Studies on nanostructured transition and post-transition metal compound electrodes for photoelectrochemical cells

Jin Soo Kang 서울대학교 대학원 2015 국내박사

Photoelectrochemical cells are electrochemical cells that generally comprise semiconductor photoelectrode, electrocatalytic counter electrode, and electrolyte. There are two types of photoelectrochemical cells according to the function of the electrolyte; (i) photovoltaic cell with regenerative redox electrolyte and (ii) photosynthetic cell with sacrificial electrolye. Mesoscopic sensitized solar cells and photoelectrochemical water splitting cells are the typical examples of each kind, respectively. In this thesis, nanostructured electrodes composed of transition and post-transition metal compound for those cells were investigated. The main issues in the development of photoelectrodes, which are mainly metal oxide films, are improvements of light harvesting and charge collection. In contrast, replacement of high-cost and rare noble metal, typically platinum, with an economic and earth-abundant material has been the main objective of the research works on counter electrodes. Therefore, in this study, nanostructured metal oxides with intrinsically fast charge transport property and state-of-the-art nanomaterials with enlarged surface area were utilized as photoelectrodes. Also, nanostructured low-cost carbide and nitride materials with facile preparation methods were employed as electrocatalytic counter electrodes. Besides the discussions on quatitative improvements, in-depth physicochemical analyses for characterization of materials and photoelectrochemical cells were performed. After brief explanations of history, backgrounds, and previous research works on photoelectrochemical cells in chapter 1, in chapter 2 and 3, nanostructured metal oxide photoelectrodes with high carrier mobility are discussed. Mesoporous ZnO nanowire arrays with large surface area were synthesized by electrochemical anodization in a mild condition, and highly uniform SnO2 nanochannels were prepared by ultrasonic-assisted anodic oxidation. These nanostructured electrodes were employed as photoanode in quasi-solid dye-sensitized solar cells (DSCs), and fair energy conversion efficiencies were obtained, with further improvements by atomic layer deposition of TiO2 shells. In the next two chapters, strategies for efficient light harvesting by increased surface area were discussed. In chpater 4, TiO2 coated wrinkled silica nanoparticles were utilized as scattering centers in photoanode of DSCs. Superior performances to the DSCs with conventional sphere-shaped TiO2 scatterers were achieved due to the enlarged surface area, and futher observations on the relationship between spectral scattering properties and interwrinkle distances were made. In chapter 5, electrochemically anodized titanium and iron foams prepared by freeze-casting were used as photoanodes of DSCs and photoelectrochemical water splitting cells, respectively. By the anodic oxidation processes, one-dimensional TiO2 nanoatube arrays were formed on the titanium foam surface, and vertically aligned two-dimensional iron oxide nanoflakse were generated on the surface of the iron foam. These multidimensional structures had following three advantages; (i) large surface area for light harvesting, (ii) low-dimensionally confined semiconductor structure for enhanced charge transport, and (iii) three-dimensionally extended current collector with very low resistnace. Based on these properties, large photocurrent densities were obtained in both DSCs and iron oxide based photoelectrochemical water splitting cells. In chapter 6, nanostructured tungsten carbide synthesized by electrochemical anodization followed by heat treatment in carbon monoxide atmosphere was used as a counter electrode for DSCs employing cobalt bipyridyl redox electrolyte. The transformation of oxide into carbide was successfully and completely done due to the amorphous nature of anodic oxide materials. Moreover, superior electrocatalytic activity and photovoltaic performances were oberseved, which were attributed to the well known platinum-like electronic structure and nanoporous morphology. In chapter 7 and 8, nanostructured nickel nitride and cobalt nitride were fabricated by reactive sputtering of nickel and cobalt in nitrogen atmosphere, respectively. By physicochemical charaterizations based on electron microscopy and X-ray analyses, the films were addressed to Ni2N and CoN with cauliflower-like morphologies. When these electrodes were used as counter electrodes of quantum dot-sensitized solar cells (QDSCs), both electrodes displayed superior performances to Pt. In addition, the energy conversion efficiency of DSC employing cobalt nitride counter electrode was comparable to that with platinum. Furthermore, in QDSCs, CoN exhibited higher stability than the state-of-the-art Cu2S counter. Photocurrent densities of QDSC with CoN counter electrode exceeded that with Cu2S within 20 min, though the initial performance was higher in QDSC employing Cu2S.

