Coumarin-based dyes have been successfully used in dye-sensitized solar cells, leading to photovoltaic conversion efficiencies of up to about 8%. Given the need to better understand the behavior of the dye adsorbed on the TiO₂ nanoparticle, we report results of density functional theory (DFT) and time-dependent DFT (TD-DFT) studies of several coumarin-based dyes, as well as complex systems consisting of the dye bound to a TiO₂ cluster. We provide the electronic structure and simulated UV-Vis spectra of the dyes alone and adsorbed to the cluster and discuss the matching with the solar spectrum. We display the energy level diagrams and the electron density of the key molecular orbitals and analyze the electron transfer from the dye to the oxide. Finally, we compare our theoretical results with the experimental data available and discuss the key issues that influence the device performance.

We present magnetic studies of three triangular quantum Heisenberg antiferromagnets (TQHAF’s) with weak additional Dzyaloshinskii-Moriya interaction, ${\mathrm{Cu}}_2{\mathrm{}\left(\mathrm{OH}\right)\mathrm{}}_3\left({\mathrm{C}}_m{\mathrm{H}}_{2m+1}\mathrm{COO}\right),$ $m=7,$ 9, and 11. Fits of the dc susceptibility data to high-temperature series expansions agree with high-temperature TQHAF behavior. At low temperatures the deviations from the TQHAF predictions suggest a canted antiferromagnetic type of ordering, while the strong peak in the second harmonic of the nonlinear ac susceptibility indicates the development of a spontaneous moment. The frequency dependence of the linear ac susceptibility and the irreversibility in the field-cooled/zero-field-cooled magnetization show that frustration is strong enough to cause glassiness. We propose that the interplay of Heisenberg and Dzyaloshinskii-Moriya exchanges leads to the unusual coexistence of glassiness and canted antiferromagnetism in these geometrically frustrated systems.