Method for measuring backside excitation-frontside detection tr and thin film longitudinal diffusion coefficient

Abstract

The application discloses a back excitation-front detection TR and thin film longitudinal diffusion coefficient measurement method, is a kind of perovskite thin film longitudinal diffusion coefficient transient reflection measurement method, belong to test method field.The application utilizes the transient reflection spectroscopy technology of back excitation-front detection, realizes the direct observation of perovskite thin film longitudinal carrier diffusion dynamics, it is a new transient reflection spectroscopy means, for the problem of carrier longitudinal diffusion.Respectively, back excitation-front detection TR can realize the direct observation of longitudinal carrier diffusion in thin film, for the perovskite thin film with the thickness of 1.2um, it can be seen that the rising edge of TR signal is about 2000ps, represents that carrier diffuses from excitation surface to detection surface.The diffusion coefficient D obtained by fitting the experimental results of back excitation-front detection TR of independent exploration is 1.47cm 2 / s, close to the diffusion coefficient of single crystal, consistent with the single grain in longitudinal direction shown by SEM.

Description

Technical Field

[0001] This invention relates to a method for testing the longitudinal diffusion coefficient of perovskite thin films. Background Technology

[0002] In recent years, organometallic halide perovskite (CH3NH3PbX3, X=Cl,Br,I) solar cells have attracted strong attention from researchers worldwide due to their simple fabrication process and excellent photovoltaic properties. In just a few years, the laboratory conversion efficiency of perovskite solar cells has rapidly increased from an initial 3.8% to over 25%, sparking a research boom internationally; the journal *Science* even named it one of the top ten scientific breakthroughs of 2013. This rapid increase in the conversion efficiency of perovskite solar cells has further fueled interest in this new material: on the one hand, applied researchers continue to explore ways to further improve its conversion efficiency using materials science and device fabrication techniques, as well as to address its inherent defects and improve its stability and non-toxicity; on the other hand, basic researchers hope to thoroughly understand the physical mechanisms of its high-efficiency photoelectric conversion from the perspectives of the photoelectric properties of perovskite materials and the working principle of the cells.

[0003] For perovskite solar cells, the carrier diffusion coefficient is one of the most important parameters affecting their performance. However, current research on the diffusion dynamics of perovskite thin films mainly focuses on lateral carrier diffusion, while the diffusion dynamics of carriers in the vertical direction (longitudinal direction) are rarely reported, and the essential difference between this and lateral carrier diffusion is not well understood. Moreover, compared to lateral diffusion, this longitudinal carrier diffusion dynamics may have a more significant impact on device performance. Since the detection depth of transient reflectance spectroscopy is only tens of nanometers from the surface, reflecting surface carrier dynamics, back-excited transient reflectance spectroscopy may be more advantageous for studying the longitudinal migration process of carriers throughout the perovskite thin film. Therefore, the purpose of this invention is to explore a back-excited transient reflectance spectroscopy method to study the longitudinal carrier diffusion dynamics within perovskite thin films, and to use this method to systematically study the factors affecting the longitudinal diffusion dynamics of perovskite thin films, providing guidance for the better application of perovskite materials in optoelectronic materials. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to propose a method for measuring the longitudinal diffusion coefficient of thin films in light of the prior art: in conventional transient reflection spectroscopy testing, the longitudinal diffusion of photogenerated carriers is studied by adjusting the incident directions of the pump light and the probe light.

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

[0006] (1) Preparation of MAPbI3 perovskite thin films: Spin coating was used. MAI, PbI2 and MACl were dissolved in a mixed solvent of NMP and GBL, with a raw material molar ratio of MAI / PbI2 / MACl = 1 / 1 / 0.2, and the mass fraction of the resulting precursor solution was 55-60%. The solution was stirred at 60℃ for 6 h. After filtration, it was spin-coated onto FTO glass at a speed of 2000-5000 rpm / s, immersed in diethyl ether for 60-90 s, dried with nitrogen, and annealed at 150℃ for 5-15 min.

