32 research outputs found
Relaxing Interfacial Dipole by Inducing PEDOT:PSS in Colloidal Quantum Dot Photovoltaics to Improve Efficiency
No full textHighly efficient aligned AgInS2 quantum dots films conjugated with self-assembled block copolymer
No full text양자점을 활용한 소자의 밴드 구조 개질을 통한 소자 기능 개선에 관한 연구
No full text학위논문(박사) - 한국과학기술원 : 신소재공학과, 2020.2,[vii, 108 p. :]Quantum dot is 0 dimensional materials that the size of the material is under few nanometers. Through confining the size of the material, quantum confinement effect can manipulate the intrinsic properties of the material. Especially, quantization of electronic band structure of the quantum dot enabled to control light absorption spectrum and light emitting wavelength, making quantum dot one of the most promising materials for photo-electronic applications such as display and solar cell. However, low understanding and underdeveloped research progress of quantum dot applied devices failed highly efficient device fabrication. In this research, band engineering of the devices of solar cell and surface enhanced Raman scattering substrates are mostly developed for the performance improvement.
In quantum dot solar cell, due to the lower hole mobility and extracting distance of carriers in band structure, the hole-extraction is mainly considered as the performance limitation. To improve hole extraction, bilayered structure of hole transport materials are applied. One is large band-gap materials to block the leakage current and the other layer for relaxing interfacial dipole to optimize band structure. Through combination of bi-layered hole transport materials, the optimized band structure is obtained. In newly designed hole transport material, the device performance is enhanced by 47% compare to conventional hole transport material.
In Surface-enhanced Raman scattering (SERS) research, to maximize localized surface plasmonic resonance of the plasmonic structure, the additional photocurrent is injected to plasmonic structure by utilizing quantum dot photovoltaic substrate. Quantum dot substrate generate carrier from incident Raman laser beam, and photo-carriers are induced to transfer to gold nano-structure. The Raman signal is enhanced by 50 % to 250 % depending on detecting materials.한국과학기술원 :신소재공학과
전도 띠 정렬 개선을 통한 양자점 태양전지의 전하 추출 개선
No full text학위논문(석사) - 한국과학기술원 : 신소재공학과, 2016.8
,[v, 55 p. :]Due to limitation of resources and environmental issue of conventional energy sources, demand for renewable energy is increasing. Though the silicon photovoltaics are highly developed device, next generation photovoltaics are still in need in terms of utility and low processing cost. Next generation photovoltaics are solution processible, and should be fabricated in thin film, applicable for flexible device. Among them, colloidal quantum dot photovoltaics (CQDPV) have great potential in terms of efficiency and stability. The development of CQDPV started in 2010, and reached the best efficiency of 10.7% in 2015, and all inorganic passivated colloidal quantum dots (CQD) are very stable in air.1 But still further efficiency improvement is required to compete silicon photovoltaics.
To prevent schottky junction formation, hole transport materials are inserted between active layer and top metal. Many materials have been studied and so far, n-type transition metal oxide, such as molybdenum oxide and vanadium oxides are found to provide the best efficiency to the device. However, molybdenum oxide is poor hole conductor and unstable in air. Due to its poor hole conductivity, the thickness of molybdenum oxide has to be delicately controlledotherwise, the device loses its performance, and current density characteristic becomes poor. Also, oxidation of vacancy oxygen site of molybdenum oxide is very critical for cell performance. In 10 days, the photovoltaic loses its performance under 25% of its own.
