EVOLUTION IN THE CHARGE INJECTION EFFICIENCY OF EVAPORATED AU CONTACTS ON A MOLECULARLY DOPED POLYMER

Injection efficiency from evaporated Au contacts on a molecularly dope d polymer (MDP) system has been previously observed to evolve from blo cking to ohmic over time. In the present article this contact forming phenomenon is analyzed in detail. The initially blocking nature of the Au contact is in contrast with that expected from the relative workfu nctions of Au and of the polymer which suggest Au should inject holes efficiently. It is also in apparent contrast to a differently prepared interface of the same materials. The phenomenon is not unique to this interface, having been confirmed also for evaporated Ag and mechanica lly made liquid Hg contacts on the same MDP. The MDP is a disordered s olid state solution of electroactive triarylamine hole transporting TP D molecules in a polycarbonate matrix. The trap-free hole-transport MD P provides a model system for the study of metal/polymer interfaces by enabling the use of a recently developed technique that gives a quant itative measure of contact injection efficiency. The technique combine s field-dependent steady state injection current measurements at a con tact under test with time-of-flight (TOF) mobility measurements made o n the same sample. In the present case, MDP films were prepared with t wo top vapor-deposited contacts, one of Au (test contact) and one of A l (for TOF), and a bottom carbon-loaded polymer electrode which is kno wn to be ohmic for hole injection. The samples were aged at various te mperatures below the glass transition of the MDP (85 degrees C) and th e evolution of current versus field and capacitance versus frequency b ehaviors are followed in detail over time and analyzed, Control measur ements ensure that the evolution of the electrical properties is due t o the Au/polymer interface behavior and not the bulk, All evaporated A u contacts eventually achieved ohmic injection. The evaporated Au/MDP interface was also investigated by transmission electron microscopy as a function of time and showed no evidence of Au interdiffusion in the MDP layer, remaining abrupt to within similar to 10 Angstrom over the course of the evolution in injection efficiency. Mechanisms related t o Au penetration into the MDP are therefore unlikely. Rapid sequence d ata acquisition enabled the detection of two main processes in the inj ection evolution. The evolving injection efficiency is very well fit b y two exponentials, enabling the characterization of time and temperat ure dependence of the evolution processes. (C) 1998 American Institute of Physics.