Electron Lifetime in Dye-Sensitized Solar Cells: Theory and Interpretation of Measurements

The electron lifetime τn in dye-sensitized solar cells (DSC) is a central quantity to determine the recombination dynamics in the solar cell. It can be measured by several methods: impedance spectroscopy, IMVS, stepped time transients, and open-circuit voltage decays. The paper aims at a better understanding of this fundamental parameter. We summarize the main models that describe the lifetime dependence on bias voltage or carrier density, and find that there are two complementary approaches to clarify the structure of the lifetime. The first is to treat the lifetime as a product of the chemical capacitance and recombination resistance. This approach is important because the resistance largely determines steady state operation characteristics of the solar cell close to open-circuit voltage. The second approach is based on a kinetic model that describes in detail the different processes governing the decay of the carrier population in a measurement of τn. The lifetime is composed of a trapping factor and a free electron lifetime. Since the diffusion coefficient contains the reciprocal of the trapping factor, it is found that a product (diffusion coefficient) × (lifetime) reveals the shape of the free electron lifetime, which contains the essential information on kinetics of electron transfer at the surface as a function of the position of the Fermi level. A model based on an exponential distribution of surface states provides a good description of the voltage and temperature dependence of free electron lifetime and diffusion lengths in high performance DSCs.