We report results of combined experimental and theoretical studies of dye‐sensitized solar cells (DSSCs) using 5‐(4‐sulfophenylazo)salicylic acid disodium salt, known as Mordant Yellow 10 (MY‐10), as TiO2 sensitizer. We focus on a single dye but vary the solvent and the pH of the solution as well as the photoelectrode preparation conditions to determine the conditions for best photovoltaic conversion efficiency. We found experimentally that the efficiency, measured under standard air mass 1.5 global (AM 1.5G) conditions, was higher in solutions of ethanol than of water, but still small (up to 0.174%), although the fill factor (FF) was large (up to 0.73). Of the dyes in ethanol, MY‐10 in alkaline solution showed the best matching of the solar spectrum but displayed the lowest efficiency. Density functional theory (DFT) calculations provided the optimized geometry, electronic structure, and electronic spectrum of the dye in fully protonated as well as partially and totally deprotonated forms, in solution. The calculated optical spectra are consistent with the experimental data, with strong absorption in the visible range only for the alkaline dye solution. The low device efficiency is very likely related to the weak optical absorption in the visible range. The much higher photovoltaic conversion efficiency of the DSSCs fabricated using acid or roughly neutral pH solutions, corresponding to the protonated and partially deprotonated forms of MY‐10, respectively, is likely caused by the better alignment of the ground state of the dye with the redox level of the electrolyte. The decrease with pH of the dye solution of the short‐circuit current was linked to a weaker charge injection from the excited state of the dye to the conduction band of the oxide, which is correlated with the shifting of the excited state of the dye deeper into the CB edge of the semiconductor. The variation of the open‐circuit voltage with the pH of the solution was linked to the adjustment of the conduction band edge of TiO2, depending on the number of protons transferred from the dye to the oxide surface. Based on our results, we analyze the relative importance of the main criteria that should be met by a dye to be used in a DSSC.

The theoretical study of chrysanthemin (cyanidin 3-glucoside) as a pigment for TiO2-based dye-sensitized solar cells (DSSCs) was performed with the GAUSSSIAN 09 simulation. The electronic spectra of neutral and anionic chrysanthemin molecules were calculated by density functional theory with B3LYP functional and DGDZVP basis set. A better energy level alignment was found for partially deprotonated molecules of chrysanthemin, with the excited photoelectron having enough energy in order to be transferred to the conduction band of TiO2 semiconductor in DSSCs. In addition, we used the raw aqueous extracts of roselle (Hibiscus sabdariffa) calyces as the source of chrysanthemin and the extracts with various pH values were tested in DSSCs. The extracts and photosensitized semiconductor layers were characterized by UV-Vis spectroscopy, and DSSCs based on raw extracts were characterized by current density-voltage measurements.

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We studied the dependence between properties and synthesis method for PEGylated ZnS nanoparticles. Thus, we proposed the PEGylation of ZnS nanopowder, in non-aqueous medium, by a facile one-pot synthesis in very mild conditions, as an alternative for the chemical precipitation of PEGylated ZnS in an aqueous solution, and we compared the properties of zinc sulfide obtained by both methods. The structure and morphology of PEGylated ZnS nanopowders were investigated by X-ray diffraction and transmission electron microscopy, and the FTIR spectra confirmed the PEGylation of ZnS nanoparticles. The values for band gap energy are in good accordance with the quantum confinement effect for nanocrystals. The mean dimension of particles was calculated, on the basis of UV–Vis spectra, by using the Brus equation and it is in good agreement with the crystallites size, determined from X-ray diffraction. The photocatalytic properties of synthesized nanopowders were tested in the degradation of Congo red azo dye, demonstrating a faster bleaching of dye in the presence of PEGylated ZnS prepared in non-aqueous medium. The photoluminescence properties are also dependent on the synthesis method and can be correlated with the surface modifications by PEG.

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