Improvement in Long-term Stability of Capacitive Deionization by Polymer Coating on Carbon Electrode

조규식 서울대학교 대학원 2020 국내박사

Capacitive deionization (CDI) is one of state-of-art technology for desalination of brackish water to supply enough water for day life. CDI has attracted attention as an efficient desalination technology in terms of energy consumption, water recovery, and environment-friendliness compared to the other desalination technology. However, CDI suffers from desalination performance degradation in long-term operation, which is critical issue for practical application. The degradation could be overcome by inhibiting the carbon oxidation, which is the main reason for degradation, but there were little research focusing on the inhibition of carbon oxidation. This dissertation aimed to provide a guideline for improving long-term stability of CDI by evaluating long-term stability of CDI and polymer-coated CDI and by investigating the working mechanisms for how polymer affects to transportation phenomena or electrical properties of the electrode. First, the salt adsorption capacity (SAC) changes of CDI using the electrode, that polydopamine (PDA) or ion exchange polymer (IEP) was coated on without the polymer layer on outer surface of the electrode, was traced. As a result, long term stability of polymer-coated CDI was improved by inhibiting carbon oxidation reactions due to the coverage of activated carbon surface by PDA and IEP without any additional polymer layer on outer surface of the electrode. PDA coating prevented the carbon oxidation reaction from occurring as much as 10% and IEP coating was more effective than PDA, resulting in approximately 80% higher long-term stability. It was because the thicker coating layer than PDA coating inhibits carbon oxidation more successfully. Second, to determine the role of IEP layer, the pre-oxidized electrode was tested in CDI and MCDI and the long-term stability of CDI using IEP-coated electrodes coated with various thicknesses (30 μm, 100 μm) were measured and compared. As major result, MCDI with pre-oxidized electrode for 24 h showed less performance reduction compared to the case of CDI. It means that ion exchange membrane has an effect on compensating the influence of carbon oxidation. This effect was investigated systemically by IEP coating with different thickness. CDI using the electrode with thicker IEP layer showed better long-term stability represented by less SAC reduction. Therefore, it can be concluded that IEP layer on outer surface of electrode also appeared to play a role as the compensation barrier that utilizes repulsed co-ions by positive PZC shift that formation of IEP layer as well as a role that inhibits the carbon oxidation more based on larger charge transfer resistance. Therefore, it was found that the stability of CDI can be further improved by coating activated carbon using various polymers and forming an IEP layer having an appropriate thickness. This research is meaningful in that it suggests a big direction to improve the stability required for the industrial application of CDI. 축전식 탈염 공정(Capacitive deionization, CDI)는 일상 생활에 깨끗한 물을 충분히 공급하기 위한 다양한 탈염 기술 중 하나이다. 축전식 탈염 공정은 다른 담수화 기술에 비해 에너지 소비, 물 회수율 및 환경 친화성 측면에서 장점을 갖는 담수화 기술로 주목을 받고 있다. 그러나, 장기 운전 시 탈염 성능의 저하로 인해 어려움을 겪고 있으며 이는 실제 현장에 적용하기 위해서는 꼭 해결해야 할, 매우 중요한 문제이다. 탈염 성능 저하의 주요 원인인 탄소 산화 및 그로 인한 표면 변화를 억제함으로써 문제를 극복할 수 있지만, 이러한 목적을 구현하거나 그 원리에 대해 규명한 연구가 현재까지는 부족한 실정이다. 이 논문에서는 CDI용 활성탄 전극에 폴리도파민과 이온교환고분자를 코팅하고, 장기 운전 시 성능이 어떻게 변화하는지를 조사하였다. 먼저, 이온 교환 고분자가 전극의 산화를 실제로 방지하는지 여부를 조사하기 위해 전극의 바깥쪽 표면에 어떠한 고분자층도 없도록 활성탄에 폴리 도파민 또는 이온 교환 고분자를 코팅하였다. 이 전극을 사용하여 CDI의 장기 운전 시 탈염 성능 변화를 추적한 결과, 폴리도파민 코팅 시 약 10 %만큼의 탈염 성능 감소 방지 효과가 관찰되었고, 이온 교환 고분자 코팅 시 80%의 성능이 50시간 후에도 유지됨으로써 폴리도파민보다 효과적이라는 결과를 얻었다. 이러한 장기 운전 내구성의 향상은 폴리도파민 및 이온교환고분자의 코팅이 탄소의 산화를 성공적으로 억제했기 때문이다. 둘째, 이온 교환 중합체 층의 역할을 조사하기 위해, 미리 산화시킨 전극에 대하여 CDI, MCDI 테스트를 진행하였고, 다양한 두께 (30 μm, 100 μm)로 이온교환고분자를 코팅한 전극을 사용하여 CDI의 장기 안정성을 측정하고 비교했다. 그 결과, 24 시간 동안 사전 산화된 전극을 사용한 MCDI는 CDI에 비해 성능 감소가 덜 나타났다. 이는 이온 교환막이 단순히 부반응을 줄이는 역할뿐만 아니라 탄소 산화에 의한 영전하 전위 영향을 상쇄하는 역할도 수행한다는 것을 의미한다. 이 효과는 다른 두께로 이온교환고분자를 코팅한 전극을 사용한 CDI 테스트를 통해 보다 체계적으로 조사되었다. 그 결과, 두께가 두꺼울수록 더 우수한 장기 안정성을 나타냈다. 두꺼운 이온교환고분자는 셀의 전기적 저항을 키움에도 불구하고 더 좋은 장기 안정성 및 더 좋은 탈염 성능이 나타난 이유는 MCDI 실험에서 확인된 바와 같이 양의 방향으로 이동한 영전하 전위에 의해 밀려난 동전하 이온이 버려지지 않고 반대전하 이온을 끌어오는 데 쓰이도록 하기 때문이다. 그와 동시에 탄소 산화를 더욱 억제하는 역할도 더 강하게 수행하기 때문에 뛰어난 장기 운전 내구성을 보임을 확인하였다. 따라서, 다양한 고분자를 사용하여 활성탄을 코팅하고 적절한 두께를 갖는 이온교환고분자층을 형성함으로써 CDI의 안정성을 추가로 개선할 수 있을 것으로 기대된다. 그러므로 이 연구는 CDI의 실제 적용에 필요한 수준의 안정성을 확보하기 위한 큰 방향을 제시한다는 점에서 의미가 있다고 사료된다.

A Study on Composite-Based Magnetorheological Fluids for Resolution of Trade-Off Between Performance and Long-Term Stability

