Using Distributed Energy States of Graphene Quantum Dots for an Efficient Hole-Injection Media in an Organic Electroluminescent Device

Abstract

In this letter, graphene quantum dots (GQDs) were used for an efficient hole-injection layer in organic electroluminescent devices. The spin-coated microplasmasynthesized GQDs had small diameters and high areal density (3.2 x 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> /cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ). The microstructure was studied using X-ray photoelectron spectroscopy, which indicated that the GQDs contained 31.5% C = O and 10.5% C-O functional groups. This indicated the highest occupied molecular orbit (HOMO) of the GQDs was approximately 5.3 eV. The photoluminescence excitation spectra suggested that the GQDs had a broad distribution of energy states (>1 eV). The material Ru(bpy) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (PF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , which has strong red-light emission at approximately 625 nm, was used as the luminescent material. The HOMO level of Ru(bpy) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> was approximately 5.7 eV. Because the HOMO level for the GQDs was 5.3 eV with an energy level distribution of 1 eV, the GQDs could adequately bridge the work function of indium-tin oxide (ITO) to the HOMO of the luminescent material (Ru(bpy) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ) for hole injection. After applying the GQDs, the emission intensity, luminous efficiency, and external quantum efficiency of the organic light-emitting electrochemical cell were enhanced approximately 45.6%, 44.2%, and 33%, respectively. The results indicated that surface-functionalizedGQDs were suitable for hole injection in organic electroluminescent devices.