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

Abstract

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: 10²–10⁴ 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.