A method for measuring the speed of sound and elastic modulus of organic semiconductor thin film materials

Abstract

The application discloses a method for measuring the sound velocity and elastic modulus of an organic semiconductor thin film material, and relates to the field of ultrafast optical technology; the method comprises the following steps: building an optical system; taking a femtosecond pulse light as pump light, periodically modulating the absorbance of the measured organic semiconductor thin film material by using the pump light, obtaining the probe light intensity under the action of the pump light and the action of no pump light, and obtaining the transient absorption kinetics spectrum of the measured organic semiconductor thin film material; analyzing the periodic change of the ground state bleaching signal in the transient absorption kinetics spectrum, measuring the frequency of coherent oscillation; measuring the sound velocity based on the relationship between the frequency and the sound velocity and the thickness of the organic semiconductor thin film material; and measuring the elastic modulus based on the relationship between the sound velocity and the elastic modulus and the material density of the organic semiconductor thin film material. In the application, the measuring method adopts a completely non-contact optical detection mode, the sound velocity and the elastic modulus of the thin film material are obtained by detecting the frequency of the coherent oscillation formed in the thin film.

Description

Technical Field

[0001] This invention belongs to the field of ultrafast optics technology, and particularly relates to a method for measuring the sound velocity and elastic modulus of organic semiconductor thin film materials. Background Technology

[0002] In recent years, organic semiconductor materials have shown great application potential in fields such as solar cells, photodetectors, field-effect transistors, and light-emitting diodes due to their advantages such as high absorption coefficients, flexibility, and solution-processability. Sound velocity and elastic modulus are important parameters affecting the performance of organic semiconductor devices. Taking organic solar cells as an example, the sound velocity and elastic modulus of the active layer material affect the device's microstructure, charge dynamics, and mechanical stability. In organic semiconductors, the transport of charge (electrons and holes) is often not completely free but rather occurs between molecules through a "hopping" mechanism. This hopping process usually requires the assistance of phonons to regulate intermolecular coupling. Higher sound velocity means faster and more efficient phonon propagation, which may facilitate the hopping transport of charge within the donor or acceptor, thereby improving charge mobility. The elastic modulus, especially Young's modulus, directly determines the mechanical properties of the active layer film in organic solar cells, which is crucial for the application of flexible organic photovoltaic devices. Therefore, determining the sound velocity and elastic modulus of organic semiconductor materials is of great significance for the development of high-performance organic semiconductor devices.

[0003] In the measurement of sound velocity in semiconductor materials, methods can be broadly categorized into contact and non-contact methods. Contact methods, such as the pulse-echo method based on piezoelectric transducers and the ultrasonic spectrum method, excite and receive sound waves through direct coupling between the probe and the sample, allowing for the calculation of acoustic parameters. However, the spatial resolution of these methods is limited by the diffraction limit, making it difficult to achieve precise localized measurements of micro / nano-scale regions or thin film materials. Organic semiconductor thin films, due to their high absorption coefficients, often have active layers with thicknesses in the hundreds of nanometers, making them unsuitable for contact measurements. Non-contact methods, such as Brillouin scattering and ultrasonic pulse-echo methods, measure the speed of sound by measuring the propagation of sound waves within the material. However, these methods also require a certain sample thickness to probe the sound wave transmission process, making them unsuitable for measuring nanometer-scale thin film samples. Therefore, methods for measuring sound velocity in organic semiconductor thin film materials are relatively scarce.

[0004] Currently, the measurement of the elastic modulus of organic semiconductors mainly relies on contact methods. These methods primarily include underwater tensile testing and nanoindentation. Underwater tensile testing involves preparing a thin film on a water-soluble substrate, transferring it to the water surface, and performing a traditional uniaxial tensile test to directly obtain the stress-strain curve. This method requires transferring the film to the water surface, a cumbersome process, and the film damage introduced during operation and the aquatic environment can affect the measurement results. Nanoindentation is another commonly used method for measuring the elastic modulus of thin films; however, this method is susceptible to the influence of the underlying substrate. It is worth noting that both underwater tensile testing and nanoindentation methods can damage the thin film sample, highlighting the scarcity of non-destructive testing methods.

