Dynamics of Interfacial Electron Transfer in Solar Energy Conversion As Viewed By Ultrafast Spectroscopy
Solar energy conversion by dye sensitized nanostructured metal oxide solar cells (Grätzel solar cells) relies on fast and efficient conversion of excited states into conduction band electrons (electron injection) and sufficiently slow electron-cation and electron-electrolyte recombination. Injection has to compete efficiently with other deactivation processes of the sensitizer excited state and recombination between conduction band electrons and oxidized dye or oxidized redox species has to be slow compared to re-reduction (regeneration) of the oxidized sensitizer by the redox system. Since for many sensitizer/semiconductor systems electron injection and recombination are reciprocal processes, very fast injection often leads to fast recombination. Optimized solar cell performance, therefore, often does not correspond to the fastest electron transfers. Distances, geometries and interactions between sensitizer and semiconductor nanoparticles control the electron transfer processes. For rational design of novel solar cell materials with predictable properties, detailed knowledge of the factors controlling interfacial electron transfer is desirable. Light triggered electron transfers occur on the femto- to nanosecond timescale, implying that time resolved ultrafast spectroscopy is a powerful tool for characterizing these processes. In this chapter we illustrate this with some results for a few different sensitizer and semiconductor systems. Electron transfer and sensitizer binding geometries and interactions are intimately connected and determine solar cell properties. Therefore, we correlate electron transfer dynamics with binding geometries obtained from surface sensitive techniques and finally link these properties to solar cell performance.