A Measuring Method for the Relative Dielectric Constant of an Organic Semiconductor Material

Abstract

The present invention discloses a method for measuring the relative dielectric constant of an organic semiconductor material. Based on the preparation of a single-layer device having a cathode / organic layer / anode, the impedance model and impedance phase model of the device are established according to the current density equation and the Poisson equation; a small sinusoidal signal is used as the excitation signal of the above-mentioned device, and the impedance spectrum data of the device under DC bias is measured using an impedance spectrometer. The unknown parameter in the impedance phase model, the carrier migration time, is fitted with the measured impedance spectrum data, and then the carrier migration time is substituted into the impedance model, and the relative dielectric constant of the organic material is calculated using the least squares method. The dielectric constant measured by this method has a small error, and this method requires the thickness of the sample to be measured to be only 600 nanometers, which is very cost-effective for many expensive new materials.

Description

Technical Field

[0001] The invention belongs to the field of characteristic parameter detection of organic semiconductor materials, and particularly relates to a method for measuring the relative dielectric constant of an organic semiconductor material. Background Art

[0002] In recent years, organic semiconductor devices, such as organic light-emitting diodes (OLEDs), organic solar cells (OPVs), and organic field-effect transistors (OFETs), have developed rapidly and are attracting increasing attention. Charge transport capability is one of the key factors affecting device performance, and carrier mobility (or migration time) is an important parameter characterizing charge transport capability. Understanding the essential properties of carrier mobility (or migration time) allows for the synthesis of desired organic semiconductor materials and optimization of the performance of organic semiconductor devices based on diverse requirements. To this end, many researchers are dedicated to measuring the carrier mobility (or migration time) of organic semiconductors. Currently, commonly used methods for measuring carrier mobility (or migration time) include time-of-flight (TOF) and impedance (admittance) spectroscopy.

[0003] However, when using the admittance (impedance) spectroscopy method to calculate the carrier mobility of organic semiconductors, it is necessary to know the relative dielectric constant of the material. The dielectric constant of some organic materials can be obtained by consulting literature. Even if the dielectric constant of a specific material can be found, the actual dielectric constant of the same material in a specific experiment may differ from that found in the literature due to reasons such as purity. The relative dielectric constant of most newly synthesized organic semiconductor materials needs to be measured using specific methods and equipment. Conventional methods for measuring dielectric constants, such as integrated circuit methods and resonance methods, require a sufficient amount of sample, so measuring the dielectric constant of some newly synthesized and expensive organic materials is costly. Summary of the Invention

[0004] The technical problem to be solved by the present invention is that the dielectric constants of some organic materials can be obtained by consulting literature. Even if the dielectric constant of a specific material can be found, the actual dielectric constant of the same material in a specific experiment may differ from the one found in the literature due to factors such as purity. Furthermore, the relative dielectric constants of most newly synthesized organic semiconductor materials require specific methods and equipment to be measured. Furthermore, conventional methods for measuring dielectric constants, such as integrated circuit methods and resonance methods, require a sufficient amount of sample, making it costly to measure the dielectric constants of some newly synthesized, expensive organic materials.

[0005] In response to the above technical problems, the present invention proposes a method for studying the performance of organic semiconductors based on the principle of admittance spectroscopy. The present invention adopts the following technical solutions:

[0006] A method for measuring the relative dielectric constant of an organic semiconductor material comprises performing the following steps on the organic semiconductor material to be measured to obtain the relative dielectric constant of the organic semiconductor material to be measured:

[0007] Step A: constructing a device to be tested based on the organic semiconductor material to be tested; the structure of the device to be tested includes an anode layer, a cathode layer, and an organic layer, wherein the organic layer is the organic semiconductor material to be tested and is arranged between the cathode layer and the anode layer;

[0008] Step B: For the device under test, based on a preset DC voltage applied to both ends of the anode and cathode of the device under test and a small sinusoidal voltage signal of preset frequencies, the impedance spectrum data of the device under test is measured and obtained;

[0009] Step C: Based on the impedance spectrum data of the device under test and the impedance phase model of the device under test, the single carrier migration time τ of the organic semiconductor material under test is obtained. dc ;

[0010] Step D: Based on the single carrier migration time τ of the organic semiconductor material to be tested dc , combined with the impedance model of the device to be tested, the relative dielectric constant of the organic semiconductor material to be tested is obtained.

