Efficient Dye-Sensitized Solar Cells with Potential-Tunable Organic Sulfide Mediators and Graphene-Modified Carbon Counter Electrodes

A new class of organic sulfide mediators with programmable redox properties is designed via density functional theory calculations and synthesized for efficient dye-sensitized solar cells (DSCs). Photophysical and electrochemical properties of these mediators derived from systematical functionalization of the framework with electron donating and withdrawing groups (MeO, Me, H, Cl, CF3, and NO2) are investigated. With this new class of organic mediators, the redox potential can be fine-tuned over a 170 mV range, overlapping the conventional I-/I(3)(-)couple. Due to the suitable interplay of physical properties and electrochemical characteristics of the mediator involving electron-donating MeO group, the DSCs based on this mediator behave excellently in various kinetic processes such as dye regeneration, electron recombination, and mass transport. Thus, the MeO derivative of the mediator is identified as having the best performance of this series of redox shuttles. As inferred from electrochemical impedance spectroscopy and cyclic voltammetry measurements, the addition of graphene into the normal carbon counter electrode material dramatically improves the apparent catalytic activity of the counter electrode towards the MeO derivative of mediator, resulting in N719 based DSCs showing a promising conversion efficiency of 6.53% under 100 mW.cm(-2) simulated sunlight illumination.