[0005] In summary, there is a lack of non-destructive measurement methods for the sound velocity and elastic modulus of organic semiconductor thin films. To address this, this invention proposes a non-contact method for measuring the sound velocity and elastic modulus of organic semiconductor thin films based on femtosecond transient spectroscopy. This method can simultaneously and non-destructively detect the sound velocity and elastic modulus of organic semiconductor thin films in a non-contact state, providing a reference for the design of high-performance organic semiconductor devices. Summary of the Invention

[0006] The purpose of this invention is to provide a method for measuring the sound velocity and elastic modulus of organic semiconductor thin film materials, in order to solve the problems mentioned in the background art, such as that the traditional methods for measuring the sound velocity of semiconductor materials are applicable to thicker semiconductor materials, but not suitable for organic semiconductor thin films with a thickness on the order of hundreds of nanometers, and that the measurement of elastic modulus lacks non-destructive testing methods.

[0007] To achieve the above objectives, the present invention employs the following technical solution:

[0008] This invention proposes a method for measuring the sound velocity and elastic modulus of organic semiconductor thin film materials, comprising the following steps: S1. Construct an optical system and use femtosecond pulse light to excite the organic semiconductor thin film material under test; S2. Using femtosecond pulse light as pump light, the absorbance of the organic semiconductor thin film material under test is periodically modulated by the pump light to obtain the intensity of the probe light under the action of pump light and without the action of pump light, and the transient absorption dynamic spectrum of the organic semiconductor thin film material under test is obtained. S3. Analyze the periodic oscillations of the ground-state bleaching signal in the transient absorption kinetic spectrum and measure the frequency of the coherent oscillations; S4. Measure the sound velocity based on the relationship between frequency and sound velocity and film thickness of organic semiconductor thin film materials; measure the elastic modulus based on the relationship between sound velocity and elastic modulus and material density of organic semiconductor thin film materials.

[0009] Preferably, S1 is specifically as follows: An optical system is constructed, comprising a femtosecond laser, several beam splitters, several mirrors, several convex lenses, attenuators, optical filters, a β-phase barium borate crystal, an optical delay line ODL1, and a spectrometer, with the organic semiconductor thin film material to be tested serving as the sample. The process of exciting the organic semiconductor thin film material under test using femtosecond pulsed light is as follows: The femtosecond laser emits an 800 nm femtosecond pulse light with a repetition rate of 1 kHz, which is first split into a probe optical path 1 and a pump optical path by the first beam splitter S1. In the pump optical path, the pump light is modulated to 500 Hz by the optical chopper (the first β-phase barium borate crystal BBO1 is placed in the optical path to make the pump light wavelength 400 nm), then passes through the first optical filter O1 (to filter out the residual 800 nm light during the conversion process), and then the light intensity is adjusted by the first attenuator F1. After being redirected by the first reflector M1 and focused by the first convex lens L1, the light finally converges onto the sample. After passing through the first beam splitter S1, the probe light path 1 enters the optical delay line ODL1 to control the time delay between the pump light and the probe light. After the probe light is emitted from the optical delay line ODL1, it is redirected by the second mirror M2. Then, after the light intensity is adjusted by the second attenuator F2, it is focused by the second convex lens L2 onto the sapphire crystal to generate supercontinuous white light. The supercontinuous white light is collimated by the third convex lens L3 and then passes through the second optical filter O2 to adjust the white light spectrum. It is then split into the reference light path and the probe light path 2 by the second beam splitter S2. The reference light path is redirected by the third reflecting mirror M3 and the fourth reflecting mirror M4, focused by the sixth convex lens L6, and then enters the spectrometer through the third beam splitter S3. The probe light path 2 is focused onto the sample by the fourth convex lens L4 and spatially coincides with the pump light; the probe light carrying sample information is focused by the fifth convex lens L5 and then reflected by the third beam splitter S3 into the spectrometer.

