INJECTION EFFICIENCY FROM VARIOUS METALS INTO TRAP FREE MOLECULARLY DOPED POLYMERS EVALUATED FROM COMBINED ANALYSIS OF CURRENT-VOLTAGE AND TIME-OF-FLIGHT DRIFT MOBILITY DATA

摘要

An experimental procedure which can provide a direct quantitative measure of contact injection efficiency into a trap free transport polymer is described. This technique combines coupled analyses of bias dependent injection current supplied by the contact under test with time-of-flight drift mobility measurements carried out on the same specimen film. This general procedure has been used in a broad range of experiments to evaluate the injection efficiencies from a series of hole injecting contacts of differing work functions into the molecularly doped polymer, TPD/Polycarbonate Two key results are emphasized: (1) Injection efficiency does scale on average with the interfacial barrier, the energy step estimated from substrate workfunction and MDP electrochemical data. In individual cases however, large variations about the norm are clearly identified and can be attributed to variations in the contact surface prior to coating, for example the relative degree of oxidation or, as TOFSIMS reveals, the interdiffusion of metal atoms from adhesive layers which secure the metal coating to a Si wafer substrate. (2) For the case of Au, the injection efficiency of a Au substrate was compared to the behavior of Au vacuum deposited onto the already formed coating under a variety of conditions. The injection efficiency from Au evaporated onto preformed coating initially varies with deposition conditions but relaxes with time under ambient conditions, eventually becoming ohmic. Significant details of this relaxation behavior are described and analyzed in an accompanying paper.