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 λmax values and electron transfer probability.
The spacer length has a red-shifted effect in λmax for the Th-, DTT-, and FL-based sensitizers due to their planar
conjugated structures; nevertheless, the λmax values
are saturated by ring number three. In contrast, the Ph-based spacers
induce a blue-shift in λmax 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 φET(λmax, TiO2) upon photoexcitation; moreover, the φET(λmax, TiO2) values decrease significantly with ring
number. On the other hand, the φET(λmax, 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 λmax values. Our study shows the Th–Th motif used as a π-spacer
balances the spectral match with solar radiation and φET(λmax, 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.