광전기화학물질로 활용하기 위한 Metal-Organic Frameworks(MOFs)의 설계 및 개선에 관한 연구

Abstract

실리콘 반도체가 전자 산업의 핵심 소재로 전자재료의 주류를 이루고 있으나, 실리콘이 가지는 많은 이점에도 불구하고 가격, 전자/정공의 전달 속도, 물성 등에 대한 개선이 요구되고 있다. 이에 대한 대안으로 산화물이나 칼코지나이드 (Chalcogenide) 화합물 반도체가 주목받고 있는데, 유기화합물 반도체를 넘어서는 탁월한 반도체 특성을 가지고 있고, 그 종류가 다양하고, 쉽게 저가로 제조 될 수 있기에, 실리콘을 대체할 수 있는 물질 군으로 최근 많은 주목을 받아왔다. 그러나 화합물 반도체의 박막형성 과정이 간단치 않고 재현성에 많은 문제가 있어서 기본적으로 물질 제조과정부터 어려움에 처하고 있다. 더 나아가 나노선과 양자점을 형성 및 도핑하고, 그 특성을 연구하는 분야는 아직도 초기단계에 있다. 실리콘 반도체의 한계를 넘어서는 다양한 요구가 반도체, 태양전지, Thin film transistor, 디스플레이, 센서, 통신, 정보저장 등 분야에서 요구되어져 왔다. 다양한 박막과 유연성, 그리고 빠른 정공/전자의 전달 속도는 새로운 전자기기의 설계를 가능하게 하고 산업의 발달을 유도할 수 있을 것이다. 아직까지 전도성 고분자를 주류로 하는 유기반도체 화합물들이 일부 적용되기는 하지만 궁극적으로는 무기물 화합물 반도체를 기반으로 하는 산업체계가 형성 될 것으로 기대된다. 본 논문에서는 새로운 반도체 물질 후보군 중에 하나로 기대되고 있는 Metal Organic Frameworks(MOFs)물질을 다루어 새로운 반도체 물질의 특성을 개발하기 위한 연구 결과를 보고 하였다. Metal Organic Frameworks(MOFs)의 전기화학적 특성 연구 Chapter III에서는 MOFs의 전기화학적 특성에 대한 연구를 보고하였다. 그 중 대표적인 차세대 2차전지로 각광 받고 있는 슈퍼케페시터 전극 물질로 MOFs를 개발하였다. MOFs의 많은 장점 중에 한가지인 기공 크기와 표면적을 다양한 리간드를 이용하여 조절할 수 있었으며, 이런 MOFs의 기공 크기와 표면적이 증가함으로써 MOFs 전극의 슈퍼케페시터 효율도 증가하는 것을 확인하였다. 해당 연구는 처음으로 MOFs를 이용하여 케페시터의 전극으로 개발된 연구이며, 약 179.2F/g의 슈퍼케페시터 효율을 나타내는 것으로 확인 되었다. Metal Organic Frameworks(MOFs)을 이용한 광원물질 개발 Chapter III은 MOFs를 태양전지의 광원 물질로 개발하기 위한 연구 결과를 보고 하였다. 전이 금속과 유기 리간드가 코디네이션 되어있는 MOFs는 d-d transition으로 인해 중심 금속원자가 빛을 받았을 때 전자가 valance band에서 conduction band로 여기 상태로 존재하게 된다. 이런 특성을 이용하여 MOFs는 처음으로 태양전지의 광원물질로 사용되기 위해 Titanium dioxide(TiO2) 반도체 전극 위해 Layer-by-layer 방법을 이용하여 코팅되었으며, 액체 태양전지의 광원물질로 개발 되었다. 기본적으로 부도체 특성을 나타내는 MOFs의 전자의 이동 능력을 높여주기 위해 아이오딘 분자를 dopant로 사용하여 MOFs 안에서 전자 이동능력을 증가 시킬 수 있었다. 비록 최고의 효율은 약 1.12%로 그 효율이 기존에 보고되고 있는 태양전지의 효율보다 높지는 않지만 적합한 리간드의 개발과 새로운 dopant를 활용한다면 그 효율은 충분히 증가할 수 있을 것이라 기대한다. 새로운 반도체 물질 후보군 : Metal Organic Frameworks(MOFs) 기존에 보고된 Co3(NDC)2 MOFs를 이용하여 반도체 특성의 연구 결과를 보고 하였다. 반도체 특성을 확인하기 위하여 MOFs을 박막으로 제작하는 것이 무엇보다도 중요한 이슈 이다. 본 연구를 통해 Co3(NDC)2 MOFs는 처음으로 Layer-by-layer 코팅 방법과 Doctor blade 코팅 방법을 이용하여 유리기판과 ITO 기판위에 코팅되어 박막으로 제작되었으며, 이를 통해 전기화학적 특성 및 반도체 특성에 대한 연구를 진행하였다. 또한 기본적으로 부도체이며 다공성 물질인 MOFs에 아이오딘 분자를 dopant로 사용하여 MOFs 기공안에 도핑하였으며, 도핑 전과 후에 MOF의 전도성이 뚜렷하게 달라지는 것을 확인하였다. 또한 Hall Effect 분석을 통해 MOFs가 p형 반도체 물질이라는 것을 확인하였으며, 1.88×10-6 s/cm-1 의 전도성을 갖고 있는 것을 확인하였다. MOFs의 전도성을 증가시키기 위하여 새로운 리가드인 DAPV (di(3-aminopropyl)violgen)를 개발하였으며, Cobalt와의 코디네이션을 통해 새로운 MOFs를 합성하였다. 앞서 보고된 연구에서 DAPV는 electron acceptor물질로 보고 되었다.[1] 해당 연구를 통해 dopant가 없이도 MOFs 안에 전자가 중심 전위 금속인 Co2+ 에서 DAPV로 전자가 이동하는 것을 확인하였으며, 이를 통해 반도체 특성을 나타내는 연구 결과를 보고하였다. 기존에 dopant를 이용하였을 때 보다 더욱 높은 전도성을 나타내었으며, 고체 태양전지의 디바이스로 제작 되어 태양전지의 새로운 광원물질로 활용하는 연구결과를 보고하였다. |In my doctoral research, I was involved in many research projects. However, my research was mainly focused on designing and modification of metal-organic frameworks (MOFs) for electro-and photochemical applications. In this study, a thin layer of metal-organic-frameworks (MOFs) has been explored as a promising material for supercapacitors and photovoltaics. The doctor bladed Co-MOF film exhibited pseudocapacitor behavior with a good specific capacitance. On the other hand, a thin film of MOFs fabricated using a layer-by-layer (LbL) technique when investigated as a light-harvesting layer in TiO2-based solar cell, exhibited a promising result. Chapter I discusses about the historical background on research about MOFs, the potential applications of MOFs, the current state of research and existing issues on MOFs, and the objective of my doctoral research on MOFs. In addition, based on the objectives of the doctoral research, so far up to what extent of research achievement was received has been briefly pictured.

