Modulation of metal free organic (MFO) molecules become imperative to researchers to obtain low-cost sensitizer for dye sensitized solar cells (DSSCs) purposes. A series of metal free phenothiazine-based (PTZ) organic dyes are designed and optimized as sensitizers for DSSCs application. Their electronic and optical properties were probed using Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) approaches. Effects of additional donor unit, π-conjugation bridges, and benzothiadiazole (BTDA) were investigated. Introducing BTDA in the acceptor unit leads to the tailoring of the energy band gap and promotion of charge transfer within donor and anchor groups with increased effective parameters in DSSC efficiency. The addition of diphenylamine, triphenylamine, or hexyloxyphenyl to the PTZ unit increased electron delocalization and enhanced intramolecular charge transfer. Changing π-spacer from phenyl to thiophene has a great effect on the electronic properties and absorption spectra of the dyes. The relationship between light harvesting efficiency (LHE) and chemical hardness (ƞ) shows that structural design that consists of BTDA and thiophene as π-spacer tends most towards excellent performance as dye sensitizers in DSSCs.

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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 -π-A 1 molecular design based on phenothiazine skeleton (D) connected with benzothiadiazole (A 2 ) linked with furan π-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 π-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 (TiO 2 ) and regenerated via redox potential (I − /I 3 - ) electrode. Attachment of 2-hexylthiophene (P2) remarkably lowered the E g , extended π-electron delocalization, hence, gives higher absorption wavelength (λ 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.

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