(485226650) oled 3

IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 50, NO. 2, MARCH/APRIL 2014 1459 1459 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 designed and investigated. This substitution OLED device is based on a circuit-equivalent OLED

model and can be used to design and test OLED dedicated drivers. 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 for the definition of strategies to compensate aging effects like luminous flux deprecia-

tion. Another advantage of such a device is its use for power supply tests as it could serve 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 equivalent 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 I O 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; accepted May 31, 2013. Date of publication July 4, 2013; date of current version March 17, 2014. Paper 2012-ILDC-738.R2, presented at the 2012 International Symposium on the Science and Technology of Lighting, Troy, NY, USA, June 24–29, and approved for publication in the IEEE T RA N SAC - T I O N S O N I NDUST RY APPL ICAT I ONS by the Industrial Lighting and Display Committee of the IEEE Industry Applications Society.

D. Buso, M. Ternisien, and C. Renaud are with the LAPLACE Laboratory, University of Toulouse, 31062 Toulouse, France (e-mail: david.buso@Laplace. univ-tlse.fr; [email protected]; cedric.renaud@Laplace. univtlse.fr).

S. Bhosle is with OLISCIE, 31520 Ramonville, France (e-mail: sounil. [email protected]).

Y. Liu and Y. Chen are with Fudan University, Shanghai 200433, China (e-mail: [email protected]; [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/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 from the

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 waveform without

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 OLED architecture.

compensate aging effects such as luminous flux depreciation.

Another use 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), “visua l

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/or rise time.

In the first section of this paper, we give a brief review of

electrical behavioral models. Then, a model matching our re-

quire m e n ts was selected, and a procedure to identify comp on e nt

values is proposed. The theoretical model is tested in pulse d

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 C A L ELE C T R I C A L EQ U I VAL 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.

From an 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, bulk conduc-

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. Simplified OLED electrical equivalent model.

Fig. 3. Selected OLED 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 electrical and

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 1461 1461 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 of simplicity.

2) It is a large signal model.

3) Two electrical time constants are represented. When

diode D is on, the time constant to consider is 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 Identi fic at io n

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 dotted lines show the leakage conductivity and the OLED threshold voltage.

Fig. 5. OLED static characteristics plotted on 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 .V rs ) (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 volta ge

was superimposed on a bias voltage to the OLED. If the

maximum value of this signal is lower than the OLED thresho ld

voltage, then the diode in the equivalent circuit is blocke d ,

and its branch is neutralized. With the help of an ident i fic at ion

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 en t .

An example of the impedance and phase versus frequency is

shown in Fig. 6.


Fig. 6. OLED impedance and phase as a function of frequency for a polariza-

tion voltage of 20 mV and an ac amplitude of 10 mV.

TABLE I

PA RA ME T E R VA L U E S F O R DI FFE RE N T BI A S VO LTAG E S

UN D E R T H E TH RE SH 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 of 4 V and an ac amplitude of 10 mV.

TABLE II

PA RA ME T E R VA L U E S F O R DI FFE RE N T BI A S VO LTAG E S

ABOV E T H E TH RE SH 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 L E N T MO D E L PA RA ME T E RS

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 fix the 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 Osram Orbeos 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 proportional to 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-mV difference 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 1 kHz does 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. HA R D W A R E ELE C T R I C A L EQ U 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 LED was 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, and 100 kHz). Right column: (Red) Normalized current through Rs and (black) measured and normalized OLED light output for operating conditions corresponding to the graph on the left on the same row.

Fig. 10. Schematic of the hardware equivalent 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) applied current to the OLED equivalent hardware. Right: (Blue) Total voltage, (orange) light output

measured with a photodiode, and (violet) applied current to the modeled OLED.

Fig. 13. OLED overdrive: Operation principle.

Fig. 12. OLED light output waveform versus normalized period (current pulse I = 200 mA and α = 50%).

IV. AP P L I C A T IO N TO IMP R O V E M E N T OF

OL E D LIGH T OUT P U T R ISE 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. 12 shows 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-o n- a -c hip ” 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 phosph or e s-

cence which can be at a different wavelength compared to the

excitation source wavelength. In this case, there is no detec t ion

problem. The radiation can also overlap the OLED spect r um ,

and here, the only way to detect it is a temporal dissoc iat io n

which therefore requires a large bandwidth OLED light sourc e .

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 techn iqu e ,


Fig. 14. Top left: Hardware equivalent model with Io = 120 mA. Top right: OLED with Io = 120 mA. Bottom left: Hardware equivalent model with Io = 300 mA. Bottom right: OLED with Io = 300 mA. Curve colors are as defined in the legend of Fig. 11.

widely used in liquid crystal displays to increase pixel response

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 I O 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.

AC K N O W L E D G M E N T

REFE R E N C E S

[1] J. Jacobs, D. Hente, and E. Waffenschm idt, “Drivers for OLEDs,” in Conf. Rec. IEEE IAS Annu. Meeting, 2007, pp. 1147–1152.

