Drivers for OLEDs Joep Jacobs, Dirk Hente, Eberhard Waffenschmidt Philips Research - SSL Aachen, Germany
[email protected] Abstract— Organic light emitting diodes (OLEDs) are expected to become important light sources in the future. In contrast to point source inorganic LEDs, OLEDs are large area device. This in combination with different materials results in differences in performance and electrical behaviour, which on their turn result in different drivers. This work gives an overview of OLED characteristics. Based on these characteristics a simplified equivalent circuit is obtained and driver requirements are derived. It is found that AM dimming is preferred. Additionally, some OLED characteristics are compared with those of inorganic LEDs. OLED, organic LED, organic light emitting diode, driver, equivalent circuit, IVL-characteristic, impedance spectrum
I. INTRODUCTION Organic light emitting diodes (OLEDs) have the potential to become an important future light source. 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, …) and the 2Dshape of the OLED can be freely chosen as well as the colour of the light. The light output can be fully dimmed. Above all, the main advantages are the cheap production and the high efficiency (30 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.
The article starts to describe typical OLED architectures, which are required to make white light for general lighting applications. In the third chapter, a very simple equivalent circuit is derived, which describes the electric behaviour of the OLED sufficiently accurate for the driver design. The model is extremely suitable for simulations in the time domain. In the fourth chapter, the recommendations for driver design are derived. They are derived from data taken from real experiments. II.
OLED ARCHITECTURE
Glass serves as substrate of most of today’s OLEDs. It is cheap and transparent, so the light can easily be emitted through the substrate. In the future, cheaper plastics can be used as substrate. On top of the glass substrate a thin transparent electrode is deposited. In most cases, indium-thinoxide (ITO) is used as electrode material. On top of the ITO electrode, different organic layers are evaporated, which all have different functions. In the case the ITO layer forms the anode, the order of deposition of the organic layers could be: hole transport layer, electron blocking layer, emitter layers, hole blocking layers and electron transporting layer. The second electrode is usually formed by a reflective aluminium layer. A second glass lid or aluminium lid, which is glued to the substrate, can be used to seal the device to prevent water and air from entering. The operation of an OLED is similar to that of an inorganic LED. Holes are generated at the anode side and electrons are generated at the cathode. Both are transported to the active region, i.e. the emitter layers, where they recombine. During recombination of electrons and holes photons are generated. The wavelengths of the photons depend on the emitter material. The photons are emitted through the substrate.
Figure 1: left: organic LED (OLED) with an active area of 1000 mm2 [2], right inorganic LED with an active area of 4 mm2 [3]
Figure 2: Schematic cross section of a monochrome (green) OLED
Primary goal of this article is to give an overview of stateof-the-art and specific OLED characteristics, which lead to recommendations for the design of the drivers for OLEDs.
For monochrome devices, such as red, blue or green, only one emitter is used. An example of a green OLED device is depicted in Figure 2. To obtain white light, multiple emitters
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have to be used. For example a combination of red, green and blue or a combination of yellow and blue can be combined resulting in white light. An example is depicted in Figure 3. Figure 3 shows a device with the three emitter layers (blue, green, red) directly deposited on top of each other.
to the OLED with a superposed sinusoidal AC voltage with low fixed amplitude UAC = 100 mV and with variable frequency. The impedance is calculated from the measured voltages and currents at the different bias voltages and frequencies using HP4194A impedance analyzer. An example is depicted in Figure 6 and Figure 7. The figures show the series resistances and series capacitances of an equivalent series RC-circuit. Both measured and calculated results are depicted. In the Figure 6, the bias voltage is set to UDC = 0 V and in Figure 7, the bias voltage is set to UDC = 7 V. At higher frequencies, i.e. higher frequencies than shown in the figure, the behaviour become a bit inductive. This is not taken into account in the model. Cs / F
OLED Impedance spectrum, #M90
Cs_fit / F Rs / ohm
A. IV-charcteristic In a first experiment, the IV-characteristic (current-voltage) of an OLED is measured. Thereto, a positive DC voltage, which is increased continuously, is applied to the electrodes of the OLED and the current through the OLED is measured at the different points of operation. An example is depicted in Figure 4. It is obtained from one dot of a small molecule monochrome OLED comprising multiple dots with an active area of 20 mm2. The IV characteristic changes with materials, size and design, the form of the graph is quite similar for all OLEDs. The characteristic can be approximated by a parabolic or exponential function as demonstrated in Figure 4. I-V Characteristic of Z84-dot2
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Figure 5: Impedance measurement of an OLED for a bias voltage UDC = 0 V Cs / F
OLED Impedance spectrum, #M90
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OLED MODELLING
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III.
