A two-photon fluorescence-based method for testing the type of semiconductor carriers

Abstract

This invention relates to a semiconductor carrier type testing method based on two-photon fluorescence, comprising the following steps: S1, selecting perovskite as the test sample and placing the test sample on the sample stage of a fluorescence spectroscopy detection system; S2, adjusting the excitation wavelength in the light source module of the fluorescence spectroscopy detection system, wherein the excitation wavelength passes through a 4f system, enters an attenuator, and generates a power I. exc1 Finally, the light is irradiated onto the sample to be tested; S3, the sample to be tested produces fluorescence, which is absorbed by the spectrometer of the fluorescence spectroscopy detection system and collected. exc1 The corresponding two-photon fluorescence intensity I 2ppl1 S4. Adjust the attenuator and repeat step S2; S5. Obtain the exponential coefficient b according to the formula, where D is a constant. This invention can conveniently, simply, and non-destructively measure the carrier type in perovskites, eliminating the influence of defects compared to single-photon excitation, and obtaining more accurate information.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of organic semiconductors, and particularly relates to a semiconductor carrier type testing method based on two-photon fluorescence. BACKGROUND

[0002] Perovskite, as a new emerging semiconductor material, has attracted extensive attention due to its excellent photoelectric properties, simple solution processability and low preparation cost. Compared with traditional semiconductor materials, perovskite has the advantages of large absorption coefficient, high carrier mobility, long carrier diffusion distance, long carrier lifetime and adjustable band gap. Therefore, the applications of light-emitting diodes (LEDs), solar cells, photodetectors, lasers and the like based on perovskite materials have rapidly developed.

[0003] Generally, when the electrons in perovskite material are excited, two types of carriers are generated: free carriers and excitons. Free carriers are free-moving excited-state electrons and holes. Excitons are bound electron-hole pairs. The difference between the two lies in the size of the Coulomb force between the excited electrons and holes. Generally, the exciton binding energy is used to describe the size of the Coulomb interaction between the corresponding electrons and holes. When the exciton binding energy is large at room temperature, the excited electrons and holes are tightly bound together to form excitons. When the exciton binding energy is small, the electron-hole pairs will quickly dissociate into free carriers in a short time. Free carriers are charged, while excitons are electrically neutral due to the tight binding of electrons and holes. The difference between them has an important influence on the photophysical processes in perovskite. The longer diffusion distance of free carriers makes them more suitable for photovoltaic cells, while the radiative recombination of excitons can achieve high quantum efficiency at a lower carrier density, and thus is more suitable for light-emitting applications. Therefore, before the application of perovskite material, it is necessary to distinguish its specific carrier type.

[0004] Generally, when studying the carriers in perovskite material, the size of the exciton binding energy is measured or theoretically calculated, and then compared with the thermal motion energy kT of about 26 eV at room temperature: when the exciton binding energy is greater than kT, it is considered that excitons are formed; and when the exciton binding energy is less than kT, the electron-hole pairs will dissociate into free carriers due to thermal motion. This method is too rough, because the situation in perovskite material is very complex, and simply comparing with the thermal motion energy at room temperature cannot correctly determine the carrier type, and in most cases it is wrong.

[0005] When the carrier type in a specific perovskite material is needed, the dependence of single-photon fluorescence intensity on excitation power is often used to determine it. This method is more inclined to the actual situation of the carriers in the perovskite than the above method. However, it also has some defects: the dependence coefficient is too close, making it difficult to determine the real situation in the perovskite; both exciton recombination and trap state induced recombination are single-molecule recombination, and it is difficult to distinguish between the two when determining the power dependence. During the preparation of perovskite materials, a large number of defects are inevitably generated on the surface. These defects will capture the generated carriers, resulting in non-radiative recombination of the carriers. Therefore, a method that can exclude the influence of defects is still needed to identify the carrier type in perovskite materials. SUMMARY

[0006] To solve the above problems, the present application provides a semiconductor carrier type testing method based on two-photon fluorescence, which comprises the following steps:

[0007] S1, selecting perovskite as the sample to be tested, and placing the sample to be tested on the sample stage of a fluorescence spectrum detection system;

