Dimesitylborane as Electron Accepting Unit in High Performance Yellow Single-Layer Phosphorescent Organic Light Emitting Diode

Abstract

A new host material for Single-Layer Phosphorescent Organic Light-Emitting Diodes (SL-PhOLED) is reported, namely SPA-2-FDMB, using the dimesitylborane (DMB) fragment as an acceptor unit. The molecular design is constructed on the general donor-spiro-acceptor architecture, which consists of connecting, via a spiro bridge, a donor and an acceptor units in order to avoid strong interaction between them. The DMB fragment is known for many electronic applications (notably Aggregation-Induced Emission) but has not been used yet for SL-PhOLED applications. This appears particularly interesting, as the development of this simplified technology has shown that only a few electron-accepting fragments such as diphenylphosphine oxide can provide high-performance devices. Herein, the yellow-emitting SL-PhOLED using SPA-2-FDMB as host presents an External Quantum Efficiency of 8.1% (Current Efficiency of 24.9 cd.A^-1) with a low threshold voltage of 2.6 V. As SPA-2-FDMB presents a sharp HOMO/LUMO difference, the good matching of HOMO and LUMO energy levels with the Fermi level of the electrodes is responsible for these performances. The low LUMO level of -2.61 eV also appears particularly important. These performances are, to date, the highest reported for a yellow/orange-emitting SL-PhOLED and show the potential of DMB unit in the single-layer technology.

Keywords: bipolar host material; single‐layer phosphorescent OLED; spiro compounds; yellow emission.

Scheme 1

Synthetic pathway of SPA‐2‐FDMB.

Figure 1

Molecular structure from single cristal X‐ray diffraction of SPA‐2‐FDMB (2 359 045), A) the perspective view, B) the donor‐donor packing interactions, C) the acceptor–acceptor packing, D) the fluorene‐acceptor interactions, E) the fluorene‐donor interactions, F) the donor‐acceptor interactions

Figure 2

Cyclic voltammetry recorded on Pt working electrode in reduction (left, DMF + Bu₄NPF₆ 0.1 M, sweep‐rate 100 mV s⁻¹) and in oxidation (middle, DCM + Bu₄NPF₆ 0.2 M, sweep‐rate 100 mV s⁻¹) of SPA‐2‐FDMB Representation of HOMO/LUMO calculated by TD‐DFT (B3LYP/6‐311+G(d,p)) (isovalue 0.04 [ebohr⁻³]^1/2) and the energy levels obtained by cyclic voltammetry (Right).

Figure 3

A) UV–vis absorption and emission spectra in cyclohexane (𝜆_exc = 310 nm) of SPA‐F‐2‐DMB. B) Normalized emission spectra at room temperature in different solvents. C) Normalized emission spectra at 77 K in 2‐MeTHF (𝜆_exc = 330 nm). D) Triplet spin density distribution (TD‐DFT, B3LYP/6‐311+g(d,p), isovalue 0.004 [ebohr⁻³]^1/2).

Figure 4

A) TGA and B) DSC (2^nd heating only) traces of SPA‐2‐FDMB, and C) representation of the energy levels and the main molecular orbitals involved in the electronic transitions obtained by TD‐DFT, B3LYP/6–311+G(d,p), shown with an isovalue of 0.04 [ebohr⁻³]^1/2. For clarity purposes, only the major contribution of each transition is shown (see Supporting Information for details).

Figure 5

Thickness‐scaled current‐voltage characteristics of SPA‐2‐FDMB hole‐A) and electron‐only B) SCLC devices.

Figure 6

SL‐PhOLED characteristics using SPA‐2‐FDMB as host material and PO‐01 as phosphorescent emitter. A) Current density (mA.cm⁻²) and luminance (cd.m⁻²) as a function of the voltage (V). B) Current efficiency (cd.A⁻¹, filled symbols) and power efficiency (lm.W⁻¹, empty symbols) as a function of the current density (mA.cm⁻²). C) Roll‐off Efficiency: EQE (%) as a function of the luminance (cd.m⁻²). D) Normalized electroluminescent spectrum at 10 mA.cm⁻² and E) Corresponding CIE diagram.