Eun-Cheol Lee

H-related defect complexes in HfO2: A model for positive fixed charge defects

Based on first-principles theoretical calculations, we investigate the hydrogenation effect on the defect properties of oxygen vacancies (VO) in HfO2. A defect complex of VO and H behaves as a shallow donor for a wide range of Fermi levels, with a positive charge state, and this complex is energetically stable against its dissociation into VO and H. We suggest that the VO–H complex is responsible for the formation of positive fixed charges, which neutralize negative fixed charges during the postannealing process of SiOx/HfO2 stack.

First-principles study of the compensation mechanism in N-doped ZnO

Abstract Based on first-principles electronic structure calculations for N-related and native point defects in Zinc Oxide (ZnO), we propose a mechanism for the compensation of N acceptors. As compared to a normal N2 source, the use of an active plasma N2 gas generally increases the N solubility limit, because the N chemical potential is enhanced. However, whenever a pure N source is used, N acceptors are greatly compensated by donor defects, which may explain the difficulty in achieving low-resistance p-type ZnO. Major compensating donors for N acceptors are found to be different at low and high N doping levels, and also depend on the type of N2 gas source.

Improvement of polycarbazole-based organic bulk-heterojunction solar cells using 1,8-diiodooctane

The use of 1,8-diiodooctane (DIO) as a solvent additive in poly[N-9′-heptadecanyl-2,7-carbazole-alt-5, 5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT)-based bulk-heterojunction solar cells increased the power conversion efficiency (PCE) by 20.9%. This improvement was due to the increase in the fill factor and open-circuit voltage (Voc), which compensated for the decrease in the short-circuit current (Jsc). DIO had the opposite effect, increasing Jsc and decreasing Voc, in the devices using other low-band gap polymers. The best sample in this study showed a PCE of 6.27% and Voc of 939u2009mV, indicating that the fabricated device is an efficient solar cell with a high Voc close to 1u2009V.