Novel D-π-A-π-A coumarin dyes for highly efficient dye-sensitized solar cells: Effect of π-bridge on optical, electrochemical, and photovoltaic performance

The direct connection of coumarin and auxiliary acceptor in D-A-pi-A construction will cause large dihedral angle between these two planes, thus affecting the intramolecular charge transfer process. Herein, with an additional pi-bridge between coumarin and auxiliary acceptor, two novel coumarin sensitizers, CS-1 and CS-2, based on D-pi-A-pi-A construction have been designed and synthesized. With this modification, the planarity of donor-to-auxiliary acceptor part is effectively improved, thus resulting in smoother ICT process and less energy loss. Along with the planarization of molecule, the energy levels of the dyes are also effectively optimized. Moreover, the introduction of additional 7c-bridge will not affect the photostability of the dye. Accordingly, a record high PCE of 8.03% is obtained by CS-2 sensitized DSSCs, with afsc of 16.38 mA cm(-2), a Voc of 694 mV, and a ff of 0.707, while only around 3.5% decline in PCE is found after 1000 h aging test. Our results demonstrate that good planarity is obviously preferred to optimize the energy level and photovoltaic performance of sensitizer, providing a powerful strategy for the development of highly efficient organic sensitizers in the future. (C) 2017 Elsevier B.V. All rights reserved.