Molecularly engineered phenothiazine sensitizer for synergistic tandem DSSCs with N719 delivering 24% indoor efficiency in tandem DSSCs

In this study, we report the design, synthesis, and application of a novel phenothiazine-based organic co-sensitizer (PHZ-1) featuring a butyl-substituted phenothiazine donor and a benzoic acid acceptor/anchoring group for high-performance dye-sensitized solar cells (DSSCs). The target compound was synthesized in high yield via base-catalyzed Claisen-Schmidt condensation and was fully characterized using various spectroscopic and elemental analyses. The photophysical, electrochemical, and photovoltaic properties of the devices were systematically evaluated using UV–Vis absorption and emission spectroscopy, density functional theory (DFT) calculations, incident photon-to-current efficiency (IPCE ), current density-voltage (J-V ), and EIS impedance spectroscopy (EIS). Co-sensitization with the benchmark N 719 dye significantly enhanced the performance, yielding a η cell of 9.98% ( V OC = 0.703 V, J SC = 20.63 mA cm −2 , FF = 68.81%) compared to 7.95% for N719 alone. A parallel tandem DSSC (PT-Tandem) configuration with N719 (top cell)/PHZ-1 (bottom cell) achieved a η cell of 11.81% ( V OC = 0.728 V, J SC = 21.74 mA cm −2 , FF = 74.55%), representing a nearly 49% improvement over the single-device N719. EIS and Bode analyses confirmed the reduced charge recombination resistance and extended electron lifetimes in the tandem device, consistent with the higher V OC and J SC . The strong donor ability of phenothiazine, combined with complementary spectral absorption and optimized tandem ordering, underpins its superior photovoltaic performance. These findings highlight the potential of phenothiazine–chalcone–triazole frameworks as robust co-sensitizers for synergistic co-sensitization in high-efficiency tandem DSSCs. • Novel phenothiazine co-sensitizer designed for high-performance tandem DSSCs. • PHZ-1 with N719 achieved a PCE of 9.98%, outperforming N719 alone in DSSCs. • Tandem DSSC with PHZ-1 reached 11.81% PCE, a 49% boost over single cells. • EIS confirmed lower recombination resistance and longer electron lifetimes in tandem devices. • Phenothiazine –chalcone –triazole frameworks aid in synergistic DSSC co–sensitization.