A spectroscopic method for measuring electron or hole diffusion coefficient

Abstract

The application discloses a kind of transient reflection spectroscopy measurement methods for separately measuring electron, hole diffusion coefficient in perovskite film.The electron and hole diffusion coefficient in perovskite film is realized separately measured by using the transient reflection spectroscopy technique of back excitation-front detection combined with acceptor quenching method.The application develops a new transient reflection spectroscopy method for studying the problem of electron and hole longitudinal diffusion.After depositing a layer of acceptor on the surface of perovskite film, the pump light excites perovskite sample from the acceptor side, and the photoinduced electron or hole is extracted by acceptor quickly, while the electron and hole diffuse to the detection surface under the action of concentration gradient, and by depositing charge acceptor, we can study the carrier diffusion process in the system with non-equilibrium distribution of electron and hole concentration.It is found that the electron and hole diffusion coefficients in MAPbI3 film are basically the same, while the hole diffusion coefficient in CsPbI3 is greater than the electron diffusion coefficient.

Description

Technical Field

[0001] This invention relates to a test method for separately measuring the diffusion coefficients of electrons and holes. Background Technology

[0002] In recent years, organometallic halide perovskite solar cells have attracted strong attention from researchers worldwide due to their simple fabrication process and excellent photovoltaic properties. In just over a decade, the laboratory conversion efficiency of perovskite solar cells has rapidly increased from an initial 3.8% to over 25%, sparking a research boom internationally. For perovskite solar cells, the carrier diffusion coefficient is one of the important parameters affecting their performance. However, reports on the electron and hole diffusion coefficients within perovskite thin films have been controversial. Some reports suggest that the electron and hole diffusion coefficients in MAPbI3 films are essentially the same, indicating bipolar diffusion, while other reports indicate significant differences, leaving no definitive conclusion. Meanwhile, all-inorganic perovskite solar cells (CsPbI3) have received increasing attention due to their high thermal stability, with cell efficiencies exceeding 20.4%. However, the properties of electron and hole diffusion in CsPbI3 are rarely reported.

[0003] Therefore, the purpose of this invention is to explore a method of combining back-excited transient reflectance spectroscopy with acceptor quenching to distinguish the diffusion coefficients of electrons and holes in perovskite thin films. The diffusion coefficients of different perovskite thin films were tested using this method, providing guidance for improving the understanding of the photophysical processes of lead halide perovskite materials and improving the efficiency of perovskite solar cells. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to propose a method for measuring the diffusion coefficients of electrons and holes in thin films separately, in contrast to the above-mentioned prior art: in the back-excited transient reflectance spectroscopy test, by spin-coating an electron transport layer or a hole transport layer on the surface of the perovskite thin film, the longitudinal diffusion of electrons and holes is studied respectively.

[0005] The objective of this invention is achieved through the following solution.

[0006] (1) Preparation of MAPbI3 perovskite thin films: Spin coating was used. Lead iodide, MAI, and MACl were dissolved in a mixed solution of NMP / GBL (volume ratio 7:3) at a molar ratio of 1:1:0.2, with a solute mass fraction of 60%. The glass substrate was ultrasonically cleaned sequentially with ultrapure water, acetone, and isopropanol, followed by UV ozone for 15 min to remove surface-adhered organic matter. 50 μL of MAPbI3 precursor solution was dropped onto the glass substrate, and then the substrate was rotated at 2500 rpm / s for 25 s, followed by an ether bath for 60-90 s. The perovskite film slowly turned black in ether. The spin-coated film was purged with nitrogen gas and finally annealed in air at 150 °C for 5-15 min.

[0007] Preparation of CsPbI3 perovskite thin films: A spin-coating method was used. CsI (0.3378 g) and PbI2 (0.5993 g) were dissolved in DMF:DMSO (volume ratio 4:1) at a molar concentration of 1.3 M and stirred at room temperature for 2 h until dissolved. 50 μL of the CsPbI3 precursor solution was dropped onto a cleaned glass substrate, and the substrate was then rotated at 1500 rpm / s for 45 s. The spin-coated film was allowed to stand in a glove box for 50 min, and finally annealed in a nitrogen glove box at 350 °C for 5–15 min. During annealing, the perovskite film slowly changed from yellow to black.

