<title>Charge carrier generation and transport in a binary donor-acceptor glass</title>

Abstract

Charge carrier generation and charge carrier transport in binary mixtures of the electron acceptor 4H-1,1-dioxo-4- dicyanomehtylidene-2-p-tolyl-6-phenylthiopyran (PTS) and the electron donor N,N'-bis(3-methylphenyl)-(1,1'-bi-phenyl)- 4,4'-diamine (TPD) have been studied. It is found that charge generation in TPD:PTS is sensitized by the formation of exciplexes and charge-transfer complexes. Moreover, TPD:PTS displays hole transport as well as electron transport. Charge carrier mobilities have been measured for different TPD:PTS ratios and are discussed within the framework of the disorder model of Baessler and coworkers. TPD:PTS displays trap-free electron transport over a wide range of electric fields, temperatures, and mixture ratios. The results demonstrate that the presence of the donor molecules has no influence on the electron transport properties. In contrast to this, the hole transport is apparently affected by dipolar interactions between TPD and PTS molecules. THus the width of DOS increases from 0.08 eV in TPD to 0.12 eV upon adding PTS. Agreement with experiment requires that the van der Waals component is dependent on the concentration of the transport molecules, increasing with increasing dilution. Surprisingly the highest non- diagonal disorder for hole transport of about (Sigma) equals 5 is found in mixtures of similar TPD and PTS ratio indicating that the formation of charge-transfer complexes causes additional disorder.