Sun-Ki Min

Achievement of 4.51% conversion efficiency using ZnO recombination barrier layer in TiO2 based dye-sensitized solar cells

The authors report the use of chemically deposited ZnO recombination barrier layer for improved efficiency of TiO2 based dye-sensitized solar cells. The ZnO layers of different thicknesses were deposited on spin coated porous TiO2. The presence of ZnO over TiO2 was confirmed by x-ray diffraction, electron dispersive x-ray analysis, and supported by x-ray photoelectron spectroscopy, proved inherent energy barrier between the porous TiO2 electrode and lithium iodide electrolyte. They found that TiO2 based dye-sensitized solar cell with 30nm ZnO layer thickness showed 4.51% efficiency due to the formation of efficient recombination barrier at electrode/electrolyte interface. Further increase in ZnO barrier thickness may leak the electrons injected from the dye due to its low electron effective mass of 0.2me.

Nanocrystalline CdS-water-soluble conjugated-polymers: High performance photoelectrochemical cells

Enhanced photoelectrochemical cell performance of nanocrystalline CdS-water-soluble conjugated-polymer sensitizers was demonstrated. The water-soluble polymers with quaternary pyridinium salts can be easily layered after dipping nanocrystalline CdS films in the aqueous polymer solution. The 2.37% energy conversion efficiency of CdS-poly(2-ethynyl-N-carboxypropylpyridiniumbromide) (LM 3) conjugated-polymer sensitizer was significantly higher than that of the bare (0.57%) and CdS-poly(2-ethynyl-N-aminopropylpyridiniumbromide) (LM 2) conjugated-polymer sensitizer (2.15%) under AM1.5 condition (80mW∕cm2).

Direct electronic transport through an ensemble of InAs self-assembled quantum dots

Electronic transport properties through an ensemble of InAs self-assembled quantum dots are reported. A metal–semiconductor–metal diode with self-assembled quantum dots has been fabricated. Clear staircases are observed in the current–voltage characteristics measured from the diode, and several peak structures are identified in the differential conductance. These conductance peaks are interpreted as due to resonant tunneling through the energy states of the self-assembled quantum dots.

In situ preparation of CdS nanoparticles imbedded in a polyelectrolyte multilayer for photocurrent generation

CdS nanoparticles (NPs) were prepared and imbedded in polyelectrolyte multilayers (PEMs) in situ and applied to the photocurrent generation. The PEM films of the poly(propylviologen)/poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) were prepared using layer-by-layer deposition. The Br− ions of the viologen groups were exchanged with S2− ion by dipping the PEMs films in a Na2S solution. Then, the films were dipped in a solution containing Cd2+ to give CdS NPs of less than 10nm in size. The CdS NP-PEM films form an efficient acceptor-sensitizer dyad system, which showed a good photocurrent of 70nA∕cm2 at 380nm for a 3.5 bilayer sample.

3.3-V baseband Gm-C filters for wireless transceiver applications

In this paper, we designed 3.3-V receiver/transmitter transconductance-C (Gm-C) filters for baseband analog front-end of wireless transceiver applications. The receiver filter consists of the 6/sup th/-order Gm-C filter including the 2/sup nd/-order group delay equalization. The filter on the transmitter path consists of the 5/sup th/-order Gm-C filter. Both filters are equipped with relevant input/output buffers and on-chip frequency tuning circuit. The filters are fabricated and verified through measurements in a 0.18-/spl mu/m CMOS technology.

Diffusion limiting mechanism in Si-delta doped GaAs grown by metalorganic chemical vapor deposition

Abstract Capacitance-voltage measurements at room temperatures have been carried out to investigate the spatial localization of Si in delta-doped GaAs grown by atmospheric pressure metalorganic chemical vapor deposition at several doping densities and temperatures. Even though the growth temperature is as high as 750°C, the capacitance-voltage profiles show a full-width at half-maximum value of carrier profiles and a Si diffusion coefficient are 44 A and 4 × 10 -17 cm 2 s -1 respectively. The value of the diffusion coefficient is an order of magnitude lower than that for molecular beam epitaxy-grown samples at the same temperature. This result will be discussed by using a diffusion limiting mechanism.

