Ultra-simple white organic light-emitting diodes employing only two complementary colors with color-rendering index beyond 90

Abstract

White organic light-emitting diodes (WOLEDs) with an ultra-high color rendering index (CRI) (≥90) are considered to be crucial for special lighting applications, such as in hospitals, art galleries, and museums. However, most reported WOLEDs with a CRI of ≥90 almost all use three or more emitters, and usually suffer from a complicated device structure. In this work, an exciplex formed between a 4,4′,4′′-tris(N-3-methylphenyl-N-phenyl-amino)triphenylamine (m-MTDATA) donor and a bis[2-(2-hydroxyphenyl)-pyridine]beryllium (Bepp₂) acceptor, exhibiting a broad-spectrum emission, was employed as a yellow emitter. And a thin 4,4′,4′′-tri(9-carbazoyl)triphenylamine (TCTA) layer (2–6 nm) as a carrier adjustment layer was inserted into the Bepp₂ layer of the exciplex to block some of the electrons at the TCTA/Bepp₂ interface, inducing a blue light emission from Bepp₂. A series of ultra-simple di-chromatic WOLEDs, using only three organic materials, were demonstrated. By changing the thickness of TCTA, the proposed WOLEDs achieve an ultra-high CRI of 92, which, to our knowledge, is by far the simplest structure for a complementary WOLED with a CRI over 90. Besides, the optimized WOLED, at a practical luminance of 3000 cd m⁻², shows an ultra-high CRI of 90, and also realizes a high maximum current efficiency and power efficiency of 8.7 cd A⁻¹ and 10.1 lmW⁻¹, respectively. This novel design concept provides a new avenue for achieving simple-structured, but ultra-high-CRI WOLEDs.