[0011] Preferably, in step C, based on the impedance spectrum data of the device to be tested, the impedance phase model of the device to be tested is fitted in combination with the following formula to obtain the single carrier migration time τ of the organic semiconductor material to be tested: dc :

[0012]

[0013] Where θ(ω) is the impedance phase, ω is the angular frequency of the sinusoidal voltage signal; τ dc is the single carrier migration time of the organic semiconductor material to be measured; ImZ(ω) is the imaginary part of the measured impedance Z(ω); ReZ(ω) is the real part of the measured impedance Z(ω).

[0014] Preferably, in step D, the single carrier migration time τ of the organic semiconductor material to be tested is dc Substitute the impedance model of the device to be tested and use the least squares method to obtain the relative dielectric constant of the organic semiconductor material to be tested, as shown in the following formula:

[0015]

[0016] Where, d is the thickness of the organic layer, S is the effective area of the electrode of the device under test, ε0 is the vacuum dielectric constant, ε r is the relative dielectric constant of the organic semiconductor material, i is the imaginary unit, ω kis the kth angular frequency in the sinusoidal voltage signal, there are n angular frequencies in total, |A(ω k )| is A(ω k ) modulus, |Z(ω k )| is Z(ω k ) modulus, Z(ω k ) is ω k The corresponding measured impedance value.

[0017] The beneficial effects of the present invention are as follows: the present invention provides a method for measuring the relative dielectric constant of an organic semiconductor material. The method is based on a simple single-layer device, based on the preparation of a device having a cathode / organic layer / anode, and the impedance model and impedance phase model of the device are established according to the current density equation and the Poisson equation; a small sinusoidal signal is used as the excitation signal of the above-mentioned device, and the impedance spectrum data of the device under DC bias is measured using an impedance spectrometer. Based on the space charge limited current theory SCLC, a theoretical impedance model and impedance phase model are established when single carrier injection and no traps are present, and then the actual relative dielectric constant of the organic semiconductor material is obtained by research. Moreover, the present invention can not only measure the relative dielectric constant of the actual organic semiconductor material, but also accurately determine the migration time of the organic semiconductor carriers. The dielectric constant measured by the method of the present invention has a small error, simple test conditions, no special requirements for experimental conditions, and can also save costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 Schematic diagram of the structure of the device under test and the test principle in an embodiment of the present invention;

[0019] Figure 2 This is a fitted impedance phase diagram of the device under test in an embodiment of the present invention. DETAILED DESCRIPTION

[0020] The present invention will be further described below in conjunction with the accompanying drawings. The following examples can enable those skilled in the art to more fully understand the present invention, but are not intended to limit the present invention in any way.

[0021] A method for measuring the relative dielectric constant of an organic semiconductor material comprises performing the following steps on the organic semiconductor material to be measured to obtain the relative dielectric constant of the organic semiconductor material to be measured:

[0022] Step A: Based on the organic semiconductor material to be tested, a device to be tested is constructed; the structure of the device to be tested includes an anode layer, a cathode layer, and an organic layer, wherein the organic layer is the organic semiconductor material to be tested and is arranged between the cathode layer and the anode layer; Figure 1 As shown, the device under test is a single-layer device.

[0023] For organic semiconductors, when measuring hole mobility, the interface between the anode and the organic layer is required to be in ohmic contact, and the interface between the organic layer and the cathode is required to block electrons. When measuring electron mobility, the interface between the anode and the organic layer is required to block holes, and the organic layer and the cathode are in ohmic contact. In one embodiment, for the constructed device to be tested, the interface between the anode and the organic layer is in ohmic contact; the interface between the organic layer and the cathode is an electron blocking layer; and organic layer traps are not considered; the organic layer thickness is 600 nanometers, and the effective area of the electrode is 0.09 cubic centimeters.