Electrochemical Synthesis of Organic and Inorganic Electrode Materials for Photoelectrochemical Cells

김진 서울대학교 대학원 2018 국내박사

The depletion of fossil fuels and various environmental issues have led to growing needs in sustainable and clean energy sources. Among renewable energy sources, solar energy has been regarded as a promising alternative to fossil fuels primarily because it can be used infinitely. Photoelectrochemical cells are powerful devices for converting solar energy into electric energy and/or solar fuels. Therefore, most of the research into photoelectrochemical cells has focused on improving cell efficiency. However, commercialization of photoelectrochemical cells has been hindered by complex fabrication processes and the high price of materials. In this thesis, a variety of organic and inorganic materials were prepared using a simple and reproducible electrochemical process and their performances as low-cost electrode materials for two types of photoelectrochemical cells; dye-senstizied solar cells and photoelectrochemical water splitting cells was investigated. Chapter 1 consists of an introduction into photoelectrochemical cells. In addition, background information and recent issues with dye-sensitized solar cells and photoelectrochemical water splitting cells, which are representative photovoltaic cells and photosynthetic cells, respectively, are introduced. Chapters 2-4 discuss the fabrication and application of electrocatalytic counter electrodes from common, inexpensive transition metal carbides. These electrodes were fabricated using an electrochemical process and applied as electrocatalytic counter electrodes for photoelectrochemical cells. Chapter 2 discusses the preparation of nanostructured amorphous iron oxides by electrochemical anodization, and successfully transformed nanostructured iron carbide/carbon composite by post heat treatment in a CO atmosphere. Superior catalytic property of nanostructured iron carbide/carbon composite to Pt in cobalt bipyridine redox electrolyte was obtained and this led to an increase in power conversion efficiency in dye-sensitized solar cells when used as an electrocatalytic counter electrode for dye-sensitized solar cells Chapter 3 focuses on using electrochemically synthesized nanoporous tungsten carbide as a reduction electrocatalyst for photoelectrochemical cells, namely dye sensitized solar cells and photoelectrochemical water splitting cells. High-crystalline and interconnected nanoporous tungsten carbide was successfully synthesized by electrochemical anodization followed by post heat treatment. It exhibited high catalytic activity for reduction of cobalt bipyridine redox electrolyte, and higher energy conversion efficiency was achieved when tungsten carbide was substituted Pt as the counter electrode for dye-sensitized solar cells. In addition, tungsten carbide exhibited decent activity as catalyst for hydrogen evolution reaction in photoelectrochemical water splitting cells. Chapter 4 discusses the preparation and application of nanoporous molybdenum carbide as an electrocatalyt for hydrogen evolution reaction. The electrocatalyst, which is prepared by a top-down electrochemical process, can also be used as electrocatalytic counter electrode in photoelectrochemical water splitting cells. During electrochemical anodization, the exfoliation along the (110) planes was discovered, which had never been previously reported. After thermal annealing under a carbon-bearing gas, interconnected nanoporous molybdenum carbide with an extremely thin carbon shell was formed. Nanoporous molybdenum carbide exhibited high activity for hydrogen evolution reaction and excellent durability with a negligible performance drop even after 3000 cycles of the accelerated durability test (ADT). It was confirmed that the surface of electrocatalysts was oxidized after ADT test via various electron and x-ray analysis, and it was suggested that the carbon shells serve as a mechanical barrier against the oxidative degradation of electrocatalysts that accompanies volume expansion. In chapter 5, poly(3,4-ethylenedioxythiophene) (PEDOT) conducting polymer was synthesized by electropolymerization and was applied as a counter electrode in bifacial dye-sensitized solar cells, which utilize incident light from both front- and back-side illumination. Highly uniform and transparent PEDOT film were deposited with few tens of nanometer thickness. By optimizing the electrocatalytic and absorbing properties of the counter electrode, high efficiency was obtained under both front- and back-illumination conditions. Especially, unprecedented power conversion efficiency among bifacial DSCs under back-illumination, which was also over 85% of that obtained under front-illumination. In chapter 6, mesoscopic nickel oxide synthesis by electrochemical anodization was demonstrated for the first time. The anodic nickel oxide had mesoporous structures with 10 nm-sized nanoparticles. Due to it’s high surface area, this structure was found to be favorable toward application as a photocathode in p-type dye-sensitized solar cells. When it was employed as a photocathode, comparable performance to conventional nickel oxide electrodes was obtained. These results confirmed that the synthesized nickel oxide had a smaller particle size than the conventional nickel oxide electrode, and thus had a high probability of electron recombination, but had a high light harvesting efficiency due to a large amount of dye adsorption. This tradeoff resulted in comparable performance of anodic nickel oxide in p-type dye-sensitized solar cells.

