Ji Hye Kim

Finite-Width Magnetic Mirror Models of Mono and Dual Coils for Wireless Electric Vehicles

Improved magnetic mirror models (IM3) for mono and dual coils with a finite width and infinite permeability are proposed in this paper. By introducing a mirror current, which is located at the same distance from a source current but with a smaller magnitude than the source current, the magnetic flux density of the mono and dual coils can be determined in a closed form. The ratio of the mirror current and source current is identified as a function of the width of the core plate and the distance between the source current and core plate, as rigorously derived from finite-element method simulations. Applying the proposed IM3 to the mono and dual coils used for wireless electric vehicles, the magnetic flux density over an open core plate is analyzed and its maximum points on the plate are found, which is crucial in the design of the coils to avoid local magnetic saturation. Furthermore, the magnetic flux density when a pick-up core plate is positioned over a primary core plate is also analyzed by introducing successive mirror currents. The proposed magnetic mirror models were extensively verified by experiments as well as site tests, showing quite promising practical usefulness.

Alignment of energy levels at the ZnS/Cu(In,Ga)Se2 interface

Further understanding of the electronic structure at the ZnS/Cu(In,Ga)Se2 interface is necessary to enhance the electron injection across the interface in Cu(In,Ga)Se2 solar cells. The valence band structure and shallow core levels were investigated by ultraviolet photoelectron spectroscopy depth profile analysis with He II line excitation. ZnS film was grown by a chemical bath deposition on a Cu(In,Ga)Se2 absorber deposited by the co-evaporation of Cu, In, Ga, and Se elemental sources. The discontinuity of 2.0 eV in the valence band edge at the ZnS/Cu(In0.7Ga0.3)Se2 interface was directly determined. This type of valence band offset yields a spike conduction band alignment of 0.25 eV. The positions of the VBM and the Zn 3d core-level emission of the buffer underwent the substantial shifts of 0.36 eV and 0.64 eV to a lower binding energy levels during the etching process. The shifts are ascribed to the contribution of the band bending in the ZnS buffer layer and its graded chemical composition. This study is the first to determine the small conduction band offset at the interface formed by the chemical bath deposited ZnS layer and the Cu(In0.7Ga0.3)Se2 absorber. Our results also provide information toward the design optimization of the optoelectronic properties of the ZnS/Cu(In0.7Ga0.3)Se2 interface. To enhance the electron injection from Cu(In0.7Ga0.3)Se2 absorber to ZnS layer further lowering of the energy barrier is required. For this purpose, the bandgap of ZnS should be reduced by controlling the crystal structure and composition or its Fermi level should be upward shifted by appropriate doping.

Study of Band Structure at the Zn(S,O,OH)/Cu(In,Ga)Se2 Interface via Rapid Thermal Annealing and Their Effect on the Photovoltaic Properties

This study focused on understanding the mechanisms of the photovoltaic property changes in Zn(S,O,OH)/Cu(In,Ga)Se2 solar cells, which were fabricated via annealing, using reflection electron energy loss spectroscopy (REELS), ultraviolet photoelectron spectroscopy (UPS), low temperature photoluminescence (LTPL), and secondary ion mass spectroscopy (SIMS). A pinhole-free Zn(S,O,OH) buffer layer was grown on a CIGS absorber layer using the chemical bath deposition (CBD). When the Zn(S,O,OH) film was annealed until 200 °C, the Zn-OH bonds in the film decreased. The band gap value of the annealed film decreased and the valence band offset (VBO) value at the Zn(S,O,OH)/CIGS interface with the annealed film increased. Both results contribute to the conduction band offset (CBO) value at the Zn(S,O,OH)/CIGS interface and, in turn, yield a reduction in the energy barrier at the interface. As a result of the annealing, the short circuit current (JSC) and quantum efficiency (QE) values (400-600 nm) of the cell increased due to the improvement in the electron injection efficiency. However, when the Zn(S,O,OH) film was annealed at 300 °C, the cell efficiency declined sharply due to the QE loss in the long wavelength region (800-1100 nm). The SIMS analysis demonstrated that the Cu content in the CIGS bulk decreased and the Cu element also diffused into CIGS/Mo interface. Through LTPL analysis, it was seen that the considerable drop of the Cu content in the CIGS bulk induced a 1.15 eV PL peak, which was associated with the transition from a deep donor defect to degrade the quality of the CIGS bulk. Accordingly, the series resistance (RS) and efficiency of the cell increased.

