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 proportionately, indicating a causal link between electron transport and recombination. The electron diffusion coefficient in dye-sensitized solar cells exhibits a power-law dependence on photocharge at all concentrations of inserted lithium in the TiO2 film. With increasing doping, the dependence of the electron diffusion coefficient on the photocharge becomes stronger, a phenomenon attributed to widening of the exponential conduction band tail resulting from disorder induced by randomly placed lithium defects in TiO2. The photovoltaic characteristics of dye-sensitized solar cells are largely unaffected by lithium intercalation, implying that intercalation has only a small effect on the charge collection efficiency and the rate of recombination. A simple model is presented that explains the observed transport-limited recombination. The results suggest that increasing the electron transport rate will not significantly improve the solar cell performance.