[0010] Preferably, the transient absorption kinetic spectrum of the organic semiconductor thin film material to be tested is obtained in step S2 as follows: A femtosecond pulse light with a repetition frequency of 1 kHz is divided into a probe light and a pump light. An optical chopper is used to modulate the repetition frequency of the pump light to 500 Hz, while the repetition frequency of the probe light is kept at 1 kHz. 2 ms is regarded as a complete measurement cycle. Within the first 1 ms after the start of each measurement cycle, the pump light and the probe light coincide in time and act together on the organic semiconductor thin film material under test. At this time, the spectrometer detects the intensity of the probe light when the pump light modulates the absorbance of the organic semiconductor thin film material under test. During the second 1 ms of the measurement cycle, the original pump light is blocked by the optical chopper. In this stage, only the probe light has the effect of the pump light and no pump light has the effect. That is, the spectrometer detects the intensity of the probe light when the absorbance of the organic semiconductor thin film material under test is not modulated. By obtaining the intensity of the probe light under pump light and without pump light, and substituting the obtained probe light intensity into the absorbance difference calculation formula, the transient absorbance change amplitude of the surface of the organic semiconductor thin film material under test at time 0 is calculated. Similarly, by continuously changing the relative time delay between the probe light and the pump light using an optical delay line, the transient absorption kinetic spectrum of the organic semiconductor thin film material under test can be obtained.

[0011] Furthermore, the formula for calculating the absorbance difference is as follows:

[0012] in, This indicates poor absorbance; and The time when there is no pump light At that moment, wavelength The absorbance and probe light intensity at the point; and These are the times after pump light excitation. At that moment, wavelength The absorbance and probe light intensity at the location.

[0013] Preferably, step S3 is as follows: Based on the thermoelastic pressure generated when the organic semiconductor thin film material under test is excited by pump light, and combined with the periodic modulation of the transient absorption kinetic spectrum with and without pump light, the periodic oscillation of the ground state bleaching signal is obtained. By analyzing the frequency of the oscillation signal, the coherent acoustic phonon resonance frequency is obtained.

[0014] Furthermore, the frequency of the analytical oscillation signal is as follows: Since the oscillation signal is a periodically fluctuating segment on the transient absorption kinetic spectrum, the adjacent averaging method is used to remove the carrier dynamics background from the transient absorption kinetic spectrum data to separate the oscillation signal; the initial phase of the oscillation signal is extracted using the sinedamp fitting formula. t c ,cycle oh Amplitude A ,frequency f、 Phonon lifetime of coherent acoustic phonons t B The frequency of the oscillation signal f This is the coherent acoustic phonon resonance frequency.

[0015] Furthermore, the parameters of the oscillating signal are extracted using the sinedamp fitting formula, specifically: For coherent acoustic phonon signals Perform fitting:

[0016] in, A Indicates amplitude. t c Indicates the initial phase. t B This represents the phonon lifetime of coherent acoustic phonons. oh Indicates the period; frequency of the oscillating signal. .

[0017] Preferably, the measurement of sound speed in step S4 is as follows: Phonon velocity is determined by the coherent acoustic phonon resonant frequency. v Specifically:

[0018] in, v For the speed of sound, d The thickness of the organic semiconductor thin film material. f The coherent acoustic phonon resonant frequency; m This represents the harmonic order of the standing wave.

[0019] Preferably, the measurement of the elastic modulus in step S4 is as follows: The elastic modulus is calculated by combining the material density. E :

[0020] in, v For the speed of sound, r This represents the material density.

[0021] Compared with the prior art, the beneficial effects of the present invention are: (1) The measurement method in this invention uses a femtosecond laser pulse to generate a thermoelastic effect that excites coherent vibrations in the material, causing periodic changes in the interatomic spacing. This change, through deformation potential coupling, periodically modulates the atomic orbital overlap intensity, thereby altering the material's energy level structure and optical absorption properties. By analyzing the periodic changes in the ground-state bleaching signal in the transient absorption spectrum, the frequency of the coherent oscillation is measured, and further, the sound velocity and elastic modulus of the material are measured.

[0022] (2) The measurement method in this invention adopts a completely non-contact optical detection method, which completely avoids the irreversible damage to the sample surface caused by traditional contact measurement (such as nanoindentation method). By detecting the coherent oscillation frequency formed inside the thin film, the interference of the substrate on the measurement results is effectively removed, thereby obtaining the sound velocity and elastic modulus of the thin film material itself.

