Suji Kang

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.

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.

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.