A computational study on the role of furan-based π-spacers in enhancing the efficiency of a DSSC

The structural parameters, frontier molecular orbital (FMO) distribution, electrostatic potential surfaces (ESP), and optical and photovoltaic parameters were investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT) to study the effects of changing the π-spacers in a sensitizer utilized for a dye-sensitized solar cell (DSSC). Three different strategies were employed for the modification of π-spacers: a) changing the spacer length, b) different spacer units, and c) maintaining the π-spacer planarity upon the introduction of multiaromatic rings. In this work, we designed eight novel dyes by changing the π-spacer in a D-π-A framework, which could be used for the fabrication of a DSSC. The effects of furan derivatives on the calculated photophysical and photovoltaic descriptors of the sensitizers were comprehensively analyzed. The charge transfer characteristics were quantified using a transition density matrix (TDM) and a charge density difference map (CDDM). The simulated absorption spectra of the dyes show strong absorption in the visible region with a bathochromic shift compared to the reference dye. Dyes 3 and 4 absorb at 506 and 453 nm in the solvent phase with high oscillator strengths above 2. The photovoltaic descriptors highlight dye 6 as a promising candidate with a relatively high open-circuit voltage of 1.2 eV, a favorable electron injection driving force (-1.64 eV), and an improved fill factor. Adsorption studies of dyes anchored onto TiO₂ clusters have been carried out. Strong chemisorption of the dye and TiO₂ cluster has been found for all the dyes, as indicated by the negative adsorption energies. Compared with the reference dye, the designed dyes presented a smaller band gap and improved LHE, identifying them as promising candidates for further experimental evaluation in DSSCs.

Quantum chemical calculations (optimization) were performed with the DFT method, B3LYP, and the 6-31G(d,p) basis set. TDDFT with CAM-B3LYP/6-31G(d,p) was adopted to obtain the optical properties of the dyes (gas and solvent phases). The effect of the implicit solvent environment was considered with the polarizable continuum model (PCM) using methanol as the solvent for DFT and TDDFT calculations. Comprehensive analysis was performed to obtain structural, electronic (HOMO-LUMO gap and energy), molecular electrostatic potential, optical (absorption spectra, excited state lifetime, and oscillator strength), and photovoltaic parameters (V_OC, LHE, ΔG_(inject), ΔG_(regen), |V_RP|, and FF). The intramolecular charge transfer (ICT) properties of all the dyes were also computed. The optimization of the dyes with the (TiO₂)₈ cluster was performed via B3LYP/6-31G(d,p) and LANL2DZ for Ti atoms. The optical properties of the dyes anchored onto the TiO₂ cluster were also studied via CAM-B3LYP, with methanol used as the solvent. These calculations were performed via Gaussian 09 W software. The charge transfer descriptors and the hole‒electron distribution of the first excited state obtained from the electron density difference from the ground state were analyzed via Multiwfn 3.8.