Transport-Limited Recombination of Photocarriers in Dye-Sensitized Nanocrystalline TiO2 Solar Cells

The effect of lithium intercalation on the transport dynamics and recombination kinetics in dye-sensitized nanoparticle TiO2 solar cells at lithium levels below 5 atom % was investigated by photocurrent and photovoltage transient and spectroelectrochemical techniques. Titanium dioxide films were doped electrochemically in the dark and under illumination. It was discovered that when Li+ is present in the electrolyte, lithium intercalates irreversibly into dye-sensitized TiO2 films at open circuit (ca. −0.7 V) under normal solar light intensities. Photocurrent transients of doped nonsensitized TiO2 films indicate that lithium doping decreases the diffusion coefficient of electrons through the nanoparticle network. Photocurrent and photovoltage transients of sensitized TiO2 films provide the first evidence that electron transport limits recombination with the redox electrolyte in working cells. As the Li density in the films increases, the diffusion and recombination times of photoelectrons increase proporti...