# Improved Charge Injection and Transport of Light-Emitting Diodes Based on Two-Dimensional Materials
## 5 Conclusions  
In conclusion, the application of 2D materials in OLEDs, QLEDs and PeLEDs were demonstrated and discussed. 2D materials, including graphene and its derivatives and TMDs, can be introduced as interlayers and dopant in composite functional layers for high-efficiency LEDs, suggesting the extensive application in LEDs.  
Firstly, 2D materials can be introduced as interlayers in LEDs, including anode, HTL, HIL and EIL. The excellent conductivity and appropriate work function of 2D materials determine their function as anode in OLEDs. For LEDs with 2D material HTL/HIL, 2D materials may enhance the performance of devices from several aspects including the improved work function, effective electron blocking, suppressed exciton quenching and reduced surface roughness of ITO. Secondly, composite layers based on 2D materials have also been widely studied due to the combined effects caused by their unique structure and morphology and can act as anode, HTL, HIL and EIL. As for composite anode based on 2D materials, the excellent conductivity and transparency of composite films based on 2D materials make them promising substitute to ITO. Meanwhile, the composite anode may exhibit an appropriate work function, leading to the excellent hole injection from anode into the organic layer. As for LEDs with composite HTL/HIL based on 2D materials, the performance of device may be enhanced from several aspects including the improved work function, effective electron blocking and reduced surface roughness of ITO. These results indicate that doping 2D materials in HTL/HIL can improve the performance of LEDs by increasing the hole injection, blocking the electron transport and reducing the surface roughness of ITO, which guarantee more holes and electrons recombining in EML. Besides, 2D materials can be introduced as dopant in EIL to enhance the electron injection ability.  
As seen, there is great progress for improving the charge injection and transport of LEDs based on 2D materials. However, there are also some problems in this field. Several factors such as thickness and doping concentration affect the performance of devices, suggesting that the appropriate introduction of 2D materials is essential for the improved efficiency and stability of LED devices. Besides, there  
Figure 9: **(a)** Schematic structure and **(b)** energy level diagram of PeLEDs with a graphene anode, reprinted with permission from [64], [PERSON], 2017; **(c)** PL spectra of MAPPbBr\({}_{3}\) film on graphene/Buf-HIL and ITO/Buf-HIL, reprinted with permission from [64], [PERSON], 2017; **(d)** Schematic structure of perovskite quantum dot LEDs with a graphene anode, reprinted with permission from [65], Royal Society of Chemistry, 2019.  
are only a few reports about their application in PeLEDs. It is expected that the introduction of 2D materials in PeLEDs and related design of device structures could obtain the enhanced performance similar to those in OLEDs and QLEDs. We believe that LEDs based on 2D materials will develop sustainably in the future.  
Conceptualization, Y.Z. and F.M.; Funding acquisition, Y.Z. and F.M.; Investigation, S.M., D.S. and N.L.; Methodology, S.M., J.X. and X.C.; Writing-original draft, Y.Z. and S.M.; Writing-review & editing, Y.Z.  
This work was supported by the Foundation of Hubei Provincial Science and Technology Department (Grant No. 2016 BKJ005), the Leading Plan of Green Industry of Hubei University of Technology (Grant No. YXQN2016003), the Open Foundation of Hubei Key Laboratory for High-efficiency Utilization of Solar Energy and Operation Control of Energy Storage System (Grant Nos. HBSEES201801, HBSEES201705) and the National Natural Science Foundation of China (Grant Nos. 51371079, 11304092 and 11305056).  
The authors declare no conflict of interest.