Field distribution and criterion for bulk-limited and injection-limited current conduction in single layer organic light-emitting devices

Abstract

A numerical model for current conduction in single layer OLEDs including both injection and bulk effect is proposed. Based upon this model, a nearly linear distribution of the electric field was found, and the slope of the distribution, or the field at the injection electrode (F₀) is dependent on the energy barrier, mobility, trap density and trap depth. F₀ equals the half of the mean field of the device (F_m), which equals the quotient of the bias to the thickness of organic layer, is proposed as the limit for bulk-limited (BL) and injection-limited (IL) conduction. OLEDs with F₀ greater than F_m/2 are considered as IL-conducting, while those with F₀ less than F_m/2 are considered as BL-conducting. It was found that, the state of current conduction is not only determined by the energy barrier at the injection electrode, but also by the mobility, trap density and trap depth of the organic semiconductor. OLEDs with high injection barrier (>0.7 eV), trap density less than 10¹⁹ cm^-3, and reduced trap depth shallower than 5, will be IL-conducting, while those with low energy barrier (<0.2 eV), low carrier mobility (<10^-6 cm² V^-1 s^-1), and trap density higher than 10¹⁷ cm^-3, will be BL-conducting.