New D−π–D−π–A Systems Based on Phenothiazine Derivatives with Imidazole Structures for Photovoltaics

Abstract

Two new compounds with an N-octyl phenothia-zine core substituted at C(7) position via a triple bond with imidazole-containing butyl or dodecyl chains bearing on the opposite side at the C(7) cyanoacrylate moiety were synthesized. Photophysical and electrochemical studies were performed to evaluate the possibility of applying them in the dye-sensitized solar cells (DSSCs). The investigations were supported by density functional theory and time-dependent DFT. The synthesized dyes showed a similar absorption profile ranging from 250 to 600 nm in solvents and a high value of light-harvesting efficiency of about 0.8. It was found that its photoluminescence quantum yield was solvent-dependent, and the highest value (about 50%) was reached in DMF solution. Electrochemical measurements revealed that both compounds showed electron injection ability from the LUMO of the molecule to the TiO2 conduction band and the possibility of regeneration by an electrolyte. DSSCs sensitized with the phenothiazine derivatives differing in TiO2 thickness and the type of electrolyte (iodide and cobalt) were prepared. Additionally, transient absorption measurements in solution and solar cells were carried out to study excited-state properties and electron transfer dynamics. The influence of the hydrophobic hydrocarbon chain length of the imidazole unit in these dyes as well as TiO2 thickness and the type of electrolyte on the device performance was observed. DSSCs sensitized with phenothiazine with dodecyl chains showed higher photocurrent, open-circuit voltage, and finally the power conversion efficiency, regardless of TiO2 thickness.