Yonsei University

About: Yonsei University is a education organization based out in Seoul, South Korea. It is known for research contribution in the topics: Population & Cancer. The organization has 50162 authors who have published 106172 publications receiving 2279044 citations. The organization is also known as: Yonsei.

What is moving in hybrid halide perovskite solar cells

Abstract: Organic-inorganic semiconductors, which adopt the perovskite crystal structure, have perturbed the landscape of contemporary photovoltaics research. In this Account, we discuss the internal motion of methylammonium lead iodide (CH$_3$NH$_3$PbI$_3$) and formamidinium lead iodide ([CH(NH$_2$)$_2$]PbI$_3$), covering: (i) molecular rotation-libration in the cuboctahedral cavity; (ii) drift and diffusion of large electron and hole polarons; (iii) transport of charged ionic defects. These processes give rise to a range of properties that are unconventional for photovoltaic materials, including frequency-dependent permittivity, low electron-hole recombination rates, and current-voltage hysteresis. Multi-scale simulations - drawing from electronic structure, ab initio molecular dynamic and Monte Carlo techniques - have been combined with neutron scattering and ultra-fast vibrational spectroscopy to qualify the nature and timescales of the motions. Recent experimental evidence and theoretical models for simultaneous electron transport and ion transport in these materials has been presented, suggesting they are mixed-mode conductors with similarities to metal oxide perovskites developed for battery and fuel cell applications. We expound on the implications of these effects for the photovoltaic action. The temporal behaviour found in hybrid perovskites introduces a sensitivity in materials characterisation to the time and length scale of the measurement, as well as the history of each sample. It also poses significant challenges for accurate materials and device simulations. Herein, we critically discuss the atomistic origin of the dynamic processes.

Effects of native defects on optical and electrical properties of ZnO prepared by pulsed laser deposition

Abstract: ZnO thin film has been deposited on a sapphire (001) at a temperature of 400°C using a pulsed laser deposition (PLD) with oxygen pressures of 50, 200, 300 and 500 mTorr. As the oxygen pressure for the thin film deposition increases, the crystallinity of the samples degrades as measured by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). In contrast, the photoluminescence (PL) intensity of ultra-violet (UV) luminescence increases as the oxygen pressure increases up to 300 mTorr. This is probably because the stoichiometry of oxygen-deficient ZnO film is improved by increasing oxygen pressure. According to the results from Hall measurements, the oxygen vacancy as a native donor defect in the ZnO decreases in concentration as the pressure increases. It is concluded that the UV luminescence intensity strongly depends on the stoichiometry in the ZnO film rather than the micro-structural quality of the crystal.

Bridging the spheres: political and personal conversation in public and private spaces

Abstract: For some theorists, talk about politics is infrequent, difficult, divisive, and, to be efficacious, must proceed according to special rules in protected spaces. We, however, examined ordinary political conversation in common spaces, asking Americans how freely and how often they talked about 9 political and personal topics at home, work, civic organizations, and elsewhere. Respondents felt free to talk about all topics. Most topics were talked about most frequently at home and at work, suggesting that the electronic cottage is wired to the public sphere. Political conversation in most loci correlated significantly with opinion quality and political participation, indicating that such conversation is a vital component of actual democratic practice, despite the emphasis given to argumentation and formal deliberation by some normative theorists.