Development of functional structure for photoelectrochemical performance

Sucheol Ju 고려대학교 대학원 2024 국내박사

광전기화학 소자는 반도체 물질이 태양에너지를 흡수하여 여기된 전하들을 이용하여 물을 산소와 수소를 분해하여 수소에너지원을 얻는 친환경에너지 기술이다. 친환경적으로 이상적이나, 상용화되기에는 효율이 낮아 촉매, 도핑, 구조화 등 다양한 연구들이 진행되고 있다. 이 논문에서는 가장 간단하고 강력한 구조화 전략을 사용하여 다기능 구조를 갖는 고효율 광전기화학 소자의 제조공정을 제안하였다. 다양한 공정법에 대한 연구는 다양한 구조 소자 제작을 가능하게 한다. 그렇기에 광전기학학 소자 물질 중 가장 유망한 바나듐산 비스무트를 스퍼터링과 전자빔 증착 공정을 이용하여 합성 및 나노구조체 제작을 진행하였다. 더 나아가 구조화된 투명전극 제작을 통해 광전소자에 적합하며, 효율을 상승시켜 주는 투명전극을 개발하였다. 위의 연구 결과들을 종합하여 다기능 구조를 제작하여 광전기화학 소자의 효율을 향상 및 효과에 대한 분석을 진행하였다. As energy consumption increases with the development of the industry, environmental pollution and global warming are accelerating due to the increase in fossil fuel consumption. Eco-friendly renewable energy is a way to solve these environmental problems while also solving the problem of finite resources. The photoelectrochemical water splitting is a technology that decomposes water using solar energy to obtain a hydrogen energy source and is a zero-CO2 emission green hydrogen technology that does not emit carbon dioxide. Recently, PEC-PSC (photoelectrochemical cells-perovskite solar cells) tandem cells have been studied, which is an advanced form of photoelectrochemical device. It is manufactured in a tandem form with a perovskite solar cell and decomposes water using only solar energy. Transparent electrodes are essential for this PEC-PSC tandem cell type. Although photoelectrochemical water splitting technology has the advantage of being the most ideal eco-friendly energy, it has the limitation of low efficiency yet to be commercialized. To solve these problems, various studies are being conducted on catalysts, doping, pattering, heterostructure, etc. In this work, we proposed various manufacturing processes for high-performance hierarchical structured photoelectrochemical devices using a simple and powerful structuring strategy. The most notable point is the fabrication of the structured FTO (fluorine-doped tin oxide). The structured FTO is a transparent and inexpensive electrode that has the effect of a pattern, so it is an electrode suitable for the energy fields, especially the PEC-PSC tandem cell. To fabricate the structured FTO, direct printing as a patterning technology and mist-CVD as a deposition technology were used. In addition, we studied the fabrication methods of light-absorbing materials such as BiVO4, WO3, and Fe2O3 to fabricate on structured FTO. By combining the results of these studies, highly efficient photoelectrochemical devices with various hierarchical structures were fabricated. The structured FTO proposed in this study offers the potential to increase the efficiency of various photovoltaic devices as well as photoelectrochemical devices. In addition, the high-efficiency hierarchical photoelectrochemical device manufacturing method provides the possibility for commercialization of photoelectrochemical devices while solving the limitations of low efficiency.

Semiconductor based micropillar patterned FTO/BiVO4 photoanodes for enhanced photoelectrochemical performance

강호정 Graduate School, Korea University 2021 국내석사

To solve the energy depletion and environmental problems, the demand for alternative energy sources is increasing more than ever. Hydrogen energy is a renewable energy source that can be used in various fields with high potential. There are various methods to generate hydrogen energy. Among them, photoelectrochemical (PEC) water splitting is the cleanest method to generate hydrogen energy. Various studies were conducted to improve the efficiency of PEC cells, such as utilizing micro/nanostructures in photoanode. In this research, we introduced periodic micropillar patterned fluorine-doped tin oxide (FTO)/ BiVO4 structured PEC cell. The newly designed pec cell was fabricated by several steps. At first, the micropillar pattern was fabricated by the direct printing process. Then, FTO was deposited by spraypyrolysis, followed by the deposition of BiVO4 via sputtering. Finally, V solution annealing was conducted for photocatalytic effect. The micropillar patterns induce the light scattering effect, reducing the reflectance of the MP/FTO/BiVO4 PEC cell. In addition, FTO material with high conductivity improves the poor charge transfer efficiency of BiVO4 material. The large area of the micropillar pattern also increases the light absorption area of the PEC cell. Due to these factors, the absorption efficiency, charge transfer efficiency of the MP/FTO/BiVO4 PEC cell was highly increase compared to those of reference cells. The photocurrent density of MP/FTO/BiVO4 at 1.23VRHE was 2.49mA/cm2, which is 67.8% higher than the reference cell. (Flat FTO/BiVO4 cell). The results showed that the newly developed PEC Cell increased the efficiency of the BiVO4 absorption layer via a simple process.

