Spacer Effects of Donor-π Spacer-Acceptor Sensitizers on Photophysical Properties in Dye-Sensitized Solar Cells

Abstract

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.

Synthesis and characterization of thiophene-based push-pull chromophores for tuning the electrical and optical properties of surfaces with controlled SAM formation

“…This can be explained by less aromatic and more quinoidal nature of DTT-relay (Table 2). 27,28 Analysis of the cyclic voltammograms of compounds 3, 12, 7, 16-18 (Fig. S2 in the Supporting Information) has revealed similar characteristics.…”

First Principles Study of Dendritic Carbazole Photosensitizer Dyes Modified with Different Conjugation Structures

“…al. has reported the red shift and blue shift by varying the number of rings and spacer lengths in D‐π spacer‐accepter dyes . The usage of Th‐, DTT‐ and FL spacers caused red shift when rings were increased to three whereas a blue shift has been observed upon usage of Ph rings with number up to 6.…”

Push‐Pull N‐Annulated Perylene‐Based Sensitizers for Dye‐Sensitized Solar Cells: Theoretical Property Tuning by DFT/TDDFT

“…Figure shows UV‐Vis absorption spectra for the dye‐TiO 2 complexes. From previous work, Tan and coworker found that using thiophene based spacers can induce red‐shifts in λmax, in contrast, using phenyl rings as spacers induces a blue‐shift in λ max due to the nonplanarity of spacers. Similarly, we found that D3‐L1‐TiO 2 with thiophene spacer exhibits λ max at 548 nm, while D3‐L2‐TiO 2 with phenyl spacer significantly induces blue‐shift in λ max to 509 nm.…”