Dye-sensitized solar cells (DSSCs) have been considered as a potential candidate to resolve the current energy crisis. Therefore, we synthesized a series of phenothiazine (PHE)-based sensitizers for the photovoltaic applications for the first time. The sensitizers include different auxiliary donors, namely hydrogen (without substituent) (PHE 1), phenyl (PHE 2), 4-octyloxy-phenyl (PHE 3), 4-dimethylamino-phenyl (PHE 4), and phenanthren-9-yl (PHE 5). The PHE 1-5 sensitizers were characterized well using NMR, HRMS, FTIR, absorption and emission spectral techniques, and DFT theoretical studies. According to the research findings, impressively, PHE 4 demonstrated significantly a higher photovoltaic efficiency of 1.00% compared to all other sensitizers and a 20-fold enhanced efficiency when compared to the PHE 1 based cell of 0.05%. The enhanced performance of the PHE 4 may be attributed to the higher ground and excited state binding ability to TiO2. The order of photovoltaic efficiency of the prepared sensitizers was PHE 4 > PHE 5 > PHE 3 > PHE 1 > PHE 2. This study revealed that the electron-donor substituent in the simple PHE-based sensitizers demonstrated a dynamic function in fine-tuning the performances of the DSSCs devices.