# 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)
### Composite Layers Based on 2D Materials
#### 2.2.3 Composite HIL Based on 2D Materials  
Park reported OLEDs with composite HILs comprised of two types of 2D materials (MoS\({}_{2}\), GO) [39]. MoS\({}_{2}\) was prepared by an intercalation method with the lateral size between a few hundreds of nanometers and 1 \(\upmu\)m, while GO was prepared by a modified Hummers method with the size ranging from 400 nm to a few micrometers. On increasing the GO concentration, the MoS\({}_{2}\)-GO composite showed improved coverage on the ITO surface accompanied with the reduced surface roughness, which can be ascribed to the large particle size of GO. The OLED with pure GO HIL showed very high power efficiency (PE, 4.94 Im/W) because of the high surface coverage and high work function of GO facilitating the hole injection. The OLED with pure MoS\({}_{2}\) HIL also showed high PE of 3.12 Im/W, suggesting the suitability of the MoS\({}_{2}\) HIL. The optimal OLED with MoS\({}_{2}\):GO (6:4) composite HIL showed the maximum PE of 3.77 Im/W, indicating that the MoS\({}_{2}\):GO composites can be employed to  
Figure 4: (**a**) Schematic structure of OLEDs with a poly(styrenesulfonate)-doped poly(3,4-ethylenedioxythiophene) (PEDOT:PSS)/Graphene Oxide (GO) composite anode, reprinted with permission from [53], Elsevier, 2015; (**b**) Schematic structure of PLEDs with a PEDOT:PSS:GO composite HTL, reprinted with permission from [52], Royal Society of Chemistry, 2014; (**c**) Schematic structure of OLEDs with a GO:MoO\({}_{r}\) composite HTL, reprinted with permission from [40], Wiley, 2018; (**d**) Schematic structure and (**e**) energy level diagram of OLEDs with different HILs, reprinted with permission from [56], AIP Publishing LLC, 2018; (**f**) Energy level diagram of ITO/MoS\({}_{2}\):PEDOT:PSS/NPB interface, reprinted with permission from [41], Royal Society of Chemistry, 2019.  
construct HILs for OLEDs. Besides, the OLEDs with GO HIL show better stability than those with PEDOT:PSS HIL. [PERSON] demonstrated phosphorescent OLEDs based on GO-doped PEDOT:PSS HILs [49]. The GO:PEDOT:PSS-based OLEDs show better performance with maximum CE of 52 cd/A, which is 27% higher than that with PEDOT:PSS HIL. The introduction of GO resulted in enhanced performance of the OLEDs because of a reduced hole injection barrier at the ITO/PEDOT:PSS interface. [PERSON] prepared OLEDs with the structure of ITO/PEDOT:PSS:GO/NPB/Alq\({}_{3}\)/LiF/Al using GO doped PEDOT:PSS as a HIL, in which the GO concentration is 0-1 wt.% [54]. High luminance and CE with a low turn-on voltage observed in OLEDs using PEDOT:PSS:GO composite HIL can be attributed to the enhanced hole injection ability, conductivity and transparency compared with the OLED with a pure PEDOT:PSS HIL. A maximum luminance of 17939 cd/m\({}^{2}\) and maximum CE of 3.74 cd/A was observed in the OLED with PEDOT:PSS:GO (0.8 wt.%) composite HIL, showing an enhancement of 46.6% and 67.6% compared with the reference OLED, respectively. [PERSON] studied Alq\({}_{3}\)-based OLEDs using GO:MoO\({}_{x}\) composite HILs with the ratios of 1:0, 1:6, 1:8, 1:10, 0:1, as shown in Figure 4c [40]. The OLED device with the ratio of 1:8 showed maximum CE of 8.6 cd/A and EQE of 3.5%, which have been enhanced by 41.0% (75.5%) and 40.0% (75.0%) compared with the control device using individual MoO\({}_{x}\) (GO) HIL, respectively. The improved device performance can be attributed to the promoted hole injection, which was caused by the improved film morphology and work function of GO:MoO\({}_{x}\) composite HILs. Besides, [PERSON] demonstrated OLEDs using MoO\({}_{3}\) nanoparticle-modified MoS\({}_{2}\) nanosheets (MoS\({}_{2}\):MoO\({}_{3}\)) as HILs, as shown in Figure 4d [56]. The device performance is greatly improved by employing the MoS\({}_{2}\):MoO\({}_{3}\) composite HIL compared with the devices with conventional PEDOT:PSS HIL or MoO\({}_{3}\) nanoparticle HIL.