(485226650) OLED 3

IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 50, NO. 2, MARCH/APRIL 2014 14591459 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 50, NO. 2, MARCH/APRIL 2014
O
OLED Electrical Equivalent Device
for Driver Topology Design
Abstract—In this paper, a hardware equivalent of an organic
light-emitting diode (OLED) was designedand investigated. This
substitution OLED device is based on a circuit-equivalent OLED
model and can be used to design andtest OLED dedicateddrivers.
Indeed, OLEDs are available on the market, but they are still
very expensive and hard to obtain. Compared to a real OLED,
the substitution device is cheap and robust and can be easily
duplicated. Moreover, a photodetector is not required to measure
the light output waveform. This can be simply done by measuring
a voltage across a resistance. This model can be used, for instance,
to simulate a large OLED panel made of several associated single
OLEDs for various series/parallel connection strategies. It can also
be used to simulate aging phenomena by changing the values of
some of its components. This might be useful forthe definition of
strategies to compensate aging effects likeluminous flux deprecia-
tion. Another advantage of such a device is its use forpowersupply
tests as it couldserve as a substitution load, at maximum deviation
from standard OLED electrical characteristics. We discuss the
theoretical model that was used as a basis for developing the
device. The accuracy of the model was then evaluated, particularly
in pulsewidth-modulation dimming conditions. Then, the hard-
ware equivalent device was compared to a real OLED. Finally, an
example of the potential use of this substitution device is given:
It was successfully used to investigate the “overdrive” technique
in order to increase OLED light output rise time. This technique
improves the light output rise time by a factor of over 4.
Index Terms—Bandwidth, dimming, drivers, electrical equiv-
alent model, lab-on-a-chip, Light Fidelity (Li-Fi), organic light-
emitting diode (LED) (OLED), overdrive, pulsewidth modulation
(PWM), rise time.
I. INT RO D U C T IO N
RGANIC light-emitting diodes (LEDs) (OLEDs) are
promising light sources as they can be thin uniform
light sources that can cover a large surface area. OLEDs are
Manuscript received March 14, 2013; revised May 18, 2013 and May 31,
2013; acceptedMay 31,2013. Date of publicationJuly 4,2013; date ofcurrent
version March 17, 2014. Paper 2012-ILDC-738.R2, presented at the 2012
International Symposium on the Science andTechnology of Lighting, Troy,
NY, USA, June 24–29,andapprovedforpublicationin theIEEE T RA N SAC -
TI O N S O N INDUSTRY APPLICAT I ONS by the Industrial Lightingand Display
Committee of the IEEE Industry Applications Society.
D. Buso, M. Ternisien, and C. Renaud are with the LAPLACE Laboratory,
University of Toulouse, 31062Toulouse, France(e-mail: david.buso@Laplace.
univ-tlse.fr; marc.ternisien@Laplace.univ-tlse.fr; cedric.renaud@Laplace.
univtlse.fr).
S. Bhosle is with OLISCIE, 31520 Ramonville, France (e-mail: sounil.
bhosle@oliscie.com).
Y. Liu and Y. Chen are with Fudan University, Shanghai 200433, China
(e-mail: ly@fudan.edu.cn; chen@fudan.edu.cn; yumingchen@fudan.edu.cn).
Color versions of one or more ofthefigures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier10.1109/TIA.2013.2272432
emerging on the lighting market and are about to achieve the
minimum performance required for commercial use.
However, these light sources have a very specific electrical
behavior. Their semiconductor nature makes their static current/
voltage characteristics similar to those of a LED. Additionally,
OLEDs consist of a large semiconductor area sandwiched be-
tween two electrodes. This architecture leads to a significant
capacitive behavior which makes the OLED electrical load
unique compared to all other light sources.
Both electrically and photometrically OLEDs have a specific
behavior which must be well understood to properly design
dedicated power supplies.
For example, it has previously been shown that the current
intensity affects the spectrum shape of white light OLEDs [1].
As a result, amplitude modulation (AM) dimming changes the
color point coordinates, which is not desirable in applications
where constant color is required.
On the other hand, pulsewidth modulation (PWM) dimming
does not affect the colorimetric behavior so much and is
therefore a preferred solution if color has to be maintained.
