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Industrialized white organic light-emitting diodes (OLEDs) currently require host-guest doping, a complicated process necessitating precise control of the guest concentration to get high efficiency and stability. Two doping-free, hybrid white OLEDs with fluorescent blue, and phosphorescent green and red emissive layers (EMLs) are reported in this work. An ultra-thin red phosphorescent EML was situated in a blue-emitting electron transport layer (ETL), while the ultra-thin green phosphorescent EML was placed either in the ETL (Device 1), or the hole transport layer (HTL) (Device 2). Device 2 exhibits higher efficiency and more stable spectrum due to the enhanced utilization of excitons by ultra-thin green EML at the exciton generation zone within the HTL. Values of current efficiency (CE), power efficiency (PE), and CRI obtained for the optimized hybrid white OLEDs fabricated through a doping-free process were of 23.2 cd/A, 20.5 lm/W and 82 at 1000 cd/m2, respectively.

The high-color-quality white emission in an AC-driven field-induced electroluminescence (FIPEL) device consisting of a white emitting ter-polymer: poly(fluorene–benzothiadiazole–quinoline) PF–BT–QL combined with a red emitting dye: Bis(2-methyl-dibenzof,hquinoxaline)(acetylacetonate)iridium (III) Ir(MDQ)2(acac) was achieved. The wide EL emission effectively covered the visible spectral region at the concentration of 5% Ir(MDQ)2(acac) in PF–BT–QL and largely enriched the color rendering capability with a CIE (0.36, 0.38) close to the ideal equal-energy white (0.33, 0.33) and a CRI as high as 97.4, close to the blackbody curve characteristic and CCT between 3034 K and 5334 K which are required for high-quality white-light illumination. When further increasing the concentration of Ir(MDQ)2(acac) to 10%, leading to a more pure white with CIE (0.36, 0.37) and a CRI as high as 97.1. Surprisingly, the FIPEL devices containing 20% and 30% Ir(MDQ)2(acac) in PF–BT–QL still exhibit high-quality white emission with CIE (0.42, 0.37) and (0.32, 0.38) and CRI 93.9 and 88.9 at high electric field, respectively. To the best of our knowledge, there are no reports of two-component FIPELs with a CRI > 90, especially with such a high concentration of the phosphor dopant. We attribute this to the unique carrier injection characteristics of the AC-driven field induced device. This further suggests its great potential application in display and solid state lighting.

A hybrid white organic light-emitting diode (WOLED) with an emission layer (EML) structure composed of red phosphorescent EML/green phosphorescent EML/spacer/blue fluorescent EML was demonstrated. This hybrid WOLED shows high efficiency, stable spectral emission and low efficiency roll-off at high luminance. We have attributed the significant improvement to the wide distribution of excitons and the effective control of charge carriers in EMLs by using mixed 4,4′,4″-tri(9-carbazoyl) triphenylamine (TCTA) and bis[2-(2-hydroxyphenyl)-pyridine] beryllium (Bepp2) as the host of phosphorescent EMLs as well as the spacer. The bipolar mixed TCTA:Bepp2, which was proved to be a charge carrier switch by regulating the distribution of charge carriers and then the exciton recombination zone, plays an important role in improving the efficiency, stabilizing the spectrum and reducing the efficiency roll-off at high luminous. The hybrid WOLED exhibits a current efficiency of 30.2 cd/A, a power efficiency of 32.0 lm/W and an external quantum efficiency of 13.4% at a luminance of 100 cd/m2, and keeps a current efficiency of 30.8 cd/A, a power efficiency of 27.1 lm/W and an external quantum efficiency of 13.7% at a 1000 cd/m2. The Commission Internationale de l’Eclairage (CIE) coordinates of (0.43, 0.43) and the color rendering index (CRI) of 89 remain nearly unchanged in the whole range of luminance.

Highly efficient fluorescent white organic light-emitting diodes (WOLEDs) have been fabricated by using three red, green and blue, separately monochromatic emission layers. The red and blue emissive layers are based on 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran (DCJTB) doped N,N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine (NPB) and p-bis(p-N,N-diphenyl-amino-styryl) benzene (DSA-ph) doped 2-methyl-9,10-di(2-naphthyl) anthracene (MADN), respectively; and the green emissive layer is based on tris(8-hydroxyquionline)aluminum(Alq(3)) doped with 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl- 1H,5H,1[H-(1)-benzopyropyrano(6,7-8-i,j)quinolizin-1]-one (C545T), which is sandwiched between the red and the blue emissive layers. It can be seen that the devices show stable white emission with Commission International de L'Eclairage coordinates of (0.41, 0.41) and color rendering index (CRI) of 84 in a wide range of bias voltages.