Chapter II discusses about the application of MOF thin films on an electrochemical device, viz., supercapacitors. For the first time, we developed a supaercapacitor electrode using doctor bladed thin film of Co-based MOFs. In this work, three dicarboxylic acids with different molecular lengths were used as organic linkers to manipulate the pore size and surface area of the frameworks. The pore size and BET surface area of the MOFs were determined and the influence of pore size and surface area on the supercapacitive performance of the MOFs was studied using cyclic voltammetry and chronopotentiometry. Among three MOFs investigated in the present study, the MOF with longer organic linker had larger pore, larger surface area and the MOF film at the electrode surface had a continuously interconnected leaflet like microstructure with less number of structural interfaces which provide the free path for charge transfer. This MOF electrode exhibited highest supercapacitive properties with 179.2 F g-1, 31.4 Wh kg-1, and 5.64 kW kg-1 of specific capacitance, energy density and power density, respectively. In the Chapter III, we discuss about the potential application of MOFs as a light harvesting active layer in solar cells. First, a thin layer of Cu-based metal−organic frameworks (MOFs, copper (II) benzene-1,3,5-tricarboxylate) is fabricated using a layer-by-layer technique, and the layer is investigated as a light-absorbing layer in TiO2-based solar cells. Iodine doping of the MOFs is performed to improve the conductivity and charge-transfer reaction across the TiO2/MOF/electrolyte interface. The HOMO and LUMO energy states of the MOF films are estimated to be −5.37 and −3.82 eV (vs vacuum), respectively, which show a well-matched energy cascade with TiO2. For the first time, a TiO2-based solar cell is fabricated successfully using iodine-doped Cu-MOFs as an active layer, demonstrating a cell performance with Jsc = 1.25 mA cm−2 and Eff = 0.26% under illumination of 1 sun radiation. In contrast, the cell with an undoped MOF layer exhibited Jsc = 0.05 mA cm−2 and Eff = 0.008%. Electrochemical impedance spectroscopy of the cells suggests that iodine doping significantly reduces the charge-transfer resistance. In order to further improve the charge transfer resistance to facilitate a facile charge transfer across the various interfaces among TiO2 nanoparticle and in between TiO2 and MOFs, multi-walled carbon nanotubes (MWCNTs) were introduced to the photoanodes. For this, TiO2 nanoparticle and multi-walled carbon nanotubes composite powder is prepared hydrothermally. After doctor blading the paste from the composite powder, the resulted composite film is sensitized with Cu-based metal-organic frameworks using a layer-by-layer deposition technique and the film is characterized using FE-SEM, EDX, XRD, UV/Visible spectrophotometry and photoluminescence spectroscopy. The influence of the carbon nanotubes in photovoltaic performance is studied by constructing a Gratzel cell with I3-/I- redox couple containing electrolyte. The results demonstrate that the introduction of carbon nanotubes accelerates the electron transfer, and thereby enhances the photovoltaic performance of the cell with a nearly 60% increment in power conversion efficiency. We further tried to improve the photovoltaic performance of the MOF-based solar cell by replacing the Cu –metal ions with Ru-metal ion in the frameworks as Ru-ions absorbs visible light more strongly than Cu-ions. For this, thin films of mesoporous TiO2 is sensitized with ruthenium based metal–organic frameworks using a layer-by-layer (LbL) technique, and the film is characterized using XRD, FE-SEM, UV/visible spectroscopy, cyclic voltammetry and photoluminance spectroscopy. Further, the feasibility of the MOF film as a sensitizer in a solar cell is investigated. The HOMO–LUMO level of the frameworks is estimated and is found to be suitable to allow the use of the frameworks as a sensitizer for TiO2. When TiO2 mesoporous film is sensitized with the LbL thin film of the frameworks and a Gratzel type liquid junction solar cell is constructed, it demonstrates the cell performance of Isc = 2.56 mA cm-2, Voc = 0.63 V, FF = 0.63, and Eff = 1.22%. Photoluminescence spectroscopy and electrochemical impedance spectroscopy show