Nevertheless, PWM dimming also has disadvantages compared
to AM dimming. First of all, PWM dimming exhibits a lower
efficiency than AM dimming, and second, light output might
not fit with the input PWM shape. Indeed, as shown in the
following sections, due to the high capacitance of OLEDs and
their voltage source behavior, light can still be generated while
no current flows through the component.
As a result, due to their very specific electrical behavior,
OLEDs need dedicated drivers to be operated in accordance
with the constraints of a specific application. Even though a
few brands market OLEDs, they are still not mass produced
and therefore are still expensive and sometimes difficult to
purchase.
OLED behavioral modeling is therefore required to design
and test dedicated OLED drivers.
In this paper, an OLED electrical model was chosen fromthe
literature and implemented in a real circuit, called an OLED
hardware equivalent device. This substitution device can be
used to design and test OLED dedicated drivers. Moreover, it
gives instantaneous access to the light output waveformwithout
the use of a photodetector.This is simply done by measuring a
voltage across a resistance.
For general lighting applications, this model can be used,
for example, to simulate a large OLED panel made up of
different series/parallel OLED associations. It can also be used
to simulate aging phenomena by changing the values of some
of its components and, consequently, to develop strategies to
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Fig. 1. Typical OLEDarchitecture.
compensate aging effects such as luminous flux depreciation.
Anotheruse of this device could be to test power supplies with
substitution loads corresponding to maximum deviations from
nominal characteristics based on tolerances given by OLED
manufacturers.
For niche applications like “Light Fidelity” (Li-Fi), “visual
light communication” (VLC) [2], and “lab-on-a-chip” based
on OLED technology, this model can be used to design very
specific drivers that would improve important characteristics of
the light source such as light output bandwidth and/orrise time.
In the first section of this paper, we give a brief review of
electrical behavioral models. Then, a model matching our re-
quirements was selected,and a procedure to identify component
values is proposed. The theoretical model is tested in pulsed
mode, and its limitations are discussed.
In the second step, the OLED hardware equivalent device is
presented and compared to a real OLED.
Third, an example of the potential use of this substitution
device is given: It has been successfully used to investigate a
specific driver technique called overdrive that increases
OLED light output rise time. A comparison between the substi-
tution device and a real OLED is also performed.
II. THE O R E T I CA L ELE CT RI CA L EQU IVAL E N T MOD E L
A. OLED Model Selected
A typical OLED architecture is presented in Fig. 1. An
OLED is a stacked structure of thin organic layers sandwiched
between an anode, generally transparent (indium tin oxide),
and a metallic cathode. The electrodes are generally deposited
on a glass or plastic substrate. Each layer has a particular
role. The electron injection layer (EIL) and hole injection layer
(HIL) improve molecule–metal interface properties in order to
optimize charge carrier injection.
A hole transport layer (HTL) and an electron transport layer
(ETL) are generally inserted in order to improve charge carrier
transport. Finally, the emissive layer(s) is(are) located at the
center of the structure.
Froman electrical point of view, this structure can be consid-
ered as an equivalent circuit combining both ohmic resistances
and a capacitor. The physical origin of ohmic losses is mainly
due to contact resistances between organic layers,bulkconduc-
tion within organic layers, and electrode resistance. The origin
of the capacitive behavior is due to the stacked structure of the
organic layers.

Fig. 2. SimplifiedOLED electrical equivalent model.
Fig. 3. SelectedOLED equivalent electrical model.
The literature mentions different types of OLED electrical
equivalent models. In our case, as the model was to be imple-
mented in hardware, it consequently had to fulfill the following
requirements:
1) be as simple as possible to provide relevant electricaland
radiative properties of a real OLED, as a load for a power
supply;
2) be transposable to real devices such as diodes, resistors,
and capacitors.
The model selected had to offer the best compromise be-
tween simplicity and accuracy. We excluded the use of elec-
trical equivalent models [3]–[5] where all transport phenomena
within each layer are taken into consideration. This approach
would have led to a very complex network of RC series and
parallel branches. In addition, we also excluded the use of
simple small signal models [6] that work only around a single
operating point.
Large signal LED models [7], [8] are generally simple and
accurate. Based on the same approach, a large signal OLED
model can be found in [1]. It is presented in Fig. 2.
