A method for preparing a bismuth sulfide thin film and its photodiode, and its application.

Abstract

This invention discloses a method for fabricating a bismuth sulfide thin film and its application in a photodiode. The method for fabricating the bismuth sulfide thin film includes: providing a Bi₂S₃ precursor solution; coating the Bi₂S₃ precursor solution onto a substrate; placing the substrate coated with the Bi₂S₃ precursor solution in a nitrogen atmosphere, first annealing at a low temperature until the solvent evaporates completely, and then annealing at 280±5℃ until surface defects of the bismuth sulfide thin film are eliminated. The Bi₂S₃ thin film prepared by this method has a very smooth surface with a surface roughness of only about 2 nm, which is very advantageous for the fabrication of thin-film diode devices. This method can be used to prepare a bismuth sulfide thin film on an electron transport layer as a light-absorbing active layer; then, a hole transport layer can be prepared on the bismuth sulfide thin film, thereby obtaining a diode-type photodetector with high light absorption capacity, high carrier mobility, and high light-dark current ratio.

Description

Technical Field

[0001] This invention relates to the field of optoelectronic device manufacturing technology, and in particular to a method for preparing a bismuth sulfide thin film and its photodiode, as well as its application. Background Art

[0002] Sulfides have attracted widespread attention from academia and industry in the fields of photovoltaics, photocatalysis, and photoelectric detection due to their excellent light absorption coefficient, tunable band gap and absorption range, strong stability (including air stability, temperature stability and humidity stability), low cost and multiple preparation methods.

[0003] Bismuth sulfide (Bi2S3) is an emerging semiconductor material. Bismuth sulfide materials prepared at room temperature typically have a band gap of 1.3–1.7 eV and a high absorption coefficient (≈10). 5 cm -1 In the visible light band, bismuth sulfide exhibits a band gap very close to the optimal bandgap width obtained from the fine balance limit, making it highly suitable for single-junction solar cell applications. Furthermore, its high mobility and carrier lifetime also make it a promising candidate for photoelectric detection. In addition, bismuth sulfide is rich in elemental components and non-toxic; these combined properties make it a very promising semiconductor material.

[0004] Currently, bismuth sulfide thin films are mostly prepared using methods such as chemical bath deposition (CBD), thermal evaporation, and hydrothermal methods, which primarily offer the advantage of high film quality. However, these methods also suffer from drawbacks such as high cost, difficulty in precisely controlling the reaction rate, difficulty in accurately controlling the doping concentration, and the presence of reactant residues on the inner surface of the experimental container, leading to cumbersome subsequent cleaning steps. In the past, research on bismuth sulfide has focused more on nanomaterial synthesis and thermoelectric applications, while further exploration is needed in optoelectronic applications. This is because the complex preparation process and low efficiency of preparation significantly limit the further development of bismuth sulfide thin film-based optoelectronic devices. Therefore, there is an urgent need to provide a suitable bismuth sulfide thin film preparation process. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention deposits a Bi2S3 precursor solution onto a substrate by coating, obtaining a bismuth sulfide thin film with a surface roughness of no more than 2 nm. Optoelectronic devices prepared based on this bismuth sulfide thin film have a significant light-to-dark current ratio and have good application prospects in the field of photodetectors.

[0006] In a first aspect, the present invention provides a method for preparing a bismuth sulfide thin film, comprising:

[0007] Provide Bi2S3 precursor solution;

[0008] The Bi2S3 precursor solution was coated onto the substrate;

[0009] The substrate coated with Bi2S3 precursor solution was placed in a nitrogen atmosphere and annealed at low temperature until the solvent evaporated completely, and then annealed at 280±5℃ until the surface defects of the bismuth sulfide film were eliminated.

[0010] In some embodiments of the present invention, the Bi2S3 precursor solution is prepared by dissolving bismuth salt and thiourea in an organic solvent at a molar ratio of bismuth to sulfur of 1:1.3 to 1:3.

[0011] In some embodiments provided by this invention, Zn is also added to the Bi2S3 precursor solution. 2+ .