In this research, we used PEDOT:PSS layer as hole transport material. Diluting PEDOT:PSS solution in methanol could successfully reserve active layer quality, and current density was increased around 20%. Additional alpha-sexithiophene layer could boost open circuit voltage by blocking leakage current. Α-6T has huge band gap of 2.2eV which forms huge potential barrier. Also could improve the stability. Due to hygroscopic PEDOT:PSS, colloidal quantum dot layer is degraded and results in poor device performance. But could block direct contact and improve stability and efficiency. Compared to Molybdenum oxide device, and PEDOT:PSS layer could improve the efficiency from 5.8% to 8.06%.한국과학기술원 :신소재공학과
4H-SiC Homoepitaxial Growth on Various SiC Substrates for Power Device Application
No full text학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2015. 8. 김형준.Silicon carbide (SiC) has been considered as a promising wide bandgap material for high power, high frequency, and high temperature devices owing to its high breakdown field (~3×106 V/cm), high thermal conductivity, high saturated electron drift velocity (~2×107 cm/s), and chemical stability. The commercialization of its large single-crystal wafer and excellent epitaxial growth technique provide a better candidate for application in high-power and high-frequency electronic devices compared with other wide-bandgap materials such as GaN, ZnO, and diamond. Among the many SiC polytypes, most of the recent work has focused on 4H-SiC, owing to its high saturated electron drift velocity and commercial availability. For growth of high quality SiC epitaxial layers, many techniques were tried such methods as molecular-beam epitaxy (MBE), liquid-phase epitaxy (LPE), and vapor-phase epitaxy (VPE), there are some remaining problems. To solve this problem, the author focuses on the homoepitaxial growth of a 4H-SiC epilayer by MOCVD using bis-trimethylsilylmethane (BTMSM, C7H20Si2). BTMSM is an organo-silicon source which has a non-toxic and non-flammable nature, thus giving it certain advantages in comparison with the normal process using silane (SiH4). Moreover, because BTMSM has a Si-C bonding structure, low temperature epitaxial growth is also possible.
Homoepitaxial growth of SiC epitaxial layers on various substrates including 4H-SiC 4°, 8° off-axis, on-axis Si-face substrates, and C-faces substrates was carried out at temperature ranging from 1240 to 1550 °C and carrier gas flow rates of 5-20 sccm for the BTMSM source.
In the case of homoepitaxial growth on various off-angle substrates, it was found that the structure perfection of SiC epilayers is improved with higher temperature and lower flow rate of BTMSM. This growth behavior can be explained by the step-controlled epitaxy model. At the nominally on-axis surface spiral growth occurs via micro-steps provided by screw dislocations intersecting the surface. When growth temperature became 1480 °C on the nominally on-axis, we conducted island growth, which possesses a number of blocks, but hexagonal shapes were sparsely appeared. In addition, epilayer began to have smooth surface in macroscopic view as well as a shape of continuous chain of hexagonal-shape, when the temperature reached 1550 °C. When epilayer grows at 1480 °C, 4H-SiC was grown to a hexagonal shape, and nearby blocks were composed of 3C-SiC. This means the application of BTMSM source enabled growth of 4H-SiC at a relatively lower temperature of 1480 °C, which is the same as the polytype of substrate. Moreover, we could identify that 4H-SiC could grow at 1550 °C, satisfying stability of 4H-SiC polytype for 100%. The whole area was grown to 4H-SiC epilayer, revealing two types of surface regions which are hexagonal chain-shape and smooth surface. The smooth surface could be obtained due to high dislocation density of the area when compared to the hexagonal chain region. Increasing the growth temperature helps to enhance the mobility of the adatoms on the surface, lowering the probability for a two dimensional nucleation of 3C.
