Maximizing Indoor Light-Harvesting Efficiency Using Molecularly Engineered Triphenylimidazole-Phenothiazine Dyes and Dual-Species Copper Electrolyte

The utilization of nonplanar molecularly engineered triphenylimidazole-phenothiazine donor-based dyes as efficient light harvesters in dye-sensitized solar cells (DSCs) was explored under both 1 sun and indoor/artificial light conditions. The two new sensitizers were designed and synthesized in the Donor–Donor-π–Acceptor (D-D-π–A, LG-P5) and Donor–Donor–Acceptor-π–Acceptor (D-D-A-π–A, LG-P6) architectures with a phenyl group as π-spacer and 4-phenybenzo[c][1,2,5]thiadiazole as an auxiliary acceptor using carboxylic acid as the anchoring group. The dyes were further characterized extensively using diverse spectroscopic, electrochemical, and theoretical studies, which revealed that the ground state potential (HOMO) of the dyes is sufficiently positive (1.1 V), enabling their compatibility with the newly introduced dual-species copper redox system ([Cu(II)(dmp)2Cl]+/[Cu(I)(dmp)2]+). With a complementary absorption peak in the visible region, the potential of LG-P5 and LG-P6 dyes as effective co-sensitizers with standard XY1b was explored, and a systematic study under both one sun and low-light conditions (100, 200, 500, 700, and 1000 lx) was carried out. The LG-P6:XY1b cosensitized devices outperformed LG-P5:XY1b devices with efficiencies above 30% under all measured intensities, achieving the best PCE of 34.4% under standard 1000 lx illumination. A comprehensive interfacial charge transfer study was adopted using various perturbation techniques to explain the obtained photovoltaic results.