Synthesis and Characterization of Nanostructured α-Fe2O3 for Photoelectrochemical Water Splitting

Aiming Mao 성균관대학교 일반대학원 2012 국내박사

제1장 (Chapter 1) 광전기분해 (photoelectrolysis)는 빛을 이용하여 전기분해 하는 것으로써 빛을 전류로 변환하고 그 전류를 이용하여 화학물질 (H2O, H2S, 등)을 수소기체와 같은 유용한 에너지 자원으로 전환시킨다.광전기화학 전지는 다양한 광전기분해 반응을 수행하는데 사용되며, 태양광 에너지를 흡수하고 물 분자를 분해하는 데 필요한 전압을 발생시키는 반도체 소자로 구성되어 있다. 광전기분해는 태양 에너지의 집광과 물의 전기분해 반응을 하나의 광전극에서 수행하며 이것은 수소 생산에 있어 가장 효율적이고 재생가능한 방법으로 사료된다.수소에너지는 화석연료와 같은 에너지 자원으로써 에너지 밀도가 높고 쉽게 운송 및 저장이 가능하지만 자원의 양에 제한이 없고 연소 생성물이 오염물질이나 기후 변화를 일으키는 물질이 배출되지 않는다. 이번 챕터에서는 물 분해에 사용되는 광전기화학(PEC) 전지의 기본 원리 및 산화철 (α-Fe2O3)과 나노 기술의 결합으로부터 오는 이점에 관해 소개될 것이다. 제2장 (Chapter 2) 분해 거동과 α-Fe2O3나노구조체 사이의 관계에 관해 연구하였다. 또한 다양한 1차원의 α-Fe2O3나노 구조체는 AAO 템플레이트 방법을 통해 합성하였고 그것의 광전기화학적 성질에 관해 조사하였다. 전착 공정을 통해 형성된 수직 성장한 α-Fe2O3나노막대와 나노 튜브 어레이의 합성 및 광전기화학적 특성에 관해 보고하였다. α-Fe2O3나노튜브는산화철나노로드에 비해 향상된 광전류 밀도, 음의 개시 전위 및 보다 나은 전하 운반체의 이동 등 훨씬 높은 광전기화학적활성도를 보였다. 게다가 금, 백금과 같이 가장 유명한 촉매로 알려진 귀금속 물질을 다양산α-Fe2O3나노구조체에 도입하였다. 금과 백금이 도입된 α-Fe2O3는 더욱 더 향상된 광전기화학적 특성을 보였다. 제3장 (Chapter 3) 이종접합 광전극은 효율을 향상시킬 수 있는 유망한 방법으로 알려져있다. α-Fe2O3나노입자가도핑된 TiO2나노튜브어레이와 메조기공을 갖는 α-Fe2O3 및 WO3나노구조체 필름이 이 챕터에서 다루어진다. 두 가지의 이종접합 광전극은 자외선부터 가시광선 영역의 빛 스펙트럼을 흡수할 수 있을 뿐만 아니라 광여기된 전자의 분리 또한 증가시켜 준다. α-Fe2O3가도핑된 TiO2나노튜브어레이는 코팅되지 않은 TiO2나노튜브어레이에 비하여 훨씬 향상된 광전류를 보인다. α-Fe2O3/WO3나노구조체의 경우에는 AM1.5 빛 조사 하에서 WO3 필름에 비해 훨씬 높은 광전류를 보여주는데, 이것은 두 물질의 이종접합이 태양광의 흡수화 광생성된 전하 이동 능력을 향상시키는 역할을 한다는 것을 나타낸다.

