Shaolong Gong

De Novo Design of Silicon-Bridged Molecule Towards a Bipolar Host: All-Phosphor White Organic Light-Emitting Devices Exhibiting High Efficiency and Low Efficiency Roll-Off

White organic light-emitting diodes (WOLEDs) continue to attract intensive interest due to their applications in full color fl at-panel displays and lighting sources. [ 1 , 2 ] Among various approaches to realize WOLEDs, adopting all phosphorescent emitters has great potential because electrophosphorescence can, in theory, achieve unity internal quantum effi ciency through harvesting both singlet and triplet excitons. [ 3 ] Allphosphor WOLEDs have made considerable progress in recent years; [ 4–6 ] however, they are still a long way from commercialization. Firstly, a major bottleneck is the lack of highly effi cient and stable blue-emitting electrophosphorescence, which is indispensable to WOLEDs. Secondly, all-phosphor WOLEDs usually exhibit signifi cant effi ciency roll-off at the luminance required for practical light sources. Thirdly, the two or multiple emitting layer (EML) structures widely adopted in WOLEDs inevitably result in higher complexity, control diffi culties and higher operational voltages during fabrication. [ 5 , 7 ] To address these issues, elegant materials and elaborate device structures have to be developed. In terms of material, a bipolar host capable of hosting blue phosphor can play a vital role in solving the above-mentioned problems because a bipolar host can not only broaden the exciton-formation zone – consequently reducing the effi ciency roll-off – but also simplify the device structure by employing a single emitting layer comprised of a bipolar host and two or more phosphors. However, the design of such a host is a signifi cant challenge because three tradeoffs have to be managed: i) the high enough triplet energy required to host blue phosphor and bipolar transporting characteristics, since the pand n-type segments integrated into the molecule structure unavoidably lower the band gap of the material due to intramolecular charge-transfer; [ 8 ] ii) the high triplet energy and minimum energy barrier between the host material and

Highly Efficient Deep-Blue Electrophosphorescence Enabled by Solution-Processed Bipolar Tetraarylsilane Host with Both a High Triplet Energy and a High-Lying HOMO Level

A new bipolar tetraarylsilane derivative with both a sufficiently high triplet energy and a high-lying highest occupied molecular orbital energy level is presented as an excellent host for the deep-blue phosphor. A solution-processed FIr6-based deep-blue phosphorescent organic light-emitting diode (PhOLED) is realized with high efficiency and low efficiency roll-off.

Simple CBP isomers with high triplet energies for highly efficient blue electrophosphorescence

Two simple CBP isomers, namely mm-CBP and ooo-CBP, were designed and synthesized by finely tuning the linking topology between the carbazole and the central biphenyl units of CBP, and their thermal, photophysical and electrochemical properties were investigated. Such simple modification of the linking topology endows the CBP isomers with high triplet energy and relative high thermal and morphological stability. The high triplet energies of mm-CBP and ooo-CBP ensure efficient energy transfer from the host to the phosphor and triplet exciton confinement on the phosphor, as indicated by the transient photoluminescence decay of 3 wt% FIrpic doped into mm-CBP and ooo-CBP. Blue phosphorescent devices employing FIrpic as guest and the two CBP isomers as hosts exhibit high efficiencies. The best EL performance is achieved for the ooo-CBP-based device, with a maximum current efficiency of 29.9 cd A−1, and a maximum external quantum efficiency of 14.2%, which are over 2 times higher than those of CBP.

Multi-carbazole encapsulation as a simple strategy for the construction of solution-processed, non-doped thermally activated delayed fluorescence emitters

Two TADF emitters are developed via the introduction of carbazole dendrons into the multi-positions of a blue TADF emissive core. By utilizing these TADF emitters as the non-doped solution-processed emissive layers, the resulting greenish-blue OLED achieves a peak EQE of 12.2%, which is among the highest values for non-doped solution-processed fluorescent OLEDs.

Optimizing Optoelectronic Properties of Pyrimidine‐Based TADF Emitters by Changing the Substituent for Organic Light‐Emitting Diodes with External Quantum Efficiency Close to 25 % and Slow Efficiency Roll‐Off

A series of green butterfly-shaped thermally activated delayed fluorescence (TADF) emitters, namely PXZPM, PXZMePM, and PXZPhPM, are developed by integrating an electron-donor (D) phenoxazine unit and electron-acceptor (A) 2-substituted pyrimidine moiety into one molecule via a phenyl-bridge π linkage to form a D-π-A-π-D configuration. Changing the substituent at pyrimidine unit in these emitters can finely tune their emissive characteristics, thermal properties, and energy gaps between the singlet and triplet states while maintaining frontier molecular orbital levels, and thereby optimizing their optoelectronic properties. Employing these TADF emitters results in a green fluorescent organic light-emitting diode (OLED) that exhibits a peak forward-viewing external quantum efficiency (EQE) close to 25 % and a slow efficiency roll-off characteristic at high luminance.