[0024] Step B: For the device under test, based on a preset DC voltage applied across the anode and cathode of the device under test and a small sinusoidal voltage signal of each frequency superimposed on the DC voltage, impedance spectrum data of the device under test is measured and obtained.

[0025] like Figure 1 As shown, in one embodiment, the preset DC voltage is 6V, the amplitude of the sinusoidal voltage small signal is 25mV, and the frequency range of the small signal is 20Hz~5×10 6 Hz, where the angular frequency point of the sinusoidal small signal is ω k Determined by the following formula: log 10 (ω k+1 )-log 10 (ω k )=0.054524,k=1,2,3,…,100,ω1=20Hz;each angular frequency point corresponds to an angular frequency;an impedance spectrometer is used to obtain impedance spectrum data of the device under test. In this embodiment, the impedance spectrometer is used to measure the impedance of the device in a swept frequency manner to obtain the measured impedance spectrum data of the device under test.

[0026] Step C: Based on the impedance spectrum data of the device under test and the impedance phase model of the device under test, the single carrier migration time τ of the organic semiconductor material under test is obtained. dc .

[0027] In step C, based on the impedance spectrum data of the device under test, the impedance phase model of the device under test shown in the following formula is fitted to obtain the single carrier migration time τ of the organic semiconductor material under test. dc , that is, the average single carrier migration time. Based on the measured impedance spectrum data of the device under test, the unknown parameter in the impedance phase model, the single carrier migration time, is fitted. The corresponding fitting curve is as follows: Figure 2 As shown:

[0028]

[0029] Where θ(ω) is the impedance phase, ω is the angular frequency of the sinusoidal voltage signal; τ dcis the single carrier migration time of the organic semiconductor material to be measured; ImZ(ω) is the imaginary part of the measured impedance Z(ω); ReZ(ω) is the real part of the measured impedance Z(ω).

[0030] Step D: Based on the single carrier migration time τ of the organic semiconductor material to be tested dc , combined with the impedance model of the device to be tested, the relative dielectric constant of the organic semiconductor material to be tested is obtained.

[0031] In the step D, based on the single carrier migration time τ of the organic semiconductor material to be measured dc , the migration time τ dc Substitute the impedance model of the device under test into the following formula:

[0032]

[0033] Where Z(ω) is the impedance, d is the thickness of the organic layer, S is the effective area of the electrode of the device under test, ε0 is the vacuum dielectric constant, and ε r is the relative dielectric constant of the organic semiconductor material, ω is the angular frequency of the sinusoidal voltage signal, and i is the imaginary unit. This impedance model is based on the space charge limited current theory (SCLC) and establishes an impedance model for single carrier injection and the absence of traps.

[0034] Based on the single carrier migration time τ of the organic semiconductor material to be tested dc , combined with the impedance model of the device under test shown in the above formula, we get

[0035]

[0036] in, d is the thickness of the organic layer, S is the effective area of the device electrode, ε0 is the vacuum dielectric constant, ε r is the relative dielectric constant of the organic semiconductor material, and i is an imaginary unit.

[0037] Then, the least squares method is used to obtain the following formula, which is the expression of the relative dielectric constant of the organic semiconductor material to be tested:

[0038]

[0039] Where, d is the thickness of the organic layer, S is the effective area of the electrode of the device under test, ε0 is the vacuum dielectric constant, ε r is the relative dielectric constant of the organic semiconductor material, i is the imaginary unit, ω k is the kth angular frequency in the sinusoidal voltage signal, there are n angular frequencies in total, |A(ω k )| is A(ω k ) modulus, and |Z(ω k)| is Z(ω k ) modulus, Z(ω k ) is ω k The corresponding measured impedance value.

[0040] In this solution, the impedance model is obtained through the following process:

[0041] Step 1: The device under test is based on an organic semiconductor to be tested. The organic semiconductor to be tested is located between the anode and cathode, known as the organic layer. In this scheme, the interface between the anode and the organic layer is an ohmic contact; the interface between the organic layer and the cathode is an electron blocking layer; and organic layer traps are not considered.

