Thin-film photodetector, preparation method thereof and heart rate sensor

Abstract

The application discloses a thin film type photoelectric detector, a preparation method thereof and a heart rate sensor, wherein the photoelectric detector comprises a substrate, a first electrode layer, a photoelectric conversion layer, a buffer layer, a window layer and a second electrode layer which are sequentially stacked, and the photoelectric conversion layer is a copper-zinc-tin-sulfur-based thin film. 14 The photoelectric detector has the advantages of fast response speed (response time reaches the order of nanoseconds), excellent reliability (can withstand high temperature of 100 DEG C and low temperature of -50 DEG C, and still has stable optical response after being placed in a room for half a year), a detectable spectral range of up to 1300 nm, a detectable light intensity as low as 708 fW, and the ability of working under ultra-weak light.

Description

Technical Field

[0001] This invention relates to the field of photoelectric detection technology, and in particular to a thin-film photoelectric detector, its fabrication method, and a heart rate sensor. Background Technology

[0002] Accurate, real-time, and non-invasive monitoring of biometric information (such as heart rate, blood pressure, and arterial oxygen saturation) is crucial for wearable medical devices. Unhealthy or sub-healthy patients wearing mobile medical devices can receive timely alerts of abnormal bodily signals and take swift action. A key component for biometric information acquisition is a highly sensitive and low-power photoelectric detection unit, typically composed of a photodetector and a light-emitting diode (LED). Light generated by the LED is transmitted through or reflected by biological tissue, carrying relevant biometric information. The photodetector detects the generated light signal and generates a related electrical signal. An integrated circuit collects the electrical signal generated by the photodetector, and finally, appropriate software programs process the signal. The processed signal can provide various important biometric information, such as heart rate, blood pressure, and arterial oxygen saturation.

[0003] Currently, the photodetectors used in wearable medical devices have low sensitivity, low detectivity, and narrow spectral response range, making them inoperable in low light conditions. Therefore, multiple photodetectors (photodiodes) and two or more high-emission LEDs are required. Consequently, these wearable medical devices have high power requirements, necessitating frequent charging, significantly reducing the single-charge usage time of mobile devices. Furthermore, the additional LEDs complicate the manufacturing process and increase product costs. In addition, the detection speed and stability of the photodetectors used in wearable medical devices need further improvement.

[0004] Therefore, existing technologies still need to be improved and developed. Summary of the Invention

[0005] In view of the shortcomings of the prior art, the purpose of this invention is to provide a thin-film photodetector, its preparation method and heart rate sensor, in order to solve the problems of low sensitivity and low detectivity of photodetectors used in existing wearable medical devices.

[0006] The technical solution of the present invention is as follows:

[0007] In a first aspect, the present invention provides a thin-film photodetector, comprising a substrate, a first electrode layer, a photoelectric conversion layer, a buffer layer, a window layer, and a second electrode layer stacked sequentially, wherein the photoelectric conversion layer is a copper-zinc-tin-sulfur-based thin film.

[0008] Optionally, the photoelectric conversion layer is a copper-zinc-tin-sulfur-based thin film doped with silver or titanium ions.

[0009] Optionally, the molar ratio of the silver ions or titanium ions to the copper ions in the copper-zinc-tin-sulfur-based thin film is (0.02-0.07):1.

[0010] Optionally, the copper-zinc-tin-sulfur-based thin film includes at least one of copper-zinc-tin-sulfur thin film and copper-zinc-tin-sulfur-selenium thin film.

[0011] Optionally, the buffer layer includes one of a CdS buffer layer and a zinc tin oxide buffer layer.

[0012] Optionally, the material of the window layer includes one of indium tin oxide and aluminum-doped zinc oxide.

[0013] Optionally, the material of the first electrode layer includes Mo; the material of the second electrode layer includes silver or gold.

[0014] Optionally, the thickness of the first electrode layer is 500-1000 nm;

[0015] The thickness of the copper-zinc-tin-sulfur-based thin film is 1-1.5 μm;

[0016] The thickness of the buffer layer is 50-100 nm;

[0017] The thickness of the window layer is 300-800 nm.

[0018] A second aspect of the present invention provides a method for fabricating the thin-film photodetector as described above, comprising the steps of:

[0019] Provide a base;

[0020] A first electrode layer, a photoelectric conversion layer, a buffer layer, a window layer, and a second electrode layer are sequentially formed on the substrate to obtain the thin-film photodetector.

