Recent progress in thermally activated delayed fluorescence emitters for nondoped organic light-emitting diodes

Abstract

Nondoped organic light-emitting diodes (OLEDs) have drawn immense attention due to their merits of process simplicity, reduced fabrication cost, etc. To realize high-performance nondoped OLEDs, all electrogenerated excitons should be fully utilized. The thermally activated delayed fluorescence (TADF) mechanism can theoretically realize 100% internal quantum efficiency (IQE) through an effective upconversion process from nonradiative triplet excitons to radiative singlet ones. Nevertheless, exciton quenching, especially related to triplet excitons, is generally very serious in TADF-based nondoped OLEDs, significantly hindering the pace of development. Enormous efforts have been devoted to alleviating the annoying exciton quenching process, and a number of TADF materials for highly efficient nondoped devices have been reported. In this review, we mainly discuss the mechanism, exciton leaking channels, and reported molecular design strategies of TADF emitters for nondoped devices. We further classify their molecular structures depending on the functional A groups and offer an outlook on their future prospects. It is anticipated that this review can entice researchers to recognize the importance of TADF-based nondoped OLEDs and provide a possible guide for their future development.

Fig. 1. Illustration of the exciton utilization channels of the TADF mechanism upon electrical excitation. GS and ES are ground and excited states, respectively; S and T are the electrogenerated singlet and triplet excitons; ISC and RISC are the intersystem crossing and reverse intersystem crossing processes; nr^S and nr^T are the nonradiative transition processes of the singlet and triplet excitons; PF and DF are the prompt and delayed fluorescence.

Fig. 2. Molecular design strategies of TADF emitters for nondoped OLEDs. (a) The AIE/AIDF mechanism. Reproduced from ref. with permission. Copyright 2020 Wiley-VCH. (b) The mechanism of introducing steric hindrance. Reproduced from ref. with permission. Copyright 2017 Wiley-VCH. (c) Self-host mechanism. Reproduced from ref. with permission. Copyright 2019 the Royal Society of Chemistry. (d) The “self-doping” mechanism. Reproduced from ref. with permission. Copyright 2021 Wiley-VCH. (e) Intermolecular hydrogen bonding mechanism. Reproduced from ref. with permission. Copyright 2019 the Royal Society of Chemistry. (f) D–spacer–A mechanism. Reproduced from ref. with permission. Copyright 2018 Wiley-VCH.

Fig. 3. Molecular structures of cyano-based TADF emitters for nondoped OLEDs.

Fig. 4. Device structure of the fully solution-processed devices and normalized EL spectra at different voltages. Reproduced from ref. with permission. Copyright 2019 American Chemical Society.

Fig. 5. Molecular structures of carbonyl-based TADF emitters for nondoped OLEDs.

Fig. 6. Schematic illustration of the main exciton dynamic processes for (a) a conventional luminogen with delayed fluorescence and (b) an AIDF emitter in nondoped OLEDs. Reproduced from ref. with permission. Copyright 2017 Wiley-VCH.

Fig. 7. Calculated spin-density distributions of the lowest-excited triplet states of (a) 84 and (b) 86 in the solid-state geometry determined by single-crystal X-ray analysis. Reproduced from ref. with permission. Copyright 2016 Wiley-VCH.

Fig. 8. Molecular structures, calculated triplet spin density distribution (TSDD) of the lowest-excited triplet states, and PL intensity versus different water fractions (f_w) of 95–97. Reproduced from ref. with permission. Copyright 2020 the Royal Society of Chemistry.

Fig. 9. Molecular structures of sulfone-based TADF emitters for nondoped OLEDs.

Fig. 10. Molecular structures of nitrogen-containing heterocycle-based TADF emitters for nondoped OLEDs.

Fig. 11. Thermal ellipsoid drawings at the 50% probability level and intermolecular geometries of (a) 118, (b) 119, and (c) 120 in the single crystals determined by X-ray analysis. Reproduced from ref. with permission. Copyright 2020 the Royal Society of Chemistry.

Fig. 12. Molecular structures of 122 and 123 and a schematic illustrating the distribution of the conformations at the ground state and corresponding energy transfer at excited states.

Fig. 13. Molecular structures of boron-based TADF emitters for nondoped OLEDs.

Fig. 14. (a) Schematic diagram for plausible RISC and TADF mechanisms involving SOC-induced spin conversion in 179. (b) EL and EQE-L characteristics of the nondoped TADF-OLEDs (devices E and F based on 179 and 180, respectively). Reproduced from ref. with permission. Copyright 2018 Wiley-VCH.