Sn(O, S) 2 thin films by chemical bath deposition for Cd-free CIGS thin film solar cells

The buffer layers of tin compound semiconductor were deposited on the Cu(In, Ga)Se2 (CIGS) substrates by chemical bath deposition. Sn(O, S)2 films were grown in an alkaline ammonia solution by reaction of tin(IV) chloride with thiourea at the bath temperature of 70oC. The smooth and conformal coverage films were achieved on the CIGS substrates with thickness of 20nm. The morphological and chemical properties were evaluated using scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). On the other hand, layer thickness of 90nm could be grown in an acidic solution with sodium sulfide (Na2S) within 60min. When we applied the Sn compound thin films as the alternative buffers in thin film CIGS solar cells, the efficiency of 9.4% small area cells were achieved. This result provided the potential for the Sn(O, S)2 films to be used as buffer or window layers in solar cell devices.

Annealing and In Interlayer Effects on the Photovoltaic Properties of CBD-In 2 S 3 /CIGS Solar Cells

In this study, chemical bath deposited (CBD) indium sulfide buffer layers were investigated as a possible substitution for the cadmium sulfide buffer layer in CIGS thin film solar cells. The performance of the /CIGS solar cell dramatically improved when the films were annealed at in inert gas after the buffer layer was grown on the CIGS film. The thickness of the indium sulfide buffer layer was 80 nm, but decreased to 60 nm after annealing. From the X-ray photoelectron spectroscopy it was found that the chemical composition of the layer changed to indium oxide and indium sulfide from the as-deposited indium hydroxide and sulfate states. Furthermore, the overall atomic concentration of the oxygen in the buffer layer decreased because deoxidation occurred during annealing. In addition, an In-thin layer was inserted between the indium sulfide buffer and CIGS in order to modify the /CIGS interface. The /CIGS solar cell with the In interlayer showed improved photovoltaic properties in the and FF values. Furthermore, the /CIGS solar cells showed higher quantum efficiency in the short wavelength region. However, the quantum efficiency in the long wavelength region was still poor due to the thick buffer layer.

Preparation of β-Cu (In,Ga) 3 Se 5 thin films for wide band gap absorber for top cell in CIGS tandem structure

Polycrystalline Cu<inf>x</inf>(In,Ga)<inf>y</inf>Se<inf>z</inf> films were deposited on Mo coated soda-lime glass substrate by three-stage co-evaporation process. Cu content x can be controlled by deposition times of each stage. The presence of β-Cu(In,Ga)<inf>3</inf>Se<inf>5</inf> phase in films was confirmed by X-ray Diffraction and Auger Electron Spectroscopy when the x decreased below 0.5. The grain size became smaller as the x decreased. The absorption edge moved to shorter wavelength and the optical transmittance of long wavelength noticeably increased in β-Cu(In,Ga)<inf>3</inf>Se<inf>5</inf> system comparing the conventional Cu(In,Ga)Se<inf>2</inf>. Its optical band gap was 1.49eV. The CdS/Cu(In<inf>0.3</inf>Ga<inf>0.7</inf>)<inf>3</inf>Se<inf>5</inf> solar cell showed the efficiency of 8.09% with an active area of 0.44cm². High transmittance and band gap are desirable to be a light absorber for top cell, but further effort is necessary to improve cell efficiency for the top cell application in CIGS tandem solar cells.