DFT computational investigation of tuning the electron donating ability in metal-free organic dyes featuring a thienylethynyl spacer for dye sensitized solar cells

The recent improvements in metal-free organic dye-sensitized solar cells (DSSCs) have been attributed to the ability to tune the optical and electronic properties through various structural modifications. Within the donor-π-conjugated spacer-acceptor (D-π-A) architecture, the electron-donating and accepting strengths have been proven to be major control variables for increasing the energy conversion efficiency. In the present study, a series of metal-free organic D-π-A dyes for DSSCs were designed and investigated. In particular, the electron donating strength was modulated by adding electron donating groups to the donor side. The HOMO energy increased with a gradual increase in donor strength which was verified by an investigation of the bond distances between the nitrogen and carbon atom of phenyl ring connected with π-conjugated spacer. The net electron transfer from the donor to acceptor, calculated from natural bond orbital (NBO) analysis, also showed quantitative correlation with the bond distances. Finally, the absorption peak shifted to a longer wavelength with the increase of donor strength as well as π-conjugated spacer. Detailed analysis of the results supported that all properties investigated has a strong correlation with the bond distance between the nitrogen and the carbon atom in the phenyl ring attached to the π-conjugated spacer. Based on this apparent correlation, this bond distance from ground-state DFT calculations may be used as a descriptor for the high throughput screening of DSSC dyes instead of using computationally expensive TDDFT calculations.