Dye-sensitized solar cells (DSSCs) are among the most promising photovoltaic technologies for both outdoor and indoor energy harvesting due to their low cost and spectral versatility. In this work, a new series of organic donor–π–acceptor (D–π–A) sensitizers (BAM-1–BAM-4) featuring a naphthalene donor and a phenyl-pyrazole π-bridge were designed to explore how structural variations in electron-withdrawing acceptors influence light absorption, charge transfer, and device efficiency. The dyes were systematically investigated using UV–Vis spectroscopy, electrochemical analysis, and density functional theory calculations, and were applied individually and jointly with the benchmark Black dye in single and parallel tandem DSSCs (PT-DSSCs). BAM-3 and BAM-4 exhibited strong intramolecular charge transfer, red-shifted absorption, and efficient electron injection, achieving power conversion efficiencies of 9.85% and 8.89%, respectively, when co-sensitized with Black dye. The optimized PT-DSSC employing BAM-3 + BAM-4 as the bottom photoanode and Black dye as the top achieved a remarkable 12.13% efficiency under AM 1.5G and 25.85% under 1000 lx indoor illumination, maintaining 95% stability after 300 h of continuous operation. These results demonstrate that molecular engineering of D–π–A dyes combined with tandem co-sensitization provides an effective pathway to achieve high-efficiency, stable DSSCs suitable for both solar and ambient-light applications.