[0008] S2, adjusting the excitation wavelength in the light source plate of the fluorescence spectrum detection system, the excitation wavelength entering the attenuating sheet through a 4f system and generating a power I exc1 , which finally irradiates on the sample to be tested;

[0009] S3, the sample to be tested generates fluorescence, which is absorbed by the spectrometer of the fluorescence spectrum detection system, and the corresponding two-photon fluorescence intensity I exc1 is collected; 2ppl1

[0010] S4, adjusting the attenuating sheet and repeating step S2, so as to generate powers I exc2 -I excn , and the corresponding two-photon fluorescence intensities I 2ppl2 -I 2ppln ;

[0011] S5, according to the formula logI 2ppl =b log D·I exc , substituting the I exc1 -I excn and the corresponding I 2ppl1 -I 2ppln , and performing fitting to obtain the exponential coefficient b, wherein D is a constant;

[0012] wherein the energy hυ of the excitation wavelength satisfies:

[0013] 0.5E g <hυ<E g ;​

[0014] E g The optical band gap of the sample to be measured.

[0015] When an electron is excited by photon energy, it jumps to an excited state and forms a carrier. After that, the carrier will undergo two main processes: transport and recombination. Transport includes diffusion and drift, which has no effect on the nature of the carrier itself. While carrier recombination generally has four main recombination channels: free carrier recombination, exciton recombination, trap state induced recombination, and Auger recombination. Free carrier recombination is a two-molecule recombination involving two freely moving free carriers: electrons and holes, which is the main source of radiative recombination. Exciton recombination is a single-molecule recombination, and the electron-hole pair that makes up the exciton itself recombines to release energy and relax to the ground state. Trap state induced recombination refers to the non-radiative recombination of free carriers (electrons or holes) trapped in defect sites in perovskite materials. Since it only involves one electron or hole, it is also a single-molecule recombination. Auger recombination is a multi-body process involving the recombination of an electron and a hole, accompanied by energy and momentum transfer to a third participant, which is also a non-radiative recombination that occurs when the carrier concentration is very high. These four recombination channels interact and compete with each other, and can be used to determine the carrier situation. In carrier dynamics, they can be related to carrier density, which can be expressed as:

[0016]

[0017] where k1 is the rate constant of single-molecule recombination, determined by the rate of exciton recombination or trap state induced recombination of a single free carrier. The rate constant k2 of bimolecular recombination reflects the recombination rate between free carriers. And k3 represents Auger recombination, which reflects the rate of three-molecule recombination. This formula represents the decay process of photo-generated carriers, i.e. the dynamics process, which is determined by the above recombination processes. And dn / dt represents the fluorescence intensity I pl .

[0018] For two-photon excitation, the carrier density A and the excitation power I exc have the following relationship:

[0019]

[0020] where A is a parameter that depends on the cross-sectional area of the spot and the laser pulse. By combining the two relationships, the relationship between the excitation power and the four recombination modes (as shown in Figure 1 ) can be derived from the two equations, and the information of the carrier can be obtained:

[0021]

[0022] The index b is a coefficient determined by the carrier recombination process, and the size of the coefficient b can be obtained through the power dependence test of the fluorescence intensity, so as to obtain the information of the carrier in the perovskite:

[0023] 1) When the photo-generated carrier is a free electron and a hole,

[0024]

[0025] At this time, the index coefficient b is equal to 4.

[0026] 2) When the photo-generated carrier is an exciton,

[0027]

[0028] At this time, the index coefficient b is equal to 2.

[0029] 3) When the recombination is induced by a trap state,

[0030]

[0031] At this time, the index coefficient b is also equal to 2.

[0032] Wherein, k1 and k2 represent single-molecule recombination rate constant and double-molecule recombination rate constant respectively; K1 and K2 represent constants containing k1 and k2 respectively.

[0033] However, the mechanism of two-photon excitation is different from that of single-photon. When two-photon excitation, the electron absorbs two photons of equal energy and jumps to the excited state. Compared with single-photon excitation, two-photon excitation focuses on the inside of the sample, while most of the defects generated during the growth of perovskite are on the surface, so the influence of surface defects on distinguishing carriers can be eliminated by two-photon excitation.

