CN112179874B - Method and device for measuring exciton orientation of luminescent material - Google Patents
Method and device for measuring exciton orientation of luminescent material Download PDFInfo
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Abstract
The invention belongs to the related technical field of photoelectric measurement, and discloses a method and a device for measuring the exciton orientation of a luminescent material, wherein the method comprises the following steps: (1) constructing a forward luminescence calculation model for distinguishing a photoluminescence spectrum by a luminescent material, wherein the forward luminescence calculation model is used for equivalent quantum mechanical description of exciton transition to classical electromagnetic theory description of electric dipole radiation in a microcavity structure, then based on the electric dipole exciton radiation model and a microcavity coherence model, adopting a transmission matrix form, and transmitting and reflecting on all related interfaces by utilizing Fresnel coefficients to solve an electromagnetic field so as to obtain an emission spectrum angularly distinguished by the luminescent material; (2) and measuring the angle-resolved spectrum of the material by adopting a measuring device for the exciton orientation of the luminescent material, and performing reverse inversion by combining a forward luminescence calculation model based on the measured angle-resolved spectrum to obtain the molecular orientation of the material. The invention can accurately measure and characterize the molecular orientation of the doped material system.
Description
Technical Field
The invention belongs to the related technical field of photoelectric measurement, and particularly relates to a method and a device for measuring the exciton orientation of a luminescent material.
Background
The exciton orientation of the light emitting material determines the physical and optical properties of the film layer, such as charge mobility, birefringence, ionization potential, etc. Therefore, the photoelectric properties of the light emitting device are closely related to the orientation of excitons of the film layer, such as device lifetime, efficiency, ionization potential, carrier mobility, and the like. Therefore, understanding and controlling the exciton orientation of a light emitting material has been a very important issue in the research of light emitting devices. In terms of optics, one of the methods to improve the light coupling efficiency of light emitting devices is to make the exciton orientation level. When the excitons are oriented horizontally, most of the light is emitted at an angle perpendicular to the substrate of the device, which can reduce the loss of reflected light within the device and thus increase the external quantum efficiency and the proportion of light extracted from the device. Among all the methods, this method is suitable for most cases and is simple and convenient because it can be used independently of the manufacturing equipment; and no extra processing steps such as gratings, high-refractive-index layers and the like are required to be introduced, and no extra processing of a substrate or other functional layers is required. Electrically, the horizontal exciton orientation not only increases the efficiency of the device, but also has an effect on the transport of charge. When the excitons of the charge transport layer of the light emitting device are in the horizontal direction, the disorder degree can be reduced, the overlap of pi-pi bonds is increased, and the mobility of charge carriers can be improved by 30 times. Therefore, understanding and controlling the exciton orientation in light emitting materials has been a very important research topic in electronics and photonics, including applications in liquid crystals, semiconductors, organic field effect transistors (OTFTs), Organic Photovoltaics (OPVs), Light Emitting Diodes (LEDs) and Organic Light Emitting Diodes (OLEDs), where pursuing horizontal exciton orientation of light emitting materials is a culprit to improve the coupling efficiency of light emitting devices.
To better explore the exciton orientation of the light-emitting material, the efficiency of the light-emitting material is improved. Many optical measurement methods are used for analyzing the molecular orientation, such as ellipsometry, excited state lifetime measurement, angle resolution electroluminescence measurement, measurement of absorption before and after thermal annealing, and angle resolution photoluminescence measurement. The excited state lifetime method is to measure the excited state lifetime of the light emitting layer at different distances from the high reflectivity layer (e.g., the OLED cathode). The method still has large errors, and the method is inaccurate when applied to triplet-triplet exigences, triplet polaron exigences and exciton generation delay of fluorescent materials. The absorption amount before and after thermal annealing is measured, namely the absorption amount before and after annealing is measured to calculate the molecular orientation. This method does not require a mold, is quite simple but requires that the luminescent material be isotropically oriented prior to thermal annealing and the results are not quite accurate. At present, ellipsometry is commonly used to measure the optical constants of materials to determine the molecular orientation of luminescent materials, which is to measure the polarization and phase changes of linearly polarized light (S light and P light) reflected by a sample for multiple times, however, this measurement method is not suitable for doped material systems.
Disclosure of Invention
In view of the above drawbacks or needs for improvement in the prior art, the present invention provides a method and an apparatus for measuring the exciton orientation of a luminescent material, which construct a forward luminescence calculation model of an angle-resolved photoluminescence spectrum of the luminescent material, and provide a theoretical method for reversely determining the exciton orientation of the material by using the angle-resolved photoluminescence spectrum. Then, an optical system and a mechanical structure of the angle-resolved photoluminescence device are designed, an angle-resolved photoluminescence spectrum measurement experimental device is built, and the function and the performance of the experimental device are tested. Finally, the established experimental device and the proposed theoretical method are utilized to carry out experimental verification on the typical luminescent material, and compared with the current commonly used ellipsometry method, the method can accurately measure and characterize the molecular orientation of the doped material system.
