Time/Date: 15:30 ~ 16:30 Friday Oct 14, 2016
Location: Meeting Room, 5F, Building No.11
Lecturer: Prof Yuguang Ma
(State Key Laboratory of Luminescent Materials and Devices, South China University of Technology)
Subject: High-lying “Hot” Excitons for OLEDs
When a singlet state(S) nonradiatively passes to a triplet state(T), or conversely a T transitions to a S, that process is known as intersystem crossing(ISC, S to T) or RISC(T to S). Because the spin of the excited electron is reversed, the time scale of ISC is relative slow with the order of 10−8 to 10−3 s, one of the slowest forms of relaxation. As a result, the probability of this process occurring is sufficiently low. In some case this process can be accelerated when the vibrational levels of both S and T states overlap or the spin/orbital interactions in such molecules are substantial. In heavy-atom molecules with strong spin-orbit coupling, ISC become more favourable, and the heavy metal complexes behavior strong phosphorescence for highly efficient OLEDs. In very few case, RISC from lowest triplet states (T1) to lowest singlet states (S1) can become a dominated process, typically as the thermally-activated delayed fluorescence (TADF) and triplet-triplet annihilation (TTA), but it is great challenging to obtain highly exciton utilization efficiency (ηr) and fluorescence efficiency (ηPL). In the end 1969, some aromatic compounds were found that the RISC can occur from the high-lying triplet energy levels to the singlet manifold(Tn (n>1) → S1)1, but the efficiency of such delayed fluorescence is too low to find a suitable application. Here, we innovatively rejuvenated this “sleeping” photophysical process by utilizing it to break the exciton statistics limit in OLEDs. Arduous efforts are made to developing fluorescent materials with combination of high photoluminescence efficiency and effective RISC through appropriate molecular design in a series of donor-acceptor (D-A) material systems. The experimental and theoretical results indicate that these materials exhibit hybridized local and charge-transfer excited state (HLCT), which achieve a combination of the high radiation from local excited state (LE) and the high Tm→Sn (m≥2, n≥1) conversion along charge-transfer excited state (CT). As expected, the devices exhibited favorable external quantum efficiency (EQE) and low roll-off, and especially the exciton utilization efficiency exceeding the limit of 25%.2,3..
[1] R.A.Keller, Chem. Phys. Lett., 3(1969), 27. [2] L.Yao, B.Yang, Y.G. Ma, Angew. Chem. Int. Ed., 53(2014), 2119. [3] L.Yao, B.Yang, Y.G. Ma, Sci. China. Chem., 57(2014), 335.
