Near-IR organic sensitizers containing squaraine and phenothiazine units for dye-sensitized solar cells

Unsymmetrical squaraine near-infrared (NIR) sensitizers with variable pi-bridge units, based on the donor-pi-squaraine core-acceptor structural motif, were synthesized and used as sensitizers for dye-sensitized solar cells. The phenothiazine as a donor group was linked to a squaraine core unit using various thiophene derivatives. Among them, three squaraine-based sensitizers have a carboxylic acid group as an electron-accepting group directly attached to the squaraine component, while one of four squaraine-based sensitizers has a cyanoacrylic acid as a more strongly electron-accepting group directly attached to the squaraine component. The sensitizers exhibited intense absorption in the NIR region and strong luminescence. The LUMO levels of the present squaraine-based sensitizers were slightly more negative than the conduction band edge of TiO2 photoanode. As a result, a solar cell based on the squaraine-based sensitizer containing a thiophene unit as a pi-bridge unit and a carboxylic acid group as an electron-accepting group exhibited better photovoltaic performance with a J(SC) of 6.0 mA cm(-2), a V-OC of 448 mV, and an FF of 0.73, corresponding to an overall conversion efficiency eta of 2.0%. (C) 2014 Elsevier Ltd. All rights reserved.