Many researchers have attempted epitaxial growth using 4H-SiC Si-face substrates, but there are few studies that have investigated the epitaxial growth on the C-face. Therefore, it is essential to determine the dependence of the quality of epitaxial layers on substrate polarity. This study has its significance in that an organosilicon source material of BTMSM was used and the epitaxial growth on C-face 4H-SiC substrates was analyzed in detail, which has not been previously attempted by other groups. In situ H2 etching and homoepitaxial growth of 4H-SiC have been carried out on 4° off-axis Si-face and C-face 4H-SiC substrates by low-pressure CVD. H2 etching characteristics and epitaxial growth behaviors on two different polarities using a BTMSM were systematically analyzed and discussed. When the temperature of in situ H2 etching was 1500 °C, the Si-face and C-face showed macro step-bunching and some clusters, respectively, whereas both faces showed fairly good quality when treated at 1450 °C for 10 min. High-quality 4H-SiC epitaxial layers with less crystallographic defects and free of step-bunching were demonstrated on both Si-face and C-face substrates. The optimal growth temperature on the Si-face substrate was 1320-1440 °C with a BTMSM source flow rate of 5-10 sccm, while the growth temperature should be increased to 1500 °C on the C-face substrate with a lower source flow rate of 5 sccm. A mechanism for the observed generation of step-bunching and surface morphological defect on both substrates depending on the growth temperature and source flow rate was also proposed.Abstract………………………………………………………………...i
Contents………………...…………...………..………………………iv
List of Tables…………………………………………………………vii
List of Figures………………………………………………………..viii
1. Introduction……………………………….…………….……….1
1.1. Overview ……………………..…..……………………………....1
1.2. Dissertation Outline………………………………..……………..2
2. Literature Review……..………...……………....………….…..3
2.1. Properties of SiC….…………………………………...……..…...3
2.1.1. Phase Equilibrium and Polytypism……....……………..…...3
2.1.2. Physical properties of SiC and Its Potential Applications.…..8
2.1.2.1. Mechanical Properties……….............................…............8
2.1.2.2. Thermal Properties…………………….……………...…..8
2.1.2.3. Optical Properties........…….…………………………….12
2.1.2.4. Electrical Properties…………..…………………...….….12
2.1.2.5. Potential Applications of SiC…………………………….15
2.2. SiC Epitaxial Growth ………………………………………...…19
2.2.1. Chemical Vapor Deposition (CVD)........……………….….19
2.2.1.1. Principle of CVD process……….……...…………….….21
2.2.1.2. Heteroepitaxial Growth of 3C-SiC….…………….….….24
2.2.1.3. Homoepitaxial Growth of 3C-SiC.....……….…………...25
2.2.1.4. Homoepitaxial Growth of α-SiC…………......…………..26
2.2.1.5. Structural Properties of α-SiC……..……………………..33
2.2.1.6. Growth Equipment and Precursor Materials for CVD.......36
2.3. SiC Doping……………………………………………..……….38
2.3.1. Site-competition Epitaxy………………..……………….38
2.3.2. Diffusion of Impurities in SiC……………………………..39
2.3.3. Ion Implantation…………………………………………...40
2.4. Step Kinetics on Vicinal Surface…………...…………………...43
2.5. SiC Power Device Application..………………….……………..49
Bibliography……………..…………………………...……………...53
3. Experimental Procedure……….…………………………….60
3.1. CVD System…….........................…………...……………….…60
3.2. Procedure for the Growth of 4H-SiC Epitaxial Layer………….62
3.3. Characterization Method…………………..………….………...63
3.3.1. Thickness Evaluation………………..………......................63
3.3.2. Surface Morphology……………………...…………..……63
3.3.3. Structure and Defect Analysis …………..…………………63
3.3.4. Electrical Properties……………………………..…………64
3.4. Precursor for Homoepitxial Growth of 4H-SiC……..…............66
3.4.1. Estimation of BTMSM vapor pressure and actual flow rate..67
Bibliography……………..…………………………...……………...69
4. Results and Discussions………….……..…………………….70
4.1. Comparative Study of 4H-SiC Epitaxial Layers Grown on 4° off-axis Si- and C-face Substrates using BTMSM……………………70
4.1.1. Intoduction……………..…………………………………...70
4.1.2. In-situ Surface Preparation …………..……...………...........73
4.1.3. 4H-SiC Epitaxial Layers Grown on Si-face Substrates .…..76
4.1.4. 4H-SiC Epitaxial Layers Grown on C-face Substrates……84
4.1.5. Background doping concentration and the growth rate of two polar face substrates ……………………………………………..97
4.2. 4H-SiC Epitaxial Growth on SiC substrates with Various Off- angles using BTMSM……………..……………………………98
4.2.1. Introduction ………………………….…………..………...98
4.2.2. 4H-SiC Epitaxial Layers Grown on 8° off-axis Substrates..100
4.2.2.1. Morphological Characteristics…………………………..100