This simplified model comprises a series resistance Re rep-
resenting electrode ohmic losses, a capacitor, and the OLED
V = f (I) characteristic. The main advantage of this model is
its simplicity. However, when the diode is blocked, no steady-
state current can flow into the structure. However, at very low
polarization voltage (OLED off), there is still a measurable
current limited by a leakage resistance. This model is therefore
not suitable for situations where the OLED is disconnected
from its driver (pulsed current source for example). Indeed,
when disconnected, the voltage V in Fig. 2 would remain
constant, but actually, in an OLED,the voltage decreases slowly
with time. In order to take into account this additional time
constant, a resistance is placed in parallel to the capacitor,
which leads to the model presented in Fig. 3 [9].
In this electrical equivalent model, Rp represents the leakage
resistance due to charge injection into the structure when diode
D is OFF. In Fig. 3, the branch containing the diode of Fig. 2
is detailed. It comprises a voltage source Vt representing the
diode threshold voltage, D (a perfect diode preventing reverse
current), and Rs (a series variable resistance expressing the
exponential link between the static OLED current and voltage).

BUSO et al.: OLED ELECTRICAL EQUIVALENT DEVICE FOR DRIVER TOPOLOGY DESIGN 14611461 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 50, NO. 2, MARCH/APRIL 2014
The main advantages in using this model compared to others
are as follows.
1) It has the advantage ofsimplicity.
2) It is a large signal model.
3) Two electrical time constants are represented. When
diode D is on, the time constant to consideris determined
by Re and C (the order of magnitude is typically a few
microseconds). When diode D is off, the time constant
is determined by Rp and C (the order of magnitude is
typically around a second).
On the other hand, the main drawback of this model is its
accuracy. We show later in this paper that the model fails to
handle one of the electrical behaviors of the OLED, particularly
when it is driven by low-frequency current pulses.
Another issue with the model selected is the dependence
of parameters on temperature. Indeed, it has been previously
shown [10] that the static V (I) characteristic is temperature de-
pendent. Nevertheless, as OLEDs are large-area light sources,
their operating temperature is far lower than that of a LED. For
an OLED, the operating temperature is typically around 40 ◦
C,
while for a LED, the junction operating temperature can be
above 100 ◦
C. Moreover, the temperature dynamics for an op-
erating OLED cover the range between room temperature and
less than 50 ◦
C (where degradations start to occur), which limits
the impact of the temperature on the OLED electrical char-
acteristics. For example, a variation of ±10 ◦
C around 40 ◦
C
generally leads to a voltage variation between ±2.5% and ±5%
[10]. It has also been shown that curves of luminance versus
current do not depend much on temperature [1].
As a result, we deliberately chose to exclude temperature
effects from this work.
B. Model Parameter Identification
Parameter identification requires only two types of
measurements:
1) static regime measurement;
2) impedance analysis.
The static V (I) curve (V and I are the voltage across the
OLED and the current flowing through it, respectively) is used
to determine Rs and Vt and also to evaluate the order of
magnitude of Rp . Rp can be estimated by measuring the V (I)
slope below the threshold voltage (i.e., when diode D is off).
This slope is clearly visible when the V (I) curve is plotted on
a semilogarithmic scale as shown in Fig. 4. The V (I) static
curve on the linear scale for the considered OLED is presented
in Fig. 5.
To extract the nonlinear relationship between the current
flowing through the component and the voltage Vrs across Rs,
a curve-fitting procedure is applied to the Vrs (I) static curve.
If we consider that the current IL drained by Rp is negligible
compared to Is drained by Rs when diode D is conducting and
that C is an open branch in the static regime, we can express
Vrs , the voltage across Rs, with the following equation:
Vrs = V − Ve − Vt = V − Re I − Vt . (1)
Fig. 4. OLED static characteristic plotted in semilogarithmic scale. The
dottedlines showtheleakage conductivityandthe OLED thresholdvoltage.
Fig. 5. OLED staticcharacteristics plottedon a linear scale.
Vt, the threshold voltage, is extracted from the V (I) curve.
Diode D is considered on as soon as the current starts to
increase strongly. From Fig. 5, it can be seen that I and Vrs are
linked by an exponential relationship similarly to a classic LED.
The analytical expression of the fitting function is naturally an
exponential function of the following form:
I = A. exp(B .Vrs ) (2)
where A and B are the fitting constants.