To achieve the above object, according to one aspect of the present invention, there is provided a method for measuring an orientation of an exciton of a light emitting material, the method mainly comprising the steps of:
(1) constructing a forward luminescence calculation model for distinguishing a photoluminescence spectrum by a luminescent material, wherein the forward luminescence calculation model is used for equivalent quantum mechanical description of exciton transition to classical electromagnetic theory description of electric dipole radiation in a microcavity structure, then based on the electric dipole exciton radiation model and a microcavity coherence model, adopting a transmission matrix form, and transmitting and reflecting on all related interfaces by utilizing Fresnel coefficients to solve an electromagnetic field so as to obtain an emission spectrum angularly distinguished by the luminescent material;
(2) and measuring the angle-resolved spectrum of the material by adopting a measuring device for the exciton orientation of the luminescent material, and performing reverse inversion by combining a forward luminescence calculation model based on the measured angle-resolved spectrum to obtain the molecular orientation of the material.
Further, the forward light emission calculation model adopts a relation between exciton orientation and angle-resolved spectrum of the light-emitting material as follows:
in the formula IinjThe number of electrons is/e; gamma is the equilibrium ratio, i.e. the ratio of injected carriers to generated excitons; etaS/TIs the ratio of radiation, i.e. the ratio of energy radiation in the excitons produced; s (λ) is the inherent spectrum, i.e. the normalized spectrum of the luminescent material; g (z) is a function of dipole distribution; q. q.seffIs the effective radiative quantum efficiency, i.e. the ratio of outgoing photons to radiative excitons; etaoutFor the outcoupling efficiency; z is the dipole position; angle-resolved spectrum P of a luminescent materialout(λ, θ) is a function related to angle and wavelength.
Further, the proportion of vertical dipoles is extracted based on the forward light emission calculation model to obtain the exciton orientation of the light emitting material.
Further, the formula adopted in the inverse inversion in the step (2) is as follows:
Pout(λ,θ)＝F(Θ)
Θ＝F-1(Pout(λ,θ))
in the formula, Pout(λ, θ) is the angle-resolved spectrum of the luminescent material; Θ is the proportion of vertically oriented dipoles to the total dipoles, so Θ is 0 when the dipoles are horizontally oriented, 0.33 when randomly oriented, 1 when vertically oriented; the operator F is a polar orientation parameter theta and a luminescent material angle-resolved spectrum Pout(λ, θ); theta is an included angle between the emergent light and the normal of the substrate;
representing the transmit power of the vertical dipole; sigma p2Represents the total transmit power; cos (chemical oxygen demand)2θ represents the average projection of the transition dipole moment to the substrate normal.
Further, in the forward light emission calculation model, different exciton orientations can obtain unique corresponding emission spectra; to quantify the transition dipole moment direction of the dipoles, inverse inversion was used to fit the exciton orientation.
Further, inverse inversion fitting is performed using a least squares method to obtain exciton orientations.
According to another aspect of the present invention, there is provided a device for measuring the exciton orientation of a luminescent material, the device being adapted to perform the method as described above.
Furthermore, the measuring device comprises a light source, an emission optical system, a receiving optical system and a spectrometer which are sequentially arranged along the horizontal direction at intervals, and the light path of the emission optical system is coaxial with the light path of the receiving optical system.
Further, the measuring device further comprises a transmission mechanism, and the emission light source system is arranged on the transmission mechanism; the transmission mechanism drives the emission light source system to rotate at multiple angles so as to achieve angle resolution.
In general, compared with the prior art, the method and the device for measuring the orientation of the excitons of the luminescent material provided by the invention have the following beneficial effects:
1. the method uses a resolution photoluminescence method to measure the angle-resolved emission spectrum of the luminescent material, then obtains the exciton orientation of the luminescent material through inverse inversion, and can not only characterize the exciton orientation of the luminescent material, but also accurately measure and characterize the exciton orientation of the luminescent material of a doping system compared with the method of measuring the optical constant of the luminescent material by a common ellipsometry method to obtain the exciton orientation.
2. The forward simulation of the invention uses a couple exciton radiation model, a film layer transmission model and a microcavity coherent model, can calculate the emergent spectrum for isotropic materials and anisotropic materials, and adopts a least square method for reverse inversion, so that the method is simple and the operation speed is high.
