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.
Because the intense absorptions of squaraine (SQ) dyes in the red/ near-infrared spectral regions closely match the spectral response of sunlight, SQ dyes have great potential for use in dye-sensitized solar cells (DSCs). In this study, we employed density functional theory (DFT) and time-dependent DFT to investigate the structural, optical, and electron transfer properties of seven recently reported SQ-derived dyes adsorbed on a (TiO 2 ) 38 cluster having an anatase (101) surface, as a model for corresponding DSCs. In particular, we calculated the proportions of the electron densities in the dye−(TiO 2 ) 38 systems that were transferred to the TiO 2 moieties upon excitation, allowing us to investigate their electron injection mechanisms. We found that the dye−(TiO 2 ) 38 systems followed different direct and indirect mechanisms of electron injection to TiO 2 depending on the localization of the excited state electron density and the driving force for excited-state electron injection. JD10 and YR6 owning two intense absorption bands, which have significant proportions of electron density delocalized into the TiO 2 moiety upon excitation and have driving forces for excited-state electron injection, followed direct electron injection mechanisms to TiO 2 . These results are compatible with their higher experimentally observed short-circuit currents (J sc ) than those of the SQ dyes. In contrast, SQ12, SQ2, and SQ4 followed the indirect electron injection mechanism due to their negligible proportion of electron density injected into TiO 2 ; in addition, SQ2 and SQ4 do not provide driving force for electron injection. SQ12, SQ2, and SQ4 dyes had lower experimentally observed J sc values than those of the other SQ dyes. The calculated probabilities of electron density being delocalized into TiO 2 and driving force for excited-state electron injection from these studied SQ dyes are compatible with their experimentally observed J sc values. This study provides insight into the electron injection mechanisms of SQ-derived dyes adsorbed on TiO 2 upon photoexcitation. Furthermore, our calculations and findings give clues for designing new SQ-derived sensitizers for DSC applications.
D–A−π–A dyes differ from the traditional D−π–A framework having several merits in dye-sensitized solar cell (DSSC) applications. With regard to D−π–A dyes, D–A−π–A dyes red-shift absorption spectra and show particular photostability. Nevertheless, the effects of internal acceptor on the charge transfer (CT) probability are unclear. We employed density functional theory (DFT), time-dependent DFT (TD-DFT), and TD-DFT molecular dynamics (MD) simulations to investigate the effects of internal acceptor on the photophysical properties of D–A−π–A dyes on DSSCs. Our calculations show the absorption bands of D–A−π–A dyes with strong electron-withdrawing internal acceptors exhibiting significant characteristics of dual CT; the excited electron density is transferred to the internal and terminal acceptors simultaneously. Particularly, the internal acceptor traps a significant amount of electron density upon photoexcitation. The TD-DFT MD simulations at 300 K show that only a small amount of excited electron density is pushing and pulling between the internal acceptor and terminal acceptor moieties; the thermal energy is not high enough to drive the electron density from the internal acceptor to the terminal acceptor. Our study reveals the nature of CT bands of D–A−π–A dyes providing a theoretical basis for further rational engineering.
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