Spectral complementarity of phenothiazine-based dyes with N719 and its application in double-sided Pt electrode tandem DSSCs

The classic N719 dye in dye-sensitized solar cells (DSSCs) exhibits insufficient spectral response in the 300–500 nm range. To address this limitation, a phenothiazine-based dye, PTZ-7, was designed by introducing an ethynyl group into PTZ-6 (a previously reported dye with partial spectral complementarity to N719 but significant overlap). Theoretical calculations revealed that this structural modification induced a redshift in the absorption spectrum, and experimental characterization confirmed that its absorption band extended to the 410–492 nm region, achieving improved spectral complementarity with N719. A tandem device architecture integrating N719 (top layer) and PTZ-7 (bottom layer) with dual-sided Pt counter electrodes was fabricated, demonstrating synergistic spectral enhancement. The optimized device exhibited a short-circuit current density (J_sc) of 20.09 mA cm⁻² and a power conversion efficiency (PCE) of 11.08 %, surpassing single-dye systems based solely on PTZ-7 (J_sc = 11.86 mA cm⁻², PCE = 6.47 %) or N719 (J_sc = 14.88 mA cm⁻², PCE = 8.52 %). This study demonstrates the effectiveness of a molecular engineering guided strategy combining spectral matching and tandem integration to overcome the performance limitations of DSSCs.