All‐Fluorescence White Organic Light‐Emitting Diodes Exceeding 20% EQEs by Rational Manipulation of Singlet and Triplet Excitons

Abstract

White organic light-emitting diodes (WOLEDs) composed of conventional fluorophores possess color purity, low efficiency roll-off, and rare metal absence, but suffer from theoretical limits due to the lack of triplet utilization. Due to the different diffusion distance for singlets and triplets, multiple Förster resonance energy transfer (FRET) channels can be adequately built up. Herein, besides the complementary component, a blue fluorescence layer, hosted by pure hydrocarbon material SF4-TPE, is put forward as the spatial exciton manipulating layer to rationally allocate singlets and triplets to the corresponding channels. Hence, singlets are captured by the blue fluorophore, diffused triplets subsequently undergo energy resonance between the blue fluorophore and green assistant, and up-conversion effect for eventual emission from the yellow fluorophore. Owing to the utilization of singlets and triplets, all-fluorescence WOLEDs exhibit high efficiency exceeding 20%, with slight efficiency roll-off even under high luminance of 5000 cd cm −2 . Moreover, CIE coordinates can be surrounding and precisely inside the American National Standard Institute (ANSI) quadrangles, as well as outstanding color stability (ΔCIE-(x, y) within (0.001, 0.012)) from 300 to 13000 cd cm −2 .The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201910633.white emission and achieved a power efficiency (PE) of 0.5 lm W −1 . However, due to the deficiency of utilizing triplets, the WOLEDs based on all-fluorescence emitters can hardly break the 25% theoretical limitation of internal quantum efficiency (IQE). [4,5] Alternatively, the exploitation of rare metallic phosphorescent complexes and thermally activated delayed fluorescence (TADF) emitters that can harvest all the electrogenerated excitons [6] become the fundamental and mainstream study on constructing high-efficiency WOLEDs in the past decade. [2,[7][8][9][10][11][12][13][14][15] However, allphosphorescence WOLEDs still suffer from the intrinsic deficiencies including rare metal reliance, color impurity, and fragile operational stability for blue phosphors, [16,17] while all-TADF WOLEDs present severe efficiency roll-off because of triplet-triplet annihilation (TTA) at high luminance and unsatisfactory Commission International de l'Eclairage (CIE) coordinates deviation. [18,19] Thus, hybrid combinations, that is, leveraging TADF/ phosphorescence, fluorescence/phosphorescence or fluorescence/TADF emitters, are also proposed for WOLEDs. [20][21][22][23][24][25] Recently, the efficiency of fluorescence OLEDs can be dramatically improved by TADF assistant technique, in which TADF materials play as "triplet exciton pump" (not emitter) to up-converse triplets in conventional fluorescence emitters to approach 100% IQEs. [26] This technique has made promising progress in monochromatic emitting dyes with relatively low-lying states, such as green, yellow, and red fluorophores. [27][28][29] Thus, it seems reasonable and accessibl...

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High-efficiency hyperfluorescent white light-emitting diodes based on high-concentration-doped TADF sensitizer matrices via spatial and energy gap effects

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