[0021] The photoelectric conversion layer is a copper-zinc-tin-sulfur-based thin film.

[0022] In a third aspect, the present invention provides an ultra-weak light optical heart rate sensor, comprising the thin-film photodetector described above.

[0023] Beneficial effects: The photodetector provided by this invention has a detectivity of up to 10. 14 Jones offers fast response (response time on the order of nanoseconds), excellent reliability (withstanding temperatures as high as 100°C and as low as -50°C, and maintaining stable optical response even after being placed indoors for six months), a detectable spectral range up to 1300nm, and detectable light intensities as low as 708fW, enabling it to operate in ultra-low light conditions. Attached Figure Description

[0024] Figure 1 (a) is a photoelectric response result diagram of the photodetector prepared in Example 1 of the present invention; (b) is a photoelectric response result diagram of the photodetector prepared in Example 1 of the present invention after being placed at 100℃ for 1 hour; (c) is a photoelectric response result diagram of the photodetector prepared in Example 1 of the present invention after being placed at -50℃ for 1 hour; (d) is a photoelectric response result diagram of the photodetector prepared in Example 1 of the present invention after being placed indoors for half a year.

[0025] Figure 2 The image shows the photoelectric response of the photodetector prepared in Example 1 of this invention under 905nm laser irradiation.

[0026] Figure 3 Figure (a) shows a schematic diagram of using the photodetector prepared in Example 1 for heart rate detection; (b) shows the results of heart rate detection when the fingertip is illuminated by an indoor incandescent lamp as a light source in a static state and after running; (c) shows the results of heart rate detection when the wrist is illuminated by a 635nm laser as a light source in a static state; and (d) shows the results of heart rate detection when the wrist is illuminated by a 905nm laser as a light source in a static state. Detailed Implementation

[0027] This invention provides a thin-film photodetector, its fabrication method, and a heart rate sensor. To make the objectives, technical solutions, and effects of this invention clearer and more explicit, the invention is further described in detail below. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0028] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of this invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0029] Currently, the photodetectors used in wearable medical devices have low sensitivity, low detectivity, and narrow spectral response range, and cannot operate in low light conditions. Therefore, multiple photodetectors (photodiodes) and two or more high-emission LED light sources are required. Consequently, these wearable medical devices have high power requirements, necessitating frequent charging, which significantly reduces the single-charge usage time of mobile devices. Furthermore, the additional LEDs complicate the manufacturing process and increase product costs. In addition, the detection speed and stability of the photodetectors used in wearable medical devices need further improvement. Based on this, this invention provides a thin-film photodetector, comprising a substrate, a first electrode layer, a photoelectric conversion layer, a buffer layer, a window layer, and a second electrode layer stacked sequentially, wherein the photoelectric conversion layer is a copper-zinc-tin-sulfur-based thin film.

[0030] The photodetector provided in this invention uses a copper-zinc-tin-sulfur-based thin film as the photoelectric conversion layer, exhibiting high sensitivity, high detectivity, wide spectral response range, and the ability to operate under ultra-weak light conditions. Specifically, the photodetector provided by this invention can achieve self-driven operation without external bias voltage, with a detectivity as high as 10. 14 Jones offers fast response (response time on the order of nanoseconds), excellent reliability (withstanding temperatures as high as 100°C and as low as -50°C, and maintaining stable optical response even after being placed indoors for six months), a detectable spectral range up to 1300nm, and detectable light intensities as low as 708fW, enabling it to operate in ultra-low light conditions.

[0031] In one embodiment, the photoelectric conversion layer is a copper-zinc-tin-sulfur-based thin film doped with silver or titanium ions. Doping with silver or titanium ions can reduce copper-zinc antisite defects, lower the bulk defect concentration of the copper-zinc-tin-sulfur-based thin film (i.e., reduce the bulk defect concentration of the photoelectric conversion layer), thereby reducing the dark current of the photodetector and further improving the detectivity of the photodetector.

[0032] In one embodiment, the molar ratio of silver ions or titanium ions to copper ions in the copper-zinc-tin-sulfur-based thin film is (0.02-0.07):1. This content allows the copper-zinc-tin-sulfur-based thin film to have a low bulk defect concentration, effectively improving the detectivity of the photodetector.

