Ultra-fast electron injection

Ultra-fast Electron Injection to Semiconductor Electrodes... 

The long effective pathlength and high surface area afforded by these colloidal semiconductor materials allow spectroscopic characterization of interfacial electron transfer in molecular detail that was not previously possible. It is likely that within the next decade photoinduced interfacial electron transfer will be understood in the same detail now found only in homogeneous fluid solution. In many cases the sensitization mechanisms and theory developed for planar electrodes" are not applicable to the sensitized nanocrystalline films. Therefore, new models are necessary to describe the fascinating optical and electronic behavior of these materials. One such behavior is the recent identification of ultra-fast hot injection from molecular excited states. Furthermore, with these sensitized electrodes it is possible to probe ultra-fast processes using simple steady-state photocurrent action spectrum. 

Durrant and co-workers performed ultra-fast interfacial electron injection studies with cM-Ru(dcb)2(NCS)2-Ti02 in 1 1 ethylene carbonate-propylene carbonate solvent and were able to observe both the oxidized sensitizer and the electron in Ti02 by time-resolved absorption spectroscopy [202]. Unfortunately, at long observation wavelengths, X = 650-900 nm, these absorption features overlapped with each other, making assignments and analysis difficult. Detailed analysis of the transient absorption data indicate that the electron injection process is at least bi-phasic with 50 % injecting in less than 150 fs and 50 % in 1.2 ps. 

The group also found that electron injection onto the Ti02 conduction band could occur on a fast or ultra fast time scale. The kinetics of electron transfer in the dye-sensitized nanocrystalline Ti02 solar cell is summarized by following equations. Recombination reaction takes place in a hundredth of a nanosecond. Regeneration of the dye cation by the iodide system is faster than the recombination reaction. 

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