Novel metal-free organic dyes constructed with the D-D|A-π-A motif: Sensitization and co-sensitization study

[Display omitted] •It was the first time that the structural motif, D-D|A-π-A, was proposed.•Three sensitizers were designed according to the aforementioned motif.•Indolo [2,3-b]quinoxaline was chosen as the main chromophore.•FS08 gave the best DSSC performance and the best co-sensitization effect.•The improved VOC was related to the suppression of the undesired charge recombination. A new structural motif, D-D|A-π-A, was suggested for the design of novel metal-free organic sensitizers, where “D|A” represents a fused donor-acceptor conjugated building block. Three organic dyes, coded FS07, FS08 and FS09, were synthesized accordingly with the incorporation of indolo [2,3-b] quinoxaline (IQ) as the main building block but different additional donors. The synthesized dyes were well-characterized and their photophysical and electrochemical properties were well-studied. Additionally, molecular modeling via DFT method was employed to probe their photovoltaic behaviors as sensitizers/co-sensitizers. The isodensity of the molecular orbitals displays the distinguished intramolecular charge delocalization in the IQ moiety, even though its donor and acceptor sections are coplanar-fused. In the photovoltaic characterization, FS08 exhibited the best DSSC performance. Moreover, FS07, FS08 and FS09 were co-sensitized along with a typical high efficient bipyridyl Ru(II) sensitizer, HD-2, in order to induce the light harvesting over expanded spectral region and hence improve the efficiency. The solar cell co-sensitized with FS08 displayed improved efficiency of 7.94% with JSC of 19.37 mA⋅cm−2, VOC of 0.654 V and FF of 62.7%, which outperformed the device employing HD-2 alone (efficiency of 7.46%). Owing to the appropriate dye loading of FS08 in the presence of HD-2 and CDCA mixed dye solution, the benign coordination for light harvesting was guaranteed and the photoexcitation was maximized, the co-sensitized JSC was improved; due to the well-modified TiO2 surface coverage with the tight attachment of FS08 and HD-2, the dye aggregation was diminished and the undesired charge recombination was suppressed, the co-sensitized VOC was improved.