[0007] (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 Ti:sapphire laser system (1030nm, 230fs, 100kHz), an optical parametric amplifier (OPA), and a transient spectrometer. The TOPAS OPA is primarily used to generate wavelength-tunable pump light (240nm–2600nm); while the transient spectrometer includes a variable delay device, a Ti:sapphire 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 340nm–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°.

[0008] (3) The surface morphology and cross-section of the perovskite film were characterized by SEM, and the number of grains in the cross-section and the thickness of the film were characterized.

[0009] (4) Test the UV-Vis absorption spectrum of the perovskite film of known thickness, and obtain the absorption coefficient α according to the formula α=(2.303·A) / L.

[0010] (5) Collect data and fit the experimental data with self-developed one-dimensional diffusion model software to obtain the diffusion coefficient of charge carriers and the surface recombination rate.

[0011] (6) Calculation of the diffusion coefficient of perovskite thin films: At lower energies (no energy dependence), the change in carrier concentration distribution throughout the film over time can be expressed as follows:

[0012]

[0013] Where N(x,t) is the carrier concentration at time t at a film depth x, D is the longitudinal diffusion coefficient, and τ B The initial conditions for Eq.(1), representing the bulk carrier lifetime, can be expressed as follows:

[0014] N(x,0)=N0·exp(-αx) (2)

[0015] N0 is the initial surface carrier concentration, and α is the absorption coefficient of the thin film. The boundary conditions can be expressed as follows:

[0016]

[0017]

[0018] Where L is the thickness of the perovskite film, and S is the surface recombination rate (SRV). Assuming the detection depth of the probe light is d, and its value is ~λ / 4πn (n is the refractive index), the TR signal of the back-excited, front-detected mode has the following relationship:

[0019]

[0020] Using a one-dimensional diffusion model, the longitudinal diffusion coefficient D and the surface recombination rate S can be extracted by fitting the TR dynamics at different excitation wavelengths. The fitted D is approximately 1.5 cm⁻¹. 2 s -1 This is consistent with the fact that the SEM cross-section only has a single grain.

[0021] This invention utilizes back-excitation-front-detection transient reflectance spectroscopy (TR) to directly observe the longitudinal carrier diffusion dynamics in perovskite thin films. This invention develops a novel transient reflectance spectroscopy method for studying longitudinal carrier diffusion. Back-excitation-front-detection TR enables direct observation of longitudinal carrier diffusion in thin films. For a 1.2 μm thick perovskite film, a rising edge of ~2000 ps can be observed in the TR signal, representing carrier diffusion from the excitation surface to the detection surface. The diffusion coefficient D obtained by fitting the experimental results of the independently explored back-excitation-front-detection TR is 1.47 cm⁻¹. 2 / s, close to the diffusion coefficient of a single crystal, consistent with the longitudinal direction of individual grains shown by SEM.

[0022] Compared with the prior art, the beneficial effects of the present invention are:

[0023] (1) Previous studies on the diffusion dynamics of perovskite thin films have mainly focused on the lateral carrier diffusion, while the carrier diffusion dynamics in the vertical direction (longitudinal direction) are rarely reported. The longitudinal carrier diffusion is the key to photovoltaic devices. This invention specifically studies the longitudinal carrier diffusion.

[0024] (2) Current research on longitudinal diffusion in perovskite thin films mainly relies on the PL quenching method, which requires the addition of an electron or hole transport layer, introducing a new interface. Since charge transfer at the interface needs to be considered, estimating the charge transfer efficiency at the interface is a significant challenge, hindering the determination of the intrinsic diffusion coefficient. Other methods, such as the Hall effect, time-of-flight (TOF), and space charge-confined current (SCLC) techniques, are also used to study carrier diffusion parameters, but these methods are affected by contact quality and slow ion migration, thus impacting the accuracy of experimental results. This invention does not introduce a new interface, avoiding the influence of electrode interfaces and ion migration.

