Abstract
This work presents the synthesis of a new hole-buffering material TAZS and its successful application in polymer light-emitting diodes to enhance device performance. The TAZS is composed of aromatic 1,2,4-triazolylcore linked with three trihydroxy tert-butyl terminals via azomethine linkages. The TAZS forms ashomogeneous film deposited by spin-coating process. The HOMO and LUMO levels of TAZS are -5.23 eV and -2.40 eV, respectively, as estimated from cyclic voltammogram. The current density results of hole-only and electron-only devices confirm strong hole-buffering capability of TAZS layer. Multilayer PLEDs with different thickness of TAZS (ITO/PEDOT: PSS/TAZS (x nm)/SY/ETL/LiF/Al) have been successfully fabricated, using spin-coating process to deposit hole-injecting PEDOT: PSS, TAZS, and emissive SY layers. The PLED with 16 nm TAZS reveals the optimal device performance, with maximum luminance and maximum current efficiency of 19,046 cd/m2 and 4.08 cd/A, respectively, surpassing those without TAZS as HBL (8484 cd/m2, 2.13 cd/A). The hole-buffering characteristic of TAZS contributes greatly to improved charges’ recombination ratio and enhanced emission efficiency.
Highlights
Compound 6 with three benzaldehyde peripherals was synthesized from 3,4,5-tri(4-bromophenyl)-4H-1,2,4-triazole (4) and 2-hydroxy5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl) benzaldehyde (5) by the Suzuki coupling reaction
The target TAZS with three azomethine linkages was synthesized by the Schiff base-forming condensation of 6 and 2-amino-2-(hydroxymethyl)-1,3-propanediol(7: tris)
The purpose of attaching high polar trihydroxy tert-butyl groups is to improve the solubility of TAZS in protic solvents such as alcohol
Summary
Some hole-buffering materials have been reported, such as copper(II) phthalocyanine (CuPC) and 4,4’,4’’-tris(N-carbazolyl)-triphenyla-mine (TATC) [9] [10] They were inserted between hole injection layer and emission layer to reduce hole-transport effectively and improve the device performance [11]. The fluorescence intensity of aromatic materials substituted with azomethine moieties is very weak due to quenching by photo-induced electron transfer (PET) [28] [29]. This characteristic is beneficiary for hole-buffering application in PLEDs, since its presence will not deteriorate color purity of the emission. Insertion of TAZS as hole-buffer layer (HBL) effectively enhanced maximum luminance and maximum current efficiency to 19,046 cd/m2 and 4.08 cd/A, respectively, from those without the HBL (8484 cd/m2, 2.13 cd/A)
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