Since the first practical multilayered organic electroluminescent (EL) device was reported in 1987, the research area of organic light-emitting diodes (OLEDs) has developed rapidly and OLEDs have been applied to advanced flat-panel displays. The OLEDs have been classified into two major categories. The first is fluorescent materials, which can harvest only the singlet excitons (25%) generated by electrical excitation. The second is phosphorescent materials, which can harvest the triplet excitons generated (75%). The branching ratio of singlet and triplet excitons is 1 : 3, which is limited by a spin statistics rule. Therefore, in recent devices, phosphorescent materials and their related technologies are indispensable to obtain high EL efficiency. However, phosphorescent materials generally contain rare metal elements such as Ir or Pt. These metals are rather expensive and are dependent on limited global resources.
Figure 1. The compounds used in the TADF OLEDs devices.
Therefore, Thermally Activated Delayed Fluorescence (TADF) OLED is proposed as a third generation luminescent material, unlike the conventional fluorescent and phosphorescent materials, which can realize the ultimate EL efficiency by efficient up-conversion from the lowest triplet excited state (T1) to the lowest singlet excited state (S1) through reverse intersystem crossing (RISC). TADF materials have a sufficiently small energy gap between S1 and T1 (DEST) to enable up-conversion of the triplet exciton from T1 to S1. This small DEST enables TADF materials to realize 100% of the exciton formation generated by electrical excitation at S1.
Figure. 2. Color online Molecular structure of PIC-TRZ and its PL characteristics.
Figure 2 shows the molecular structure of 2-biphenyl-4,6-bis(12-phenylindolo[2,3-a]carbazole-11-yl-1,3,5-triazine(PIC-TRZ) containing an indolocarbazole donor unit and a triazine acceptor unit. Figure 1a is the molecular structures of PIC-TRZ and its HOMO and LUMO calculated by Gaussian 03. Figure 1b shows the absorption at RT, fluorescence, and phosphorescence spectra (at T=5 K) of a 6 wt % PIC-TRZ: m-CP codeposited film. Figure 1c is the two-dimensional transient decay streak image of transient PL of a 6 wt % PIC-TRZ: m-CP codeposited film showing the prompt component of fluorescence red, light color and the delayed component TADF black, dark color. In the streak image, the spots correspond to the intensity of PL: red, yellow, and green signal strong, medium and weak emission, respectively.
1. Zhang Q, Li B, Huang S, et al. Efficient blue organic light-emitting diodes employing thermally activated delayed fluorescence[J]. Nature Photonics, 2014, 8(4): 326.