No abstract

Charge transfer plays an important role in ruling the exciton dynamics of organic emitters. Most likely, intermolecular/intramolecular charge transfers occur when a donor meets a matched acceptor, which shapes the radiative decays of excitons. The generation and utilization of excitons are vital for organic light‐emitting devices (OLEDs). Thermally activated delayed fluorescence (TADF) emitters, which harvest the triplets via reverse intersystem crossing and thus have attracted intensive attention in the last decade, are very promising to address the issue of cost‐effectiveness in the commercialized OLEDs. Conventionally, intramolecular charge transfer via through‐bond interaction is ubiquitous in the state‐of‐the‐art TADF materials. Alternatively, through‐space charge transfer via either intermolecular or intramolecular interaction is found to be unique and effective in modulating the luminescent properties, such as the emissive colors, exciton lifetimes, and quantum yields. The emerging approaches to evoke the TADF mechanisms beyond through‐bond charge transfer in a single molecule and the related applications of the emitters in OLEDs are highlighted here. The timely and fashionable molecular design and device engineering regarding through‐space charge transfer to the emitting layers in OLEDs are compared and discussed. Finally, a conclusion and the perspectives of the electroluminescence of novel through‐space systems are proposed.

We have demonstrated an organic light-emitting diode based on molybdenum oxide ͑MoO x ͒ doped 4,4Ј ,4Љ-tris͑3-methylphenylphenylamino͒triphenylamine ͑m-MTDATA͒ as a p-type doping hole injection layer. The tris-͑8-hydroxyquinoline͒ aluminum ͑Alq 3 ͒-based organic light-emitting diodes show high brightness at very low operating voltage, 100 cd/ m 2 at 3.2 V and 1000 cd/ m 2 at 4.4 V, corresponding to a low turn-on voltage of 2.4 V. Such improved properties are attributed to the formation of the charge transfer complex produced by doping MoO x into m-MTDATA, which provides much more free hole carriers, and the introduction of an efficient electron-injecting layer to improve the performance.

We have demonstrated two kinds of stacked white organic light-emitting diodes (WOLEDs) employing tri(8-hydroxyquinoline) aluminum:20 wt %Mg/MoO3 as charge generation layer. White light emission can be obtained by mixing blue fluorescence and orange phosphorescence. Stacked WOLED with individual blue fluorescent and orange phosphorescent emissive units has better color stability and higher efficiency than that with double white emissive units, which is attributed to the avoidance of the movement of charges recombination zone and elimination of the Dexter energy transfer between blue and orange emission layers occurring in the latter. The efficiency of the stacked WOLED is 35.9 cd/A at 1000 cd/m2.