[0008] (2) Construction of the transient reflection optical path for back-side excitation and front-side detection: This system mainly consists of three parts: a femtosecond Yb:KGW laser system (1030nm, 230fs, 100kHz) manufactured by Light Conversion, an optical parametric amplifier (OPA), and a transient spectrometer. The OPA is mainly used to generate wavelength-tunable pump light (240nm~2600nm); while the transient spectrometer includes a variable delay device, a YAG crystal window, a chopper, and a detector. This system can achieve a time resolution of 230fs, a maximum delay time of 8ns, and a probe light (white light) wavelength covering 380nm~1600nm. The incident angle of the probe light on the thin film sample is adjusted to approximately 45°, while the pump light is incident on the back side of the thin film sample at an incident angle of 0-10°.

[0009] (3) The absorbance of the perovskite film was tested by UV-Vis steady-state absorption spectroscopy, and the thickness of the film was estimated.

[0010] (4) The UV-Vis absorption spectrum of the perovskite film of known thickness was measured, and the absorption coefficient α was obtained according to the formula α=(2.303·A) / L. For the MAPbI3 film, the absorption coefficient of the 400nm excitation light is 394000cm. -1The absorption coefficient of the CsPbI3 thin film under 380 nm excitation light is 217100 cm⁻¹. -1 .

[0011] (5) Collect ΔR / R data.

[0012] (6) Calculation of the diffusion coefficient of perovskite thin films: The change in carrier concentration distribution throughout the film over time can be expressed as follows:

[0013]

[0014] Where N(x,t) is the carrier concentration at depth x and time t, where x is in nm and t is in ps, D is the carrier diffusion coefficient, and τ is the carrier diffusion coefficient. B This indicates the bulk carrier lifetime, measured in nanoseconds (ns).

[0015] The initial and boundary conditions of Eq.(1) are respectively

[0016] N(x, 0) = N0·exp(-αx) (2)

[0017]

[0018]

[0019] Where N0 is the surface carrier concentration at the initial moment, α is the absorption coefficient of the thin film, L is the thickness of the perovskite thin film, and S is the surface recombination rate (SRV). Assuming the detection depth of the probe light is d, its estimated value is ~λ / 4πn (n is the refractive index), the TR signal (ΔR / R) of the back-excited, front-detected mode is expressed as:

[0020]

[0021] Using a one-dimensional diffusion model, the back-excited TR dynamics can extract the diffusion coefficients D of electrons and holes.

[0022] This invention utilizes a back-excitation-front-probe transient reflectance spectroscopy technique combined with a acceptor quenching method to achieve separate measurements of the electron and hole diffusion coefficients in perovskite thin films.

[0023] This invention develops a novel transient reflectance spectroscopy method for studying the longitudinal diffusion of electrons and holes. It was found that the diffusion coefficients of electrons and holes are essentially the same in MAPbI3 films, while the diffusion coefficient of holes is greater than that of electrons in CsPbI3.

[0024] Compared with the prior art, the beneficial effects of the present invention are:

[0025] (1) Previous methods for testing the diffusion coefficients of electrons and holes in perovskite thin films relied on the mathematical models used, and the diffusion coefficients of electrons and holes obtained from the literature varied greatly. This invention combines the back-side excitation TR with the acceptor quenching method, which can test the diffusion coefficients of electrons and holes separately, and is more intuitive than previous methods.

[0026] (2) Current electrical methods for distinguishing electron and hole diffusion coefficients in perovskite thin films, such as space charge confinement current (SCLC) technology, require the construction of a device structure and, in principle, ohmic contact between the perovskite layer and the deposited electrode to reduce contact resistance. However, the deposition of electrode materials inevitably alters the surface of the perovskite thin film, introducing defects that hinder the accurate measurement of electron and hole mobility within the perovskite layer. This invention eliminates the need for electrode deposition and device structure construction, simplifies the testing method and operating procedures, and avoids the influence of additional factors such as electrode deposition on the test results. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the back-side excitation TR of receptor quenching in Example 1.

[0028] Figure 2 The UV-Vis absorption spectra of the MAPbI3, MAPbI3@PCBM, and MAPbI3@Spiro-OMeTAD films in Example 1 are shown.

[0029] Figure 3 The images show the fluorescence kinetic curves of the three thin film samples in Example 1.

[0030] Figure 4 The image shows the back-side excitation-TR kinetic curve after spin-coating the charge acceptor in Example 1, with an excitation wavelength of 400 nm.

[0031] Figure 5 The image shows the back-side excitation-TR kinetic curve after spin-coating the charge acceptor in Example 1, with an excitation wavelength of 340 nm.

[0032] Figure 6 The image shows the back-side excitation-TR kinetic curve after spin-coating the charge acceptor in Example 1, with an excitation wavelength of 470 nm.