최준석 서울대학교 대학원 2021 국내박사

자기유변유체는 자성입자가 비자성 매개액에 분산된 현탁액 형태의 스마트물질이다. 외부 자기장 하에서 자성입자들 사이의 쌍극자로 인한 정자기성 상호작용으로 체인 형태의 구조가 형성되고, 이 구조가 유체의 흐름을 막아 매우 짧은 시간 내에 점도가 크게 향상되게 된다. 이러한 성질로 인해 자기유변유체의 유변특성을 외부 자기장을 통해 쉽게 조절하는 것이 가능하다. 이러한 외부자장에 대한 톡특한 반응성으로 인해, 햅틱 디바이스 파워스티어링 펌프, 그리고 자동차, 다리, 건물 등의 충격 방지 시스템에 자기유변유체를 이용할 수 있다. 하지만 자기유변유체의 활용은 자성입자의 침전에 대한 안정성의 부족함으로 인해 크게 제한 될 수 있다. 밀도가 높은 자성입자와 밀도가 낮은 매개액 사이의 큰 밀도차이로 인해 자성입자가 빠르게 가라앉게 되면, 자기유변유체의 수명이 크게 감소하게 된다. 이러한 문제를 해결하기 위한 한가지 방법으로 자성물질과 밀도가 낮은 물질(고분자, 실리카 탄소물질 등)을 결합하여 자성복합입자를 합성함으로써, 자성입자의 밀도를 낮추고 자기유변유체의 침강안정성을 높이는 연구들이 진행되어 왔다. 하지만 이러한 경우 복합자성입자의 자기적 성질이 저하되기 때문에 자기유변유체의 침강안정성과 성능이 서로 상충관계에 있다는 문제점을 가지고 있다. 본 논문에서는 뛰어난 성능과 침강안정성을 가지는 자기유변유체를 제조하기 위해 다양한 물질구성과 구조를 가지는 합성하였다. 첫 단계로 실리카를 템플레이트로 사용하여 할로우 구조를 가지는 고분자-Fe3O4 복합자성입자를 합성하였다. 할로우 구조 내부의 공동으로 인해, 입자의 밀도가 순수 Fe3O4 대비 40 % 수준까지 감소하였고, 이로 인해 자기유변유체의 침강안정성이 크게 상승하였다. 다음 연구로, 비자성 고분자로 인한 자기유변유체 성능의 감소를 최소화하기 위해, 간단한 전기방사 방법을 통해 계층구조를 가지는 Fe3O4 나노입자들을 제조하였다. 앞의 연구와 대비하여, 고분자의 배제를 통해 높은 자화값을 가지는 Fe3O4 나노구조입자들을 얻을 수 있었고, 이를 자기유변유체에 적용하여 할로우 고분자-Fe3O4 입자 기반 자기유변유체 대비 3배 이상의 성능을 가지는 자기유변유체를 얻을 수 있었다. 이와 동시에 Fe3O4 나노구조입자 내부에 생성된 기공들로 인해 순수 Fe3O4 대비 밀도가 약 23 % 정도 감소하였고, 이로 인해 침 Fe3O4 나노구조입자기반 자기유변유체의 침강안정성이 향상됨을 확인할 수 있었다. 자기유변유체의 성능과 침강안정성사이의 상충성을 최소화 하기 위해서 비구형의, CoFeNi 합금기반 자성 복합입자를 합성하고 자기유변유체에 적용하였다. 먼저 개질된 카본나노튜브 표면에 CoFeNi를 합성하는 방법을 통해 카본나노튜브-CoFeNi 복합체를 합성하였다. Fe3O4 대비 높은 CoFeNi의 자화값으로 인해 카본나노튜브-CoFeNi 복합체 기반 자기유변유체는 Fe3O4 복합체기반 유체 대비 3배에서 10배 이상의 뛰어난 유변성능을 보였다. 또한 종횡비가 높은 카본나노튜브로 인해 복합체가 유체 내에서 3차원 네트워크 구조를 형성하여, 빛 투과도 22 %의 매우 뛰어난 침강안정성을 보였다. 마지막으로 비자성 물질인 카본나노튜브를, 자성물질인 플레이크형 센더스트로 대체한 센더스트-CoFeNi 복합입자를 합성하여 자기유변유체에 적용하였다. 플레이크형 센더스트의 높은 종횡비로 인해 나타나는 높은 항력계수로 인해, 해당 자기유변유체는 빛 투과도 23 %의, 높은 입자밀도 대비 매우 뛰어난 침강안정성을 보였다. 동시에, 센더스트 CoFeNi 모두 높은 자화값을 가지는 자성물질이기 때문에, 센더스트-CoFeNi 기반 자기유변유체의 성능이 카본나노튜브-CoFeNi 유체 대비 크게 향상되는 것을 확인할 수 있었다. Magnetorheological (MR) fluids are smart materials composed of magnetic particles dispersed in magnetically-insulating carrier medium. With magnetic field, chain-like structures are formed due to dipole-induced magnetostatic interaction between magnetic particles, and the structures inhibit the flow and increase the viscosity of MR fluids in very short time. This characteristic enables the rheological properties of MR fluids to be easily tailored with magnetic field strength. Due to this unique response, MR fluids can be used for actuator systems like power steering pumps haptic devices, and active suspensions, and damper systems in automobile, bridges, buildings and so on. A huge obstacle for application of MR fluids is their poor long-term stability against the sedimentation of magnetic particles. The large difference in the density between heavy magnetic particles and light medium make magnetic particles quickly go down to bottom, reducing the MR fluids’ length of life. One of the strategies for improvement of the long-term stability was to reduce the density of magnetic particles by synthesizing magnetic composite materials. Fabrication of magnetic composites using light materials such as polymer, silica, carbon materials efficiently have reduced the density mismatch between magnetic materials and carrier medium, enhancing the long-term stability of MR fluids. However, there was trade-off between long-term stability and performance of MR fluids because use of light materials is equivalent to the deterioration in magnetic properties. In this study, various magnetic composites with different composition and structures were fabricated for the objective of producing MR fluids having excellent performance and long-term stability simultaneously. As a first step, hollow structured polymer-Fe3O4 composite particles were synthesized using SiO2 as sacrificial template. With cavity inside, the hollow magnetic composite particles showed the density only 40 % of bare Fe3O4 and the large improvement in long-term stability of MR suspensions could be observed. To avoid huge decline in MR performance with non-magnetic polymers, hierarchically-structured Fe3O4 nanoparticles were prepared with simple electrospraying process. By excluding polymers, hierarchically-structured Fe3O4 had magnetization value very closed to its primary nanoparticles, leading to MR performance higher more than 3 times of hollow structured polymer-Fe3O4 suspensions. At the same time, the pores inside reduced the density of the structured particles by 23 %, resulting in better long-term stability of hierarchically-structured Fe3O4 suspension than bare Fe3O4 suspension. To minimize the trade-off between MR performance and long-term stability (density of magnetic particles), non-spherical, CoFeNi-based magnetic composites were fabricated and applied for MR fluids. CNT-Co0.4Fe0.4Ni0.2 composite was produced by synthesizing Co0.4Fe0.4Ni0.2 on the surface of functionalized CNTs. Much higher magnetization of Co0.4Fe0.4Ni0.2 compared to Fe3O4 enabled CNT-Co0.4Fe0.4Ni0.2 suspension to have much superior MR performance than Fe3O4 composite-based MR fluids. Also, due to high aspect ratio of CNTs, outstanding long-term stability of 22 % light transmission was observed with formation of 3-dimensional network structures. Finally, magnetically non-active CNTs were replaced by magnetizable, flake-shaped sendust. The high drag coefficient of flake sendust, combined with roughened surface due to attached Co0.4Fe0.4Ni0.2 nanoparticles, resulted in excellent stability with 23 % of light transmission despite of the high density of sendust-Co0.4Fe0.4Ni0.2 composite particles. Also, because both constituents of sendust-Co0.4Fe0.4Ni0.2 are both magnetic materials with high magnetization value, the MR fluids retained very high yield stress value

Passivation Engineering of Perovskite Solar Cells for Long-term Stability : Passivation Engineering for Photovoltaic devices