3. The invention also provides a measuring device for realizing the method, the measurement of the orientation of the excitons of the luminescent material based on angle-resolved photoluminescence can be effectively realized through the research and design of the specific structure and the specific assembly mode of the device, the optical path system is greatly simplified, the measurement is accurate and quick, and the corresponding technical standard is as follows: angle range: 0-80 °; angular resolution: <1 °; sample size range: <30mm X30 mm. The wave band range is as follows: 400-800 nm; optical resolution: 1 nm.
4. The invention also provides the method that before measurement, the measurement device is calibrated to ensure the alignment of the light path and the setting of the rotation angle, the accuracy of the measurement device is ensured, and the accurate characterization of the orientation of the subsequent exciton is facilitated.
Drawings
FIG. 1 is a schematic flow chart of a method for measuring the orientation of excitons in a light-emitting material provided by the present invention;
FIG. 2 is a schematic measurement of an angle-resolved photoluminescence spectroscopy device;
FIG. 3 is a schematic diagram of a system architecture of an angle-resolved photoluminescence spectroscopy experimental apparatus;
FIG. 4 is a graph of the angle-resolved spectrum (full band) of an organic light-emitting material RD measured using an angle-resolved photoluminescence instrument of example 1;
FIG. 5 is a PL spectrum of an organic luminescent material RD of example 1;
FIG. 6 is a graph of example 1 using an angle-resolved photoluminescence spectroscopy device in combination with theoretical methods to measure and characterize the RD angle-resolved spectra (single wavelength) of organic photonic materials;
fig. 7 is a graph showing the measurement result of the optical constants of the organic luminescent material RD measured using the ellipsometer in example 1;
FIG. 8 is a graph of example 2 using an angle-resolved photoluminescence spectroscopy device to measure the RHRRD angle-resolved spectrum (full band) of the organic light emitting material;
FIG. 9 is a PL spectrum of the organic light emitting material RHRD of example 2;
FIG. 10 is a graph of example 2 using an angle-resolved photoluminescence spectroscopy device in combination with theoretical methods to measure and characterize the RFRD angle-resolved spectra (single wavelength) of organic optical materials;
fig. 11 is a graph illustrating the measurement result of the optical constants of the organic light emitting material RFRD measured using the ellipsometer of example 2.
The same reference numbers will be used throughout the drawings to refer to the same or like elements or structures, wherein: 1-transmitting optical system, 2-transmission mechanism, 3-receiving optical system, 101-light source, 102-first optical filter, 103-focusing lens, 104-diaphragm, 105-lens, 106-electric rotating platform, 107-wire grid polarizer, 108-second optical filter, 109-spectrometer.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
Referring to fig. 1, fig. 2 and fig. 3, in order to solve the problem of accurate measurement of molecular orientation of a doped material system in an organic light emitting device, the measurement method and apparatus provided by the present invention develop an angle-resolved photoluminescence measurement apparatus based on an angle-resolved spectrum, and construct a forward optical model and a reverse parameter reconstruction method to realize accurate measurement and characterization of molecular orientation in a doped light emitting film layer.
The invention makes a luminous sample emit light by a photoluminescence method, a light source obtains light spots through a light filter, a focusing lens and a diaphragm to irradiate the luminous sample, polarization vectors of light emitted by a dipole in the vertical direction are all parallel to an incident plane, polarization light parallel to horizontal incidence (namely P light: the polarization vector of light is in the plane) is collected through a polaroid, an angular distribution spectrum of far-field emission corresponding to a linearly polarized light mode (P light) is detected by a spectrometer, and the aim of angle resolution is achieved by rotating the light source and an objective table. And extracting the proportion of vertical dipoles based on a forward light-emitting calculation model according to the angle-resolved spectrum obtained by the device so as to obtain the exciton orientation of the luminescent material.
The method for measuring the exciton orientation of the luminescent material mainly comprises the following steps:
firstly, constructing a forward luminescence calculation model for distinguishing a photoluminescence spectrum by a luminescent material, wherein the forward luminescence calculation model is equivalent to a classical electromagnetic theory description of electric dipole radiation in a microcavity structure by quantum mechanical description of exciton transition, then based on the electric dipole radiation model and a microcavity coherence model, and adopting a transmission matrix form, and transmitting and reflecting on all related interfaces by utilizing Fresnel coefficients to solve an electromagnetic field so as to obtain an emission spectrum of the luminescent material with angle resolution.
