Relating the Photodynamics of Squaraine-Based Dye-Sensitized Solar Cells to the Molecular Structure of the Sensitizers and to the Presence of Additives

Dye-sensitized solar cells (DSSCs) fabricated with TiO₂ nanoparticle thin films and sensitized with four types of indole-based squaraines, SQs (symmetric or asymmetric and varying the length and nature of the alkyl side chain substituents), have been prepared. We have studied the influence of the presence of different additives in the electrolyte solutions (tert-butyl pyridine and/or Li⁺ cations) on the electron transfer dynamics by means of femtosecond transient absorption spectroscopy and flash photolysis. We obtained the rate constants for the electron injection, k_ei = 2, 3, 8, and 14 × 10¹⁰ s^–1, for complete solar cells with an iodide-based electrolyte. The asymmetric SQ showed the largest k_ei value, 14 × 10¹⁰ s^–1, in line with a unidirectional flow of electrons from the lowest unoccupied molecular orbital (LUMO) orbital of the SQ to the sub-bandgap states of the TiO₂, which leads to a more efficient electron injection than that in the symmetric SQs. Addition of tert-butyl pyridine to the electrolyte solution (I^–/I₃^– in acetonitrile) causes a 5–10-fold deceleration of the electron injection (for example, τ_obs = 2–11 ps in SQ 41). When including the Li⁺ cation together with the tert-butyl pyridine, the injection is still slower than in cells without any additive (τ_obs = 2 vs 7 ps in SQ 41), which reflects a stronger influence of the tert-butyl pyridine in the electron injection process. The effective lifetimes for the charge regeneration reaction, τ_obs, range from 2 to 25 μs for the complete cells with an iodide-based electrolyte. The fastest regeneration occurs in the SQs with the CF₃– groups anchored to the side chains and, especially in SQ 26, with two CF₃– groups. This result suggests that the inductive effect of the CF₃– groups in the structure of SQ 26 and SQ 41 leads to a higher positive charge density in the π-conjugated system, which promotes a higher local concentration of iodide near the oxidized dye and therefore faster regeneration kinetics. Moreover, addition of Li⁺ cations to the electrolyte accelerates the regeneration reaction, which is ascribed to its interaction with the backbone of the SQ, favoring the approach of the I^– species. Using the transient absorption results, we calculated the electron injection efficiency, φ_ei, and compared it with the short-circuit current density, J_sc, of the complete cells. Thus, in the complete cells sensitized with SQ 41 and SQ 4, φ_ei are the highest ones and present comparable values, 0.93 and 0.90, respectively. On the contrary, cells sensitized with SQ 26 and SQ 2 present lower values, 0.47 and 0.75, respectively. A similar tendency is observed for the values of J_sc. On the basis of this good correlation (φ_ei vs J_sc), we can suggest that the electron injection reaction is partially responsible for the photon losses and derive the reasons why this occurs.