Charge recombination suppression in dye-sensitized solar cells by tuning the dielectric constant of triphenylamine dyes with altering π-bridges from naphthalene to anthracene units.

Charge recombination reactions (CRRs) are responsible for the major loss of power conversion efficiency (PCE) in dye-sensitized solar cells (DSSCs). This study tracks the impact of the dielectric constant ( ε ') of two D-π-A types of triphenylamine-based organic dyes (TpAzo 1 and TpAzo 2) with naphthalene and anthracene π-bridges on CRRs, respectively, and these dyes are sandwiched as dielectric layers of capacitors between Al electrodes via rf-sputtering deposition. The structure and thickness of components of capacitors were considered by applying field emission scanning electron microscopy (FESEM), and ε ', ε '', tan  δ , ac conductivity ( σ ac ) and electric modulus ( M* ) of capacitors were analyzed by LCRmetry at different frequencies ( f : 10 2 -10 4 Hz) and temperatures ( T : 299-390 K). Cyclic voltammetry (CV) and UV-visible spectroscopy analysis of dyes were performed for revealing the optical profiles, highest occupied molecular orbitals (HOMO), lowest unoccupied molecular orbitals (LUMO), and band gap ( E g ); the results were found to be highly consistent with density functional theory (DFT) quantum calculation outputs. The f - and T -dependent relationships were detected for ε ', ε '', tan  δ and σ ac , which experienced an upward trend and a downward trend with the increase in T and the decrease in f , respectively. The results indicated higher ε ', ε '' M ' and M '' values for TpAzo 2 than those of TpAzo 1, which from the molecular point of the view could be related to the more extending π-conjugated structure of anthracene in comparison to naphthalene π-bridges that not only provided appropriate dipole moments and exciton binding energy, but also facilitated powerful intramolecular charge transfer (ICT). Therefore, this study proposes ε ', ε '', M ' and M '' as novel physiochemical characters in dye design, which have been neglected so far and can open a new synthesis paradigm for developing more efficient organic sensitizers for DSSCs.