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Self-host thermally activated delayed fluorescence (TADF) materials have recently been identified as effective emitters for solution-processed nondoped organic light-emitting diodes (OLEDs). However, except for the carbazole unit, few novel dendrons have been developed to build self-host TADF emitters. This study reports two self-host blue materials, tbCz-SO and poCz-SO, with the same TADF emissive core and different dendrons. The influence of the peripheral dendrons on the photophysical properties and electroluminescent performances of the self-host materials were systematically investigated. The transient fluorescence and electroluminescence spectra indicated that the diphenylphosphoryl carbazole units could effectively encapsulate the emissive core to reduce the concentration quenching effect and to enhance reverse intersystem crossing. By using tbCz-SO and poCz-SO as host-free blue emitters, the performance of the solution-processed nondoped OLED device demonstrated that a more balanced charge transfer from the bipolar dendrons would offer a better current efficiency of 10.5 cd A−1 and stable color purity with Commission Internationale de L'Eclairage units of (0.18, 0.27). [ABSTRACT FROM AUTHOR]

Two self-host dendrimers TZ-Cz and TZ-3Cz with TADF nature were designed and synthesized for solution-processed nondoped fluorescent organic light-emitting diodes (OLEDs). The first and second generation carbazole dendrons were conveniently introduced into the TADF core through non-conjugated aliphatic chains, which favourably make the fluorescence of the core independent of the peripheral dendrons. Photophysical investigation showed that the prevalent concentration quenching effect of the TADF materials can be effectively restrained by the encapsulation of the emissive core. Moreover, the key role of the end-capping moieties on the molecular frontier orbital of the dendrimers was revealed by electrochemical testing and theoretical calculations. Solution-processed OLEDs featuring TZ-3Cz as the host-free emitter achieved a superior external quantum efficiency of 10.1%, which is almost ten times higher than that of the emissive core TZ (1.09%). Thus, a facile strategy to design a solution-process with highly emissive self-host TADF materials for nondoped OLEDs by flexible alkyl chain is demonstrated. [ABSTRACT FROM AUTHOR]

A series of bipolar hosts based on carbazole and phenyl benzimidazole (PBI) moieties, collectively named xCz–nPBI, were designed and synthesized. On the basis of different numbers, ratios and link-configurations of the functional groups, the influence of substitution on the chemical, photophysical and electrochemical properties of the host materials were investigated in detail. Both DFT calculations and single carrier devices demonstrate that the strategy of introducing more electron-withdrawing PBI groups in the molecules can effectively enhance the electron injection and transport ability of the bipolar host, while an increased number of carbazole units endows the hosts with a much smaller ΔEST for efficient hole injection at the cost of sacrificing their charge balance property. As a result, the solution-processed green-emitting PHOLEDs based on Cz–6PBI show an extremely low turn on voltage of 2.9 V and the highest current and power efficiency of 47.8 cd A−1 and 29.6 lm W−1, respectively. Even at luminance as high as 1000 cd m−2, their efficient roll-off was only 4.2%, which was far better than the 6Cz–PBI host device. Because the T1 energy levels and triplet state locations of these hosts are similar, their ΔEST and charge balance property should be the main factors that influence their EL performances. We conclude that it is not necessary to achieve a very small ΔEST by introducing more carbazole moieties at the cost of weakening of electron transporting ability. As for solution-processed devices, which suffer from solvent impurities and oxygen diffusion induced strong electron trapping effect, a systemic increase in the number of electron-withdrawing PBI groups in their host materials can significantly enhance the charge balance of their emission layers (EMLs) for highly power efficient solution-processed PHOLEDs. [ABSTRACT FROM AUTHOR]

g-C 3 N 4 -assisted interface engineering has been developed as an effective method to improve the efficiency and stability of perovskite solar cells (PSCs). However, most of the reported works used g-C 3 N 4 -induced single-interface modification, which is difficult to passivate the bilateral interfaces of the perovskite layer at the same time. In this paper, we fabricated two kinds of C 3 N 4 materials simultaneously (w-CN and y-CN) after the twice calcination of melamine and used them in the bilateral interface modification toward all-inorganic PSCs. The two kinds of C 3 N 4 play different roles in different interface engineering. On the front interface, w-CN could optimize band level arrangement and improve the perovskite film quality, which contributes to the efficiency of the device. On the back interface, y-CN could also improve the film quality of the perovskite layer, accelerating the extraction of charge carriers. The champion efficiency of the CsPbIBr 2 -based device treated by the bilateral interface is significantly enhanced from 7.8 to 10.1%. Moreover, the modified perovskite film exhibits negligible degradation after 40 min of exposure in the ambient environment with a relative humidity of 70%, while the pristine perovskite film has a rapid degradation within 20 min.

Highly efficient solution-processable emitters are greatly desired to develop low-cost organic light-emitting diodes (OLEDs). The recently developed thermally activated delayed fluorescence (TADF) materials are promising candidates, but blue TADF materials compatible with the all-solution-process have still not been achieved. Here, a series of TADF materials, named X-4CzCN, are developed by introducing the bulky units through an unconjugated linker, which realizes high molecular weight to enhance the solvent resistance ability without disturbing the blue TADF feature. Meanwhile, the peripheral wrapping groups efficiently inhibit the triplet-triplet and triplet-polaron quenching by isolating the energy-transfer and charge-transporting channels. The photophysical measurements indicate that a small variation in peripheral unit will have a noticeable effect on the luminescence efficiency. The enlarged volume of peripheral units will make the electroluminescent spectra blueshift, while enhancing the energy transfer of exciplex and blocking the energy leakage of electromer can facilitate the exciton utilization. As a result, the fully solution-processed blue OLED achieves a CIE of (0.16, 0.27), a low turn on voltage of 2.9 eV, and a high external quantum efficiency of 20.6 %. As far as we known, this is the first report of all-solution-processed TADF OLEDs with blue emission, which exhibits a high efficiency even comparable to the vacuum-deposited devices., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)