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Current materials leaders in OLED technology are largely based on phosphorescent iridium complexes and Thermally Activated Delayed Fluorescence (TADF) materials which emit by harvesting light from all excited states ensuring nearly 100% internal quantum efficiency (IQE). Although, high efficiency red, green and blue OLEDs were realized, very short operating stability remains a fundamental challenge for blue OLEDs. Here we present our materials design strategy.
We have recently designed numerous linear coinage metal complexes with efficient photo- and electroluminescent properties.[1,2] Our materials are composed of the donor and acceptor ligands which are linked by a coinage metal atom. Linear geometry of coinage metal complexes enables rotational flexibility. Rotation about the metal-ligand bond allowed us to tune the energy gap between singlet and triplet excited states. When the gap is close to zero, facile intersystem crossing and reversed intersystem crossing are possible which enables efficient singlet and triplet excited state harvesting. Depending on the value of the energy gap we have designed various functional materials with phosphorescent or delayed fluorescence properties. As a proof of concept, we fabricated OLED devices with exceptionally high external quantum efficiencies (>28% EQE) in both solution-processed and vacuum-deposited OLEDs.[3] Power and current efficiency are comparable to or exceeding state-of-the-art phosphorescent OLEDs and quantum dot LEDs. Our materials possess short excited state lifetime (100-300 ns) for the delayed emission which is highly important for the fabrication of the long-lived OLEDs.
[1] A.S. Romanov, D. Di, L. Yang, J. Fernandez-Cestau, C.R. Becker, C.E. James, B. Zhu, M. Linnolahti, D. Credgington, M. Bochmann, Chem. Commun., 52, 6379 (2016)
[2] A.S. Romanov, C.R. Becker, C.E. James, D. Di, D. Credgington, M. Linnolahti, M. Bochmann, Chem. Eur. J., 23, 4625 (2017).
[3] D. Di, A.S. Romanov, L. Yang, J.M. Richter, J.P.H. Rivett, S. Jones, T.H. Thomas, M.A. Jalebi, R.H. Friend, M. Linnolahti, M. Bochmann, D. Credgington, Science, 356, 159 (2017)
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