[2] D. O’Brien, H. L. Minh, L. Zeng, G. Faulkner, K. Lee, D. Jung, Y. Oh, and E. T. Won, “Indoor visible light communications: challenges and prospects,” in Proc. SPIE, 2008, vol. 7091, pp. 709106-1–709106-9.

[3] J. Drechsel, M. Pfeiffer, X. Zhou, A. Nollau, and K. Leo, “Organic Mip-diodes by p-doping of amorphous wide-gap semiconductors: CV and impedance spectroscopy,” Synthetic Metals, vol. 127, no. 1–3, pp. 201–205, Mar. 2002.

[4] S. Nowy, W. Ren, A. Elschner, W. Lövenich, and W. Brütting, “Impedance spectroscopy as a probe for the degradation of organic light emitting diodes,” J. Appl. Phys., vol. 107, no. 5, pp. 054501- 1–0 5450 1- 9, Mar. 2010.

[5] H. Park, H. Kim, S. K. Dhungel, and J. Yi, “Impedance spectroscopy analysis of organic light-emitting diodes fabricated on plasma-treated indium-tin-oxide surfaces,” J. Korean Phys. Soc., vol. 51, no. 3, pp. 1011– 1015, Sep. 2007.

[6] J. Ahn, D. Chung, and J. Lee, “Equivalen t-c irc uit analysis of organic light- emitting diodes by using the frequency-dependent response of an ITO/ Alq3/Al device,” J. Korean Phys. Soc., vol. 46, no. 2, pp. 546–550, Feb. 2005.

[7] W. N. Cheung, P. J. Edwards, and G. N. French, “Determination of LED equivalent circuits using network analyser measurements,” in Proc. Opto- electron. Microelectron. Mater. Dev., 1998, pp. 232–235.

[8] R. L. Lin and Y. F. Chen, “Equivalent circuit model of light-emitting- diode for system analyses of lighting drivers,” in Conf. Rec. IEEE IAS Annu. Meeting, 2009, pp. 1–5.

[9] C. Pinot, “Modélisation électrique des diodes électroluminescentes or- ganiques multicouches dopées. Application à De Nouvelles Architec- tures,” Ph.D. dissertation, Ecole Polytechnique, Palaiseau, France, 2008.

[10] A. Poppe, L. Pohl, E. Kollár, Z. Kohári, H. Lifka, and C. Tanase, “Method- ology for thermal and electrical characterization of large area OLEDs,” in Proc. 25th Annu. IEEE SEMI-THERM, 2009, pp. 38–44.

[11] S. Nowy, W. Ren, J. Wagner, J. A. Weber, and W. Brutting, “Impedance spectroscopy of organic hetero-layer OLEDs as a probe for charge carrier injection and device degradation,” in Proc. SPIE, Organic Light Emitting Materials Devices XIII , Aug. 27, 2009, vol. 7415, p. 74 150G.

[12] M. Tsai, T. C. Chang, P. Liu, C. W. Ko, C. J. Chen, and K. M. Lo, “Short- diode like diffusion capacitance of organic light emission devices,” Thin Solid Films, vol. 498, no. 1/2, pp. 244–248, Mar. 2006.

[13] V. Shrotriya and Y. Yang, “Capacitance-voltage characterization of poly- mer light-emitting diodes,” J. Appl. Phys., vol. 97, no. 5, p. 054504, Mar. 2005.

[14] M. H. Le, D. O’Brien, G. Faulkner, L. Zeng, K. Lee, D. Jung, Y. Oh, and E. T. Won, “100-Mb/s NRZ visible light communications using a postequalized white LED,” IEEE Photon. Technol. Lett., vol. 21, no. 15, pp. 1063–1065, Aug. 1, 2009.

[15] A. H. Azhar, T.-A. Tran, and D. OBrien, “A gigabit/s indoor wireless transmission using MIMO-OFDM visible-light communications,” IEEE Photon. Technol. Lett., vol. 25, no. 2, pp. 171–174, Jan. 2013.

[16] J. W. Park, D. C. Shin, and S. H. Park, “Large-area OLED lightings and their applications,” Semicond. Sci. Technol., vol. 26, no. 3, pp. 034002-1– 034002-9, Mar. 2011.

[17] Z. Ghassemlooy, “OLED-based visible light communications,” in Proc. IEEE Photon. Soc. Summer Top. Meet. Ser., 2012, pp. 102–104.

[18] J. Park, “Speedup of dynamic response of organic light-emitting diodes,” J. Lightw. Technol., vol. 28, no. 19, pp. 2873–2880, Oct. 2010.

[19] H. L. Minh, “Equalization for organic light emitting diodes in visible light communications,” in Proc. IEEE GLOBECOM Workshops, 2011, pp. 828–832.

[20] ORBEOS CDW-031 datasheet. [Online]. Available: http://www.osram. com/

http://www.osram/

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