Bulky Phenanthroimidazole–Phenothiazine D−π–A Based Organic Sensitizers for Application in Efficient Dye-Sensitized Solar Cells

Metal-free organic sensitizers are important in dye-sensitized solar cells (DSSCs) to overcome the existing concerns such as synthetic feasibility and cost effectiveness. Herein, we developed four phenanthroimidazole–phenothiazine organic sensitizers based on the concept of donor−π–acceptor (D−π–A) systems, in which the donor phenanthroimidazole is at the C-7 position with either cyano acetic acid (D1 and D2) or rhodanine 3-acetic acid (D3 and D4) acceptor groups at the C-3 position of a phenothiazine moiety. All four sensitizers were characterized using various spectroscopic and electrochemical techniques. UV–visible absorption studies revealed that all four molecules exhibit absorption bands in the range of 220–650 nm in tetrahydrofuran and 400–750 nm in the film state, when adsorbed on nanocrystalline TiO2. The quenched emission and lifetime indicate the electron transfer between the donor (phenanthroimidazole–phenothiazine) and the acceptor (cyanoacrylic acid or rhodanine-3-acitic acid). Electrochemical studies suggest that each new sensitizer has a quasi-reversible oxidation wave at ∼0.76 V versus SCE. Theoretical calculations (density functional theory (DFT)/time-dependent DFT) indicate the electron distribution of the highest molecular orbital (HOMO) over the donor phenanthroimidazole–phenothiazine and the lowest molecular orbital (LUMO) over the acceptor part of all four sensitizers. The power conversion efficiency (PCE) of these four dyes was in the range of 7.8–9.0% using a liquid redox couple, while the PCE of all sensitizers enhanced to 8.4–10.2% by the addition of the coadsorbent 3a,7a-dihydroxy-5b-cholic acid (chenodeoxycholic acid, CDCA). When compared to other (D1, D3, and D4) sensitizers, D2 has showed an enhanced PCE of 10.2% because of the rapid regeneration of the oxidized dye from the redox couple which is an important factor in excitonic solar cells. Hence, our results indicate that the design of diverse molecular structures that pave the way for improved efficiency and durability of photovoltaic devices is a crucial factor for DSSC efficiency.