4.2.2.2. Structural Characteristics………………..……..………..105
4.2.3. 4H-SiC Epitaxial Layers Grown on 4° off-axis Substrates...111
4.2.3.1. Morphological Characteristics…………………………..111
4.2.3.2. Structural Characteristics………………..……..………..116
4.2.4. 4H-SiC Epitaxial Layers Grown on On-axis Substrates …..122
4.2.4.1. In-situ surface preparation………….…………........……...122
4.2.4.2. 4H-SiC Epitaxial Growth on On-axis Substrates..……....126
4.2.4.3 Growth Rate…………………..…………………….……140
Bibliography……………..…………………………...…………….142
5. Conclusions...………………………………….………………146
List of Publications……………………….……………………..149
Abstract (in Korean)……………………….…………………...156Docto
Economic Analysis of USN-Based Data Acquisition Systems in Tall Building Construction
No full textThe successful construction of tall buildings requires effective construction management based on various quantitative data. The recent development of ubiquitous sensor networks (USNs) enables massive amounts of data to be collected in real-time. However, the application of USN-based data acquisition systems to repetitive tasks on typical floors of tall buildings can be inefficient, because this may involve the repetitive reinstallation of sensors and the repositioning of data loggers and routers to enable continuous data transfer. To minimize this cumbersome work, a modified data acquisition method using reusable sensor nodes and mobile devices can be a useful solution. This study analyzes the economic aspects of the USN-based systems for concrete temperature monitoring by using the activity-based costing technique. The case study shows that the modified system can reduce the process cost by about 19%. It can also reduce the resource input time of management by about 55%, freeing up time for other management activities. Moreover, the cost benefits should scale up as projects increasingly require more measurement and monitoring. This study should facilitate the application of USN-based information management systems, particularly for tall building construction
Suppressing Interfacial Dipoles to Minimize Open‐Circuit Voltage Loss in Quantum Dot Photovoltaics
No full textThis is the peer reviewed version of the following article:
Lim, H., Kim, D., Choi, M. J., Sargent, E. H., Jung, Y. S., Kim, J. Y., Suppressing Interfacial Dipoles to Minimize Open‐Circuit Voltage Loss in Quantum Dot Photovoltaics. Adv. Energy Mater. 2019, 1901938. https://doi.org/10.1002/aenm.201901938,
which has been published in final form at https://onlinelibrary.wiley.com/doi/full/10.1002/adfm.201908200. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions.Quantum‐dot (QD) photovoltaics (PVs) offer promise as energy‐conversion devices; however, their open‐circuit‐voltage (VOC) deficit is excessively large. Previous work has identified factors related to the QD active layer that contribute to VOC loss, including sub‐bandgap trap states and polydispersity in QD films. This work focuses instead on layer interfaces, and reveals a critical source of VOC loss: electron leakage at the QD/hole‐transport layer (HTL) interface. Although large‐bandgap organic materials in HTL are potentially suited to minimizing leakage current, dipoles that form at an organic/metal interface impede control over optimal band alignments. To overcome the challenge, a bilayer HTL configuration, which consists of semiconducting alpha‐sexithiophene (α‐6T) and metallic poly(3,4‐ethylenedioxythiphene) polystyrene sulfonate (PEDOT:PSS), is introduced. The introduction of the PEDOT:PSS layer between α‐6T and Au electrode suppresses the formation of undesired interfacial dipoles and a Schottky barrier for holes, and the bilayer HTL provides a high electron barrier of 1.35 eV. Using bilayer HTLs enhances the VOC by 74 mV without compromising the JSC compared to conventional MoO3 control devices, leading to a best power conversion efficiency of 9.2% (>40% improvement relative to relevant controls). Wider applicability of the bilayer strategy is demonstrated by a similar structure based on shallow lowest‐unoccupied‐molecular‐orbital (LUMO) levelsThis work was supported by KIST Institutional Program (2E28271) and Creative Materials Discovery Program through the National Research Foundation of Korea (NRF-2016M3D1A1021140 and NRF-2016M3D1A1900035) and the Hydrogen Energy Innovation Technology Development Program of the National Research Foundation of Korea (NRF) funded by the Korean government (Ministry of Science and ICT (MSIT)) (No. 2019M3E6A1063674)
Development of an advanced composite system form for constructability improvement through a Design for Six Sigma process
