Organic light-emitting diodes (OLEDs) are increasingly used in displays replacing traditional flat panel displays; e.g., liquid crystal displays. Especially, the paradigm shifts in displays from rigid to flexible types accelerated the market change from liquid crystal displays to OLEDs. However, some critical issues must be resolved for expansion of OLED use, of which blue device performance is one of the most important. Therefore, recent OLED material development has focused on the design, synthesis and application of highefficiency and long-life blue emitters. Well-known blue fluorescent emitters have been modified to improve their efficiency and lifetime, and blue phosphorescent emitters are being investigated to overcome the lifetime issue. Recently, thermally activated delayed fluorescent emitters have received attention due to the potential of high-efficiency and long-living emitters. Therefore, it is timely to review the recent progress and future prospects of high-efficiency blue emitters. In this feature article, we summarize recent developments in blue fluorescent, phosphorescent and thermally activated delayed fluorescent emitters, and suggest key issues for each emitter and future development strategies.
Transistors On page 5875, J. H. Cho and co-workers demonstrate a new device architecture for flexible vertical Schottky barrier (SB) transistors and logic gates based on graphene-organic-semiconductor-metal heterostructures and ion gel gate dielectrics. The devices show well-behaved p-and n-type characteristics under low-voltage operation (<1 V), yielding high current densities (>100 mA cm-2) and on-off current ratios (>10 3). Biosensors On page 6034, P. K. Wong and co-workers demonstrate a nanorod-based biosensor for dynamic single-cell analysis in native tissue microenvironments. The biosensor is capable of monitoring spatiotemporal mRNA expression in primary human cells, capillary networks, and animal tissues, including the skin, retina, and cornea, challenged mechanically and biochemically. Conjugated Polymers M. Xue and co-workers describe an in situ polymerization method for yielding single-crystal-conjugated polymer (SCCP) arrays on page 5923. As-fabricated SCCP micro-arrays exhibit a smooth surface, excellent environmental stability, and enhanced electron sensitivity, which may bring high performances for CP-based devices, such as supercapacitors, organic solar cells, polymer super-conductors, organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs), or some flexible electronics. Photocatalysts Well-designed hetero-nanostructural plasmonic photo-catalysts with a multichannel sensitization effect on the charge-carrier dynamics process are developed by B. Dong and co-workers, as described on page 5906. The rational combination of the semiconductor hetero-junction effect and a surface plasmon resonance (SPR) coupling effect of the plasmonic dimers, as well as the nanostructural property of electrospun nanofibers, results in a remarkable enhancement in the efficiency of sol ar to fuels conversion. Carbazole-and triazine-derived thermally activated delayed fl uorescent (TADF) emitters , with three donor units and an even distribution of the highest occupied molecular orbital, achieve high external quantum effi ciencies of above 25% in blue and green TADF devices.
Highly efficient green thermally activated delayed fluorescent organic light-emitting diodes with an external quantum efficiency of 31.2% were investigated by using 3-(3-(carbazole-9-yl)phenyl) pyrido[3',2':4,5]furo[2,3-b]pyridine (3CzPFP) derived from carbazole and pyrido[3',2':4,5]furo[2,3-b]pyridine. The host material showed well-matched photoluminescence emission with absorption of the green dopant material, (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) and harvested all excitons of 4CzIPN. The 3CzPFP:4CzIPN film exhibited high photoluminescence quantum yield of 100%, and the green delayed fluorescence device employing the 3CzPFP host showed high maximum quantum efficiency of 31.2 ± 0.5% at 1% doping after optimization of the device structure.
OLEDs and TADF OLEDs are essential. One method to overcome those issues is to harvest singlet excitons by triplet-triplet fusion (TTF) process which converts triplet excitons into singlet excitons. [18][19][20] The TTF approach has been used in the blue fluorescent OLEDs, but the triplet to singlet conversion efficiency is limited, resulting in relatively low EQE compared to the phosphorescent and TADF OLEDs. The other method is to apply singlet-exciton-harvesting technology of fluorescent OLEDs by energy transfer to achieve high EQE and long device lifetime by overcoming the deficiencies of the two technologies. The EQE can be comparable to that of phosphorescent and TADF OLEDs if singlet excitons can be fully harvested, and the device lifetime can be better than that of those devices due to the intrinsic stability of the molecular structures and triplet excitons critical to the device lifetime are excluded in the final light-emission process.Herein, recent progress on the singlet harvesting approach of fluorescent emitters by an energy transfer process from host or sensitizer is discussed. Basic concepts, material requirements, and device architecture for the singlet-exciton-harvesting technology are covered to understand the light-emission process and to develop an ideal emitting layer system for both high EQE and long device lifetime. Additionally, future prospects of singletexciton-harvesting fluorescent OLEDs are proposed. Basic Concept of Singlet-Exciton-Harvesting Fluorescent OLEDsFluorescent emitters can emit light by photoluminescence (PL) or electroluminescence (EL) processes although other processes such as chemiluminescence and mechanoluminescence can induce light emission. [21][22][23][24] In the PL process, fluorescent emitters can exhibit 100% photon conversion efficiency by light excitation if there is no intersystem crossing from the singlet excited state to triplet excited state and no loss during the radiative transition from singlet excited state to ground state. However, an ideal 100% PL efficiency is not realized in the EL process because 75% triplet excitons generated by the electrical carrier injection process are lost by nonradiative pathways. [25][26][27][28][29][30] Only 25% of the singlet excitons contribute to the EL emission process, and the internal quantum efficiency (IQE) of fluorescent OLEDs is limited to 25%. However, the low IQE of fluorescent OLEDs can be improved if triplet exciton The external quantum efficiency (EQE) of organic light-emitting diodes (OLEDs) has been dramatically improved by developing highly efficient organic emitters such as phosphorescent emitters and thermally activated delayed fluorescent (TADF) emitters. However, high-EQE OLED technologies suffer from relatively poor device lifetimes in spite of their high EQEs. In particular, the short lifetimes of blue phosphorescent and TADF OLEDs remain a big hurdle to overcome. Therefore, the high-EQE approach harvesting singlet excitons of fluorescent emitters by energy transfer processes from the host or sensit...
Room-temperature phosphorescence from metal-free purely organic molecules has recently gained much interest. We devised metal-free organic phosphors by incorporating selenium (Se) to promote spin–orbit coupling by its nonmetal heavy atom effect. The Se-based organic phosphors showed bright phosphorescent emission in organic light-emitting diodes (OLEDs) and photo-excited phosphorescence in an amorphous film state. Large orbital angular momentum change (ΔL) during the electron transition process and heavy atom effect of Se render a PL quantum yield of 0.33 ± 0.01 and a high external quantum efficiency (EQE) of 10.7 ± 0.14% in phosphorescent LEDs. This work demonstrates the rational molecular design of metal-free organic phosphorescent emitters with Se as an alternative novel class of materials to the conventional organometallic phosphors for OLEDs.
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