New Generation of High Efficient OLED Using Thermally Activated Delayed Fluorescent Materials

Abstract

The search for efficient materials for organic light emitting diodes is one of the most imperative research area. The focus is to obtain a bright large area emitter, limited by the low internal quantum efficiency of conventional organic emitters. Recently, a new generation of the organic materials (TADF) with a theoretical internal quantum efficiency up to 100%, opened new frameworks. However, significant challenges persist to achieve full understanding of the TADF mechanism and to improve the OLEDs stability. Starting from the photo-physical analysis, we show the relationship with the molecular electrical carrier dynamics and internal quantum efficiencies. The OLED structure, fabrication, and characterization are also discussed. Several examples for the full color emitters are given. Special emphasis on experimental results is made, showing the major milestones already achieved in this field.

Towards Highly Efficient TADF Yellow-Red OLEDs Fabricated by Solution Deposition Methods: Critical Influence of the Active Layer Morphology

“…In solution-processed devices, an EQE of 31% has been reported with the use of a cross-linkable polymer in blue-green TADFs, but the process again involves a complex processing method [20]. Recently, reports on the solution-processed and thermally evaporated TADF-based OLEDs have summarized recent advances in such processes, but they have failed to deliver effective charge transport properties and recombination processes inside the emissive layer [15,[21][22][23][24]. Typically, research in this area is still in an early stage, as there is a lack of standardization for the fabrication and performance evaluation of these devices.…”

Effect of the Host on Deep-Blue Organic Light-Emitting Diodes Based on a TADF Emitter for Roll-Off Suppressing

“…The thermally activated delayed fluorescence (TADF) process has recently emerged as one of the most attractive methods for harvesting both singlet and triplet states in metal-free organic materials for application in highly efficient OLEDs because they can achieve a theoretical maximum internal quantum efficiency (IQE) up to 100% [2,3,4,5,6,7,8]. In TADF materials, the triplet excitons (T 1 ) are readily upconverted to the singlet state (S 1 ) by virtue of the reverse intersystem crossing (RISC) due to their near-zero singlet-triplet splitting (ΔE ST ) [3,6,8,9,10].…”

“…To date, many deep-blue TADF emitters have been reported for their use in various applications with a maximum external quantum efficiency (EQE) over 30% [2,3,7,9,12,13]. Generally, deep-blue TADF emitters possess high triplet energy T 1 over 2.8 eV [3].…”

“…First, the hosts should have T 1 around or above 3.0 eV to prevent exciton quenching [6]. Second, it is also essential that in a host-guest system, the host should ensure the excitons transfer to the guest [2,3,6,8,17]. Third, suitable charge carrier transporting abilities are essential to increase the chances of the hole and electron recombination within the emitting layer [18].…”

“…At present, many studies of blue TADF emitters have been carried out in different n-type or p-type hosts, such as host bis[2-(diphenylphosphino) phenyl] ether oxide (DPEPO), 2,8-bis(diphenylphosphine oxide) dibenzofuran (DBFPO), 4,4′-bis( N -carbazolyl)-1,1′-biphenyl (CBP), and 1,3-bis( N -carbazolyl)benzene (mCP) [3,7,8,21,22,23]. In a carbazole based host such as CBP (mCP with the optimum T 1) , the carbazole unit acts as a hole transfer unit, while in a phosphine–oxide based host (i.e., DPEPO), the phosphine unit acts as an electron transport unit [2,8]. In phosphine–oxide based hosts, the narrow-induced emission zone arises owing to unbalanced charge transportation, which increases the probability of TTA [24]; while in carbazole unipolar hosts, the lower triplet-triplet resonance between both host and guest due to the lower T 1 (<3.0 eV) impacts overall device performance.…”