Analysis Of Ac Impedance Characteristics Of Dye-sensitized Solar Cells Based On Diffusion Theory

As a new type of photoelectrochemical cell, dye-sensitized solar cell (DSSC) has attracted much attention these years. Compared to traditional Si-based solar cells, DSSCs are well known as low-cost, convenient to preparation, energy-saving, flexible, wide applications and so on. Currently, there are two bottlenecks for the progress of DSSCs-low efficiency and weak long term stability. It is urgent to clarify the mechanics of DSSC operation to solve these problems. Electrochemical impedance spectroscopy is a common-used and effective method to study DSSCs. We study the physical process and reaction in the DSSCs by establishing equivalent circuit models compared with experimental results. This paper starts from diffusion theory, discusses the boundary conditions of photo anode/electrolyte interface, and derives a formula of the total impedance, based on which impedance coupling, dispersive effect in medium and high frequencies and negative capacitance effect are found.Impedance coupling figures out that the view of series connection of photo anode and electrolyte impedances is inappropriate in previous theoretical models. The motion of elections in the photo anode and the electrolyte is restrained by continuity of election transport in the interface, so the total impedance cannot be simply described by series connection. Our research showes that total impedance can be split into two OT circuits, which are in series. In high frequency limit, the phase elements of the four equivalent circuit elements are all π/4with completely impedance coupling; as frequency steps down, the coupling goes weak and the impedance is decoupled in a dc condition.With an in-depth analysis of electron concentration oscillation model in medium and high frequencies, we point out that the capacitance is derived from double-layer-like election accumulation zone and electron depletion zone while concentration oscillating. Yet since the two zones are time-dependent, the impedance deviates from pure capacitance, that is, dispersive effect.Electron concentration oscillation under ac perturbation is separated into two parts, the forward transport and the backward transport, which respectively correspond to the forward impedance and the backward impedance, connected in parallel. In medium and high frequencies, the total impedance shows characteristic of capacitance most, for it is dominated by forward impedance, which shows characteristic of capacitance only. Backward impedance shows lots of peculiar properties, including periodicity of phase shift in medium and high frequencies and negative capacitance in low frequencies. For its impedance is so great in medium and high frequencies, its influence can be ignored considering its parallel connection with forward impedance; in low frequencies, the backward impedance only shows negative capacitance characteristic and is becoming more and more remarkable, which leads the total impedance to shift from capacitance type to resistance type, and even to negative capacitance type at times. By resolving the total impedance in this way, we can confirm the major factors of the impedance in different frequency zones, get correct information from the impedance spectroscopy, and clarify the mechanics of electron transport.