1次元 티타늄 옥사이드의 合成 및 應用

맹헌혜 성균관대학교 일반대학원 2012 국내박사

Titanium dioxide (TiO2) is one of the most widely studied semiconductors due to their excellent optical, photocatalystic and photovoltaic properties, biocompatible and low cost. This work explores several new method for synthesis, characterization and application of TiO2 nanostructure, especially one-dimensional (1-D) nanotube, nanofiber and nanorod arrays with controllable morphology. Large area of self-organized, free standing anodic titanium oxide (ATO) nanotube membranes with clean surfaces were facilely prepared to desired lengths via electrochemical anodization of highly pure Ti sheets in an ethylene glycol electrolyte by using a recycling process. After annealing the anatase ATO nanotube arrays could be use to enhance performance of dye sensitized solar cell (DSSC) and photoelectrochemical cell (PEC). With a pre-deposited anatase layer, different size of well aligned rutile nanorods arrays could be grown on arbitrary substrate in a single hydrothermal reaction in Ti-HCl solution. Depending on the temperature and reaction time the diameter and length could be adjusted. After hydrothermal reaction for SiO2/Si substrate an anatase-brookite two layer film was formed, while, on the sodium lime glass only anatase layer was formed. The growth mechanism is correlated with Cl/Ti ratio, evaporation of HCl from solution and seeds crystal size. Finally, it was demonstrated that three-dimensional hierarchical nanostructures of TiO2, consisting of high density nanorods on nanofibers, could be synthesized by the combination of electrospinning, thermal annealing and hydrothermal methods. The growth mechanisms of these rutile rods were discussed on the basis of (i) surface roughness of the anatase nanofibers, (ii) presence of the twin planes of anatase layer with structure similar to rutile phase and (iii) Cl/Ti ratio in the solution. The applicability of three-dimensional hierarchical TiO2 as photocatalyst was discussed. 이산화 타이타늄은 뛰어난 광학 특성과 인체에 무해하며, 저렴한 가격으로 광촉매와 광전지로 많은 연구와 응용이 되고 있습니다. 본 논문은 이산화 타이타늄 의 나노구조인 1차원 나노 튜브, 나노 섬유 그리고 나노 막대를 모양이 제어가 가능한 새로운 방법으로 합성, 분석, 응용에 관련된 논문입니다. 전기 화학 방법으로 고순도 타이타늄 판과 에틸렌 글리콜 전해액을 이용하여, 대면적의 자가 결합, 깨끗한 표면의 단독으로 서 있는 ATO 나노 튜브 막이 합성 되었습니다. 특히, 재활용 공정을 이용하여, 시간과 전압을 조절하면 튜브의 두께와 직경을 조절할 수 있습니다. 열처리를 한 후, ATO 나노튜브집합체는 연료 전지와 광전기화학 전지에 응용 했습니다. 낮은 온도에서 FTO 유리 위에 이산화 타이타늄 나노 막대집합체를 합성하는 원리를 발견 하였습니다. 먼저 증착된 anatase 층을 통해서, 실리콘 웨이퍼와 일반 유리등 여려 기판 위에 다양한 크기의 이산화 타이타늄 나노 막대집합체가 Ti-HCl 용매에서 합성 되었습니다. 열수 반응의 온도와 시간에 따라 직경과 길이를 조절할 수 있습니다. 핵생성 원리에 따라서, 열수 반응후, SiO2/Si 기판위에 anatase-brookite 층이 합성되고, 일반 유리 위에는 anatase 층만 생깁니다. 마지막으로, electrospinning과 열수 반응의 기술을 합쳐서, 나노 막대를 나노 섬유를 위에 성장시켜 3차원 구조를 생성할 수 있었습니다. 막대와 섬유의 크기를 조절하여, 이산화 타이타늄의 밴드갭을 조절할 수 있습니다. 이렇게 새로운 3D 구조로 합성된 재료는 단상 재료 보다 더 좋은 광총매 특성을 얻었습니다.