that iodine doping into the frameworks is essential to facilitate the photo-generated electron transfer from the frameworks to TiO2. Owing to the mater of cost of Ru-precursor, we tried to design a photovoltaic device using a commonly available and low cost metal ion precursor which absorbs visible light strongly. In this regards, Co (II) ions was chosen as it belongs to d7 electronic configuration. A thin film of Co-based metal organic frameworks (MOFs) is synthesized using layer-by-layer technique, and the film is employed successfully for the first time as a light harvesting active layer in a fully devised solar cell. Further, two different organic linkers with different molecular lengths were used to bridge the Co (II) ions in the frameworks. The Co3(NDC)2 frameworks which has higher porosity exhibited the capturing of higher amount of iodine molecule while doping as compared to the Co3(BDC)2 frameworks. As a result, the photoanode consisting of iodine doped Co3(NDC)2 frameworks demonstrated the higher photovoltaic performance with the power conversion efficiency of 1.12%. This efficacy is comparable to the above mentioned Ru-MOFs based solar cell. In the Chapter IV, we discussed about MOFs a new candidate for electronic materials. First, thin films of Co-based metal organic frameworks (MOFs) with improved electronic conductivity are synthesized using layer-by-layer and doctor blade coating techniques followed by iodine doping. The undoped and doped films are characterized using FE-SEM, EDX, UV/Visible spectroscopy, XPS, current-voltage measurement, PL spectroscopy, EIS, Hall Effect measurement, cyclic voltammetry and photovoltaic measurements. It is shown that the iodine doped MOF film behaves as a p-type semiconductor due to oxidative doping of iodine. In addition, it is demonstrated that upon illumination of the MOF film on an ITO substrate, electrons of cobalt undergo d-d transition followed by the metal to ligand charge transfer (MLCT) via iodine, and finally the excited electrons transfer to the ITO substrate. The charge transfer phenomenon and HOMO-LUMO measurement imply that the doped MOF film can be employed to construct a photovoltaic device to harvest solar radiations. Finally, we developed a water stable and intrinsically conductive MOFs. For this, Co(II)-precursor was chosen as metal ions due to the promising photovoltaic performance shown above by these metal ions in MOFs. These metal ions were bridged with amine type organic linker viz., di-(3-diaminopropyl)-viologen (DAPV). A thin film of Co-DAPV MOFs is fabricated on a non-conducting glass substrate using a layer-by-layer (LbL) technique. This film without any external doping exhibited p-type semiconductor character. The energy gap and the HOMO-LUMO positions are found to be suitable to use as sensitizer for TiO2. On the other hand, the hole mobility of this MOF film was found to be higher than that of P3HT hole transporting polymer. Therefore, the Co-DAPV MOFs were used in all solid state solar cells. When a thin film of gold is evaporated to the surface of 500 nm thick mesoporous TiO2 layer sensitized with LbL film of Co-DAPV MOFs, the device demonstrated a power conversion efficiency of 2.10%, which we believe as a great achievement in the field of MOFs as photovoltaic applications.; In my doctoral research, I was involved in many research projects. However, my research was mainly focused on designing and modification of metal-organic frameworks (MOFs) for electro-and photochemical applications. In this study, a thin layer of metal-organic-frameworks (MOFs) has been explored as a promising material for supercapacitors and photovoltaics. The doctor bladed Co-MOF film exhibited pseudocapacitor behavior with a good specific capacitance. On the other hand, a thin film of MOFs fabricated using a layer-by-layer (LbL) technique when investigated as a light-harvesting layer in TiO2-based solar cell, exhibited a promising result. Chapter I discusses about the historical background on research about MOFs, the potential applications of MOFs, the current state of research and existing issues on MOFs, and the objective of my doctoral research on MOFs. In addition, based on the objectives of the doctoral research, so far up to what extent of research achievement was received has been briefly pictured.

