[Two mechanisms of the color-tunable organic electroluminescence by the voltage applied]

Two devices structures of ITO/PVK: Rubrene/Al and ITO/Alq3:MN-PPV/Al were developed. Their characters of photoluminescence and electroluminscence were studied. The result showed that there were great difference between the photoluminescence and electroluminscence at a doping ratio of 2 wt% in ITO/PVK: Rubrene/Al. It is concluded that energy transfer in these dye-doped devices occurred by Förster energy transfer processes. And the trap effect of electrons of Rubrene on holes of PVK resulted in different combination probability of PVK excitation in photoluminescence and electroluminescence. At the same time, the two EL devices were color-tunable when the bias voltage was increased. The mechanisms which cause color variation are different. In the first device, it indicates that the energy transfer process from PVK to Rubrene is not complete. The combination probability of PVK excitation which does not contribute to the emission of Rubrene is increased when the bias voltage increases. But in the second device, there are many micro-interfaces due to the phase-separation just like the interface in a double layer device. The tunneling interface barriers of carriers is the main cause of color variation in the second device.