# Improved Charge Injection and Transport of Light-Emitting Diodes Based on Two-Dimensional Materials
## 3 Two-Dimensional Materials in Quantum Dot Light Emitting Diodes (QLEDs)  
QLEDs, in which EML is composed of quantum dots (QDs) such as CdSe@ZnS QDs, have attracted much attention because of their superior properties such as tunable emission wavelengths manipulated by the particle size of QDs, high color purity, solution processability compatible with flexible substrates [15; 16; 17]. Many strategies have been explored to improve the performance of QLEDs,including synthesizing high-quality QDs and engineering QLED device structures, in which the energy band structure among functional layers is essential for the achievement of high efficiency devices. Solution processable PEDOT:PSS is widely used as HIL/HTL in QLEDs, which can reduce the surface roughness of ITO and lower the energy barrier at ITO/EML interface. However, an energy barrier at the PEDOT:PSS/EML interface and exciton quenching may also exist. Therefore, an interlayer or modified HTL is required to optimize the structure and performance of QLEDs. 2D materials are promising candidates to replace or modify PEDOT:PSS. In QLEDs, 2D materials have been introduced as HTL, HIL and dopant in composite HIL/HTL to enhance the performance of devices [58; 59; 60; 61]. Figure 5 shows some related structures of QLED devices based on 2D materials. The device parameters of QLEDs based on 2D materials were summarized in Table 3.  
[PERSON] reported QLEDs with the structure of ITO/GO/QDs/TPBi/LiF/Al utilizing GO HTL, as shown in Figure 6a [58]. A maximum luminance of 165 cd/m\({}^{2}\) was found in the QLED device consisting of the 2 ML QDs EML and 2 nm GO HTL. In addition to the role of HTL, the thin GO films can also act as the electron blocking layer in the QLED devices. [PERSON] reported a QLED using GO doped PEDOT:PSS (PEDOT:PSS:GO, 0.23-1.5 wt.%) composite HIL to optimize the energy barrier at HTL/EML interface, as shown in Figure 6b [60]. On increasing the doping concentration of GO, the valence band edge of the PEDOT:PSS:GO shifts downwards by 0.53-0.66 eV, which is close to that of the QDs. The QLEDs with PEDOT:PSS:GO (1.0 wt.%) composite HILs showed a maximum luminance of 4200 cd/m\({}^{2}\), which is six times higher than that of the control device without GO. The improved performance can be attributed to the enhanced hole injection, while a reduced turn-on voltage can be explained by the direct exciton recombination within the QDs. [PERSON] inserted a GO HIL between ITO and PEDOT:PSS to improve the hole injection ability of QLED devices, as shown in Figure 6c [59]. The QLED device with the GO HIL exhibited a maximum luminance of 6741 cd/m\({}^{2}\), CE of 1.34 cd/A and EQE of 1.61, which are 282%, 60% and 39% higher than those of the control device without GO HIL. Besides, a turn-on voltage of 5.35 V was obtained, which is lower than the value of 8.35 V for the device without GO HIL. The improved performance was ascribed to the energy step after the insertion of GO HIL between ITO and PEDOT:PSS. In detail, the work function of the GO layer is measured to be 4.98 eV, which reduces the energy barrier between ITO and PEDOT:PSS from 0.49 eV to 0.22 eV, as shown in Figure 6d. [PERSON] further investigated the performance of QLEDs with a GO/rGO HIL and found that the hole current in reduced GO (rGO) HIL is one order of magnitude larger than that in GO HIL mainly originated from the increased conductivity of rGO [61]. The enhanced hole injection leads to the maximum CE and EQE of 14.99 cd/A and 4.23%, which are 112% and 114% compared with the control device with GO HIL, respectively. According to these  
Figure 5: Schematic structures of quantum dot light emitting diodes (QLEDs), in which quantum dots as EML and 2D materials as (**a**) HTL, (**b**) HIL, (**c**) dopant in HTL.