강병수 과학기술연합대학원대학교 한국과학기술연구원(KIST) 2024 국내박사

Passivation Engineering of Perovskite Solar Cells for Long- term Stability The rapid increase in fossil fuel use has caused global warming, which is accelerating. To replace this, research on new and renewable energy is emerging. Since solar cells use sunlight as an energy source, there is no problem of resource depletion, making it possible to generate sustainable renewable energy, which is why ongoing interest and research and development are underway. In particular, perovskite, known as a next-generation solar cell material, is attracting worldwide attention by achieving a photoelectric conversion efficiency of more than 25% through much interest and groundbreaking research. In addition, as energy demand increases due to increased population density, the proportion of solar-based small- scale distributed power generation is increasing. Therefore, the demand for solar cell technology applicable to real life will increase, and accordingly, the need to develop flexible solar cell technology is also emerging. Perovskite solar cells have high efficiency, but long-term stability issues must be resolved for commercialization. In general, factors that impede long-term stability are divided into external factors (moisture, humidity) and internal factors (light, heat, voltage). In addition, the ITO transparent electrode used in flexible solar cells has the disadvantage of being mechanically weak as it is prone to breakage at certain bends. In order to commercialize perovskite solar cells, it is necessary to solve problems arising from two factors, along with the vulnerability of the mechanical strength of transparent electrodes used in flexible solar cells applicable to everyday life. In this study, we propose to develop a process technology that can ensure the stability of solar cells by adding or using specific materials to the perovskite layer and its surrounding layers. First, hydrophobicity was given to the perovskite film and moisture stability was investigated. Perovskite films were used with additive of hydrophobic nature to investigate the correlation. There are numerous defects on the surface or interface of perovskite films made through a solution process. Moisture present in the atmosphere permeate into these defects and accelerates the degradation of the perovskite. To solve this problem, the stability of perovskite solar cells was confirmed through surface and interface treatment using perfluorinated alkylammonium salt. It was confirmed that the surface hydrophobicity increased through the contact angle, and it was confirmed that the efficiency was maintained at more than 80% of the initial efficiency for 1000 hours at a relative humidity of about 40%, showing excellent moisture stability. Second, to investigate device performance and driving stability, the interface between the perovskite film and the electron transport layer was passivated with RMS material. SnO2, commonly used in perovskite solar cells as an electron transport layer, has many advantages such as high permeability, low- temperature processing, and high conductivity. However, defects arising from oxygen vacancies and interstitial tin (Sn) atoms exist in SnO2 films, which can lead to charge carrier accumulation and non-radiative recombination at the interface. These defects degrade perovskite device performance. To address this issue, we attempted to reduce surface oxygen vacancies and interstitial tin defects by passivating the electron transport layer film. Using X-ray photoelectron spectroscopy, it was confirmed that chemical bonding occurred between the electron transport layer and RMS. Therefore, it was found that defects in the electron transport layer were reduced by RMS. Through X-ray diffraction analysis, it was confirmed that the crystallinity of the perovskite film processed from the surface-treated electron transport layer was improved. As a result of manufacturing a perovskite solar cell based on this, a photoelectric conversion efficiency of more than 21% was obtained in the surface-treated device. It was found that surface treatment improved the electron extraction ability from the perovskite to the electron transport layer, thereby improving device performance. In addition, stability was secured to maintain 90% of the initial efficiency even after 600 hours in air without encapsulation. Through this study, it was confirmed that defects on the surface of the electron transport layer have a negative effect, and it was suggested that surface passivation is important for long-term stability of perovskite solar cells. Third, the introduction of alternative conductive materials to replace the conventional transparent electrode, ITO, is pursued. While commercialized ITO electrodes are widely used in flexible solar cells, they suffer from brittleness under bending radii of less than 5mm, leading to decreased conductivity and thus reduced solar cell efficiency. In contrast, silver nanowires offer advantages such as high conductivity, transparency, and flexibility, making them suitable for replacing ITO electrodes with flexible conductors due to their facile processing. However, silver nanowires are characterized by a highly rough surface, posing a significant obstacle to replacing ITO electrodes in transparent electrodes for solar cells. Indeed, penetration of the active layer of solar cells (≈500 nm) by silver nanowires can lead to severe leakage current (or even electrical shorts), thereby degrading device performance. Additionally, poor adhesion of the silver nanowire network to the substrate and corrosion due to external factors are also significant concerns. In this study, by incorporating conductive materials with silver nanowires, we aimed to overcome the drawbacks of conventional transparent electrodes by enhancing conductivity and mechanical strength. Consequently, critical drawbacks were addressed. Thus, by applying this technology, we sought to advance the development of recyclable underlying electrode processes by mechanically bonding detachable electrodes and solar cell components. Dry deposition of the conductive polymer PH1000 as a sacrificial layer was introduced to prevent damage to the ETL or photoactive layer. After fabricating the components, detachment was initiated, resulting in detachment occurring between the dummy substrate and PH1000. The surface resistance of the detached film was measured to be approximately 600 Ω/□, and SEM morphology measurements confirmed that the PH1000 layer prevented damage to the photoactive layer. Through this approach, the feasibility of driving solar cells with a composite of silver nanowires and a sub-assembly electrode mechanically bonded was confirmed. Key words : Perovskite solar cells, Passivation, Long-term stability, Silver nanowire-composite, Flexibility, Mechanical junction Passivation Engineering of Perovskite Solar Cells for Long- term Stability The rapid increase in fossil fuel use has caused global warming, which is accelerating. To replace this, research on new and renewable energy is emerging. Since solar cells use sunlight as an energy source, there is no problem of resource depletion, making it possible to generate sustainable renewable energy, which is why ongoing interest and research and development are underway. In particular, perovskite, known as a next-generation solar cell material, is attracting worldwide attention by achieving a photoelectric conversion efficiency of more than 25% through much interest and groundbreaking research. In addition, as energy demand increases due to increased population density, the proportion of solar-based small- scale distributed power generation is increasing. Therefore, the demand for solar cell technology applicable to real life will increase, and accordingly, the need to develop flexible solar cell technology is also emerging. Perovskite solar cells have high efficiency, but long-term stability issues must be resolved for commercialization. In general, factors that impede long-term stability are divided into external factors (moisture, humidity) and internal factors (light, heat, voltage). In addition, the ITO transparent electrode used in flexible solar cells has the disadvantage of being mechanically weak as it is prone to breakage at certain bends. In order to commercialize perovskite solar cells, it is necessary to solve problems arising from two factors, along with the vulnerability of the mechanical strength of transparent electrodes used in flexible solar cells applicable to everyday life. In this study, we propose to develop a process technology that can ensure the stability of solar cells by adding or using specific materials to the perovskite layer and its surrounding layers. First, hydrophobicity was given to the perovskite film and moisture stability was investigated. Perovskite films were used with additive of hydrophobic nature to investigate the correlation. There are numerous defects on the surface or interface of perovskite films made through a solution process. Moisture present in the atmosphere permeate into these defects and accelerates the degradation of the perovskite. To solve this problem, the stability of perovskite solar cells was confirmed through surface and interface treatment using perfluorinated alkylammonium salt. It was confirmed that the surface hydrophobicity increased through the contact angle, and it was confirmed that the efficiency was maintained at more than 80% of the initial efficiency for 1000 hours at a relative humidity of about 40%, showing excellent moisture stability. Second, to investigate device performance and driving stability, the interface between the perovskite film and the electron transport layer was passivated with RMS material. SnO2, commonly used in perovskite solar cells as an electron transport layer, has many advantages such as high permeability, low- temperature processing, and high conductivity. However, defects arising from oxygen vacancies and interstitial tin (Sn) atoms exist in SnO2 films, which can lead to charge carrier accumulation and non-radiative recombination at the interface. These defects degrade perovskite device performance. To address this issue, we attempted to reduce surface oxygen vacancies and interstitial tin defects by passivating the electron transport layer film. Using X-ray photoelectron spectroscopy, it was confirmed that chemical bonding occurred between the electron transport layer and RMS. Therefore, it was found that defects in the electron transport layer were reduced by RMS. Through X-ray diffraction analysis, it was confirmed that the crystallinity of the perovskite film processed from the surface-treated electron transport layer was improved. As a result of manufacturing a perovskite solar cell based on this, a photoelectric conversion efficiency of more than 21% was obtained in the surface-treated device. It was found that surface treatment improved the electron extraction ability from the perovskite to the electron transport layer, thereby improving device performance. In addition, stability was secured to maintain 90% of the initial efficiency even after 600 hours in air without encapsulation. Through this study, it was confirmed that defects on the surface of the electron transport layer have a negative effect, and it was suggested that surface passivation is important for long-term stability of perovskite solar cells. Third, the introduction of alternative conductive materials to replace the conventional transparent electrode, ITO, is pursued. While commercialized ITO electrodes are widely used in flexible solar cells, they suffer from brittleness under bending radii of less than 5mm, leading to decreased conductivity and thus reduced solar cell efficiency. In contrast, silver nanowires offer advantages such as high conductivity, transparency, and flexibility, making them suitable for replacing ITO electrodes with flexible conductors due to their facile processing. However, silver nanowires are characterized by a highly rough surface, posing a significant obstacle to replacing ITO electrodes in transparent electrodes for solar cells. Indeed, penetration of the active layer of solar cells (≈500 nm) by silver nanowires can lead to severe leakage current (or even electrical shorts), thereby degrading device performance. Additionally, poor adhesion of the silver nanowire network to the substrate and corrosion due to external factors are also significant concerns. In this study, by incorporating conductive materials with silver nanowires, we aimed to overcome the drawbacks of conventional transparent electrodes by enhancing conductivity and mechanical strength. Consequently, critical drawbacks were addressed. Thus, by applying this technology, we sought to advance the development of recyclable underlying electrode processes by mechanically bonding detachable electrodes and solar cell components. Dry deposition of the conductive polymer PH1000 as a sacrificial layer was introduced to prevent damage to the ETL or photoactive layer. After fabricating the components, detachment was initiated, resulting in detachment occurring between the dummy substrate and PH1000. The surface resistance of the detached film was measured to be approximately 600 Ω/□, and SEM morphology measurements confirmed that the PH1000 layer prevented damage to the photoactive layer. Through this approach, the feasibility of driving solar cells with a composite of silver nanowires and a sub-assembly electrode mechanically bonded was confirmed. Key words : Perovskite solar cells, Passivation, Long-term stability, Silver nanowire-composite, Flexibility, Mechanical junction 화석연료 사용의 급증으로 인하여 지구 온난화를 야기했으며, 점점 가속화가 진행되고 있다. 이를 대체하기 위해 신재생에너지가 주목 받고 있다. 태양전지는 에너지원을 태양빛으로 사용하기 때문에 자원의 고갈문제가 없어 지속가능한 재생에너지 생성이 가능하여 지속적인 연구개발이 진행되고 있다. 특히, 차세대태양전지 소재로 알려진 페로브스카이트는 많은 관심과 비약적인 연구를 통해 25% 이상의 광전변환 효율을 달성하여 전세계적으로 주목받고 있다. 더불어, 인구밀도증가에 따른 에너지 수요가 늘어남에 따라 태양광 기반 소규모 분산형 발전 비중이 확대되고 있다. 따라서 실생활에 적용가능한 태양전지 기술 수요가 증가할 것이며, 이에 따라 유연 태양전지 기술 개발의 필요성 또한 대두되고 있다. 페로브스카이트 태양전지는 높은 효율을 가지고 있음에도 불구하고 상업화를 이루기 위해서는 장기안정성 문제점을 해결해야 한다. 일반적으로 장기 안정성을 저해하는 요소는 외부적인 요인(수분, 습도)과 내부적인 요인 (빛, 열, 전압)로 구분된다. 또한, 유연태양전지에서 사용되는 ITO 투명전극은 특정한 굽힘에서 부러지기 쉬운 특성을 지니고 있어 기계적으로 취약한 문제점을 지니고 있다. 페로브스카이트 태양전지 상업화를 위해서는 두 가지 요인에서 야기되는 문제점과 더불어 유연태양전지의 사용되는 투명전극의 기계적 강도의 취약점을 해결해야 된다. 해당 연구는 페로브스카이트층과 주변을 구성하는 층에 특정한 물질을 첨가 또는 추가로 사용하여 태양전지의 안정성을 확보하기 위한 공정 기술 개발을 제안하고자 한다. 첫째, 페로브스카이트 필름에 소수성을 부가하여 수분안정성을 조사했다. 상관관계를 조사하기 위해 소수성 첨가유무에 따른 페로브스카이트 필름을 사용하였다. 용액 공정으로 만들어진 페로브스카이트 표면이나 계면에 수많은 결함들이 존재한다. 이 결함을 통해 대기중에 존재하는 수분이 침투하게 되고 페로브스카이트의 열화를 가속시킨다. 해당 문제를 해결하고자 과불화된 암모늄염으로 표면 및 계면처리를 통해 페로브스카이트 태양전지 안정성을 확인하였다. 접촉각을 통해 표면이 소수성이 증가됨을 확인하였으며, 상대습도 약 40%에서 1000시간동안 초기효율대비 80%이상 효율이 유지됨으로써 좋은 수분안정성을 보였다. 둘째, 페로브스카이트 필름과 전자수송층 사이를 RMS 물질로 부동화함으로써 소자 성능 및 구동안정성을 조사했다. 페로브스카이트 태양전지에서 흔히 사용되는 SnO2는 전자수송층으로 높은 투과율, 저온공정 그리고 높은 전도도 등 많은 장점을 가지고 있다. 하지만, SnO2 필름에서는 산소 결손과 격자간 주석 (Sn) 원자에서 발생하는 결함들이 존재하며, 이는 계면에서 전하 캐리어 축적 및 비방사성 재결합 손실로 이어질 수 있다. 이러한 결함은 페로브스카이트 소자 성능을 저하시킨다. 이를 해결하고자 전자수송층 필름 위에 부동화를 하여 표면에 존재하는 산소 결손과 격자간 주석 결함을 줄이고자 하였다. X선 광전자 분광법을 통해 전자수송층과 RMS 사이에서 화학적으로 결합이 의해 전자수송층에 존재하는 결함이 줄어듦을 알 수 있었다. X선 회절분석으로 표면처리된 전자수송층에서 페로브스카이트 필름의 결정성이 좋아짐을 확인하였다. 이를 기반으로 페로브스카이트 태양전지를 제작한 결과, 21% 이상의 광전변환효율을 얻을 수 있었다. 표면처리로 인해 페로브스카이트에서 전자수송층으로 전자추출 능력이 향상되어 소자 성능이 좋아졌다. 그리고, 대기중에서 봉지처리없이 600시간 후 초기 효율대비 90% 유지하는 안정성을 확보하였다. 이를 통해 전자수송층 표면에 존재하는 결함에 부동화처리는 페로브스카이트 태양전지 장기안정성에 중요하다는 것을 시사하였다. 셋째, 기존의 투명전극인 ITO를 대체할 전도성 물질을 도입하는 것이다. 상용화 된 ITO 전극은 유연태양전지에도 많이 사용되고 있지만, 5mm 이하의 굽힙반경에서는 부서지기 쉬운 특성을 지니고 있어 전도성이 저하되며, 이는 태양전지 효율저하를 야기한다. 이에 반해 은 나노와이어는 높은 전도성, 투과도, 그리고 굽힙성과 같은 장점을 가지고 있고, 공정이 용이하여 ITO 전극을 대체하여 유연전극으로 사용이 가능하다. 하지만, 은 나노와이어의 매우 거친 표면기는 ITO 전극을 대체하는 큰 장애물이 된다. 은 나노와이어 기반 태양전지는 활성층 (≈500 nm)을 쉽게 침투하여 심각한 누설 전류로 인해 장치 성능이 저하될 수 있다. 게다가, 기판에 대한 은 나노와이어 네트워크의 부족한 접착력과 외부 요인으로 인한 부식도 간과할 수 없는 문제이다. 본 연구에서는 은 나노 와이어와 함께 전도성 물질을 접목하여 높은 전도성과 기계적강도를 향상시킴으로써 기존 투명전극의 단점을 보완할 수 있으며, 기존과 지닌 치명적인 단점을 해결할 수 있었다. 추가로, 액체 금속을 은 나노와이어와 접합을 통해 합금이 만들어지며, 만들어진 합금은 우수한 전도성뿐만 아니라 접착성을 띈다. 그리고, 액체 금속표면에 얇은 산화층이 형성되어 은 나노와이어의 부식을 방지할 수 있다. 따라서, 해당 기술을 적용하여 박리가능한 전극과 태양전지 소자를 기계적으로 접합해 재활용가능한 하부전극 공정을 발전시키고자 하였다. 건식방식으로 박리가 가능한 전도성 고분자인 PH1000을 희생층으로 도입해 ETL 또는 광활성층의 손상을 방지하고자 하였다. 소자를 제작 후 박리를 진행하면, 더미기판과 PH1000 사이에서 박리가 일어나게 된다. 박리된 필름의 면저항을 측정한 결과 ~약600 Ω/□을 확인하였으며, SEM으로 morphology를 측정하여 PH1000 층에 의해 광활성층이 손상을 방지했다. 이 접근 방식을 통해, 은 나노와이어 복합체가 함침된 하부전극과 소자를 기계적으로 접합하여 태양전지 구동가능성을 확인할 수 있었다. 주요단어(Key words) : 페로브스카이트 태양전지, 부동화, 장기 안정성, 은나노와이어 복합체, 유연성, 기계적 접합