Specifically, exciton radiation in the quantum mechanics category can be equivalent to electric dipole radiation in classical mechanics, a luminescent exciton equivalent model is proposed by Chance et al, an oscillating dipole near an interface is regarded as a forced damping harmonic oscillator, and the motion equation of the dipole is shown in formula (1):
wherein m is the mass of the dipole; x is the displacement of the dipole; b0Is the decay rate of free space; omega0A resonant angular frequency; e is a charge constant; eRIs the component of the reflected electric field acting on the dipole that is oriented parallel to the dipole's momentum; t is time.
The electromagnetic field is solved mainly by three models: a couple exciton radiation model, a film layer transmission model and a microcavity correlation model;
wherein, the couple exciton radiation model is shown as a formula (2):
wherein A represents the retardation potential generated by an electric dipole at r ═ (x, y, z), and ksIs the wavevector, j is the unit of imaginary number, ω is the angular frequency of oscillation of the electric dipole, μ0Is permeability in air, P0Is the dipole vector moment.
The film transport model is shown in equation (3):
Ψ(d)＝ejΩdΨ(0) (3)
in the formula, d is the thickness of the film layer, Ω d is the phase difference, and Ψ is the phase.
The microcavity coherent model is shown in equation (4):
in the formula (I), the compound is shown in the specification,
is the actual electric field of the light-emitting layer,
is the original electric field of the light-emitting layer, r+(k) Is the reflectivity of the upper surface of the film layer, r-(k) Is the reflectivity, k, of the lower surface of the filmzIs wave vector ksAnd d is the film thickness.
The formula of the emission spectrum obtained by combining the formulas (2) to (4) for the angle resolution of the luminescent material is as follows:
wherein, IinjThe number of electrons is/e; gamma rayThe equilibrium ratio, i.e., the ratio of injected carriers to generated excitons; etaS/TIs the ratio of radiation, i.e. the ratio of energy radiation in the excitons produced; s (λ) is the inherent spectrum, i.e. the normalized spectrum of the luminescent material; g (z) is a function of dipole distribution; q. q.seffIs the effective radiative quantum efficiency, i.e. the ratio of outgoing photons to radiative excitons; etaoutFor the outcoupling efficiency; z is the dipole position; angle-resolved spectrum P of a luminescent materialout(λ, θ) is a function that is related to angle and wavelength, where λ is the wavelength in the angle-resolved spectrum and θ is the angle in the angle-resolved spectrum.
In this embodiment, the proportion of the vertical dipoles is extracted based on the forward light emission calculation model to obtain the exciton orientation parameter of the light emitting material, and then the relationship between the exciton orientation parameter of the material and the angle-resolved spectrum is established.
And step two, measuring the angle-resolved spectrum of the material by adopting an angle-resolved photoluminescence spectrum measuring device, and performing reverse inversion by combining a forward luminescence calculation model based on the measured angle-resolved spectrum to obtain the molecular orientation of the material.
Specifically, the steps of obtaining the exciton orientation of the luminescent material to be tested are as follows:
(1) monochromatic ultraviolet light of the light source passes through the optical filter, the focusing lens and the diaphragm to obtain light spots with the size of less than 1mm multiplied by 1 mm.
(2) The luminous material film is irradiated by the light spot, the luminous material film is excited to emit light waves, the semi-cylindrical quartz lens couples the light emitted from the luminous material, and linear polarized light (P light) is obtained through the wire grid polarizing plate.
(3) After the linearly polarized light passes through the optical filter to filter light of the light source, light waves are coupled and enter the optical fiber to be transmitted into the spectrometer, and therefore the spectrum of the linearly polarized light is recorded.
(4) The purpose of angle resolution is achieved by rotating the light source and the object stage by using a rotating motor, so that the angle-resolved spectrum of the luminescent material is obtained.
(5) And (3) according to the angle-resolved spectrum of the experimental measurement luminescent material, obtaining the exciton orientation of the measured luminescent material by combining an inversion method after data processing.
The angle-resolved photoluminescence spectrum measuring device is the measuring device for the exciton orientation of the luminescent material, and the measuring device comprises a light source 101, an emission optical system 1, a receiving optical system 3 and a spectrometer 109 which are sequentially arranged along the horizontal direction at intervals. The light source 101 is a light-emitting layer for irradiating a light-emitting material to be measured so that the light-emitting layer emits light. The light source 101 selects a single-wavelength ultraviolet LED light source with higher energy to ensure the power, and the volume of the whole device is reduced.
The emission light source system 1 includes a first optical filter 102, a focusing lens 103, and a diaphragm 104, wherein stray light is filtered from light emitted from the light source 101 through the first optical filter 102, light beams of the light source are focused through the focusing lens 103, an interface agent of the light is adjusted through the diaphragm 104, and the stray light is prevented from entering, so that the purpose of controlling the size of a light spot is achieved, and the size of the light spot is smaller than 1mm × 1 mm.
