Benzimidazole derivatives in the electrolyte of new-generation organic dye-sensitized solar cells with an iodine-free redox mediator

The effect of the addition of benzimidazole derivatives (xBI) to new-generation organic dye-sensitized solar cells (DSSCs) was investigated by measuring the photovoltaic performance and by using a theoretical approach. An organic indoline dye (D149) and a stable organic radical, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), were employed as the sensitizer and redox mediator, respectively, for the fabrication of DSSCs. Three different xBI additives were added to the electrolyte with and without bis(trifluoromethane) sulfonimide lithium salt (LiTFSI) in D149-DSSCs with a TEMPO/TEMPO+ redox mediator. Competitive adsorption occurred significantly between D149 and xBI on TiO2 particles in acetonitrile without the presence of LiTFSI in the electrolyte, implying a role of Li+ in the electrolyte of D149-DSSCs. The addition of xBIs increased the open-circuit voltage (Voc) by the band-edge shift upward when a negative surface charge was built up on the surface of the TiO2 particles treated with Lewis base-xBIs. The addition of xBIs in the presence of LiTFSI/acetonitrile electrolytes would increased the photocurrent by more than 40% due to the suppression of recombination on the bare surface of the TiO2 particles by the effective shielding of Li+ on TiO2. The value of Voc decreased due to the decreased redox potential of the redox mediators in the presence of xBI, which was confirmed by the theoretical and experimental methods.