Three new phenothiazine organic dyes containing thiophene, 3-(5-(3-(4-(bis(4-methoxy phenyl)amino)phenyl)-10-octyl-10H-phenothiazin-7-yl)thiophen-2-yl)-2-cyanoacrylic acid (P1), 3-(5-(3-(4-(diphenylamino)phenyl)-10-octyl-10H-phenothiazin-7-yl)thiophen-2-yl)-2-cyanoacrylic acid (P2) and 2-cyano-3-(5-(10-octyl-3-(4-(2,2-diphenylvinyl)phenyl)-10H-phenothiazin-7-yl)thiophen-2-yl) acrylic acid (P3) were designed and synthesized as sensitizers for application in dye-sensitized solar cells (DSSCs). For these dyes, the phenothiazine derivative moiety and the cyanoacetic acid take the roles of electron donor and electron acceptor, respectively. The absorption spectra, electrochemical and photovoltaic properties of P1–P3 and the cell long-term stability were extensively investigated. It was found that HOMO and LUMO energy level tuning can be conveniently accomplished by alternating the donor moiety. The DSSCs based on dye P2 showed the best photovoltaic performance: a maximum monochromatic incident photon-to-current conversion efficiency (IPCE) of 84.9%, a short-circuit photocurrent density (J_sc ) of 10.84 mA cm⁻², an open-circuit photovoltage (V_oc) of 592 mV, and a fill factor (ff) of 0.69, corresponding to an overall conversion efficiency of 4.41% under standard global AM 1.5 solar light conditions. These results demonstrated that the DSSCs based on phenothiazine dyes could achieve both high performance and good stability.

In this review, intra- and intermolecular engineering strategies for improving the efficiencies of porphyrin based dye-sensitized solar cells are briefly summarized, revealing the in-depth structure–photovoltaic performance correlations.