Abstract:
This invention discloses a solid-based synthesis of cesium tin tri-iodide (CsSnI 3 ). More specifically, the CsSnI 3  is fabricated in a 3 zone high temperature resisting tube by the solid-phase sintering method. CsSnI 3  are ideally suited for a wide range of applications such as light emitting and photovoltaic devices.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention generally relates to photovoltaic technologies and, more specifically, to a method of synthesizing CsSnI 3 , especially in large scale. 
     2. Description of the Prior Art 
     The current photovoltaic technologies can be classified by the ten different materials used for the light absorption in a solar cell. These materials include amorphous and polycrystalline silicon, CdTe, CuIn,Ga 1-x Se 2  (CIGS), GaAs, and photosensitive organic dyes. A transformative technology may emerge when a new and better material is discovered for photovoltaic applications. 
     CsSnI 3  is a unique phase-change material that exhibits four polymorphs. [1, 2] The black polymorph of CsSnI 3  could be obtained through a phase transition from the yellow polymorph CsSnI 3  by increasing its temperature above 425 K. It was further demonstrated by differential thermal analysis and X-ray diffraction that during the cooling of the black CsSnI 3  from 450 K, its ideal cubic perovskite structure (B-α) deformed to a tetragonal structure (B-β) at 426 K, and became an orthorhombic structure (B-γ) below 351 K. [3] The CsSnI 3  is unique in combining two generally contra-indicated properties, strong photoluminescence (PL) and high electrical conductivity. [4, 5] 
     A need still exists in the industry for developing synthesis methods for CsSnI 3 , especially in large scale. The successful implementation of these materials for various applications requires a detailed understanding of both their processing and material properties. 
     BRIEF SUMMARY OF THE INVENTION 
     This invention is directed to the synthesis of cesium tin tri-iodide (CsSnI 3 ) by a solid-phase sintering method. 
     According to one embodiment of the invention the process of forming CsSnI 3  in a high temperature healing unit, comprises steps of: 
     providing a 3 zone high temperature resistant heating unit; 
     providing CsI powder; 
     providing SnI 2  powder; 
     mixing CsI and SnI 2  powders; 
     vacuum pumping to create a vacuum and sealing the mixed powder in a side or end zone of the heating unit; and 
     heating the end zone containing the mixed powder in the heating unit to form CsSnI 3 . 
     Yet, another embodiment of this invention is directed to synthesizing CsSnI 3  by any one of the processes disclosed above. 
     In the process, the CsI and SnI 2  powders, which are preferably milled, are mixed in a protective environment of N 2  gas; the molar ratio of the SnI 2  and CsI in the mixture is from about 1/1 to 5/1. 
     The length of the 3 zone heating unit depends on the dimension of the heating chamber of the furnace. The material forming the 3 zone heating unit can be quartz, Pyrex, ceramics or metals. 
     The degree of vacuum in the 3 zone heating unit is from about 10 −5  to 10 −2  Pa. 
    
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE FIGURES 
       While the specification concludes with claims particularly pointing out and distinctly claiming that which is regarded as the present invention, advantages of this invention may be more readily ascertained from the following description of the invention when read in conjunction with the accompanying drawings in which: 
         FIG. 1  shows the schematic diagram of synthesizing CsSnI 3  in accordance with the invention; 
         FIG. 2(   a ) shows a 3 zone heating unit in the form of a quartz tube filled with CsI and SnI 2  powders before sintering and, in  FIG. 2(   b ), after sintering; 
         FIG. 3(   a ) shows the X-ray diffraction data (XRD) profile taken from CsSnI 3  (synthesized by Csl and SnI 2  powder) and  FIG. 3(   b ) shows the standard XRD pdf card (43-1162) of black-δ phase of CsSnI 3 ; and 
         FIG. 4  shows the photoluminescence (PL) characteristics of CsSnI 3  powder. 
     
