In this paper, we have demonstrated the time-dependent distribution of recombination-rate of a mixed-host (MH)
organic light-emitting devices (OLEDs) by co-evaporating an ultra thin red-emitting doped layer (probe). With various
probe position, the intensity ratio of red to green directly indicates the exciton distribution in MH layer. If the position of
probe insertion is that of maximum recombination-rate, the driving voltage is also reduced which can be explained by the
increase of the recombination current. From spectral and J-V analyses, the maximum recombination-rate position is 10
nm to the hole transporting layer when MH-OLED is not aged. After 48 hours of the DC aging test, the changes in the
red to green intensity ratio of different devices are different. After 96 hours aging, this ratio does not change further
among all devices, indicative of the achievement of steady state of recombination-rate distribution. The organic materials
degrade more when it locates near the maximum of the recombination-rate.
In this paper, we demonstrated methods for determining the recombination zone in a mixed-host (MH) organic light-emitting device (OLED). The host of the emitting layer material in this device consists of a hole transport layer and an electron transport layer fabricated by co-evaporation. By comparing the spectra shift between bilayer and MH OLEDs, the recombination position with different mixing concentration can be determined. It showed the recombination zone shifts from the anode to the cathode side with increasing NPB mixing ratio.