In this section, a simple equivalent electrical model of the OLED is presented. The model can be used to simulate the electrical behaviour of the OLED.
Series Capacity Cs / F
Figure 3: Schematic cross section of an OLED
Parameters: DC Bias = 0.00 V Rito = 19.50 Ohm Rdiode = 2.3E+6 Ohm Coled = 4.40 nF
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Figure 6: Impedance measurement of an OLED for a bias voltage UDC = 7 V
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Figure 4: Exponential approximation of the IV-characteristic
B. Impedance Spectra Measurement In a second experiment, the OLED impedance spectrum is measured. This is done by applying a constant bias voltage UDC
From these figures it can be seen that an OLED has a capacitive behaviour. This is due to the form factor. The OLED has a large area and the thickness of the organic layers between the electrodes is only 100 … 200 nm. Hence, OLEDs have a large internal capacitance COLED. A typical value for this capacitance is C’OLED = 200 … 400 pF/mm2. Please keep in mind that the internal capacitance is not constant, but depends on the voltage and frequency. Nevertheless, calculations and simulations with a constant value have proven to give good results. The calculated value of the OLED capacitance of the abovementioned example is given in the figures. It is kept constant for both bias points.
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Today’s OLEDs have an active area of a few square centimetres resulting in an internal capacitance of a few microfarads. In the future, OLEDs with an area of one square meter are envisaged.
C. Equivalent model With the derived parameters, a simple equivalent circuit can be obtained. The simple model is depicted in Figure 7. It comprises the ITO resistance RITO § 15 ȍ/, OLED capacitance COLED § 200…400 pF/mm2 and the OLED IVcharacteristic, which can be described with a parabolic or exponential equation.
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Figure 8: VL-characteristics of a small molecule OLED with an active area of 20 mm2 Phosphorescent SM material 3000 2500 Luminance in cd/m2
Additionally, the impedance spectrum demonstrates that the OLED behaves a bit resistive. Hence, a resistance is in series with the internal OLED capacitance. This is predominantly the resistance of the transparent ITO layer. Since the thickness of the ITO layer is more or less fixed, the ITO resistance is given as square resistance and it has a typical value of RITO § 15 ȍ/. Consequently, the resistance depends strongly on the design of the ITO electrode (anode). Again the calculated values for the ITO resistance are given in the figures. The figures show that calculation and measurement correspond very well to each other.
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Figure 9: IL-characteristics of a small molecule OLED with an active area of 20 mm2
Figure 7: Simplified equivalent circuit of an OLED
The obtained model is well suitable to use for the design of the OLED driver, especially for simulations, e.g. with PSpice, Matlab/Simulink and Simplorer. IV.
OLED DRIVER RECOMMENDATIONS
This section gives some helpful recommendations for the design of OLED drivers. A. Voltage or Current Control Typical examples of a voltage-luminance-characteristic and a current-luminance-characteristic of a small molecule OLED are depicted in Figure 8 and Figure 9. The characteristics are measured at different temperatures. From Figure 9, it can be clearly seen that the relationship between current and luminance is quasi-linear. This is not the case for the voltage and luminance relationship (cf. Figure 8). Hence, the best way to control the light output of the OLED is by controlling the current and not the voltage. Additionally, the IL-characteristic does not depend much on the temperature. Only at high temperatures, the luminance changes with temperature. Again, it can be concluded that control of the current through the OLED is preferred.
