Charge carrier mobilities in dark-conductive organic thin films determined by the surface acoustoelectric travelling wave (SAW) technique

The surface acoustic wave (SAW) method in its most simple form [R. Adler, D. Janes, B.J. Hunsinger, S. Datta, Appl. Phys. Lett. 38 (1981) 102], determines in-sheet mobilities in semiconducting thin films by effecting an electric field force on the charge carriers. The electric field force is generated piezoelectrically by a mechanical wave propagating along the surface of a piezoelectric wafer (e.g., LiNbO3) placed underneath. Because of the unidirectional propagation of the so generated alternating electric field wave, the (slower) forced motion of the charge carriers (with respect to a fixed point) is not symmetric in the negative and in the positive half-wave of the field. On the average this is equivalent to a net constant (dc) driving field, which, in the short circuit case gives rise to a direct current. In the open circuit situation, the field is compensated by the build-up of a counter field, which — for the finally established equilibrium — can be measured and attributed to a mobility in a rather straightforward manner. We have successfully applied this method on semiconducting organic thin films and single crystals and wish to present as an example the temperature dependence of the electron mobility in vapor-deposited thin films of the radical ion salt bipyridyl-ammonium+: (tetracyano-quinodimethane)2−, BIPA+:(TCNQ)2−.