The transmission mechanism 2 includes an electric rotating platform 106, the emission optical system 1 is disposed on the electric rotating platform 106, and the light source 101, the first optical filter 102, the focusing lens 103, the diaphragm 104, and the semi-cylindrical fused silica lens 105 rotate together with the electric rotating platform 106. The light spot irradiates on the organic light-emitting sample, and the sample emits light.
The receiving optical system 3 comprises a wire grid polarizer 107, a second optical filter 108 and an optical fiber, after a sample is excited, emergent light passes through the semi-cylindrical fused quartz lens 105, polarization vectors of light emitted by dipoles in the vertical direction are all parallel to an incident plane, polarized light (namely P light) parallel to horizontal plane incident is collected through the polarizer, then light of a light source is filtered through the optical filter, light waves are coupled into the optical fiber, and light intensity of the P polarized light recorded by only the spectrometer 109 is transmitted. The polarizing plate is used to convert light emitted from the organic light emitting material into linearly polarized light (P light). The optical filter is used to filter out the influence of the light from the light source on the angle-resolved spectrum, and the optical fiber is used to guide the emitted light to a fixed spectrometer 109, wherein the spectrometer 109 is used to obtain spectral information of the sample.
The transmission mechanism is mainly used for providing power to realize multi-angle rotation, and the purpose of angle resolution is achieved. The rotating arm mechanism is used for bearing the emission optical system and the sample table and providing installation positions for the optical elements, so that all the light source elements of the emission optical system and the sample table can be on the same straight line. The rotating arm mechanism consists of a mounting bracket, a support rod, a sleeve, a cage plate connecting rod and the like.
For the compactness of the angle-resolved photoluminescence spectrum measuring device, the sample stage is set to be capable of installing a semi-cylindrical fused quartz lens and reasonably placing a sample. The sample stage is the center of rotation and should be on the central axis. By installing the bracket on the rotating motor and fixing the sample stage and the plano-convex cylindrical mirror at the same time, the sample and the emission light source system are ensured to rotate together and be in a straight line.
The measuring device performs a calibration before the measurement. The light source enters the emission optical system from a certain angle, the light wave generated by the sample after being excited is emitted through the semi-cylindrical fused quartz lens, and the receiving optical system receives signals; meanwhile, when the transmitting optical system rotates a certain angle, the receiving optical system can receive signals. To achieve this, the angle-resolved photoluminescence spectroscopy measurement system needs to meet the following requirements: firstly the optical paths of the transmitting optical system must be coaxial and secondly the optical paths of the receiving optical system must also be coaxial. The optical path of the receiving optical system is defined to be 0 °, and when the transmitting optical system is also 0 °, the optical path of the transmitting optical system must be coaxial with the receiving optical system. Finally the sample must be coaxial with the emission optics. To achieve this, the calibration method is described with reference to fig. 2, and the specific measures are as follows: the light source 101, the optical filter 102, the focusing lens 103, the aperture 104, and the semi-cylindrical fusion lens 105 are aligned in this order with the optical path, and the polarizing plate 107 and the optical filter 108 are also aligned in this order with the optical path. By covering the reticle alignment target on the optical mount of polarizer 107, only the light wave from the light source is detected without placing a sample. If the light from the light source strikes exactly the center of the reticle alignment target, the transmit optics are aligned with the receive optics. Or detecting the spectrum of-90 degrees by a spectrometer. The light intensity is highest at 0 degree, the spectra of 0-90 degrees and-90-0 degrees are symmetrical according to the 0 degree axis, and the measuring device can be calibrated by using isotropic luminescent materials to fix the 0 degree angle of the rotating motor.
And obtaining a reverse inversion theoretical method of exciton orientation according to the forward luminescence calculation model.
Pout(λ,θ)＝F(Θ) (6)
Θ＝F-1(Pout(λ,θ)) (7)
Wherein, Pout(λ, θ) is the angle-resolved spectrum of the luminescent material; Θ is the proportion of vertically oriented dipoles to the total dipoles, so Θ is 0 when the dipoles are horizontally oriented, 0.33 when randomly oriented, 1 when vertically oriented; the operator F is a polar orientation parameter theta and a luminescent material angle-resolved spectrum Pout(λ, θ); theta is an included angle between the emergent light and the normal of the substrate;
representing the transmit power of the vertical dipole; sigma p2Represents the total transmit power; cos (chemical oxygen demand)2θ represents the average projection of the transition dipole moment to the substrate normal. Parameters such as dipole orientation, a light emitting area, an optical constant, thickness and the like are necessarily known in the emission spectrum obtained according to a forward light emission calculation model of the emission spectrum of the light emitting material, and in the forward light emission calculation model, the parameters are used as input parameters, and different exciton orientations can obtain unique corresponding emission spectra. To quantify the transition dipole moment direction of the dipoles, inverse inversion was used to fit the exciton orientation. The angle-resolved spectrum of the luminescent material is obtained by the angle-resolved photoluminescence device, and the molecular orientation can be reversely solved according to the light intensity data obtained by experiments by utilizing a least square method.
