Molecularly Engineered Naphthalene–Carbazole Schiff Base Dyes for Co-Sensitized DSSCs Achieving 29.4% Indoor Efficiency and Exceptional Stability

Two novel donor–donor π–anchored naphthalene–carbazole dyes (NCA-1 and NCA-2) are designed, synthesized, and evaluated as co-sensitizers with N719 in dye-sensitized solar cells (DSSCs). Optical and electrochemical analyses supported by DFT calculations reveal that donor orientation and anchoring topology critically influence intramolecular charge transfer (ICT) and orbital distribution. Under standard illumination, NCA-1 and NCA-2 achieve moderate efficiencies of 3.08% and 3.80%, respectively. Co-sensitization with N719 and CDCA significantly broadens spectral coverage and suppresses recombination, leading to enhanced efficiencies of 9.90% (NCA-1 + N719) and 10.56% (NCA-2 + N719). Under indoor lighting (1000 lx, 0.283 mW·cm−2), the NCA-2 + N719 device attains a record power conversion efficiency of 29.4%, demonstrating excellent indoor energy-harvesting capability. Electrochemical impedance spectroscopy (EIS) and open-circuit voltage decay (OCVD) confirm reduced charge recombination and prolonged electron lifetimes, while thermogravimetric analysis (TGA) verifies superior thermal stability. The results highlight molecular engineering of donor–donor architectures and synergistic co-sensitization as an effective strategy for achieving highly efficient and stable DSSCs suitable for both outdoor and indoor photovoltaic applications.