A reappraisal of the electron diffusion length in solid-state dye-sensitized solar cells

The performance of dye-sensitized solar cells (DSCs) depends on the competition between transport and interfacial recombination of electrons. A key parameter in this context is the electron diffusion length, which is given by L-n = (D tau)(1/2), where D and tau are, respectively, the diffusion coefficient and lifetime of mobile electrons. A new approach to the reliable estimation of L-n is described, which involves use of a titanium contact to measure the short-circuit value of the electron quasi-Fermi level on the side of the TiO2 film furthest from the anode contact. This information is used to define conditions under which the effects of electron trapping/detrapping can be eliminated from the calculation of L-n. The method is illustrated by measurements of a solid-state DSC based on the organic hole conductor 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)9,9'-spirobifluorene (spiro-OMeTAD), which show that L-n is significantly greater than had been thought previously.