In the optical model used to quantify the molecular orientation, the refractive index and thickness of the sample stack are the necessary input parameters. With variable angle ellipsometry, the relevant optical constants and thickness, i.e. refractive index n, extinction coefficient k, and thickness d, can be determined. The inverse inversion is to obtain the exciton orientation by using the exciton orientation as a unique fitting parameter (knowing a plurality of parameters such as optical constants, thicknesses, dipole distribution areas and the like of each layer of the light-emitting device) through a least square method after an actual emission spectrum of the light-emitting material is obtained through experiments, and the error between the angle-resolved spectrum in the orientation and the angle-resolved spectrum obtained through the experiments is minimized.
The method in which the ellipsometer characterizes the exciton orientation of the luminescent material is as follows: after the ellipsometry parameters of a sample are measured by a multi-angle spectrum ellipsometer, a Cauchy dispersion dielectric function is adopted in a transparent wave band in the fitting process, the thickness of a luminescent material is preliminarily determined, then a B-spline model is used for expanding to a full wave band, a proper step length is required to be selected at this moment, an oscillator model is selected for fitting according to a dielectric function curve, a combination of Tauc-Lorentz and a plurality of Gaussian oscillators is generally selected, and finally the optical constant of the obtained material is obtained.
The exciton orientation of a material is determined by measuring the anisotropic optical constants of the material.
(1-Θ)：Θ＝(sin2θ):(cos2θ) (10)
Wherein k ise maxRepresents the maximum value of the extinction coefficient in the vertical direction due to the transition dipole moment; k is a radical ofo maxRepresents the maximum value of the extinction coefficient in the horizontal direction due to the transition dipole moment; s is an exciton orientation parameter with a value range of [ -0.5,1 [)](ii) a Theta represents the average value of the angle between the moment vector of the transition dipole and the normal vector perpendicular to the substrate, and the value range is 0-90 DEG]。
And step three, comparing the obtained molecular orientation result with the result of the ellipsometry to verify the correctness of the measurement method.
The accuracy of the measuring instrument is verified as follows: the ellipsometry measurement of the luminescent material (pure material is not doped) is accurate and well-recognized, so that the data of incidence angle in the range of 60 ° to 70 ° every 5 ° is recorded by an ellipsometer, and the optical constants and thickness of the luminescent material are determined in combination with the analysis software thereof, and the molecular orientation of the pure luminescent material is found according to the analyzed optical constants; the spectral data of the luminescent material is measured by the built angle-resolved photoluminescence instrument to further obtain the molecular orientation of the luminescent material. And comparing the results of the two, and correspondingly debugging the angle-resolved photoluminescence spectroscopy instrument to ensure that the measurement result of the instrument is accurate and reliable.
For the present method, the substrate of the light emitting layer must be a glass substrate, and the sample size range: <30mm X30 mm.
The present invention is further described in detail below with reference to several specific examples.
Example 1
This example relates to the exciton orientation of the pure luminescent material RD. Sample A is a luminescent film made of material RD by vacuum evaporation, and the substrate is quartz glass; the theoretical thickness of material RD is 50nm and sample A is shaped with square dimensions of about 20mm by 20 mm. The method specifically comprises the following steps:
(1) the luminescent layer material with glass as the substrate is coated with refractive index matching oil and is fixed on a sample stage of an angle-resolved photoluminescence instrument. The LED light source (375nm) was turned on, the rotary table was rotated 80 °, the angle resolved spectral data was measured every 1 ° through a semi-cylindrical fused silica lens using a spectrometer and the spectral data was recorded using a spectrometer as shown in fig. 4.
(2) The spectrum data with the wavelength of 400nm to 800nm in each angular spectrum range of 0 to 80 degrees is obtained, the band with the strongest luminescence, namely 617nm, is selected from the spectrum data according to the PL spectrum of the luminescent material as shown in figure 5, the spectrum is normalized, the abscissa is the angle, and the ordinate is the intensity after normalization. An angle resolved spectrum (single wavelength) is obtained as shown by the dots in FIG. 6.
(3) The experimental data are imported into MATLAB, and also the thickness and optical constants of the sample to be tested and the glass are imported. The exciton orientation parameter S of the luminescent material RD is-0.0635, theta is 0.291, namely the vertical dipole ratio is 0.291, and the angle resolution spectrum corresponding to the orientation is shown in a curve of FIG. 6.
