Patent Application: US-201313756730-A

Abstract:
a method of forming a czt thin film from a quaternary target involves sputtering a quaternary target onto a substrate , wherein the quaternary target comprises copper , zinc , tin , and selenium and / or sulfur , wherein each component through is present in the quaternary target within ± 50 % of a 2 : 1 : 1 : 4 molar ratio , respectively , thereby forming a czt thin film on the substrate , wherein the czt thin film has a kesterite crystalline phase and a band gap of about 1 . 0 to 1 . 5 ev . in an embodiment , a ternary target is employed .

Description:
before describing the present invention in detail , it is to be understood that the terminology used in the specification is for the purpose of describing particular embodiments , and is not necessarily intended to be limiting . although many methods , structures and materials similar , modified , or equivalent to those described herein can be used in the practice of the present invention without undue experimentation , the preferred methods , structures and materials are described herein . in describing and claiming the present invention , the following terminology will be used in accordance with the definitions set out below . as used herein , the term “ czt ( s , se )” refers to the copper - zinc - tin - chalcogenide materials cu 2 znsns 4 and cu 2 znsnse 4 , including combinations thereof , and including materials wherein the elements are present within ± 50 % of the nominal 2 : 1 : 1 : 4 molar ratio . as used in this specification and the appended claims , the singular forms “ a ”, “ an ,” and “ the ” do not preclude plural referents , unless the content clearly dictates otherwise . as used herein , the term “ and / or ” includes any and all combinations of one or more of the associated listed items . as used herein , the term “ about ” when used in conjunction with a stated numerical value or range denotes somewhat more or somewhat less than the stated value or range , to within a range of ± 20 % of that stated . disclosed herein is technique for fabricating a sputtering target with a desired stoichiometry and subsequently forming a thin czt ( s , se ) film through single - step quaternary sputtering for use in a photovoltaic device . this approach is an improvement over other vacuum deposition techniques . first , pre - synthesis of czt ( s , se ) enables precise stoichiometric control and target homogeneity . second , films sputtered from the pre - synthesized target can be produced with a preferred stoichiometry ( by adjusting the stoichiometry of the sputtering target ) without the need for post - processing treatments ( annealing and sulfurization / selenization ), a significant advantage over other deposition techniques . the resulting thin film has a composition that is more reproducible than that formed by prior art methods . quaternary sputtering is a simpler and more economical deposition technique than existing vacuum deposition methods , such as multi - target sputtering and thermal evaporation . typically , czt ( s , se ) with desired stoichiometry will be prepared by heating in elements in the desired proportions an ampule , then grinding the czt ( s , se ) into a powder . the czt ( s , se ) powder may be hot pressed into a puck , then machined into the desired final target dimensions suitable for deposition by sputtering , for example rf sputtering , dc sputtering , or pulsed dc sputtering . in forming the target , it is possible to employ elemental cu , zn , sn , and se / s , and / or compounds thereof such as cu 2 s , zns , sns 2 , or their selenide analogues . a combination of these can be added to a quartz ampoule and reacted to form czt ( s , se ). it is also possible to reduce the number of steps required to fabricate the target by forming it directly during the initial czt ( s , se ) formation by using an appropriately - shaped ampoule , negating the need for grinding and hot pressing . non - stoichiometric quantities of each element can be added to create an off - stoichiometric czt ( s , se ) compound , i . e . copper - poor or tin - rich compounds . in one embodiment , the target is formed that is relatively rich in zn , sn , and / or ( s , se ) in order to account for relative loss of such elements during processing , thus achieving a czt ( s , se ) thin film having the desired stoichiometry . instead of a single target , multiple ternary or quaternary targets , preferably with differing stoichiometries can be used . use of multiple targets of differing stoichiometry is expected provide varied stoichiometry throughout the active layer . elemental copper , zinc , tin and selenium were combined in a 2 : 1 : 1 : 4 molar ratio and sealed inside a quartz ampoule . the ampoule was then heated gradually to 750 ° c . and held at temperature for 30 hours to form cztse . the ampoule was then broken and the cztse is ground to a fine powder using a mortar and pestle . the powder was then placed in a hot press for one hour at 650 ° c . with a 10 - ton ram force to form a compressed target that was subsequently machined to 3 ″ diameter by ⅛ ″ thick as seen in fig1 . a ruler measured in inches ( 2 . 54 cm ) is also shown for scale ). the target was indium - bonded to a copper backing plate prior to installation in a sputter deposition system . to form films , deposition was carried out onto a molybdenum - coated soda lime glass substrate , where the molybdenum served as the bottom electrode ( anode ) of the photovoltaic device . cztse was deposited onto the substrate using rf magnetron sputtering in a sputter - up geometry in an ar atmosphere at a pressure of 1 - 5 mt with an energy density on the order of 0 . 5 - 2 w / cm2 with the substrate held at ˜ 500 ° c . the nominal thickness of the deposited cztse layer was 500 - 2000 nm . the substrate was rotated at ˜ 10 rpm during deposition . after cztse deposition , cds , a transparent conducting oxide of zno / aluminum - doped zno , and nickel / aluminum grids were deposited to finish the device in accordance with standard industry practice . fig2 and 3 show top - down and cross section electron microscope images , respectively , of the resulting cztse film . all documents mentioned herein are hereby incorporated by reference for the purpose of disclosing and describing the particular materials and methodologies for which the document was cited . although the present invention has been described in connection with preferred embodiments thereof , it will be appreciated by those skilled in the art that additions , deletions , modifications , and substitutions not specifically described may be made without departing from the spirit and scope of the invention . terminology used herein should not be construed as being “ means - plus - function ” language unless the term “ means ” is expressly used in association therewith . d . a . r . barkhouse , o . gunawan , t . gokmen , t . k . todorov , and d . b . mitzi , “ device characteristics of a 10 . 1 % hydrazine - processed cu 2 znsn ( se , s ) 4 solar cell ,” prog . photovolt : res . appl . i . repins , c . beall , n . vora , c . dehart , d . kuciauskas , p . dippo , b . to , j . mann , w .- c . hsu , a . goodrich , and r . noufi , “ co - 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