In-situ 복합 물리 기상 증착법을 활용한 kesterite Cu2ZnSnS4 적층 박막 형성

Abstract

현재 전 세계적으로 에너지원에 대한 수요는 점차 증가하고 있으며, 고도화된 산업화 및 도시화 과정을 걸친 선진국을 비롯하여, 비약적인 속도로 발전하고 있는 개발도상국에게도 안정적인 에너지 공급 및 사용이 중요한 이슈 및 국가 생존 전략과 연관되어 있다. 산업혁명 이래로 인류의 화석 연료 사용에 따른 부작용으로 인하여, 각종 환경오염 발생, 매장량 감소 및 고갈에 대한 두려움, 에너지원을 둘러싼 국지적인 분쟁 등의 심각한 문제가 나타나고 있다. 따라서 이러한 각종 문제점을 극복하고, 친환경적이면서 보다 안정적인 공급이 가능한 지속 및 재생 가능한 새로운 에너지원에 대한 관심 및 연구개발 활동이 활발해지고 있는 추세이다. 태양광 발전은 전도유망한 신재생에너지 중 하나로서 태양전지 소자를 통하여 태양광 에너지를 전기에너지로 변환하여 사용할 수 있어 주목을 받고 있다. 청정, 무한한 에너지원을 사용하고, 소자 수명이 길며, 유지 보수가 용이하다는 장점을 가지고 있기 때문이다. 전통적으로 실리콘 재료를 기반으로 한 실리콘 태양전지는 저가, 고효율 특성을 가짐으로써 상용화되었으나 기존의 화석에너지를 기반으로 한 화력발전 및 원자력 발전에 비하여 경제성 우위를 가지지 못한 상황이다. 실리콘 태양전지는 웨이퍼 제조 공정이 복잡하여 이 과정 속에서 많은 에너지 투입이 필요하다. 게다가 실리콘을 국내에서 조달할 수 있고, 해당 업계에 유리한 규제 환경이 유지되며, 저렴한 노동력을 쉽게 얻을 수 있는 중국 위주로 시장이 재편되어 있는 실정이다. 실리콘 원자재 생산 설비에 대한 과잉 투자, 장기적인 경기 불황으로 인한 시장 위축, 태양광 모듈 공급량과 설치량 차이에 따른 모듈 가격의 하락 등으로 태양전지 시장이 고전을 하고 있다. 이에 대응하여 보다 저가의 셀과 모듈을 제조할 수 있는 기술 및 연구개발 활동이 필요하게 되어 태양광 시장의 주류를 차지하고 있는 실리콘 태양전지를 대체할 수 있는 차세대 태양전지 개발이 필요하게 되었다. 이러한 연구, 개발 필요성에 따라 각국에서 해당 분야에 대한 연구 및 투자 지원 활동을 활발하게 진행하고 있다. 박막 태양전지는 실리콘 태양전지를 대체할 수 있는 후보군으로서 각광을 받아오고 있으며, 대량 생산 및 저가 모듈제작이 가능하다는 장점을 가지고 있다. 박막 태양전지의 대표군 으로는 CdTe(카드뮴 텔룰라이드), CIGS(Se), (구리, 인듐, 갈륨, 황 또는 셀레늄) 화합물 반도체 태양전지가 있다. 높은 광흡수율을 바탕으로 고효율을 달성할 수 있으며, 생산단가가 저렴하다. 그러나 CdTe 태양전지는 각국의 유해 환경 물질 사용 제한에 대한 규제 움직임으로 인하여, 인체와 환경에 유해한 Cd(카드뮴) 원소 사용 소자에 대한 논란이 확산되고 있다. CIGS 박막 태양전지는 적은 원료 사용, 공정 단계에서 상대적으로 제조 가격이 적게 들고, 약 20%의 높은 효율을 달성할 수 있다는 장점을 가진다. 그러나 희소금속인 In(인듐), Ga(갈륨)을 사용함에 따라 잠재적으로 가격이 상승할 수 있다는 단점을 가지고 있으며, 특히 인듐은 터치스크린 및 디스플레이 시장의 팽창으로 인하여 그 수요가 점차 늘어나고 있고, 중국 당국의 자원 무기화 정책 추진으로 인하여 가격 상승에 대한 부담이 큰 상황이다. 따라서 CIGS 박막 태양전지에 비하여 보다 저렴한 원소를 사용할 수 있는 태양전지 개발이 필요하게 되었다. CZTS (Cu₂ZnSnS₄, 구리, 아연, 주석, 황) 4성분계 화합물 태양전지는 CIGS 태양전지를 대체할 수 있을 것으로 주목을 받고 있다. 값싼 원료를 이용하고 환경 친화적이면서 ~1.5 eV 에너지 대역을 가지는 직접 천이형 반도체로서 CIGS 박막 태양전지의 제조 공정을 동일하게 사용하여 제조할 수 있다는 장점을 가진다. 그러나 CZTS 태양전지의 상업화를 위해서는 광변환 효율 향상 및 대면적화 실현이 기술적으로 난제로 손꼽히고 있다. CIGS 태양전지는 최고 20%에 달하는 높은 효율을 달성하였지만, CZTS 태양전지는 불과 11% 남짓한 효율이 현재까지 기록한 세계 최고 변환 효율이기 때문이다. CZTS 태양전지의 효율을 높이기 위해서 태양빛을 흡수하는 역할을 하는 핵심적인 역할을 하는 CZTS p-type 광흡수층 부분에 대한 연구가 요구된다. 따라서 본 연구에서는 물리적 기상 증착법으로 전구체를 형성한 후, 이를 고온로에서 열처리 하는 2-step, 복합공정을 이용하여 CZTS 태양전지 광흡수층 제작에 대한 연구를 진행하였다. 소다라임 유리 (Soda-lime glass, SLG)위에 열증착법 및 스퍼터링법이 결합된 물리적 기상증착법을 이용하여 Sn, Cu, ZnS 층을 각각 stack 구조로 증착한 후, 이 전구체를 고온에서 N₂ + H₂S (5%) 황화수소 혼합가스 분위기에서 열처리를 하여 CZTS 광흡수층을 제작하였다. 이러한 2-step 합성법의 장점은 증착조건에 따라 박막의 재현성, 화학적 조성, 두께를 제어할 수 있다는 장점을 가진다. 따라서 최종적으로 완성한 박막의 화학양론, 투과도, 에너지 밴드갭 등을 선택적으로 제어하여 박막형 태양전지 광흡수층에 적합하도록 설계할 수 있다. 본 실험은 진공법으로 Cu, ZnS, Sn 및 Sn, Cu, ZnS의 두 가지 구조의 스택 전구체를 합성한 후, 황 원소가 CZTS의 박막 성장에 미치는 영향을 조사하기 위해 황 원소 유/무에 따른 열처리 공정을 실시하였다. 우선적으로, 질소 단일 분위기에서 공정온도를 500℃ 및 550℃, 두 가지 조건으로 열처리를 한 후, 그 구조적 특성을 조사하였다. 황 원소 영향을 조사하기 위하여, 질소 분위기에서 첫 번째 열처리를 실시한 후, 연속적으로 N₂ + H₂S (5%) 황화수소 혼합가스 기체를 흘려주어 550℃고온에서 sulfurizaiton을 하여 그 특성을 분석하였다. 합성한 CZTS 박막을 평가하기 위하여 surface profiler로 두께 측정을, XRD로는 구조적 특성을, Raman 분광법은 XRD 측정으로부터 peak이 서로 겹치거나 구분하기 곤란한 제 2상을 명확하게 구별하기 위하여 사용되었다. 박막의 morphology는 FE-SEM으로 분석되었고, 화학적 조성은 EDS를 이용하였으며, 광학적, 전기적 특성은 UV-visible-nir 및 Hall measurement system으로 각각 확인되었다. 증착된 2가지 stack 구조의 precursors를 연속적 N₂ 어닐링, Sulfurization을 수행한 실험에서 N₂ 어닐링을 처리하였을 때, Cu_xSn_yS_z와 같은 제 2상의 성장이 두드러졌으며, N₂ +H₂S(5%) 혼합가스 분위기를 처리하였을 때에 CZTS 박막 성장을 구조적 분석으로 확인할 수 있었다. 박막은 Cu-rich, Zn-poor 특성을 보여주는 데, 열처리과정에서 가스 분위기 및 온도가 아연 및 황 원소 증발에 영향을 미친 것으로 생각되며, 그 결과로 1.25 ~ 1.29 eV의 에너지 밴드 갭을, 2.01 x 10^(18) ~ 3.47 x 10^(19) /cm³의 carrier concentration을 가짐을 보여주었다. 