Modifying the AIE-TADF chromophore with host-substituents to achieve high efficiency and low roll-off non-doped OLEDs
Graphical abstract
Introduction
Organic light-emitting diode (OLED) was considered to be the most promising candidate for the next-generation display and lighting technology [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10]]. In recent years, thermally activated delayed fluorescence (TADF) based OLEDs have been widely paid attention to by researchers in academic and industry on account of 100% excitons utilizing peculiarity by upconversion from excited triplet state (T1) to excited singlet state (S1), without using noble heavy metal elements [[11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]]. Nowadays, although TADF emitting materials have achieved extremely high maximum external quantum efficiency (EQE) of 32%, but the serious efficiency roll-off at high luminance is a lethal factor impede its commercialization. It is still pivotal problem urgently needed to be resolved [[23], [24], [25], [26], [27], [28]]. Traditionally, a host-guest doping system is adopted to mitigate this trend due to the host can reduce the exciton density [29,30]. Unfortunately, the manufacturing process of the host-guest doping system is relatively complicated. Therefore, this prompted us to research self-doping system to manufacture non-doped device when designing molecules. Non-doped OLEDs involve only intrinsic emitting materials with nonexistence of phase separation and crystallinity can maintain film uniformity and better device stability during device fabrication and operation [31,32]. In addition, it has advantages in simplifying manufacturing process and reducing costs. However, efficient TADF emitting materials suitable for non-doped devices are still uncommon and it is a challenge to construct non-doped TADF-OLED with high efficiency and improved efficiency roll-off simultaneously.
Recently, Tang et al. reported a series of high performance TADF materials with aggregation-induced emission (AIE) properties, and a maximum EQE of 22% was obtained [[33], [34], [35], [36], [37], [38]]. It motivated us to believe that the AIE-TADF materials could provide high EL efficiency by enhancing fluorescence emission in pure film, thus we would carry out desired non-doped OLEDs [[39], [40], [41]]. Furthermore, emulating the host-guest doping mechanism, introducing host-substituents into a TADF core chromophore to form self-host TADF derivates might further enhance the efficiency and alleviate the efficiency roll-off for non-doped OLEDs [42,43]. Accordingly, self-host AIE-TADF materials based non-doped devices would pave the way to approaching excellent OLEDs. However, the selection of core chromophore and modificatory periphery host-substituents are key issues. Meanwhile, the effect of binding mode between ligand and core radicals on AIE properties is not well known yet. The intrinsic mechanism of this system should be further explored. As previously reported, diphenyl sulfone-phenoxazine (DPS-PXZ) AIE-TADF chromophore exhibited significant AIE properties due to a large twist angle between DPS and PXZ [17]. In addition, DPS-PXZ had good thermal and morphological stability, which was suitable for OLED fabrication and operation. Considering that the common hosts based on carbazole are frequently used in TADF devices to improve the performance. Here, four host-substituents 1,4-di(9H-carbazol-9-yl)benzene (DCB), 1,3-di(9H-carbazol-9- yl)benzene (mCP), 3-(4-(9H-carbazol-9-yl)phenyl)-9H-carbazole (pPhDCz) and 3-(3-(9H-carbazol-9-yl)-phenyl)-9H-carbazole (mPhDCz) were introduced to modify the AIE-TADF core chromophore DPS-PXZ. It is promising in the exploiture of advanced materials exhibit outstanding luminescent properties and achieve high efficiency and low efficiency roll-off non-doped OLEDs.
Based on above concept, a series of new self-host AIE-TADF emitters, namely DCB-DPS-PXZ, mCP-DPS-PXZ, pPhDCzDPSPXZ and mPhDCzDPSPXZ were designed and synthesized. All of four molecules exhibited AIE properties. The increase margin of fluorescence emission for pPhDCzDPSPXZ and mPhDCzDPSPXZ reached a fairly high level (6.4–6.7 times in the situation of water added to 90% compared to that of pure THF solution), however, the fluorescence enhancement for DCB-DPS-PXZ and mCP-DPS-PXZ is barely satisfactory. The study in this paper provides more insight into how substituted groups and linking positions effect on the material characteristics. Owing to strong AIE enhancement, mPhDCzDPSPXZ and pPhDCzDPSPXZ obtained high PLQY in the neat film, which were 56% and 55%, respectively and performed much better than that of DCB-DPS-PXZ (40%) and mCP-DPS-PXZ (47%), respectively. The maximum EQEs achieved by the non-doped OLEDs were 18.1% for mPhDCzDPSPXZ and 17.1% for pPhDCzDPSPXZ, and the efficiency roll-off at 1000 cd/m2 were 7.7% and 9.9% respectively. However, the maximum EQEs of 13.9% and 14.7% were obtained based on DCB-DPS-PXZ and mCP-DPS-PXZ non-doped devices, respectively.
Section snippets
Synthesis and thermal properties
Scheme 1 depicted the chemical structures and synthetic routes of DCB-DPS-PXZ, mCP-DPS-PXZ, pPhDCzDPSPXZ and mPhDCzDPSPXZ. The specific synthetic routes were outlined in the Supporting Information (Schemes S1–S3, ESI†). DCB-DPS-PXZ and mCP-DPS-PXZ were synthesis using 10-(4-((4-(9H-carbazol-3-yl)phenyl)sulfonyl)phenyl)-10H-phenoxazine and 9-(4-bromophenyl)-9H-carbazol and 9-(3-bromophenyl)-9H-carbazol via Ullmann reaction. The synthesis of pPhDCzDPSPXZ and mPhDCzDPSPXZ could be obtained by the
Conclusions
In summary, a series of TADF emitters featured aggregation-induced emission properties were designed and synthesized by modifying the DPS-PXZ with different periphery substituent groups. The introduced peripheral substituents act as host to disperse excitons. The twist angles in the molecules were adjusted by changing the bonding positions of host-substituents, which could affect the AIE and PL properties. Non-doped devices based on mPhDCzDPSPXZ and pPhDCzDPSPXZ have achieved outstanding
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This research work was supported by the NSFC/China (51573065, 51727809), China Postdoctoral Science Foundation (2017M620321), the science and technology support program of Hubei Province (2015BAA075). Thanks to SCTS/CGCL HPCC of HUST for providing computing resources and technical support. The Analytical and Testing Center at Huazhong University of Science and Technology is acknowledged for characterization of new compounds.
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