Engineering Strategy for Robust Stability of All-Inorganic Black CsPbI3 Perovskites

조남광 서울대학교 대학원 2020 국내박사

The halide perovskite with an adjustable direct bandgap, high absorption coefficient, and long exciton diffusion length has attracted considerable attention in the field of solar cell and photodetector as the next generation light absorber. Recently, researches have been intensively conducted to secure structural long-term stability under ambient conditions of a black colored (α, γ phase) halide perovskite with low-bandgap (~1.72 eV), which has excellent absorption property along the entire visible light region. However, there is still a challenge to further methodological improvement of the perovskite long-term stability issue. In this thesis, two engineering strategies for securing long-term stability of black phase perovskite under atmospheric conditions were proposed sequentially, and photoelectric properties were evaluated by applying them to phototransistor and photoresistor devices respectively. Firstly, I introduced the implementation of all-inorganic α-CsPbIxBr3-x, one of the partial substitution methods, that secured long-term stability under atmospheric conditions by introducing the multi anion strategy with low temperature process. Additionally, phototransistors were fabricated through the heterojunction of CsPbIxBr3-x and IGZO oxide semiconductor, and the characteristics of the phototransistor were evaluated. Throughout this study, I also found that the CsPbIxBr3-x/IGZO heterojunction significantly cushions the photoillumination stress of IGZO. Furthermore, I suggest a robust strategy to ensure the structural, electrical long-term stability of γ-CsPbI3 under ambient conditions through the ultraviolet (UV) curable oligo(ethylene glycol) dimethacrylate (OEGDMA). Oxygen lone pair electrons of the OEGDMA capture Cs+ and Pb2+ cations, improving the crystal growth of γ-CsPbI3 around OEGDMA. Additionally, polymer network of OEGDMA processed by UV irradiation, strongly contributes to maintaining the black phase of γ-CsPbI3 structure tight for more than 35 days in ambient conditions. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analysis were performed to understand the effect of UV-cured polymer network of OEGDMA which is introduced for maintaining the black phase of γ-CsPbI3. In order to evaluate the optoelectronic properties, two-terminal photoresistors were fabricated in the lateral direction and the effective photocurrent under the visible light was confirmed. The optimized UV-cured 3 wt% OEGDMA/γ-CsPbI3 perovskite film showed that ~90% of the initial electrical properties responding to red, green, and blue light were maintained even after 35 days. These results provide that introducing the photocurable polymer network is an innovative strategy that can effectively contribute to the long-term stability of the halide perovskite. 할라이드 페로브스카이트는 높은 흡광계수와 긴 엑시톤 확산길이 및 밴드갭을 조절 가능한 특징들로 인하여 태양전지 및 광 검출기 분야에서 차세대 광 흡수제로써 상당한 관심을 끌었다. 이러한 할라이드 페로브스카이트에 대한 연구는, 최근에는 가시광 영역 전체에 걸쳐 우수한 흡광 특성을 갖는 좁은 밴드갭 (~1.72 ev)의 흑색 (α, γ 상) 할라이드 페로브스카이트 구현에 초점이 맞추어져 왔으며, 이와 더불어 대기 조건 하에서 흑색 상의 장기안정성을 확보하기 위한 연구가 집중적으로 수행되었다. 그럼에도 불구하고 페로브스카이트를 대기 조건 하에서 장기적으로 흑색 상을 유지시키기 위한 공정 방법론적인 문제는 여전히 남아있으며 추가적인 개선의 여지가 분명히 존재한다. 본 논문에서는 대기 조건 하에서 흑색 상 페로브스카이트의 장기 안정성을 확보하기 위한 두 가지 공정 전략을 순차적으로 제안하였으며, 이 전략들을 포토트랜지스터 및 포토레지스터 소자에 각각 적용하여 광전기적 특성을 평가하였다. 먼저, 브롬 이온을 요오드 위치에 부분적으로 치환시킨 α-CsPbIxBr3-x를 저온공정을 통하여 대기중에서 안정적으로 흑색 상을 유지할 수 있는 방법을 제안하였다. 이 방법을 통하여 제작한 α-CsPbIxBr3-x를 IGZO 산화물 반도체와 이종 접합시킴으로써 포토트랜지스터를 제작하였고 광전기적 특성을 평가하였다. 또한, 이 연구를 통하여 α-CsPbIxBr3-x/IGZO 이종접합이 IGZO 본래의 photoillumination stress를 크게 완화시키는 것을 확인하였다. 그러나, 요오드 위치에 부분적으로 브롬 이온을 치환시키게 되면, 밴드갭이 CsPbI3 고유의 밴드갭 보다 넓어지는 근본적인 한계점을 내포하고 있으므로, 이러한 문제를 극복하기 위하여 자외선 경화가능한 폴리에틸렌 글리콜 디 메타크릴레이트 (PEGDMA)를 도입하여 대기 조건 하에서 γ-CsPbI3의 구조적, 전기적 장기 안정성을 보장하는 강력한 전략을 추가적으로 제안하였다. PEGDMA 내부에 존재하는 다량의 산소 고립전자쌍들이 세슘 및 납 양이온들을 포획하여 PEGDMA 주위에서부터 γ-CsPbI3의 결정 성장을 유도하였으며, 추가적으로 자외선 경화를 통해 PEGDMA 네트워크가 γ-CsPbI3구조를 단단하게 고정시켜 줄 수 있도록 하였다. 또한, 2 단자 포토레지스터 소자를 구현하여 가시광 조사 시, 유효한 광전류를 35일간 대기중에서 노출되어도 90%이상 유지됨을 확인하였다. 이러한 결과들은 광 경화성 고분자 네트워크를 적용한 공정방법이 할라이드 페로브스카이트의 장기안정성에 효과적이고 범용적으로 기여할 수 있는 혁신적인 전략이 될 수 있음을 시사한다.