후속실험은, 단일 스택 박막, Sn/Cu/ZnS precursor 증착단계에서 Cu 두께 조절을 통하여 2-step 방법으로 CZTS 광흡수층을 합성하여, Cu 조성비 영향에 대해 연구하였다. 그 결과, CZTS 4성분계 화합물의 화학양론인 Cu: Zn: Sn: S = 2: 1: 1: 4에 근접한 조성비를 얻을 수 있었고 Kestrite 단일 상을 가진 CZTS 박막의 합성을 XRD 분석을 통하여 확인할 수 있었다. Cu 조성 증가는 광투과도 감소에 따른 에너지 밴드 갭 감소, 전기적 특성 변화 (carrier concentration 및 mobility 증가, 비저항 감소)를 보여준다. 본 연구로 합성된 p-type CZTS 광흡수층 박막은 저가 범용 박막 태양전지 셀 제조의 핵심적인 재료로 사용되어 광변환 효율을 개선하는 데 도움이 될 것으로 기대한다.| Crystalline silicon (c-Si) have maintained a dominant world market share of solar cell owing to cheap module price and high efficiency. However, it have a difficult time because of overinvestment to production facilities for silicon raw material, a lot of energy consumption during the production process, global economic crisis and loss of price competitiveness. Thin film photovoltaics (TFPV) including CdTe (Cadmium telluride), CIGS (Copper, Indium, gallium and selenium) solar cell are expected to substitute for crystalline silicon (c-Si) solar cell. They take advantage of the developed technologies in order to reduce manufacturing cost per surface area than that of c-Si base solar cell/module. CdTe thin film solar cell have reached a ~16.7% cell efficiency by NREL (National Renewable Energy Laboratory). But it use a toxic material, Cd which could influence human body and environment. CIGS thin solar cell have 20.8% cell efficiency by ZSW(The Baden-Württemberg Center for Solar Energy and Hydrogen Research) and received a lot of attention its potential as a candidate for next generation solar cell. Nevertheless, it might have trouble in the production process because In price have been increased due to the rising its demand, in particular, display and touch screen industries. So, CdTe and CIGS PV need to overcome using harmful element and potential scarcity of constituent, respectively. CZTS (Copper, Zinc, Tin and Sulfur) thin film solar cell have been vigorously investigated using abundance and availability of materials, Cu, Zn, Sn and S. It could substitute In, Ga to Zn, Sn, respectively, and it is equally applied the CIGS fabrication process. CZTS solar cell have moderate prices, non-toxic and possesses a direct band gap near 1.5eV. However, it have recorded only 11% cell efficiency which was conducted by IBM and this record value was a long way from the efficiencies of other thin film photovoltaics. Therefore, the researches to CZTS absorber layer that could perform a role of optical absorption layer are requires to improve CZTS solar cell efficiency. In this study, we could fabricate CZTS thin films using 2-step process, precursor deposition and annealing treatment. At first experiment, we examined sulfur gas inclusion or not during annealing process. So, we applied two types of heat treatment N₂ ambient annealing and sequential N₂ annealing and sulfurization. Next, we investigated Cu element effect inner CZTS thin layer. Thus, Stack precursor was deposited by varying Cu metal layer thickness to synthesis precursor and then sulfurized at 550℃ for 1hour. First, consecutive N₂ annealing and sulfurization was conducted to Cu/ZnS/Sn and Sn/Cu/ZnS two stack precursors. They were deposited by physical vapor deposition methods, evaporation and sputtering. Sn, Cu and ZnS layer were deposited by thermal evaporator, DC-magnetron sputtering and RF-magnetron sputtering, respectively, on soda lime glass substrate. The thickness of metal or metallic sulfide was fixed to 300nm, equally. In N₂ annealing treatment, the precursors sample was heated at 