickness and the aluminum cathode thickness. The experimental results show
that a 125 nm-thick cavity OLED ended by a 20 nm thick aluminum cathode, exhibits a
resonance at 609 nm, with a full width at half maximum of 13nm, and with current-densities up
to (exceeds) 500 m A/cm2. Without a top high quality mirror the uncompleted microcavity
OLED heterostructure exhibits a clear luminance enhancement at normal incidence.
1. Introduction
Although optically-pumped organic solid-state organic lasers have been reported as early as 1996
[i,ii,iii]
, the electrically pumped organic laser diode, has, despite several attempts, not been
demonstrated so far. It remains a major challenge for the organic optoelectronics. An abundant
litterature has identified the electrical issues and the optical issues that prevent the electrically
driven laser operation[iv,v].
From the electrical point of view, the main limitation with the organic semiconductors
remains the low carrier mobility of the organic films resulting in OLED current-densities in the
range 0.1A/cm2 to 1A/cm2[vi][vii] under DC excitation. However higher current density have
been reported under pulsed excitation [viii,ix]. By comparison, the current-densities required to
achieve the same excitation level than the lowest thresholds of the optically-pumped organic
laser experiment is estimated in the range 1kA/cm2 to 10 kA/cm2 [ref needed]. Among the
reasons that explains these high value of the current-densities, are several type of losses like the
thermal effects, the exciton-exciton annihilation (EE-A) and the polaronic absorption. Several
losses existing only under electrical excitation like the absorption in the metal electrodes [x, xi]
increase even more the laser threshold.
From the optical point of view, the main issue is the low refractive index of the organic
material (typically n~1.7), which results in a reflectivity at the air/organic interface of only
R=6%. The consequence is a low light confinement and, obviously an optical resonator medium
made of a higher index material has to be used to realize high quality-factor-laser-cavities
favorable to low-laser-threshold. As an example, Koschorreck et al. [xii] reported an optically
pumped organic laser cavity based on TiO2/SiO2 multilayer mirrors resulting in a quality factor
Q=4500.
The choice of the type of cavity is in fact limited by the necessity to achieve both a high
quality factor and an electrical excitation with a high current-density. The literature gives some
insight of the different types of microcavities compatible with an electrical excitation and their
respective current density. In 1996, in the context of color selectivity Dodabalpur et al. reported
OLEDs deposited on a quarter-wave stack of SiO2/Si3N4 with quantum efficiencies of about 0.2 0.7 % and injection current of about 0.25 mA [xiii]. These type of experiments were also done on
thermally oxidized silicon substrates[xiv]. Tokito et al. reported 1.5 to 5 intensity enhancement at
the resonance in cavity OLED made with SiO2/TiO2 dielectric bottom mirror and aluminum top
mirror [xv]. Noteworthy, the intensity enhancement observed in few experiments is favourable to
even further decrease the laser threshold. Top emitting OLEDs with 20 nm thick metallic mirrors
reported by Chen et al exhibit 1% quantum efficiency [and current density up to Chen JAP 101,
1113107, 2007]. Recently, Liu et al. [xvi] reported an OLED in a microcavity made of two quaterwavelength multilayer mirrors. The electrical excitation is achieved with a semitransparent
aluminum cathode and an ITO anode that plays also the role of a high index layer of the bottom
mirror. This microcavity OLED exhibits a Q=237 with a 860 mA/cm2 pulsed current density
which remains lower than the state of the art current density in OLED [xvii]. In spite of their
claim, it is not considered as a demonstration o an organic diode laser [xviii]. As a matter of fact,
these microcavity OLED experiments provide quality factors lower than the optically pumped
experiments (Q << 1000) and current densities lower than the state of the art OLED current
densities [xix]. We believe there is a room for a better design of the microcavity OLEDs with
enhanced electrical and optical performances. On this particular topic, only numerical study
exists, whereas systematical studies are compulsory to pave the way towards the first organic
laser diode demonstration[ref needed].