[0033] Figure 7 The image shows the back-side excitation-TR kinetic curve after spin-coating the charge acceptor in Example 2, with an excitation wavelength of 380 nm. Detailed Implementation

[0034] The present invention will be further described in detail below with reference to embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto. All reagents used in the embodiments are commercially available.

[0035] Example 1:

[0036] The method for measuring the electron and hole diffusion coefficients of MAPbI3 thin films by back-side excitation and front-side probe (TR) includes the following steps:

[0037] (1) Preparation of MAPbI3 perovskite thin films: Spin coating was used. Lead iodide, MAI (methylamine iodine), and MACl (methylamine chloride) were dissolved in a mixed solution of NMP (1-methyl-2-pyrrolidone) / GBL (γ-butyrolactone) at a molar ratio of 1:1:0.2 (volume ratio 7:3), with a solute mass fraction of 60%. The glass substrate was ultrasonically cleaned sequentially with ultrapure water, acetone, and isopropanol, followed by UV ozone for 15 min to remove surface-adhered organic matter. 50 μL of MAPbI3 precursor solution was dropped onto the glass substrate, and then the substrate was rotated at 2500 rpm / s for 25 s and bathed in ether for 90 s. The perovskite film slowly turned black in ether. The spin-coated film was purged with nitrogen and finally annealed in air at 150 °C for 15 min. Two films were prepared. The absorbance of the perovskite film was measured by UV-Vis steady-state absorption spectroscopy, and the film thickness was calculated to be 1 μm.

[0038] (2) Spin-coating of Spiro-OMeTAD (2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirodifluorene) layer: The Spiro-OMeTAD solution contained 72.5 mg Spiro-OMeTAD, 17.5 μL of 520 mg / mL lithium bis(trifluoromethanesulfonyl)imide in acetonitrile, 28.5 μL of 4-tert-butylpyridine, and 1 mL of chlorobenzene. The solution was stirred overnight at 50 °C. It is important to note that after spin-coating the Spiro-OMeTAD solution, the film should be placed in a desiccator for 12 h to ensure sufficient oxidation time for Spiro-OMeTAD and improve its ability to extract vacancies. 50 μL of Spiro-OMeTAD solution was dropped onto a glass substrate coated with the perovskite film, and the substrate was then rotated at 2000 rpm / s for 30 s and placed in a glove box to air dry. The acceptor layer thickness was 50 nm.

[0039] (3) Spin-coating PCBM layer: 10 mg PCBM ([6,6]-phenyl C 61 Methyl butyrate was dissolved in 1 mL of chlorobenzene and stirred overnight at room temperature. 50 μL of PCBM solution was dropped onto a glass substrate coated with a perovskite film, and the substrate was then rotated at 2000 rpm / s for 30 s and air-dried in a glove box. The acceptor layer thickness was 50 nm.

[0040] (4) Construction of the transient reflection optical path for back-side excitation and front-side detection: This system mainly consists of three parts: a femtosecond Yb:KGW laser system (1030nm, 230fs, 100kHz) manufactured by Light Conversion, an optical parametric amplifier (OPA), and a transient spectrometer. The OPA is mainly used to generate wavelength-tunable pump light (240nm~2600nm); the transient spectrometer includes a variable delay device, a YAG crystal window, a chopper, and a detector. The incident angle of the probe light on the glass surface of the thin film sample is adjusted to approximately 45°; the angle of the reflector is adjusted so that the pump light is incident perpendicularly on the other side of the thin film sample at an incident angle of 0°. In this experiment, 340nm, 400nm, and 470nm are used as excitation light.

[0041] Figure 1 This is a schematic diagram of the back-excitation, front-probe TR-based acceptor quenching method. After depositing an acceptor layer on the surface of a perovskite film, pump light excites the perovskite sample from the acceptor side. The photoinduced electrons or holes are rapidly extracted by the acceptor, and at the same time, electrons and holes diffuse towards the probe side under the action of the concentration gradient. By depositing charge acceptors, we can study the carrier diffusion process in a system where the electron and hole concentrations are in non-equilibrium distribution, and reflect the carrier diffusion dynamics by detecting changes in the intensity of the light signal.

[0042] Figure 2 The figure shows the UV-Vis absorption spectrum of the synthesized MAPbI3 film, with an absorption edge at ~760 nm, consistent with reports in the literature. The figure also shows the UV-Vis absorption spectra of the MAPbI3 film after spin-coating with PCBM (electron acceptor) and Spiro-OMeTAD (hole acceptor). The absorption edges are the same as those of the pure MAPbI3 film, indicating that the spin-coating process did not alter the properties of the perovskite layer.

