# Improved Charge Injection and Transport of Light-Emitting Diodes Based on Two-Dimensional Materials
## 2 Two-Dimensional (2D) Materials in Organic Light Emitting Diodes (OLEDs)  
OLEDs, in which EML is an organic material such as Tris-(8-hydro-xyquinoline)aluminum (Alq\({}_{3}\)), have attracted intensive interest because of their excellent properties such as low cost, high brightness and flexibility [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14]. The organic material employed in OLEDs is an organic semiconductor, which contains small organic molecules and organic polymers. The OLEDs can be used in the commercialization of low-cost displays with low driving voltage, high contrast, wide viewing angle and color gamut. In OLEDs, 2D materials have been introduced as anode, HTL, HIL, electron injection layer (EIL) and dopant in functional layers such as composite HTL/HIL in order to obtain the enhanced performance of devices [35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57]. Figure 2 shows some related structures of OLED devices based on 2D materials. The device parameters of OLEDs with 2D materials interlayers were summarized in Table 1.  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**2D Materials** & **Device Structure** & **L\({}_{max}\) (cd/m\({}^{2}\))** & **CE\({}_{max}\) (cd/A)** & **EQE\({}_{max}\) (\%)** & **References** \\ \hline rGO & ITO/rGO/super yellow/LE/AI & 8300 & 5.0 & 1.8 & [35] \\ GO & ITO/GO/super yellow/LE/AI & 39,000 & 19.1 & 6.7 & [35] \\ GO & ITO/GO/TP/Alq\({}_{3}\)/LiF or Li\({}_{3}\)/NI & 53,635 & 22.13 & 2.27 & [36] \\ GO & ITO/GO/NPB/Alq\({}_{3}\)/LiF/AI & - & - & 0.62 & [37] \\ GO & ITO/SO/NPB/Alq\({}_{3}\)/LiF/AI & 15,770 & 4.4 & - & [38] \\ GO & ITO/SO/NPB/Alq\({}_{3}\)/C457/BCD/Alq\({}_{3}\)/AI & 20,000 & 11.4 & - & [39] \\ GO & ITO/SO/NPB/Alq\({}_{3}\)/Bben/LI/AI & 14.99 & 4.9 & 2.0 & [40] \\ MoS\({}_{2}\) & ITO/MoS\({}_{2}\) &NPB/Alq\({}_{3}\)-C5457/BCD/Alq\({}_{3}\)/AI & 23,000 & 11.95 & - & [39] \\ MoS\({}_{2}\) & ITO/MoS\({}_{2}\)/NPB/Alq\({}_{3}\)/Bben/LI/AI & 8003 & 7.0 & - & [41] \\ Graphene & PE/graphene/PE/DC/PSS/NPB/Alq\({}_{3}\)/LiF/AI & - & 1.09 & - & [42] \\ MoS\({}_{2}\)CO & ITO/MoS\({}_{2}\)/SO/NPB/Alq\({}_{3}\)/C4557/BCD/Alq\({}_{3}\)/AI & 20,500 & 11.38 & - & [39] \\ MoS\({}_{2}\) & ITO/MoS\({}_{2}\)/NPB/Alq\({}_{3}\)/C4557/BCD/Alq\({}_{3}\)/LiF/AI & 18,900 & 12.01 & - & [43] \\ WS\({}_{2}\) & ITO/MoS\({}_{2}\)/NPB/Alq\({}_{3}\)/C4557/BCD/Alq\({}_{3}\)/LiF/AI & 19,300 & 12.44 & - & [43] \\ TS\({}_{2}\) & ITO/MoS\({}_{2}\)/NPB/Alq\({}_{3}\)/C4557/BCD/Alq\({}_{3}\)/AI & 18,400 & 12.66 & - & [43] \\ MoS\({}_{2}\) & ITO/MoS\({}_{2}\)/NPB/Alq\({}_{3}\)/C5457/BCD/LiF/AI & 16,700 & 9.44 & - & [44] \\ WS\({}_{2}\) & ITO/WS/NPB/Alq\({}_{3}\)/C4557/BCD/Alq\({}_{3}\)/EI/AI & 19,000 & 10.82 & - & [44] \\ MoS\({}_{2}\) & ITO/MoS\({}_{2}\)/NPB/Alq\({}_{3}\)/C4557/BCD/Alq\({}_{3}\)/AI/AI & 21,100 & 11.3 & - & [45] \\ WS\({}_{2}\) & ITO/WS\({}_{2}\)/NPB/Alq\({}_{3}\)/C4557/BCD/Alq\({}_{3}\)/AI/AI & 23,300 & 14.7 & - & [45] \\ MoS\({}_{2}\) & ITO/MoS\({}_{2}\)/Bben/Sc\({}_{2}\)O\({}_{3}\)/Bben/NDAD-SDA-ph/NPB/MoO\({}_{3}\)/AI & 9141 & 14.8 & 7.3 & [46] \\ \hline \end{tabular}
\end{table}
Table 1: Summary of the device parameters of OLEDs with 2D materials interlayers.  
Figure 2: Schematic structures of organic light emitting diodes (OLEDs), in which organic materials such as emission layers (EML) and 2D materials such as **(a)** hole transport layer (HTL), **(b)** hole injection layer (HIL), (c) dopant in HTL.