Justly, in a previous work[xx], we proposed the design of a high quality factor microcavity
OLED allowing a low-laser-threshold approach so as to go around some of the electrical and
optical issues. On the basis of a quantitative analysis of the laser threshold for a set of the
reported optically-pumped-organic-laser experiments [reference needed], we first extrapolated
that laser cavities with quality factors Q>10,000 may lead to a substantial threshold reduction
down to the level of the state of the art OLED pulsed current-densities. We also demonstrated
numerically that a quality factor Q>10,000 can be obtained with a quarter-wavelength multilayer
microcavity incorporating an OLED heterostructure with a transparent and conductive anode and
a thin absorbing metallic cathode [xxi].
The current work aims at validating experimentally the previously proposed design. More
precisely, at this step, the current challenge is to design a half-wavelength thick microcavity
while minimizing the impact of the OLED heterostructure thickness on the current density. A
second challenge is to minimize the aluminium cathode absorption without a drastic reduction of
the current density.
1. OLED and cavity structures
The microcavity considered in this work is shown on fig 1.a. It consists of glass substrate
covered with a bottom multilayer mirror, on which is deposited an OLED heterostructure ended
with an aluminum layer. The bottom mirror is a multilayer mirror made of 10 pairs of alternated
quarter-wavelength optically thick high index layer made of titanium oxide (TiO2) and low index
layer made of silicon oxide (SiO2). It is ended with an Indium Tin Oxide (ITO) transparent and
conductive layer playing both the role of a high index layer and of a transparent cathode. The
mirror reflectance is centered at 630 nm with a maximum reflectivity better than 99,5%, and with
a stop-band of 100 nm. After the mirror deposition, the substrates have been cleaned
ultrasonically by acetone and ethanol, and then rinsed with deionized water.
The OLED heterostructure layers were deposited in high vacuum (10 -7) system and it
consists (figure 1) of a 4,4’,4’’,tris-(3-methylphenylphenylamino) triphenylamine (m-MTDATA)
as hole injection layer (HIL) with a thickness varying from 10 nm to 60 nm, a 30 nm of N,N'diphenyl-N,N'-bis(1-naphthyl)-1,1'-biphenyl-4,4-diami (NPD) as a hole transport layer (HTL), a
30 nm 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) doped at 2
wt% into the Tris(8-hydroxyquinolinato) aluminum (Alq3) host material is used as the emitting
layer (EL) with a peak emission centered at 620 nm. The emitting layer is followed by a 10 nm
Bathocupuroine (BCP) as a hole blocking layer (HBL) and a 25 nm of Alq3 as an electron
transport layer (ETL). The organic layers were followed by a 1 nm of lithium fluoride (LiF) and
a 20 nm of aluminum (Al) layer as a cathode that was vapor deposited at the same background
pressure. Note that the energy transfer process between the DCM (guest) and Alq3 dipoles (host)
is very efficient (the characteristic transfer time is below 1ps) for two reasons: first of all, there is
a good overlapp between the Alq3 emission and the DCM absorption spectra. Second of all the 2
wt% of DCM concentration reduces the quenching due to the bimolecular annihilation reactions
and the intermolecular aggregation while maintaining an average spacing between the host/guest
dye molecules lower than the Forster radius.