(4) The data of the incidence angle measurement RD in the range of 60 ° to 70 ° were recorded by an ellipsometer, and the optical constants thereof were analyzed in conjunction with the software thereof. And (4) carrying out analysis on the transparent waveband by using a Cauchy model, extending the band to the whole waveband through a b-spline model, and meeting the consistency of a k-k relation. The optical constants of the sample to be measured were obtained by the additive oscillator matching analysis, and as shown in fig. 7, the solid line indicates the index of refraction and extinction coefficient of the luminescent material RD in the horizontal direction by the subscript o, and the broken line indicates the index of refraction and extinction coefficient of the luminescent material RD in the vertical direction by the subscript e. The exciton orientation parameter S of the luminescent material RD was determined to be 0.008 from the analyzed optical constants, and Θ is 0.3386, i.e., the vertical dipole ratio 0.3386.
Because it is a pure luminescent material, the results of the default ellipsometer measurement and characterization of the molecular orientation of RD are accurate. By comparing the results of measuring and characterizing the RD luminescent material with the ellipsometer and the angle-resolved photoluminescence device, the error between the molecular orientation of the angle-resolved spectroscopic molecules and the ellipsometric analysis parameter Θ is about 0.04. The reason for the error may be inaccurate fitting at low angles in the inverse simulation. But this error is within an acceptable range. The deviation of the fit from the experiment occurs mainly at low and high angles. The low angle error may be the effect of stray light from the light source on the angle-resolved spectrum, and the high angle error is the sensitivity of the spectrometer.
Example 2
This example relates to the molecular orientation of the doped luminescent material RHRD. RHRD is a luminescent material RH as a main material, the RD doping proportion is 3 percent as a luminescent film, the substrate is glass, and the RHRD is prepared by vacuum evaporation, and the area size of the RHRD is about 20mm multiplied by 20 mm. The method specifically comprises the following steps:
(1) the luminescent layer material with glass as the substrate is coated with refractive index matching oil and is fixed on a sample stage of an angle-resolved photoluminescence instrument. The LED light source (375nm) was turned on, debugged to determine and fix the 0 ° position of the rotary stage, the rotary stage was rotated 80 °, the angle resolved spectral data was measured every 1 ° using a spectrometer through a semi-cylindrical fused silica lens and the spectral data was recorded using a spectrometer as shown in fig. 8.
(2) The spectrum data with the wavelength of 400nm to 800nm in each angular spectrum range of 0 to 80 degrees is obtained, the band with the strongest luminescence is selected from the spectrum data according to the PL spectrum of the luminescent material as shown in FIG. 9, the spectrum is normalized, the abscissa is the angle, and the ordinate is the intensity after normalization. An angle resolved spectrum (single wavelength) is obtained as shown by the dots in FIG. 10.
(3) The experimental data are imported into MATLAB, and also the thickness and optical constants of the sample to be tested and the glass are imported. The molecular orientation parameter S of the organic luminescent material RHRD obtained by the inverse inversion method is-0.2750, theta is 0.15, namely the vertical dipole ratio is 0.15, and the angle-resolved spectrum corresponding to the orientation is shown in a curve of FIG. 10.
(4) The data of the incidence angle measurement RHRD in the range of 60 ° to 70 ° were recorded by an ellipsometer and the optical constants thereof were analyzed in conjunction with the software thereof. And (4) carrying out analysis on the transparent waveband by using a Cauchy model, extending the band to the whole waveband through a b-spline model, and meeting the consistency of a k-k relation. The optical constants of the sample to be measured obtained by the additive oscillator matching analysis are shown in fig. 11, in which the solid line indicates that the refractive index and the extinction coefficient of the light-emitting material RHRD in the horizontal direction are indicated by the subscript o, and the broken line indicates that the refractive index and the extinction coefficient of the light-emitting material RHRD in the vertical direction are indicated by the subscript e. Obtaining an exciton orientation parameter S of the luminescent material RHRD according to the analyzed optical constants to be 0.0543, wherein theta is 0.2971, namely the vertical dipole ratio 0.2971;
from the analysis of the RD material, it was confirmed that the angle-resolved photoluminescence device combined with the inverse fit could yield the correct molecular orientation. When the ellipsometer measures the doped material, the measurement result is inaccurate because the doping ratio of the luminescent material is too small. At this time, the molecular orientation result of the ellipsometry analysis is greatly different from the result of the angle-resolved spectroscopic analysis, which proves the superiority of the angle-resolved photoluminescence device and the inverse fitting thereof in the aspect of the doped luminescent material.
