Abstract
The design of thermally activated delayed fluorescence (TADF) materials both as emitters and as hosts is an exploding area of research. The replacement of phosphorescent metal complexes with inexpensive organic compounds in electroluminescent (EL) devices that demonstrate comparable performance metrics is paradigm shifting, as these new materials offer the possibility of developing low-cost lighting and displays. Here, a comprehensive review of TADF materials is presented, with a focus on linking their optoelectronic behavior with the performance of the organic light-emitting diode (OLED) and related EL devices. TADF emitters are cross-compared within specific color ranges, with a focus on blue, green-yellow, orange-red, and white OLEDs. Organic small-molecule, dendrimer, polymer, and exciplex emitters are all discussed within this review, as is their use as host materials. Correlations are provided between the structure of the TADF materials and their optoelectronic properties. The success of TADF materials has ushered in the next generation of OLEDs.
Highlights
The design of thermally activated delayed fluorescence (TADF) materials both cence, the process of converting electrical as emitters and as hosts is an exploding area of research
This review aims to provide a comprehensive summary of the development of organic TADF emitters, together with their monochromatic device performance and their use in white organic light-emitting diode (OLED)
Zhang et al.[181] attempted to construct a fluorescent OLED using a TADF material as the host. They noted that while triplet excitons can be upconverted to singlets via reverse intersystem-crossing (RISC), the small exchange integral present in the TADF materials intrinsically results in a low kr and a low PLQY
Summary
The first purely organic TADF emitter PIC-TRZ (Figure 4) was reported in 2011 by Adachi et al.[13]. In another publication by the same group, a structurally similar blue TADF emitter DMAC-DPS (λmax: 464 nm; PLQY: 80%; τd: 3.1 μs in 10 wt% mCP; ΔEST: 0.08 eV) device (ITO/α-NPD/TCTA/CzSi/10 wt% emitter:DPEPO/DPEPO/TPBI/LiF/Al) showed an EQE of 19.5%.[15b] The choice of the dimethylacridan donor results in a higher 3LE state than 3CT state, producing a small ΔEST of 0.08 eV. A blue-greenish TADF emitter CzT (λmax: 502 nm; PLQY: 40%; τd: 42.6 μs in 3 wt% DPEPO; ΔEST: 0.09 eV) (Figure 10) was employed in an OLED device (ITO/α-NPD/ TCTA/CzSi/3 wt% emitter:DPEPO/DPEPO/TPBi/LiF/Al), which showed an EQE of 6% at CIE coordinates of (0.23, 0.40).[51] a structurally similar emitter PhCzTAZ (PhCzTAZ = 3-(2′-(4,6-diphenyl-1,3,5-triazin-2-yl)-[1,1′biphenyl]-2-yl)-9-phenyl-9H-carbazole) does not show TADF because of the absence of charge-transfer emission, probably due to limited HOMO and LUMO communication restricted by steric hindrance around the biphenyl bridge.
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