[0034] The relevant mechanism diagram is shown in Figure 2 .

[0035] Therefore, it can be concluded that:

[0036] If b = 2, the carrier type is an exciton;

[0037] If 2<b<4, excitons and free carriers coexist;

[0038] If b = 4, the carrier type is a free carrier.

[0039] Further, the fluorescence spectrum detection system comprises the following blocks: a light source block, a 4f system, an attenuation sheet, a spectrometer, and a plurality of mirrors.

[0040] Further, the perovskite is selected from a three-dimensional perovskite or a quasi-two-dimensional perovskite.

[0041] Further, the Iexcn and I 2ppln In the formula, n is a positive integer greater than or equal to 3.

[0042] Further, the excitation wavelength of the light source plate is in the range of 600-1300 nm.

[0043] Further, the index coefficient b is in the range of 2≤b≤4.

[0044] Further, the 4f system comprises a front convex lens and a rear convex lens, wherein the focal length of the front convex lens is smaller than that of the rear convex lens.

[0045] Further, the fluorescence spectrum detection system further comprises a dichroic mirror, a CCD and a color filter.

[0046] The present application has the following advantages:

[0047] The present application discloses a semiconductor carrier type testing method based on two-photon fluorescence. Figure 3 Compared with single-photon excitation, the present application has the following advantages: single-photon excitation is absorbed by the material on the surface, and the excitation intensity exponentially decays with the increase of the depth, so that the photon can only stay on the surface of the perovskite sample to be measured.

[0048] In summary, the present application can conveniently, simply and non-destructively measure the carrier type in the perovskite, and the influence of defects is excluded relative to single-photon excitation, so that more real information is obtained. BRIEF DESCRIPTION OF DRAWINGS

[0049] Figure 1 Fig. 4 shows four main composite channels of carriers;

[0050] Figure 2 Fig. 5 shows a comparison diagram of single-photon excitation and two-photon excitation;

[0051] Figure 3 Fig. 6 shows the positions of excited electrons on the sample when single-photon is incident on the perovskite sample, and when two-photon is incident on the perovskite sample;

[0052] Figure 4 Fig. 7 shows the XRD results of three perovskite single crystals in the preparation example;

[0053] Figure 5The microscope optical bright field images of three perovskite single crystals in the preparation example are shown.

[0054] Figure 6 The structural schematic diagram of the fluorescence spectrum detection system used in the application is shown. Wherein

[0055] 1 - light source; 2 - 4f system; 3 - attenuation sheet; 4 - mirror; 5 - sample; 6 - color filter; 7 - CCD; 8 - spectrometer.

[0056] Figure 7 The statistical experimental results of the single and two-photon excitation fluorescence intensity power dependence test of the three perovskite single crystals are shown. DETAILED DESCRIPTION

[0057] In order to more clearly illustrate the technical solutions of the application, the following examples are listed. The raw materials, reactions and post-processing means appearing in the examples are all common raw materials on the market and technical means familiar to those skilled in the art, unless otherwise stated.

[0058] Preparation example

[0059] Due to the natural existence of grain boundaries in perovskite polycrystals, the transport, diffusion length and lifetime of carriers in the crystal will be affected by the grain boundaries. At the same time, the random orientation of the grains in the polycrystal will lead to uneven carrier transport. Compared with polycrystals, single crystals have the advantages of high-quality crystal structure, clear physical and optical properties, high stability, etc. Under the influence of environmental factors such as humidity, high temperature and ultraviolet light, single crystal perovskite materials are more stable and less likely to degrade. At the same time, the uniform orientation of the grains in the single crystal material can produce uniform optoelectronic properties, which is more conducive to the study of the optoelectronic properties of perovskite materials. Therefore, perovskite single crystals are selected as the research object of this experiment.

