Spacer effects of donor-π spacer-acceptor sensitizers on photophysical properties in dye-sensitized solar cells

The donor (D)-π spacer-acceptor (A) framework with electronic push-pull effects provides suitable molecular architectures for molecular design used as efficient light-harvesting sensitizers in dye-sensitized solar cells (DSSCs). Efficiencies of light harvesting and electron injection to the semiconductor of sensitizers play critical roles in DSSC performance. Here, we employed density functional theory to systematically and comparatively investigate the effects of π-spacers of D-π spacer-A types of dyes in solution and adsorbed on a (TiO₂)₃₈ anatase cluster on various photophysical properties. The absorption spectra, electron transfer probability, and related photophysical properties of D-π spacer-A types of dyes were investigated as functions of different types (thiophene (Th)- and phenyl (Ph)-based), lengths, and planarity (bridging two neighboring rings; dithieno-thiophene (DTT) and fluorene (FL)-based) of π-spacers, while the D (diphenylamine) and A (cyano-acrylic acid) moieties remained the same. Spacers could significantly influence the λ_max values and electron transfer probability. The spacer length has a red-shifted effect in λ_max for the Th-, DTT-, and FL-based sensitizers due to their planar conjugated structures; nevertheless, the λ_max values are saturated by ring number three. In contrast, the Ph-based spacers induce a blue-shift in λ_max with spacer length due to their nonplanar structures. Interestingly, the Th- and DTT-based spacers with lower LUMO energy levels trap more electron density and thus reduce the probability of electron density transfer to TiO₂ ψ_ET(λ_max, TiO₂) upon photoexcitation; moreover, the ψ_ET(λ_max, TiO₂) values decrease significantly with ring number. On the other hand, the ψ_ET(λ_max, TiO₂) values for the Ph- and FL-based sensitizers are less sensitive to the spacer length. Interestingly, the orders of theoretical maximum short-circuit current density of four studied families of molecules are correlated with their λ_max values. Our study shows the Th-Th motif used as a π-spacer balances the spectral match with solar radiation and ψ_ET(λ_max, TiO₂) suitable for DSSC applications. Our results based on molecular and electronic structures could be used for rational sensitizer design of organic dyes for DSSC applications.