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Organic light-emitting devices (OLEDs) are already widely used for common applications like OLED TVs or smartphone displays. Nevertheless, it is still a challenge for both science and industry to develop OLED systems for lighting applications that combine true-color white light, high efficiencies and high brightness at the same time. Since white emission in OLEDs is usually a combination of two or more different emitters with individual colors it is necessary that all included systems are efficient. It has been shown that the concept of thermally activated delayed fluorescence (TADF) allows to synthesize very efficient light-emitting molecules with various emission colors.
In our work, we use the sky-blue TADF emitter 9-[2,3,4,5-tetra(carbazol-9-yl)-6-(trifluoromethyl)phenyl]carbazole (5CzCF3Ph) with an emission maximum at a wavelength of 495 nm in thin films, reaching a photoluminescence quantum yield of 70 %. In an OLED, the emitter delivered up to 18 % external quantum efficiency (EQE). This is beyond the theoretical limit of conventional fluorescent OLEDs. To achieve warm-white emission, we combine the sky-blue emission of 5CzCF3Ph with the red emission of the common phosphorescent emitter Ir(MDQ)2(acac) within one emission layer. Due to the very broad blue emission (FWHM ~ 95 nm), a dedicated deep blue emitter becomes obsolete and it is possible to tune the combined two-color spectrum in such a way, that a high color rendering index of over 80 and correlated color temperatures about 2800 K can be obtained by this strategy. EQEs of up to 17 % and luminous efficacies of 16 lm/W have been measured for the hybrid white OLEDs. This two-color concept paves the way towards future utilization of TADF emitters in lighting applications by simplifying the required sequence of organic layers inside the OLED.
In our approach, the excitons are formed mostly on the TADF emitter itself. To achieve a suitable amount of red light for the white emission, it is necessary to enable efficient exciton transition pathways between 5CzCF3Ph and Ir(MDQ)2(acac). Due to the variety of potential local and charge-transfer excited states in the emitter system, there are several probable scenarios for the energy transfer. Utilizing time-correlated single photon counting (TCSPC) with a wavelength-sensitive detection, we study the exciton decay of both the TADF prompt and delayed fluorescence as well as the phosphorescent emission channel in detail. With this technique, we deliver a thorough investigation of the exciton transfer and exchange mechanisms in the emitter system of our warm-white hybrid OLEDs.
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