[0060] In this experiment, (PEA)2PbI4, MAPbBr3 and CsPbBr3 micro single crystals were synthesized by anti-solvent vapor assisted crystallization method. This synthesis method has unique advantages. Compared with perovskites synthesized by other methods, it has higher uniformity and fewer defects. This can improve the performance and stability of perovskite devices (such as solar cells). At the same time, the anti-solvent vapor assisted crystallization method is low in cost because it does not require special equipment or complex processing steps, and has a high success rate.

[0061] The related synthesis steps are as follows:

[0062] The synthesis experimental steps of MAPbBr3 single crystal are as follows:

[0063] S1, Preparation of precursor solution: 0.1505 g (1.5 mmol) of PbBr2 and 0.1680 g (1.5 mmol) of MABr were weighed respectively. Then 10 mL of DMF solution was weighed and poured into a glass bottle with a lid (volume 20 mL). Then 1.5 mmol of PbBr2 and 1.5 mmol of MABr were poured into 10 mL of DMF solution, and the solution was left still at room temperature and ultrasonicated for 10 min to ensure that the two raw materials were completely dissolved in DMF. Finally, the completely dissolved precursor solution was filtered using a PTFE filter with a pore size of 0.22 μm to obtain a clear precursor solution.

[0064] S2, Preparation of growth substrate: The glass substrate was cleaned by sequentially ultrasonically treating with detergent, acetone, deionized water and ethanol. Then the substrate was blown dry with high-purity nitrogen and placed in a vacuum drying box for vacuum drying, and finally naturally cooled to room temperature for standby use.

[0065] S3, Single crystal growth: The treated 15 mm x 15 mm glass substrate was placed on a Teflon sample holder, and the sample holder was placed in a 500 mL beaker. 5 μL of precursor solution was added to the glass substrate using a 20 μL pipette. An appropriate amount of dichloromethane was added to the beaker as an anti-solvent. Dichloromethane is volatile, and the solute in the precursor is difficult to dissolve in dichloromethane. The gas volatilized from dichloromethane will promote the crystallization of the precursor solution. Finally, the beaker was sealed and placed in a cool place for 36 hours. It can be seen that under the action of the anti-solvent dichloromethane, orange crystals gradually precipitated on the glass substrate, and a layer of visible MAPbBr3 three-dimensional perovskite single crystals was formed on the glass substrate after the precursor solution dried.

[0066] Synthesis of (PEA)2PbI4 and CsPbBr3 single crystals:

[0067] The synthesis steps are consistent with the above-mentioned MAPbBr3 single crystal, except that toluene is used as an anti-solvent in the synthesis of (PEA)2PbI4 two-dimensional perovskite single crystals, and the time required for complete crystal growth is about 72 hours, which is longer than the growth process of MAPbBr3 single crystals, which may be related to the volatility of toluene. In the synthesis of CsPbBr3 single crystals, a mixture of dichloromethane (DCM) and toluene is used as an anti-solvent to promote crystal growth, which can obtain high-quality CsPbBr3 three-dimensional perovskite single crystals.

[0068] The above samples were characterized, and the results are as follows:

[0069] Firstly, the X-ray diffraction (XRD) of (PEA)2PbI4, MAPbBr3 and CsPbBr3 perovskite single crystal samples were measured to characterize the crystal results of the prepared materials, and the results are shown in FIG. 1. Figure 4 This shows that the perovskite single crystals synthesized by the anti-solvent vapor assisted crystallization method have good quality and crystallinity. The periodic peaks in the XRD results of (PEA)2PbI4 single crystals also prove that a clear two-dimensional structure is formed. The three perovskite single crystal samples all meet the requirements of this experiment.

[0070] Figure 5 The microscope optical bright field images of the prepared (PEA)2PbI4, MAPbBr3 and CsPbBr3 perovskite single crystal samples are shown. As can be seen from the figure, the three single crystal samples are relatively light orange, and the crystal surface is smooth and the inside is uniform, showing high crystal quality.