The second type of measurement was performed with a
Solartron Modulab MTS impedance analyzer: An ac voltage
was superimposed on a bias voltage to the OLED. If the
maximum value of this signalis lower than the OLED threshold
voltage, then the diode in the equivalent circuit is blocked,
and its branch is neutralized. With the help of an identification
software tool, it is then possible to derive the values of Re and
C. Note that, as Rp is very high, its determination requires a
very low frequency that was not attainable with our equipm ent.
An example of the impedance and phase versus frequency is
shown in Fig. 6.

Fig. 6. OLED impedance andphase as a functionoffrequencyfor a polariza-
tion voltage of 20 mV and an ac amplitude of 10 mV.
TABLE I
PA RA METE R VA LU ES FO R DI FFEREN T BI A S VO LTAG ES
UN D ER TH E TH RESH O L D VO LTAG E
It can be seen that, for low frequencies, the OLED be-
haves like a pure capacitor with a −90◦
phase. As frequency
increases, the impedance decreases with an increasing phase.
When the phase crosses zero, the OLED is purely resistive,
and the electrode resistance at this point can be derived. For
higher frequencies, the phase becomes positive, indicating a
global inductive behavior. This inductive behavior is only due
to the inductance of the wiring and is not linked to the OLED
behavior itself. The equivalent inductance value derived from
measurements was typically few hundreds of nanohenries.
Table I shows the values of these parameters for an Osram
Orbeos CDW-031 commercial OLED, with an ac component
of 10 mV and different bias voltages below the diode threshold
voltage.
The results show that parameter values in this operating
mode do not depend on the applied bias voltage and can
be considered constant. As no charges are injected since the
bias voltage is under the threshold voltage, the capacitance
corresponds to the geometric capacity given by
C =
ε0 εr S
(3)
d
where ε0 is the vacuum permittivity, εr is the relative permit-
tivity of the active layer (3.5 for most organic materials [11]),
S is the OLED surface area, and d is the active layer thickness.
Note that, for a circular OLED, the capacitance is proportional
to the OLED radius squared.
When the bias voltage is above the OLED threshold voltage,
diode D in the equivalent circuit is on, and its branch is active.
In this regime, the current is high, and impedance measurement
was performed with the help of a booster current module
Fig. 7. OLED impedance and phase versus frequency for a polarization
voltage of4 Vandan ac amplitude of 10mV.
TABLE II
PA RA METE R VA LU ES FO R DI FFEREN T BI A S VO LTAG ES
ABOV E TH E TH RESH O L D VO LTAG E
Fig. 8. OLED equivalent capacitance versus bias voltage.
coupled to the impedance analyzer. An example of measure-
ments is given in Fig. 7.
Parameters were identified for several values of the polar-
ization voltage above the threshold voltage. The results are
summarized in Table II.
Unlike in the previous case, the parameters here are not con-
stant, except Re , which was kept as constant as possible during
the optimization process. Indeed, there is no reason that this
parameter should change as it represents electrode and contact
resistance. On the other hand, it can be observed that capaci-
tance values vary by more than a factor of 2 when the OLED
is on compared to measured capacitances when the OLED is
off. Fig. 8 shows the capacitance variation as a function of
bias voltage. Capacitance increases until a maximum value is
reached and then decreases sharply. This is in agreement with

TABLE III
EQU I VA LEN T MO D EL PA RA METERS
previous work [11]–[13]. Below 1.8 V, the OLED is off, and
the capacitance is the geometric one. At 1.8 V, majority charge
carriers start to be injected and accumulate within the structure
to form a space charge. The distance between the two charged
regions is therefore reduced, and the capacitance increases.
At around 2.5 V, when the capacitance is maximal, minority
charge carriers start to be injected into the structure. Holes and
electrons can then progressively recombine, and charges are
annihilated. Since there are fewer free charges as the voltage
increases,capacitance decreases.
In order to keep the model simple, in this work, we chose
to fixthe value of the capacitance. Some numerical simulations
in the dynamic regime, not reported in this paper, have shown
that the best compromise is obtained for a capacitance value of
4.5 μF (bias voltage = 3 V).