[0025] (3) Although transient reflection TR has been applied to the measurement of carrier diffusion coefficient, generally speaking, TR spectrum can only reflect the carrier dynamics process of tens of nanometers on the surface. However, conventional TR is affected by many factors, while back-excited and front-probe TR are affected by fewer factors, and the fitting results are more accurate. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the TR optical path in Example 1.

[0027] Figure 2 The UV-Vis absorption spectrum and steady-state PL spectrum of the MAPbI3 perovskite thin film in Example 1 are shown.

[0028] Figure 3 The absorption coefficient is the thin film of Example 1.

[0029] Figure 4 TA kinetics and fitting lifetime of perovskite thin film in Example 1

[0030] Figure 5 The TR time-division spectrum is shown in Example 1.

[0031] Figure 6 The TR dynamics and fitted curves in Example 1 are shown.

[0032] Figure 7 The SEM surface morphology and cross-section of the perovskite thin film prepared in Example 1 are shown. Detailed Implementation

[0033] 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.

[0034] Example 1:

[0035] The method for measuring the longitudinal diffusion coefficient of a thin film by back-side excitation and front-side probe (TR) includes the following steps:

[0036] (1) Preparation of MAPbI3 perovskite thin films: Spin coating was used. MAI (methyl iodide), PbI2 and MACl (methyl iodide) were dissolved in a mixed solvent of NMP (N-methylpyrrolidone) and GBL (γ-butyrolactone) in a volume ratio of 7:3. The molar ratio of raw materials MAI / PbI2 / MACl was 1 / 1 / 0.2, and the mass fraction of (MAI, PbI2 and MACl) in the resulting precursor solution was 60%. The solution was stirred at 60℃ for 6 h. It was then spin-coated onto FTO glass at a speed of 3000 rpm / s for 25 s, followed by immersion in diethyl ether for 90 s, drying with nitrogen, and annealing at 150℃ for 15 min.

[0037] (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 Ti:sapphire laser system (1030nm, 230fs, 100kHz), an optical parametric amplifier (OPA), and a transient spectrometer. The TOPAS OPA is mainly used to generate wavelength-tunable pump light (240nm~2600nm); while the transient spectrometer includes a variable delay device, a Ti:sapphire window, a chopper, and a detector. The incident angle (angle between the incident light and the normal perpendicular to the film surface) of the probe light (white light 450nm~950nm) on the thin film sample was adjusted to 45°, and the pump light was incident perpendicularly on the back side of the thin film sample at an incident angle of 0°. Two excitation wavelengths were selected for testing; in this experiment, 635nm and 400nm excitation light were used for the pump light. The probe light reflected from the thin film sample entered the detector to obtain the TR spectrum. The probe light and pump light correspond to each other on the two sides of the thin film. They are symmetrically set on the two sides of the thin film. Finally, the diffusion coefficient is obtained by extracting the dynamic global fitting from the experimental data.

[0038] Figure 1 This is an optical path diagram of a conventional TR and our developed back-excited-front-probe TR. The difference between the two lies in the incident direction of the excitation light. In a conventional TR, the excitation light is incident from the front of the thin film and detected from the front; while in the back-excited-front-probe TR of this invention, the excitation light is incident from the back of the thin film, and the dynamics reflect the process of charge carriers diffusing from the front to the back.

[0039] Figure 2 The UV-Vis absorption spectrum and steady-state fluorescence emission spectrum are typical of MAPbI3 thin films, with an absorption band edge of ~770 nm, consistent with those reported in the literature. Furthermore, the absorption spectrum exhibits a very low baseline at longer wavelengths, which is attributed to the smoothness of the synthesized perovskite film, resulting in low light scattering.

[0040] Figure 3 The absorption coefficient of the perovskite film is obtained by first measuring the ultraviolet-visible absorption spectrum and then using the formula α=(2.303·A) / L.

[0041] Figure 4 This is the transient absorption kinetics of the thin film used in this invention, and the lifetime obtained from the fitted curve is 3.1 ns.

