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.Graphical abstractHighlights► External quantum efficiency increases from 13.4% at 100 cd/m2 to 13.7% at 1000 cd/m2. ► The CIE (0.43, 0.43) and CRI of 89 remain nearly unchanged. ► Mixed host and spacer are the key factors in improving the performance.

We present a simple hybrid white organic light-emitting diodes (WOLED) consisting of only two layers, i.e., a hole-transporting layer and an emitting layer. The emitting layer is formed by simply co-doping a green phosphor and a red phosphor in bis[2-(2-hydroxyphenyl)-pyridine]beryllium (Bepp2), which acts as the blue emitter, electron-transport material, and high triplet energy host for the phosphors, i.e., a multifunctional chromophore. This simple device exhibits a maximum power and quantum efficiency of 46.8 lm W−1 and 16.5%, respectively, with a good CRI up to 90. The versatile experimental techniques are performed to gain a deep understanding of the emission mechanism. We believe that this simple design concept can provide a new avenue for achieving ultrahigh performance WOLEDs for lighting application.Graphical abstractAn ultra-simple hybrid WOLED with high efficiency (46.8 lm W-1 and 16.5%) and CRI (90) is reported. This is one of the best hybrid WOLEDs’ stories so far considering the comprehensive properties. Versatile experimental techniques are performed to gain a deep understanding of the emission mechanism. We believe that this simple design concept can provide a new avenue for achieving ultrahigh-performance hybrid WOLEDs for lighting application.Highlights► Highly efficient hybrid white organic light-emitting diode was proposed. ► The simple device consists of a hole-transporting layer and an emitting layer. ► The maximum power efficiency of 46.8 lm W−1 with CRI up to 90 was achieved. ► This concept offers a low-cost fabrication process for hybrid white device.

•Green TWOLED with enhanced coupling efficiency is realized by coupling layer design.•The refractive indices of the coupling layers reduce smoothly.•The doping layer acts as a diffuser film to gives stable colors over angles.By means of refractive index gradually changed coupling layers, a highly efficient green top-emitting OLED (TOLED) with enhanced light coupling efficiency and stable colors over angles has been realized. The refractive index transition of the coupling layers including the doping layer smoothes light extraction from the semitransparent cathode metal to the air, which is the reason for the enhancement of light coupling efficiency. The doping layer in the coupling layers also acts as a microparticle diffuser to eliminate the shift in EL spectra with viewing angles. A universal simulation has also been carried out, and the result suggests that the light coupling efficiency will be enhanced further if the refractive index transition of the coupling layers is continuous.Graphical abstract