This choice to fix the capacitance is a limiting factor for
model accuracy. From a dynamic point of view, as this capac-
itance is bias voltage dependent, the circuit time constant is
also bias voltage dependent. This means that, for example, if
we consider a pulsed-current-driven OLED, switched on and
off periodically, the time constant is over- or underestimated
depending on the bias voltage. If we assume a rising current
edge and a 3-V bias voltage when the OLED is switched
on, this time constant will be overestimated, and the voltage
across the OLED equivalent device will increase slower than
the actual voltage during the transient.In contrast,if the OLED
is at the nominal operating point and a falling current edge is
considered, the time constant will be overestimated as long as
the bias voltage is above 3 V and underestimated when it is
below. Voltage decay will be slower than the actual one if it is
above 3 V and faster if it is below.
Parameter values for the OLED tested are presented in
Table III.
C. Model Accuracy
To check the accuracy of the OLED equivalent model in the
dynamic regime, it was implemented with the parameters given
earlier in PSIM software. Of course, the model can also be
implemented within any other circuit software.
The results obtained are compared to experiment: A pulsed
current source with variable duty cycle, variable frequency, and
variable current was used to drive an OsramOrbeos CDW-031.
A photodiode was used to measure OLED light output.
The left column of Fig. 9 shows a comparison between the
voltage measured across the OLED terminals and the calcu-
lated voltage for three different driving frequencies (1, 10, and
100 kHz), for a duty cycle of 50% and a current of 200 mA.
With the OLED light output being directly proportionalto Is
(the current flowing through the variable resistance branch), an
image of the light output can be obtained by measuring this cur-
rent. As the experimental setup was not calibrated in absolute
units, the light output and the current Is were normalized to 1
in order to compare the two waveforms. The results are shown
in the right column of Fig. 9 for the same frequencies as above.
The maximum delay observed between the normalized light
output and the normalized simulated current (graphs on the
right column) is 5 μs for 1-kHz pulses. This maximum delay
is only 1 μs at 10 kHz and is almost nonexistent at 100 kHz.
Considering the period for each frequency, these delays are
negligible.
If we now consider the OLED voltage (graphs on the left
column), at 100 kHz, the maximum deviation of the simulated
voltage is only 50 mV (1.5%) compared to the measured volt-
age (except for the switching voltage, the applied current has
a perfect waveform in simulation). At 10 kHz, on the current
rising edge, the simulated voltage is slightly delayed (maximum
delay is 2 μs) compared to the measured voltage. Once the
voltage is stabilized, simulated and measured voltages are in
perfect agreement. On the current falling edge, the simulated
voltage is also slightly delayed compared to the measured
voltage. After around 80 μs, the simulated voltage becomes
lower than the measured voltage.At the end of the period,there
is a 100-mVdifference between the measured and the simulated
voltage (3%). The model behavior is therefore acceptable at that
frequency.
At 1 kHz, on a current rising edge, the behavior is similar to
the 10-kHz case. The simulated voltage delay is around 10 μs.
On the falling edge, the agreement is correct until 550 μs, but
after, the simulated voltage becomes lower than the measured
voltage. This discrepancy increases with time.At the end of the
period, the voltage deviation is 300 mV (11%).
This behavior is in line with the comment made in
Section II-B concerning the dynamic behavior of the model and
the time constant which is bias voltage dependent.
At frequencies lower than 1 kHz, the model is therefore
less accurate and has to be used with care because voltage
simulation may lead to errors. On the other hand, it can be also
noticed that the divergence seen in the voltage at 1kHzdoes not
lead to a strong divergence in the light output waveform.Indeed
once the voltage is lower than around 2.8 V, Is is already very
low (see the static characteristic) and therefore has very little
impact on the light output.
III. HAR D WA R E ELE CT R IC A L EQU I VAL E N T MOD E L
From the theoretical electrical equivalent model, it is possible
to design a hardware equivalent OLED. The implementation of
the hardware equivalent model is presented in Fig. 10. Passive
components were chosen according to values given in Table III.
To simulate the branch composed of Vt, the perfect diode, and
the series variable resistance (see Fig. 3), a LED associated to
Schottky diodes and a resistance were used.The LEDwas used
to reproduce the nonlinear shape of the V (I) characteristic.
