Charge Transport Limitations of Redox Mediators in Dye-Sensitized Solar Cells: Investigation Based on a Quasi-Linear Model

Abstract

A simplified reaction−diffusion model is proposed to show the origins of inefficient charge transport in dye-sensitized solar cells (DSCs) based on bulky complex redox shuttles. In this model, diffusion length of the oxidized species in electrolyte solution is defined and assumed to depend on the average density of the conduction band electrons. The resulting quasi-linear equations enable the density profiles of electrons and redox shuttles to be calculated directly. When the charge transport of a DSC is under diffusion limitation, diffusion overpotential is shown to serve as an additional potential, which maintains a high level of electron density in TiO 2 film, even at short circuit. The photocurrent output is therefore considerably limited because of the accelerated recombination reaction. Moreover, short circuit current density is confined by the diffusion limited current density under inefficient charge transport, and close to the photocurrent density at high diffusivity of redox shuttles. For the DSC with slow redox shuttles, a critical thickness of TiO 2 film is observed. This is the maximum thickness allowing the charge transport to be free from diffusion limitation. The optimized film thickness yielding the maximum power conversion efficiency is shown to be slightly less than the critical thickness, which implies the potential usage of the critical thickness in designing the state-of-art DSC.

Unraveling the Nonideal Recombination Kinetics in Cobalt Complex Based Dye Sensitized Solar Cells: Impacts of Electron Lifetime and the Distribution of Electron Density

“…Thereby, the results of J sc are not influenced by the diffusion limitation of cobalt complexes. Otherwise, the J sc – I 0 plot should be curved in event of inefficient mass transport of redox shuttles . Thus, it is concluded that the employment of J sc in place of U inj in eq could give reasonable estimation for the steady-state lifetime.…”

Correlating Photovoltaic Performance of Dye-Sensitized Solar Cell to the Film Thickness of Titania via Numerical Drift-Diffusion Simulations

“…These equations were solved numerically in one dimension by using Clark-Nicolsen method. In these numerical calculations, we applied reflective boundary conditions for n c , el ox , and [26]. At the contact between TiO 2 film and TCO sheet (x=0), the interfacial density of the conduction band electron (n 0 c ) is related to its Fermi-level (E 0 F,n ).…”

Electrospun carbon nano-felt derived from alkali lignin for cost-effective counter electrodes of dye-sensitized solar cells

“…Since reported by O'Regan and Gratzel in 1991, dye-sensitized solar cells (DSSCs) have attracted growing interests as important alternatives to conventional silicon-based solar cells due to low cost, simple fabrication process, and high energy conversion efficiency. [1][2][3][4][5][6] In general, a DSSC is composed of three main components including a dye-sensitized photoanode, an electrolyte containing a redox couple such as iodide/triiodide (I À / I 3 À ), and a counter electrode. The photoanode can be made of a thin layer of transparent conducting oxide (TCO) on a glass substrate, and the TCO layer is typically coated with a porous lm of semiconductor nanoparticles (e.g., the nanoparticles of anatase-phase TiO 2 ).…”