# Improved Charge Injection and Transport of Light-Emitting Diodes Based on Two-Dimensional Materials
## 1 Introduction  
Light-emitting diodes (LEDs) are considered to be the most promising energy-saving technology for future lighting and display. The appropriate use of LEDs can lower the world's electricity use for lighting significantly. LED is a semiconductor light source operating through an effect called electroluminescence (EL). In detail, electrons and holes recombine in the semiconductor emitter, accompanied with the releasing of energy in the form of photons. The wavelength (or color) of the generated light depends on the energy band gap of the semiconductors employed in LEDs. By utilizing several kinds of semiconductors corresponding to different colors or the additive of light-emitting phosphor, white light can be generated. Compared with incandescent light sources, LEDs have many advantages, including longer lifetime, lower energy consumption and smaller size. Up to now, several types of LEDs have been studied and commercialized in the last decades. Ideal LEDs are characterized by several factors, including high efficiency, stability, purity and tunability of color. Furthermore, solution-processable manufacturing technology of LEDs will be favorable to obtain the LEDs with reduced energetic and economic cost, which is essential to the commercialized manufacturing of LEDs. To achieve this purpose, the optimized research and development of emitter materials is of great importance to increase radiative recombination and light extraction efficiency, as well as reduce the losses caused by non-radiative recombination. Although inorganic LEDs based on III-V semiconductors such as GaN and InGaN have made semiconductor solid state lighting popular, accompanied with the significant saving of energy, the high-temperature, expensive vacuum-based manufacturing process employed to fabricate these semiconductors restricts their widespread application. The substitutes of inorganic LEDs contain organic light emitting diodes (OLEDs) [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14], quantum dot light emitting diodes (QLEDs) [15; 16; 17] and perovskite light emitting diodes (PeLEDs) [18; 19; 20; 21; 22; 23; 24; 25], which take advantage of the high photoluminescence quantum efficiency (PLQE), solution processability, good purity and tunability of color. For these new types of LEDs, high-efficiency and stable devices are still the objectives of research works. Two-dimensional (2D) materials, a class of materials comprised of monolayer (ML) or a few layers of atoms (or unit cells), have attracted much attention in recent years, due to their excellent physical and chemical properties, which are originated from their unique structure and morphology. Excellent electrical, optical and other properties have been found in many conventional 2D materials including graphene and its derivatives, transition metal dichalcogenides (TMDs) such as MoS\({}_{2}\), WS\({}_{2}\), MoSe\({}_{2}\), WSe\({}_{2}\), as shown in Figure 1[26; 27; 28; 29; 30; 31; 32; 33; 34]. These 2D materials have been widely used as functional layers for optoelectronic devices. For example, some graphene derivatives such as solution-processable graphene oxide (GO) and reduced graphene oxide (rGO) can be utilized as efficient electron or hole transport layers for optoelectronic devices, such as solar cells, photodetectors and LEDs. By treating graphite with strong oxidizers, graphite oxide can be obtained, which is a compound of carbon, hydrogen and oxygen. Oxygen functional groups are introduced in the graphite structure during the oxidation process, which expands the distance between layers and makes the material hydrophilic, suggesting that they can be dispersed in water. Therefore, the graphite oxide can be easily exfoliated in water to obtain single or few-layer graphene, which is the so-called GO, exactly a few-layer graphite oxide. GO behaves as an electrical insulator with a large band gap of \(\sim\)3.6 eV due to the disruption of its sp\({}^{2}\) bonding networks.