# 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
#### 2.2.1 Composite Anode Based on 2D Materials  
PEDOT:PSS doped with 2D materials can be introduced as an anode to substitute ITO, which facilitates the implementation of ITO-free OLEDs. [PERSON] demonstrated ITO-free OLEDs with improved performance by doping PEDOT:PSS with GO (0:1, 5:1, 15:1, 25:1, 35:1, 1:0) as a composite anode, as shown in Figure 4a [53]. The OLED with PEDOT:PSS/GO (15:1) composite anode showed the maximum CE of 5.71 cd/A, which is 55% higher than that with pure PEDOT:PSS anode. It can be explained by the enhanced hole injection, because the PEDOT:PSS/GO composite anode exhibits a higher work function compared with the pure PEDOT:PSS anode. Moreover, composite anode possesses high transmittance and good surface morphology similar to that of the pure PEDOT:PSS film. [PERSON] reported a highly transparent and conductive film by doping PEDOT:PSS with GO and sodium dodecyl benzene sulfonate (SDBS) [50]. The conductivity of PEDOT:PSS/GO composite anode increases because the weakened coulombic attraction between PEDOT and PSS by functional groups in GO nanosheets, accompanied with the expanded conductive network by linking PEDOT chains with GO nanosheets. Using the optimized composite film as an anode, ITO-free OLEDs showed better performance than that with conventional ITO anode, suggesting that PEDOT:PSS:GO:SDBS hybrid films are promising alternatives to ITO for flexible OLEDs. [PERSON] prepared flexible OLEDs with a graphene/PEDOT:PSS composite anode, namely graphene composite electrode (GCE) [51]. The conductivity and transparency of GCE are comparable with the ITO anode and the performance of two kinds of devices are also at the same level. Compared with ITO with a work function of \(\sim\)4.7 eV, GCE may have a higher work function between 4.7 eV and 5.2 eV, suggesting the smaller energy barrier for hole injection, which would lead to an enhanced hole-electron recombination. As for composite anode based on 2D materials, the excellent conductivity and transparency of composite films based on 2D materials make them a promising constituent of ITO. Meanwhile, the composite anode may exhibit a higher work function compared with that without 2D materials, leading to the enhanced hole injection from the anode into the organic layer and correspondingly the improved performance. According to these reports, the solution-processable PEDOT:PSS/GO composite film represents a promising anode for OLEDs and other organic optoelectronic devices which require smooth and transparent anode.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**2D Materials** & **Device Structure** & **L\({}_{\text{max}}\) (cd/m\({}^{2}\))** & **CE\({}_{\text{max}}\) (cd/A)** & **EQ\({}_{\text{max}}\) (\%)** & **References** \\ \hline GO & ITO/GO:PEDOT:PSS/ITPD)/CBP:IT(ppy)\({}_{\text{1/B}}\)/BPhen/LiQ/Ca/Al & 103,500 & 52 & - & [49] \\ GO & PEN or Glass/GO:PEDOT:PSS/NPB/Alq\({}_{\text{3/B}}\)/BPhen/Bphen/ScCo\({}_{\text{3/A}}\)I & - & 2.7 & & [50] \\ Graphene & PET/Graphene:PEDOT:PSS/PEDOT/ITPD/Alq\({}_{\text{3/B}}\)/LiF/Al & 33 & 3.9 & - & [51] \\ GO & ITO/GO:PEDOT:PSS/MEH:PPV/LiF/Al & 725.6 & - & - & [52] \\ GO & Glass/GO:PEDOT:PSS/NPB/Alq\({}_{\text{3/B}}\)/LiF/Al & - & 5.71 & - & [53] \\ GO & Glass/GO:PEDOT:PSS/NPB/Alq\({}_{\text{3/B}}\)/LiF/Al & 17,939 & 3.72 & - & [54] \\ GO & ITO/GO:PEDOT:PSS/ADS231 BE/Cs\({}_{\text{2C}}\)O\({}_{\text{3/A}}\)I & - & 0.156 & 0.102 & [55] \\ GO & ITO/GO:Mo\({}_{\text{1/N}}\)/IPB/Alq\({}_{\text{3/B}}\)/BPhen/LiF/Al & 27,185 & 8.6 & 3.5 & [40] \\ Mo\({}_{\text{5/2}}\) & ITO/Mo\({}_{\text{3/B}}\):TD/TiO\({}_{\text{3/B}}\)/Nb/Alq\({}_{\text{3/B}}\)/DiF/Al & 24,064 & 8.1 & - & [41] \\ MoS\({}_{\text{2}}\) & ITO/Mo\({}_{\text{3/C}}\)/Mo\({}_{\text{3/C}}\)/Nb/Alq\({}_{\text{3/B}}\)/LiF/Al & - & 4.36 & - & [56] \\ graphene & ITO/NPB/Alq\({}_{\text{3/C}}\)/Cs\({}_{\text{2C}}\)O\({}_{\text{3/B}}\)/graphene/Al & 7690 & 2.02 & - & [57] \\ \hline \end{tabular}
\end{table}
Table 2: Summary of the device parameters of OLEDs with composite layer doped with 2D materials.