# Improved Charge Injection and Transport of Light-Emitting Diodes Based on Two-Dimensional Materials
## 4 Two-Dimensional Materials in Perovskite Light Emitting Diodes (PeLEDs)  
Perovskite materials refer to a type of crystalline materials possessing an ABX\({}_{3}\) configuration. At present, the perovskite molecule applied in optoelectronic devices is a long-range ordered structure and A, B and X represent monovalent organic cations (CH3 NH3+, NH2 CH = NH2+, Cs+), divalent metal cations (Pb2+, Sn2+) and halogen anions (Cl-, Br-, I-), respectively [22]. Because of many unique properties such as high photoluminescence quantum efficiency (PLQE), excellent charge mobility and good color purity, perovskite materials have been widely used in solution-processed LEDs, although the related reports are much less compared with perovskite solar cells. The EQE value of PeLEDs increased rapidly from 0.1% to exceeding 20% in the past few years [18; 19; 20; 21; 22; 23; 24; 25; 62]. Although PeLEDs have obtained the high EQE comparable with OLEDs and QLEDs, many problems also exist for the future commercialization, including efficiency and stability of PeLEDs. As reported, PeLEDs can be fabricated according to two typical structures including p-i-n and n-i-p, as shown in Figure 7. For the p-i-n structure shown in Figure 7a, the LED device is prepared on top of a p-type HTL such as PEDOT:PSS and covered by an n-type electron transport layer (ETL), which is the typical structure of OLEDs. While for the n-i-p structure shown in Figure 7b, the LED is constructed on top of an n-type ETL such as ZnO and TiO2 and covered by a p-type HTL, which is similar to the structure of planar perovskite solar cells. The preparation of high-quality, pinhole-free perovskite films is of great importance for the achievement of high-efficiency PeLEDs. Among these two structures, the p-i-n configuration is employed more frequently due to the better coverage of perovskite films on PEDOT:PSS compared with those on TiO2 or ZnO. In p-i-n PeLEDs, solution processable PEDOT:PSS is widely used as HIL/HTL, which can reduce the surface roughness of ITO and lower the energy barrier at ITO/perovskite interface. However, an energy barrier and exciton quenching may also exist at the PEDOT:PSS/perovskite interface. Therefore, an interlayer or modified HTL is required to optimize the performance of PeLEDs. Figure 6: (**a**) Schematic structure of QLEDs with a GO HTL, reprinted with permission from [58], American Chemical Society, 2012; (**b**) Schematic structure of QLEDs with a PDEOT:PSS:GO composite HTL, reprinted with permission from [60], Wiley, 2015; (**c**) Schematic structure and (**d**) energy level diagram of QLEDs with a GO HIL, reprinted with permission from [59], Royal Society of Chemistry, 2017. Conventional 2D materials such as graphene, GO and MoS\({}_{2}\) have been widely used in photodetectors and solar cells based on perovskite materials [23; 63]. Referred to the results in OLEDs and QLEDs, 2D materials are promising candidates to replace or modify PEDOT:PSS through constructing the device structures partly shown in Figure 8. In detail, 2D materials are expected to enhance the performance of PeLEDs through functionalizing as anode, HIL, HTL, EIL and dopant in these functional layers. However, there are only a few reports about their application in PeLEDs [64; 65; 66]. The device parameters of PeLEDs based on 2D materials were summarized in Table 4. [PERSON] developed ITO-free PeLEDs, in which four-layer graphene was employed as an anode, as shown in Figure 9a,b [64]. The graphene-based device exhibited the maximum CE of 18 cd/A and EQE of 3.8%, which are higher than those (10.6 cd/A and 2.2%) of ITO-based device. It can be attributed to the reduced exciton quenching by using the chemically inert graphene anode, as shown in Figure 9c. [PERSON] reported perovskite quantum dot (PQD) and nanocrystal (NC) LEDs with a two-layer graphene/PEDOT:PSS conductive film, in which monolayer graphene is an anode and PEDOT:PSS is a HTL, as shown in Figure 9d [65; 66].