[0033] In one embodiment, the copper-zinc-tin-sulfur-based thin film includes at least one of copper-zinc-tin-sulfur thin film (CZTS thin film) and copper-zinc-tin-sulfur-selenium thin film (CZTSSe thin film). The CZTS thin film has a band gap of 1.5 eV and a detection spectrum of 300-1000 nm, while the introduction of selenium into the CZTSSe thin film can lower the band gap to as low as 1.0 eV, further broadening the detection spectrum of the photodetector to 300-1300 nm. In this embodiment, the general chemical formula of the copper-zinc-tin-sulfur-based thin film is Cu₂ZnSn(S₂S₂)₃. 1-x Se x )4, where 0≤x≤1; preferably 0.8≤x≤1.

[0034] In one embodiment, the thickness of the copper-zinc-tin-sulfur-based thin film is 1-1.5 μm. For example, it can be 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, or 1.5 μm. Copper-zinc-tin-sulfur-based thin films of this thickness have high responsivity.

[0035] In one embodiment, the substrate includes, but is not limited to, a glass substrate or a stainless steel substrate.

[0036] In one specific embodiment, the glass substrate is a soda-lime glass substrate, where sodium diffuses into the photoelectric conversion layer, thereby improving the performance of the photodetector.

[0037] In one embodiment, the thickness of the buffer layer is 50-100 nm, for example, it can be 50 nm, 60 nm, 70 nm, 80 nm, 90 nm or 100 nm. This thickness of buffer layer is beneficial for the transmission of incident light and the collection of electrons.

[0038] In one embodiment, the photoelectric conversion layer and the buffer layer form a heterojunction. In another embodiment, the buffer layer includes, but is not limited to, a CdS buffer layer and a zinc-tin-oxide buffer layer (i.e., a ZTO buffer layer, wherein the zinc-tin molar ratio is 1:1 to 4:1). Currently, most photodetectors require an external bias voltage as a driving force to prevent the recombination of photogenerated electron-hole pairs, which generates a large dark current, posing challenges to application scope and service life. In this embodiment, the copper-zinc-tin-sulfur-based thin film forms a PN junction with the CdS buffer layer or the ZTO buffer layer. Electrons and holes generated under illumination can spontaneously separate, allowing the photodetector to generate the photoelectric conversion process without an external bias voltage. In this case, the thin-film photodetector is a self-driven thin-film photodetector.

[0039] In one embodiment, the thickness of the window layer is 300-800 nm, for example, it can be 300 nm, 400 nm, 500 nm, 600 nm, 700 nm or 800 nm. This thickness of window layer is beneficial for light transmission and has a low resistance value.

[0040] In one embodiment, the material of the window layer includes, but is not limited to, indium tin oxide or aluminum-doped zinc oxide.

[0041] In one embodiment, the material of the first electrode layer includes, but is not limited to, Mo. Mo has good adhesion to copper-zinc-tin-sulfur-based thin films, meaning that the copper-zinc-tin-sulfur-based thin film can adhere well to the Mo material. The first electrode layer (Mo layer) is closely bonded to the copper-zinc-tin-sulfur-based thin film, which helps to improve the performance of the photodetector.

[0042] In one embodiment, the thickness of the first electrode layer is 500-1000 nm.

[0043] In one embodiment, the material of the second electrode layer includes, but is not limited to, silver or gold. The present invention does not limit the thickness of the second electrode layer, which can be set according to actual needs.

[0044] This invention also provides a method for fabricating the thin-film photodetector described above, comprising the following steps:

[0045] S1, Provide the substrate;

[0046] S2. A first electrode layer, a photoelectric conversion layer, a buffer layer, a window layer, and a second electrode layer are sequentially formed on the substrate to obtain the thin-film photodetector.

[0047] The photoelectric conversion layer is a copper-zinc-tin-sulfur-based thin film.

[0048] The fabrication method provided in this invention is simple and efficient, and the resulting photodetector can achieve self-driven operation without external bias voltage, with a detectivity as high as 10. 14 Jones has a response speed in the nanosecond range, excellent reliability (it can withstand high temperatures of 100°C and low temperatures of -50°C, and still has a stable optical response after being placed indoors for six months), can detect light intensity as low as 708 fW, and can detect a spectral range of up to 1300 nm (300-1300 nm), and can operate under indoor lighting conditions.

[0049] In step S1, the specific type of the substrate can be found above and will not be repeated here.

[0050] In step S2, in one embodiment, a first electrode layer is formed on the substrate using magnetron sputtering. The substrate containing the first electrode layer can also be purchased directly. The material for the first electrode layer is described above and will not be repeated here.