[0012] In some embodiments provided by the present invention, the process of first annealing at low temperature until the solvent evaporates completely, and then annealing at 280±5°C until the defects on the surface of the bismuth sulfide film are eliminated includes: first annealing at 100±5°C for 10 to 20 minutes, and then annealing at 280±5°C for 15 to 25 minutes.

[0013] Secondly, the present invention provides a bismuth sulfide thin film prepared by the above-described method for preparing bismuth sulfide thin films.

[0014] Thirdly, the present invention provides the application of the above-mentioned bismuth sulfide thin film in the optoelectronic field.

[0015] Fourthly, the present invention provides a method for fabricating a diode-type photodetector, comprising:

[0016] A bismuth sulfide thin film is prepared on the electron transport layer using the above-described method, serving as a light-absorbing active layer; then a hole transport layer is prepared on the bismuth sulfide thin film.

[0017] Fifthly, the present invention provides a diode-type photodetector prepared by the above-described method for preparing a diode-type photodetector.

[0018] In some embodiments provided by the present invention, the electron transport layer is SnO2, TiO2 or CdS; the hole transport layer is PTAA, P3HT or spiro-OMeTAD.

[0019] In some embodiments provided by the present invention, the thickness of the electron transport layer is 10-80 nm; the thickness of the bismuth sulfide film is 20-500 nm; and the thickness of the hole transport layer is 5-50 nm.

[0020] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0021] (1) Compared with the existing bismuth sulfide films prepared by hydrothermal method, evaporation method and chemical bath deposition method, the bismuth sulfide film prepared by the present invention has a very smooth surface with a surface roughness of only about 2 nm, which is very advantageous for thin film diode devices. At the same time, the present invention avoids the problem of difficult reaction rate control in hydrothermal method, evaporation method and chemical bath deposition method, and the experimental process is simple, the equipment requirements are simple, and the cost is low.

[0022] (2) The photodiode-type detector prepared by this invention has a quality exceeding 10 2 The light sensitivity is less than 1 nA / Hz under bias voltage. 1 / 2 The noise current is significantly reduced, and cation-doped detectors exhibit even lower dark current and noise current. Furthermore, the devices demonstrate good environmental stability, showing tolerance to both temperature and humidity, thus providing more favorable conditions for future applications. Attached Figure Description

[0023] The present invention will be further described below with reference to the accompanying drawings, but the embodiments in the drawings do not constitute any limitation on the present invention.

[0024] Figure 1 a is a schematic diagram of the photodiode-type detector prepared by spin-coating a bismuth sulfide thin film using a solution method in Example 1. From bottom to top, the structure consists of an Ag electrode, a MoO2 electrode, and a bismuth sulfide film. x Hole transport layer, light-absorbing active layer Bi2S3, electron transport layer, transparent conductive thin film ITO; Figure 1 b shows the photo-dark current of the device.

[0025] Figure 2 a is the photocurrent and dark current curve of the thin film obtained by spin coating of precursor solutions with different bismuth-sulfur molar ratios in Example 1; Figure 2 b is the photocurrent curve of a diode device prepared by thermally evaporating a Bi2S3 light-absorbing active layer while keeping other layers completely unchanged.

[0026] Figure 3 The noise spectrum of the photodetector prepared by the bismuth sulfide thin film obtained by the precursor solution with a bismuth-sulfur molar ratio of 1:1.5 in Example 1 under different bias voltages.

[0027] Figure 4 The current change of the device with a bismuth-sulfur molar ratio of 1:1.5 in Example 1 under continuous illumination.

[0028] Figure 5 The image shows the photoresponse curve of the device with a bismuth-sulfur molar ratio of 1:1.5 in Example 1.

[0029] Figure 6The current density-voltage curves of Bi2S3 diode devices with different hole transport layers are shown in dark and light conditions, using CdS deposited by chemical bath deposition as the electron transport layer in Example 2.

[0030] Figure 7 The current density-voltage curves of Bi2S3 diode devices with different hole transport layers are shown in dark and light conditions, using TiO2 prepared by spin coating in Example 2 as the electron transport layer.

[0031] Figure 8 The image shows the current density-voltage curves of Bi2S3 diode devices with different hole transport layers, using SnO2 prepared by spin coating as the electron transport layer in Example 2, under dark and light conditions.