Ultra-thin, lightweight, long-term stable, flexible InGaP/GaAs-based optoelectronic devices for wide field applications

김태수 Graduate School, Yonsei University 2024 국내박사

Flexible solar cells are gaining attention not only for their potential applications as power sources in vehicles, drones, and satellites but also as environmentally friendly energy solutions. Flexible solar cells composed of perovskite, organic materials, and amorphous silicon are actively researched due to their reasonable light absorption properties and cost-effective manufacturing. However, these materials tend to degrade over time, and their disordered and defective structures result in a short carrier lifetime, making them less than ideal for high-efficiency solar cells. Solar cells made from III-V compound semiconductors, known for their excellent optical and electrical properties, offer a solution by enabling the creation of thin geometrical structures with high energy conversion efficiency, overcoming these limitations. To be used effectively in the mentioned fields, these solar cells need to provide high power-to-weight ratios called specific power and long-term stability. Therefore, this paper presents research on flexible InGaP/GaAs solar cells that achieve high power-to-weight ratios and long-term stability. Firstly, research was conducted to maximize the specific power of flexible InGaP/GaAs solar cells. The specific power is a crucial metric for comparing the performance of flexible solar cells, especially when considering their use as power sources for mobile devices. Traditional flexible solar cells based on III-V compound semiconductors were fabricated on copper or metal foils, which are thick and heavy, limiting their flexibility. Recent research has explored the use of lightweight and flexible polymer films as substrates to reduce the weight and increase the flexibility of solar cells. However, even with polymer substrates, the specific power of solar cells remains limited due to the thickness of the substrate and the bonding materials. To overcome these limitations, a study proposed reducing the thickness of both the substrate and the bonding materials using a wafer bonding technique known as metal bonding. By using a 13 μm thin polyimide substrate and a thin metal stack (Pt/Au/Pt, 10/120/10 nm) as the bonding materials, a high specific power over 5.36 kW/kg was achieved. This is the highest level among solar cells made of III-V compound semiconductors. Additionally, a protective encapsulation layer was developed to protect the solar cells from temperature and humidity, serving as an anti-reflective coating and maintaining the solar cells' performance for over 500 hours under conditions of 85 °C temperature and 85% relative humidity. Secondly, research was conducted to ensure the long-term stability of flexible InGaP/GaAs solar cells. Long-term stability is a critical factor for solar cells to be used as environmentally friendly energy sources. Various materials have been investigated as encapsulation layers to protect the components from moisture. However, most of these materials have pinholes that do not provide perfect protection against moisture. In contrast, thermally grown silicon dioxide does not have pinholes and can offer complete protection against moisture. By transferring thermally grown silicon dioxide on the solar cells, it was confirmed that the solar cells remained stable even when submerged in water at 70°C for ten days. These results demonstrate the potential for the produced solar cells to be used as power sources for mobile devices and environmentally friendly energy solutions, meeting the requirements for high specific power and long-term stability. 본 학위 논문은 다음과 같은 내용을 다룬다. 1장에서는 III-V족 화합물 반도체 태양전지에 대해 소개한다. III-V족 화합물 반도체 태양전지의 연구의 필요성에 설명하고, 태양전지의 성능의 평가지표에 대한 설명을 한다. 그 후, 태양전지에서 무게 대비 출력과 장기 안정성의 중요성에 대해 설명을 하고, 이 연구의 전체 목표에 대하여 서술하였다. 2장에서는 본격적으로 무게 대비 출력을 극대화한 장기 안정성을 갖춘 InGaP/GaAs 유연 태양전지에 대하여 서술하였다. 자세한 공정과정과 함께 무게 대비 출력을 극대화한 방법과 장기 안정성을 갖춘 방법, 그리고 그 결과에 대하여 서술하였다. 3장에서는 장기 안정성을 극대화한 높은 무게 대비 출력을 갖는 InGaP/GaAs 유연 태양전지에 대하여 서술하였다. 자세한 공정과정과 함께 장기 안정성을 극대화한 방법과 높은 무게 대비 출력을 갖춘 방법, 그리고 그 결과에 대하여 서술하였다. 4장에서는 본 연구를 정리하여 끝맺음하였다.

Study on robust all-inorganic perovskite phase structure for enhancing long-term stability of photodetector

나현재 서울대학교 대학원 2022 국내박사

The halide perovskite with an adjustable high absorption coefficient, direct bandgap, and long exciton diffusion length has attracted considerable attention in the field of the photodetector and solar cell as the next generation light absorber. Recently, research has been intensively conducted to secure structural long-term stability under ambient conditions of a black colored halide perovskite with low-bandgap (~1.7 eV), which has excellent absorption property along the entire visible light region. However, there are still difficulties in the further methodological improvement of the long-term stability problem of perovskite. In this thesis, two engineering strategies were sequentially proposed to ensure the long-term stability of black phase perovskite under atmospheric conditions, and their photoelectric properties were evaluated by applying them to phototransistors and photoconductor devices, respectively. Firstly, I introduce the implementation of all-inorganic α-CsPbIxBr3-x, one of the partial substitution methods, which proposed the multi-anion strategy of low-temperature processes to ensure long-term stability in atmospheric conditions. Additionally, heterojunction phototransistors were fabricated with CsPbIxBr3-x and oxide IGZO semiconductor, and the characteristics of the phototransistor were evaluated. Throughout this study, I also discovered that the CsPbIxBr3-x/IGZO heterojunction significantly relieves the photo-illumination stress of IGZO. Furthermore, I propose a robust strategy for the fabrication of highly stable lead-free nanometer-thick Cs2SnI6 film with broadband absorption optical properties via UV-cured polyethylene glycol dimethacrylate (PEGDMA) in the perovskite precursor solution. The UV-cured PEGDMA captures CsI, and the presence of relatively excessive SnI2 has a significant influence on the formation of nanoscale Cs2SnI6 film with high crystallinity and high yield. The oxygen lone pair electrons of PEGDMA capture Cs+, and significantly improve the crystal growth of Cs2SnI6 around PEGDMA. In addition, the optimal amount of PEGDMA (5 wt%) had formed polymer networks by UV irradiation, which strongly contributed to the maintained black phase of the Cs2SnI6 film without phase transition over 1 month under ambient conditions. In particular, by using the fairly low bandgap (1.24 eV) of this lead-free Cs2SnI6 film, the photoconductor with the Au/Cs2SnI6/Au structure exhibits the broadband photodetector that operates not only in the visible light region but also in the near-IR region. These strategies provide insights and novel approaches to the development of photosensitive nanoscale materials that are highly stable all-inorganic perovskite under ambient conditions. 할라이드 페로브스카이트는 긴 엑시톤 확산길이와 높은 흡광계수 및 밴드갭을 조절 가능한 특징들로 인하여 태양전지 및 광검출기 분야에서 차세대 광 흡수제로써 상당한 주목을 받고 있다. 이러한 할라이드 페로브스카이트에 대한 연구는, 최근 가시광 전 영역에 걸쳐 흡수성이 우수한 낮은 밴드갭(~1.72eV)을 갖는 흑색(알파상) 할로겐화물 페로브스카이트의 구현하기 위한 방법 및 장기 안정성을 확보하기 위한 연구가 집중적으로 수행되고 있다. 그럼에도 불구하고 대기조건 하에서 페로브스카이트의 장기 안정성 확보에 대한 추가적인 방법론적 개선에는 여전히 과제로 남아있다. 본 논문에서는 대기 조건 하에서 흑색상 페로브스카이트의 장기 안정성을 확보하기 위한 두 가지 공학적 전략을 제안하고 있으며, 이를 포토트랜지스터와 포토레지스터 소자에 각각 적용하여 광전 특성 및 장기 안정성을 평가하였다. 먼저, 다중 음이온 전략으로 브롬 이온을 요오드 위치에 부분적으로 치환시킨 α-CsPbIxBr3-x 페로브스카이트는 장기 안정성을 향상시키 위한 방법으로 제안하였다. CsPbI3 전구체 용액에 CsBr과 PbBr2를 첨가하여 I3 대신에 이중 음이온 IxBr3-x로 대체되어 CsPbIxBr3-x 필름이 대기중에서 장기적으로 안정적으로 흑생 상을 유지 하는 것을 확인하였다. 이 방법을 통하여 제작한 α-CsPbIxBr3-x와 IGZO 산화물 반도체를 이종 접합시킴으로써 포토트랜지스터를 제작하였고 광전기적 특성의 장기 안정성을 평가하였다. 또한, 이 연구를 통하여 α-CsPbIxBr3-x/IGZO 이종접합이 IGZO 본래의 photoillumination stress를 크게 완화시키는 것을 확인하였다. 또한, 페로브스카이트 전구체 용액에서 UV 경화 폴리에틸렌 글리콜 디메타크릴레이트(PEGDMA)를 통해 광대역 흡수 광학 특성을 가진 매우 안정적인 무연 나노미터 두께의 Cs2SnI6 필름을 제조하기 위한 전략을 제안하였다. UV 경화 PEGDMA는 CsI를 포착하며 상대적으로 높은 SnI2의 존재는 높은 결정도와 높은 수율을 갖는 나노 스케일 Cs2SnI6 필름의 형성에 상당한 영향을 미쳤다. PEGDMA의 산소 고독쌍 전자는 Cs+를 포착하고 PEGDMA 주변의 Cs2SnI6의 결정 성장을 크게 향상시킨다. 또한, 최적량의 PEGDMA(5wt%)는 UV 조사에 의해 폴리머 네트워크를 형성하여 주변 조건에서 1개월 동안 상전이 없이 Cs2SnI6 필름의 흑색 상을 유지하는 데 크게 기여하였다. 특히, 상당히 낮은 밴드갭(1.24 eV)을 가진 무연 Cs2SnI6 광전도체는 가시광선 영역뿐만 아니라 IR 영역까지 작동하는 광대역 광검출기 특성을 보였으며 대기 중에서도 장시간 안정적인 특성이 확인되었다. 이러한 제조 공법들은 무기 페로브스카이트의 장기 안정성 확보를 위한 감광성 재료의 개발에 대한 통찰력과 새로운 접근 방식을 제공 할 수 있다.

