Effect of ions added by mordant and brightener on the signal structure and energy storage properties of natural dye-based DSSCs

Abstract

It has been observed experimentally that the output voltage profiles are exponential of two types: rising or decaying; the structure of these profiles is controlled by the mobility of charges: ions and electrons. A comparison between the arrival times of electrons at electrode τe, and ions at double-layer τi, allows determining the shape of these signals. The electrons produced by the photon–dye interaction reach the cathode increasing the output voltage. The ions in the electrolyte are moved by the charge on the electrodes forming the electrical double layer; these ions reduce the output voltage: electrons and ions produce opposite effects. These two charge accumulations produce double layers on both sides of the electrode–electrolyte interface forming double-layer capacitors. The numerical values of several cell parameters (energy, electric field, charge density, capacitance) were estimated, and it was found that they depend on κ; the inverse of the thickness of the double layer λ = κ−1; because κ is a large number, double-layer capacitors are suitable for storing large amounts of energy which is associated with chemical capacitance Cμ. Two dyes were used: brazilwood and cochineal, Al3+ as a mordant, and Na+-metasilicate as brightener. A model was included to describe and fit oscillations in the voltage profiles that appear due to an abrupt illumination. An expression was obtained for Cμ as a function of the voltage.