In order to optimize the microcavity OLED heterostructure two design rules have been
applied. First of all, the OLED heterostructure optical thickness is to be a multiple of the halfwavelength, which leads to an OLED heterostructure thicker than the optimized OLEDs. For an
emission at 620 nm with an averaged refractive index n = 1,7 for the organic materials, the
required geometrical thickness should be of e = /2n = 183 nm. As we will see below, this
approximation neglects the penetration depth of the electric-field in the multilayer mirror. Due to
generally low conductivity and low carrier nobilities of organic materials, a delicate attention is
taken so that the thickness increase do not results in a dramatic reduction of the OLED currentdensity. To take this problem into account, only the m-MTDATA layer (HIL), and the Alq3
(ETL) thicknesses have been increased while the rest of the layers are kept constant. The choice
of the m-MTDATA is due to its hole motility (2*10-4 cm².V-1.S-1) larger than those other HIL
materials which limits the impact of an increase of the M-mtDATA thickness on the currentdensity and on the threshold voltage. Note [reference needed] that the m-MTDATA can form an
excellent amorphous film, without pinholes, leading to easier hole injection at the Anode/HIL
and HIL/HTL interfaces. The bulk trap levels of the m-MTDATA are closed to the HOMO level
which is favorable to the detrapping of the holes into the HOMO level and enhance the carrier
transport [xxii]. Moreover, the m-MTDATA has an intermediate HOMO level in between the NPD
HOMO level and the ITO work-function which facilitates the hole injection both at the ITO/HIL
and at the HIL/HTL interface. Moreover, the m-MTDATA has a low residual absorption at 620
nm [xxiii] which will reduce the losses at the resonance. For all these reason the m-MTDATA is an
interesting HIL material for the design of a optimized half-wavelength thick OLED
heterostructure.
A second design rule, consists in locating the emitting layer at an antinode so as to
maximize the coupling of the organic emitting dipoles with the field and to increase the light
emission. Given the diffusion length of the excitons, the active layer consists in the part of the
Alq3/DCM2 Guest-Host (G-H) system located between the HTL/EL interface and I the limite of
15 nm away from it. In order to increase the total heterostructure thickness while keeping the
active layer at the antinode, both the HIL and the ETL are increased. On the ETL side the Alq3 is
used to increase the OLED heterostructure thickness. In total, the OLED heterostructure
thickness is varied between 105 nm and 165 nm, the m-MTDATA layer being varied from 10 nm
to 70 nm in order to define experimentally the proper thickness for an emission at 620 nm.
The setup is completed with a home-made automatized acquisition system characterizing
electrically and optically the vertical microcavity OLEDs. The current-voltage curve (I-V) are
measured with a sourcemeter Keithley A2640. Spectra are simultaneously acquired using a
pigtailed spectrophotometer from Ocean Optics with a 8 nm resolution. The optical power
emitted by the OLED for the current-power characterization is measured in a 4 inch integrating
sphere with an optical power meter Newport 1930-C and corrected so as to take into account the
sensitivity difference of the calibrated photodiode on the entire OLED spectra.
Angular
measurements use a rotation stage on which the samples are mounted, and a fixed pigtailed fiber
is used to collect the light emitted by the OLED for different angles. Note that all the
measurements were performed in ambient atmosphere at room temperature.
3- Experimental results
Figure 2.a shows the OLED electroluminescent spectra emitted through the top-side
(through the aluminum mirror) for different OLED thickness, and the normalized intrinsic
emission of the Alq3:DCM. These spectra show a clear single peak with a Full Width at Half
Maximum (FWHM) of 14 nm centered at the resonance wavelength, which indicates a
significant linewidth narrowing compared to the reference OLED (FWHM = 80 nm). By
changing the thickness of the m-MTDATA from 10 nm to 70 nm which correspond to a overall
thickness of the OLED hetero-structures from 105 to 165 nm, the peak resonance is shifted from
582 nm to 662 nm. Note that a resonance at the maximum of the reference OLED
electroluminescence spectra (606 nm) is obtained with a 125nm thick OLED (m-MTDATA
thickness is 30 nm) whereas in first approximation a 183 nm distance between the two mirror is
expected. The 58 nm difference can be explained by the penetration of the optical field into the
mirrors.This value of the penetration depth is consistent with the 80 nm penetration depth into
20 nm thick metallic mirrors reported by Ma et alxxiv.
