Co-Sensitization Effects of Indoline and Carbazole Dyes in Solar Cells and Their Neutral–Anion Equilibrium in Solution

Abstract

Co-sensitization of two or more light-absorbing compounds on a TiO2 surface has recently become one of the most successful strategies in the development of dye-sensitized solar cells (DSSCs). The specific structure of the dyes for DSSCs implies that they can partly exist in anionic forms in popular solvents used for sensitization. Our study concerns the above two issues being analyzed in detail using the example of the popular carbazole (MK2) and indoline (D205) dyes, studied by stationary absorption and emission, femtosecond transient absorption (in complete cells and in the solutions), current-voltage measurements, DFT and TD-DFT theoretical calculations. After the addition of D205 to DSSC with MK2, the fill factor of the cells was improved, and the electron recombination between TiO2 and the dyes was blocked (observed on sub-nanosecond time scales). Thus, the active co-adsorbent can take the role of the typically used passive additive, like chenodeoxycholic acid. Evidence of the concentration-dependent equilibrium between neutral and anionic forms of dyes with different lifetimes was found in acetonitrile solutions (the best for sensitization), while in ethanol solution the dominant form was the anion (worse for sensitization). Our findings should help in better understanding the operation and optimization of DSSC.

Effect of Mixture Composition on the Photophysics of Indoline Dyes in Imidazolium Ionic Liquid-Molecular Solvent Mixtures: A Femtosecond Transient Absorption Study

“…We observed a similar process across all of our samples. Interestingly, Ziolek et al also noted this behavior while investigating the excited state dynamics of D205 in pure ACN …”

“…However, in essence, all of the global analysis time components can be viewed as mixed-time components, incorporating multiple excited state processes. For instance, both τ 1 and τ 2 may involve solvation dynamics ,,− alongside intramolecular vibrational relaxation (IVR), dye aggregation, and the ultrafast decay of the locally excited (LE)/Franck–Condon state, while τ 3 may primarily involve excited state vibrational relaxation and ultrafast twisting. Lastly, τ 4 can be attributed to the decay process of the charge transfer (CT) excited state.…”

“…Various reports discuss the origin of the shortest time constant τ 1 , following TA global analysis, particularly, with regard to pure components. Previous studies identify a few ultrafast processes within this time scale: solvation dynamics, ,,− the ultrafast decay process of the LE state, and intramolecular vibrational relaxation (IVR) …”

“…Interestingly, Ziolek et al also noted this behavior while investigating the excited state dynamics of D205 in pure ACN. 99 Furthermore, Figure 6A−C shows that within 1 ps after excitation, the SE contribution is significantly red-shifted, and during its time progressions, it affects the shape of the TA spectra. Indeed, the overlap between SE and GSB decreases; The red shift of the SE band can be due to solvent relaxation as well as intramolecular vibrational relaxation (IVR) and geometric relaxation of the dye.…”

Unexpected Impact of N-Alkyl Chain Length in Bis-2-cyanoacrylic Acid Substituted Phenothiazines on the Photovoltaic Response of DSSCs

“…The relative photocurrent of the best PTZ dyes reaches the high values of more than 80%, much better than for the N719 dye in the same solar cell configuration. The synergetic effect of coadsorption of PTZ and N719 dyes was observed, yielding higher values of PCE for the dye mixtures than for the separate PTZ or N719 dyes, Utilization of PTZ-A and N719 mixture increased PCE by 52% compared to cell with a neat N719. − ,− …”