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

다국어 초록 (Multilingual Abstract)

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

다국어 초록 (Multilingual Abstract)

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

국문 초록 (Abstract)

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

목차 (Table of Contents)

• Chapter1. Introduction 1
• 1.1 Background 1
• 1.1.1 Organic inorganic lead halide perovskite solar cell 1
• 1.1.2 Device structure and working mechanism of perovskite
• solar cell 4
• 1.1.3 Stability of organic-inorganic PSCs 8
• 1.2 Research objectives 10
• Chapter 2. Surface and grain boundary co-passivation using
• perfluorinated alkylammonium salt for long-term durable
• perovskite solar cells
• 13
• 2.1 Introduction 13
• 2.2 Experimental process 16
• 2.2.1 Materials 16
• 2.2.2 Perfluorinated alkylammonium salt synthesis 16
• 2.2.3 F7 device process 17
• 2.2.4 Characterization 18
• 2.3 Results and discussions 19
• 2.3.1 Properties of perovskite film 19
• 2.3.2 J-V characteristics of PSCs 33
• 2.3.3 Optoelectronic properties of PSCs 39
• 2.3.4. Long-term durability of PSCs 53
• 2.4 Conclusion 55
• Chapter 3. Surface defect passivation of SnO2 layer with
• liquid crystal monomer for improved air stability of
• perovskite solar cell
• 56
• 3.1 Introduction 56
• 3.2 Experimental section 58
• 3.2.1 RMS preparation and device fabrication 58
• 3.2.2 Characterization 61
• 3.3 Results and discussions 62
• 3.3.1 Effect of RMS passivation of perovskite film 62
• 3.3.2 Properties of RMS based thin film 65
• 3.3.3 Device characteristic of RMS perovskite solar cell 76
• 3.3.4 Operational stability of PSCs in air 85
• 3.4 Conclusions 89
• Chapter 4. Ultrathin metal conductors assisted by EGaIn
• nanoparticles for flexible perovskite solar
• cells
• 90
• 4.1 Introduction 90
• 4.2 Experimental methods 94
• 4.2.1 The fabrication of AgNWs-metal composite 94
• 4.2.2 Fabrication of ultra-thin AgNWs-metal conductor 94
• 4.2.3 Device fabrication based on ultra-thin AgNWs-metal
• conductor 95
• 4.3 Results and discussions 96
• 4.3.1 Solution strategies of drawbacks of
• AgNWs 96
• 4.3.2 Device characteristic based on AgNWs-metal
• composite 108
• 4.3.3 Properties of Ultra-thin metal composite conductor
• and device performance 112
• 4.4 Dry transfer process for mechanical junction for flexible
• solar cells 125
• 4.4.1 Dry transfer for perovskite solar cell 125
• 4.4.2 Transfer for metal composite 140
• 4.5 Conclusions 147
• Conclusions 148
• References 150