[0051] In one embodiment, the method for preparing the copper-zinc-tin-sulfur-based thin film includes the following steps:

[0052] S211. Copper salt, zinc salt, tin salt and sulfur source are added to solvent and mixed to obtain precursor solution;

[0053] S212. At a preset temperature, the precursor solution is coated onto the first electrode layer to prepare a copper-zinc-tin-sulfur thin film on the first electrode layer.

[0054] In step S211, in one embodiment, the copper salt is selected from at least one of cuprous chloride and copper acetate, but is not limited thereto.

[0055] In one embodiment, the zinc salt is selected from at least one of zinc acetate and zinc chloride, but is not limited thereto.

[0056] In one embodiment, the tin salt is selected from at least one of tin tetrachloride and stannous chloride, but is not limited thereto.

[0057] In one embodiment, the sulfur source includes, but is not limited to, thiourea.

[0058] In one embodiment, the solvent includes, but is not limited to, ethylene glycol methyl ether.

[0059] In step S211, silver salt or titanium salt may be added to the solution in the precursor to prepare copper-zinc-tin-sulfur thin films or copper-zinc-cadmium-tin-sulfur thin films doped with silver ions or titanium ions. The silver salt may be silver chloride, and the titanium salt may be titanium chloride, but is not limited thereto.

[0060] In step S212, the preset temperature is 270–300°C. For example, it can be 270°C, 280°C, 290°C, or 300°C, etc.

[0061] Step S212 is followed by the following steps:

[0062] A copper-zinc-tin-sulfur thin film is obtained by selenization treatment at a temperature of 500-600℃; or,

[0063] Copper-zinc-tin-sulfur thin films doped with silver or titanium ions are selenized at a temperature of 500-600℃ to obtain copper-zinc-tin-selenium-sulfur thin films doped with silver or titanium ions.

[0064] In practice, the aforementioned film can be placed inside a graphite box, a quartz boat containing selenium particles (or selenium powder) can be placed inside the graphite box, and then the graphite box can be placed in a tube furnace and argon gas can be introduced at a temperature of 500-600℃ for selenization treatment.

[0065] In step S2, in one embodiment, the method for preparing the buffer layer includes the following steps:

[0066] S221. Cadmium sulfate, thiourea, ammonia, and water are mixed to obtain a mixed solution;

[0067] S222. The substrate containing the first electrode layer and the photoelectric conversion layer is placed in the mixture and chemically deposited in a water bath at a preset temperature to obtain a CdS buffer layer.

[0068] In step S221, in one embodiment, the concentration of cadmium sulfate in the mixture is 0.01-0.02 mol / L, the concentration of thiourea is 0.5-1 mol / L, the mass content of NH3 in the ammonia water is 28-30%, and the mass ratio of ammonia water to water is 1:7.

[0069] Cadmium sulfide is insoluble in water but soluble in ammonia. As the heating reaction proceeds, the ammonia evaporates, causing cadmium sulfide to slowly precipitate and deposit on the photoelectric conversion layer.

[0070] In step S222, in one embodiment, the preset temperature is 75-85°C. For example, it can be 75°C, 80°C, or 85°C.

[0071] In step S2, the window layer can be prepared by magnetron sputtering. Magnetron sputtering is a commonly used method in the prior art and will not be described in detail here. The material for the window layer is as described above and will not be repeated here.

[0072] The second electrode layer can be prepared by thermal evaporation. Specifically, the Ag electrode can be prepared by thermal evaporation. Thermal evaporation is a commonly used method in the prior art, and will not be elaborated here.

[0073] Currently, wearable medical devices typically employ multiple photodetectors (photodiodes) and are equipped with two or more high-emission LED light sources. Therefore, these wearable medical devices have high power requirements, necessitating frequent charging, significantly reducing the single-charge usage time of the mobile device. Furthermore, the additional LEDs complicate the manufacturing process and increase product costs. Based on this, embodiments of the present invention also provide an ultra-low light optical heart rate sensor, which includes the thin-film photodetector described above. The ultra-low light optical heart rate sensor provided by this invention can achieve heart rate detection at the fingertip under indoor lighting conditions, and can also achieve transmitted light heart rate detection at the wrist (near-infrared (NIR) light can penetrate deep into human skin / tissue; oxyhemoglobin / deoxyhemoglobin have different absorption rates for NIR light, thus heart rate changes can be observed, and blood oxygen content can be obtained by calculating the transmittance), with accurate and reliable detection results. The ultra-low light optical heart rate sensor provided by this invention can achieve self-driven operation without external bias voltage, can operate under indoor lighting conditions, does not require an additional high-emission LED light source, has low power requirements, does not require frequent charging, has a longer single-charge usage time, and is less expensive.

