Accepted Manuscript Impedance-based temperature measurement method for Organic Light-Emitting Diodes (OLEDs) L.H.J. Raijmakers, M. Büchel, P.H.L. Notten PII: DOI: Reference:
S0263-2241(18)30244-6 https://doi.org/10.1016/j.measurement.2018.03.058 MEASUR 5377
To appear in:
Measurement
Received Date: Revised Date: Accepted Date:
25 October 2017 21 March 2018 22 March 2018
Please cite this article as: L.H.J. Raijmakers, M. Büchel, P.H.L. Notten, Impedance-based temperature measurement method for Organic Light-Emitting Diodes (OLEDs), Measurement (2018), doi: https://doi.org/10.1016/ j.measurement.2018.03.058
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Impedance-based temperature measurement method for Organic Light-Emitting Diodes (OLEDs) L.H.J. Raijmakersa,b, M. Büchelc, P.H.L. Nottenb,d* a
b
Delft University of Technology, 2629 JB, Delft, The Netherlands.
Eindhoven University of Technology, 5600 MB, Eindhoven, The Netherlands. c
d
OLEDWorks GmbH, D-52068, Aachen, Germany.
Forschungszentrum Jülich (IEK-9), D-52425, Jülich, Germany.
* Corresponding author, +31 (0)40 247 3069;
[email protected] Abstract This short communication presents a method to measure the integral temperature of organic light-emitting diodes (OLEDs). Based on electrochemical impedance measurements at OLEDs, a non-zero intercept frequency (NZIF) can be determined which is related to the OLED temperature. The NZIF is defined as the frequency at which the imaginary part of the impedance is equal to a predefined (non-zero) constant. The advantage of using an impedance-based temperature indication method through an NZIF is that no hardware temperature sensors are required and that temperature measurements can be performed relatively fast. An experimental analysis reveals that the NZIF is clearly temperature dependent and, moreover, also DC current dependent. Since the NZIF can readily be measured this impedance-based temperature indication method is therefore simple and convenient for many applications using OLEDs and offers an alternative for traditional temperature sensing. Keywords Organic Light Emitting Diode, Temperature measurement, Electrochemical Impedance Spectroscopy 1. Introduction Electronic systems and devices increasingly make use of displays to enhance the human-device interface. Organic light-emitting diodes (OLEDs) are considered as one of the most promising display and light-source devices due to their advantages with respect to related systems [1]. Since OLEDs generate heat during operation, or are heated by emission of surrounding electronics, parameters such as efficiency, brightness, colour and lifetime may be negatively affected [2–6]. Therefore, thermal management is an essential element during OLED display utilization [7,8]. Thermal sensors [9], infrared [10,11] or thermoreflectance imaging [7] can be used to measure the temperature of an OLED or a temperature profile across multiple OLEDs. Using the OLED temperature as an input signal, a controller can compensate the brightness and/or colour. However, infrared or thermoreflectance imaging is a technique which is well suited for experiments in the laboratory but not in displays or light source applications. Moreover, this measurement equipment is expensive and less suitable for mass-produced
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OLED devices. From an application point of view, thermal sensors would be a better option. However, since OLEDs can have large surface areas, the question is where to locate a thermal sensor at the surface. If multiple OLEDs are used also multiple thermal sensors should be introduced, which can lead to a complex structure of sensors and wiring. Moreover, conventional thermal sensors often sense with a delay since heat transfer is not infinitely fast. To accommodate these shortcomings an impedance-based temperature measurement system would be a good alternative. Electrochemical Impedance Spectroscopy (EIS) is a non-destructive technique to characterize electro(chemical) materials and devices [12–14]. Therefore, impedance-based temperature measurements do not cause any damage to devices under test. In the field of Li-ion batteries EIS has been successfully applied as an integral temperature measurement technique [15–17]. However, the application of EIS-based temperature indication to OLEDs is a new concept and therefore introduced in the present communication. The complex impedance, which can be obtained by EIS, is defined as
Z j
V j I j
,
(1)
where Z is the complex impedance at constant temperature, V the measured or applied voltage, I the measured or applied current,
the angular frequency and
j is the imaginary unit satisfying j 2 1 .
