Phenothiazine (or phenoxazine) based (D–π–A)-L2-(A–π–D–π–A)2-type organic dyes with five anchors for efficient dye-sensitized solar cells

Developing organic dyes with di-/multi-anchors has aroused increasing attention in recent years due to these dyes had many advantages in comparison with the analogues of single acceptor. Herein, four (D–π–A)-L2-(A–π–D–π–A)2 dyes (TQ1−2, OQ1−2) along with a reference D−(π–A)2 dye (TD), comprising phenothiazine/phenoxazine as a donor, thiophene as a π space, cyanoacetic acid as an acceptor and saturated n-hexyl/n-dodecyl as a flexible linker have been prepared and characterized. The correlative between the variation of structure and the optical, electrochemical properties in addition to photovoltaic performances of the dyes have been explored. Besides, the ultraviolet–visible spectra in solution and on the TiO2 film, cyclic voltammetry, photocurrent-voltage scan, incident photon-to-current conversion efficiency and electrochemical impedance spectroscopic of the presented dyes have been investigated. Compared with the reference dye TD, TQ1−2 and OQ1−2 exhibit higher molar extinction coefficient, broader and stronger incident photon-to-current conversion efficiency response spectrum, and higher density of light-absorbing units adsorbed on the TiO2 surface. Among the cells based on the pristine dyes, the relatively high photocurrent of OQ1-2 by comparison with the counterparts of TQ1−2 is attributed to the incorporation of phenoxazine donor into the (A–π–D–π–A)2 units. An increase open-circuit voltage of TQ2 and OQ2 compared to TQ1 and OQ1 is ascribed to the higher charge recombination resistance and the longer electron lifetime in their cells. Consequently, a maximum power conversion efficiency of 6.77% is attained by the TQ2-based cells with iodide/triiodide electrolyte under the illumination of 100 mW cm−2.