Numerical Model to Simulate Electrochemical Charging of Nanocrystal Films

Electrochemical charging of nanocrystal films opens up new possibilities for designing quantum dot-based device structures, but a solid theoretical framework of this process and its limitations is lacking. In this work, drift-diffusion simulations are employed to model the charging of nanocrystal films and gain insight into the electrochemical doping process. Through steady state simulations it is shown that the Fermi level and doping density in the nanocrystal film depend on the concentration of the electrolyte in addition to the value of the applied potential. Timeresolved simulations reveal that charging is often limited by transport of electrolyte ions. However, ion transport in the film is dominated by drift, rather than diffusion, and the concentration profile of ions differs substantially from concentration profiles of diffusing redox species at flat electrodes. Classical electrochemical theory cannot be used to model this type of mass transport limited behavior in films of nanocrystals, so a new model is developed. We show that the Randles-Sevcik equation, which is derived for electrochemical species diffusing in solution, but is often applied to films as well, results in a significant underestimation of the diffusion coefficients of the charge compensating electrolyte ions.

Automated summary

In this study, drift-diffusion simulations are used to model the electrochemical charging of nanocrystal films and investigate the limitations of this process. It is found that the concentration of the electrolyte and the applied potential affect the Fermi level and doping density in the film. The results also show that ion transport in the film is dominated by drift, rather than diffusion, which is different from classical electrochemical theory.