Hongsik Choi

The effects of 100 nm-diameter Au nanoparticles on dye-sensitized solar cells

Gold nanoparticles of ∼100 nm in diameter were incorporated into TiO2 nanoparticles for dye-sensitized solar cells (DSSCs). At the optimum Au/TiO2 mass ratio of 0.05, the power-conversion efficiency of the DSSC improved to 3.3% from a value of 2.7% without Au, and this improvement was mainly attributed to the photocurrent density. The Au nanoparticles embedded in the nanoparticulate-TiO2 film strongly absorbed light due to the localized surface-plasmon resonance, and thereby promoted light absorption of the dye. In the DSSCs, the 100 nm-diameter Au nanoparticles generate field enhancement by surface-plasmon resonance rather than prolonged optical paths by light scattering.

Effect of Gd implantation on the structural and magnetic properties of GaN and AlN

Gd+ ions were implanted at total doses of 3–6×1014cm2 into single-crystal GaN or AlN epilayers grown on sapphire substrates and annealed at 700–1000°C. The implanted Gd showed no detectable diffusion in either material after annealing, as measured by secondary ion mass spectrometry, corresponding to a diffusion coefficient <8×10−12cm2s−1. Under all annealing conditions, x-ray diffraction shows the formation of second phases. In the case of GaN, these include Gd3Ga2, GdN, and Gd, while for AlN only Gd peaks are observed. Both the GaN and AlN show high saturation magnetization after annealing at 900°C (∼15emucm−3 for GaN and ∼35emucm−3 for AlN). The magnetization versus temperature characteristics of the Gd-implanted GaN show a blocking behavior consistent with the presence of precipitates, whereas the AlN shows a clear difference in field-cooled and zero-field-cooled magnetization to above room temperature which may also be due to Gd inclusions.

Photoluminescence enhancement in CdS nanoparticles by surface-plasmon resonance

To examine the influence of metal nanoparticles on the photoluminescence of semiconductors, colloidal mixtures of CdS and Au nanoparticles were prepared with different CdS/Au fractions. Compared to the cadmium-sulfide nanocrystals (quantum efficiency ≅ 7%), the CdS/Au mixtures showed enhanced luminescence properties (quantum efficiency ≅ 14%). The existence of an optimum ratio of metal to semiconductor nanoparticles for the photoluminescence intensity indicates that interactions between the metal and semiconductor nanoparticles induced by surface-plasmon resonance occur constructively at appropriate distances.

Graded bandgap structure for PbS/CdS/ZnS quantum-dot-sensitized solar cells with a PbxCd1−xS interlayer

To suppress the electron-hole recombination in the multishell sensitizer for quantum-dot-sensitized solar cells (QDSCs), the PbxCd1−xS interlayer is incorporated between the PbS core and CdS shell. The PbS/PbxCd1−xS/CdS structure enhances the cell efficiency by ∼60% compared to PbS/CdS QDSCs, and consequently shows a power-conversion efficiency of 1.37% with ZnS coating. Open-circuit voltage decay confirmed that the PbxCd1−xS interlayer effectively reduces the recombination at the PbS/CdS interface. Furthermore, with respect to the peak shift of incident photon-to-current conversion efficiency, the interlayer also increases the light-harvesting efficiency in the higher-wavelength region by reducing the exciton confinement within the PbS sensitizer.

Photoluminescence enhancement in CdS quantum dots by thermal annealing

The photoluminescence behavior of CdS quantum dots in initial growth stage was studied in connection with an annealing process. Compared to the as-synthesized CdS quantum dots (quantum efficiency ≅ 1%), the heat-treated sample showed enhanced luminescence properties (quantum efficiency ≅ 29%) with a narrow band-edge emission. The simple annealing process diminished the accumulated defect states within the nanoparticles and thereby reduced the nonradiative recombination, which was confirmed by diffraction, absorption, and time-resolved photoluminescence. Consequently, the highly luminescent and defect-free nanoparticles were obtained by a facile and straightforward process.

Interplay between carrier and impurity concentrations in annealed Ga1- xMnxAs : Intrinsic anomalous hall effect

Investigating the scaling behavior of annealed Ga1-xMnxAs anomalous Hall coefficients, we note a universal crossover regime where the scaling behavior changes from quadratic to linear. Furthermore, measured anomalous Hall conductivities in the quadratic regime when properly scaled by carrier concentration remain constant, spanning nearly a decade in conductivity as well as over 100 K in T_[C] and comparing favorably to theoretically predicated values for the intrinsic origins of the anomalous Hall effect. Both qualitative and quantitative agreements strongly point to the validity of new equations of motion including the Berry phase contributions as well as the tunability of the anomalous Hall effect.

Surface-plasmon resonance for photoluminescence and solar-cell applications

The surface plasmon of metal nanostructures influences surrounding semiconductors in various ways. In particular, a surface plasmon modifies recombination rates of excitons in a semiconductor, and intensifies photon flux in the vicinity of metal nanostructures. These phenomena have contributed to the improvement of photoluminescent properties both by enhancement of radiative recombination and by electromagnetic-field amplification, even though the degree of nonradiative energy dissipation is sensitively dependent on the metal-semiconductor distance. Strong light absorption induced by surface plasmons is also attractive for photovoltaic applications, so metal nanostructures can be incorporated into diverse solar-cell systems with a reduced solar-cell thickness.

A simple template-free 'sputtering deposition and selective etching' process for nanoporous thin films and its application to dye-sensitized solar cells

A facile and straightforward method is suggested to synthesize nanoporous-TiO₂ thin films for dye-sensitized solar cells (DSSCs). Silver/TiO₂ co-sputtering led to the formation of nanocomposite films which consisted of silver nanoclusters with surrounding TiO₂ matrices, and metal particles were subsequently etched by just immersing in nitric acid. Nanoporous-TiO₂ DSSCs fabricated by this simple and effective process showed power-conversion efficiencies of up to 3.4% at a thickness of only 1.8 μm, which is much superior to that of conventional nanoparticulate-TiO₂ DSSCs with similar thickness.

Evidence of metallic clustering in annealed Ga1−xMnxAs from atypical scaling behavior of the anomalous Hall coefficient

The anomalous Hall coefficient (Rs) and longitudinal resistivity (ρxx) scaling relationship (RS=cρxxn) for a series of annealed Ga1−xMnxAs epilayers (x≈0.055) are investigated. As-grown samples exhibit a scaling parameter n∼1 and samples annealing near the optimal annealing temperature, n∼2. For annealing temperatures far above the optimum they observe scaling with n>3, which is similar to the behavior of certain inhomogeneous systems. Optical spectroscopy measurements also reveal an enhancement of the absorption coefficient in these samples for photons of energy around 1eV. These atypical behaviors are characteristic of spherical resonance from metallic inclusions.

Improving scattering layer through mixture of nanoporous spheres and nanoparticles in ZnO-based dye-sensitized solar cells

A scattering layer is utilized by mixing nanoporous spheres and nanoparticles in ZnO-based dye-sensitized solar cells. Hundred-nanometer-sized ZnO spheres consisting of approximately 35-nm-sized nanoparticles provide not only effective light scattering but also a large surface area. Furthermore, ZnO nanoparticles are added to the scattering layer to facilitate charge transport and increase the surface area as filling up large voids. The mixed scattering layer of nanoparticles and nanoporous spheres on top of the nanoparticle-based electrode (bilayer geometry) improves solar cell efficiency by enhancing both the short-circuit current (Jsc) and fill factor (FF), compared to the layer consisting of only nanoparticles or nanoporous spheres.