Influence of Lithium Ion Concentration on Electron Injection, Transport, and Recombination in Dye-Sensitized Solar Cells

Varying Li+ concentration in the electrolyte of dye-sensitized solar cells equipped with compact TiO2 blocking layers is found to alter the mean slopes of semi logarithmic open-circuit photovoltage-intensity and dark current-voltage plots. Almost identical values of ideality factor or transfer coefficient are required to fit data in the dark and under illumination for each Li+ concentration. It is found that cell characteristics become progressively more "ideal" as Li+ concentration is increased, with a transfer coefficient of ca. l for 1 M Li+ in the electrolyte. We find that trends in photovoltage-intensity data are well fitted using a model which assumes that electron transfer to acceptor species in the electrolyte occurs from both the conduction band of the TiO2 and an exponential distribution of band gap surface states. Changes in the mean ideality factor and linearity of semilogarithmic photovoltage-intensity plots can be rationalized by considering the variation in overlap between occupied donor states (conduction band and surface states) with electron acceptor states in the electrolyte, as the conduction band edge is shifted positive by increasing Li+ concentration. In accordance with previous studies, this positive shift in conduction band edge is also found to cause a dramatic increase in the photocurrent generation efficiency of the cells, especially in the long-wavelength region of the photocurrent action spectrum. It is argued that this improvement in photocurrent is predominantly due to an increase in wavelength-dependent electron injection efficiency, as opposed to an increase in electron collection efficiency.