500 or 550℃ for 1hour, respectively. Sequential annealing process, the samples preferentially annealed with N₂ambient at 500 or 550℃ for 1hour and then they were immediately sulfurized at 550℃ with N₂+ H₂S (5%) mixed gas atmosphere for 1hour. The crystal properties were analyzed by X-ray diffraction (XRD) regarding 4 kinds of CZTS thin films(Precursor: Cu/ZnS/Sn and Sn/Cu/Zn, Two-types annealing process: one is N₂ annealing at 500℃ or 550℃ and the other is a consecutive N₂ and sulfurization). The films were identified kesterite CZTS and/or CTS (Cu_xSn_yS_z) dominant phase but there were existed a kind of secondary phase, for example, copper sulfide, copper tin alloy and so forth. Raman spectroscopy was utilized to distinguish overlapped or uncertain phases from XRD analysis. At the results, thin films show the co-existence of CZTS and other secondary phases and the higher N₂ annealing treated, the more secondary phases were occurred even though they were finished sulfurization. From EDS results, the films show Cu-rich and Zn-poor condition. The optical properties revealed energy band gap of thin films the range from 1.25 ~ 1.29eV and this values were slightly decreased direct band gap, 1.5eV. The surface morphology was observed by a FE-SEM(Filed effect scanning electron microscope). The top-view images showed the combination of large grain and small particles with little defects. The hall measurement system could help confirm p-type semiconductors as a light absorber layer. In conclusion, Sulfur elements play a crucial role to compose a quaternary compound, CZTS and CTS phase groups were mainly made with N₂ ambient. Clearly, sulfurization process help enhance the crystallinity of thin films. Second, CZTS thin films were fabricated by 2-step process, too. We designed with varying Cu contents at Sn/Cu/ZnS stack order. The precursor was deposited on soda lime glass(SLG) or bilayer-Mo/SLG substrate. Sn layer was fabricated by thermal evaporation and then Radio-Frequency (RF) magnetron sputtering was used to deposite Cu and ZnS layers. The thickness of Cu layer was controlled during deposition process. Next, the stack structure precursor was treated by sulfurization at 550℃ for 1hour. The fabricated samples display kesterite structure of CZTS and the preferred (112) orientation was observed by using XRD. The secondary phase, Cu₂S was vanished following moderate Cu contents which could contribute to growth single crystal of kesterite CZTS structure. Also, increasing Cu/(Zn+Sn) ratio improves the interface adhesion between CZTS absorber layer and Mo back contact layer. In addition, The increment of Cu element ratio enhanced the surface morphology that grains were more dense and could effect on the grain size. The optical band gap was calculated by extrapolating method using tauc's equation. The Cu-poor chemical composition lead to direct band gap, around 1.5eV but increasing Cu contents caused reappearance of copper disulfide phase which lower the band gap of CZTS films to near 1.2eV. The hall measurement using van der Pauw method system could identify electrical properties including carrier concentration, hall mobility and resistivy. Cu-rich state lead to increase carrier concentration with decreasing resistivity and increase hall mobility at Cu/(Zn+Sn) ratio over 1, dramatically. CZTS thin films as a p-type semiconductor is suitable for as a absorber layer in order to replace conventional c-Si PV or original CIGS thin film solar cell owing to using earth abundance, non-toxic