Computational study of the effect of π-spacers on the optoelectronic properties of carbazole-based organic dyes

In this article, we studied a series of dye-sensitized solar cells (DSSCs) type Donor-π-Acceptor involving carbazole as donors and cyanoacrylic acid as acceptors of the electrons. These cells are linked by different π-spacer unit's, with the aim to develop new organic dyes with high-performance optoelectronic properties. Different units have been introduced in the π-bridge in order to investigate their effects on the structural and optoelectronic properties of the studied compounds, as well as their adsorbed compounds-titanium dioxide (TiO₂) semi-conductor. We evaluated and assessed the important relevant parameters that influence the performance of photovoltaic cell to measure their involvement in the short-circuit photocurrent density (J_sc). Using Density Functional Theory (DFT) and Time-Dependent-BHandHLYP, the geometrical and optoelectronics properties have been predicted theoretically. The results obtained indicate that introducing the oxazole (S5) and thiazole (S6) molecules in the π-spacer have significant impact on the geometric properties for D5-D6 dyes. This results in the fact that dye D5 has a planar structure. Also, the insertion of the thiophene, oxazole and thiazole units improves the energies of the HOMO and LUMO molecular orbitals of D1, D5, and D6 dyes. Moreover, these results show the ability of electron transfer and regeneration from the studied sensitizers (D1-D6). Also, it can be noted that the application of the pyrrole group in the π-spacer moiety of the dye (D2) improves the electron's transfer performance with a lower regeneration motive force ΔG^reg, a more negative injection driving forces (ΔG^inject), and a higher values of open circuit-voltage (Voc).