[0023] (3) The measurement method in this invention utilizes a focused femtosecond laser spot, which can measure the micro-region of the sample at the micrometer scale. The transmission optical path design is fully compatible with the standard optoelectronic device structure fabricated on a transparent substrate, and supports the characterization of the active layer in the complete device.

[0024] (4) The measurement method in this invention can simultaneously and directly obtain the two key parameters of the material, namely sound velocity and elastic modulus, by analyzing the detection of ground state bleaching signal oscillation in transient absorption signal. It uses the resonant frequency measurement in the frequency domain to avoid the timing problem and signal attenuation problem of the time-of-flight method in traditional sound velocity measurement, and has higher accuracy and anti-interference ability.

[0025] (5) The intrinsic elastic modulus measured by the measurement method in this invention directly corresponds to the mechanical environment of the material in the ultrafast photoelectric process. The key steps in organic solar cells (such as exciton diffusion and charge transfer) occur on the scale of picoseconds to nanoseconds. What this invention reveals is the instantaneous stiffness of the material within this time window, providing an indispensable mechanical dimension parameter for understanding the microscopic physical mechanism of the photoelectric conversion process, and establishing a direct correlation between the mechanical properties of the material and its photoelectric function.

[0026] (6) The measurement method in this invention differs from existing technologies (such as the transient reflection method), which require samples to have extremely high surface flatness, a single acoustic interface, and a homogeneous bulk phase in order to obtain an effective oscillation signal. This is usually difficult to meet for semiconductor thin films with rough surfaces, multiphase blends, and complex structures, resulting in extremely low signal-to-noise ratios and inability to decouple signals, thus limiting its application in practical research. The transient absorption method used in this invention derives its signal from the light absorption changes in the bulk phase of the thin film, and is insensitive to surface morphology and internal micro-interfaces, providing a stable and universal experimental scheme for measuring the intrinsic elastic modulus of such complex functional thin films. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the optical system in this invention; Figure 2 The diagram shows the TA kinetics curve of the D18:L8-BO blend film in this invention after subtracting the carrier background. Detailed Implementation

[0028] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] Example 1: The method for measuring the sound velocity and elastic modulus of organic semiconductor thin film materials mainly includes the following steps: Step 1: Build an optical system and use femtosecond laser pulses to excite organic semiconductor thin film materials.

[0030] like Figure 1 As shown, in the optical system, M is a mirror, L is a convex lens, S is a beam splitter, A is a positioning stop, F is an attenuator, O is an optical filter, and BBO is a β-phase barium borate crystal.

[0031] The optical system includes a femtosecond laser (Fs-Laser), beam splitters, mirrors, convex lenses, attenuators, optical filters, a β-phase barium borate crystal, an optical delay line ODL1, and a spectrometer. Specifically, the beam splitters include a first beam splitter S1, a second beam splitter S2, and a third beam splitter S3; the mirrors include a first mirror M1, a second mirror M2, a third mirror M3, and a fourth mirror M4; the convex lenses include a first convex lens L1, a second convex lens L2, a third convex lens L3, a fourth convex lens L4, a fifth convex lens L5, and a sixth convex lens L6; the attenuators include a first attenuator F1 and a second attenuator F2; and the optical filters include a first optical filter O1 and a second optical filter O2.

[0032] The 800 nm femtosecond pulse light emitted from the femtosecond laser with a repetition rate of 1 kHz is first split into a probe optical path and a pump optical path by the first beam splitter S1. In the pump optical path, the pump light is modulated to 500 Hz by an optical chopper (placing a first β-phase barium borate crystal BBO1 in the optical path can change the pump light wavelength to 400 nm), then passes through the first optical filter O1 to filter out the residual 800 nm light during the conversion process, and then the light intensity is adjusted by the first attenuator F1, redirected by the first reflector M1 and focused by the first convex lens L1, finally converging onto the sample.

