Energy-Level Control via Molecular Planarization and Its Effect on Interfacial Charge-Transfer Processes in Dye-Sensitized Solar Cells

As the critical property of the organic dye, the energy level determines the thermodynamic possibilities and the efficiencies of multiple interfacial charge-transfer processes in dye-sensitized solar cells. Thus, a suitable energy level is highly required, and selective energy control becomes a quite important and systemic objective. Herein, a novel planar carbazole unit, which is synthesized through simple aryl immobilization, is applied as the donor segment in the D-A-pi-A organic dye. The considerable dihedral angle between benzene and carbazole is almost eliminated, thus resulting in effective improvement of molecular planarity. As the planarity of donor segment enhances, the highest occupied molecular orbital level of the dye increases, whereas its lowest unoccupied molecular orbital level remains around the same value, with respect to the twisted dye. Besides, with good molecular planarity, the interfacial charge-transfer processes, including charge injection, charge recombination, and dye regeneration, are efficiently improved. Consequently, the optimization of molecular planarity can selectively control the energy level of the dye, while multiple interfacial charge-transfer processes can also be finely optimized, providing a reasonable strategy to develop an efficient organic sensitizer with long-term photostability.