Dynamic Control Point Simulation of OLEDs - IEEE Xplore

Dynamic Control Point Simulation of OLEDs Joep Jacobs, Carsten Singer, Dirk Hente and Hans-Peter Loebl Philips / Research Aachen, Germany [email protected]

Abstract—Organic light emitting diodes (OLEDs) for lighting applications are still in a research phase, but improvements are made continuously. In the last years, colour variable OLEDs have been reported by research. For the design of drivers and control systems for colour variable OLEDs, it is important to have accurate models to reduce the development costs of these components. In this work, a simulation method is proposed. Although the model uses statically measured input data, it is possible to model dynamic waveforms very accurately. This work gives an overview of colour variable OLED characteristics. The proposed model is presented and its results are compared to experimental results. Organic light source; OLED; stacked OLED; colour control; driver; dynamic contro; AM; PWM

I. INTRODUCTION Organic light emitting diodes (OLEDs) have the potential to become an important future light source. Examples of OLEDs are depicted in Figure 1. The depicted OLEDs have an active area of 50 cm2 each. Similar to inorganic LEDs, OLEDs have to be supplied by a DC current [1]. Hence, a driver is required. The current through the OLED, i.e. the output current of the driver, may have different forms, such as a pure DC, a DC with ripple (sinusoidal, triangular, etc.) or a pulse width modulated current. This current determines to a large extend the light output of the OLED. In applications with colour control, the current also determines the colour point or correlated colour temperature.

Figure 1: Examples of OLEDs, left: white OLEDs, right: coloured OLEDs [source: Philips Research]

In the presented study, a simulation model for colour variable OLEDs has been developed. This model uses input data obtained from stationary measurements. Although the input data are taken from stationary measurements, it is shown that the model is also able to simulate dynamic or transient waveforms very accurately. This work starts with a general overview of colour variable OLEDs and their characteristics. Examples of typical stationary characterisation measurements are given. In the following section, the model is described in detail. Lastly, the simulation results are compared to experimental results. II.

COLOUR VARIABLE OLEDS

OLEDs have many advantages compared to other light sources. They are thin, flat and light weighted large area light sources, which can be made bendable and flexible. This form factor makes them very nice to look at, especially compared to their inorganic sisters, which are point sources. Furthermore, the off-state appearance is freely selectable (black, silver, transparent, …), the 2D-shape of the OLED can be freely chosen as well as the colour of the light. This light output can be fully dimmed. Above all, the main advantages are the cheap production and the high efficiency (40 lm/W now, 50-75 lm/W in 2010). Additionally, the low-voltage technology enables a safe operation and requires no extra isolation. All used materials are fully recyclable. In the last decade, monochrome coloured and white OLEDs have been reported [2] (cf. Figure 1), even colour variable OLEDs have been reported [3, 4]. Two examples are depicted in Figure 2 to Figure 4. The first example is a so called striped OLED. The difference with standard monochrome OLEDs is that stripes of two different colours, e.g. yellow and blue, are deposited adjacently on a substrate. In addition, a glass for mixing of the light and a scattering foil are used to obtain a homogeneous colour. The combination of yellow and blue light results in white light. A picture of such an OLED device is depicted in Figure 3. With such an arrangement all colour points between the blue and the yellow colour point are possible. The second example shows a so-called stacked OLED. Multiple OLEDs are stacked on top of each other and are separated by so-called charge generation (CG) layers. These charge generation layers can also be contacted. In the example, this results in a total of 4 electrodes for an RGB stacked OLED. Important advantages are the thin overall structure and homogeneity of the OLED. Major drawbacks are creation of the electrodes in the middle and the driver complexity.

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Figure 4: Schematic of a stacked colour tunable OLED (top) and its equivalent electric circuit (bottom) Figure 2: Schematic of a blue-yellow-white striped OLED: top view (top), side view (middle) and equivalent electric circuit (bottom)

Other techniques to obtain a colour tunable OLED are the use of voltage dependant characteristics or the use of a combination of standard OLED with transparent OLEDs. For details on colour variable OLED it is referred to literature, e.g. [2-5]. III.

OLED CHARACTERISATION

In most cases, two measurements are performed to characterise the OLEDs after construction. In a first measurement current I, voltage V and luminance L are measured for different points of operation. This IVLmeasurement is a stationary measurement, because a DC current or voltage is applied and the other two parameters are monitored. An example is depicted in Figure 5 and Figure 6. Figure 5 shows the measurement results as obtained for 16 quasi-identical OLEDs. The IL-characteristic can be described with a simple linear equation. A mathematical description of the 16 measurements is given in Figure 6. L = m⋅I

Figure 3: Pictures of a blue-yellow-white striped OLED, on the left-hand side, the scattering foil has been removed [source: Philips Research]

with m =

L I

In the second measurement, the spectrum of the light is measured at different points of operation. An example for a white OLED is depicted in Figure 7. From these spectra, the correlated colour temperature CCT and the colour points can be calculated. For the example depicted in Figure 7, both are given in Figure 8.

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Figure 5: Measured luminance as a function of current for sixteen OLEDs

Figure 8: Chromaticity diagram 1931 with the indicated colour points

IV.

SIMULATION MODEL

For the design of OLED drivers and control systems, accurate models of the OLED are required to reduce the development costs of these components. The measured data presented in the previous section can be implemented in the simulation environment. However, the measured data are obtained from stationary measurements, whereas the simulation with for example PSpice or MatlabTM are dynamic simulations. Question is if the stationary data can be used. To this end, a model has been created to investigate this question.

