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 e lectrons and holes in organic films prepared on silicon or indium-tin-oxide (ITO)-coated glass substrates have been determined. For the samples on sil icon, the silicon was also used as a carrier-generating layer This substant ially increased the number of charge carriers generated and thus resulted i n a higher intensity electrical signal. Consequently, the thickness of the organic layers can be reduced to less than 1/10 of the typical values (seve ral microns) required in the conventional TOF measurement. The typical thic kness of the organic layer in the present work is 400 nm. For organic mater ials with a high optical absorption coefficient, samples for the TOF measur ement 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 use d in organic electroluminescent devices. The signal, and thus the accuracy, in the present measurement were much improved over those of the convention al TOF measurement. The logarithm of the drift mobility changed linearly wi th the square root of the applied electric field.