[0032] Figure 9 a shows the carrier dynamics curves of the time-resolved microwave conductivity of bismuth sulfide films after doping with different types of cations. Figure 9 b shows the absolute values ​​of the change in bismuth sulfide film mobility after using different cation doping methods.

[0033] Figure 10 In Example 3, when the bismuth-sulfur molar ratio in the Bi₂S₃ precursor solution was 1:1.5, different concentrations of Zn were introduced. 2+ Current density-voltage curves of doped diode devices in the dark and under illumination. Detailed Implementation

[0034] To enable those skilled in the art to more easily understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below.

[0035] This invention uses spin coating to deposit Bi2S3 thin films and finds that the resulting bismuth sulfide thin films have high carrier mobility and high light absorption capacity. They are used as light-absorbing active layers to prepare photodetectors. The resulting photodetectors have advantages such as low noise density, fast response, and a wide range of selectable detection wavelengths.

[0036] This invention further adjusts the bismuth-sulfur molar ratio in the Bi2S3 precursor solution to obtain Bi2S3 thin films with different photoelectric properties, thereby further optimizing the performance of bismuth sulfide diode-type photodetectors.

[0037] This invention also attempts to use bismuth sulfide thin films in combination with different electron transport layers and hole transport layers. It was found that the device using SnO2 as the electron transport layer and spiro-OMeTAD as the hole transport layer has the best performance and can improve the photoelectric detection performance of the device.

[0038] The present invention also involves incorporating Cs into the Bi2S3 precursor solution. +Cu + Ag + Cd 2+ Sn 2+ Zn 2+ Ni 2+ Pb 2+ Sb 3+ Different types of cations were found, including Zn. 2+ The migration rate of the bismuth sulfide film itself was most effectively improved, and further improved by changing Zn 2+ By adjusting the doping concentration, a diode-type photodetector with lower dark current and higher signal-to-noise ratio was obtained.

[0039] Based on the above research, this invention provides bismuth sulfide thin films with high carrier mobility and high light absorption capacity, their preparation methods, and applications:

[0040] The method for preparing bismuth sulfide thin films provided by the present invention includes: providing a Bi2S3 precursor solution; coating the Bi2S3 precursor solution onto a substrate; placing the substrate coated with the Bi2S3 precursor solution in a nitrogen atmosphere, first annealing at a low temperature until the solvent evaporates completely, and then annealing at 280±5℃ until the surface defects of the bismuth sulfide thin film are eliminated.

[0041] This invention utilizes a simple precursor solution method to prepare high-performance bismuth sulfide thin films with extremely high efficiency. The surface roughness of the obtained bismuth sulfide thin films does not exceed 2 nm, which is highly advantageous for the fabrication of photosensitive sensors. Furthermore, this method is applicable to various simple or promising thin film preparation methods, such as spin coating, blade coating, and spray coating. Due to the diversity of preparation methods, it is also compatible with rigid or flexible substrates, showing great application potential. In addition, this method offers advantages such as simple operation, low cost, readily available materials, simple equipment requirements, adjustable film formation speed, and low environmental and equipment requirements.

[0042] Furthermore, the method for providing a Bi₂S₃ precursor solution according to the present invention includes: dissolving bismuth salt and thiourea in an organic solvent at a molar ratio of bismuth to sulfur of 1:1.3 to 1:3 to prepare a Bi₂S₃ precursor solution. Preferably, the organic solvent is ethylene glycol monomethyl ether, the bismuth salt is bismuth nitrate or its hydrate, and the Bi₂S₃ precursor solution contains Bi₂S₃... 3+ The concentration of [agent] is 0.5 mol / L, and the concentration of sulfur is 0.65–1.5 mol / L, more preferably, the concentration of sulfur is 0.75 mol / L.

[0043] Specifically, the method for providing a Bi₂S₃ precursor solution in this invention involves gradually and directly dissolving the bismuth and sulfur sources in a solvent by adjusting the molar ratio of bismuth to sulfur. This includes the following steps: quantitatively dissolving Bi(NO₃)₃·5H₂O in ethylene glycol monomethyl ether, resulting in a solution containing Bi₂S₃ precursor solution. 3+The concentration of thiourea is 0.5 mol / L. The resulting solution is stirred thoroughly for about 2 to 5 minutes until all the solute is dissolved. Then, a certain amount of thiourea is added so that the concentration of sulfur in the solution is 0.65 to 1.5 mol / L. Stirring is continued for 2 to 5 minutes to obtain a clear yellow Bi2S3 precursor solution. The higher the proportion of thiourea, the darker the color of the obtained solution.

