DIRECT EVALUATION OF CONTACT INJECTION EFFICIENCY INTO SMALL-MOLECULEBASED TRANSPORT LAYERS - INFLUENCE OF EXTRINSIC FACTORS

Studies of interface formation on conventional semiconductor materials are typically carried out under relatively pristine conditions. Howev er, for devices based on the use of electronic polymers there is also compelling interest in exploring the variations in contact behavior th at might result under realistic manufacturing conditions like multilay er device assembly based on solution coating technology. Small molecul e doped polymers (MDPs) developed principally as large area coatings f or electrophotographic use are now finding wider device applications. These polymers are insulators capable of transporting excess injected charge with a unipolar drift mobility which can be tuned over a wide r ange by varying the concentration of transport active species. Most si gnificant in the present context, MDPs can be rendered trap free by mo lecular design. These unique characteristics of MDPs make it possible to analyze the relative injection efficiencies of their interfaces wit h various contacts simply by a direct comparison of current-voltage ch aracteristics with time of flight drift mobility measurements carried out on the same film coatings. In this way, and apart from their intri nsic interest and practical value, MDPs and closely related polymeric media provide the ideal venue for the study of contact phenomena on mo lecular solids. Almost all the present measurements were carried out b y measuring dark hole injection from various preformed metal substrate s into the MDP film TPD/polycarbonate. Under these circumstances it wa s found that while injection efficiency nominally scaled with the esti mated interfacial energy step then was significant variance that in so me cases could be clearly associated with the specific details of inte rfacial chemistry. For one exceptional case where Au was evaporated on the free surface of an already cast film a time and temperature depen dent contact forming process could be delineated in which the interfac e systematically evolved from emission limited to ohmic. (C) 1998 Amer ican Institute of Physics.