[0033] The probe light path passes through the first beam splitter S1 and then enters the optical delay line ODL1 to control the time delay between the pump light and the probe light. After exiting the optical delay line ODL1, the probe light is redirected by the second reflecting mirror M2, and then its intensity is adjusted by the second attenuator F2 before being focused onto the sapphire crystal by the second convex lens L2 to generate supercontinuum white light. The supercontinuum white light is collimated by the third convex lens L3, and then passes through the second optical filter O2 to adjust the white light spectral type. It is then split into a reference light path and a probe light path by the second beam splitter S2.

[0034] The reference light is redirected by the third reflecting mirror M3 and the fourth reflecting mirror M4, focused by the sixth convex lens L6, and then enters the spectrometer through the third beam splitter S3. The probe light is focused onto the sample by the fourth convex lens L4 and spatially coincides with the pump light. The probe light carrying sample information is focused by the fifth convex lens L5 and then reflected by the third beam splitter S3 into the spectrometer.

[0035] Step 2: Obtain the transient absorption kinetic spectrum of the semiconductor material.

[0036] In an optical system, in order to obtain the transient absorption dynamic spectrum of a semiconductor material, this invention uses femtosecond pulsed light with photon energy higher than the bandgap of the semiconductor material as pump light. First, the pump light excites the sample material, and then supercontinuous white light generated by sapphire crystal is used as probe light, so that the probe light and the pump light achieve spatial and temporal overlap. Temporal overlap means controlling the light pulses of the probe light and the pump light to arrive at the sample at the same time.

[0037] After the sample is excited by the pump light, over time... Variation in absorbance Specifically, it is represented as shown in equation (1):

[0038] and Time without pump At that moment, wavelength The absorbance and probe light intensity at the point; where and The time after pump excitation are respectively At that moment, wavelength The absorbance and probe light intensity at the location.

[0039] To achieve a comparative measurement of the states with and without pump light in the above theory, this invention adopts the following scheme: The repetition frequency of the pump light is modulated to 500 Hz using an optical chopper, while maintaining the original repetition frequency of the probe light at 1 kHz. In this case, 2 ms can be considered a complete measurement cycle. In the first 1 ms after the start of each cycle (denoted as time 0), the pump light and probe light coincide in time and act together on the material. This stage corresponds to the state with pump light action, and the spectrometer detects the intensity of the probe light modulated by the pump light absorbed by the sample. Subsequently, in the second 1 ms of the same cycle, the optical chopper blocks the pump light, and only the probe light acts on the material. This stage corresponds to the state without pump light action, and the spectrometer detects the intensity of the probe light when the sample is not modulated. Through this method, the intensity of the probe light on the material with and without pump light can be obtained within the same 2 ms cycle. Substituting these two sets of intensity values ​​into formula (1), the transient absorbance change amplitude of the material at time 0 can be calculated.

[0040] Step 3: Analyze the periodic oscillations of the ground-state bleaching signal in the transient absorption kinetic spectrum and measure the frequency of the coherent oscillations.

[0041] (1) Pump light generates a thermoelastic effect that excites coherent oscillations in the material; After the pump light pulse is absorbed by the semiconductor thin film material, its energy is almost entirely converted into the thermal energy of lattice vibrations within a picosecond timescale through the rapid thermalization and nonradiative recombination of hot carriers, resulting in a transient local temperature jump in the absorption layer. Since the thin film is tightly attached to a rigid substrate, its in-plane thermal expansion is completely constrained, while the film thickness direction is movable on the free surface side. This constrained geometry converts the thermal energy into a huge compressive thermoelastic stress, which is expressed by the following equation (2):

[0042] in, For the first j The phonon number increment of each phonon mode; For the first j The quantum energy of a phonon mode is the energy carried by the phonon in that mode; Strain is the relative deformation of a material. This formula describes the microscopic quantum mechanical mechanism of stress generated by photothermal excitation and explains the microscopic origin of macroscopic thermal stress generated when a material is instantaneously heated by absorbing light energy.

