KR100605121B1 - Method and device for measuring capacitance of organic light emitting device - Google Patents

Abstract

The present invention provides an organic electroluminescent device based on an alternating current impedance characteristic obtained by superimposing and applying a predetermined constant voltage or constant current direct current bias and an AC signal having a predetermined amplitude and frequency to the organic light emitting device. Provided are a method and apparatus for measuring capacitance.

Description

Method and apparatus for measuring capacitance of organic light emitting device

1 is a flowchart illustrating a process of measuring capacitance of an organic light emitting diode by measuring an AC impedance spectrum;

2 is a graph showing the shape of an AC signal superimposed on a DC bias

3 is an equivalent circuit corresponding to an impedance characteristic of an organic light emitting diode.

4 is a schematic configuration diagram of an impedance measuring apparatus for measuring deterioration characteristics of an organic light emitting diode.

5 is a graph showing the impedance spectrum of the organic light emitting device according to the driving bias measured in the [Example] (20) 0 V, (21) 1 V, (22) 2 V, (23) 3 V, (24) 4 V

The present invention relates to a method and an apparatus for measuring capacitance, which is one of internal characteristics of a device, which greatly affects light emission characteristics when the organic light emitting device is driven.

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The present invention provides a method for quantitatively measuring capacitance, which is one of nonlinear internal characteristics of an organic light emitting device, under a predetermined bias condition, and experiments for developing an organic light emitting device having excellent driving characteristics by measuring capacitance. It is an object of the present invention to provide a test apparatus for securing quality by strictly selecting good and defective products by measuring capacitance in the production of devices and organic light emitting devices.

The organic light emitting device is a self-light emitting device in which electrons and holes injected into the anode and the cathode are moved into the device by the potential difference applied from the outside and recombine in the organic light emitting layer to emit light. The luminescence properties of the organic light emitting display device such as luminescence brightness and chromaticity vary depending on the properties of the material constituting the device and the structure of the device. In particular, since the light emission luminance of the organic light emitting diode is directly controlled by the driving voltage or the current, the relationship between the driving voltage, the current, and the light emission luminance of the device is the basis for defining the characteristics of the device. In order to implement a specific voltage or current to drive the device.

The organic light emitting display device having a laminated structure of materials having different electronic properties as described above is a nonlinear device whose driving characteristics vary depending on the driving bias and the frequency of the driving signal. In other words, even when the same device is driven with the same voltage bias, the current signal applied to the drive signal is a direct current or a pulse signal having a predetermined period due to the difference in capacitance.

Therefore, the method using a conventional LCR measuring device for measuring the capacitance of the organic light emitting device is difficult to measure the nonlinear characteristics according to the driving bias. The charge transport factor of the organic light emitting diode having a stacked structure has various factors such as contact resistance, ohmic resistance, charge transfer and storage at the interface between layers. Therefore, a single AC resistance value measured for an AC signal corresponding to a specific single frequency is also measured. Internal characteristics of the device cannot be expressed entirely.

The technical problem of the present invention is to measure the capacitance reflecting the driving characteristics of the organic light emitting diode regardless of the measurement frequency at a predetermined bias, the impedance spectrum obtained by measuring the impedance for a plurality of frequencies in a predetermined frequency region The present invention provides a method and apparatus for measuring the capacitance of an organic light emitting diode by corresponding to an equivalent circuit composed of linear elements such as resistors, capacitors, and inductors.