Harnessing solar energy through perovskite for energy conversion systems

이준규 Graduate School, Yonsei University 2024 국내박사

과거 10 년간 유기-무기 혼합된 납 할라이드 페로브스카이트(LHP)는 유연성, 높은 광전력 효율성, 저비용, 빠른 전하 분리, 가변 밴드갭, 가벼움, 용이한 가공 등의 이점으로 인해 다음 세대 광흡수체로 큰 관심을 받았습니다. 이러한 이점을 활용하여 태양전지 및 광전기화학(PEC) 셀과 같은 기기에 LHP 를 적용하는 것이 관심사입니다. 첫째, 페로브스카이트는 유연성이 높아 차세대 웨어러블 전자 제품에 적용하는 연구가 많이 진행되고 있습니다. 태양전지의 유연성과 신축성은 웨어러블 기기의 실제 응용을 향상시키는 중요한 요소입니다. 그러나 폴리(3,4-에틸렌다이옥시티오펜):폴리(스티렌술포네이트) (PEDOT:PSS)는 유기나 페로브스카이트 태양전지의 HEL 로 흔히 사용되지만, PEDOT:PSS 의 신축성에 대한 내재적인 특성은 아직 확신할 수 없습니다. 본 연구에서는 높은 신축성과 기계적 안정성을 갖는 PEDOT:PSS 기반 박막과 이를 이용한 페로브스카이트 태양전지의 응용을 보고하고 있습니다. PEDOT:PSS 는 기계적 안정성이 합리적으로 보유하고 있지만, 전통적인 PEDOT:PSS HELs 는 임계 적인 인장 변형에서 전도도가 현저히 감소하고 형태적 붕괴가 나타난 것으로 나타났습니다. 이 문제를 해결하고 보다 유연한 태양전지를 구현하기 위해서는, PEDOT 기반 HELs 에 대한 도펀트로서 내재적으로 신축성이 있으며 첨가물이 없고 물에 용해되는 PSS 기반 기능성 공중 합체를 개발할 필요가 있습니다. 따라서 우리는 PSS 와 tetrafluoropropylmethacrylate (TFPMA)로 이루어진 화학적으로 결합된 공중합체인 P(SS-co-TFPMA)를 합성했고, 이를 poly(ethyleneglycol) methyl ether methacrylate (PEGMA)와 그래프트-공중합으로 변환하여 PEDOT HEL 을 위한 P(SS-co-TFPMA)-g-PEGMA 도핑제를 형성했습니다. PEDOT:P(SS-co-TFPMA)-g-PEGMA (PEDOT:PTP) 공중합체 용액은 우수한 균일성과 높은 상 안정성을 가지고 있으며, 개발된 HEL 필름은 탁월한 신축성을 나타냅니다. 300% 신장 후에도 PEDOT:PTP 필름은 초기 전도도의 80% 이상을 유지하며, 이는 전통적인 PEDOT:PSS 필름이 300%의 변형 후에 전도성을 완전히 잃는 것과 대조됩니다. 더불어 PEDOT:PTP 가 포함된 유연한 페로브스카이트 태양전지는 미개척된 셀과 비교하여 기계적 안정성이 향상되어 7mm 반경에서 1500 번의 굴곡 주기 후에도 초기 변환 효율의 92%를 유지합니다. 이로써 유연하고 내구성 있는 태양전지의 개발이 가능해져, 재생 에너지 산업에 상당한 영향을 미칠 수 있습니다. 둘째로, LHP 기반의 PEC 수분해 연구는 수소 생산의 효율적인 방법을 개발하는 목적으로 큰 발전을 이루었습니다. 수소는 무한한 가능성을 제공할 수 있는 지속 가능한 연료로, 더 효율적인 태양광 수소 생산을 위해 잘 알려진 LHP 광흡수체와 경제적인 수분해 촉매를 결합하여 두 가지 유형의 단 일체 LHP 기반 PEC 장치를 개발하였습니다. 이러한 장치는 외부 바이어스 없이 수소 발생 반응(HER)을 위한 광전극과 산소 발생 반응(OER)을 위한 광양극 역할을 수행하여 효율적인 전체 수분해를 이루어 냅니다. 우리는 잘 알려진 광흡수성 LHP(Lead-Halide Perovskite)와 비용 효과적인 수분해 촉매를 결합하고, 수소 발생 반응(HER) 및 산소 발생 반응(OER)을 위한 광전극 및 광양극으로 작용하는 두 가지 유형의 단일체 LHP 기반 PEC 장치를 소개합니다. 이 두 가지 단일체 LHP 기반 광전극의 통합을 통해 10.64%의 외부 바이어스 태양-수소 (STH) 전환 효율과 8.65 mA cm-2의 광전류 밀도를 달성하였습니다. In the past decade, organic-inorganic lead halide perovskite (LHP) has drawn much attention as next generation photo-absorber because of its flexibility, high photovoltaic efficiency, low cost, fast charge separation, tunable bandgap light weight, and facile processing. Therefore, utilizing the LHP into devices such as solar cells and photoelectrochemical (PEC) cell are of interest. Firstly, perovskite have high flexibility, much research on applying them to wearable electronics came to rise in interest. Flexibility and stretchability of solar cells are crucial factors for enhancing their real-life application for wearable devices. Even though poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been conventionally employed as a hole extraction layer (HEL) in flexible organic or perovskite solar cells, the inherent nature of PEDOT:PSS for stretchability has yet to be convinced. Here we report a highly stretchable and mechanically stable PEDOT:PSS-based thin film and its application for perovskite solar cells. PEDOT:PSS is a commonly used conductive polymer in inverted type hole extraction layers due to its high optical transparency and ease of processing at low temperatures. However, it has several drawbacks that need to be addressed, such as its low work function, strong acidity of PSS, hygroscopic nature, and low conductivity. Various methods such as doping, copolymerization, and post-treatment have been employed to improve its properties. While PEDOT:PSS is considered to have reasonable mechanical stability, conventional PEDOT:PSS HELs have shown a significant decrease in conductivity and morphological breakdown under critical tensile strains. To address this issue and achieve more flexible solar cells, there is a need to develop intrinsically stretchable, additive-free, and water-soluble PSS-based functional copolymers as dopants for PEDOT-based HELs. The use of these copolymers could help maintain the conductive nature of PEDOT-based HELs while providing enhanced mechanical properties. Therefore, we synthesized a chemically linked copolymer, P(SS-co-TFPMA), consisting of PSS and tetrafluoropropylmethacrylate (TFPMA), followed by graft-copolymerization with poly(ethylene glycol) methyl ether methacrylate (PEGMA) to form a P(SS-co-TFPMA)-g-PEGMA dopant for the PEDOT HEL. The PEDOT:P(SS-co-TFPMA)-g-PEGMA (PEDOT:PTP) copolymer solution has excellent homogeneity and high phase stability and its developed HEL film exhibits outstanding stretching capability. After stretching of 300%, PEDOT:PTP films sustain conductivity of over 80% of its original conductivity whereas the conventional PEDOT:PSS films completely lose their conductivity after the strain of 300 %. In addition, the PEDOT:PTP incorporated flexible perovskite solar cells exhibited improved mechanical stability compared with the unassisted cells, retaining 92% of the initial power conversion efficiency after 1500 bending cycles at a 7 mm radius. This could lead to the development of more flexible and durable solar cells, which could have a significant impact on the renewable energy industry. Secondly, research into PEC water splitting based on LHPs has advanced significantly with the goal of more efficient solar hydrogen production since hydrogen is a promising sustainable fuel that has the potential to provide boundless opportunities for the future. Researchers have combined a well-known LHP photo-absorber with cost-effective water splitting catalysts to develop two types of monolithic LHP-based PEC devices. These devices act as a photocathode and a photoanode for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively, leading to efficient overall water splitting without any external bias. we unite a well-known photo absorbing LHP with cost-effective water splitting catalysts, and we introduce two types of monolithic LHP-based PEC devices that act as a photocathode and a photoanode for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), leading to efficient unbiased overall water splitting. Through the integration of these two monolithic LHP-based photoelectrodes, an unbiased solar-to-hydrogen (STH) conversion efficiency of 10.64% and a photocurrent density of 8.65 mA cm-2 are achieved.

