Abstract:
The present invention relates generally to a near-infrared light-emitting diode (LED) and the method for manufacturing the same. When preparing the light-emitting layer of the near-infrared LED according to the present invention, the CsSnXX′ 2  solution is coated on the substrate having the hole transport layer. Then, by a drying process, the solvent is moved away and the CsSnXX′ 2  solution is solidified, crystallized to CsSnXX′ 2  in the perovskite structure, which is used as the light-emitting layer of the near-infrared LED and emits near infrared. X and X′ are identical or different halogen elements. In addition, according to the present invention, lead can be used to replace a part of tin. By adjusting the ratio of lead and tin or adopting different combination of halogen elements, the wavelength of the generated near infrared varies.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a light-emitting diode (LED) and the method for manufacturing the same, and particularly to a near-infrared LED capable of emitting near infrared and the method for manufacturing the same, which can be formed by a simple method without using complicated chemical synthesis method or rare element. 
     BACKGROUND OF THE INVENTION 
     The near infrared technology plays an important role in defense industries, geological prospecting, optical fiber communication, environmental monitoring, biological imaging, food and drugs, and agricultural inspecting. The key to the development of the near infrared technology lies in the development of materials and the fabrication of light-emitting devices. Due to their small size, low power consumption, high stability, and long lifetime, near-infrared LEDs are critical media for implementing the infrared technology. 
     Near-infrared LEDs are commonly seen in general applications such as remote controllers and automatic doors of shops. In addition, they are also frequently adopted in transmission and storage of information as well as in the domain of optical fiber communication. Furthermore, in night-vision monitors, near-infrared light sources are applications of near-infrared LEDs. Although the near infrared cannot be sensed by human eyes, it can be captured by cameras or video recorders. Thereby, the near infrared is usually used for supplementing light at night or in places with insufficient brightness, so that monitoring cameras can catch images. 
     In addition to the night-vision or optical communication applications as described above, near-infrared LEDs can be applied to the biomedical domain. For example, the near infrared emitted by a near-infrared LED can be used to illuminate the skin of a user. Next, a spectrum detector is used for receiving the reflection light, and then the user&#39;s blood glucose level can be evaluated after calculations. Thereby, near-infrared LEDs can be applied to a non-invasive blood glucose meter; they can be applied to therapies of skin diseases. 
     Nonetheless, in the fabrication of near-infrared LEDs, very few of the light-emitting materials are soluble. Moreover, complicated chemical synthesis methods are mostly required, or complicated methods are required for synthesizing quantum dot materials with different diameters, which mostly contains lead and thus against the regulations of environmental protection. Besides, the manufacturing processes mostly require costly vacuum equipment, demanding substantially high fabrication investment. 
     The China Patent Application Number CN 200780017078.2 disclosed a light-emitting material capable of emitting light in the visible or near-infrared region. The dehydrated SnI 2  and CsI are mixed in a standard mold filling machine with the ratio of 1 wt %˜99 wt % and ground for 1 to 24 hours. After testing, the ground product can be used as the light-enabled light-emitting material in the near infrared region. The adopted material is a potential material for the light-emitting layer of near-infrared LEDs. Nonetheless, the technology is still immature for application. Further researches are still required on the fabrication method of the light-emitting layer. 
     Accordingly, owing to the wide applications of near-infrared LEDs and difficulties in fabrication, the present invention provides a novel improvement on the composition of the light-emitting layer of a near-infrared LED and the method for manufacturing the same. 
     SUMMARY 
     An objective of the present invention is to provide a method for manufacturing near-infrared LED. After preparing a solution using related materials and coating, drying methods such as draining and baking can be used directly to form a solid-state film as the light-emitting layer. The method is simple and effective. 
     Another objective of the present invention is to provide a method for manufacturing near-infrared LED. The concentration of the prepared solution can be adjusted by dilution. Thereby, light-emitting layers with different thickness can be formed after drying. This is a novel technique for controlling the property of the light-emitting layer. 
     Still another objective of the present invention is to provide a method for manufacturing near-infrared LED. A specific solution is prepared and then drained. After solidification and crystallization, the formed light-emitting layer includes the perovskite structure, which is the key point for electron-hole recombination and emitting near infrared with a wavelength between 710 nm and 950. 
     A further objective of the present invention is to provide a near-infrared LED. The material of the light-emitting layer contains no heavy metal such as mercury, cadmium, chrome, cobalt, nickel, or arsenic. In addition, the lead can be used selectively as the means for adjusting the wavelength of the emitted light, which is beneficial to environmental protection. Moreover, different combinations of halogens can be used for changing the color of the light. 
