Copper Zinc Tin Selenide (CZTSe) thin film solar cell is the most promising source of energy production of the future. It is free from rare and toxic elements which are used in other thin films solar cells like Cadmium Telluride (CdTe) and Copper Indium Gallium Selenide (CIGS). CZTSe is basically a replacement for high performing CIGS thin film solar cells. Indium and Gallium are rare in Earth’s crust which is the major factor of the price increase. However, replacing Indium and Gallium with Zinc and Tin will give the benefit of huge cost reduction because both the elements are present in abundance on Earth.
CZTSe thin film solar cells are relatively new born as compared to CIGS thin film solar cells. CZTSe Research was started on these solar cells in late 1990s, almost 35 to 40 years later than CIGS research. But the research in this area is growing and many photovoltaic research groups have taken interest in development of this low cost alternative of permanent energy generation source.
There are two major issues making serious hurdles in the progress of CZTSe thin film solar cells, which are a) tin loss during annealing and b) poor adhesion at the interface of absorber layer and the molybdenum. In this study, these two issues has been addressed by using binary selenides precursor sources and annealing in close space sublimation equipment and thermal evaporator, both having selenium atmosphere.
In the beginning, six types of stacking orders were prepared to have check which stacking order is more suitable to address the issue mentioned above with good morphology and grain growth. These six stacking orders were prepared with reference to the tin selenide on the soda lime glass substrate. In the first stacking order (A), tin selenide was kept at the top and followed by copper selenide and zinc selenide. Second stacking order (B) also had tin selenide at the top but followed by zinc selenide and copper selenide. Third stacking order (C) had zinc selenide at the top followed by tin selenide and copper selenide. Fourth stacking order (D) had copper selenide at the top followed by tin selenide and zinc selenide. In third and fourth stacking both tin selenide is at the center of other two binary selenides. Fifth stacking order (E) has copper selenide at the top followed by zinc selenide and then tin selenide. Sixth stacking order (F) has zinc selenide at the top followed by copper selenide and tin selenide. In fifth and sixth stacking order, tin selenide is at the bottom.
All six of these stackings were annealed in close space sublimation equipment individually, at 250oC, 300oC, 350oC, 400oC, 450oC, 500oC and 550oC for three minutes in selenium atmosphere. Stacking A, B and C were not stable under annealing conditions, and evaporated from the glass. Stacking D, E and F were studied in detail. In the first three stackings (A, B and C) tin selenide was directly or indirectly above the copper selenide layer. Through this, it was cleared that zinc selenide layer do not act as a protector layer. Copper selenide layer act as a protector layer. Reaction of CZTSe proceeds in a manner that first copper selenide reacts with tin selenide to form copper tin selenide and then this copper tin selenide reacts with the zinc selenide to from copper zinc tin selenide.
After studying the morphology, atomic concentration, xrd patterns, raman spectra and auger depth profile, stacking E and F were eliminated from the further research. Stacking E did not form the film; instead it formed isolated copper selenide grains. Stacking F grains appeared separated and pores were also visible. Stacking D was selected for further studies. Copper selenide sputtering target was burned, so it was deposited through thermal evaporation using copper selenide powder. Copper metal was also used in the stacking order D to compare the effect of copper metal and copper selenide on grain growth and morphology and selenization was performed in selenium evaporator. Molybdenum coated soda lime glass was used to conduct deeper studies.
Stacking order D, with copper and copper selenide both, was annealed in selenium evaporator for 400oC, 450oC and 500oC for 5 and 10 minutes. Study of sem, xrd, raman, and auger depth profile suggested that the use of copper metal instead of copper selenide gave better results in terms of crystallnity, phase stability and morphology specially at 500oC for 5 minutes. Secondary phase generation is very big issue when it comes to CZTSe, but with copper metal, secondary phase generation is also quite low.
Copper Zinc Tin Selenide (CZTSe)박막태양전지는미래의유망한에너지원이다. CZTSe물질은Cadmium Telluride (CdTe)에서쓰이는 Cd같이독성인원소도사용하지않고, Copper Indium Gallium Selenide (CIGS)에서의 In 과Ga처럼희귀한원소를사용하지않는다. 또한CZTSe는높은효율을기록하고있는 CIGS 박막태양전지를대체할수있다고여겨지고있다. 최근지표면의희귀금속인 In 과Ga지구상에풍부한 Zn 와Sn으로대체한다면가격절감에도매우유리할것이다.
CZTSe박막태양전지는 CIGS 박막태양전지와비교하여연구가잘이루어지지않았다. CZTSe연구는 CIGS 에비하여 35~40년정도늦은 1990년대후반부터시작되었다. 그러나이영역에서의연구는저비용과영구적인에너지원에대한관심이커지면서점점활발해졌고, 많은연구그룹들이참여하게되었다.