비스무트 기반 반도체를 통한 광촉매적 에너지 전환-실험적 리뷰와 물질 합성 및 조사 : Development of Bi based semiconductor for photocatalytic energy conversion – experimental review for material synthesis & analysis

김진현 포항공과대학교 일반대학원 환경공학부 2017 국내박사

Photoelectrochemical cell for solar energy conversion is rapidly raising research subject as one of the renewable energy technology. Similarity with photosynthesis of nature utilizing sun’s energy provoked numerous researches for more than half of century. While now there are specific requirements for this technology to be practical like photovoltaic cell – cost balance of 1 kg solar hydrogen/10 $ and efficiency over 10 % (10 mW/cm2 from 100 mW/cm2 of 1 sun condition), it takes the proper material to realize such goals. Yet there are mere few (less than 10) reports claiming ideal reaction of complete infra independent photocatalysis to harvest solar energy and produce hydrogen and oxygen, which are carbon neutral, completely contaminant free energy, photoelectrochemical cell itself proposed much larger window of various material application by half-cell approach. Thus, material only has to meet one of the reaction amongst reduction or oxidation reaction by harvesting solar energy. While there are also countless materials to be effective, there are only few meeting all requirements for practical application – 1) price of material, 2) inherent stability, 3) environmental inertness and 4) photon harvesting & utilization efficiency by having favorable electronic structure (preferably 1.2~2.0 eV as band gap energy (Eg)). Recent development of BiVO4 (Eg = 2.6,direct~2.4 eV,indirect) as photoanode (photocatalyst loaded conductive substrate, for PEC oxidation reaction) showed that mere metal oxide could achieve unpresented solar energy utilization efficiency of 7.0 %. Characteristics of BiVO4 revealed that such functionality was from BiVO4’s specific chemical and electronic property caused by Bi element. Therefore, other not well studied Bi based semiconductors would probably have similar characteristics that can be next generation material for photoelectrochemical cell. Indeed, literature survey showed a lot of potent candidates –BiFeO3 (2.2 eV), BiOI (2.1 eV), CuBi2O4 (1.8 eV) et al as visible light active photocatalysts. This research’s goal is to confirm those materials’s property that whether they are suitable for PEC energy conversion or not and preferably find the proper successor of BiVO4, which is currently the champion metal oxide for PEC cell application.

