Binary redox electrolytes used in dye-sensitized solar cells

Fast dye-regeneration and slow charge recombination are prerequisites for selecting superior redox couples of electrolytes to obtain highly efficient dye-sensitized solar cells (DSSCs). Although the ubiquitous combination of the I-/I-3(-) redox couple demonstrates high power conversion efficiency (PCE), it suffers from several limitations such as a large potential difference of approximately 560 mV between the Fermi level of I-/I-3(-) and the HOMO level of the N719 dye as well as high visible light absorption. These limitations cause inefficient dye-regeneration and significantly enhance the back reaction rate of photoelectrons to I-3(-) in the electrolyte. This review discusses recent progress in the conception and device performance of different binary redox couples in DSSCs based on lowering potential differences, the back reaction of photo-induced electrons, the absorption of visible light, and improvement of dye regeneration. We specifically focus on recent strategies targeted for effectively increasing both the open circuit voltage of DSSCs up to (similar to)100 mV and the PCE to above 10%; these strategies include introduction of binary redox couples or additional redox species to conventional iodine-based electrolytes. Moreover, we propose future directions for the further development of binary redox couples with advanced concepts for achieving DSSCs with high performance and high stability. (C) 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.