Molecular Engineering of Phenothiazine Sensitizers Using Trihexyl-Dithieno [2,3]Pyridine as a Novel Heterocyclic π-Bridge for Enhanced Single and Tandem DSSC Performance

Dye-sensitized solar cells (DSSCs) are promising alternatives to conventional silicon-based photovoltaics, but the impact of π-bridge extension and auxiliary-acceptor positioning on device performance remains insufficiently understood. Herein, we designed and synthesized four phenothiazine-based sensitizers (PTT-1–PTT-4) featuring a highly conjugated dithienopyridine (π-DTP) π-bridge and positionally varied benzothiadiazole (BTD) or benzotriazole (BTZ) auxiliary acceptors, anchored via cyanoacrylic acid to form D–A–π–A and D–π–A–A architectures. Experimental and theoretical studies revealed that BTD-containing dyes (PTT-1, PTT-3) exhibit stronger intramolecular charge transfer, lower band gaps (2.13–2.25 eV), and higher light-harvesting efficiency than BTZ analogues. PTT-1 achieved the highest single-cell efficiency of (PCE = 7.25%, JSC = 16.85 mA.cm−2, VOC = 0.828 V, FF = 0.553) with maximal recombination resistance (Rrec = 18.3 Ω) and electron lifetime (τeff = 10.98 ms). Moreover, tandem DSSC with N719 (top) and PTT-1 (bottom) achieved PCE of 9.31% (VOC = 0.848 V, JSC = 20.20 mA·cm−2, FF = 0.543) with the highest Rrec (22.12) and τeff (14.15 ms). PTT-1 and PT-N719/PTT-1 devices demonstrated remarkable long-term stability and retained 94% and 98% of their initial efficiency, respectively. These results demonstrate that strategic π-DTP engineering and auxiliary acceptor placement effectively tune optical and electronic properties, providing a clear strategy for high-performance, metal-free DSSCs.