The impedance of the device is obtained by applying an AC voltage or a current signal having a predetermined frequency and amplitude to the organic light emitting device in a superimposed manner and measuring the voltage and current response signals. The impedance value of the same device in the same state depends on the frequency of the superposition alternating signal, and the difference depends on the internal characteristics of the device. In addition, the measured impedance value of the device changes according to the organic light emitting device driving bias.
Since the internal characteristics of the organic light emitting diode change with frequency with respect to a constant voltage or current bias, it cannot be a standard that consistently reflects the driving characteristics of the device. Therefore, an equivalent circuit composed of a plurality of linear elements such as a resistor, a capacitor, and an inductor is measured by measuring an impedance spectrum obtained by measuring impedances of a plurality of frequencies in a predetermined frequency range. Calculate the capacitance corresponding to the impedance function indicated by. Since the capacitance obtained from the impedance spectrum is not an amount that varies with the measurement frequency, it can be a standard representing the internal characteristics of the device. A method of obtaining capacitance of an organic light emitting display device using an equivalent circuit from an impedance spectrum measured for a plurality of alternating frequency frequencies under constant voltage or constant current bias conditions is briefly shown in FIG. 1.
In order to measure the AC impedance of the organic light emitting diode, an AC signal having a predetermined amplitude and frequency f is applied to the constant voltage bias V _b or the constant current bias I _{b as} shown in FIG. 2. In this case, the measured voltage and current signals have the same type of signal in which the AC signal is superimposed on the bias or the amplitude (V ₀ and I ₀ ) and the phase difference ((φ)) of the predetermined AC component. From the AC component of the displayed and measured signal, the impedance (Z) is obtained as follows.

The impedance value of the organic light emitting display device varies depending on the frequency of the AC signal used for the measurement. If the impedance is measured continuously at different frequencies in a given frequency range, the impedance of the device can be represented by an impedance characteristic curve expressed as a function of frequency, and the organic light emitting device generally has a real part of impedance on the X axis and an imaginary part. A semicircle Randle curve is shown in the coordinate system on the Y axis.

In order to extract the resistance component from the measured impedance characteristic curve, an equivalent circuit composed of a plurality of linear elements such as a resistor, a capacitor, and an inductor is set. An equivalent circuit composed of three linear elements, a series resistor (R _s ), a parallel resistor (R _p ), and a parallel capacitor (C _p ), is shown in FIG.

When the impedance spectrum measured as a function of frequency in a predetermined frequency range is approximated to an impedance function having an irregular component such as R _s , R _p , and C _p as shown in [Equation 2], the capacitance component of the organic light emitting display device C _p can be determined.

By calculating the initial R _s , R _p , and C _p by approximating the impedance spectrum to the impedance function, the impedance for each frequency is calculated from the impedance function, and the square of the difference between this value and the measured value summed over all frequencies. After calculating the deviation, the R _s , R _p , and C _p values are changed according to a certain rule, and then the nonlinear regression approximation method using the least squares method that determines the R _s , R _p , and C _p values representing the minimum square deviation is repeated. Can be used.

The apparatus for measuring the internal resistance of the organic light emitting display device is configured as shown in FIG. Among the output signals set by the digital control unit 10, which is the main control unit, the constant current or constant voltage bias is output by the bias output unit 11, and the AC signal having a predetermined amplitude and frequency is respectively converted by the AC signal generator 12 by the digital-analog converter. The output is controlled to the organic light emitting device through the input and output terminal 14, the change in voltage and current of both ends of the organic light emitting device is converted into a digital signal in the signal detector 13 is provided to the digital control unit. The measurement signal is calculated by the impedance calculation unit 16 as a complex impedance value and provided to the data storage unit 15, and also output to the digital file, printer or display device through the data output unit 17. The impedance measuring apparatus continuously measures impedance with respect to a plurality of AC signal frequencies, and stores an impedance spectrum in the data storage unit 15 and outputs the data to the data output unit 17. In addition, the impedance function setting unit 18 sets a predetermined equivalent circuit to provide the impedance function to the impedance calculating unit 16, and the impedance calculating unit 16 approximates the measured impedance spectrum to the impedance function to configure the impedance function. Extract and calculate the capacitance can be stored in the data storage unit 15 and output to the data output unit 17.