The resonance wavelength as a function of the OLED heterostructure thickness e is
plotted on fig 2.b. A linear regression (offset + .e ) indicates that the wavelength varies
linearly with the thickness e. The slope  e = 1.21 obtained experimentally is different
from what is expected from a half-wavelength microcavity. Indeed, the phase relationship at
normal incidence for a standing wave to exists in between the two mirrors is xxv:
m λ = 2.n.e+ (πbottom+ πtop ) Eq. 1.
where m is the order,  is the resonance wavelength of the standing wave inside the
microcavity ended by the two mirrors, e is the distance between the two mirrors, and bottom, top
are the penetration depth of the optical field in the dielectric (bottom) and the metallic (top)
mirrors. For m=1, a slope =2.n=3.4 is expected. Obviously, a higher order is to be expected and
the penetration depth in the mirror is large. Assuming the metallic mirror to induce a constant
penetration depth of the field top, and the dielectric mirror to induce a phase shift bottom the eq 1
rewrites :
λ (m−
ϕbottom
)= 2.n.e+ ptop Eq.2.
(2 π)
2.n ϕbottom
The order m is related to the the slope  by : m= γ + (2 π) Eq.3.
The first term gives 2.80, and since m is an integer, a possible solution is m=3 if
bottom = 0.4.
The m=3 order is confirmed by another experimental results. Indeed, the
electroluminescence spectra (fig.2.a) from the 165 nm device shows a second peak at 2=530 nm
which correspond to a 128 nm free spectral range (FSR) microcavity. Such a FSR is possible if
the distance between the mirrors is m+1=4 times the wavelength 2. In this case, the optical
thickness of the microcavity is too long, and a second modes of the cavity overlap with the
DCM:Alq3 free-space emission. Noteworthy, the resonances can be obtained on the entire
Alq3/DCM electroluminescence spectra including the tails of the DCM electroluminescence.
Indeed an emission at 575 nm has been observed whereas the electroluminescence of the
reference OLED at this wavelength is only 8% of the maximum intensity. Note also that, these
results indicate a quality factor Q = 43 for the cavity OLED instead of Q =7.5 for non cavity
OLED.
Based on the previous results, we proceed with the investigation using the 125 nm thick
micro-cavity OLED. Both the up-side and bottom-side electroluminescence spectra of a 125 nm
cavity are studied and compared to a reference OLED sample. Results are reported on figure 3.
All the spectra of this figure are normalized with respect to the bottom side emission of the
reference OLED. Fig 3.a shows, the bottom side emission of the non-cavity OLED, the
calculated transmittance and the measured bottom emission of the microcavity OLED
normalized to that of the reference OLED. The calculations are based on the matrix transfer
model. The complex refractive index used for the simulations have been measured by
ellipsomety system. The measured spectrum of the microcavity OLED exhibits two extra peaks
centered at 708 nm and 780 nm which correspond to the components existing in the far tail of the
reference OLED emission spectrum filtered by the microcavity. Since the top ad bottom mirrors
differ the top and bottom emission are different.
The calculated transmittance is in good
agreement with the experiment since the different peaks are centered on the same wavelength.
This clearly indicates that a pure filtering effect can explain the existence of the two extra peaks.
Figure 3.b shows the experimental data and the calculated emission of the structure
(cavity OLED) from the up-side. There is a good agreement between the calculated wavelength
resonance and the experimental one (λres = 606 nm). However the calculated spectrum is larger
(FWHM = 30 nm, Q = 20.2) than the measured spectrum (FWHM = 14 nm, Q = 7.5). This can
be explained by the absorption of the multiple reflected light by the organic layers.
In Fig.3.b, the top emission of the microcavity OLED is 15 times higher in amplitude
than the emission of the reference OLED with the same aluminum thickness. The comparison of
the sum of the bottom side emissions and the top side emissions of the reference OLED and of
the microcavity OLED is required to calculate the external efficiency enhancement (Gext). The
total forward emission (from two sides) of each OLED is presented in the Fig 3.c. This figure
shows that the light emitted from the microcavity OLED is a factor of 1.3 higher than that
emitted from the non cavity OLED at the resonance wavelength (606 nm). This amplitude
difference cannot be explained with a pure filtering effect. Obviously dipole’s spontaneous
emission rate has been modified. The latter effect, which is called Purcell effect, can alter the
radiative and nonradiative decay rate, resulting in a change of the internal emission efficiency
[Ref]. The enhancement of the spontaneous emission is favorable to a laser threshold reduction.
