A.S. Romanov, P.J. Conaghan, D. Di, L. Yang, S.T.E. Jones, M. Linnolahti, D. Credgington, M. Bochmann
University of East Anglia,
United Kingdom
Keywords: OLED, electroluminescence, gold, silver, carbene
Summary:
Highly efficiency red and green organic light emitting diodes (OLEDs) have been successfully commercialized, but very short operating stability remains a fundamental challenge for deep blue OLEDs. Phosphorescent Iridium(III) complexes occupy a leading position as materials for OLED technology. Recently, two main competing molecular design approaches based on purely organic materials show-cased 100% energy efficiency. Light emission is achieved by harvesting all singlet and “dark” triplet excited states. These organic materials operate via thermally activated delayed fluorescence (TADF) and hyperfluorescence light emission mechanisms. But, organic materials still have not solved the main market problem while iridium is the third rarest transition metal in the Earth's crust. This motivated us to develop novel materials and light emitting concept to accelerate implementation of OLED technology in various displays and lighting applications. Here we present our materials design strategy and OLED prototypes based on coinage metal (copper, silver and gold) complexes. We designed a new family of materials with donor (amide) and acceptor (carbene) moieties linked by a metal – carbene metal amides (CMA). CMA materials exhibit the energy gap between singlet and triplet excited states with a charge-transfer character. The energy gap can be tuned effectively to zero via the rotation of donor and acceptor about the metal-amide bond. When the gap is close to zero, facile reversed intersystem crossing process is possible, enabling highly efficient light emission. We report efficient electroluminescence (EL) from a group of mononuclear copper, silver and gold complexes. As a proof of concept, we fabricated OLED devices based on gold complexes with exceptionally high external quantum efficiencies (>28% EQE) in both solution-processed and vacuum-deposited OLEDs. Surprisingly silver complexes show good light, oxygen/moisture and thermal stability. Silver-based materials display green emission, good photoluminescence quantum yields up to 74% while delayed emission is one of the shortest known to date for silver emitters (200–400 ns). First OLEDs using silver emitting molecules as dopants demonstrate moderate external quantum efficiencies up to 11 and 14% via solution processing or vacuum deposition. Comparison and key differences for the gold- and copper-based carbene-metal-amides for wide colour range OLEDs will be presented. Our work demonstrates high potential for silver and gold-based complexes as next-generation materials for bright and energy efficient OLEDs.