Figure 6: Mathematical description of the measured data prersented in the previous figure, red: measurements, blue: mathematical description

The model is implemented in MatlabTM. In a first step, the data obtained with the two characterisation measurements are read by the MatlabTM program. Secondly, the current waveforms that have to be investigated need to be defined. This can be done in an analytical way or by using an electrical circuit entered in the Simulink or PLECS environments. With these currents, an amplitude modulate (AM) current driving signal can be for example compared to a pulse width modulated (PWM) signal. The luminance is calculated for every sample. For the calculation of the luminance L, the linear relationship between current and luminance is used. Afterwards, the sum of the luminance calculated for every sample n over an entire period T is accumulated and divided by the number of the samples N. The obtained value is the luminance one perceives. N

L=

N

¦ I (n) ⋅ m ¦ L(n) n =1

=

N

n =1

N

The colour values and the CCT are calculated in the same manner. The derived values (x, y, CCT) are weighted with the current through the OLED i. N

x= Figure 7: Spectrum of the OLED at different points of operation

¦ x (n ) ⋅ n =1

i(n ) i

N

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Figure 9: Simulation current with exponential discharge current N

y=

¦ y (n ) ⋅ n =1

N N

CCT =

i(n ) i

¦ CCT (n) ⋅ n=1

N

Figure 10: Simulated (o) and measured (*) colour point using PWM-dimming

i(n) i

Additionally, the electrical characteristics of the OLED have to be taken into account, in particular the internal capacitance. This can be done by using a simulation model to obtain the internal OLED current contributing to the light output [1] or by using different analytical models. An example is depicted in Figure 9. The quasi exponential slope, due to the discharge of the internal OLED capacitance [1], is added to the PWM current as shown in the figure. The resulting simulation results as well as experimental results for verification are given in Figure 10. The model is already quite accurate as can be seen from the table on the bottom of this page. Similar results are presented in Figure 11 for AM dimming. The larger deviations using this dimming technique are caused

Figure 11: Simulated (o) and measured (*) colour point using AM-dimming

by aging of the OLED. Measuring the test device again after the measurements confirms this assumption. Figure 12 shows Table 1: Comparison between measurement and PWM simulation

Reference (measured)

Target (simulated)

xref

yref

xtgt

ytgt

ǻx / %

ǻy / %

PWM 10% PWM 12.5% PWM 25% PWM 50% PWM 75% PWM 80% PWM 90% PWM 100%

PWM 10% PWM 12.5% PWM 25% PWM 50% PWM 75% PWM 80% PWM 90% PWM 100%

0.2841 0.2953 0.3121 0.3236 0.3275 0.3283 0.3361 0.3327

0.3167 0.3227 0.3318 0.3381 0.3403 0.3411 0.3462 0.3432

0.2911 0.2932 0.3020 0.3138 0.3208 0.3218 0.3233 0.3238

0.3169 0.3182 0.3234 0.3307 0.3353 0.3359 0.3369 0.3373

2.5 -0.7 -3.2 -3.0 -2.0 -2.0 -3.8 -2.7

0.1 -1.4 -2.5 -2.2 -1.5 -1.5 -2.7 -1.7

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the deviation between the colour points of the test OLED before the measurements (black asterisk) and after the measurements (red circles). One can see that the colour points differ from each other. The average deviation is 1.56 % for the x-coordinate and 0.89 % for the y-coordinate. These deviations might not be the only reason for the difference between the measurements and the simulation, but it is one important factor that leads to an inaccurate simulation. Although the simulation results differ from the measurement one can see a tendency for the change in colour if using different dimming methods. If one uses AM-dimming, between a ten percent dimming and the undimmed state, the colour difference ǻE is nearly three times higher than for an OLED device dimmed in the same range with PWM.

V. CONCLUSIONS A simple simulation tool has been created to simulate dynamic cases using data from stationary characterisation measurements. At first, an IVL-measurement is made, which gives data about the relationship between current and luminance. With the spectral characterisation one receives the relationship between current and colour coordinates and CCT. If these data are available for an OLED device, it is possible to simulate the colour point, luminance and CCT in dependency of the driving current. The simulation software is verified with measurements. The deviation between the simulated colour coordinate results and the measurement data for AM- and PWM-dimming is smaller than 4 %. ACKNOWLEDGEMENTS The authors would like to thank the German Federal Ministry of Education and Research “Bundesministerium für Bildung und Forschung (BMBF)” for supporting this work under the “OPAL” Program. REFERENCES [1]

[2] [3]

[4] [5]

J. Jacobs, D. Hente, E. WaffenSchmidt: "Drivers for OLEDs", Conference record of the 2007 IEEE 42nd IAS annual meeting, pp. 11471152, 2007 http://www.research.philips.com/technologies/intsol/oled/index.html P. Burrows, G. Gu, V. Bulovic, Z. Shen, S. Forrest, M. Thompson: "Achieving full-color organic light-emitting devices for lightweight, flatpanel displays", IEEE Transactions on Electron Devices, Volume 44, Issue 8, Page(s):1188 – 1203, August 1997 P. Burrows, S. Forrest: "Color-tunable organic light-emitting devices", Applied Physics letters 69 (20), 11 November 1996 P. Destruel, G. Ablart, P. Jolinat, I. Seguy, J. Farenc: "White organic light-emitting diodes (WOLEDs)", Conference record of the 2006 IEEE 41nd IAS annual meeting, pp. 694-697, 2006

Figure 12: Colour points before the measurements (*) and after the measurements (o)

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