Origin of Nonlinear Recombination in Dye-Sensitized Solar Cells: Interplay between Charge Transport and Charge Transfer

Electron transfer between nanostructured semiconductor oxides and redox active electrolytes is a fundamental step in many processes of technological interest, such as photocatalysis and dye-sensitized solar cells. It has been shown that the transfer kinetics in the dye-sensitized solar cell are determined simultaneously by trap-limited transport and by the relative energetics of donor and acceptor states in the semiconductor and electrolyte. In this work, the transport and recombination mechanisms of photogenerated electrons in dye-sensitized solar cells are modeled by random walk numerical simulations with explicit description of the electron transfer process in terms of the Marcus-Gerischer model. The recombination rate is computed as a function of Fermi level in order to extract the electron lifetime and its influence on the electron diffusion length. The simulation method allows one to relate the recombination reaction order to the trap distribution parameter, the band edge position, and the reorganization energy. The results show that a model involving electron transfer from both shallow and deep traps, coupled with transport via shallow states, adequately reproduces all the experimental phenomena, including the dependence of the electron lifetime and the electron diffusion length on the open-circuit voltage when either the conduction band or the redox potential are displaced. Nonlinear recombination is predicted when the electron diffusion length increases with Fermi level, which is correlated with a reaction order different from one, in an open-circuit voltage decay experiment. The results reported here are relevant to the understanding of the effect of using new electrolyte compositions and novel redox shuttles in dye-sensitized solar cells.