Spin-Dependent Kinetics in Dye-Sensitized Charge-Carrier Injection into Organic Crystal Electrodes
Dye-sensitized charge-carrier injection into semiconductor electrodes1 is a fascinating and literally colorful topic in the field of photoelectrochemistry. The phenomenon is easily described. Charge carriers are injected into a semiconductor or suitable insulator electrode after light absorption by adsorbed dye molecules. The excitation spectrum of the photocurrent resembles the absorption spectrum of the adsorbed dye layer or, at least, that of certain dye species on the electrode. It is fairly easy to measure the corresponding stationary photocurrent in an electrochemical cell. However, unambiguous information revealing details of dye-sensitized charge-carrier injection is virtually impossible to obtain with standard photoelectrochemical techniques, having a time resolution typically of a microsecond or even slower and very poor spectral resolution due to the high temperature and inhomogeneous broadening. Since dye-sensitized charge-carrier injection involves excited electronic states of dye molecules and charge carriers in high electric fields at the surface of the electrode, the relevant processes will proceed on a time scale of nanoseconds or faster. Thus, any realistic attempt to unravel details of the injection mechanism must provide information with this high time resolution. With this in mind, we have developed in recent years three different measuring techniques with the appropriate time resolution on the nanosecond and picosecond time scale in photoelectrochemical systems: first, measurement of the fluorescence decay of the adsorbed dye molecules with 10-ps time resolution2; second , measurement of the photocurrent in a photoelectrochemical cell with 1-ns3 and, recently, 100-ps4 time resolution; and third, measurement of the stationary magnetic-field-modulated photocurrent as a function of the electric field at molecular crystal electrodes.5,6 Complementary information collected from such different types of time-resolved measurements has to be brought together to reach a better understanding of the very complicated overall process of dye-sensitized charge-carrier injection.
KeywordsElectric Field Strength Surface Trap Spin Motion Delayed Fluorescence Magnetic Field Modulation
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