Magnetotransport and electric subband studies of Si-delta-doped Al0.27Ga0.73As/GaAs single quantum wells grown by metalorganic chemical vapour deposition

Shubnikov-de Haas and Van der Pauw Hall effect measurements at 1.5 K have been carried out to investigate the existence of a two-dimensional electron gas and to determine subband energies in an Si-delta-doped Al0.27Ga0.73As/GaAs single quantum well. Capacitance-voltage profiling measurements have been performed to characterize the properties of centre delta-doped Al0.27Ga0.73As/GaAs quantum wells. The subband energies, energy wavefunctions and subband carrier densities were determined using the experimental results and a self-consistent calculation taking into account the exchange correlation effects.

Growth of ZnO-Nanorod Grating on the Seed Grating Produced by Femtosecond Laser Pulses

In this research, we successfully fabricated ZnO-nanorod grating by carrying out femtosecond-laser modification of the seed layer. First, a Ag-doped ZnO seed layer was deposited on a glass substrate by dc/rf magnetron co-sputtering, in which rf and dc power sources were utilized for the ZnO and the Ag targets, respectively. Next, a seed grating was produced on the seed layer by using the two-beam interference of femtosecond-laser pulses. Finally, a ZnO-nanorod grating was grown on the seed grating by chemical bath deposition in an aqueous solution of Zn(NO3)2 and hexamethyltetramine. The scanning-electron-microscopy images indicate that the ZnO-nanorod grating can be regarded as a spatially periodic structure consisting of alternating bands of ZnO nanorods with relatively large and small diameters. The selected-area electron-diffraction patterns of the seed grating reveal that the formation of the ZnO-nanorod grating is attributable to the spatially selective modification of the seed layer produced by femtosecond-laser pulses.

Room temperature synthesis of nanostructured mixed-ordered-vacancy compounds (OVCs) and chalcopyrite CuInSe2 (CIS) thin films in alkaline chemical bath

Room temperature synthesis of ordered-vacancy-compounds (OVCs) and copper indium diselenide (CuInSe2, CIS) by cation and anion exchange reactions of solid CdS thin films with CIS ionic solution in an alkaline chemical bath is reported. The growth parameters such as pH, deposition time and concentration of the solutions were optimized to achieve uniform thin films. Nanostructured CdS thin films (150 nm thick) prepared by chemical bath deposition are used for the deposition of OVC and CIS thin films. The ion exchange reaction between the CdS thin film and the CIS ionic solutions transforms the yellow colour CdS film into faint black, indicating the formation of OVC and CIS film. The resultant films were annealed in air at 200 °C for 1 h and further subjected to characterization using the x-ray diffraction, transmission electron microscopy, energy dispersive x-ray analysis, x-ray photoelectron spectroscopy, scanning electron microscopy, atomic force microscopy, optical absorption and electrical measurement techniques. The OVC and CuIn3Se5 nanodomains are observed in chalcopyrite CIS thin films and these films have nanostructured morphology onto amorphous/nanocrystalline phase of CdS. The OVC–CIS films are p-type with a band gap energy of 1.453 eV.

Electric subbands in Si-delta-doped GaAs grown by metalorganic chemical vapor deposition

Abstract Shubnikov-de Haas and Van der Pauw Hall effect measurements at 1.5 K have been carried out to investigate the existence of a two-dimensional electron gas and to determine subband energies in Si-delta-doped GaAs grown by atmospheric pressure metalorganic chemical vapor deposition. The Shubnikov-de Haas measurements clearly show multiple oscillation periods, which vary dramatically with the angle between the magnetic field and the normal to the sample surface, indicative of the occupation of several subbands by quasi-two-dimensional electrons in a potential well. With the magnetic field in the plane of the interface, magnetoresistance oscillations are also clearly observed that are not periodic in the reciprocal of the magnetic field. Using these experimental results and a self-consistent numerical method which took into account the exchange-correlation effects, the electrical subband energies were also determined.