[0044] In some embodiments provided by this invention, Zn is also added to the Bi2S3 precursor solution. 2+ Zn 2+ It can effectively improve the migration rate of bismuth sulfide films, preferably Zn 2+ The concentration is 1% to 3%.

[0045] In some embodiments provided by this invention, the process of first annealing at a low temperature until the solvent has completely evaporated, and then annealing at 280±5℃ until the surface defects of the bismuth sulfide film are eliminated, includes: first annealing at 100±5℃ for 10 to 20 minutes, and then annealing at 280±5℃ for 15 to 25 minutes. If annealing is performed directly at 280±5℃, a large number of pores will appear on the surface of the bismuth sulfide film.

[0046] This invention uses a coating method to deposit a precursor solution onto a corresponding substrate to prepare a simple thin film or optoelectronic device. The bismuth sulfide thin film prepared by the above-mentioned method has a very smooth surface with a surface roughness of no more than 2 nm. It has high light absorption capacity and high carrier mobility, which is very advantageous in the preparation of thin film diode devices.

[0047] The aforementioned bismuth sulfide thin films can be used in the optoelectronic field to prepare photoconductive devices and transistors.

[0048] The method for fabricating a diode-type photodetector provided by the present invention includes: fabricating a bismuth sulfide thin film on an electron transport layer using the above-described method for fabricating a bismuth sulfide thin film, which serves as a light-absorbing active layer; and then fabricating a hole transport layer on the bismuth sulfide thin film.

[0049] In some embodiments, the thickness of the bismuth sulfide film is 20–500 nm, preferably 80–150 nm.

[0050] The present invention prepares a bismuth sulfide thin film on an electron transport layer using the above-mentioned method. The steps include: depositing a Bi2S3 precursor solution onto an ITO glass plated with an electron transport layer by spin coating. During spin coating, the substrate, which has been treated with an ozone cleaner for 15 minutes, is adsorbed into a spin coater (EZ4), and an appropriate amount of Bi2S3 precursor solution is dropped onto the substrate and uniformly covered. Then, the substrate is rotated at 3000 rpm for 60 seconds. After the rotation ends, the substrate is removed and can proceed to the annealing stage. During annealing, the substrate coated with the Bi2S3 precursor solution is first pre-annealed on a hot plate at 100°C for 15 minutes to remove the solvent, and then annealed at 280°C for 20 minutes. The entire process is carried out in an N2 atmosphere. The thickness of the bismuth sulfide thin film is approximately 110 nm.

[0051] The electron transport layer on ITO glass is prepared by depositing an electron transport layer material onto a patterned ITO conductive layer on a glass substrate using a solution method (spin-coating or chemical bath deposition). In some embodiments, the electron transport layer is SnO2, TiO2, or CdS; the hole transport layer is PTAA, P3HT, or spiro-OMeTAD. The thickness of the electron transport layer is 10–80 nm.

[0052] The hole transport layer is prepared on the bismuth sulfide thin film by spin coating, and the thickness of the hole transport layer is 5-50 nm, preferably 10-35 nm.

[0053] The method for fabricating a diode-type photodetector provided by this invention further includes: evaporating an 8 nm thick layer of MoO by thermal evaporation. x The electrode consists of a layer and 100nm thick metallic silver.

[0054] The diode-type photodetector prepared by the above-described method of fabricating a diode-type photodetector has a density exceeding 10. 2 The light sensitivity is less than 1 nA / Hz under bias voltage. 1 / 2 The noise current is significantly reduced, and cation-doped detectors exhibit even lower dark current and noise current, eliminating the need for a high-voltage power supply. Furthermore, the devices demonstrate good environmental stability, exhibiting tolerance to both temperature and humidity, thus providing more favorable conditions for future applications.