The Schottky diode was used to adjust the threshold voltage,
and the series resistance was used to adjust the V (I) charac-
teristic slope. The current through this branch can be simply
determined by measuring the voltage VI across R. This voltage
is then an image of the OLED light output. Fig. 11 shows a
comparison between the OLED supplied with a current source
delivering pulses at 200 mA, 10 kHz, and 50% duty cycle

Fig. 9. Left column: Simulated voltage (red) and measured voltage (black) as a function of time for (blue) current pulses at different frequencies (from top
to bottom,1, 10, and100kHz). Right column: (Red) Normalizedcurrent through Rs and(black) measuredandnormalized OLED light output for operating
conditions corresponding to the graph on the left on the same row.
Fig. 10. Schematic of thehardwareequivalent model implemented.
and the hardware equivalent supplied with the same operating
conditions.
A good agreement can be seen between the hardware equiv-
alent circuit and the OLED electrical characteristics. As dis-
cussed before, discrepancies come from parameters like the
capacitor that is voltage dependent in a real OLED but kept
constant in this hardware equivalent device. We can also note
that the waveform of VI is similar to the OLED light output
waveform.

Fig. 11. Left: (Blue) Total voltage, (orange) VI, and(violet) appliedcurrent tothe OLED equivalent hardware. Right: (Blue)Total voltage,(orange) light output
measuredwith a photodiode, and(violet)appliedcurrent tothe modeledOLED.
Fig. 13. OLED overdrive: Operationprinciple.
Fig. 12. OLED light output waveform versus normalized period(current pulse
I = 200 mA andα = 50%).
IV. APPL IC AT ION TO IMPRO V E M E N T OF
OLED LIGH T OUT PU T RISE TIM E
OLED light output cannot be modulated as easily as it can
be with a LED. Indeed, the OLED internal capacitance, due
to very low charge carrier mobility and long exciton lifetime,
forms a low-pass filter with a relatively low cutoff frequency
(typically some tenths of a kilohertz compared to tenths of a
megahertz for LED). One consequence of this low-pass filter
behavior is the poor dynamics of light output.Fig.12shows the
light output measured on an Osram Orbeos CDW-031 OLED
driven by a pulsed current source at 200 mA with 50% duty
cycle for different frequencies. The period was normalized in
order to see how the light output waveform varies with signal
frequency. The light output signal was measured with a large
bandwidth photodiode placed in front of the OLED.
This poor dynamic behavior might limit the use of OLEDs in
certain applications. We can, for example, mention applications
like wireless information transmission using white light (Li-Fi
and VLC). In this kind of emerging application, light generated
by a light source used for general lighting is modulated and
sensed by a detector. With this technique, the information
transfer rate is mainly limited by the bandwidth of the light
source. Due to their large bandwidth and the fact that they
are easy to control, LEDs are very good candidates for this
kind of application. Some examples showing the feasibility and
expected performance of this kind of application using LED
light sources can be found in the literature [2], [14], [15].
Nevertheless, OLEDs are also attractive candidates because
they are expected to be intensively used in general lighting
in the near future. They therefore present a huge potential to
become a strong vector of wireless information transmission
by white light, in spite of their lower bandwidth compared
to LEDs. Although some works already report OLED-based
systems [16], [17], techniques to improve OLED bandwidth are
in progress [18], [19].
Another application where light output bandwidth is also
very important is “lab-on-a-chip” devices.Indeed some of these
devices use light to detect substances or particles (bacteria for
example) in a liquid. The principle is to excite the target with
a given wavelength and detect its fluorescence or phosphores-
cence which can be at a different wavelength compared to the
excitation source wavelength.In this case,there is no detection
problem. The radiation can also overlap the OLED spectrum ,
and here, the only way to detect it is a temporal dissociation
which therefore requires a large bandwidth OLED light source.
The model hardware device developed was used in this
context to evaluate the possibility to increase OLED light
output rise time with the overdrive technique. This technique,

Fig. 14. Top left: Hardware equivalent model with Io = 120 mA. Top right: OLED with Io = 120 mA. Bottomleft: Hardware equivalent model with Io =
300 mA. Bottom right: OLED withIo = 300 mA. Curve colors are as definedin thelegendof Fig. 11.
widely used in liquid crystal displays to increase pixelresponse
time, consists in charging the intrinsic capacitor of the OLED
faster by applying a high current during a short period of time
(overdrive pulse Io ) before returning to the nominal current
during the remainder of the period (main pulse Im ).
To evaluate the contribution of this technique, two syn-
chronized current sources were used. The first one provided
a main current pulse, and the second one delivered a short
pulse (overdrive pulse) superimposed on the main pulse. The
principle of this technique is presented in Fig. 13.
