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

Abstract

We present a theoretical investigation of the aggregation effects of two 4H-Pyran-4-ylidene-based organic sensitizers on the TiO2(101) 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.