Research Seminar of Dr. Di Dawei fromCavendish Laboratory, University of Cambridge

Lecture Time: 14:30 on 22nd November, 2017

Lecture Room: C501 of Science and Innovation Building

Lecture Title: Next-Generation OLEDs: New Materials and Emission Mechanisms

Abstract

The efficiency of organic light-emitting diodes (OLEDs) is fundamentally determined by the spin of excited state electrons. For conventional fluorescent OLEDs [1, 2], the radiative recombination of triplet excitons is spin-forbidden, limiting the maximum internal quantum efficiency of electroluminescence to the generation probability of singlet excitons. In the past decades, high-efficiency phosphorescent [3, 4] and thermally activated delayed fluorescence [5] OLEDs, which utilise triplet excitons effectively, were demonstrated. In our recent work [6], using a new class of emissive molecules, carbene-metal-amides, we present an unusual emission pathway based on rotationally accessed spin-state inversion (RASI), which results in extremely efficient spin-state interconversion and photoemission. To investigate this unique emission mechanism, detailed time-resolved luminescence spectroscopy and quantum chemical calculations have been carried out. The performance metrics of our best solution-processed OLEDs (with peak external quantum efficiencies of 27.5%) are comparable to, or exceeding those of state-of-the-art OLEDs and quantum-dot LEDs. Considerations for the design of molecular materials based on this principle, and new approaches for achieving fully solution-processed multi-layer device architectures will be addressed. Besides, an overview of other relevant research activities in our group, including the investigation of triplet exciton fusion [7] for next generation optoelectronics, will be presented.

[1] C. W. Tang & S. A. VanSlyke,Appl. Phys. Lett.51, 913 (1987).

[2] J. H. Burroughes, R. H. Friendet al,Nature347, 539–541 (1990).

[3] Y. Maet al,Synth. Met.94, 245-248 (1998).

[4] M. A. Baldo, S. Forrestet al,Nature395, 151–154 (1998).

[5] H. Uoyama, C. Adachiet al,Nature492, 234–238 (2012).

[6] D. Di, A. S. Romanov, L. Yang, R. H. Friend, M. Linnolahti, M. Bochmann, D. Credgingtonet al, “High-performance light-emitting diodes based on carbene-metal-amides”,Science356, 159-163 (2017).

[7] D. Di, L. Yang, R. H. Friendet al, “Efficient triplet exciton fusion in molecularly-doped polymer light-emitting diodes”,Advanced Materials29, 1605987 (2017).