     In order to achieve the objectives described above, the present invention discloses a method for manufacturing a near-infrared LED. The method with no lead involved is a typical embodiment. The near-infrared LED comprises a light-emitting layer, which is formed by the following steps. Mix a first powder, a second powder, and a solvent to form CsSnXX′ 2  solution. The first powder is CsX; the second powder is SnX′ 2 ; X and X′ are halogen elements. Coat the CsSnXX′ 2  solution on a substrate. Then, dry and solidify the CsSnXX′ 2  solution for crystallizing to CsSnXX′ 2  in the perovskite structure, which is just the light-emitting layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a structural schematic diagram according to a preferred embodiment of the present invention; 
         FIG. 2  shows a flowchart according to a preferred embodiment of the present invention; 
         FIG. 3  shows a schematic diagram of the light-emitting layer in the perovskite structure according to a preferred embodiment of the present invention; and 
         FIGS. 4A ˜ 4 C show microscopic diagrams of the light-emitting layer according to a preferred embodiment of the present invention, which are used for showing the light-emitting layers formed by CsSnI 3  solution in different concentrations after draining and drying. 
     
    
    
     DETAILED DESCRIPTION 
     In order to make the structure and characteristics as well as the effectiveness of the present invention to be further understood and recognized, the detailed description of the present invention is provided as follows along with embodiments and accompanying figures. 
     Please refer to  FIG. 1 . The near-infrared LED according to the present invention comprises a substrate  10 , a hole transport layer  20 , a light-emitting layer  30 , an electron transport layer  40 , and an electrode layer  50 . The hole transport layer  20  is disposed on the substrate  10 . The light-emitting layer  30  is disposed on the hole transport layer  20 . The electron transport layer is disposed on the light-emitting layer  30 . The electrode layer  50  is disposed on the electron transport layer  40 . 
     Based on the above structure, when the near-infrared LED according to the present invention is operating, holes can be injected into the valence band of the material of the light-emitting layer  30  via the hole transport layer  20 , and electrons can be injected into the conduction band of the material of the electron transport layer  40  via the electrode layer  50 . Because the energy level of the conduction band of the material of the electron transport layer  50  is higher, the electrons in the electron transport layer  40  can be transported to the conduction band of the light-emitting layer  30 . When electron-hole recombination occurs in the light-emitting layer  30 , because the material is Cs(Sn y Pb 1-y )X 3-Z X′ z  in the perovskite structure, where X and X′ are identical or different halogen elements, 0&lt;y≦1 and 0≦z≦3, if the formed light-emitting layer is CsSnI 3 , the wavelength of the emitted light is approximately 950 nm. Nonetheless, because the wavelength of the red light emitted by CsPbI 3  is approximately 710˜720 nm, addition of lead can be adopted for adjusting the color of the light. By controlling the added amount of lead, the wavelength of the light emitted by the near-infrared LED according to the present invention can be adjusted within the range between 710 nm and 950 nm. As more lead is added, the wavelength shifts 710 nm, approaching the lower limit of the wavelength of near infrared. In addition to changing the color of light by using the ratio of tin and lead, according to the present invention, by using different halogen elements or changing the combination of halogen elements, the wavelength of near infrared can be changed, and thus achieving the purpose of adjusting the color of light. 
     According to the method for manufacturing the near-infrared LED of the present invention, a patterned indium tin oxide (ITO) substrate is prepared first. Because residual oil, dust, photoresist, and water can be left on the surface of the patterned ITO glass substrate during storage and etching processes, before disposing the hole transport layer, the surface should be cleaned first. The cleaning process includes: (1) Immerse the patterned ITO glass substrate into the acetone solvent, place it in an ultrasonic oscillator, and oscillate for 10 minutes. (2) Immerse the substrate in the step (1) into iospropanol, place it in an ultrasonic oscillator, and oscillate for 10 minutes. (3) Immerse the substrate in the step (1) into deionized water, place it in an ultrasonic oscillator, and oscillate for 10 minutes. (4) Take out the substrate, blow it by a nitrogen gun on the workbench of the clean room, place it in an ultraviolet-ozone machine, and expose for 30 minutes. Then subsequent layer coating can be performed. 
     For forming the hole transport layer on the substrate through spin coating method, the mixture of conductive polymers poly(3,4-ethylenedioxythiophene) (PEDOT) and polystyrene sulfonate (PSS) is coated on the substrate and baked at 120° C. for 20 minutes. 
     Next, dispose the hole transport layer on the light-emitting layer. Please refer to  FIG. 2 . According to the present invention, the lead-free light-emitting layer is used as an example. The method comprises the following steps.
     Step S 1 : Mix the first powder, the second powder, and the solvent for forming the CsSnXX′ 2  solution, where the first powder is CsX; the second powder is SnX′ 2 ; and X and X′ are halogen elements.   Step S 2 : Coat the CsSnXX′ 2  solution on the hole transport layer.   Step S 3 : Dry the CsSnXX′ 2  solution, thus the CsSnXX′ 2  solution solidifies and crystallizes into CsSnXX′ 2  in the perovskite structure, and form the light-emitting layer.   