[0043] (2) Thermoelastic pressure transmission modulates the material energy level, generating periodic oscillations; When a material is excited by external energy (such as pump light), the excess energy is transferred to the thin film as heat, causing thermal expansion and generating thermoelastic pressure. sThe thermoelastic effect excites coherent vibrations in the thin film, inducing periodic changes in the interatomic spacing. This change, coupled by deformation potential, periodically modulates the atomic orbital overlap intensity, thereby altering the material's energy level structure and optical absorption properties. This periodic modulation of the energy level structure is mapped in real-time to the material's optical absorption characteristics: when the band gap increases due to aggregate compression, the absorption spectrum exhibits a blue shift; when the band gap decreases due to aggregate stretching, the absorption spectrum exhibits a red shift. This dynamic process is captured by the transient absorption spectrum and directly manifests as periodic oscillations of the ground-state bleaching signal. This principle can be expressed by the following equation:

[0044] in, d The thickness of the thin film material; To reduce Planck's constant; The absorption coefficient; To detect the angular frequency of light; The band gap of the material; For the strain of the material; In time t The average strain along the entire thickness direction of the film. This equation describes the strain within the film. The resulting relative change in instantaneous transmittance.

[0045] By analyzing the frequency of this oscillation signal, the intrinsic vibrational frequencies of the coherent phonons can be directly obtained. f To extract the key parameters of the oscillation signal, the carrier dynamics background in the TA data is first subtracted to separate the oscillation signal. Then, the coherent acoustic phonon signal is fitted using the following function:

[0046] in, A Indicates amplitude. t c Indicates the initial phase. t B This represents the phonon lifetime of coherent acoustic phonons. oh Indicates period; coherent acoustic phonon resonance frequency .

[0047] Step 4: Obtain the sound velocity and elastic modulus of the semiconductor thin film material.

[0048] These coherent acoustic phonons travel at the speed of sound. v Phonon propagates within the thin film and is reflected between the upper and lower interfaces, forming a stable acoustic standing wave. Phonon velocity. v It can be determined by the coherent acoustic phonon resonant frequency The specific representation is shown in equation (5) below:

[0049] in, v For the speed of sound, d For the thickness of semiconductor materials, f It is the fundamental resonant frequency of coherent acoustic phonons. m Let be the harmonic order of the standing wave, here m For baseband mode, m =1.

[0050] After obtaining the phonon propagation speed of the semiconductor material, further, in conjunction with the material density... r The elastic modulus is calculated using formula (6). E :

[0051] Femtosecond laser pulses excite coherent vibrations in materials through thermoelastic effects, inducing periodic changes in interatomic spacing. These changes, coupled by deformation potentials, periodically modulate the atomic orbital overlap intensity, thereby altering the material's energy level structure and optical absorption properties. By analyzing the periodic oscillation signals in the transient absorption dynamics, the phonon velocity and elastic modulus of the material can be calculated.

[0052] Experimental verification: This invention verifies the measurement of sound velocity and elastic modulus of semiconductor thin film materials. If the fitting results of the parameters in Example 1 are consistent with the theoretical predictions based on the thermoelastic pressure model, they can serve as key experimental evidence for the existence of thermoelastic stress transmission and modulation in the material. The sound velocity and elastic modulus of the D18:L8-BO organic blend film are used as an example.

[0053] This invention discovers that, under the influence of coherent acoustic phonons generated by thermoelastic stress, an oscillating signal is superimposed on the transient absorption dynamics of the material. Subsequently, the carrier background is removed using the adjacent averaging method, and the oscillating signal attributed to the coherent acoustic phonon effect is extracted, as shown below. Figure 2 As shown. The initial phase of the oscillation signal was then extracted using the sinedamp fitting formula. t c ,cycle oh Amplitude A ,frequency f、 Phonon lifetime of coherent acoustic phonons t B See Table 1.

[0054] Table 1. Relevant parameters of sound velocity and elastic modulus of D18:L8-BO blend film

[0055] The data in the table represent the sound velocity and elastic modulus of the D18:L8-BO blend film calculated from the oscillating signal. A It is an oscillation signal, amplitude t c It is the initial phase. t B It is the phonon lifetime of coherent acoustic phonons. oh It is the period of the oscillation signal. f It is the frequency of the oscillation signal. d It is the thickness of the material. v It is the phonon velocity. E It is the elastic modulus of the material.

