Direct evaluation of injection efficiency from metals into trap free small molecule based transport layers: Probing the details of interface formation

Far devices based on the use of electronic polymers thee is practical interest in exploring the variations in contact behavior that might result under realistic manufacturing conditions like mutilayer device assembly based on solution coating technology. Small molecule doped polymers (MDPs) developed principally as large area coatings for 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 range by varying the concentration of transport active species. Most significant in the present context, MDPs can be rendered trap free by molecular design. These unique characteristics of MDPs make it possible to analyze the relative injection efficiencies of their interfaces with various contacts simply by a direct comparison of current voltage characteristics with time of night drift mobility measurements carried out on the same film coatings Measurements were carried out by measuring dark hole injection into the MDP film TPD/polycarbonate and a polymeric analog, PTPB, from various preformed metal substrates as well as evaporated top contacts For preformed metal substrates under fully relaxed conditions, it was found that while injection efficiency nominally scaled with the estimated interfacial energy step there was significant variance that in some cases could be clearly associated with the specific details of interfacial chemistry. Time and temperature dependent phenomenon were also delineated and analyzed. Here contact injection efficiencies were observed to increase monotonically, following initial formation, from an emission limited to final steady state which could be ohmic.