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.
Keywords: co-sensitization; dye-sensitized solar cells; neutral–anion equilibrium study; solvatochromic studies.