B. OLED Dimming Many applications, such as colour variable lamps for general lighting or LCD backlighting require dimming of the OLED. From Figure 9 it becomes clear that, similar to their inorganic sisters, OLEDs are perfectly suited to dim. The spectrum of inorganic LEDs changes with current and temperature. Therefore, pulse-width-modulation (PWM) is used to dim inorganic LEDs in the vast majority of applications. However, PWM dimming applied to OLEDs has some disadvantages. At first, the efficacy (in lm/W) of OLEDs is lower at higher currents. An example is depicted in Figure 10. In the example, the light output has to be reduced to 25%. Using PWM dimming, the efficacy of the OLED is ηPWM § 1.5 lm/W. However, using amplitude modulation, i.e. AM dimming, the efficacy is ηPWM § 1.85 lm/W. Hence, the efficacy is approximately 25% higher for AM dimming. So in general, a higher efficacy can be achieved at lower light levels using AM dimming. Secondly, the internal capacitance of the OLED COLED causes a delay in the voltage decay. A measurement is depicted in Figure 11. A square wave current is generated by a DC-toDC converter and is applied to the OLED. The current through the OLED and the voltage across the OLED are measured. One
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recognizes that, although the OLED current is zero, the OLED voltage is not zero. This voltage results in an internal current, i.e. a current through the diode in the equivalent circuit, so light is generated. This is not shown in the figure, but as been validated experimentally. M123, dot A1
Fourthly, the frequent charging and discharging of the large OLED capacitance causes large switching losses in the interconnections, e.g. the ITO plane. Hence, dimming using a switch in parallel to the OLED (Figure 12) is also not possible due to a reduced life-time. For similar reasons, PWM dimming using voltage control can not be applied to OLEDs.
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Figure 10: OLED efficacy as a function of the OLED current
Hence, it becomes extremely difficult and/ or expensive to control the light output, i.e. flux and spectrum, exactly in application where it is desired. An example of such an application is LCD backlighting.
Figure 12: PWM dimming using a switch in parallel to the OLED
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From the above it can be concluded that AM dimming is preferred for OLEDs. However, it is important to know what happens with the colour point of the produced light. Therefore, first measurements have been made. The spectrum of different monochrome and white devices has been measured at different current levels. Experimental results of two monochrome devices are depicted in Figure 13 to Figure 16. It is interesting to notice that the spectrum of these devices is a lot wider compared to that of monochrome inorganic LEDs. Figure 13 shows the spectrum of a red device with an area of 2300 mm2. In Figure 14, the spectra of Figure 13 are normalized to the peak intensity. Similar spectra of a yellow/green OLED with an area of 850 mm2 are depicted in Figure 15 and Figure 16.
Figure 11: PWM dimming Spectrum of red smOLED 606.001.1 200 180 160
Power in μW/nm
Thirdly, the internal capacitance COLED causes current spikes in the OLED if a step voltage is applied. A measurement is given in Figure 12. A constant current is applied to the OLED using a DC-to-DC converter. An electronic switch is placed in parallel to the OLED to enable PWM dimming. A pulse pattern is applied to the gate of the switch resulting in a sequentially closing and opening of the switch. At the instant the switch closes, the internal capacitance is quickly discharged and the voltage across the OLED (green curve) drops rapidly to zero. This results in a huge current spike (red curve), which is only limited by the ITO resistance RITO. In the example, the peak of the current spike is approximately 20 times higher than the nominal current. This spike may damage the OLED and reduce the lifetime.
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Figure 13: Spectra of a red OLED with an active area of 2300 mm2 at different current levels
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In contrast to single emitter monochrome devices, white OLEDs use multiple emitters (cf. section II). Examples of spectra of a three-emitter white OLED are depicted in Figure 17 for different OLED voltages. The spectrum changes with OLED voltage or current, due to the fact that each emitter has a specific IVL-characteristic (depending on architectures and other materials). In this example, the colour point changes to blue with increasing output power. Figure 18 shows the colour points in the chromaticity diagram. It is interesting to note that the colour points this OLED are closely to the black body line. Furthermore, similar to light bulbs, the colour changes towards warm white when dimming, this is something the human perception is used to.