Figure 4 presents the electroluminescence spectra of the 125 nm thick microcavity OLED
as a function of the observation angle related to the normal incidence: 10°, 20°, 30°, 40°, 50° and
60°. The resonance wavelength appears to blue-shift continuously with the external observation
angle, which is related to the internal angle by the Snell’s law. This indicates that the short
wavelength light, favored at large solid angle is trapped in the device. The inset in figure 4
compares the polar radiation pattern of the reference OLED (solid line) and the microcavity
OLED (dashed line). It shows the highest brightness along the normal direction indicating an
improvement of the directivity of the microcavity-OLED emission. The reference device shows
emission pattern close to the Lambertian distribution.
Finally, let us consider the investigation of the maximum available current-density for
different values of the aluminum layer thickness. Figure 5.a, presents the voltage-current (V-I)
characteristics for the different OLED consisting of the same OLED hetero-structure as the 125
nm thick cavity OLED but made with different cathode thickness (5 nm, 10 nm 20 nm and 100
nm). The 100 nm thick aluminum layer allows a of 583 mA/cm2 and a threshold voltage of 7.2 V
(to inject 1 mA/cm²), whereas the OLED with the 20 nm (respectively 10 nm and 5 nm) thick
aluminum layer is limited to a 524 mA/cm2 (respectively 120 mA/cm2 and 1.47 mA/cm2)
current-density and a 7.2 V (respectively 15.5 V and 21.7 V) threshold voltage. Clearly, the
OLEDs with an aluminum cathode thickness between 10 nm and 5 nm restrict the currentdensity to very low values and an optimum is to be find between 10 nm and 20 nm.
On the inset of fig.5.b is presented the normalized top emission spectra of different
OLEDs with different thick aluminum layers. The 5 nm, 10 nm, and 20 nm of aluminum cathode
thickness lead to the spectra with a FWHM of 37 nm, 21 nm and 14 nm respectively. The
corresponding quality factors (Q = 17, Q = 30 and Q = 44, respectively) is increased with the
layer thickness as a result of a better reflectivity despite a larger absorption. In the context of an
external mirror to be added on top of the aluminum cathode, the reflectivity and the absorption of
the aluminum layer have to be minimized and therefore the optimization is achieved if the
thickness is decreased down to the limit of the desired current density.
4.
Conclusion
In the context of the organic laser diode we investigate the design and the optimization of
an OLED heterostructure in a uncompleted vertical microcavity. The latter consists of a bottom
multilayer
dielectric
mirror
on
which
is
deposited
an
MTDATA/NPD/Alq3:DCM/BCP/Alq3/Lif based OLED hetero-structure ended with
Ma
semitransparent aluminum cathode. Before the cathode is covered with a high reflectivity
multilayer top mirror, the heterostructure is investigated and two key parameters are studied: the
OLED hetero-structure thickness and the aluminum layer thickness. Our results indicate that the
emission wavelength change from 588 nm to 660 nm by varying the hetero-structure thickness
from 105 nm to 165 nm. An emission at 606 nm corresponding to the maximum of the DCM2
electroluminescence is achieved for a 125 nm-thick OLED hetero-structure. At this thickness, the
comparison with a reference OLED shows that the considered uncompleted-microcavity exhibits
a better directivity and an enhancement of the spontaneous emission by a factor of 1.3 at the
resonance. The metallic cathode induces a parasitic reflection and a microcavity effect that is
reduced with an aluminum layer less than 20 nm thick. However, the current-density is decreased
from 524 mA/cm2 down to 120 mA/cm2 and 1,5 mA/cm2 when the metallic cathode is reduced
from 20 nm to 10 nm and 5 nm.
