Organic photosensitizers based on terthiophene with alkyl chain and double acceptors for application in dye-sensitized solar cells

We designed and synthesized organic photosensitizers carrying terthiophene as the electron donor and cyanoacrylic acid as the electron acceptor for application in dye-sensitized solar cells (DSSCs). The photovoltaic performances of the DSSCs based on these dyes markedly depended on the molecular structures of the dyes in terms of the intermolecular interaction and the length of π conjugation. The suppressed intermolecular interaction with the presence of n-hexyl chain linked to thiophene gave rise to an enhanced electron transfer. Meanwhile, the structure with double electron acceptors of cyanoacrylic acid increased the dye adsorption on the TiO₂ surface and the length of π conjugation. As a result, an apparent improvement of photocurrent (short circuit current; J_sc) and power conversion efficiency was achieved upon the addition of n-hexyl chain and double cyanoacrylic acids to the terthiophene moiety. These dyes, however, had inferior performances (η ∼ 2.0%) in general, despite the apparent advantages such as broad and intense absorption spectra in the visible region. This is attributed to the ineffective electron injection to TiO₂ due to their inherent low lowest unoccupied molecular orbital (LUMO) levels, probably down to the conduction band edge of TiO₂.