Non-ideal behavior in a model system: Contact degradation in a molecularly doped polymer revealed by variable-temperature electric force microscopy

摘要

We present an electric force microscope and transport study of the degradation of the contact between Au and TPD, a triarylamine widely employed as a hole transporting layer in light emitting diodes. TPD was dispersed into a polystyrene (PS) binder and spin casted onto a quartz substrate with coplanar gold electrodes. Electric force microscopy was used to map the electrostatic potential drop in the device channel while a voltage was applied and the current was measured. Two contact degradation mechanisms were observed. When the TPD-PS film was allowed to age in high vacuum, the TPD crystallized out of solution. We show that the observed loss of current is the result of both a decrease in bulk mobility and a decrease in injection efficiency. The operating temperature of a freshly prepared device was then varied from 296 K to 330 K to simulate heating that might occur during light emitting diode operation. While the current increased in an apparently smooth way as the temperature was raised, electric force microscopy revealed that the underlying injection efficiency had undergone a dramatic change. Above a temperature of 310K, running current through the device led to a dramatic decrease in injection efficiency which was found to be associated with the creation of a dipole layer at the injecting contact. Upon decreasing temperature, we found that a measurable charge remained in the device channel when the applied voltage was switched to zero. The decay of the associated electrostatic potential, which appears to be governed primarily by charge-charge repulsion and not diffusion, provides an estimate the zero-field mobility of the holes in the film.