Thermally Activated Delayed Fluorescent Gain Materials: Harvesting Triplet Excitons for Lasing

Abstract

Thermally activated delayed fluorescent (TADF) materials have attracted increasing attention because of their ability to harvest triplet excitons via a reverse intersystem crossing process. TADF gain materials that can recycle triplet excitons for stimulated emission are considered for solving the triplet accumulation problem in electrically pumped organic solid-state lasers (OSSLs). In this mini review, recent progress in TADF gain materials is summarized, and design principles are extracted from existing reports. The construction methods of resonators based on TADF gain materials are also introduced, and the challenges and perspectives for the future development of TADF gain materials are presented. It is hoped that this review will aid the advances in TADF gain materials and thus promote the development of electrically pumped OSSLs.

Keywords: electrically pumping; gain material; organic solid-state laser; reverse intersystem crossing; thermally activated delayed fluorescent.

Figure 1

a) Schematic of the electron–hole combination in organic electroluminescent devices. b) Proposed energy diagram of harvesting triplet excitons for lasing via the RISC process for TADF gain materials.

Figure 2

Chemical structures of reported TADF gain materials.

Figure 3

Design principle of a) conventional TADF materials and b) MR‐TADF materials. Schematics of c) J‐aggregation and d) H‐aggregation of compound 5.

Figure 4

a) Fluorescence microscopy image of compound‐3‐doped PS microspheres. b) Laser spectrum of compound‐3‐doped PS microspheres. Inset: fluorescence microscopy image of a compound‐3‐doped PS microsphere above the lasing threshold. Reproduced with permission.^{[ 45 ]} Copyright 2020, Wiley‐VCH . c) Schematics of the poly‐(dimethylsiloxane) (PDMS) template‐confined solution‐growth method. d) Bright‐field (left) and fluorescence (right) microscopy images of compound‐6‐doped mCBP microring arrays. Reproduced with permission.^{[ 50 ]} Copyright 2019, American Chemical Society. e) Fluorescence microscopy images of mCBP microplates doped with different materials (compounds 1b, 3, and 7 from left to right) used as microlaser displays. Reproduced with permission.^{[ 52 ]} Copyright 2021, American Chemical Society.

Figure 5

a) Schematics of the solution self‐assembly method and fluorescence microscopy images of microcrystals of compounds 4a–4c (from top to bottom on the right side). b) Normalized lasing spectra of microcrystals of compounds 4a–4c (from left to right). Inset: corresponding fluorescence microscopy images. Reproduced with permission.^{[ 46 ]} Copyright 2021, American Chemical Society.