Effects of different electron donating groups on dye regeneration and aggregation in phenothiazine-based dye-sensitized solar cells

A series of phenothiazine-based dyes containing different auxiliary chromophores (TP, TTP, EP, and EEP) bring about unusual power conversion efficiency (PCE) for the corresponding dye-sensitized solar cells (DSSCs): EEP with the best electron-donating capability provides the lowest PCE of 2.24%, while TP with the weakest electron-donating capability leads to the highest PCE of 8.07%. The underlying influencing factors have been investigated by considering the electronic structures and aggregation properties based on density functional theory and Marcus theory. We found that the energy-mismatch between electron-donating units and the PTZ moiety results in poor EEP dye regeneration. Additionally, molecular dynamics simulations illustrate that the increased intermolecular interaction energy induced by preferable electron-donating groups aggravates the intermolecular aggregation. Especially, the calculated average values of the time-dependent intermolecular lateral charge transfer rate k for (EEP)₂ are nearly one order of magnitude higher than those of (TP)₂, revealing a more robust π-π stacking interaction induced by the donor unit of EEP. Importantly, the dye-TiO₂ interactions have been taken into account, which are absent in many previous theoretical work but crucial for accurate describing the aggregations. These deeper insights into the regeneration process and the aggregation mechanism induced by different donor units encourage researchers to balance various properties in designing novel components for photovoltaic devices.