The aim of this short communication is to introduce an impedance-based temperature measurement method which can be applied to OLEDs. EIS measurements are performed on commercial OLEDs and a temperature indication method through a non-zero intercept frequency (NZIF) is discussed. By applying impedance-based temperature measurements complex arrangements of thermal sensors and wiring can be avoided. An additional advantage is that OLED aging can be measured by EIS simultaneously [4]. Moreover, EIS measurements can be performed quickly and a fast temperature indication is therefore guaranteed. 2. Experimental 2
Red-light emitting OLEDs with a glass substrate and an active surface of 35x35 mm are used in this experimental study. The maximum DC operating current of the OLEDs is specified at 184 mA. The corresponding voltage depends on the OLED temperature. Current-voltage characteristics are measured with an Autolab PGSTAT30 (Metrohm Autolab) up to a current of 184 mA. EIS measurements are performed in galvanostatic mode with an Autolab PGSTAT30 (Metrohm Autolab). A frequency range from 950 kHz to 100 Hz is applied in which the measured frequencies are distributed logarithmically across 50 frequencies. The AC excitation signal is set to 5 mA. EIS is measured under various DC operating currents (10, 15, 30, 50, 70, 90, 110, 130, 150 and 175 mA). For temperature calibration purposes OLEDs are measured in a temperature range from -20 to 60 oC with o
increments of 10 C. A stainless steel double-walled climate chamber is used which is temperature-controlled by coolant. The temperature of the coolant is controlled by a circulation pump (Lauda RK20-KS). The OLEDs are taped to
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the flat surface of the climate chamber in order to have the most optimal heat transfer between OLED and stainless steel wall. Simulations and experiments confirmed that the OLED fully adapts the climate chamber temperature during all operating conditions. Therefore, the climate chamber temperature is taken for NZIF calibration purposes. 3. Results and discussion 3.1. Impedance as a function of temperature Fig. 1a. shows the current-voltage characteristics of an OLED device at all measured temperatures. From these results it can clearly be seen that for a given voltage the current increases as the temperature increases. This behaviour is expected because the mobility of the charge carriers depends on the temperature [18,19]. Higher temperatures lead to better mobility and the current therefore increases with increasing temperature. From Eq. 1 it can then be expected that the impedance Z ( j) decreases as the temperature increases. An experimental proof is shown in Fig. 1b, where the impedance spectra are plotted at various temperatures in the complex plane. From these results, it can be concluded that the characteristic semicircles decrease in diameter with increasing temperature, which indeed means that the impedances decrease. Since the impedance is a function of temperature, a non-zero intercept frequency (NZIF) can be applied as temperature indication parameter [16]. In the following subsections a temperature measurement method through the NZIF is discussed. 3.2. Non-zero intercept frequency An NZIF can be determined from the measured impedance data. The NZIF is defined as the frequency at which the imaginary part of the impedance is equal to a predefined (non-zero) constant [16]. It can be determined by interpolating the frequency to a specified non-zero imaginary value ( Z im ). In the field of Li-ion battery systems this method has been successfully applied in the high frequency range of the impedance spectra to avoid State-of-Charge (SoC) dependencies and measurement interferences [15,16]. The inductive part ( Zim 0 ) was advantageously used to determine NZIFs. However, for the present OLEDs the inductive part could not be reached as those measurement frequencies are too high. Therefore, the lower frequency range is now selected to determine NZIFs (at Zim 0 ). The lower NZIF frequencies are located at the right-hand side of the semicircles. NZIFs interpolated at imaginary values of Zim 0.2 Ω and Zim 0.6 Ω are determined from the impedance spectra shown in Fig. 1b. For convenience dotted lines are shown at Zim 0.2 and 0.6 Ω. Fig. 2 shows these NZIFs as a function of temperature. From the results it can be seen that the NZIFs increase with increasing temperature. This means that the frequency at negative imaginary values increases with temperature. Note that this only holds for the right-hand side of the spectra. However, the temperature dependence of the NZIF at Zim 0.6 Ω is much higher than at Zim 0.2 Ω. This implies that the NZIF at Zim 0.6 Ω is more sensitive to temperature changes and therefore more accurate for temperature indication than at 0.2 Ω. An accuracy analysis, as performed by Beelen et al. [20], can be used to determine the most optimal NZIF. Since NZIFs are obtained at high frequencies, measurements can be done relatively fast (time=1/NZIF).