and inexpensive materials having the outstanding potential. However, it must achieve cell/module efficiency, production cost reduction and productivity to become main leader of next generation photovoltaics.; Crystalline silicon (c-Si) have maintained a dominant world market share of solar cell owing to cheap module price and high efficiency. However, it have a difficult time because of overinvestment to production facilities for silicon raw material, a lot of energy consumption during the production process, global economic crisis and loss of price competitiveness. Thin film photovoltaics (TFPV) including CdTe (Cadmium telluride), CIGS (Copper, Indium, gallium and selenium) solar cell are expected to substitute for crystalline silicon (c-Si) solar cell. They take advantage of the developed technologies in order to reduce manufacturing cost per surface area than that of c-Si base solar cell/module. CdTe thin film solar cell have reached a ~16.7% cell efficiency by NREL (National Renewable Energy Laboratory). But it use a toxic material, Cd which could influence human body and environment. CIGS thin solar cell have 20.8% cell efficiency by ZSW(The Baden-Württemberg Center for Solar Energy and Hydrogen Research) and received a lot of attention its potential as a candidate for next generation solar cell. Nevertheless, it might have trouble in the production process because In price have been increased due to the rising its demand, in particular, display and touch screen industries. So, CdTe and CIGS PV need to overcome using harmful element and potential scarcity of constituent, respectively. CZTS (Copper, Zinc, Tin and Sulfur) thin film solar cell have been vigorously investigated using abundance and availability of materials, Cu, Zn, Sn and S. It could substitute In, Ga to Zn, Sn, respectively, and it is equally applied the CIGS fabrication process. CZTS solar cell have moderate prices, non-toxic and possesses a direct band gap near 1.5eV. However, it have recorded only 11% cell efficiency which was conducted by IBM and this record value was a long way from the efficiencies of other thin film photovoltaics. Therefore, the researches to CZTS absorber layer that could perform a role of optical absorption layer are requires to improve CZTS solar cell efficiency. In this study, we could fabricate CZTS thin films using 2-step process, precursor deposition and annealing treatment. At first experiment, we examined sulfur gas inclusion or not during annealing process. So, we applied two types of heat treatment N₂ ambient annealing and sequential N₂ annealing and sulfurization. Next, we investigated Cu element effect inner CZTS thin layer. Thus, Stack precursor was deposited by varying Cu metal layer thickness to synthesis precursor and then sulfurized at 550℃ for 1hour. First, consecutive N₂ annealing and sulfurization was conducted to Cu/ZnS/Sn and Sn/Cu/ZnS two stack precursors. They were deposited by physical vapor deposition methods, evaporation and sputtering. Sn, Cu and ZnS layer were deposited by thermal evaporator, DC-magnetron sputtering and RF-magnetron sputtering, respectively, on soda lime glass substrate. The thickness of metal or metallic sulfide was fixed to 300nm, equally. In N₂ annealing treatment, the precursors sample was heated at 500 or 550℃ for 1hour, respectively. Sequential annealing process, the samples preferentially annealed with N₂ambient at 500 or 550℃ for 1hour and then they were immediately sulfurized at 550℃ with N₂+ H₂S (5%) mixed