Kinetics Of Charge Separation In Dye-Sensitized Solar Cells: Analysis And Simulation

Solar energy is inexhaustible and renewable power source available to human being.In recent years,thin-film photovoltaic solar cell has drawn vast attentions because of its low-cost,easy fabrication and high photoelectric conversion efficiency.Deeper understanding on the kinetics of charge transport and recombination is fundamental to realize persisting improvement of photovoltaic performance in dye-sensitized cell.In this thesis,charge recombination kinetics in dye sensitized solar cells based on the electrolytes of cobalt complex and iodine redox couple is elucidated by using the measurements on electron lifetime,the distribution of electron density,and charge recombination rate.It was found that the relationship between “steady-state” lifetime and the Fermi-level of TiO₂ film could be well described by using the Butler-Volmer?B-V?model.And there exists two characteristic regions at high and low bias voltage showing different charge recombination behaviors.Such phenomenon can be interpreted by either the unpinning of the conduction band or biexponential electron density distribution.Both of them imply that the configurations of the adsorbed dyes and small molecules vary with the Fermi-level.Moreover,recombination reaction in cobalt complex based dye sensitized solar cell is dominated by the trapping-state electrons rather than the conduction band electrons.And recombination reaction orders in the two regions are shown to depend on the apparent charge transfer rate constant and the average of density distribution parameters.This indicates the validity of B-V model in mimicking the charge recombination at TiO₂/dye/electrolyte interfaces.Additionally,the operation of dye-sensitized cell was modeled based upon reaction-diffusion equations.And the effectiveness of atomic layer deposition?ALD?technology in improving photovoltaic performance of dye-sensitized cell is investigated.Combining the tunneling model with the reaction-diffusion equations,we elucidated the influences of atomic layer deposition on cell parameters,such as open-circuit voltage,short circuit current,photoelectric conversion efficiency,transient photovoltage,and electron lifetime,etc.Two strategies of atomic layer deposition treatment were analyzed by simulation.Based upon the results,the optimized thickness of atomic layer deposition is predicted,which indicates the validity of those ALD methods in improving cell efficiency.