[0074] The following detailed description uses specific examples.

[0075] Example 1

[0076] A thin-film photodetector includes a soda-lime glass substrate, a Mo layer, a silver ion-doped CZTSSe thin film, a CdS buffer layer, an ITO window layer, and an Ag electrode layer, which are sequentially stacked. The soda-lime glass substrate has a thickness of 2 mm, the Mo layer has a thickness of 700 nm, the silver ion-doped CZTSSe thin film has a thickness of 1.2 μm, the CdS buffer layer has a thickness of 60 nm, the ITO window layer has a thickness of 500 nm, and the Ag electrode layer has a thickness of 500 nm.

[0077] The fabrication method of the thin-film photodetector includes the following steps:

[0078] Provides a soda-lime glass substrate;

[0079] A 700 nm thick Mo layer was formed on a soda-lime glass substrate using magnetron sputtering.

[0080] 0.0195 mol cuprous chloride, 0.01 mol zinc acetate, 0.01 mol tin tetrachloride, 0.04 mol thiourea, and 0.5 mmol silver chloride were added to 20 mL of ethylene glycol methyl ether and mixed thoroughly to obtain the precursor solution.

[0081] A glass substrate containing a Mo layer was placed on a heating stage at 280°C, and the above precursor solution was spin-coated onto the Mo layer to form a 1.2 μm thick silver ion-doped CZTS precursor film.

[0082] A soda-lime glass substrate containing a silver ion-doped CZTS precursor film and a Mo layer was placed in a graphite box, and a quartz boat containing selenium particles was placed inside. The graphite box was then placed in a tube furnace and heated to 555°C while argon gas was introduced to supersaturate the silver ion-doped CZTS precursor film, resulting in a silver ion-doped CZTSSe film with a Se to S molar ratio of 5:1, i.e., Cu. 1.95 Ag 0.05 ZnSn(S 0.1667 Se 0.8333 )4;

[0083] Cadmium sulfate, thiourea, and ammonia were added to water to form a mixture (wherein, the concentration of cadmium sulfate was 0.015 mol / L, the concentration of thiourea was 0.75 mol / L, and the mass content of NH3 in the ammonia was 28%, and the mass ratio of ammonia to water was 1:7). The mixture was then deposited on a silver ion-doped CZTSSe thin film in a chemical water bath at 80°C for 9 minutes to obtain a CdS buffer layer with a thickness of 60 nm.

[0084] A 500 nm thick ITO window layer was deposited on a CdS buffer layer by magnetron sputtering under a pressure of 0.4 Pa and a power of 120 W.

[0085] An Ag electrode with a thickness of 500 nm was thermally evaporated on an ITO window layer.

[0086] The photodetector prepared in Example 1 was tested at 905 nm and 1 mW / cm. 2 Photoelectric performance tests were conducted under illumination, and the results are as follows: Figure 1 As shown in (a)-(d), by Figure 1 (a) It can be seen that the photodetector has a photoelectric response, from Figure 1 As shown in (b), the photodetector can withstand a high temperature of 100℃ and can still function normally after being placed in a high temperature of 100℃ for 1 hour, demonstrating high temperature stability. Figure 1 As shown in (c), the photodetector can withstand a low temperature of -50℃. After being placed at -50℃ for 1 hour, it can still work normally, demonstrating low-temperature stability. Figure 1 As shown in (d), the photodetector has high stability and can still work normally after being placed indoors for six months (with stable optical response), demonstrating long-term stability.

[0087] The photoelectric response was tested under 905nm laser irradiation, and the results are as follows: Figure 2As shown, this photodetector can detect light intensities as low as 708 fW.

[0088] Application Example 1

[0089] The photodetector prepared in Example 1 was used for heart rate detection. Specifically, the photodetector was connected to the source meter with copper wire to perform heart rate detection.

[0090] Place the photodetector on the pad of your index finger, and illuminate the pad of your index finger with the light from an indoor incandescent lamp. (See diagram below.) Figure 3 As shown in (a), the output signal of the photodetector is collected using a source meter, which is the heart rate signal. Heart rate was detected in a resting state and after running, and the results are as follows. Figure 3 As shown in (b), heart rate detection at the fingertip under indoor lighting conditions has been successfully achieved.