Chapter II discusses about the application of MOF thin films on an electrochemical device, viz., supercapacitors. For the first time, we developed a supaercapacitor electrode using doctor bladed thin film of Co-based MOFs. In this work, three dicarboxylic acids with different molecular lengths were used as organic linkers to manipulate the pore size and surface area of the frameworks. The pore size and BET surface area of the MOFs were determined and the influence of pore size and surface area on the supercapacitive performance of the MOFs was studied using cyclic voltammetry and chronopotentiometry. Among three MOFs investigated in the present study, the MOF with longer organic linker had larger pore, larger surface area and the MOF film at the electrode surface had a continuously interconnected leaflet like microstructure with less number of structural interfaces which provide the free path for charge transfer. This MOF electrode exhibited highest supercapacitive properties with 179.2 F g-1, 31.4 Wh kg-1, and 5.64 kW kg-1 of specific capacitance, energy density and power density, respectively. In the Chapter III, we discuss about the potential application of MOFs as a light harvesting active layer in solar cells. First, a thin layer of Cu-based metal−organic frameworks (MOFs, copper (II) benzene-1,3,5-tricarboxylate) is fabricated using a layer-by-layer technique, and the layer is investigated as a light-absorbing layer in TiO2-based solar cells. Iodine doping of the MOFs is performed to improve the conductivity and charge-transfer reaction across the TiO2/MOF/electrolyte interface. The HOMO and LUMO energy states of the MOF films are estimated to be −5.37 and −3.82 eV (vs vacuum), respectively, which show a well-matched energy cascade with TiO2. For the first time, a TiO2-based solar cell is fabricated successfully using iodine-doped Cu-MOFs as an active layer, demonstrating a cell performance with Jsc = 1.25 mA cm−2 and Eff = 0.26% under illumination of 1 sun radiation. In contrast, the cell with an undoped MOF layer exhibited Jsc = 0.05 mA cm−2 and Eff = 0.008%. Electrochemical impedance spectroscopy of the cells suggests that iodine doping significantly reduces the charge-transfer resistance. In order to further improve the charge transfer resistance to facilitate a facile charge transfer across the various interfaces among TiO2 nanoparticle and in between TiO2 and MOFs, multi-walled carbon nanotubes (MWCNTs) were introduced to the photoanodes. For this, TiO2 nanoparticle and multi-walled carbon nanotubes composite powder is prepared hydrothermally. After doctor blading the paste from the composite powder, the resulted composite film is sensitized with Cu-based metal-organic frameworks using a layer-by-layer deposition technique and the film is characterized using FE-SEM, EDX, XRD, UV/Visible spectrophotometry and photoluminescence spectroscopy. The influence of the carbon nanotubes in photovoltaic performance is studied by constructing a Gratzel cell with I3-/I- redox couple containing electrolyte. The results demonstrate that the introduction of carbon nanotubes accelerates the electron transfer, and thereby enhances the photovoltaic performance of the cell with a nearly 60% increment in power conversion efficiency. We further tried to improve the photovoltaic performance of the MOF-based solar cell by replacing the Cu –metal ions with Ru-metal ion in the frameworks as Ru-ions absorbs visible light more strongly than Cu-ions. For this, thin films of mesoporous TiO2 is sensitized with ruthenium based metal–organic frameworks using a layer-by-layer (LbL) technique, and