Characterization of Electron Trapping in Dye-Sensitized Solar Cells by Near-IR Transmittance Measurements

In situ near-IR transmittance measurements have been used to characterize the density of trapped electrons in dye-sensitized solar cells (DSCs). Measurements have been made under a range experimental conditions including during open-circuit photovoltage decay and during recording of the IV characteristic. The optical cross section of electrons at 940 nm was determined by relating the IR absorbance to the density of trapped electrons measured by charge extraction. The value, σn = 5.4 × 10−18 cm2, was used to compare the trapped electron densities in illuminated DSCs at open and short circuit in order to quantify the difference in the quasi Fermi level, ΔnEF under the two conditions. It was found that ΔnEF for the cells studied was 250 meV over wide range of illumination intensities. IR transmittance measurements have also been used to quantify shifts in conduction band energy associated with dye adsorption.