Experimental Determination of Carrier Mobilities in Organic Molecular Crystals

The measurement of carrier mobilities in organic molecular crystals enables one to understand the basic mechanisms of charge-transport in these systems. This, in turn, has direct bearing on electronic conduction in biologically important molecules such as 5-carotene, chlorophyll, nucleic acids, etc. However, because of many inherent difficulties (for example: low quantum–efficiency of carrier–generation; severe trapping effects; development of space-charge regions; electrode crystal contact and other barrier phenomena; etc.) progress toward better understanding remains limited. In view of these difficulties, it is not surprising that the first successful determination of drift mobilities in atypical molecular crystal, namely anthracene, was not completed until 1960 when LeBlanc' and Kepler independently measured mobilities using a method now known as the “pulse photocurrent technique”. Despite this apparent initial success, no consistent approach or the general application of such methods to organic materials has evolved. As a result, the list of organic molecular crystals for which reliable mobility data exists is quite small.