A Propeller-Shaped, Triazine-Linked Porphyrin Triad as Efficient Sensitizer for Dye-Sensitized Solar Cells

A propeller-shaped, triazine-linked, unsymmetrical porphyrin triad - consisting of two zinc-metallated porphyrin units and one free-base porphyrin unit that is functionalized by a carboxylic acid - has been synthesized by stepwise amination reactions of cyanuric chloride. Photophysical, electrochemical, and DFT studies of the triad revealed no significant electronic interactions between the porphyrin units in the triad ground state but frontier orbital energy levels suitable for use as sensitizer in dye-sensitized solar cells (DSSCs). Furthermore, the triad can be described as a 2D--A system (D: donor, A: acceptor) that has the potential to promote electron transfer and injection into the TiO2 electrode. Solar cells sensitized by the triad were fabricated with two different TiO2 photoanodes, one processed by the paste-coating (PC) method and the other by the electrophoretic deposition (EPD) method. They were found to exhibit power conversion efficiencies (PCEs) of 3.80 and 4.91%, respectively. The higher PCE value of the latter is attributed to its larger dye loading, as well as to its enhanced short-circuit current (J(sc)) and higher open-circuit voltage (V-oc) and fill factor (FF) values. In addition, differences in the surface morphology between the two different TiO2 photoanodes result in different electron-transport kinetics. Electrochemical impedance spectra (EIS) demonstrated that the solar cell with the EPD-processed photoanode exhibits a shorter electron-transport time ((d)), a longer electron lifetime, and higher charge-recombination resistance (R-ct). The PCE of the latter solar cell was further improved up to 5.56% by incorporating chenodeoxycholic acid in the dye solution as co-sensitizer.