No full textSystem form is widely used when constructing concrete buildings and structures because it has high productivity and good concrete casting quality compared with traditional hand-set form. However, from a worker’s perspective, system form is still very harsh to handle because of its heavy weight, noise generation, and use of releasing agent, and it also attenuates the productivity of system formwork. Therefore, this study proposes the use of an advanced composite material-based concrete form for workers using a Design for Six Sigma (DFSS) process to improve constructability of system formwork. User requirements are systematically reflected in the technical characteristics of concrete form, and innovative principles are scientifically organized through the DFSS process that mainly consists of quality function deployment and theory of creative problem-solving methods. The proposed composite form showed improved performance in deriving high-quality formwork and worker-friendly working conditions compared with previous system forms. Additionally, this study demonstrated how the DFSS will be a valuable tool for technology development and systematic decision-making in building construction
An Optimal Layout Model of Curved Panels for Using 3D Printing
No full textRecently, the application of 3D printing in the production of curved panels has increased due to the irregular shape of free-form buildings. In general, 3D printing based on additive manufacturing (AM) methods requires various supports that cause a waste of printing materials and an increase in production time. In this study, we proposed a method for printing a pair of panels that can hold each other through the minimal support connected between each panel. However, this printing method causes an additional non-productivity factor called the non-printing path for the nozzle to move between the pair of panels. Therefore, we also developed an optimal layout model that can minimize non-printing paths and used the genetic algorithm (GA) for its calculation. As a result of applying the optimization model proposed in this study through the case study, the non-printing path was reduced by 18.54% compared with that from the existing method, and the non-printing time was reduced by 34.41 h. The total production time, including non-printing time and printing time, was reduced by 3.89%, and the productivity was improved by 4.04%. The model proposed in this study is expected to minimize unproductive factors that occur in the process of manufacturing curved panels and reduce the energy consumption
Tuning Solute‐Redistribution Dynamics for Scalable Fabrication of Colloidal Quantum‐Dot Optoelectronics
No full textThis is the peer reviewed version of the following article:
Choi, M. J., Kim, Y., Lim, H., Alarousu, E., Adhikari, A., Shaheen, B. S., Kim, Y. H., Mohammed, O. F., Sargent, E. H., Kim, J. Y., Jung, Y. S., Tuning Solute‐Redistribution Dynamics for Scalable Fabrication of Colloidal Quantum‐Dot Optoelectronics. Adv. Mater. 2019, 31, 1805886. https://doi.org/10.1002/adma.201805886,
which has been published in final form at https://onlinelibrary.wiley.com/doi/full/10.1002/adma.201805886. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions.Solution-processed colloidal quantum dots (CQDs) are attractive materials for the realization of low-cost and efficient optoelectronic devices. Although impressive CQD-solar-cell performance has been achieved, the fabrication of CQD films is still limited to laboratory-scale small areas because of the complicated deposition of CQD inks. Large-area, uniform deposition of lead sulfide (PbS) CQD inks is successfully realized for photovoltaic device applications by engineering the solute redistribution of CQD droplets. It is shown experimentally and theoretically that the solute-redistribution dynamics of CQD droplets are highly dependent on the movement of the contact line and on the evaporation kinetics of the solvent. By lowering the friction constant of the contact line and increasing the evaporation rate of the droplets, a uniform deposition of CQD ink in length and width over large areas is realized. By utilizing a spray-coating process, large-area (up to 100 cm2 ) CQD films are fabricated with 3-7% thickness variation on various substrates including glass, indium tin oxide glass, and polyethylene terephthalate. Furthermore, scalable fabrication of CQD solar cells is demonstrated with 100 cm2 CQD films which exhibits a notably high efficiency of 8.10%.This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT). (NRF-2017R1A2B2009948
