Theoretical investigation of the influence of π-spacer on photovoltaic performances in carbazole-based dyes for dye-sensitized solar cells applications

• Optoelectronic properties of carbazole dyes with different π-spacers are studied. • The designed dyes show small HOMO-LUMO energy gap. • The variation of π-spacer group could increase the performance of dyes used in DSSCs. • Dye adsorption on TiO 2 surface is analyzed. A series of D-A-π-A metal-free organic dyes (D i , i = 1–8) have been taken into account to study the influence of different π-spacer groups on their efficiency in dye-sensitized solar cells (DSSCs). Density functional theory (DFT) and time-dependent DFT (TD-DFT) methods have been used to investigate the geometrical structures, absorption properties, molecular electrostatic potential, and some important parameters in relation with the short-circuit current density (J SC ) and the open-circuit photovoltage (V OC ), such as nonlinear optical properties (NLO), light-harvesting efficiency (LHE), electron injection driving force (ΔG inject ), total reorganization energy (λ total ), and chemical reactivity. The results of theoretical calculations of these dyes showed that the variation of π-spacer group could increase the open-circuit photovoltage, enhance light absorption ability and intramolecular charge transfer properties, reduce energy gap, thus leading to improved photovoltaic performance. In addition, the effects of the dyes adsorption on TiO 2 surface, absorption spectra and energy levels were evaluated. Accordingly, we can assume that this theoretical investigation is predictable to provide guidance for experimental synthesis of greatly efficient metal-free organic dyes based on carbazole for DSSC applications.