[0071] Embodiment

[0072] A two-photon fluorescence-based semiconductor carrier type testing method, comprising the following steps:

[0073] S1, selecting the above-prepared perovskite as a to-be-tested sample, and placing the to-be-tested object on a sample stage of a fluorescence spectrum detection system (the specific structure is shown in FIG. 2) ; Figure 6

[0074] S2, adjusting the excitation wavelength in the light source plate of the fluorescence spectrum detection system to 800 nm (converted to hv=1.55 eV), the excitation wavelength enters the attenuation sheet through the 4f system, and generates a power I exc1 , which finally irradiates on the to-be-tested sample;

[0075] S3, the to-be-tested sample generates fluorescence, and the fluorescence is absorbed by the spectrometer of the fluorescence spectrum detection system, and the corresponding two-photon fluorescence intensity I exc1 is collected; 2ppl1

[0076] S4, adjusting the attenuation sheet, and repeating step S2, thereby generating powers I exc2 -I excn , and two-photon fluorescence intensities I 2ppl2 -I 2ppln ;

[0077] S5, according to the formula logI 2ppl =blogD·I exc , the I exc1 -I excn and the corresponding I 2ppl1 -I 2ppln ​​Substitute, and get the exponential coefficient b by fitting, where D is a constant.

[0078] The above-mentioned relevant experimental parameters are shown in Table 1 below.

[0079] Table 1 Parameters involved in the two-photon excitation of three perovskites in the examples

[0080]

[0081]

[0082] According to the formula in step S5, the value of b is shown in Table 2.

[0083] Table 2 The final b value of three perovskites in the examples

[0084] (PEA)2PbI4 MAPbBr3 CsPbBr3 2.0 4.0 3.6

[0085] Comparative example

[0086] The three perovskite samples were subjected to single-photon excitation, and the steps were consistent with the examples, only the excitation wavelength in the light source block of the fluorescence spectrum detection system was adjusted to 400 nm (converted to hυ = 3.10 eV).

[0087]

[0088] According to the formula in step S5, the value of b' is shown in Table 2.

[0089] Table 2 The final b' value of three perovskites in the examples

[0090] (PEA)2PbI4 MAPbBr3 CsPbBr3 1.1 1.8 1.7

[0091] For single and two-photon excitation fluorescence intensity power dependence test of (PEA)2PbI4, MAPbBr3 and CsPbBr3 three perovskite single crystals, we repeated many times and counted all the experimental results, as shown in Table 3. Figure 7The results are shown in FIG. 6. First is (PEA)2PbI4, as a two-dimensional perovskite, has a huge exciton binding energy. Whether single-photon excitation or two-photon excitation, the difference in the value of the power-dependent fluorescence intensity power exponent coefficient b is very small. Single-photon is about 1, and two-photon is about 2. It has a pure exciton characteristic. The exciton binding energy of the MAPbBr3 single crystal at room temperature is smaller. Under single-photon excitation, the power exponent coefficient b fluctuates between 1.6 and 2.1. Under two-photon excitation, the coefficient b is concentrated around 4. This result shows that the excitation species of MAPbBr3 is a pure free carrier, and the defect density on different surfaces causes the coefficient b to be smaller to different degrees under single-photon excitation. The case of CsPbBr3 is special. Previous experimental results show that it has both excitons and free carriers inside, and the two coexist. The coefficient b fluctuates between 3.2 and 4, reflecting the different proportions of excitons and free carriers in different samples. The single-photon coefficient is between 1.5 and 2, with the influence of trap state induction, so it is less than the expected range of 1.6-2. According to theoretical derivation, this is caused by the combined action of single-molecule recombination and two-molecule recombination. However, it is impossible to analyze whether the single-molecule recombination is exciton recombination or trap state induction recombination or both from the single-photon excitation power dependence experiment. From the figure, we can also re-summarize that when we use two-photon excitation fluorescence intensity power dependence to judge the carrier type, when the two-photon fluorescence intensity is quadratically correlated with the excitation power, the carrier inside the material is a pure exciton. When the power exponent coefficient b is greater than 2 and less than 4, excitons and free carriers coexist. When the power exponent coefficient is equal to 4, free carriers dominate in the material.