Long-Term Stability of Wechsler Intelligence Scale for Children-Fifth Edition (WISC-V) with Special Education Students

Purisic, Seniha ProQuest Dissertations & Theses Fairleigh Dickinso 2022 해외박사(DDOD)

소속기관이 구독 중이 아닌 경우 오후 4시부터 익일 오전 9시까지 원문보기가 가능합니다.

Intelligence tests are important tools in psychology and education; however, the stability of scores and use of intelligence tests has always been surrounded by some controversy. Since its conception in 1905, intelligence testing has become one of the most prominent tasks for school psychologists worldwide. Today’s need for frequent use of intelligence testing is immeasurable. In schools alone, intelligence testing helps predict student’s academic attainment, helps determine the need for special education services, guides program placement, and helps identify students’ strengths and weaknesses to drive the direction of the intervention. Other important predictors include job performance, type of occupation, income level, and social status. Thus, stability of intelligence scores is critical as they are often used to guide diagnostic and program-based decisions. Although intelligence test scores are assumed to be relatively stable in typically developing individuals of average intelligence, it is unclear if the same long-term test-retest stability is evidenced with students receiving special education services. While there are many studies that have explored stability of IQ scores with different instruments, there has been limited research investigating long-term stability of intelligence scores using Wechsler Intelligence Scale for Children-Fifth Edition (WISC-V), bearing in mind that the WISC-V is a fairly new measure. Therefore, the following study examines the long-term stability of WISC-V scores with children that have previously been identified as meeting a special education classification.

Long-term light stability of photovoltaic device based lead iodide perovskite light absorber

진영운 Sungkyunkwan university 2017 국내석사

지구상의 화석 에너지원 사용 빈도가 전체적으로 급증하고, 그 결과 자원 보유량의 한계와 환경 오염의 문제에 직면하게 된 현 시점의 인류는 차세대 에너지원 개발에 큰 주목을 할 수 밖에 없는 실정에 이르렀다. 사람들은 지속적인 개발이 가능한 차세대 에너지원들 중 무한한 양과 환경오염을 전혀 일으키지 않는 태양에너지에 대해 끊임없이 연구를 진행해왔는데, 최근 페로브스카이트 광흡수체(CH3NH3PbI3)를 기반으로 한 태양전지의 개발은 값싼 재료 및 공정과 높은 광전 변환 효율을 자랑하며 전 세계적으로 주목받았다. 이 태양전지는 현재 20% 이상의 광전 변환 효율에 도달했으며 앞으로도 계속 높아질 것으로 기대된다. 하지만 이 물질은 광원에 취약하기 때문에 태양전지에 실질적으로 적용하기가 힘들다. 특히 자외선 파장의 광원을 장시간 광흡수체 물질에 조사할 시 쉽게 광분해반응이 일어나 소자의 구동이 어려워진다. 그러나 주 흡광 영역인 가시광 파장대의 광원을 광흡수체 물질에 조사했을 때, 물질 또는 소자가 어떤 화학적 반응이나 특별한 변화를 일으키는 지에 대해 아직 연구가 거의 진행되지 않았다. 게다가 실질적인 태양광이 조사될 때, 어떤 메커니즘이 분해의 주요 원인이 되는 지에 대한 연구도 크게 진행된 바가 없다. 따라서 이번 연구에서는 페로브스카이트 흡광체에 400nm~700nm 사이의 다양한 광원을 수분이 없는 환경에서 장시간 조사해 일어나는 변화를 FE-SEM Image와 UV-Vis Absorption, PL Intensity 등을 통하여 분석하고, 파장의 차이에 의해 나타나는 변화를 비교하였다. 그 결과, 470nm 파장대의 광원에서 광흡수체의 Morphology가 변하고 광학적 특성이 변화하였다. 이는 가시광 영역의 광원에서도 페로브스카이트 광흡수체의 분해를 야기한다는 것을 시사하며, 광흡수체에 유해한 광원의 발견을 통해 위 결과는 페로브스카이트 태양전지의 장기-광안정성 연구에 매우 중요한 연구 결과라는 것을 반증하였다. 그리고 페로브스카이트 광흡수체 아래에 각기 다른 전자 전달층 및 정공 전달층을 증착하여 계면을 형성하였고, Solar Simulator에서 발생되는 모의 태양광을 지속적으로 조사시킬 때 나타나는 변화를 비교하였다. 그 결과, 전달층의 정공 추출률에 따라 흡광체의 광분해도가 달라지는 것을 발견하였다. 따라서 본 연구에서는 페로브스카이트와 계면을 형성하는 정공 전달층 소재에 변화를 주어 흡광체의 광안정성을 개선하였다. 본 연구에서는 광원의 종류나 전도층의 종류에 따라 페로브스카이트 광흡수체 소재와 페로브스카이트 기반 태양전지에서 나타나는 안정성을 비교하였다. 이는 페로브스카이트 기반 태양전지의 장기-광안정성을 확보하기 위한 새로운 연구 방향을 분명히 제시할 수 있고, 페로브스카이트 소재가 사용되는 다양한 분야에 적용될 수 있다. Research of solar cells based on the perovskite light absorber (Methylammonium lead iodide, CH3NH3PbI3) has attracted worldwide attention due to their low cost of materials and processes and high power conversion efficiency. Perovskite solar cell has reached a power conversion efficiency of more than 20% and is expected to continuously increase in the future. However, since the perovskite light absorber is easily decomposed to light exposure, it is difficult to practically apply on the solar cells. Particularly, when a light source of ultraviolet wavelength is irradiated to perovskite light absorber for a long time, photodegradation of perovskite is rapidly proceeded and power conversion efficiency of the solar cell is drastically declined. Research on the light stability of perovskite to ultraviolet light is proceeding actively. Simultaneously, when the perovskite light absorber is irradiated with a light source of the visible light which is the main light absorption range of perovskite, there has been little research into whether a perovskite causes any chemical reaction or any particular morphological change. In addition, there has been little research into the chemical mechanism of decomposition in sunlight exposure. (AM 1.5G) In this study, we analyzed FE-SEM image, UV-Visible absorption spectra, and photoluminescence intensity, which are caused by long-time irradiation of various visible light sources between 400nm and 700nm of wavelength in a perovskite. As a result, the morphology of the perovskite changed in the light exposure of 470 nm wavelength and the optical properties were changed. This demonstrates that the visible light causes the photo-decomposition of the perovskite light absorber. Because of the study of a detrimental effect due to wavelength of visible light on the perovskite, the results are very important for the long-term stability study of the perovskite solar cell. Then, the electron transport layer and the hole transport layer were deposited under the perovskite light absorber to form the interface, and the changes of the perovskite were compared in light exposure generated in the solar simulator. As a result, it has been found that the photo-decomposition rate of the perovskite varies depending on the hole extraction rate of the hole transfer layer. Therefore, the light stability of perovskite was improved via deposition of the hole transfer material with excellent hole extraction rate.

