Electronic and optical properties' tuning of phenoxazine-based D-A2-π-A1 organic dyes for dye-sensitized solar cells. DFT/TDDFT investigations

Modulation of molecular features of metal free organic dyes is important to present sensitizers with competing electronic and optical properties for dye sensitized solar cells (DSSCs). The D-A(2)-pi-A(1) molecular design based on phenothiazine skeleton (D) connected with benzothiadiazole (A(2)) linked with furan pi-spacer and acceptor unit of cynoacrylic acid (A(1)) were fabricated and examined theoretically for possible use as DSSCs. Density functional theory (DFT) and time dependent density functional theory TDDFT were used to study the effect of additional donors on the photophysical properties of the dyes. Eight (8) different donor subunits were introduced at C7 of phenoxazine based dye skeleton to extend the pi-conjugation, lower HOMO-LUMO gap (E-g) and improve photo-current efficiency of the dye sensitizer. All the dye sensitizers (except P3 and P4) exhibited capability of injecting electrons into the conduction band of the semiconductor (TiO2) and regenerated via redox potential (I-/I-3(-)) electrode. Attachment of 2-hexylthiophene (P2) remarkably lowered the E-g, extended pi-electron delocalization, hence, gives higher absorption wavelength (lambda(max)) at 752 nm. The donor subunit containing 2-hexylthiophene (P2) presented the best chemical hardness, open circuit voltage (V-oc), and other comparable electronic properties, making P2 the best DSSC candidate amongst the optimized dyes. The reported dyes would be interesting for further experimental research.