the film is characterized using XRD, FE-SEM, UV/visible spectroscopy, cyclic voltammetry and photoluminance spectroscopy. Further, the feasibility of the MOF film as a sensitizer in a solar cell is investigated. The HOMO–LUMO level of the frameworks is estimated and is found to be suitable to allow the use of the frameworks as a sensitizer for TiO2. When TiO2 mesoporous film is sensitized with the LbL thin film of the frameworks and a Gratzel type liquid junction solar cell is constructed, it demonstrates the cell performance of Isc = 2.56 mA cm-2, Voc = 0.63 V, FF = 0.63, and Eff = 1.22%. Photoluminescence spectroscopy and electrochemical impedance spectroscopy show that iodine doping into the frameworks is essential to facilitate the photo-generated electron transfer from the frameworks to TiO2. Owing to the mater of cost of Ru-precursor, we tried to design a photovoltaic device using a commonly available and low cost metal ion precursor which absorbs visible light strongly. In this regards, Co (II) ions was chosen as it belongs to d7 electronic configuration. A thin film of Co-based metal organic frameworks (MOFs) is synthesized using layer-by-layer technique, and the film is employed successfully for the first time as a light harvesting active layer in a fully devised solar cell. Further, two different organic linkers with different molecular lengths were used to bridge the Co (II) ions in the frameworks. The Co3(NDC)2 frameworks which has higher porosity exhibited the capturing of higher amount of iodine molecule while doping as compared to the Co3(BDC)2 frameworks. As a result, the photoanode consisting of iodine doped Co3(NDC)2 frameworks demonstrated the higher photovoltaic performance with the power conversion efficiency of 1.12%. This efficacy is comparable to the above mentioned Ru-MOFs based solar cell. In the Chapter IV, we discussed about MOFs a new candidate for electronic materials. First, thin films of Co-based metal organic frameworks (MOFs) with improved electronic conductivity are synthesized using layer-by-layer and doctor blade coating techniques followed by iodine doping. The undoped and doped films are characterized using FE-SEM, EDX, UV/Visible spectroscopy, XPS, current-voltage measurement, PL spectroscopy, EIS, Hall Effect measurement, cyclic voltammetry and photovoltaic measurements. It is shown that the iodine doped MOF film behaves as a p-type semiconductor due to oxidative doping of iodine. In addition, it is demonstrated that upon illumination of the MOF film on an ITO substrate, electrons of cobalt undergo d-d transition followed by the metal to ligand charge transfer (MLCT) via iodine, and finally the excited electrons transfer to the ITO substrate. The charge transfer phenomenon and HOMO-LUMO measurement imply that the doped MOF film can be employed to construct a photovoltaic device to harvest solar radiations. Finally, we developed a water stable and intrinsically conductive MOFs. For this, Co(II)-precursor was chosen as metal ions due to the promising photovoltaic performance shown above by these metal ions in MOFs. These metal ions were bridged with amine type organic linker viz., di-(3-diaminopropyl)-viologen (DAPV). A thin film of Co-DAPV MOFs is fabricated on a non-conducting glass substrate using a layer-by-layer (LbL) technique. This film without any external doping exhibited p-type semiconductor character. The energy gap and the HOMO-LUMO positions are found to be suitable to use as sensitizer for TiO2. On the other hand, the hole mobility of this MOF film was found to be higher than that of P3HT hole transporting polymer. Therefore, the Co-DAPV MOFs were used in all solid state solar cells. When a thin film of gold is evaporated to the surface of 500 nm thick mesoporous TiO2 layer sensitized with LbL film of Co-DAPV MOFs, the device demonstrated a power conversion efficiency of 2.10%, which we believe as a great achievement in the field of MOFs as photovoltaic applications.