(The) Study on high performance MR fluid with enhanced sedimentation stability

한상석 서울대학교 대학원 2020 국내박사

자기유변유체는 물 또는 비수계(실리콘 오일 등)의 유체에 자화 가능한 미세입자(철 마이크로 입자)를 분산시킨 현탁액으로서, 외부로부터 제공되는 강한 자기장에 따라 짧은 시간안에 탄성, 소성, 점도 같은 자기유변효과를 나타내는 유체를 말한다. 자기유변유체는 외부 자기장에 의해 유변효과를 조절할 수 있기 때문에 다양한 응용분야로의 적용 가능성에 대한 관심이 증가하고 있다. 그러나 자성입자와 현탁 유체와의 밀도 차에 의해 발생하는 침전현상으로 인해 자기유변유체의 실제적인 응용이 제한되고 있다. 본 연구에서는 자기유변유체의 거동을 예측하는 구성방정식을 제안하고, 침전 문제를 극복하기 위해 연자성 복합체로 구성된 자기유변유체를 조사한다. 재료 설계를 최적화하기 위해 필수적으로 광범위한 전단 속도에 걸친 흐름 동작을 설명하고 정적 항복 응력과 동적 항복 응력을 구분하여야 한다. 또한, 현탁액의 유전학적 특성과 변형률, 적용된 자기장 강도 및 구성과 같은 변수 사이의 관계에 대한 정확한 지식이 필요하다. 따라서, 자기유변유체의 흐름을 설명하기 위한 기존의 제안된 구성방정식을 분석하고 새로운 구성방정식을 제안한다. 새롭게 제안한 구성 방정식인 서-서 모델은 기존에 존재하는 구성방정식과 비교하여 유체의 흐름을 정확하게 예측하였고, 비교적 적은 실험 값을 바탕으로 자기유변유체의 흐름에 대한 정량적, 질적으로 정밀한 설명을 도출하였다. 침전 문제를 극복하기 위해 피커링 에멀전 중합을 및 초임계 이산화탄소를 이용한 발포공정의 이중 공정 처리를 통해 코어-쉘 구조의 발포 스타이렌 고분자-철 복합체를 합성하였다. 이중 공정 처리를 통해 복합체의 밀도가 현저히 떨어지고 장기 안정성이 향상되었다. 또한, 스타이렌이 코어 부분에 위치하여, 철 입자가 직접적인 접촉을 통해 높은 자력 특성을 얻었다. 코어-쉘 구조의 발포 스타이렌 고분자-철 복합체의 자력 특성이 상당한 수준을 보였음에도 불구하고, 스타이렌이 자력적으로 비활성화 물질이므로 순수한 철에 비해 자력 특성은 약화되었다. 따라서 자력적으로 비활성화 물질인 스타이렌을 제거하여 지지대가 없는 중공형상의 철 입자를 합성하였다. 그 결과, 코어-쉘 구조의 발포 스타이렌 고분자-철 복합체에 비해 중공형상의 철 입자는 밀도가 약간 상승하였으나 높은 자력특성을 보였고 장기 안정성이 유지되었다. 추가적으로 마이크로/나노 크기의 철 입자를 사용하여 피브릴 구조의 보강효과를 검증하였다. 입자 크기가 증가함에 따라 자기 포화 수준의 상승으로 자력특성이 개선되었다. 그러나, 나노 크기의 철 입자의 비율에 따라 자력특성과 자기 포화 현상의 역전현상이 관찰되었다. 이 현상은 마이크로 크기의 철 입자의 피브릴 구조를 형성시에 철 입자 사이의 공동때문이다. 마이크로 크기의 철 입자는 비교적 거친 피브릴 구조를 형성한다. 혼성 자기유변체는 마이크로 크기의 철 입자와는 다른 피브릴 구조를 형성한다. 나노 크기의 철 입자들이 마이크로 크기의 철 입자 사이의 공동을 채움으로 인해서 자력특성이 향상되었다. 마지막으로, 벌크형과 박리형의 센더스트를 이용하여 자기입자의 모양이 유변적 특성에 끼치는 영향을 조사하였다. 박리형 센더스트의 자구는 한 방향으로 정렬되어 있어 작은 감자율을 갖고, 이 특징은 저자기장에서 피브릴 구조로의 빠른 전환을 가능하게 한다. 또한, 박리형 센더스트의 높은 종횡비로 인한 항력계수는 장기 안정성을 향상시켰다. Magnetorheological (MR) fluids are typically consist of magnetic particles (Carbonyl Iron, Fe2O3, Fe3O4 and so on) in a magnetically insulating fluid (water, silicon oil and so on). When a magnetic field induces attractive interactions between the magnetic particles, these particles form a solid-like network of fibril shapes within a few milliseconds oriented along the direction of the magnetic field. Reverse transition occurs as soon as the magnetic field is switched off. These features lead to remarkable changes in the rheological properties of the fluid which shows wide potential applications such as dampers, brakes, shock observers, drug delivery, and robotics, etc and could be controlled by adjusting the strength of the magnetic field depending on applications. Despite substantial advanced in commercialization, MR fluids have long-term stability issues that significantly limit their usefulness and also need to be predicted the precise flow behavior. In this thesis, we propose the constitutive equation to predict the flow behavior of MR fluid and investigate a number of MR fluid composed of soft-magnetic composite particles to overcome the sedimentation drawback. Firstly, as modeling and analysis are essential to optimize material design, describe the flow behavior over a wide range of shear rate and distinguish between static yield stress and dynamic yield stress, the precise knowledge of the relationships between the suspension rheological properties and such variables as the deformation rate, the applied magnetic field strength, and the composition are required. So we re-analyze the constitutive equation proposed before to describe the MR fluids flow and propose new constitutive equation. The proposed Seo-Seo model predicted the flow behavior precisely compared to pre-exist constitutive model and also yielded a quantitatively and qualitatively precise description of MR fluid rheological behavior based on relatively few experimental measurements. To overcome sedimentation drawback, the core/shell structured Foamed polystyrene/Fe3O4 Particles were synthesized by applying a dual-step processing comprising pickering emulsion polymerization, subsequently by the foaming of polystyrene core using the supercritical carbon dioxide fluid foaming process. Through these processes, the density of composite was dropped significantly and the long-term stability was improved. As polystyrene located core part and magnetic particle contact directly, the magnetorheological properties of the Foamed polystyrene/Fe3O4 were considerable compared to pure Fe3O4. Even though the core/shell structured Foamed polystyrene/Fe3O4 showed considerable level, the magnetorheological properties got worsen because polystyrene is magnetically non-active. So, we synthesized hollow shape Fe3O4 particles without any magnetically non-active template. As a result, compared to the core/shell structured Foamed polystyrene/Fe3O4, the density of hollow shape Fe3O4 particles rise slightly and the magnetorheological properties reached outstanding level, and the long-term stability maintained. Also, the conformation of solid-like network of fibril shapes changes were investigated by using micro/nano size Fe3O4 particles to verify the reinforcement effect. As the particle size increases, the magnetorheological properties improve due to a rise of the magnetic saturation level. However, depending on the ratio of the nano size Fe3O4 particles, an overturning of the magnetorheological properties and the magnetic saturation was observed. This phenomenon is because of the cavity among the micro size Fe3O4 particles. The micro size Fe3O4 particles develops a relatively coarse solid-like network of fibril shapes. The chain conformation of a bidisperse MR fluid shows quite different from that of the micron size Fe3O4 particles-based fluids. The nano size Fe3O4 particles appear to fill in the cavity among the micro size Fe3O4 particles. As a result, this distinct conformation reinforced the magnetorheological properties. Finally, the shape effect of the magnetic particle on magnetorheological properties and sedimentation stability was investigated by using two types of sendust which are bulk and flake type. The flake type sendust has a small demagnetization factor because its domain orients one direction. This feature lead to extraordinary behavior which is a rapid transition to solid-like network at low magnetic field. Also, its high aspect ratio leads to a large drag coefficient which improve the long-term stability.