[0042] Figure 5 This is a time-division spectrum of TR (transverse oscillation) excited from the back and detected from the front, with an excitation wavelength of 635 nm. The shaded area represents the interference region, which needs to be avoided when extracting the kinetics. To completely avoid this region, we chose the kinetics at 720 nm.

[0043] Figure 6 These are the experimentally measured kinetics and global fitting curves. Back-excitation-front-probe TR (transient current) allows for direct observation of longitudinal carrier diffusion in thin films. For a 1200 nm thick perovskite film, a TR signal rising edge of approximately 2000 ps can be observed, representing carrier diffusion from the excitation surface to the probe surface. Figure 6 ). TR experimental fitting (see literature (1) or (2); (1) Yang, Y.; Yang, M.; Moore, David T.; Yan, Y.; Miller, Elisa M.; Zhu, K.; Beard, Matthew C.Top and bottom surfaces limit carrier lifetime in lead iodide perovskite films.Nature Energy2017, 2.; (2) Wu, B.; Zhou, Y.; Xing, G.; Xu, Q.; Garces, HF; Solanki, A.; Goh, TW; Padture, NP; 2017, 27, 1604818.)

[0044] The obtained diffusion coefficient D is 1.47 cm⁻¹. 2 / s, close to the diffusion coefficient of MAPbI3 single crystal, consistent with the longitudinal direction of individual grains shown by SEM ( Figure 7 ).

[0045] Calculation of the diffusion coefficient of perovskite thin films: At lower energies (no energy dependence), the change in carrier concentration distribution throughout the film over time can be expressed as follows:

[0046]

[0047] Where N(x, t) is the carrier concentration at time t at a film depth x, D is the longitudinal diffusion coefficient, and τ B The initial conditions for Eq.(1), representing the bulk carrier lifetime, can be expressed as follows:

[0048] N(x, 0) = N0·exp(-αx) (2)

[0049] N0 is the initial surface carrier concentration, and α is the absorption coefficient of the thin film. The boundary conditions can be expressed as follows:

[0050]

[0051]

[0052] Where L is the thickness of the perovskite film, and S is the surface recombination rate (SRV). Assuming the detection depth of the probe light is d, and its value is ~λ / 4πn (n is the refractive index), the TR signal of the back-excited, front-detected mode has the following relationship:

[0053]

[0054] Using a one-dimensional diffusion model, the longitudinal diffusion coefficient D and the surface recombination rate S can be extracted by fitting the TR dynamics at different excitation wavelengths. The fitted D is approximately 1.5 cm⁻¹. 2 s -1 This is consistent with the fact that the SEM cross-section only has a single grain.

[0055] Figure 7 The surface morphology and cross-section of the perovskite film were characterized by SEM. It was found that the MAPbI3 film with large grains of about 2 μm was synthesized, and the cross-section showed a single grain without grain boundaries. The thickness of the film could also be obtained by SEM.

[0056] Example 2:

[0057] To examine the effect of the perovskite film on the fitted diffusion coefficient, all reaction conditions were the same as in Example 1, except for the perovskite precursor concentration and spin-coating speed. The mass fraction of the precursor solution was 55%. The solution was stirred at 60°C for 6 hours. After filtration, it was spin-coated onto FTO glass at 4000 rpm / s for 25 seconds. The resulting film measured 550 nm using SEM. The longitudinal diffusion coefficient D was found to be 1.73 cm⁻¹ using back-excitation-front-detection (TR) testing and fitting. 2 / s, which is close to the diffusion coefficient of MAPbI3 single crystal.

[0058] Example 3:

[0059] To examine the effect of the perovskite film on the fitted diffusion coefficient, all reaction conditions were the same as in Example 1, except for the perovskite precursor concentration and spin-coating speed. The mass fraction of the precursor solution was 55%. The solution was stirred at 60°C for 6 hours. After filtration, it was spin-coated onto FTO glass at 3000 rpm / s for 25 seconds. The resulting film measured 753 nm using SEM. The longitudinal diffusion coefficient D was found to be 1.70 cm⁻¹ using back-excitation-front-detection (TR) testing and fitting. 2 / s, which is close to the diffusion coefficient of MAPbI3 single crystal.