[0042] Step 2: Apply a forward DC bias voltage V to both ends of the device. dc , in V dc Under the action of , holes are injected from the anode and move to the cathode. dc unchanged, and then add a small AC signal v with a frequency of f = ω / 2π to the device ac =V m sin(ωt), will generate a small AC current signal i ac =I m sin(ωt+θ), where θ is the phase difference between the current and voltage signals. Admittance Y(ω)=i ac / v ac =G+iB=G+iωC, where G is the device conductance, B is the device susceptance, and C is the device capacitance.

[0043] Step 3: By solving the Poisson equation and the current density equations, we can derive the impedance model with single carrier injection and no traps. Under the space charge limited current (SCLC) model, the Poisson equation and the total current density equation of the device are:

[0044] Poisson's equation:

[0045]

[0046] Total current density equation:

[0047]

[0048] Where E(x, t) is the electric field intensity, ρ(x, t) is the carrier concentration, μ(t) is the carrier mobility, and ε = ε r ε0(ε r is the relative dielectric constant of the organic semiconductor, and ε0 is the dielectric constant of vacuum).

[0049] Under SCLC conditions, the Motto-Gunet equation is satisfied:

[0050]

[0051] where μ dc is the carrier mobility under DC bias, V is the DC bias, and d is the thickness of the organic layer of the single-layer device.

[0052] Step 4: Substitute formula (1) into formula (2), ignoring the effect of the sinusoidal small signal on the carrier mobility, that is, μ(t) = μ dc , using Taylor series expansion for formula (2), ignoring small terms above the second order, we get:

[0053]

[0054] Substituting formula (3) into (4), we get

[0055]

[0056] in B=iwε,

[0057] Perform Fourier transform on formula (5) to get

[0058]

[0059] Solve equation (2-25) and get

[0060]

[0061] Through points, get

[0062]

[0063] Multiply both sides of formula (8) by Combined with the impedance calculation formula:

[0064]

[0065] The impedance model of organic semiconductor in the trap-free state is obtained:

[0066]

[0067] Where S is the effective area of the organic semiconductor, ε is the dielectric constant, d is the thickness of the organic semiconductor, τ dc is the carrier migration time, ω=2πf is the angular frequency of the small signal sinusoidal voltage.

[0068] The impedance phase model is obtained by the following steps: in order to estimate the carrier migration time without being affected by the dielectric constant of the organic semiconductor material, the thickness of the organic layer and the effective area of the electrode, the impedance model is converted into an impedance phase model.

[0069] Step 1: Use Euler's formula to convert the exponential term in formula (10) Breaks down to:

[0070]

[0071] Step 2: Substitute formula (11) into formula (10), compare the imaginary part with the real part, and take the inverse tangent transformation to obtain the impedance phase model:

[0072]

[0073] There is only one unknown quantity in formula (12): carrier migration time τ dc Compared with formula (10), it does not contain the three parameters of relative dielectric constant, electrode effective area and organic layer thickness, so formula (12) is used to fit τ dc , it is not affected by the above three parameters.

[0074] This scheme is based on the principle of impedance spectroscopy. Under sinusoidal small signal perturbation, based on the space charge limited current theory (SCLC), a theoretical impedance model and impedance phase model of the organic semiconductor device under test are established. Under DC bias, the device under test is stimulated by sinusoidal small signals of different frequencies, and an impedance spectrometer is used to collect the impedance spectrum of the single-layer device. The carrier migration time τ in the impedance phase model is fitted. dc , then the carrier migration time τ dc Substitute the impedance model and use the least squares principle to fit the relative dielectric constant ε of the organic semiconductor material r Furthermore, in one embodiment, the specific steps of the experiment of this scheme are as follows:

[0075] ⑴Prepare the device to be tested for measuring organic semiconductor materials: evaporate the organic sample to be tested and the Ag electrode on the treated ITO glass in sequence, such as Figure 1 As shown;

[0076] (2) Apply a 6V forward DC bias to the device;

[0077] ⑶ Apply a small AC voltage signal to the device with an amplitude of 25mV and a frequency range of 20Hz to 5×10 6 Hz, the impedance of the device was measured using an impedance spectrometer in a frequency sweep mode;

[0078] (4) Fit the impedance phase model to obtain the carrier migration time τ dc ; τ dc Substituting the impedance model, the relative dielectric constant of the organic semiconductor material is calculated using the least squares method:

[0079]

[0080] in, |A(ω k )| and |Z(ω k )|are A(ω k ) and Z(ω k ) modulus value.

