Hyeong Seop Shim

Defect states of ZnO nanoparticles: Discrimination by time-resolved photoluminescence spectroscopy

ZnO nanoparticles with different shapes and sizes were prepared by changing coordinating ligands. Hexagonal cones presented UV and green (∼500 nm) emissions, which originated from excitons and defects, respectively. Oxygen vacancies were found to be major defects in the hexagonal cones. Blue emission at ∼440 nm was observed for hexagonal plates, and analyses of time-resolved photoluminescence spectra showed that two transitions were responsible for this blue emission, because transitions from Zni to the valence band (430 nm) and from Zni to VZn (480 nm) were distinguished by emission wavelengths and lifetimes. The visible emissions from defects were related to the roles of coordinating ligands.

Chalcogenization-Derived Band Gap Grading in Solution-Processed CuInxGa1–x(Se,S)2 Thin-Film Solar Cells

Significant enhancement of solution-processed CuIn(x)Ga(1-x)(Se,S)2 (CIGSSe) thin-film solar cell performance was achieved by inducing a band gap gradient in the film thickness, which was triggered by the chalcogenization process. Specifically, after the preparation of an amorphous mixed oxide film of Cu, In, and Ga by a simple paste coating method chalcogenization under Se vapor, along with the flow of dilute H2S gas, resulted in the formation of CIGSSe films with graded composition distribution: S-rich top, In- and Se-rich middle, and Ga- and S-rich bottom. This uneven compositional distribution was confirmed to lead to a band gap gradient in the film, which may also be responsible for enhancement in the open circuit voltage and reduction in photocurrent loss, thus increasing the overall efficiency. The highest power conversion efficiency of 11.7% was achieved with J(sc) of 28.3 mA/cm(2), V(oc) of 601 mV, and FF of 68.6%.

Unexpected Size Effect Observed in ZnO-Au Composite Photocatalysts

Semiconductor-metal nanocomposites prepared with well-defined gold nanoclusters, such as Au25, Au144, and Au807, showed size-dependent photocatalytic activities for the reduction of nile blue and azobenzene. Whereas the photoreduction of nile blue was directly related with the charge separation and transfer rate from the photoexcited ZnO to gold nanoclusters, the photoreaction of azobenzene showed unexpected size effect with a clear threshold. Mechanistic investigations revealed that the photoreduction of azobenzene proceeded via a proton-coupled electron transfer process. The photocatalytic activity of the ZnO-Au nanocomposites was also dependent on the excitation intensity, demonstrating that the multielectron/multiproton process was controlled by the charge separation and transfer in the nanocomposites.