[0055] As a preferred embodiment of the above technical solution, SnO2 can be used for the electron transport layer of the diode-type photodetector, and Spiro-OMeTAD can be used for the hole transport layer of the diode-type device, which can achieve better photoelectric detection performance.

[0056] The technical solution of the present invention will be described below through specific embodiments. Unless otherwise specified, the Bi2S3 precursor solution in the following embodiments is obtained by dissolving Bi(NO3)3·5H2O quantitatively in ethylene glycol monomethyl ether, and the resulting solution contains Bi 3+ The concentration of thiourea is 0.5 mol / L. The resulting solution is stirred thoroughly for about 2 to 5 minutes until the solute is completely dissolved. Then, a certain amount of thiourea is added to make the sulfur concentration in the solution 0.75 mol / L. Stirring is continued for 2 to 5 minutes to obtain a clear yellow Bi2S3 precursor solution.

[0057] Example 1

[0058] Spin-coating preparation of bismuth sulfide thin films: This invention first studied a diode-type detector using Bi2S3 precursor solutions with different bismuth-sulfur molar ratios as the light-absorbing active layer. SnO2 was used as the electron transport layer during device fabrication, and then the corresponding bismuth sulfide thin film was spin-coated as the light-absorbing active layer. The Bi2S3 precursor solution was deposited on ITO glass with an electron transport layer by spin-coating. During spin-coating, the substrate, which had been treated with an ozone cleaner for 15 minutes, was adsorbed onto a spin coater (…). Inside EZ4), an appropriate amount of Bi2S3 precursor solution is dropped onto the substrate and uniformly covered. Then, it is rotated at 3000 rpm for 60 seconds. After the rotation ends, the substrate is removed and can proceed to the annealing stage. During annealing, the substrate coated with Bi2S3 precursor solution is first annealed at 100℃ for 15 minutes, and then annealed at 280℃ for 20 minutes. The entire process is carried out in a N2 atmosphere. The thickness of the bismuth sulfide film is about 110 nm, the surface is very smooth, and the surface roughness is ≤2 nm.

[0059] Preparation of bismuth sulfide thin films by vapor deposition: For comparison, this invention also attempted to prepare bismuth sulfide thin films as light-absorbing active layers by vapor deposition: the evaporation process is carried out directly on an atmosphere with a pressure of ~10 -4 The bismuth sulfide bulk material is directly heated in a vacuum chamber of Pa to melt it, forming upward vapor that is then deposited onto the fixed substrate above. After evaporation, it is annealed at 280°C for 20 minutes on a hot stage in a glove box under N2 atmosphere.

[0060] After the light-absorbing active layer was prepared, spiro-OMeTAD was spin-coated as a hole transport layer, followed by the evaporation of MoO2. x The specific structural diagram of the Ag electrode is shown below. Figure 1 As shown in a.

[0061] Figure 2A shows the current density-voltage curves of various devices using bismuth sulfide films spin-coated with Bi2S3 precursor solutions of different bismuth-sulfur molar ratios as the light-absorbing active layer in the dark and under the same white LED illumination. From the dark current, it can be seen that the dark current levels of the various devices are not significantly different, but it is obvious that the devices with bismuth-sulfur molar ratios of 1:1.3 and 1:1.5 have weaker leakage current behavior. Furthermore, when the bismuth-sulfur molar ratio is 1:1.5, it has the highest photocurrent response. Therefore, the optimal bismuth-sulfur molar ratio is determined to be 1:1.5. Figure 2 Figure b shows the photocurrent and dark current curves of a Bi2S3 diode device with the same device structure prepared by evaporation of bismuth sulfide thin film. As can be seen from the figure, the evaporation-prepared bismuth sulfide thin film is highly conductive, resulting in a near-short circuit in the device, exhibiting a very high dark current and a low photocurrent / dark current on / off ratio. Furthermore, the device has no opening voltage and does not possess the unidirectional conduction performance of a diode.

[0062] Figure 3 The noise spectrum of the device with a bismuth-sulfur molar ratio of 1:1.5 under different bias voltages is shown. It can be seen that at a bias voltage of -0.5V, the noise density is only at the picoampere level, which can greatly improve the device's performance for light detection.