[0091] Place the photodetector on one side of the wrist, and use a 635nm laser as the light source to illuminate the other side of the wrist (the side without the photodetector), as shown in the diagram. Figure 3 As shown in (a), the output signal of the photodetector is collected by a source meter, which is the heart rate signal. The heart rate detection result in the resting state is as follows. Figure 3 As shown in (c).

[0092] Place the photodetector on one side of the wrist, and use a 905nm laser as the light source to illuminate the other side of the wrist (the side without the photodetector), as shown in the diagram. Figure 3 As shown in (a), the output signal of the photodetector is collected by a source meter, which is the heart rate signal. The heart rate detection result in the resting state is as follows. Figure 3 As shown in (d). From Figure 3 As can be seen from (c) and (d), this invention is the first to realize the transmission light heart rate detection at the wrist (near-infrared (NIR) light can penetrate deep into human skin / tissue, and oxyhemoglobin / deoxyhemoglobin have different absorption rates for near-infrared light, so heart rate changes can be seen, and blood oxygen content can be obtained by calculating the transmittance), which is different from the traditional reflective detection method, and the detection results are fast and accurate.

[0093] In summary, this invention provides a thin-film photodetector, its fabrication method, and a heart rate sensor. The photodetector provided in this embodiment can achieve self-driven operation without external bias voltage, with a detectivity of up to 10. 14The Jones optical heart rate sensor boasts a fast response speed (nanosecond-level response time) and excellent reliability (withstanding temperatures as high as 100°C and as low as -50°C, maintaining stable optical response even after six months of indoor placement). It can detect light intensities as low as 708 fW and a spectral range up to 1300 nm, enabling operation even in extremely low light conditions. This ultra-low light optical heart rate sensor can detect heart rate at the fingertips under indoor lighting and can also detect heart rate at the wrist using transmitted light (a first in this invention), providing accurate and reliable results. The ultra-low light optical heart rate sensor provided by this invention can achieve self-driven operation without external bias voltage and can work under indoor lighting conditions. It does not require an additional high-emission LED light source, has low power requirements, does not need frequent charging, has a longer operating time per charge, and is less expensive.

[0094] It should be understood that the application of the present invention is not limited to the examples above. Those skilled in the art can make improvements or modifications based on the above description, and all such improvements and modifications should fall within the protection scope of the appended claims.

Claims

1. An application of an ultra-low light optical heart rate sensor in heart rate detection, characterized in that, The ultra-low light optical heart rate sensor includes a thin-film photodetector, which comprises a substrate, a first electrode layer, a photoelectric conversion layer, a buffer layer, a window layer, and a second electrode layer stacked sequentially. The photoelectric conversion layer is a silver ion-doped copper-zinc-tin-sulfur-based thin film. The copper-zinc-tin-sulfur-based thin film includes at least one of copper-zinc-tin-sulfur thin film and copper-zinc-tin-sulfur-selenium thin film; The molar ratio of silver ions to copper ions in the copper-zinc-tin-sulfur-based thin film is (0.02-0.07):1; The buffer layer includes one of a CdS buffer layer and a zinc tin oxide buffer layer; The ultra-low light optical heart rate sensor can detect light intensities as low as 708 fW; The ultra-low light optical heart rate sensor can operate self-driven without external bias voltage; The heart rate detection methods include: Place a thin-film photodetector on the fingertip of your index finger, illuminate the fingertip with an indoor incandescent lamp, and collect the output signal of the thin-film photodetector using a source meter, which is the heart rate signal. Place a thin-film photodetector on one side of the wrist, and use a 635nm or 905nm laser as a light source to illuminate the other side of the wrist where the thin-film photodetector is not located. Use a source meter to collect the output signal of the thin-film photodetector, which is the heart rate signal.

2. The application according to claim 1, characterized in that, The window layer is made of either indium tin oxide or aluminum-doped zinc oxide.

3. The application according to claim 1, characterized in that, The material of the first electrode layer includes Mo; the material of the second electrode layer includes silver or gold.

4. The application according to claim 1, characterized in that, The thickness of the first electrode layer is 500-1000 nm; The thickness of the copper-zinc-tin-sulfur-based thin film is 1-1.5 μm; The thickness of the buffer layer is 50-100 nm; The thickness of the window layer is 300-800 nm.

5. The application according to claim 1, characterized in that, The fabrication method of the thin-film photodetector includes the following steps: Provide a base; A first electrode layer, a photoelectric conversion layer, a buffer layer, a window layer, and a second electrode layer are sequentially formed on the substrate to obtain the thin-film photodetector.

Images