[0060] Example 4:

[0061] To examine the effect of the perovskite film on the fitted diffusion coefficient, all reaction conditions were the same as in Example 1, except for the perovskite precursor concentration and spin-coating speed. The mass fraction of the precursor solution was 55%. The solution was stirred at 60°C for 6 hours. After filtration, it was spin-coated onto FTO glass at 2000 rpm / s for 25 seconds. The resulting film measured to be 1011 nm using SEM. The longitudinal diffusion coefficient D was found to be 1.20 cm⁻¹ using back-excitation-front-detection (TR) testing and fitting. 2 / s, which is close to the diffusion coefficient of MAPbI3 single crystal.

[0062] Example 5:

[0063] To examine the effect of the perovskite film on the fitted diffusion coefficient, all reaction conditions were the same as in Example 1, except for the perovskite precursor concentration and spin-coating speed. The mass fraction of the precursor solution was 60%. The solution was stirred at 60°C for 6 hours. After filtration, it was spin-coated onto FTO glass at 2000 rpm / s for 25 seconds. The resulting film measured 1540 nm using SEM. The longitudinal diffusion coefficient D was found to be 1.65 cm⁻¹ using back-excitation-front-detection (TR) testing and fitting. 2 / s, which is close to the diffusion coefficient of MAPbI3 single crystal.

Claims

1. A method of measuring the backside excitation-frontside detection transient reflectance (TR) and the film longitudinal diffusion coefficient, characterized in that: The thickness of the perovskite film sample is 1-2 um, the pump light is incident from the back of the film sample, the incident angle is 0-30°, the probe light is incident from the front of the film sample, the incident angle is 30-90°, the reflected probe light on the film sample enters the detector to obtain the TR spectrum, the longitudinal diffusion coefficient of the perovskite film is obtained by TR analysis, and the one-dimensional diffusion model is used. By fitting the TR kinetics of different excitation wavelengths, the longitudinal diffusion coefficient D and the surface recombination rate S can be extracted.

2. The measurement method according to claim 1, characterized in that, The probe light is white light 450-950 nm; the pump light is 400-635 nm excitation light.

3. The measurement method according to claim 1, characterized in that, The probe light and the pump light correspond to the two side surfaces of the film, and finally the diffusion coefficient is obtained by extracting the kinetic global fitting of the experimental data.

4. The measurement method according to claim 1, characterized in that, The test sample is a MAPbI3 perovskite film.

5. The measuring method according to claim 1 or 2, characterized in that, The pump light is incident from the back of the sample, the incident angle is 0-10°, and the probe light is incident from the surface of the sample, the incident angle is 45°.

6. The measurement method according to claim 1, characterized in that, The longitudinal diffusion coefficient of the perovskite film is obtained by TR analysis, and the calculation of the diffusion coefficient of the perovskite film is as follows: there is no energy dependence at a lower energy, The concentration distribution of the carrier in the whole film changes with time and can be expressed as: ， wherein N x, t is the carrier concentration at time t at film depth x, D is the longitudinal diffusion coefficient, τ B denotes the bulk carrier lifetime, the initial condition for equation (1) can be expressed as:​ ， N 0 is the surface carrier concentration at the initial moment, α is the absorption coefficient of the film, the boundary conditions can be expressed as follows: ， ， wherein L is the thickness of the perovskite film, S is the surface recombination velocity SRV, assuming a probing depth of the probing light of d， with a value of λ / 4π n , n is the refractive index, the TR signal of the backside excitation-frontside probing mode has the following relation: ， D of about 1.5 cm was obtained by fitting 2 s -1 consistent with the SEM cross-section having a single grain.

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