Inheriting the Characteristics of TADF Small Molecule by Side‐Chain Engineering Strategy to Enable Bluish‐Green Polymers with High PLQYs up to 74% and External Quantum Efficiency over 16% in Light‐Emitting Diodes

Thermally activated delayed fluorescence (TADF) polymers are designed and synthesized by grafting the TADF emitter to the side chain of the polycarbazole backbone. By employing these TADF polymers with large ratio of delayed fluorescence component and high photoluminescence quantum yield as the emitters, the solution-processed light-emitting diodes achieve a maximal external quantum efficiency of 16.1% at a luminance of around 100 cd m-2 .

Boosting reverse intersystem crossing by increasing donors in triarylboron/phenoxazine hybrids: TADF emitters for high-performance solution-processed OLEDs

Three triarylboron-based TADF emitters are developed by integrating electron-donating phenoxazine units and electron-accepting triarylboron units. Employing these TADF emitters in the solution-processed organic light-emitting diodes achieves a maximum external quantum efficiency of 13.9% and slight efficiency roll-off.

In Situ Solid-State Generation of (BN)2-Pyrenes and Electroluminescent Devices

New BN-heterocyclic compounds have been found to undergo double arene photoelimination, forming rare yellow fluorescent BN-pyrenes that contain two BN units. Most significant is the discovery that the double arene elimination can also be driven by excitons generated electrically within electroluminescent (EL) devices, enabling the in situ solid-state conversion of BN-heterocycles to BN-pyrenes and the use of BN-pyrenes as emitters for EL devices. The in situ exciton-driven elimination (EDE) phenomenon has also been observed for other BN-heterocycles.

Achieving Nearly 30% External Quantum Efficiency for Orange–Red Organic Light Emitting Diodes by Employing Thermally Activated Delayed Fluorescence Emitters Composed of 1,8‐Naphthalimide‐Acridine Hybrids

The combination of rigid acridine donor and 1,8-naphthalimide acceptor has afforded two orange-red emitters of NAI-DMAC and NAI-DPAC with high rigidity in molecular structure and strongly pretwisted charge transfer state. Endowed with high photoluminescence quantum yields (ΦPL ), distinct thermally activated delayed fluorescence (TADF) characteristics, and preferentially horizontal emitting dipole orientations, these emitters afford record-high orange-red TADF organic light-emitting diodes (OLEDs) with external quantum efficiencies of up to 21-29.2%, significantly surpassing all previously reported orange-to-red TADF OLEDs. Notably, the influence of microcavity effect is verified to support the record-high efficiency. This finding relaxes the usually stringent material requirements for effective TADF emitters by comprising smaller radiative transition rates and less than ideal ΦPL s.

High-performance blue and green electrophosphorescence achieved by using carbazole-containing bipolar tetraarylsilanes as host materials

A series of carbazole-containing tetraarylsilane compounds, namely p-BISiPCz (1), m-BISiPCz (2), p-OXDSiPCz (3) and m-OXDSiPCz (4) were designed and synthesized by incorporating electron-donating carbazole and electron-accepting benzimidazole or oxadiazole into one molecule via a silicon-bridge linkage mode. Their thermal, photophysical and electrochemical properties can be finely tuned through the different groups and linking topologies. The di-para-position compounds 1 and 3 display higher glass transition temperatures and slightly lower triplet energies than their di-meta-position isomers 2 and 4, respectively. The four compounds exhibit similar HOMO levels (5.60–5.63 eV), while the LUMO level of 3 (2.36 eV) is slightly lower than that of 4 (2.28 eV). The silicon-interrupted conjugation of the electron-donating and electron-accepting segments endows these materials with relative high triplet energies, good thermal and morphological stability, and bipolar transporting ability. For FIrpic-based blue PhOLEDs, the di-meta-position compounds 2 and 4 display better device performances than their di-para-position analogues 1 and 3, respectively. Device B using 2 as the host exhibits the best performance with a maximum current efficiency of 29.3 cd A−1, a maximum power efficiency of 19.8 lm W−1, and a maximum external quantum efficiency of 11.4%. Green phosphorescent devices using (ppy)2Ir(acac) as guest and 1–4 as hosts show excellent EL performances with maximum external quantum efficiencies of 18.3–22.2%. Remarkably, device H hosted by 4 still exhibits an external quantum efficiency of 19.4% at the extremely high luminance of 10 000 cd m−2. These efficiencies are significantly higher than those of blue and green control devices using mCP as host, respectively.