Dyes based on the D/A-acetylene linker-phenothiazine system for developing efficient dye-sensitized solar cells

Two novel sensitizers, 2a and 2b, with D/A--D--A architectures were designed and synthesized based on a phenothiazine framework connected via ethylene with 3,5-bis(trifluoromethyl)phenyl (A) and p-methoxyphenyl (D), and with cyanoacrylic acid as an anchoring group, acting as an acceptor (A). A comparative analysis was performed to investigate the effect of the electronic character of end-capped D and A groups on the photophysical properties and photovoltaic performance of 2a and 2b. The optical, thermal, electrochemical and photovoltaic properties of the dyes were thoroughly investigated and confronted with DFT and TD-DFT calculations. Compound 2b of D--D--A type showed better photovoltaic parameters, which verify the general trends, although the parameters for compound 2a of A--D--A type do not differ significantly from those for 2b. The metal-free 2a and 2b-based DSSC devices exhibited higher power conversion efficiencies of 6.11% (2a) and 5.03% (2b) compared to 3.56% for the N719-based device.