High colour rendering index and colour stable hybrid white efficient OLEDs with a double emitting layer structure using a single phosphorescence dopant of heteroleptic platinum complexes

Abstract

Four heteroleptic platinum complexes (FPtXND) bearing 4-hydroxy-1,5-naphthyridine derivatives functionalized with dimethyl (X = mm), phenoxy (X = OPh), piperidine (X = pp), or carbazole (X = Cz) units as one ligand (XND) and 2-(2,4-difluorophenyl)pyridine as the other common ligand (F) were newly synthesized and characterized. The crystal structures of FPtOPhND and FPtCzND were determined by single-crystal X-ray diffraction crystallography. Although they have a short plane-to-plane packing distance of 3.62 and 3.39 Å, respectively, both platinum complexes have different molecular packing patterns, which affect their photoluminescence (PL) in solution and electroluminescence (EL) in the solid state. Due to the contribution from both monomers and excimers/aggregates, all of the platinum complexes exhibited broad and red-shifted PL in concentrated solutions as well as in doped thin films. In the monochromatic organic lighting diode (OLED) testing, FPtXND doped in 4,4′-di(9H-carbazol-9-yl)-1,1′-biphenyl (CBP) exhibited greenish yellow or orange yellow EL, of which FPtOPhND has the highest EL efficiency mainly due to its high solution PL quantum yield of 21%. Hybrid white OLEDs were first achieved with a single emitting layer configuration, of which highly fluorescent blue N,N′-di-1-naphthalenyl-N,N′-diphenyl-[1,1′:4′,1′′:4′′,1′′′-quaterphenyl]-4,4′′′-diamine (4P-NPD) was used as the host material for all four platinum complexes. To improve the performance of the FPtOPhND-based hybrid white OLEDs, double emitting layer configurations were adopted with CBP and 4P-NPD as the host material. Virtually voltage independent, a white EL having CIE_x,y (0.33, 0.31) and a CRI as high as 91 was obtained. The maximum EL efficiency of 11.8%, 25.9 cd A⁻¹, or 11.6 lm W⁻¹ was acquired with FPtOPhND doped in the double emitting layer configuration of an OLED.