Efficient Heterojunction Light Harvesting Devices with Tailored Nanostructures

김정규 성균관대학교 일반대학원 2015 국내박사

Due to carbon dioxide emission and global warming, worldwide researches have been focused on developing clean and renewable energy. Among the several renewable energy sources, the solar energy is much useful than others because we can use the solar energy anywhere we live. Moreover, we can say the sun is the eternal source of energy and it has tremendous quantity of energy. The energy that can be absorbed on the surface of the earth for 1 hour is corresponding to that of world energy consumption for 1 year. In order to convert the solar energy into the energy we can easily use, the light harvesting devices have been extensively utilized. Therefore, my research have been focused on the efficient light harvesting devices with tailored nanostructures. In chapter 2, a novel cost- and time-effective vacuum-free process to fabricate bulk-heterojunction (BHJ) organic photovoltaics (OPVs) via layer-by-layer selective stamping transfer of all layers is described. By controlling the surface properties of polyurethane acrylate (PUA) stamping molds with ultraviolet−ozone (UVO) exposure, poly-(3,4- thylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS), BHJ layer, and metal cathode were uniformly transferred layer by layer onto each of the bottom layers. Among several interfaces between each layer, we found that the interface between the active layer and metal cathode is a critical factor in obtaining conventional device-like efficiency. To enhance the interfacial connectivity between the BHJ layer and metal cathode and increase electron extraction from the BHJ layer, a titanium oxide (TiOx) interlayer was introduced. Cell performance was optimized by controlling the concentration of TiOx solution. The poly(3-hexylthiophene-2,5-diyl)/[6,6]-phenyl-C61-butyric acid methyl ester (P3HT/PC60BM) BHJ device fabricated by transferring PEDOT/PSS, TiOx/active layer, and Al cathode showed 2.01% power conversion efficiency. This efficiency is not comparable to those of conventional OPVs, but our approach shows the possibility of fabricating OPVs via the layer-by-layer transfer method for the first time. Furthermore, significant improvements of Voc and FF in Sb2S3 quantum dot (QD)-based, solid-state heterojunction solar cells were achieved from the solid transfer of preformed PEDOT:PSS hole extraction layers. Despite the moderate optical properties of Sb2S3 QDs, the solid state QD solar cells suffer from poor power conversion efficiency (PCE) resulting from the disappointing Voc and the high series resistance since there is inefficient charge extraction from QDs to the metal top electrode. In order to improve the hole extraction performance, a significantly uniform PEDOT:PSS (poly(3,4-ethylenedioxythiophene) /poly(styrenesulfonate)) layer was transferred on the hole transport layer (P3HT, poly(3-hexylthiophene-2,5-diyl)) by using a simple solid-transfer method. In contrast with conventional spin-cast methods, the hydrophilic PEDOT:PSS layer was uniformly coated on the hydrophobic P3HT layer without any significant detriment to P3HT film properties. Due to improved contact surface for the Au top electrode and hole conductance resulting in significantly improved charge extraction, the power conversion efficiency was dramatically enhanced. Furthermore, the thickness of the PEDOT:PSS film was precisely optimized by layer-by-layer solid transfer, and thereby the PCE of the PEDOT:PSS solid-transfer device (30 nm) was improved by 25.7% in comparison to the PEDOT:PSS spin-cast device and by 76% in comparison to the PEDOT:PSS free device. In chapter 3, high efficient organic electronic devices with enhanced a long term stability by tailoring hole conducting PEDOT:PSS interlayer were described. At first, a novel concept where the inorganic ultrathin nanoclay (organo-treated montmorillonite) film is introduced as a buffer layer is exploited to enhance the performance and the stability of the devices. To precisely control the thickness of organoclay films within ~1 nm scale, which can be a critical length in organic electronics research, we employ the charge-controlled deposition technique wherein the molecular interactions between nanoclay sheets and charged polymeric surface (mixture of poly(3,4-ethylenedioxythiophene) and poly(styrenesulfonate)) are elaborately manipulated. As a result, significantly improved device performance is observed for the case with the 5 nm thick clay films as compared to those without buffer layer, wherein the enhanced charge transport and the suppression of performance decay over time can be obtained. Eventually, device lifetime is greatly enhanced and the performance is maintained while minimizing the occurrence of dark spots. This substantial improvement in environmental stability of the device can be attributed to the role of the ultrathin nanoclay film which prevents the active polymer layer from directly contacting the indium contaminated and strongly acidic PEDOT:PSS bottom layer. In addition, it also works as a barrier film against the oxygen and moisture. At second, polymer solar cells with enhanced initial cell performances and long-term stability were fabricated by performing a simple dry transfer of a hole extraction layer [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)] onto an indium tin oxide (ITO) substrate. Due to the very flat surface of the polyurethane acrylate/polycarbonate (PUA/PC) film, which was used as a mold and resembled the surface of the original substrate (silicon wafer), the transferred layer had a very smooth surface morphology, resulting in enhancement of the interfacial characteristics. The work function of the PEDOT:PSS layer and the morphology of bulkheterojunction (BHJ) layer were tuned by controlling the position of PSS enrichment in the PEDOT:PSS layer using the dry transfer. The power conversion efficiency of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b’]-dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl) carbonyl]-thieno[3,4-b]thiophenediyl]] : [6,6]-phenyl-C71-butric acid methyl ester (PTB7:PC71BM) BHJ device prepared by the dry transfer was 8.06 %, which was significantly higher than that of the spin-cast device (7.32 %). By avoiding direct contact between the ITO substrate and the PEDOT:PSS solution in the dry transfer system, etching and diffusion of indium in the ITO substrate were greatly reduced, thereby improving the stability. In chapter 4, organic optoelectronic devices by incorporating graphene quantum dots into polymer films with conjugated carbons were intensively studied. At first, polymer light emitting diodes (PLEDs) using quantum dots (QDs) as emissive materials have received much attention as promising components for next-generation displays. Despite their outstanding properties, toxic and hazardous nature of QDs is a serious impediment to their use in future eco-friendly opto-electronic device applications. Owing to the desires to develop new types of nano-material without health and environmental effects but with strong opto-electrical properties similar to QDs, graphene quantum dots (GQDs) have attracted great interest as promising luminophores. However, the origin of electroluminescence (EL) from GQDs incorporated PLEDs is unclear. Herein, we synthesized graphene oxide quantum dots (GOQDs) using a modified hydrothermal deoxidization method and characterized the PLED performance using GOQDs blended poly(N-vinyl carbazole) (PVK) as emissive layer. Simple device structure was used to reveal the origin of EL by excluding the contribution of and contamination from other layers (e.g., electron transporting and hole blocking layers). The energy transfer and interaction between the PVK host and GOQDs guest were investigated using steady-state PL, time-correlated single photon counting (TCSPC) and density functional theory (DFT) calculations. Experiments revealed that white EL emission from the PLED originated from the hybridized GOQD-PVK complex emission with the contributions from the individual GOQDs and PVK emissions. At second, high-efficiency bulk-heterojunction solar cells by incorporation of graphene quantum dots (GQDs) in polymer layers are described. GQDs have been considered as a novel material because their electronic and optoelectronic properties can be tuned by controlling the size and the functional groups of GQDs. Reduction-controlled GQDs were synthesized and it was conducted that their application to bulk heterojunction (BHJ) solar cells with enhanced power conversion efficiency (PCE). Three different types of GQDs; graphene oxide quantum dots (GOQDs), 5 h reduced GQDs, and 10 h reduced GQDs; were tested in BHJ solar cells, and the results indicate that GQDs play an important role in increasing optical absorptivity and charge carrier extraction of the BHJ solar cells. The enhanced optical absorptivity by rich functional groups in GOQDs increases short-circuit current, while the improved conductivity of reduced GQDs leads to the increase of fill factors. Thus, the reduction level of GQDs needs to be intermediate to balance the absorptivity and conductivity. Indeed, the partially reduced GQDs yielded the outstandingly improved PCE of 7.60% in BHJ devices compared to a reference device without GQDs (6.70%). Moreover, a GQD-incorporated organic photovoltaic device shows enhanced power conversion efficiency (PCE) of 8.67%, where the oxygen-related functionalization of GQDs enabled good dispersity in a PEDOT:PSS hole extraction layer, leading to significantly improved short circuit current (Jsc). To maximize the PCE of organic photovoltaic devices, hydrophobic GQDs that are hydrothermally reduced (rGQD) were additionally incorporated in a bulk-heterojunction layer, which is found to promote a synergistic effect with the GQD-incorporated hole extraction layer. In chapter 5, a conformal coating strategy with nanocarbon to enhance photoelectrochemical responses and the long-term stability of ZnO quantum dots is described. Strong anchoring bonds between a ZnO core and nanocarbon shell ameliorate the poor electrochemical stability of ZnO (such as photocorrosion) in liquid electrolyte. The conjugation of the graphene and C60 to the ZnO QDs leads to 71 % and 99 % quenching of the UV photoluminescence (PL) emission, respectively. Also, the decay time of the nanocomposites at UV wavelengths measured much faster than that for the reference of bare ZnO QDs. The moderate energy states and good charge conductance of the nanocarbons result in ultrafast charge transport from the ZnO core to the nanocarbon shell. Thereby, the ZnO core - nanocarbon shell quantum dots shows significantly improved light harvesting performance. The PEC cell test for water oxidation and conventional degradation test using organic dyes exhibited that the photoelectrochemical activities could be significantly improved. At 1.23 V (vs. RHE) in pH 6.9 electrolyte, 6 times enhanced photocurrent density was achieved by the conformal coating with C60 (0.235 mA/cm2 for ZnO-C60 photoanodes). In particular, the strong Zn-O-C bond structures on the ZnO surface prevented photoinduced holes from being consumed by the photocorrosion reaction of ZnO, thereby improving long-term stability.