## 2 Results and Discussion
### Diphenylsulfone Based Emitters
Employing a similar device structure and preparation conditions, **4**-based device gave a maximum EQE of 14.5% accompanied by a small efficiency roll-off. This improvement can be assigned to its small \(\Delta E_{\text{ST}}\) and the shorter lifetime of the delayed luminescence. The same group has also proposed **5**, a thermally cross-linkable and solution processable version of **4**[67]. If the strategy seems to be promising to address the stability issue, device performances of **5**-based TADF OLEDs remained low compared to the vacuum-processed OLEDs and an EQE only peaking at 2.0% could be measured. For the small molecular deep-blue emitters, the use of the host material with a larger energy gap is often needed to suppress emission quenching in the devices [68; 69; 70]. This doping strategy could be the source of inefficient hole and/or electron injection into the emitting layer [71]. In addition, it requires the precise control of the doping concentration, a rather complicated structure, and high fabrication costs. For this reason, developing efficient blue emitters with solution processability and reduced \(\Delta E_{\text{ST}}\) values for non-doped device is of importance. As mentioned above, **3** furnished deep-blue OLED achieving a high EQE of 10%, but the devices suffered from efficiency roll-off at high current density, which was primarily caused by its relatively large \(\Delta E_{\text{ST}}\) (0.32 eV, Table 1) [66]. To overcome this issue, [PERSON] et al. proposed to use sulfone based dendrimers (**6-7**, Figure 2) [72]. Dendrimers exhibit great advantages owning to their well-defined molecular structure, precisely controllable molecular size, as well as the inhabited molecular interactions [73; 74]. However, until now, the reported blue TADF dendrimers are still rare [75], while TADF dendrimers displaying green and greenish-blue emissions have been utilized for the fabrication of non-doped solution-processed OLEDs while exhibiting satisfying electroluminescent performance [75; 76; 77; 78]. **6** and **7** inherited blue emitting and similar triplet energy level to **3**, but superior in smaller \(\Delta E_{\text{ST}}\) to facilitate the RISC process. As expected, the specific molecular design of **6** and **7** prevented from strong intermolecular interactions, while providing high thermal and morphological stabilities and good solution processability. Authors investigated these dendrimers for the design of non-doped solution processed devices and efficient deep-blue OLEDs with low efficiency roll-off at high current density were prepared. OLEDs employing **6** as the non-doped emissive layer achieved a deep blue emission with a maximum CE of 4.1 cd/A and colour coordinates of (0.15, 0.12). Similar devices based on 7 displayed a negligible efficiency roll-off at high current density. This study provides several interesting elements for the molecular conception of TADF emitters and the deep blue dendrimers that are reported in this work are promising candidates for the design of solution-processed OLEDs. In the search for new structures, [PERSON] et al. used azasiline as a donor in combination with DPS to construct a push-pull deep blue emitter (compound **8**) with colour coordinates of (0.15, 0.11) [79]. Carbazole has been considered during decades as an excellent donor for the design of light-emitting materials and a huge number of carbazole-based blue emitters have been reported in the literature. However, carbazole exhibits a central five-membered ring that lowers the steric hindrance generated by the lateral aromatic rings. For this reason, the substitution of the 1,8-positions of carbazole has long been considered as an opportunity to increase the bulkiness of this group and this specific substitution of the carbazole core thus constitutes an effective strategy to maintain a large dihedral angle. In contrast to this, azasiline has a six-membered central ring and this group is thus capable of inducing the same encumbrance without taking recourse to a substitution.