Drift mobility of holes in vacuum-deposited films of zinc tetraphenylporphyrin

Drift mobilities (mu) of holes in zinc tetraphenylporphyrin (ZnTPP) films were measured using the time-of-flight method. The plot of log mu vs. a square root of electric field (E(1/2)) fit a straight line and its extrapolation to E=O yielded an extremely small mu value of the order 10(-10) cm(2) V-1 s(-1) at 20 degrees C. The field dependencies of the hole mobilities at different temperatures were analyzed on the basis of the disorder formalism developed by Bassler and co-workers. As a result, the slow hole transport in the ZnTPP films was ascribed to a greater energetic disorder parameter(sigma=0.15 eV) resulting from a nearly amorphous structure of the ZnTPP films.