Improved efficiency of p-type Nickel Oxide photocathode in photoelectrochemical energy conversion cells

박민아 영남대학교 대학원 2014 국내석사

염료 감응 태양전지 혹은 양자점 감응 태양전지에 대한 대부분의 연구가 n-type 반도체를 위주로 이루어지고 있다. 그러나 백금을 상대전극으로 사용하는 기존 광음극에서 얻을 수 있는 개방전압은 0.7V 내외로 고작동전압을 요구하는 전자소자용 에너지원으로는 미흡하다. 백금 상대전극을 대체할 광양극을 적용하게 되면 전자-전공 쌍극자를 생성하고 개방전압을 상승시킴으로써 고효율 텐덤 광전기화학 태양전지를 구현할 수 있다. 현재 활발한 연구가 이루어지고 있는 n-type 반도체를 이용한 광음극과의 결합을 위해 p-type 반도체인 니켈 산화물을 이용한 광양극 연구 개발은 필수적이다. 본 연구에서는 p-type 반도체인 니켈 산화물을 기반으로 CdS 혹은 CdSe 양자점을 올리고이를 양자점 감응 태양전지와 photoelectrochemical water splitting cell에 적용하였다. 광양극의 전기적, 광학적, 구조적 특성을 연구함으로써 광양극에 적합한 광감응 물질을 설계 및 합성하여 효율을 향상시키고자 한다.

Charge transfer in semiconductor photoelectrode for solar water splitting

채상윤 Korea University 2017 국내박사

Hydrogen production by utilizing solar energy with semiconductor photoelectrodes has been intensively studied due to renewable energy requirement. When solar light is absorbed in a semiconductor, photo-generated charge carriers (electrons/holes) have to be separated and transported to the interfaces. Therefore, studying charge transportation behavior within the semiconductor photoelectrode is important to achieve efficient solar to hydrogen production. In this thesis, the phenomena of the charge transfer in semiconductor photoelectrodes are investigated to develop strategies for efficient charge transfer and water splitting systems. After introduction the general principles of photoelectrochemical (PEC) cell for solar to hydrogen production, in chapter 2, the morphology effects on the charge transfer are discussed through a heterojunction photoelectrode system whose cascade band structure can improve charge separation efficiency. As a model heterojunction, WO3/BiVO4 photoanodes demonstrate that charge transfer at the semiconductor/semiconductor interface is critical to determine the PEC activity. In the morphology aspect, top porous BiVO4 and bottom 1-dimensional (1D) nanorod WO3 show significantly improved photoactivity due to the high surface area of the porous structure and fast electron transportation along1D structure. In chapter 3, the effect of the surface state on PEC activity is discussed in the case of a novel photocathode material. CuInxGa1-xS2Se2-y (CIGS) photocathode is synthesized on a transparent substrate by a non-vacuum based process, and ZnS passive layer is demonstrated to suppress the surface recombination. Furthermore, the internal charge transfer issue is investigated by varying the light illumination directions proposing the importance of the charge transfer of minority carriers (electrons). In chapter 4, beyond the semiconductor photoelectrode itself, charge transfer between the photoelectrode and a dye-sensitized solar cell (DSSC) is discussed by designing a monolithic PEC/PV tandem cell. The integration of DSSC and the photoelectrode allows potential gaining by ~800 mV; finally, 1.34 % of solar to hydrogen conversion efficiency by spontaneous water splitting. In chapter 5, a redox couple is investigated for charge carrier shuttle in a DSSC-CIGS tandem solar cell, applied for solar water splitting cell. The charge transfer with minimal voltage loss is ideal in tandem cell because of the requirement for a large voltage output for overall water splitting reaction. Therefore, a cobalt based redox couple (Co(bpy)32+/3+ or Co(phen)32+/3+) is applied instead of the iodide redox couple, and the stability of DSSC-CIGS tandem cell is also substantially increased, only 5% decrease in power conversion efficiency due to the non-corrosive character of the cobalt redox couple. 4.65% of solar to hydrogen conversion efficiency is obtained when DSSC-CIGS tandem solar cell and electrolyzer is combined. The acquired knowledge of charge carrier transportation can be utilized for improving charge transfer property of various semiconductors, and ultimately can contribute to achieving high solar to hydrogen conversion efficiency with a PEC cell.