Electrical Properties of Au/Polymer Interfaces

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EXPERIMENTAL The sample configurations used for the measurements are shown in Fig. la and an illustration of graphical mehods in Fig. lb. The TOF measurement is performed using a semitransparent Al top contact [5a-b,61 enabling the calculation of the trap-free space charge limited current, which is the maximum current the bulk will support, according to 121 J'n:sc1.c = 9/8 PEE. V2/d3 (1) Where d=MDP thickness (typically 201tm), V= potential, t=drift mobility, E=dielectric constant and E0=vacuum permitivity. Any injection current measured can then be compared against the maximum value calculated so that injection efficiency of that contact is simply the ratio of measured to calculated value. Details of the rationale for this method are described in an accompanying paper as well as in 15,61. At the same time, monitoring the mobility during the evolution and during the sample manipulations to be described ensures that any eflects noted are not due to changes in the bulk transport properties. The injection current is measured for the contact under test, evaporated Au, as well as the bottom contact which then acts as a control contact as well. The ox)ttom contact is an ohmic contact for hole injection into this MDP, as seen in Fig. lb where the TFSCLC current density calculated from the mobility data coincides with the injection measured from the bottom contact. Equivalently therefore, the injection from the Au contact can be compared to that from the bottom contact to obtain the injection efficiency. Au contacts were evaporated by resitive heating of a Au source, producing films of 220-250A at a rate of 10A/s, unless otherwise indicated.

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Fig. I. a. Sample configuration; b. Illustration of graphical methods RESULTS AND DISCUSSION Evolution parameters - time constants and thermal activation The hole injection current from evaporated Au contacts was studied as a function of applied field, E, over a one month period for three samples aged at 23, 40 and 65"C. Results were reproducible for two subsequent sample sets. The hole injection current from the control contact, which was the commercially available carbon-filled polyimide Myst-R®, was monitored during this time, as was the TOF drift mobility. Current density from Myst-R®, Jm,remained constant throughout the experimental time scale, as did the drift mobility, yielding constant Jrsclc results, such that Jm-JTnScw throughout this study. Beginning at 5 minutes after Au deposition, J-E curves were obtained ever -15 minutes for the first few hours, tapering off to every few hours and finally every 2 days as the rate of change in injection current decreased. Indicative results are illustrated in Fig.2 for a sample aged at 40"C. The initial behavior of the Au contact is fairly blocking, with the injecti

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