The invention selects an angle-resolved photoluminescence spectrum method to measure the angle-resolved spectrum of the organic luminescent material, utilizes photoluminescence to excite the luminescent material to emit light, and uses an optical component, namely a semi-cylindrical fused quartz lens, to ensure that the emergent angle of the optical component is not deflected and a substrate mode and an emergent mode are extracted. The angular distribution spectrum of the far field emission corresponding to all polarization modes (p-and s-light) is detected for angular resolution by rotating the light source and stage. The vertical dipole only contributes to the TM polarization (i.e., p-light), and the polarization parallel to the horizontal plane of incidence (i.e., p-light) is collected by the polarizer, and the spectrometer records the intensity of the p-polarized light. Therefore, the proportion of vertical dipoles can be extracted based on the inversion of the emergent spectrum obtained by experiments so as to obtain the exciton orientation of the luminescent material. The device can accurately measure and characterize the molecular orientation of various luminescent materials. Compared with an ellipsometry method, the angle-resolved spectroscopic measurement realizes accurate measurement of the organic luminescent material of the doping system, can meet the requirement of the optical field on characterization of exciton orientation, provides support for simulation analysis and optimization design of a luminescent device, and has the advantages of simplicity, compactness, high speed, low cost and the like.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (6)
1. A method of measuring the orientation of excitons in a light-emitting material, the method comprising the steps of:
(1) constructing a forward luminescence calculation model for distinguishing a photoluminescence spectrum by a luminescent material, wherein the forward luminescence calculation model is used for equivalent quantum mechanical description of exciton transition to classical electromagnetic theory description of electric dipole radiation in a microcavity structure, then based on the electric dipole exciton radiation model and a microcavity coherence model, adopting a transmission matrix form, and transmitting and reflecting on all related interfaces by utilizing Fresnel coefficients to solve an electromagnetic field so as to obtain an emission spectrum angularly distinguished by the luminescent material;
(2) and measuring the angle-resolved spectrum of the material by adopting a measuring device for the exciton orientation of the luminescent material, and performing reverse inversion by combining a forward luminescence calculation model based on the measured angle-resolved spectrum to obtain the molecular orientation of the material.
2. The method of measuring the orientation of an exciton of a light-emitting material according to claim 1, wherein: the forward light-emitting calculation model adopts a relational expression of exciton orientation and angle-resolved spectrum of a light-emitting material as follows:
in the formula IinjThe number of electrons is/e; gamma is the equilibrium ratio, i.e. the ratio of injected carriers to generated excitons; etaS/TIs the ratio of radiation, i.e. the ratio of energy radiation in the excitons produced; s (λ) is the inherent spectrum, i.e. the normalized spectrum of the luminescent material; g (z) is a function of dipole distribution; q. q.seffIs the effective radiative quantum efficiency, i.e. the ratio of outgoing photons to radiative excitons; etaoutFor the outcoupling efficiency; z is the dipole position; angle-resolved spectrum P of a luminescent materialout(λ, θ) is a function related to angle and wavelength.
3. The method of measuring the orientation of an exciton of a light-emitting material according to claim 1, wherein: and extracting the proportion of vertical dipoles based on the forward light-emitting calculation model so as to obtain the exciton orientation of the light-emitting material.
4. The method of measuring the orientation of an exciton of a light-emitting material according to claim 1, wherein: the formula adopted in the reverse inversion in the step (2) is as follows:
Pout(λ,θ)＝F(Θ)
Θ＝F-1(Pout(λ,θ))
in the formula, Pout(λ, θ) is the angle-resolved spectrum of the luminescent material; Θ is the proportion of vertically oriented dipoles to the total dipole, so Θ is 0 when the dipoles are horizontally oriented, and randomly orientedΘ is 0.33, and in vertical orientation Θ is 1; the operator F is the exciton orientation parameter theta and the angle-resolved spectrum P of the luminescent materialout(λ, θ); theta is an included angle between the emergent light and the normal of the substrate;
representing the transmit power of the vertical dipole; sigma p2Represents the total transmit power; cos (chemical oxygen demand)2θ represents the average projection of the transition dipole moment to the substrate normal.
5. The method for measuring the orientation of an exciton of a luminescent material as claimed in any one of claims 1 to 4, wherein: in the forward light emitting calculation model, different exciton orientations can obtain unique corresponding emergent spectra; to quantify the transition dipole moment direction of the dipoles, inverse inversion was used to fit the exciton orientation.
6. The method of measuring the orientation of an exciton of a luminescent material as claimed in claim 5, wherein: and performing inverse inversion fitting by using a least square method to obtain the exciton orientation.
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