Aggregation of metal-free organic sensitizers on TiO₂(1 0 1) surface for use in dye-sensitized solar cells: A computational investigation

We present a theoretical investigation of the aggregation effects of two 4H-Pyran-4-ylidene-based organic sensitizers on the TiO₂(1 0 1) surface, which are used in dye-sensitized solar cells (DSSCs). Our results combining density functional theory (DFT), time-dependent DFT, and density functional tight binding (DFTB) methods show that 4H-Pyran-4-ylidene-based organic sensitizers are prone to aggregate on the TiO₂ surface due to the strong intermolecular π…π interactions. The aggregation has obvious effects on electronic structures and absorption spectra of the organic sensitizers. The incorporation of an electron-deficient benzothiadiazole (BT) unit in the spacer part of the organic dye leads to a broader absorption band in the red portion of the UV–visible spectra, which is desirable for enhanced spectral responses. However, the BT moiety also leads to more aggregation on the TiO₂ surface and stronger electron coupling within the dimers, which might retard the electron transfer at the dye-TiO₂ interface and therefore limit the photovoltaic efficiency. This study sheds light on the aggregation effects of metal-free organic dyes and is expected to assist in the rational design of novel and efficient all-organic sensitizers for the further optimization of DSSCs.