Highly-twisted pyrene derivative for pure-blue organic light emitting diodes

Abstract

The new fluorescent blue host material, 1,3,6,8-tetrakis(2,5-dimethylbiphenyl-4-yl)pyrene (BD4PP), was synthesized for pure-blue organic light-emitting diodes (OLEDs). BD4PP has highly twisted and steric structure for efficient fluorescence, and has high thermal stability of 492 °C. BD4PP host doped with 5 wt.% 4,4′-bis(9-ethyl-3-carbazovinylene)-1,1′-biphenyl (BCzVBi) or 5 wt. % 1,4-bis[2-(3-N-ethylcarbazoryl)-vinyl]benzene (BCzVB) showed high device efficiencies due to wide bandgap (˜ 3.1 eV) and electron-rich host. An OLED with the BD4PP:BCzVB emitter showed higher current and external quantum efficiencies of 4.41 cd/A and 3.80% with color coordinates (0.16, 0.13) compared to those of the BD4PP:BCzVBi (3.63 cd/A and 3.62%) with color coordinates (0.16, 0.11) and non-doped BD4PP (2.01 cd/A and 1.61%) with color coordinates (0.18, 0.13).

Introduction

The development of a wide bandgap of ˜ 3 eV and pure blue color of Commission Internationale d’Énclairage (CIE) y-coordinate <0.15 can improve the organic light emitting diodes (OLEDs) efficiency by reducing device power-consumption, and it can be utilized to generate emission of other colors by energy transfer in full-color OLEDs [1], [2], [3]. However, the symmetrical and plain molecules for blue host material can easily form eximer emission in the solid state with long wevelength and it could be resulted in low external quantum efficiency although many fluorescent blue emitters, such as anthracene, xylene, phenylene, pyrene, fluorene, cabazole, triarylamine derivatives, and aromatic hydrocarbon have been reported [1], [2], [3], [4], [5], [6], [7], [8], [9]. To resolve the excimer emission problem, many pyrene derivatives with highly steric groups have been engineered: 1,3,6,8-tetra(phenyl)pyrene with meta-tolyl and ortho-tolyl, fluorene derivatives with pyrene group at the C2,7 positions, 9-phenyl-9-pyrenylfluorene subsitituted pyrenes with the noncoplanar and steric tortional hindrance structure, pyrene-based diarylbenzenes, highly soluble and processable polypyrene with a defect-free structure, pyrene-fuctionalized carbazole derivatives, and 2,7-functionalized pyrene scaffold with low orbital coefficients [1], [2], [4], [5], [6], [9]. Pyrene lead to red-shift or low quantum efficiency by strong intermolecular interaction due to π–π stacking of the pyrene moiety, wheras it has electron-rich property. However, the strong π–π stacking can be reduced by twist structure for high device efficiency [2], [5]. Pyrene derivatives phenyl or naphthyl end groups undergo intermolecular interaction that disrupts recrystallization, and thereby purify the color [3]. This emitting material has well-aligned molecular structure that increases OLED efficiency by strengthening outcoupling and increasing carrier transport.

In this study, we synthesized a new pyrene derivative, 1,3,6,8-tetrakis(2,5-dimethylbiphenyl-4-yl)pyrene (BD4PP) that contains xylene units, as a host material for fluorescence blue OLEDs. Since the xylene units inhibit intermolecular interaction, this inhibition yields both color purity and high efficiency. We have studied the properties of non-doped BD4PP device and 5 wt. % 4,4′-bis(9-ethyl-3-carbazovinylene)-1,1′-biphenyl (BCzVBi) or 5 wt.% 1,4-bis[2-(3-N-ethylcarbazoryl)-vinyl]benzene (BCzVB) doped BD4PP devices. The scattered X-ray intensities of the non-doped host and host:dopant compounds along with the spectra of in-plane and out-of-plane scattering for a plot of orientation distribution were analyzed using grazing incidence wide-angle X-ray diffraction (GI-WAXD) measurement.