The 298th MANA & the 129th ICYS Joint Seminar
Dr. Liwen Sang & Dr. Ryo Tamura
| Date | November 16, Friday |
| Time | 15:30-16:30 |
| Place | Auditorium, 1F, WPI - MANA Building, NAMIKI Site, NIMS |
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15:30-16:00
Thin-film photovoltaic cells based on III-Nitride semiconductors
The re-evaluation of bandgap of InN extends the fundamental bandgap energy of III-Nitrides from near infrared (InN at 0.65 eV) to deep-ultraviolet (AlN at 6.2 eV), which provides an almost perfect match to the solar spectrum, opening up an interesting opportunity for the high-efficiency photovoltaic application. The power conversion efficiency of a four-junction solar cell based on InGaN was expected to be more than 50% according to the balance modeling estimation. Additionally, InGaN alloys have the advantages of high drift velocity, high radiation resistance, large absorption coefficient, and high carrier mobility. These characteristics enable the InGaN solar cells to operate also in severe environments such as the desert or space, in which Si-based cells are degraded. However, due to the difficulty in high-quality In-rich InGaN and p-type doping, the development of InGaN solar cells are still in their early stage. In this presentation, we aim to improve the conversion efficiency of the InGaN solar cells by using a novel structure. A super-thin AlN layer is inserted between the intrinsic InGaN and p-InGaN in the InGaN solar cell structure to improve the photovoltaic properties. The conversion efficiency was doubled due to the novel structures. Temperature-dependent measurement was also performed to investigate the nature of the photovoltaic in the InGaN-based homojunction solar cells.
Speaker
Dr. Liwen Sang, ICYS-MANA Researcher, MANA, NIMS
Chair
Dr. Toyohiro Chikyow, MANA PI, NIMS
16:00-16:30
Control of Order of Phase Transition by Distortion Effect in Frustrated System
The phase transition occurs between phases having different symmetries, and anomalies of physical quantities appear at the phase transition point. According to anomaly in the free energy, phase transitions can be categorized into two types: first-order phase transition (first-derivation of free-energy is discontinuous) and second-order phase transition (second-derivation of free-energy is discontinuous or diverges). The nature of phase transitions has been exhaustively investigated by using theoretical magnetic models, and study of phase transitions is central topic in the statistical physics. In this talk, I will first easily introduce fundamental concepts of phase transition and frustration effect. Subsequently, I will explain to control the order of phase transition by an external field in frustrated spin systems. The nature of phase transitions is directly connected to the underlying symmetry of system. Thus, in order to change the order of phase transition, it is only necessary to change the underlying symmetry. In this talk, we focus on the lattice distortion effect as an external field. In the Heisenberg spin (three-component vector) system on equilateral triangular lattice with some competing interactions, the spiral-spin structure where the “threefold” symmetry is broken appears by frustration effect1, 2. Based on this symmetry, this model exhibits a “first-order” phase transition at finite temperature. Here, we introduce the uniaxial lattice distortion effect for this model. Then, the spiral-spin state where the “twofold” symmetry is broken appears. We find that the uniaxially distorted model exhibits a “second-order” phase transition3. In this way, the order of phase transition can be changed by the uniaxial lattice distortion effect in frustrated systems. The collaborators are Prof. Naoki Kawashima (Institute for Solid State Physics, University of Tokyo) and Dr. Shu Tanaka (Department of Chemistry, University of Tokyo).
References
- R. Tamura and N. Kawashima, J. Phys. Soc. Jpn. 77, 103002 (2008).
- R. Tamura and N. Kawashima, J. Phys. Soc. Jpn. 80, 074008 (2011).
- R. Tamura, S. Tanaka, and N. Kawashima, arXiv: 1209.2520.
Speaker
Dr. Ryo Tamura, ICYS-Sengen Researcher, NIMS
Chair
Dr. Hideaki Kitazawa, Group Leader, Neutron Scattering Group, Quantum Beam Unit, NIMS