    
    
     DETAILED DESCRIPTION 
     The CsSnI 3  exhibits outstanding optical, electrical, and ferro electric properties. These features make CsSnI 3  ideally suited for a wide range of applications such as light emitting and photovoltaic devices. 
     More specifically, CsSnI 3  is a promising material in the application of solar cells, since CsSnI 3  was found to possess a direct band gap of 1.32 eV at room temperature, right in the narrow region of optimal band gaps for the Shockley-Queisser maximum efficiency limit of a solar cell. 
     An effective method of synthesizing large domain size high quality Perovskite semiconductor are disclosed in the present invention. 
     More specifically, a solid-phase sintering method to synthesize CsSnI 3  is disclosed in the present invention. The CsSnI 3  can be fabricated in a 3 zone heating unit, such as a high temperature resistant tube, by the solid-phase sintering method as shown in  FIG. 1 . This synthesis method of the CsSnI 3  further enhances the likelihood of using CsSnI 3  as a new absorption material for solar cells. 
     EXAMPLES 
     The procedures of synthesizing polycrystalline CsSnI 3  using reaction raw materials include mixing and filling the raw materials in a 3 zone heating unit such as a high temperature resistant tube under a protective environment of N 2  gas under ambient conditions. 
     The ambient conditions refer to: room temperature or temperature of 298.15 K (or 25 ° C., 77 F.); and an absolute pressure of 100 kPa (or 14.504 psi, 0.986 atm). 
     I. Example 1 
     Preparation of CsI and SnI 2  Mixed Powder 
     13 gram of CsI (99.999% purity) powder was placed in an agate mortar and milled for 1 hour. 19 grams of SnI 2  (99% purity) powder was placed in an agate mortar and milled for 1 hour. The milled CsI and SnI 2  powders were then placed in a container to mix and stir the powder for 30 minutes to insure homogeneity of the mixture. 
     The molar ratio of the SnI 2  and CsI in the mixture is from about 1/1 to 5/1. 
     II. Example 2 
     Vacuum Pumping and Sealing Mixed Powder in a Side Zone of the Tube 
     4 gram of mixed powder was placed at one end or at one end zone of a heating unit, shown by way of example to be a 3 zone high temperature resistant tube as shown in  FIG. 2(   a ). Then the tube was pumped to create a vacuum by using a conventional vacuum system, and the tube was sealed by using CO gas fire equipment. The length of the 3 zone tube depends on the dimension or length of the heating chamber of the furnace. The material of the 3 zone tube can be formed of quartz, Pyrex, ceramic or metal. The degree of vacuum of the 3 zone tube is from about 10 -5  to 10 -2  Pa. 
     III. Example 3 
     Sintering to Form CsSnI 3    
     The sealed tube was placed in a tube furnace. The temperature of the reaction zone must be maintained above 450° C., the temperature of the middle zone may be maintained between 320 and 450° C., and the temperature of the last zone or other end is preferably maintained under 320° C. as shown in  FIG. 1 , a temperature gradient being maintained from one to the other of the opposing zones. 
     The mixed powder reacted to form black CsSnI 3  in the reaction zone when the temperature increased above 450° C., and a small amount of SnI 2  vapor was transported to the last or other end zone (right end as viewed in  FIG. 1 ) of the tube and re-crystallized as shown in  FIG. 2(   b ). 
     The chemical reaction for the mixed powder could be described as the following: 
     CsI+SnI 2 →CsSnI 3    
     The reaction was verified by identifying the end products of CsSnI 3  using the X-ray diffraction (XRD) data. XRD data was used not only to further verify the crystal structure of CsSnI 3  but importantly also to identify the side products of this reaction (as shown in the reaction).  FIG. 3(   a ) showed the XRD data profile taken from CsSnI 3  (synthesized by CsI and SnI 2  powder).  FIG. 3(   b ) showed the standard XRD pdf card (43-1162) of black-δ phase of CsSnI 3 . All the measured peaks were well matched to the black-δ phase of CsSnI 3 . 
     Photoluminescence characteristics or properties of CsSnI 3  powder is shown in  FIG. 4 . It can be seen that the characteristic peak at 950 nm belongs to the black-δ phase of CsSnI 3 . 
     In summary, CsSnI 3  in powder or bulk form was synthesized using CsI and SnI 2  powders by using a solid-phase sintering method. 
     A solid-phase sintering method was employed to fabricate CsSnI 3  in powder or bulk form, especially suitable for mass production of CsSnI 3  for solar cell applications. The polycrystalline quality of the resulting powder or bulk form was confirmed by being characterized by the XRD data. 
     The method in accordance with the invention assures efficient production of CsSnI 3  in powder or bulk form without the impurities associated with the prior art methods. Also, the method allows recycling of unused SnI 2  powder further decreasing the cost of production. 
     While the invention has been described in detail and with reference to specific examples and the embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.