Spectrum of red smOLED 606.001.1 1.2
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Figure 14: Normalized spectra of a red OLED with an active area of 2300 mm2 at different current levels Spectrum of yellow PolyOLED 56 1
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Figure 15: Spectra of a yellow OLED with an active area of 850 mm at different current levels
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Figure 16: Normalized spectra of a yellow OLED with an active area of 850 mm2 at different current levels
The normalized spectra of both monochrome devices demonstrate that the colour point is quasi-independent of the OLED current or OLED voltage. Please note that this is the case for the majority of monochrome devices so far. In a few devices a change in the spectrum could be observed. Additionally, the impact of temperature and aging has not been investigated yet. As a result, AM dimming can be used in most applications where exact colour control is required.
Figure 18: Colour points of the OLED spectra of Figure 17 in the Chromaticity diagram
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From the above results, it becomes obvious that PWM dimming results in a more stable colour point for multiple emitter devices, such as white OLEDs, compared to AM dimming. However, AM dimming is more efficient and could still be tolerated, because the colour shift matches the user’s experience. V.
OLED DRIVER TOPOLOGIES
For organic LEDs, similar converter topologies as for inorganic LEDs can be used. In most cases these are simple topologies, such as buck converter, boost converter, buck-boost converter, flyback converter, SIPEC converter or CUK converter [4]. However, the aforementioned features and recommendations have to be taken into account in the driver design. Hence, the control of these topologies has to be adjusted.
capacitance can be used as converter output filter. Hence, the output filter of the driver becomes smaller and cheaper or can be completely eliminated. An example is depicted in Figure 19 and Figure 20. In the last figure, the OLED internal capacitance replaces the output capacitance Cout of the buck converter. Figure 21 shows the results of a measurement. The current through the OLED (green) is injected by a driver without output filter. The voltage across the OLED (yellow) is measured. One recognizes that, although the current ripple is quite large, the voltage ripple is only small, due to the internal capacitance of the OLED. VI.
CONCLUSIONS
From the above, the following conclusions can be drawn: - A simple equivalent OLED model has been derived. - Current control is preferred for OLEDs, because the relationship between light and current is linear and its relationship does not depend much on temperature.
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- OLEDs have a large internal capacitance limiting methods to dim and to control the colour point.
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- AM dimming results in higher efficacies compared to PWM dimming.
Figure 19: Buck converter with output filter capacitor Cout
- Switches connected in parallel to OLED for dimming should be avoided. - The spectrum of monochrome devices are quasi independent of the OLED current. Hence, AM dimming can be used for monochrome devices in applications which require colour control.
uin uout
- If designed suitably, the internal capacitance of OLEDs can replace completely or partly the output filter of the driver the OLED is connected to.
Figure 20: Buck converter without output filter capacitor Cout
ACKNOWLEDGEMENT The authors wish to acknowledge the work of Christoph Martiny, Jie V. Shen, Michael Bragard, Peter Loebl and Edward Young of Philips Research Europe, who cared for a large amount of measurements and made many data available. REFERENCES [1] [2] [3] [4]
[5] Figure 21: Experimental result, OLED current (green) and OLED voltage (yellow)
D. Bertram: “Philips Lighting actively investigating business potential of OLEDs”, press release: http://pww.lighting.philips.com, March 2005 www.philips.com www.lumileds.com N. Mohan, T. Undeland, W. Robbins: “Power Electronics: Converters, Applications and Design”, second edition, John Wiley & Sons, New York, 1995 S. Xiong; W. Xie; Y. Zhao; J. Wang; E. Liu; C. Wu: “A simple and flexible driver for OLED”, Proceedings of the 5th Asian Symposium on Information Display 1999, ASID, 1999
Additionally, the internal capacitance of the OLED can be considered during the driver design. The OLED internal
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