Changwoo Nahm

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.

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.

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.

An effective oxidation approach for luminescence enhancement in CdS quantum dots by H2O2

The effects of surface passivation on the photoluminescence (PL) properties of CdS nanoparticles oxidized by straightforward H2O2 injection were examined. Compared to pristine cadmium sulfide nanocrystals (quantum efficiency ≅ 0.1%), the surface-passivated CdS nanoparticles showed significantly enhanced luminescence properties (quantum efficiency ≅ 20%). The surface passivation by H2O2 injection was characterized using X-ray photoelectron spectroscopy, X-ray diffraction, and time-resolved PL. The photoluminescence enhancement is due to the two-order increase in the radiative recombination rate by the sulfate passivation layer.

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.