## 1 Introduction  
During the past two (plus) decades, organic light-emitting diodes (OLEDs) have been extensively studied as potential devices for full-color flat-panel displays and lighting sources [1; 2; 3; 4; 5]. OLEDs have attracted a great deal of interest and possess the potential to replace the traditional inorganic technology. OLEDs are attractive for manifold reasons. OLEDs can be operated at relatively low driving voltages and they can display high brightness, wide viewing angles. The response time can be extremely short and the power consumption can be considerably reduced relative to that of inorganic light-emitting devices. Contrarily to inorganic diodes that can only fabricated on rigid substrates (i.e., glass), OLEDs can be prepared on flexible substrates, such as polymers and the resulting devices can thus be light-weight [6; 7; 8]. Fully flexible OLEDs can also be elaborated using solution-processing methods, thus becoming particularly promising, owing to large-area low-cost fabrication potential and compatibility with flexible plastic substrates. For a full-color display and white lighting, it is essential to have the three primary colors: red, green, and blue. However, progress in blue OLEDs is far behind that of green and red OLEDs. It relies on the difficulty to develop blue emitters exhibiting high photoluminescence quantum yields (PLQY), the desired color purity and a sufficient operational lifetime for the device to be viable. Deep-blue emitters that exhibit high PLQYs are thus needed to effectively reduce the power consumption and expand the color gamut of full-color OLEDs. In addition, the development of deep blue emitters is of crucial interest as these materials can also be utilized to generate the other colors by a mechanism of energy cascade inside the emissive layer from the wide bandgap blue emitters to emitters of lower and adapted energy levels. At present, efficient and stable deep-blue OLEDs that satisfy the required Commission Internationale de l'Eclairage (CIE) coordinates of (0.14, 0.08), as defined by the National Television System Committee (NTSC) or (0.15, 0.06) defined by the High-Definition Television (HDTV) and the European Broadcasting Union (EBU) are still rare [9]. Therefore, the development of saturated deep-blue emitters with high efficiencies is crucial to produce efficient OLEDs and this topic has become an important research subject. At present, the development of deep-blue emitting materials is still challenging because of an intrinsic large band-gap and the encountered problems of stability [10; 11; 12; 13]. In the last decade, a huge number of efficient blue emitters, both fluorescent and phosphorescent ones, have been reported [14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26]. However, most of them give a light with CIE \(y\)-coordinate \(>\) 0.10, thus far from adequate with respect to the present blue standards [27]. One reason for the scarcity of reports is that it is difficult to simultaneously inject electrons and holes into such wide band-gap organic semi-conductors. Notably, for fluorescent emitters, deep blue emission is obtained by restricting the \(\pi\)-conjugation length within the materials to retain the color purity of the emitting part, a strategy that simultaneously inhibits carrier injection and transport [28; 29]. Therefore, a trade-off has to be found between an extended \(\pi\)-conjugation that favors charge transport, but at the same time impairs the emission color, and the complete isolation of the emitting part to perfectly match the NTSC or EBU blue standards but that drastically limits charge transport and injection. Carbazole is commonly used to designed blue emitters. To illustrate this, [PERSON] et al. reported OLEDs comprising a carbazole based emitter that furnished a blue light with CIE coordinates of (0.15, 0.09), a maximum external quantum efficiency (EQE) of 4.3%, and a turn-on voltage of 6.5 V [30]. [PERSON] et al.