Effect of the alkyl chain length on the optoelectronic properties of organic dyes: theoretical approach

In recent years, investigating organic compounds for Dye Sensitized Solar Cells (DSSCs) applications has received particular attention for its interesting optoelectronics properties. In this article, we have conducted a theoretical study on a series of D–π–A molecules, which have carbazole as a donor (D) and cyanoacrylic acid as acceptor (A) linked by a conjugated bridge. Theoretical calculations were conducted using Density Functional Theory (DFT) and Time-Dependent-DFT to evaluate the substitution effect of linear chains and to elucidate the effects of alkyl chain length (R: H, CH₃ to C₆H₁₃). We have studied the geometrical structures, the electronic and optical properties, the conduction band shift, as well as the charge transfer parameters: IP, EA, PEE, HEP, λ, LHE, Voc, ΔG^inject and ΔG^reg of the studied dyes. According to the results obtained, the substitution of hydrogen H (Dye 1) by the methyl group CH₃ (Dye 2) has, on the one hand, a significant effect on the values of the energies HOMO and LUMO, and on the other hand, the absorption peak is redshifted and the contribution of the electronic transition (H → L) has been improved (664.17 nm and 70%, respectively). However, the substitution of CH₃ (Dye 2) by the other alkyl groups C_nH_2n+1 (n = 2 to 6) (Dye 3 to Dye 7) has low effects. The best results are obtained by substitution with methyl CH₃, and the increase in chain length, which will require additional computing time, has a low effect on the properties of the dyes studied.