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200 -20 C -10 C 0 C 10 C 20 C 30 C 40 C 50 C 60 C
Current [mA]
150
100
T
50
(a)
0
0
1
2 3 Potential [V]
4
5
2.4
-Zim []
T
2 1.6 1.2 0.8 0.4
(b)
0
Zim=-0.6 Zim=-0.2
1
1.5
2
2.5 Zre []
3
3.5
4
4.5
Fig. 1. Current-voltage (a) and impedance characteristics in complex plane at I dc 175 mA (b) for various temperatures. During OLED operation the impedance can be easily measured by electronics and an NZIF can be determined by interpolating to a predefined Z im . The NZIF can then be converted to temperature with help of lookup tables or by solving an equation which approximates the NZIF curve (Fig. 2). Note that it is not necessary to measure the complete impedance spectrum. Two frequencies around the NZIF is sufficient to interpolate, enabling fast sampling. Furthermore, it should be emphasized that a temperature indicated by the NZIF is an integral temperature since the impedance is measured across the whole active material of the OLED. As the integral temperature is measured, it is to be expected that local temperature changes like hotspots can be detected as well.
140 Zim =-0.2 120
Zim =-0.6
NZIF [kHz]
100 80 60 40 20 0 -20
0
20 Temperature [C]
40
60
Fig. 2. Non-zero intercept frequency (NZIF) at Zim 0.2 and 0.6 Ω as a function of temperature.
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3.3. DC current Although OLEDs are normally driven at predetermined and thus known (high) DC currents, it is scientifically interesting to investigate NZIF dependence as a function of DC current. In Fig. 3 the impedance spectra are shown for o
various DC currents at a temperature of 20 C. From the results, it can be seen that the characteristic semicircles decrease with increasing DC current. This behaviour is expected because the charge mobility increases with an increasing electric field, as the energy barrier in the direction of the field is lowered [18,19,21].
10 mA 15 mA 30 mA 50 mA 70 mA 90 mA 110 mA 130 mA 150 mA 175 mA
10
-Zim []
8
I
dc
6 4 2 0
5
10 Zre []
15
Fig. 3. Impedance spectra in the complex plane for various DC currents at 20 oC. A changing impedance as a function of DC current has direct consequences on the NZIF. Therefore, the NZIF is determined at Zim 0.2 and 0.6 Ω and plotted as a function of DC current in Fig. 4. From these results, it can be seen that the NZIF increases linearly on a log-scale for both Zim 0.2 and 0.6 Ω. Similar to the results shown in Fig. 2, the NZIF at Zim 0.6 Ω shows higher values than at Zim 0.2 Ω, although they both have the same slope. These results reveal that the NZIF is DC current dependent, which implies that the NZIF should be calibrated for the DC current as well. Therefore, an NZIF matrix can be generated for all temperatures and DC currents. A three-dimensional representation of the NZIF interpolated at Zim 0.6 Ω is shown in Fig. 5 and can be used as a lookup table or function to indicate OLED temperatures under a large range of operating conditions.
11 Zim =-0.2 Zim =-0.6
log(NZIF) [Hz]
10
9
8
7
6
5
2
2.5
3
3.5 4 4.5 log(DC current) [mA]
5
5.5
5
Fig. 4. Non-zero intercept frequency (NZIF) at Zim 0.2 Ω and Zim 0.6 Ω as a function of DC current at 20 oC. 4. Conclusions Electrochemical impedance measurements at OLEDs revealed that OLED impedance spectra are temperature dependent. Non-zero intercept frequencies (NZIFs), i.e. frequencies at which the imaginary part of the impedance is equal to a predefined constant, are determined from the measured impedance spectra. Experimental results showed that NZIFs at Zim 0.2 and 0.6 Ω are temperature and DC current dependent. However, the NZIF at Zim 0.6 Ω is more sensitive to temperature changes and therefore more accurate. The temperature dependence of the NZIF makes it a good candidate to use as temperature indication parameter. Since the NZIF can be measured in an application it favourably can be used as a temperature indication parameter and, therefore, this method it is readily applicable to OLEDs.
Fig. 5. Non-zero intercept frequency (NZIF) at Zim 0.6 Ω as a function of DC current and temperature. Acknowledgements This work was supported by ADEM, A green Deal in Energy Materials of the Ministry of Economic Affairs of The Netherlands. References [1]
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Figure Captions Fig. 1. Current-voltage (a) and impedance characteristics in complex plane at I dc 175 mA (b) for various temperatures. Fig. 2. Non-zero intercept frequency (NZIF) at Zim 0.2 and 0.6 Ω as a function of temperature. Fig. 3. Impedance spectra in the complex plane for various DC currents at 20 oC. o Fig. 4. Non-zero intercept frequency (NZIF) at Zim 0.2 and 0.6 Ω as a function of DC current at 20 C.
Fig. 5. Non-zero intercept frequency (NZIF) at Zim 0.6 Ω as a function of DC current and temperature.
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