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We developed dual photosensitization, in which CdS quantum dots serve as a sandwich layer between the ZnO nanoparticles (NPs) and N3 dye molecules, alleviating the serious issues of CdS photocorrosion and the Zn 2+ /N3 dye complex formation leading to insulating surface aggregation. The improvement in the solarto-electrical conversion efficiency from 0.11 and 0.85 to 3.04% when compared to ZnO/N3 and ZnO-CdS, respectively, is attributed to the effective coupling between the ZnO NPs/N3 dye molecules in the presence of CdS QDs, which in turn enhances the rate of electron injection due to the faster electron injection and transfer rate. Strong ZnO NPs-CdS QDs linking is explored systematically using UV-vis spectroscopy and is interpreted with a charge transfer resistance obtained from impedance spectroscopy studies.

A novel and highly efficient dye, Ru(H 2 dcbpy)(4-(4-(N,N-di(p-hexyloxyphenyl)amino)styryl)-4′-methyl-2,2′bipyridine)(NCS) 2 , HMP-2, with extinction coefficients of about 33 260 L • mol -1 • cm -1 at 534 nm and about 20 000 L • mol -1 • cm -1 at 490 nm, is designed and further applied onto ZnO nanocrystals for dye-sensitized solar cells application. ZnO nanocrystals of a few nanometers are spin coated onto an indium-tin oxide substrate for forming about 2.5 µm uniform film thickness. Change in the surface appearance of ZnO in the presence of N3 and in newly designed HMP-2 dye is illustrated. Structural elucidation of the newly developed HMP-2 dye is presented in depth. A ZnO nanoparticle electrode is dipped in HMP-2 dye and in commercially used N3 dyes for 24 h to see the influence of dye structure on the surface aggregation effect due to insulating Zn + /dye complex layer formation. Interestingly, the ZnO nanocrystal electrode surface with HMP-2 dye is almost free of this efficiency-diminishing effect, and a solar-to-electrical conversion efficiency of 4.03%, which is about 40-fold times higher than that of N3 dye, is obtained.