CZTSe박막태양전지연구에는a) 열처리중의Sn loss 문제그리고 b) Mo 후면전극과CZTSe흡수층계면에서의 adhesion 문제두가지쟁점이있다.이연구에서이러한두쟁점을 binary selenide전구체를이용한 close space sublimation과 thermal evaporation 의selenization을통하여해결하고자하였다.
먼저, 좋은 morphology 와대결정립을가진박막에대해연구하기위해 Cu2Se와ZnSe, SnSe2의 binary selenide타겟을이용해스퍼터하여소다라임유리위에이렇게준비된세개층의박막이쌓아져있게전구체를제조하였다. 기본적으로소다라임유리기판에 SnSe2박막의위치를기준(최상위, 중앙, 최하위)으로Cu2Se와ZnSe박막의 stacking order를달리하여 6가지전구체를준비하였다.첫 stacking order는 (A) tin selenide를최상위에위치하고그밑에 copper selenide와 zinc selenide박막의순서로된구조이고, (B) tin selenide박막이역시최상위에위치하고그밑에 zinc selenide와 copper selenide박막의순서로된구조, (C) tin selenide박막이중앙에위치하고그위에 zinc selenide박막이있고최하위층에는 copper selenide박막이있는구조, (D) tin selenide박막이중앙에위치하고그위에copper selenide박막이있고최하위층에는 zinc selenide박막이있는구조, (E) tin selenide박막이최하위층에있고최상위층에는 copper selenide박막이있고중앙에는 zinc selenide박막이있는구조, (F) tin selenide박막이최하위층에있고최상위층에는 zinc selenide박막이박막이있고중앙에는 copper selenide박막이있는구조이다.
위에서와같은구조로되어있는전구체는모두 close space sublimation을이용하여selenization을하여CZTSe박막을제조하였다. Selenium 분위기에서 3분동안기판온도를 250℃와 300℃, 350℃, 400℃, 450℃, 500℃, 550℃ 로 변화시켜 가며 실험을 하였다. Stacking A, B, C 구조로준비된전구체는selenization처리동안유리기판에서모두기화되어없어지는현상이일어나전구체구조로써적합하지않았다. Stacking D, E, F 구조로준비된전구체에대한selenization실험은뒤에서자세히언급할것이다. 우선첫 3개의구조 (Stacking A, B, C)는직접적또는간접적으로 copper selenide층위에 tin selenide층이있는구조이었다. 이것을통해서 zinc selenide층이selenization시CZTSe로의반응이일어날때, 전구체가기화되지않게막는보호막으로서역할을못한다는것을발견하였다. 반면 Copper selenide층은보호막으로서역할을하였다. 그래서CZTSe박막제조는첫번째로 copper selenide가 tin selenide와반응하여 copper tin selenide가형성되고그다음으로 copper tin selenide가 zinc selenide와반응을이용하여준비되었다.
앞에서기술한 Morphology에대한연구와 atomic concentration, xrd patterns, raman spectra, auger depth profile을통해 stacking E 와 F 보다 D의구조가CZTSe흡수층을제조하기에적합하다고판단되었다. Stacking E는selenization후잔류하는 copper selenide상이표면에존재하였고, Stacking F로selenization된박막은표면이불규칙하게형성되었고, pore도또한존재하였다. 이렇게선택된 stacking D는소다라임유리위에 Mo이코팅된기판을이용하여전구체를제조하였다. 이번실험에서Cu2Se 타겟의손상으로인해 Cu2Se 분말을이용한 thermal evaporator 로 Cu2Se 층을증착하였으며, CZTSe박막형성에서반응성을비교하기위해 Cu2Se 층대신 Cu 타겟을이용하여 stacking D와비슷한 Cu/SnSe2/ZnSe/Mo구조로전구체를제조하여CZTSe박막의결정립성장과 morphology 특성을연구하였다. 또한앞서selenization에쓰인 close space sublimation 대신 thermal evaporator 를이용하여기판온도와 selenium 의공급을정교하게조절하고자하였다.
Cu2Se or Cu/SnSe2/ZnSe/Mo 구조로준비된전구체는기판온도 400℃와 450℃, 500℃ 에서 각각 5분 그리고 10분 동안 selenization 되었다. sem, xrd, raman, and auger depth profile 의분석을통해 copper selenide대신 copper metal이결정성과 morphology,CZTSe상의안정성에서좋은특성을가졌다. 이차상은CZTSe에서큰쟁점이지만 copper metal을이용하여만들어진CZTSe박막은이에대한문제를개선할수있었다.