[0092] In summary, in the present application, we have derived the relationship between the carrier dynamics process in perovskite single crystals and the excitation intensity, and finally obtained the relationship between the two-photon fluorescence intensity and the excitation power by connecting the relationship between the two-photon excitation intensity and the photo-generated carrier density. In the relationship between the two, the four carrier recombination channels together determine the size of the exponent coefficient b, so we can infer the carrier type in the perovskite material by the coefficient b.

[0093] Moreover, we have tested the single-photon and two-photon fluorescence intensity power dependence of the three different perovskite single crystals prepared. The experimental results prove the limitations of single-photon excitation and the ability of two-photon excitation to well exclude the influence of defects and provide real perovskite photo-generated carrier information. The different sizes of the coefficient b obtained in the experiment reflect the different carrier types of the three perovskite crystals.

[0094] It will be obvious to a person skilled in the art that the application is not limited to the details of the above-described exemplary embodiments, but that the application can be implemented in other embodiments without departing from the scope of the application. The embodiments are therefore to be seen as exemplary and in no way restrictive, the scope of the application being defined by the claims below rather than by the above description, and all variations falling within the meaning and range of equivalency of the essential characteristics of the claims are therefore intended to be embraced therein.

[0095] Furthermore, it should be understood that although the description is made according to embodiments, not every embodiment contains only one independent technical solution, and the description is made in this way only for the sake of clarity, and a person skilled in the art should consider the description as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by a person skilled in the art.

Claims

1. A semiconductor carrier type testing method based on two-photon fluorescence, characterized in that, The method for testing the type of semiconductor carriers based on two-photon fluorescence includes the following steps: S1. Select perovskite as the sample to be measured and place the object to be measured on the sample stage of the fluorescence spectroscopy detection system; S2. Adjust the excitation wavelength in the light source module of the fluorescence spectroscopy detection system. The excitation wavelength passes through the 4f system, enters the attenuator, and generates a power I. excl Finally, the light is irradiated onto the sample to be tested; S3. The sample to be tested generates fluorescence, which is absorbed by the spectrometer of the fluorescence spectroscopy detection system and collected. excl The corresponding two-photon fluorescence intensity I 2ppll ; S4. Adjust the attenuator and repeat step S2 to generate power I. exc2 -I excn And the corresponding two-photon fluorescence intensity I 2ppl2 -I 2ppln ; S5. According to the formula logI 2ppl =b logD·I exc , will the I excl -I excn and the corresponding I 2ppl1 -I 2ppln Substitute the values ​​and perform a fitting to obtain the exponential coefficient b, where D is a constant. If b = 2, the carrier type is exciton; if 2 < b < 4, excitons and free carriers coexist; if b = 4, the carrier type is free carrier; Among them, the energy hυ of the excitation wavelength satisfies: 0.5E g <hυ<E g ; E g The optical band gap of the sample under test is denoted as .

2. The semiconductor carrier type testing method based on two-photon fluorescence according to claim 1, characterized in that, The fluorescence spectroscopy detection system includes the following components: a light source component, a 4f system, an attenuation film, a spectrometer, and several reflectors.

3. The semiconductor carrier type testing method based on two-photon fluorescence according to claim 1, characterized in that, The perovskite is selected from three-dimensional perovskite or quasi-two-dimensional perovskite.

4. The semiconductor carrier type testing method based on two-photon fluorescence according to claim 1, characterized in that, The I excn and I 2ppln In this context, n takes the value of a positive integer greater than or equal to 3.

5. The semiconductor carrier type testing method based on two-photon fluorescence according to claim 1, characterized in that, The excitation wavelength of the light source component ranges from 600 to 1300 nm.

6. The semiconductor carrier type testing method based on two-photon fluorescence according to claim 1, characterized in that, The value range of the exponential coefficient b is: 2 ≤ b ≤ 4.

7. The semiconductor carrier type testing method based on two-photon fluorescence according to claim 1, characterized in that, The 4f system includes a front convex lens and a rear convex lens. Among them, the focal length of the front convex lens is less than that of the rear convex lens.

8. The semiconductor carrier type testing method based on two-photon fluorescence according to claim 1, characterized in that, The fluorescence spectroscopy detection system further includes a dichroic mirror, a CCD, and a color filter.

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