[0056] As shown in Table 1, the oscillation frequency of the oscillation signal is obtained. f Then, the material thickness d With oscillation frequency f Substituting these values ​​into formula (5), the phonon velocity of the D18:L8-BO blend membrane can be calculated. v (2412 m / s). Subsequently, the phonon velocity of the D18:L8-BO blend membrane was... v (2412 m / s) and mass density (1 mg / cm³) 3 (Since the mass density of this type of organic blend active layer is relatively constant, based on literature, the mass density is 1 mg / cm³) 3 Substituting into formula (6), the elastic modulus of the D18:L8-BO blend film was finally calculated to be 5.8 GPa.

[0057] The intrinsic elastic modulus of materials measured by this invention using ultrafast spectroscopy yields significantly higher values ​​than those reported by traditional nanoindentation or water surface stretching methods. This is not a measurement error, but rather demonstrates the superior accuracy and advancement of this invention's method in probing the true, intrinsic mechanical properties of materials. Traditional methods are limited by interference from macroscopic defects, plastic flow, and slow molecular relaxation, resulting in the measurement of "apparent modulus." This invention, however, directly probes the instantaneous response of the lattice or molecular framework at the picosecond scale, obtaining an intrinsic modulus that eliminates these interfering factors, thus yielding a higher value that better reflects the material's essential mechanical properties. Finally, the experimentally measured parameters, such as coherent phonon oscillation frequency, sound velocity, and modulus, are consistent with the expected high-frequency mechanical response of the material, and the observed oscillation characteristics are consistent with the predictions of the thermoelastic stress-driven mechanism. This, from both theoretical and experimental perspectives, demonstrates the reliability and effectiveness of the measurement method of this invention.

[0058] The above description is only for the purpose of helping to understand the method and core essence of the present invention, but the scope of protection of the present invention is not limited thereto. For those skilled in the art, any equivalent substitutions or modifications made to the technical solution and inventive concept disclosed in the present invention within the scope of the technology disclosed in the present invention should be covered within the scope of protection of the present invention. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for measuring the sound velocity and elastic modulus of an organic semiconductor thin film material, characterized in that, Includes the following steps: S1. Construct an optical system and use femtosecond pulse light to excite the organic semiconductor thin film material under test; S2. Using femtosecond pulse light as pump light, the absorbance of the organic semiconductor thin film material under test is periodically modulated by the pump light to obtain the intensity of the probe light under the action of pump light and without the action of pump light, and the transient absorption dynamic spectrum of the organic semiconductor thin film material under test is obtained. S3. Analyze the periodic oscillations of the ground-state bleaching signal in the transient absorption kinetic spectrum and measure the frequency of the coherent oscillations; S4. Measure the sound velocity based on the relationship between frequency and sound velocity and film thickness of organic semiconductor thin film materials; measure the elastic modulus based on the relationship between sound velocity and elastic modulus and material density of organic semiconductor thin film materials.

2. The method for measuring the sound velocity and elastic modulus of organic semiconductor thin film materials according to claim 1, characterized in that, S1 is specifically as follows: The process of setting up an optical system, using the organic semiconductor thin film material under test as a sample, and exciting the organic semiconductor thin film material under test with femtosecond pulsed light is as follows: The femtosecond laser emits femtosecond pulses, which are split into a probe optical path 1 and a pump optical path by the first beam splitter S1. In the pump optical path, the pump light is adjusted by the first attenuator F1, redirected by the first reflector M1, and focused by the first convex lens L1, and finally converges on the sample. After passing through the first beam splitter S1, the probe light path 1 enters the optical delay line ODL1 to control the time delay between the pump light and the probe light. After the probe light is emitted from the optical delay line ODL1, it is redirected by the second mirror M2. Then, after the light intensity is adjusted by the second attenuator F2, it is focused by the second convex lens L2 onto the sapphire crystal to generate supercontinuous white light. The supercontinuous white light is collimated by the third convex lens L3 and then passes through the second optical filter O2 to adjust the white light spectrum. It is then split into the reference light path and the probe light path 2 by the second beam splitter S2. The reference light path is redirected by the third reflecting mirror M3 and the fourth reflecting mirror M4, focused by the sixth convex lens L6, and then enters the spectrometer through the third beam splitter S3. The probe light path 2 is focused onto the sample by the fourth convex lens L4 and spatially coincides with the pump light; the probe light carrying sample information is focused by the fifth convex lens L5 and then reflected by the third beam splitter S3 into the spectrometer.

