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

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

Charge separation at disordered semiconductor heterojunctions from random walk numerical simulations

Many recent advances in novel solar cell technologies are based on charge separation in disordered semiconductor heterojunctions. In this work we use the Random Walk Numerical Simulation (RWNS) method to model the dynamics of electrons and holes in two disordered semiconductors in contact. Miller–Abrahams hopping rates and a tunnelling distance-dependent electron–hole annihilation mechanism are used to model transport and recombination, respectively. To test the validity of the model, three numerical ‘‘experiments’’ have been devised: (1) in the absence of constant illumination, charge separation has been quantified by computing surface photovoltage (SPV) transients. (2) By applying a continuous generation of electron–hole pairs, the model can be used to simulate a solar cell under steady-state conditions. This has been exploited to calculate open-circuit voltages and recombination currents for an archetypical bulk heterojunction solar cell (BHJ). (3) The calculations have been extended to nanostructured solar cells with inorganic sensitizers to study, specifically, non-ideality in the recombination rate. The RWNS model in combination with exponential disorder and an activated tunnelling mechanism for transport and recombination is shown to reproduce correctly charge separation parameters in these three ‘‘experiments’’. This provides a theoretical basis to study relevant features of novel solar cell technologies