[0043] Figure 3 These are the results of time-resolved fluorescence kinetics tests after spin-coating pure perovskite films with PCBM (electron acceptor) and Spiro-OMeTAD (hole acceptor). The fluorescence quenching was obvious and the fluorescence lifetime was shortened after spin-coating the acceptor, indicating that charge transfer occurred from the perovskite film to the acceptor.

[0044] Figure 4 The figures show the kinetic curves obtained from the back-side excitation-transfer (TR) experiment after spin-coating charge acceptors onto the MAPbI3 thin film. The excitation wavelength was 400 nm. The TR kinetics were completely coincident, indicating that electrons and holes in the MAPbI3 thin film have the same diffusion coefficient. The absorption coefficient α of the 400 nm excitation light is 394000 cm⁻¹. -1 The film thickness is 1140 nm, τ BThe diffusion coefficient D was found to be 1.26 cm⁻¹ after fitting the data at 154 ns. 2 s -1 .

[0045] Figure 5 This is the kinetic curve obtained from the back-side excitation-TR experiment of the MAPbI3 thin film after spin-coating a charge acceptor, with an excitation wavelength of 340 nm. The excitation light is incident from one side of the charge acceptor layer and focused onto the surface of the perovskite layer. Some of the generated photogenerated carriers are extracted by the charge transport layer, while others diffuse to the other side due to the concentration gradient, and are then detected by the probe light as a ΔR / R signal. The TR kinetics of the pure MAPbI3 thin film and the film after spin-coating the acceptor are completely identical, indicating that electrons and holes in the MAPbI3 thin film have the same diffusion coefficient.

[0046] Figure 6 The curves are obtained from the back-side excitation-TR experiment after spin-coating charge acceptors onto the surface of the MAPbI3 thin film. The excitation wavelength is 470 nm. The TR kinetics are completely coincident, which also indicates that electrons and holes in the MAPbI3 thin film have the same diffusion coefficient.

[0047] Example 2:

[0048] To examine the electron and hole diffusion coefficients of different types of perovskite thin films, the electron and hole diffusion coefficients in CsPbI3 thin films were measured using back-side excitation and front-side probe (TR) testing. The specific steps included:

[0049] (1) Preparation of CsPbI3 perovskite thin films: Spin coating was used. CsI (0.3378 g) and PbI2 (0.5993 g) were dissolved in DMF:DMSO (volume ratio 4:1) with a solute molar concentration of 1.3 M and stirred at room temperature for 2 h until dissolved. The glass substrate was ultrasonically cleaned sequentially with ultrapure water, acetone, and isopropanol, and then subjected to UV ozone for 15 min to remove surface-adhered organic matter. 50 μL of CsPbI3 precursor solution was dropped onto the cleaned glass substrate, and then the substrate was rotated at 1500 rpm / s for 45 s. The spin-coated film was allowed to stand in a glove box for 50 min, and finally annealed in a nitrogen glove box at 350 °C for 10 min. During the annealing process, the perovskite film slowly changed from yellow to black. Two films were prepared. The absorbance of the perovskite film was measured by UV-Vis steady-state absorption spectroscopy, and the film thickness was calculated to be 1 μm.

[0050] (2) Spin-coating Spiro-OMeTAD layer: The Spiro-OMeTAD solution contains 72.5 mg Spiro-OMeTAD, 17.5 μL of 520 mg / mL lithium bis(trifluoromethanesulfonyl)imide in acetonitrile, 28.5 μL of 4-tert-butylpyridine, and 1 mL of chlorobenzene. The solution is stirred overnight at 50 °C. It should be noted that after spin-coating the Spiro-OMeTAD solution, the film should be placed in a humidity-controlled desiccator for 12 h to ensure sufficient oxidation time for Spiro-OMeTAD and improve its ability to extract vacancies. 50 μL of Spiro-OMeTAD solution is dropped onto a glass substrate coated with a perovskite film, and then the substrate is rotated at 2000 rpm / s for 30 s and placed in a glove box to dry. Thickness: 50 nm. (3) Spin-coating PCBM layer: 10 mg PCBM is dissolved in 1 mL of chlorobenzene and stirred overnight at room temperature. 50 μL of PCBM solution was dropped onto a glass substrate coated with a perovskite film. The substrate was then rotated at 2000 rpm for 30 s and placed in a glove box to air dry. The thickness was 50 nm.

