Study of multi-wavelength emission organic electroluminescence devices

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Optical characteristics of multi-color organic light emitting diodes (OLEDs) were studied to contribute the development of highly efficient OLD devices for display. We studied using the following three kinds of multi-layer OLEDs. The first one is OLED with one emitting layer containing one guest molecule doped in host. Two color emissions were obtained by electronic transitions from the S_1 and T_1 states at 10 K, but only one color emission was obtained at room temperature. To enhance the T_1 emission at room temperature for fluorescent molecules, selection of host molecule was important. The second OLED has one emitting layer containing more than two guest molecules that give different emission bands. The intensity ratio depends on the concentration ratio of the two guests and on the T_1 energy of host molecule. The third OLED has two stacked emitting layers of different color emission. The emission intensity ratio depends on the thickness of the layers and structure of the two layers. Besides these OLEDs, the absorption, photoluminescence and ellipsometric properties of single layer films evaporated or spin-coated on glass substrate were investigated. Blue-emitting FIrpic, green Ir(ppy)_3 and red PtOEP phosphors were mainly studied, while uv-emitting CBP, polystyrene and polycarbonate fluorescent molecules were used as host materials. Temperature dependence of emission intensity is observed for the guest and host molecules. It is clarified, from quantum-mechanical quantitative analysis, that the temperature dependence is cause by balance among (1)spin-phonon relaxations in three zero-field splitting substates of the T_1 state, (2)endothermic and exothermic energy transfer between the T_1 states of guest and host, and (3)energy dissipation from the lowest-energy T_1 state of either guest or host. The quantum efficiency obtained from OLEDs is determined by such a balance at room temperature.