# 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)
### 2D Materials as Interlayers
#### 2.1.2 2D Materials as Hole Injection Layer (HIL)  
Shi reported fluorescent OLEDs with a simple device structure of ITO/GO/TPD/Alq\({}_{3}\)/LiF or Li\({}_{3}\)N/Al, in which GO with different concentration (0.02, 0.1, 0.4 mg/mL) was introduced as an HIL locating between ITO anode and TPD HTL, as shown in Figure 3d [36]. The transmitances of ITO/GO substrates with low concentration GO are near-identical, suggesting that the introduction of low-concentration GO HIL does not impede the light emission. The device with 0.1 mg/mL GO obtained a maximum luminance of 53,635 cd/m\({}^{2}\), maximum CE of 22.13 cd/A and maximum EQE of 2.27%, showing an enhancement of 110%, 44% and 44% compared with the reference device without GO HIL, respectively. Because the work function of GO (4.9 eV) is higher than that of ITO (4.7 eV), the hole injection barrier can be reduced effectively, leading to the improved hole injection.  
Figure 3: **(a)** Schematic structure and **(b)** energy level diagram of polymer light emitting diodes (PLEDs) with a Graphene Oxide (GO) HTL; **(c)** Photoluminescence (PL) spectra of SY films on different substrates, reprinted with permission from [35], American Chemical Society, 2012; **(d)** Schematic structure of OLEDs with a GO HIL, reprinted with permission from [36], Royal Society of Chemistry, 2013; **(e)** Schematic structure of OLEDs using TMDs as HILs, reprinted with permission from [43], Wiley, 2015; **(f)** Schematic structure of OLEDs with a graphene anode, reprinted with permission from [42], Elsevier, 2014.  
Furthermore, the large band gap of GO hinders the electron transport from EML to ITO, acting as an effective electron blocking layer, which also induce the improved hole-electron recombination in the EML [35]. [PERSON] reported the improved performance of OLEDs with a simple device structure of ITO/GO/NPB/Alq\({}_{3}\)/LiF/Al, in which GO (0, 1.8, 2.4, 3.2, 3.6, 4.0, 4.8, 5.4 nm) was introduced as HIL [38]. The maximum luminance of 15,770 cd/m\({}^{2}\) and CE of 4.4 cd/A were achieved in OLED with an optimal 3.6 nm GO HIL, which are higher than the values of 4735 cd/m\({}^{2}\) and 1 cd/A in reference device without GO HIL, respectively. It should be noted that the surface roughness of the ITO anode can also be reduced after the introduction of GO HIL [38]. The improvement of devices' performance can be attributed to the smooth ITO surface after the introduction of GO HIL and the reduced hole-injection barrier due to the high work function of GO. We also found the similar result in green OLED devices with the structure of ITO/GO/NPB/Alq\({}_{3}\)/LiF/Al [37].  
TMDs can be also employed as HILs in OLEDs [39; 41; 43; 44; 45]. [PERSON] reported OLEDs with a structure of ITO/TMDs/NPB/Alq\({}_{3}\):C545T/BCP/Alq\({}_{3}\)/LiF/Al, in which MoS\({}_{2}\), WS\({}_{2}\) and TaS\({}_{2}\) nanosheets were used as HILs, as shown in Figure 3e [43]. After UVO treatment, the work function of the MoS\({}_{2}\), WS\({}_{2}\) and TaS\({}_{2}\) nanosheets increases from 3.9, 4.9, 4.4 eV to 4.9, 5.1, 4.9 eV, respectively. These TMDs have similar work function comparable with PEDOT:PSS. The modification of work function leads to the reduced energy barrier for hole injection and the improved efficiency close to the device with PEDOT:PSS HIL, suggesting that TMDs are promising HILs in OLEDs. Besides, the devices with UVO-treated TMD nanosheet HILs showed longer lifetime than that with PEDOT:PSS HIL because of their nonacidic and stable properties. Similar results can be found in other literatures [44; 45].