Thermally activated injection limited conduction in single layer N,N′-diphenyl-N,N′-bis(3-methylphenyl)1-1′-biphenyl-4,4′-diamine light emitting diodes

Impedance, current-voltage-luminosity and spectral measurements have been carried out on indium-tin-oxide/N,N'-diphenyl-N,N'-bis(3-methylphenyl)1-1'-biphenyl-4,4'-diamine (TPD)/Al light emitting diodes. The devices have a blue/violet emission with a spectrum peaked at 404 nm. Capacitance-voltage measurements show that at zero bias the devices are fully depleted. The impedance measurements show that the devices can be modeled on a single, frequency-independent parallel resistor-capacitor RPCP circuit with a small series resistance R-S. R-P changes with applied bias and temperature, while C-P remains constant. The values of C-P give epsilon(r)=3.0 +/- 0.3. Analysis of the current-voltage (J-V) characteristics show that the dominant conduction mechanism cannot be either ohmic, trap free space charge limited, or tunneling injection. The temperature and thickness dependence indicate that it must be either thermionic emission or thermally assisted tunneling, the carrier density varying from about 10(10)/10(11) to 3x10(13) cm(-3) over the measured bias range. The EL efficiency increases 20 fold upon cooling but shows little variation with bias at all temperatures, indicating the same mechanism is responsible for the injection of both holes and electrons. Modeling the results with thermionic emission suggests that image force lowering is responsible for the variation of the current with applied bias, but the calculated injection barrier height and Richardson constant are much smaller than expected. This cannot be explained by models based on a backflowing surface recombination current due to the high carrier mobility found in TPD. (C) 1999 American Institute of Physics. [S0021-8979(99)01420-6].