An Ab Initio Molecular Dynamics Study of the Mechanism and Rate of Dye Regeneration by Iodide Ions in Dye-Sensitized Solar Cells

In the ambition to improve the power conversion efficiency (PCE) of dye-sensitized solar cells (DSSCs), it will be essential to understand the mechanisms and rates of dye regeneration. Although the mechanism of dye regeneration has been studied by static density functional theory (DFT) and classical molecular dynamics (CMD) simulations, ab initio molecular dynamics simulation (aiMD) has the potential to combine the insights from both methods for a deeper understanding. In this work, a series of aiMD simulations has been performed to study the interaction between an oxidized organic model dye, LEG4, and an electrolyte containing iodide ions as reducing agents. Dynamic Mulliken and natural spin population analyses show that two iodide ions, I–···I–, are required for dye regeneration. It was found that a distance between I–···I– of less than 6.5 Å at site 1 benefits from the electrostatic environment of the triphenylamine group of the LEG4 dye, and a corresponding distance of 4.8 Å at site 2 is essential for the dye regeneration process to take place. The rate constants of the LEG4 regeneration by two iodine ions range from 105 to 1012 s–1, spanning a window in which results from both experimental and static theoretical calculations fall. It is also verified that the probability of electron transfer from a radical I2– to the oxidized LEG4 dye is extremely low due to the rapid electron back-transfer. However, it has been found that the addition of an additional iodide ion at a distance of 5 Å with respect to the radical I2– opens the pathway for the reduction of the oxidized LEG4 dye with an associated formation of I3–. The current results highlight the necessity for a dynamical approach for a full understanding of the regeneration process.