Dependence of TiO₂ Film Thickness on Photocurrent-Voltage Characteristics of Dye-Sensitized Solar Cells

The dependence of increasing thickness of nanocrystalline anatase TiO₂ film of DSSC on J-V characteristics and IPCE was studied. The films were formed by repetitive coating of anatase TiO₂ colloid and characterized by FE-SEM and SEM. With increasing film thickness from 6 to 22 μm, the J_sc increases by 96%, but V_oc and FF decrease monotonically. The η of the cell increases linearly with thickness increases up to 15 μm, due mainly to the surface area increase. The V_oc decrease is related to the increase of back electron transfer between I₃^- ions and conduction band electrons in the TiO₂ electrode. It is suggested that optimum η can be attained with around 15 μm thickness for N719 dye. To realize high energy conversion efficiency, mesoporous TiO₂ nanocrystalline electrodes of a large surface area have been investigated extensively as a key material for dyesensitized solar cells (DSSC). One of the strategies to improve conversion efficiencies is to increase light harvesting efficiency by increasing the amount of dye in the electrodes using thick TiO₂ films. A thick nanocrystalline film may, however, increase charge recombination between injected electrons and electroactive agents arising from low drift mobility of electrons in the film (10^-4-10^-7cm/Vs), which limits the conversion efficiency. Furthermore, surface states increase with increasing film thickness, offsetting the injection efficiency achieved by the large surface area