Phenoxazine-Based Dyes with Dual Electron Donating Moiety for Organic Dye-Sensitized Solar Cells

Highly effective phenoxazine (POZ)–based dyes (dye1, dye2, dye3, dye4 and dye5) for dye-sensitized solar cells (DSSCs) were designed and studied theoretically by using density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations. When the electronic properties of dye2 were further analyzed in terms of the coupling among its moieties, it was found that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the dye2 was originated from the HOMO of the POZ and the triphenylamine moieties and the LUMO of the cyanoacrylic acid moiety, respectively. It was found that HOMO = >LUMO (and HOMO-1 = >LUMO) transition could be considered as an intramolecular charge-transfer (ICT) transition and HOMO = >LUMO+1 transition, however, as the mixture of ICT and π–π* excitation. It was shown that through the additional electron donating group, absorption bands due to HOMO = >LUMO transition were red-shifted and another strong absorption bands appeared as peaks between 400 and 450 nm. Of the POZ-based dyes, we showed that dye5 would have the best photovoltaic properties in terms of light-harvesting efficiency and energy-antenna consideration. These results suggested organic dye sensitizers with dual electron donating group would give good photovoltaic performance for DSSCs.