EXAMPLE

Hole transport layer TDP (N, N'-diphenyl-N, N'-bis (3-methylphenyl) -1,1-biphenyl-4,4'-diamine) on organic indium tinoxide transparent electrode, Alq The driving characteristics of the organic light emitting device having an effective light emitting area of 2 mm 2 laminated with ₃ (Tris-8-hydroxyqinolinato Aluminum) and aluminum cathode were measured. In constant voltage driving, the device showed a light emission luminance of 120 Cd / m 2 at a voltage of 10V. Impedance measuring devices configured as shown in FIG. 4 were connected to both ends of the organic light emitting diode, and the impedance was measured by applying a constant voltage bias in the range of 0 to 4 V and a sinusoidal AC signal having a amplitude of 50 mV. At this time, the frequency range of the AC signal was 10 Hz to 1 MHz. 5 is a result of outputting the measured impedance spectrum as a Nyquist graph with an impedance real part on the X axis and an imaginary part on the Y axis. The impedance characteristic curve of the organic light emitting diode showed a form of a ladle curve, and the capacitance was extracted by performing a nonlinear regression approximation by the least square method on the complex impedance function of the lance equivalent circuit. The capacitance of the device at 4 V in the luminescent state increased about 9% compared to the capacitance at 0 V in the non-luminescent state.

The present invention provides a method and apparatus capable of quantitatively measuring a clear capacitance that does not change depending on frequency characteristics when driving an organic light emitting diode without directly measuring the luminance of the organic light emitting diode. It can be used to measure the lifetime of the device by quantitatively evaluating the deterioration characteristics of the device when driving for a long time, and it is possible to implement a production test apparatus for efficiently securing the quality by strictly selecting good and defective products by measuring capacitance during mass production.

Claims (10)

A first process of superimposing and applying an alternating current signal having a predetermined frequency to the constant voltage or constant current bias for driving the organic light emitting element and measuring the alternating current impedance from the measured voltage and current response signals; A second process of measuring the impedance spectrum of the organic light emitting diode by repeating the first process with respect to an AC signal of a plurality of frequencies belonging to the measurement region; A third process of calculating the capacitance by approximating the impedance spectrum to an equivalent circuit composed of a linear element including a resistor, a capacitor, and an inductor The capacitance measurement method of the organic electroluminescent device by measuring the capacitance of the organic light emitting device. The method according to claim 1, wherein the AC signal, A capacitance measuring method of an organic light emitting display device, characterized in that the sinusoidal signal represented by a single frequency. The method according to claim 1, wherein the AC signal, 4. A method for measuring capacitance of an organic light emitting display device, characterized in that it is a Fourier transform signal having a plurality of sinusoids having a single frequency component evenly superimposed. The frequency of the said AC signal is any one of Claims 1-3. A method for measuring capacitance of an organic electroluminescent device, characterized in that it ranges from as low as 10 Hz to as high as 1 MHz. delete delete The method of claim 1, wherein the third process, And a complex function having the frequency as an independent variable and the linear element constituting the equivalent circuit as a parameter. The approximation of the impedance spectrum to the equivalent circuit of claim 1, A method for measuring capacitance of an organic light emitting device, characterized in that it is a nonlinear regression approximation by least squares method. The approximation of the impedance spectrum to the equivalent circuit of claim 1, A method for measuring capacitance of an organic light emitting device, characterized in that for calculating the capacitance from the geometric feature of the impedance spectrum shown in a complex coordinate system. A bias output unit 11 for providing a constant voltage or a constant current direct current bias to the organic light emitting diode, An AC signal generator 12 for generating a constant voltage or a constant current AC signal to be superimposed on the driving bias; A signal detector 13 for detecting a voltage and a current signal at both ends of the organic light emitting diode and converting the signal into a digital signal; An input / output terminal 14 which contacts both ends of the organic light emitting element and transmits a control and measurement signal; An impedance calculator (16) for calculating a complex impedance from the voltage and current signals measured by the signal detector (13) and approximating the measured impedance spectrum to an impedance function; An impedance function setting unit 18 for setting a predetermined equivalent circuit and providing the impedance function; A data storage unit 15 for storing the calculation result, A data output unit 17 for outputting the calculation result to a display apparatus or a printing apparatus or outputting the digital data; Digital control unit 10 for overall control of the parts Capacitance measuring device of an organic light emitting device comprising a.

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