Improved time-of-flight technique for measuring carrier mobility in thin films of organic electroluminescent materials

Using an improved time-of-flight (TOF) technique, the drift mobilities of electrons and holes in organic films prepared on silicon or indium-tin-oxide (ITO)-coated glass substrates have been determined. For the samples on silicon, the silicon was also used as a carrier-generating layer This substantially increased the number of charge carriers generated and thus resulted in a higher intensity electrical signal. Consequently, the thickness of the organic layers can be reduced to less than 1/10 of the typical values (several microns) required in the conventional TOF measurement. The typical thickness of the organic layer in the present work is 400 nm. For organic materials with a high optical absorption coefficient, samples for the TOF measurement can be prepared by directly depositing these materials onto ITO glass substrates with a thickness of about 1000 nm. For both types of substrate, the thickness of the organic layer is much closer to the typical value used in organic electroluminescent devices. The signal, and thus the accuracy, in the present measurement were much improved over those of the conventional TOF measurement. The logarithm of the drift mobility changed linearly with the square root of the applied electric field.