[0063] Figure 4 The device's response curve to light illumination under a 0V bias voltage, as tested by an oscilloscope, is shown. It can be seen that the device has a fast light response on the order of microseconds.

[0064] Example 2

[0065] In this embodiment, a Bi₂S₃ precursor solution with a bismuth-sulfur molar ratio of 1:1.5 was used to fabricate the device via spin coating as described in Example 1. Three different electron transport layers and three different hole transport layers were employed. Aside from the different types of transport layers used, the specific structural diagrams are shown below. Figure 1 As shown in a.

[0066] Figures 6-8 This demonstrates a comparison of optical dark currents using different transport layers, from Figure 6 As can be seen, when CdS is used as the electron transport layer, only devices using spiro-OMeTAD as the hole transport layer exhibit a significant on-state voltage, thus constituting diode-type devices; from Figure 7 As can be seen, when TiO2 is used as the electron transport layer, only devices using spiro-OMeTAD as the hole transport layer exhibit significant on-state voltage; for example... Figure 8 As shown, when SnO2 is used as the electron transport layer, there is a relatively obvious on-state voltage. It can also be found that the device with spiro-OMeTAD as the hole transport layer has a weaker leakage current phenomenon.

[0067] Example 3

[0068] In this embodiment, a Bi2S3 precursor solution with a bismuth-sulfur molar ratio of 1:1.5 was selected. Different types of cations were doped into the Bi2S3 precursor solution, and devices were prepared by spin coating according to the method in Example 1 to investigate the effect of different types of cation doping on device performance.

[0069] In the experiment, different types of cation doping were first used. Appropriate amounts of doped cations (with a concentration of Bi2S3) were added to the Bi2S3 precursor solution. 3+ The solutes included lead acetate (1% concentration), antimony acetate, zinc acetate dihydrate, nickel nitrate, cesium carbonate, cuprous chloride, cadmium chloride, stannous chloride, and silver nitrate. After the solutes were completely dissolved, they were coated onto glass slides using the same spin-coating process as in Example 1. The migration rate of the films was then characterized by time-resolved microwave conductivity (TRMC). The analysis results showed that Zn... 2+ Doping with this agent has the most significant effect on improving the mobility of bismuth sulfide films. Figure 9 a shows the carrier dynamics curves of the time-resolved microwave conductivity of bismuth sulfide films after doping with different types of cations. Figure 9 b shows the absolute values ​​of the change in bismuth sulfide film mobility after using different cation doping methods. It can be found that using Zn... 2+ Doped bismuth sulfide films showed the highest improvement.

[0070] This embodiment also uses different concentrations of Zn 2+ Different diode-type devices were fabricated by doping, and their specific structural schematic diagrams are shown below. Figure 1 As shown in a. Figure 10 It demonstrates how to change Zn 2+ A series of devices were fabricated using varying doping concentrations of Zn, and it was found that... 2+ Subsequently, the device exhibited a significant reduction in dark current level while maintaining a slightly higher photocurrent response, resulting in a noticeable performance improvement for the detector.

[0071] The above are merely preferred embodiments of the present invention and do not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method for fabricating a diode-type photodetector, characterized in that, include: A bismuth sulfide thin film was prepared on the SnO2 electron transport layer as a light-absorbing active layer: Bismuth salt and thiourea were dissolved in an organic solvent at a molar ratio of bismuth to sulfur of 1:1.3 to 1:3 to prepare a Bi₂S₃ precursor solution; Zn was also added to the Bi₂S₃ precursor solution. 2+ ; Bi2S3 precursor solution was spin-coated onto SnO2 electron transport layer; The SnO2 electron transport layer coated with Bi2S3 precursor solution was placed in a nitrogen atmosphere and annealed at 100 ± 5 °C for 10 to 20 min, and then annealed at 280 ± 5 °C for 15 to 25 min. Then, a spiro-OMeTAD hole transport layer was prepared on a bismuth sulfide film.

2. A diode-type photodetector, characterized in that: It is prepared by the method of preparing the diode-type photodetector according to claim 1.

3. The diode-type photodetector according to claim 2, characterized in that: The electron transport layer has a thickness of 10-80 nm; the bismuth sulfide film has a thickness of 20-500 nm; and the hole transport layer has a thickness of 5-50 nm.

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