[0081] The present invention designs a method for measuring the relative dielectric constant of an organic semiconductor material. Based on a simple single-layer device, a device comprising a cathode / organic layer / anode is prepared. An impedance model and an impedance phase model for the device are established according to the current density equation and Poisson's equation. A small sinusoidal signal is used as the excitation signal for the device, and the impedance spectrum data of the device under a DC bias is measured using an impedance spectrometer. Based on the space charge limited current theory (SCLC), a theoretical impedance model and an impedance phase model for single carrier injection and the absence of traps are established, thereby studying and obtaining the actual relative dielectric constant of the organic semiconductor material. Furthermore, the present invention can not only measure the relative dielectric constant of actual organic semiconductor materials, but also accurately determine the migration time of organic semiconductor carriers. The dielectric constant measured by this method has a small error, simple testing conditions, no special experimental requirements, and can also save costs.

[0082] The above are only preferred embodiments of the present invention, but do not limit the scope of the patent of the present invention. Although the present invention has been described in detail with reference to the above embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the above embodiments or to replace some of the technical features therein with equivalents. Any equivalent structure made by using the contents of the present invention specification and drawings, directly or indirectly applied to other related technical fields, is also within the scope of protection of the patent of the present invention.

Claims

1. A method for measuring the relative dielectric constant of an organic semiconductor material, characterized by: For the organic semiconductor material to be tested, perform the following steps to obtain the relative dielectric constant of the organic semiconductor material to be tested: Step A: constructing a device to be tested based on the organic semiconductor material to be tested; the structure of the device to be tested includes an anode layer, a cathode layer, and an organic layer, wherein the organic layer is the organic semiconductor material to be tested and is arranged between the cathode layer and the anode layer; Step B: For the device under test, based on a preset DC voltage applied to both ends of the anode and cathode of the device under test and a small sinusoidal voltage signal of preset frequencies, the impedance spectrum data of the device under test is measured and obtained; Step C: Based on the impedance spectrum data of the device under test and the impedance phase model of the device under test, the single carrier migration time τ of the organic semiconductor material under test is obtained. dc ; In step C above, based on the impedance spectrum data of the device under test, the impedance phase model of the device under test shown in the following formula is fitted to obtain the single carrier migration time τ of the organic semiconductor material under test: dc : Where θ(ω) is the impedance phase, ω is the angular frequency of the sinusoidal voltage signal; τ dc is the single carrier migration time of the organic semiconductor material to be measured; ImZ(ω) is the imaginary part of the measured impedance Z(ω); ReZ(ω) is the real part of the measured impedance Z(ω); Step D: Based on the single carrier migration time τ of the organic semiconductor material to be tested dc , combined with the impedance model of the device to be tested, the relative dielectric constant of the organic semiconductor material to be tested is obtained; In the above step D, the single carrier migration time τ of the organic semiconductor material to be tested is dc Substitute the impedance model of the device to be tested and use the least squares method to obtain the relative dielectric constant of the organic semiconductor material to be tested, as shown in the following formula: Where, d is the thickness of the organic layer, S is the effective area of the electrode of the device under test, ε0 is the vacuum dielectric constant, ε r is the relative dielectric constant of the organic semiconductor material, i is the imaginary unit, ω k is the kth angular frequency in the sinusoidal voltage signal, there are n angular frequencies in total, |A(ω k )| is A(ω k ) modulus, |Z(ω k )| is Z(ω k ) modulus, Z(ω k ) is ω k The corresponding measured impedance value.

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