As a conclusion, the 125nm thick OLED heterostructure with 30nm of m-MTDATA HIL,
and a 20nm thick aluminum cathode allowing an emission at the maximum of the Guest-Host
system 606nm , with more than 500mA/cm2 current density constitutes a basic trade-off. As a
perspectives, both the quality factor and the Purcell factor can be increased by adding a top high
reflectivity multilayer mirror on top of the aluminum mirror.
Il manqué des commentaries sur le trade-off et la suite de ce travail …
a)
LiF
OLED Hetero structure
λ/4
λ/4
Dielectric
Mirror
λ/4
λ/4
λ/4
λ/4
Glass substrate
b)
1.8
3.5
NPD
5
5.4
3.2
3.1
Alq3
4,7 ev
ITO
3. 2
DCM2
m-MTDATA
2.3
2.9 ev
LiF/Al
Alq3
BCP
5.2
5.7
5.7
6.7
Fig 1 : Structure of the uncompleted microcavity : a) Structure of the microcavity b) Energy
diagram of the OLED heterostructure.
El intensity (a.u.)
1,2
1,0
m_MTDATA thickness, whole OLED thickness:
Alq3:DCM normalized intrinsic emission
10 nm, 105 nm
30 nm, 125 nm
50 nm, 145 nm
70 nm, 165 nm
0,8
0,6
0,4
0,2
0,0
500
600
700
Wavelength (nm)
800
fig. 2 : a) Resonance wavelength for different OLED heterostructure thickness. The intrinsic
ALQ3:DCM emission is plotted for comparison. b) réintorduire la figure 2b avec la régression
linéaire ou au moins la mettre en inset.
1,4
a)
non cavity OLED
Cavity OLED; (exp)
Cavity OLED :
calculated
EL intensity (a.u)
1,2
1,0
0,8
0,6
0,4
0,2
0,0
El intensity (a.u)
1,2
1,0
b)
0,8
Non cavity OLED
Cavité OLED:
measured
Cavity OLED:
Calculated
Mirror transmittance
0,6
x20
0,4
0,2
EL intensity (a.u)
0,0
1,6
1,4
1,2
1,0
0,8
0,6
0,4
0,2
0,0
500
c)
Non Cavity OLED
Cavity OLED
600
700
Wavelength (nm)
800
Fig. 3. Emission of the uncomplete microcavity OLED, the spectra are normalized with respect
to the reference OLED bottom side emission : a) Top side emission. The light emitted through
the aluminum cathode is measured (square) and compared with the reference OLED spectrum
(circle). b) Emission from the bottom side : Reference (non-cavity) OLED normalized to 1
(square).measured EL of the cavity OLED (circle ) calculated emission of the cavity
OLED(square) experimental measurement of the bottom mirror transmittance (triangle).
EL Intensity (a.u)
50
-30
-15
0
-45
30
45
60
-60
40
15
-75
75
-90
90
Cavity OLED
Non Cavity OLED
30
20
x5
10
0
0°
10°
20°
30°
40°
50°
60°
500
600
Wavelength (nm)
700
Figure 4 : Emission spectra for different view angles. Inset is the emitting diagram for the
reference OLED (dashed line ) and the microcavity OLED (solid line) .
5 nm
10 nm
20 nm
100 nm
100
10
1
1,0
0,8
EL (a.u.)
Current density (mA/cm²)
1000
0,1
0,6
20 nm
10 nm
5 nm
0,4
0,2
0,0
0
10
20
Tension (V)
600 Wavelength (nm)
30
700
40
Fig. 5. Electrical and optical properties for different thickness of the aluminum cathode a):
OLED Current densities for different aluminum thickness : current-voltage characteristics for a
100nm thick aluminum cathode ( ) , 20nm ( ) , 10nm ( ). Inset shows the different top-sideemission spectra for 20nm (
). b)
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