Estimate of carrier balance and exciton distribution in organic light-emitting diode

Organic light-emitting diodes (OLEDs) have attractive advantages as flat panel display and flat lighting device. The EL performance of OLED is often described as the external quantum efficiency. The external quantum efficiency is defined as the product of light out-coupling factor, PL quantum efficiency, exciton factor (singlet, 0.25), and carrier balance factor. Carrier balance factor is described as the ratio of recombination current to the current flowing in a external circuit. We estimate the above parameters by means of assuming the exciton distribution due to concentration diffusion. We use naphthyl-substituted diamine derivative as a hole transport material and aluminum quioline as an emission material. The external quantum efficiency is enhanced from 0.92% to 1.1% by introducing hole-blocking layer. When the exciton distribution in steady state is assumed to be a simple exponential defined by the diffusion length, EL intensity can be calculated. By comparing the experimental value with the calculated value, we can estimate the range of the above parameters.