3. The method for measuring the sound velocity and elastic modulus of organic semiconductor thin film materials according to claim 1 or 2, characterized in that, In step S2, the transient absorption kinetic spectrum of the organic semiconductor thin film material to be tested is obtained, as follows: A femtosecond pulse light with a repetition frequency of 1 kHz is divided into a probe light and a pump light. An optical chopper is used to modulate the repetition frequency of the pump light to 500 Hz, while the repetition frequency of the probe light is kept at 1 kHz. 2 ms is regarded as a complete measurement cycle. Within the first 1 ms after the start of each measurement cycle, the pump light and the probe light coincide in time and act together on the organic semiconductor thin film material under test. At this time, the spectrometer detects the intensity of the probe light when the pump light modulates the absorbance of the organic semiconductor thin film material under test. Then, in the second 1 ms of the same measurement cycle, the original pump light is blocked by an optical chopper. In this stage, only the probe light has the effect of not the pump light. That is, the spectrometer detects the intensity of the probe light when the absorbance of the organic semiconductor thin film material under test is not modulated. By obtaining the intensity of the probe light under pump light and without pump light, and substituting the obtained probe light intensity into the absorbance difference calculation formula, the transient absorbance change amplitude of the surface of the organic semiconductor thin film material under test at time 0 is calculated. Similarly, by continuously changing the relative time delay between the probe light and the pump light using an optical delay line, the transient absorption kinetic spectrum of the organic semiconductor thin film material under test can be obtained.

4. The method for measuring the sound velocity and elastic modulus of organic semiconductor thin film materials according to claim 3, characterized in that, The formula for calculating the absorbance difference is as follows: in, This indicates poor absorbance; and The time when there is no pump light At that moment, wavelength The absorbance and probe light intensity at the point; and These are the times after pump light excitation. At that moment, wavelength The absorbance and probe light intensity at the location.

5. The method for measuring the sound velocity and elastic modulus of organic semiconductor thin film materials according to claim 1, characterized in that, S3 is specifically as follows: Based on the thermoelastic pressure generated when the organic semiconductor thin film material under test is excited by pump light, and combined with the periodic modulation of the transient absorption kinetic spectrum with and without pump light, the periodic oscillation of the ground state bleaching signal is obtained. By analyzing the frequency of the oscillation signal, the coherent acoustic phonon resonance frequency is obtained.

6. The method for measuring the sound velocity and elastic modulus of organic semiconductor thin film materials according to claim 5, characterized in that, The frequency of the analytical oscillation signal is as follows: Since the oscillation signal is a periodic fluctuating curve segment on the transient absorption kinetic spectrum, the adjacent averaging method is used to remove the carrier dynamic background from the transient absorption kinetic spectrum data to separate the oscillation signal. The initial phase of the oscillating signal is extracted using the sinedamp fitting formula. t c ,cycle ω Amplitude A ,frequency f、 Phonon lifetime of coherent acoustic phonons τ B The frequency of the oscillation signal f This is the coherent acoustic phonon resonance frequency.

7. The method for measuring the sound velocity and elastic modulus of organic semiconductor thin film materials according to claim 6, characterized in that, The measurement of sound speed in S4 is as follows: Phonon velocity is determined by the coherent acoustic phonon resonant frequency. v Specifically: in, v For the speed of sound, d The thickness of the organic semiconductor thin film material. f The coherent acoustic phonon resonant frequency; m This represents the harmonic order of the standing wave.

8. The method for measuring the sound velocity and elastic modulus of an organic semiconductor thin film material according to claim 1 or 7, characterized in that, The measurement of the elastic modulus in S4 is as follows: The elastic modulus is calculated by combining the material density. E : in, v For the speed of sound, ρ This represents the material density.

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