Abstract
Abstract In this work, a series of three and four-color tandem white organic light-emitting diodes (WOLEDs) are developed by using the optimized charge generating unit (CGU) to connect two electroluminescence (EL) units with symmetrical emitting layers, in which symmetrical emitting layers are constructed based on the mixed hosts; sandwiched between hole and electron-transporting hosts and the light emitted from two EL units that are absolutely complementary for forming white emission. All resulting tandem WOLEDs realize good white emission with maximum color rendering index (CRI) beyond 77 and 90 for three and four-color white devices and extremely high EL spectra stability, and also achieve high device efficiency with maximum external quantum efficiency (EQE) exceeding 33.10%. For example, for the optimized four-color tandem WOLED, the maximum CRI and EQE of 91 and 34.78% are demonstrated and only very slight CIE (Δx, Δy) variation of (0.002, −0.010) was observed at a wide luminance range from 170.9 cd/m2 to 13,870 cd/m2. To the best of our knowledge, this is the first tandem WOLED with only two EL units realizing such high device performances. More importantly, the proposed tandem WOLEDs here avoid introducing carrier or exciton blocking layer or using more EL units to realize high color quality white emission that provides a novel approach to develop simple, but high-performance tandem WOLEDs.
Highlights
White organic light-emitting diodes (WOLEDs) have attracted great attentions owing to their unique merits such as self-emitting, surface light source, flexibility, transparency, low cost, and high efficiency etc., which indicate that they have great potential applications in full-color flat-panel display and solid-state lighting fields [1,2,3,4,5,6]
In this work, a series of three and four-color tandem white organic light-emitting diodes (WOLEDs) are developed by using the optimized charge generating unit (CGU) to connect two electroluminescence (EL) units with symmetrical emitting layers, in which symmetrical emitting layers are constructed based on the mixed hosts; sandwiched between hole and electron-transporting hosts and the light emitted from two EL units that are absolutely complementary for forming white emission
To the best of our knowledge, this is the first tandem WOLED with only two EL units realizing such high device performances
Summary
White organic light-emitting diodes (WOLEDs) have attracted great attentions owing to their unique merits such as self-emitting, surface light source, flexibility, transparency, low cost, and high efficiency etc., which indicate that they have great potential applications in full-color flat-panel display and solid-state lighting fields [1,2,3,4,5,6]. Miao et al.: Tandem WOLEDs with two EL units achieving superior efficiency/CRI/color stability the materials in EMLs are more oxidized [20]; (ii) The LT of OLEDs usually follows a power law dependence with respect to the applied current density (J) (LT ∝ J-β, 1.5 < β < 3) [21, 22], to reach a practical luminance of 3000−5000 cd/m2, these white devices need higher current density, which drastically causes a reduced device lifetime Another main issue for single and multiple-EMLs white devices is poor color stability originated from spectral variation with increasing operating voltage, especially for three or more color WOLEDs [1, 3, 23, 24]. For the optimized four-color tandem WOLED, the maximum CRI and EQE of 91 and 34.78% are demonstrated, and only very slight CIE (Δx, Δy) variation of (0.002, −0.010) is observed at a wide luminance range from 170.9 cd/m2 to 13,870 cd/m2
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