Recent Progress of Singlet‐Exciton‐Harvesting Fluorescent Organic Light‐Emitting Diodes by Energy Transfer Processes

Abstract

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...

Routes for Efficiency Enhancement in Fluorescent TADF Exciplex Host OLEDs Gained from an Electro‐Optical Device Model

“…This discovery opened the possibility to fabricate highly efficient fluorescence-based OLEDs with external quantum efficiencies (EQEs) similar to phosphorescence-based OLEDs, but with the advantages of a broader variety of emitter materials (no iridium and platinum complexes), higher color purity, and higher operational stability. [9] By adding TADF assistant dopants to fluorescent emitters in conventional hosts [5,10,11] or in a TADF exciplex host, [12] EQEs of up to 24% were reported. With TADF emitters in a conventional host, [6] an exciplex host, [7] or in a TADF exciplex host, [8] EQEs of up to 37% were obtained without light outcoupling structures, which are similar to the values obtained with the best phosphorescent OLEDs.…”

“…[5] Different TADF approaches are presently employed to increase the efficiency of fluorescent OLEDs including TADF emitter molecules, TADF assistant dopant molecules, TADF exciplex hosts, and different combination of them. The third class uses TADF exciplex hosts with conventional fluorescent emitters, and EQEs of >10% [10,13] are currently reached. [9] By adding TADF assistant dopants to fluorescent emitters in conventional hosts [5,10,11] or in a TADF exciplex host, [12] EQEs of up to 24% were reported.…”

High‐Performance Nonvolatile Organic Photonic Transistor Memory Devices using Conjugated Rod–Coil Materials as a Floating Gate

“…Organic materials have emerged as potential candidates for use in flexible or stretchable electronic device applications, [1] such as field-effect transistors (FETs), [2][3][4][5] light-harvesting devices, [6][7][8][9] light-emitting electronics, [10][11][12][13] and memories. [14][15][16][17] This is due to their superior simple processability, thin-film flexibility, and tunable electrical properties.…”

TADF Technology for Efficient Blue OLEDs: Status and Challenges from an Industrial Point of View

“…If both, the rates of RISC and FRET are high, the overall excited state lifetime in this system is decreased, and hence the operational lifetime of the whole system is expected to be increased. Hyper-fluorescence with all of its potential benefits is a very promising technology, which is not easy to engineer, because many criteria have to be met [54][55][56][57][58]. The two main criteria, which have to be fulfilled in order to achieve efficient hyper-fluorescence are an optimal spectral overlap of the TADF molecule's emission spectrum with the absorption spectrum of the fluorescent molecule and a high PLQY of both molecules.…”