To generate the currents presented in Fig. 13, commercial
LED drivers (LT3517 in buck–boost mode) were used. These
drivers were connected in parallel and controlled by two syn-
chronized PWM signals generated by a microcontroller (FTDI
VNC2). The overdrive pulse duty cycle αs = t1 /T and main
pulse duty cycle αm = t2 /T were software controlled and can
be set between 0 and 1. Currents Io and Im can be indepen-
dently set between 0 and 450 mA by applying a dc control
voltage to both drivers.
Fig. 11 shows the initial situation for both the hardware
electrical equivalent and a real OLED when no overdrive pulse
is applied.
Fig. 14 shows two examples of results obtained for the same
operating conditions as presented in Fig. 11 but for overdrive
pulses of 120 mA (peak of 320 mA) and 300 mA (peak of
500 mA).
When no overdrive pulses are applied over the main pulse,
we can see from Fig. 11 that the light output rise time (i.e.,
voltage across R rise time) is around 13 μs. From Fig. 14, it
can be seen that, as the overdrive pulse amplitude increases,
the light output rise time decreases (around 10 μs at a 120-mA
overdrive pulse and around 3 μs at a 300-mA overdrive pulse).
These measurements demonstrate the ability of this method to
shorten the light output rise time. The light output rise time in
our work has been increased by a factor greater than 4.
Moreover, the OLED light output waveform (orange curve)
is well reproduced by the waveform of VI . This voltage can be
considered as a good indicator of the total light output emitted
by the OLED. A good agreement can also be found between the
measured voltage across the hardware equivalent circuit (blue
curve) and the OLED.
According to the application considered, lifetime can be a
critical issue or not. Lifetime is critical, for example, for Li-Fi
applications, whereas it is not for single-use “lab-on-a-chip”

devices. Whatever the application considered, as the overdrive technique requires a strong initial current pulse, it may affect the
device lifetime.
The data sheet of the OLED used [20] indicates a maximum continuous admissible current of 400 mA and a nominal current
of 186 mA. In our experiment, the OLED was driven with 200 mA, and the maximum overdrive pulse tested was
300 mA (peak intensity of 500 mA). No data are available concerning the impact of such driver methods on OLED lifetime.
Nevertheless, we can discuss lifetime issues and make some assumptions. The total current undergoes a transitory split into a
conduction (OLED branch) and a displacement current (capacitor branch). The main factors affecting OLED lifetime are
temperature and electric field strength. Temperature depends on the average conduction current and may shorten lifetime if it is too
high; field strength depends on the instantaneous voltage across the component and may lead to OLED breakdown.
The conduction current can be directly measured on the hardware electrical equivalent (orange curves on the right in Fig. 14).
The maximum overdrive current spike (see Fig. 14) was set to avoid conduction current overshoot. In these conditions, it can be
reasonably assumed that overdrive current pulse has only a slight or even no influence on OLED lifetime. On the other hand,
overdrive current induces an overvoltage across the OLED. This overvoltage can be seen in the blue curves of Fig. 14 when the
overdrive pulse is applied. If this voltage is too high, it can lead to OLED breakdown due to too high an electric field across the
organic layers and interfaces. Several tests, not reported here, with overdrive current up to 800 mA did not lead to any
device breakdown. We can therefore also assume that, in this case, overvoltage does not have a strong influence on lifetime.
V. CONC L U S IO N
An OLED electrical equivalent device has been proposed. It is able to quite accurately reproduce the static and dynamic
behavior of a real OLED. Nevertheless, discrepancies can be observed more particularly during transients when the OLED is
driven by current pulses at frequencies below 1 kHz. It was shown that these discrepancies are due to the choice of a fixed
capacitance value. More accurate results can be obtained using a capacitor value that depends on the polarization voltage.
This OLED equivalent device can be used as a tool to develop OLED drivers according to application-specific requirements. It is
cheap and robust and can be easily reproduced. It gives indi- rect access to the light output waveform simply by measuring the
voltage across a resistance.
The device was used to evaluate the so-called overdrive technique. It was shown that, with this technique, it is possible to
increase OLED light output rise time in the pulsed regime by a factor of over 4.
ACK N O W L E D G M E N T
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