Enhancement of Förster energy transfer from thermally activated delayed fluorophores layer to ultrathin phosphor layer for high color stability in non-doped hybrid white organic light-emitting devices**Project supported by the National Natural Science Foundation of China (Grant Nos. 61675041 and 61605253), the Foundation for Innovation Research Groups of the National Natural Science Foundation of China (Grant No. 61421002), and the Science

Abstract

Fluorescence/phosphorescence hybrid white organic light-emitting devices (WOLEDs) based on double emitting layers (EMLs) with high color stability are fabricated. The simplified EMLs consist of a non-doped blue thermally activated delayed fluorescence (TADF) layer using 9,9-dimethyl-9,10-dihydroacridine-diphenylsulfone (DMAC-DPS) and an ultrathin non-doped yellow phosphorescence layer employing bis[2-(4-tertbutylphenyl)benzothiazolato-N, C2’] iridium (acetylacetonate) ((tbt)2Ir(acac)). Two kinds of materials of 4,7-diphenyl-1,10-phenanthroline (Bphen) and 1,3,5-tris(2-N-phenylbenzimidazolyl) benzene (TPBi) are selected as the electron transporting layer (ETL), and the thickness of yellow EML is adjusted to optimize device performance. The device based on a 0.3-nm-thick yellow EML and Bphen exhibits high color stability with a slight Commission International de l’Eclairage (CIE) coordinates variation of (0.017, 0.009) at a luminance ranging from 52 cd/m2 to 6998 cd/m2. The TPBi-based device yields a high efficiency with a maximum external quantum efficiency (EQE), current efficiency, and power efficiency of 10%, 21.1 cd/A, and 21.3 lm/W, respectively. The ultrathin yellow EML suppresses hole trapping and short-radius Dexter energy transfer, so that Förster energy transfer (FRET) from DMAC-DPS to (tbt)2Ir(acac) is dominant, which is beneficial to keep the color stable. The employment of TPBi with higher triplet excited state effectively alleviates the triplet exciton quenching by ETL to improve device efficiency.