     According to a preferred embodiment of the present invention, the first powder is CsI and the second powder is SnI 2 . The amount of CsI is 0.2054 g and that of SnI 2  is 0.2946 g. In other words, 0.0008 moles of both are mixed with the solvent. The solvent adopted according to the present invention is a mixture of multiple solvents. The mixture contains methoxyactonitrile (MOAN), dimethylformamide (DMF), and acetonitrile (AN) in the ratio of 1:3:2. After mixing the first powder, the second powder, and the solvent according to the present embodiment, a CsSnI 3  solution with a concentration of 330 mg/ml is given. 
     According to the previous embodiment, CsI is used as the first powder and SnI 2  is used as the second powder. The iodine can be replaced by other halogen elements, such as fluorine, chlorine, bromine, and astatine. On the other hand, tin can be replaced by lead. In addition, both can be adopted at the same time. For example, a portion of the second powder SnI 2  is replaced by a third powder PbI 2 . However, the mole value of the first powder should be kept equal to the sum of the mole values of the second and third powders. Take the same halogen element X for example. As the first powder is CsX, the second powder is SnX 2 , and the third powder is PbX 2 , by mixing the first, second, third powders with the solvent, a Cs(Sn y Pb 1-y )X 3  solution is formed, where 0&lt;y&lt;1. If different halogen elements are uses, for example, the first powder is CsX, the second powder is SnX 2  or SnX′ 2 , and the third powder is PbX 2  or PbX′ 2 , where X and X′ are different halogens, by mixing the first, second, third powders with the solvent, a Cs(Sn y Pb 1-y )X 3-Z X′ z  solution is formed, where 0&lt;y&lt;1 and 0&lt;z&lt;2. No matter forming the CsSnXX′ 2  solution, the Cs(Sn y Pb 1-y )X 3  solution, or the Cs(Sn y Pb 1-y )X 3-Z X′ z  solution, the main difference is only that the wavelength of the light emitted by the prepared light-emitting layer in the perovskite structure is different, though all in the near-infrared region. The user can adjust the wavelength by changing the quantity of the added lead. 
     After the step S 1  and forming the CsSnXX′ 2  solution (in the following, CsSnI 3  solution is used as an example) and before coating on the hole transport layer, the CsSnI 3  solution can be diluted first and reducing its concentration to 15˜70 mg/ml. Then spin coating (2500˜3500 rpm), blade coating, or drop casting can be used for coating the CsSnI 3  solution on the hole transport layer uniformly. Next, drying methods such as draining or baking can be used to move the solvent away and solidify the CsSnI 3  solution, which crystallize to form CsSnI 3  in the perovskite structure, giving the light-emitting layer of the present invention. Please refer to  FIG. 3 , which shows a schematic diagram of the cubic perovskite structure according to a preferred embodiment of the present invention. The elements A, B, and C represent Cs, Sn, and I, respectively. 
     Please further refer to  FIGS. 4A to 4C , which show microscopic diagrams of coating the CsSnI 3  solution on the hole transport layer, where the prepared CsSnI 3  solution are diluted to 16.6 mg/ml, 33 mg/ml, and 66 mg/ml before they are coated using drop casting. It is seen that the thickness of the light-emitting layer is influence by the concentration of the CsSnI 3  solution after draining. 
     After forming the CsSnI 3  in the perovskite structure as the light-emitting layer, the 2-(4-biphenylyl)-5-(4-tert-butylphenyl)1,3,4-oxadiazole (PBD) solution is spin coated on the light-emitting layer uniformly and acting as the electron transport layer. 
     Finally, lithium fluoride and aluminum are deposited on the electron transport layer using vapor deposition and used as the electrode layer, which acts as the cathode of the near-infrared LED according to the present invention. 
     By forming the stack of films following the above steps, the near-infrared LED according to the present invention is produced. The perovskite CsSnI 3  or other chemical composition described above is used as the light-emitting layer. Depending on the ratio of the added lead, the recombination of electron-hole pair occurring in the layer can emit near infrared with a wavelength approximately between 701 nm and 950 nm. The near-infrared LED prepared according to the method can be further applied to night vision, optical communication, and biomedical domains. To sum up, the present invention discloses a near-infrared LED and the method for manufacturing the same. Owing to it low-cost nature and ease in controlling the conditions, the present invention undoubtedly provides a near-infrared LED and the method for manufacturing the same with economical and practical values. 
     Accordingly, the present invention conforms to the legal requirements owing to its novelty, nonobviousness, and utility. However, the foregoing description is only embodiments of the present invention, not used to limit the scope and range of the present invention. Those equivalent changes or modifications made according to the shape, structure, feature, or spirit described in the claims of the present invention are included in the appended claims of the present invention.