gas atmosphere for 1hour. The crystal properties were analyzed by X-ray diffraction (XRD) regarding 4 kinds of CZTS thin films(Precursor: Cu/ZnS/Sn and Sn/Cu/Zn, Two-types annealing process: one is N₂ annealing at 500℃ or 550℃ and the other is a consecutive N₂ and sulfurization). The films were identified kesterite CZTS and/or CTS (Cu_xSn_yS_z) dominant phase but there were existed a kind of secondary phase, for example, copper sulfide, copper tin alloy and so forth. Raman spectroscopy was utilized to distinguish overlapped or uncertain phases from XRD analysis. At the results, thin films show the co-existence of CZTS and other secondary phases and the higher N₂ annealing treated, the more secondary phases were occurred even though they were finished sulfurization. From EDS results, the films show Cu-rich and Zn-poor condition. The optical properties revealed energy band gap of thin films the range from 1.25 ~ 1.29eV and this values were slightly decreased direct band gap, 1.5eV. The surface morphology was observed by a FE-SEM(Filed effect scanning electron microscope). The top-view images showed the combination of large grain and small particles with little defects. The hall measurement system could help confirm p-type semiconductors as a light absorber layer. In conclusion, Sulfur elements play a crucial role to compose a quaternary compound, CZTS and CTS phase groups were mainly made with N₂ ambient. Clearly, sulfurization process help enhance the crystallinity of thin films. Second, CZTS thin films were fabricated by 2-step process, too. We designed with varying Cu contents at Sn/Cu/ZnS stack order. The precursor was deposited on soda lime glass(SLG) or bilayer-Mo/SLG substrate. Sn layer was fabricated by thermal evaporation and then Radio-Frequency (RF) magnetron sputtering was used to deposite Cu and ZnS layers. The thickness of Cu layer was controlled during deposition process. Next, the stack structure precursor was treated by sulfurization at 550℃ for 1hour. The fabricated samples display kesterite structure of CZTS and the preferred (112) orientation was observed by using XRD. The secondary phase, Cu₂S was vanished following moderate Cu contents which could contribute to growth single crystal of kesterite CZTS structure. Also, increasing Cu/(Zn+Sn) ratio improves the interface adhesion between CZTS absorber layer and Mo back contact layer. In addition, The increment of Cu element ratio enhanced the surface morphology that grains were more dense and could effect on the grain size. The optical band gap was calculated by extrapolating method using tauc's equation. The Cu-poor chemical composition lead to direct band gap, around 1.5eV but increasing Cu contents caused reappearance of copper disulfide phase which lower the band gap of CZTS films to near 1.2eV. The hall measurement using van der Pauw method system could identify electrical properties including carrier concentration, hall mobility and resistivy. Cu-rich state lead to increase carrier concentration with decreasing resistivity and increase hall mobility at Cu/(Zn+Sn) ratio over 1, dramatically. CZTS thin films as a p-type semiconductor is suitable for as a absorber layer in order to replace conventional c-Si PV or original CIGS thin film solar cell owing to using earth abundance, non-toxic and inexpensive materials having the outstanding potential. However, it must achieve cell/module efficiency, production cost reduction and productivity to become main leader of next generation photovoltaics.