[0051] (4) Construction of transient reflection optical path for back excitation and front detection: The optical path used for testing is the same as in Example 1. In this experiment, 380nm is used as the excitation light.

[0052] Figure 7 This is the kinetic curve obtained from a back-side excitation-TR experiment of a CsPbI3 thin film after spin-coating a charge acceptor layer. The excitation wavelength was 380 nm. The excitation light was incident from one side of the charge acceptor layer and focused onto the surface of the perovskite layer. Some of the generated photogenerated carriers were extracted by the charge transport layer, while others diffused to the other side due to the concentration gradient, and were subsequently detected by the probe light as a ΔR / R signal. Figure 7 The results showed significant differences in the kinetics of the three thin film samples. CsPbI3@PCBM exhibited the fastest upward trend, followed by pure CsPbI3 film, while CsPbI3@Spiro-OMeTAD showed the slowest increase, indicating a substantial difference in the diffusion coefficients of electrons and holes. A 1D diffusion model was then used to simulate the back-side excitation-TR kinetics and fit the experimental results, where the absorption coefficient α of the 380nm excitation light was 217100 cm⁻¹. -1 The film thickness L is 665 nm, τ B The diffusion coefficient of the pure CsPbI3 thin film was found to be 0.70 cm⁻¹ after fitting the data at a time interval of 10 ns. 2 s -1 This reflects the average diffusion coefficient of electrons and holes; the diffusion coefficient fitted to the CsPbI3@Spiro-OMeTAD thin film is 0.49 cm⁻¹. 2 s -1This primarily reflects the diffusion of electrons; while the diffusion coefficient fitted by the CsPbI3@PCBM thin film is 1.03 cm⁻¹. 2 s -1 This primarily reflects the diffusion of holes. Therefore, the hole diffusion coefficient in CsPbI3 thin films is much greater than the electron diffusion coefficient.

Claims

1. A spectroscopic method of measuring the diffusion coefficient of electrons or holes, characterized in that: After depositing an acceptor layer on the surface of the perovskite thin film to form the acceptor surface, the back-excited-front probe (TR) is performed. The pump light is incident from the acceptor surface at an incident angle between 0 and 30°, while the probe light is incident from the perovskite surface away from the acceptor, i.e., the probe surface, at an incident angle between 30 and 90°. The incident angle refers to the angle between the incident light and the normal. Collecting the TR signal in the front detection mode Data; fitting the data gives the diffusion coefficient D of the carriers.

2. The spectroscopic method for measuring the diffusion coefficient of electrons or holes according to claim 1, characterized in that, Pump light excites the perovskite sample from the acceptor surface. The photoinduced electrons or holes are extracted by the acceptor. At the same time, the electrons and / or holes diffuse toward the detector surface under the action of the concentration gradient. By depositing charge acceptors, the carrier diffusion process in a system with non-equilibrium distribution of electron and / or hole concentrations can be studied. The test samples were MAPbI3 perovskite films or CsPbI3 perovskite films, with an electron transport layer PCBM or a hole transport layer Spiro-OMeTAD spin-coated on their surface as the acceptor layer; the acceptor layer thickness was 50 nm, and the thickness of the perovskite film samples ranged from 1 μm to 1.2 μm.

3. The spectroscopic method for measuring the diffusion coefficient of electrons or holes according to claim 2, characterized in that, The thickness of the perovskite film sample was 1 μm, and the perovskite film was MAPbI3, FAPbI3 or CsPbI3.

4. The spectroscopic method for measuring the electron or hole diffusion coefficient according to claim 1, characterized in that, The pump light is incident from the receiver surface at an angle between 0 and 10°, nearly perpendicular to the surface, while the probe light is incident from the glass surface at an angle of 45°. The excitation light range is 340-470 nm; the detection light range is 500 nm-950 nm. The formula for calculating the diffusion coefficient of perovskite thin films is as follows: （1）， in N ( x, t () represents the carrier concentration at a film depth x and time t, where x is in nm and t is in ps. D It is the carrier diffusion coefficient. τ B This represents the bulk carrier lifetime, measured in nanoseconds (ns). The initial and boundary conditions of Eq. (1) are as follows: N ( x, 0 ) = N 0 ·exp ( -αx ) （2）， （3）， （4）， in N 0 It is the surface carrier concentration at the initial moment. α It is the absorption coefficient of the thin film. L It refers to the thickness of the perovskite thin film. S For surface recombination rate (SRV), The TR signal in back-excited, front-detected mode is represented as: （5）， in d The detection depth of the probe light is l / 4p. n , n is the refractive index.

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