**1. Introduction**

After QLED was first published in 1994, a lot effort had been spent to improve the reliability and performance of QLED devices [1, 2]. The first QLED device uses thick QDs acting as both the emission layer and electron transport layer, which can be referred to in **Figure 1(a)** and **(b)**. The luminous efficiency of QLED device has been improved by the use of electron injection layer. Coe et al. published a sandwiched QLED structure in 2002, which consisted of two organic thin films with QDs as the emission layer. The luminescence was improved 25-fold over the best results of the previous QLED device, as shown in **Figure 1(d)** [3]. The QLEDs use Alq3 and poly-TPD as the electron transport layer, as shown in **Figure 1(c)**. However, the organic thin layers were sensitive to moisture and oxygen. Thus, replacing organic material with inorganic material seems to be the best solution to improve the reliability of the QLED device. Mueller et al. fabricated an all-inorganic

#### **Figure 1.**

*Structure designs and material designs for efficient QLEDs. (a) Energy-level structure of the first QLED. (b) Electroluminescence of the first QLED [1]. (c) Electroluminescence spectra and structures for a 40-nm-thick film of Alq3, followed by a 75-nm-thick Mg:Ag cathode with a 50 nm Ag cap. (d) The corresponding external quantum efficiency [3]. (e) QLED structure consists of p-type GaN and n-type GaN. (f) Electroluminescence of QLED consists of p-type GaN and n-type GaN [4]. (g) QLED structure with metal oxide as the electron transport layer with layer-by-layer structure in emission layer. (h) Electroluminescence and photoluminescence of the QLED with metal oxide as the electron transport layer with layer-by-layer structure in emission layer [5].*

**69**

**Figure 2.**

*(d) organic/inorganic electron transport layer [1–10].*

*Quantum Dot Light-Emitting Diode: Structure, Mechanism, and Preparation*

QLED in which QDs were sandwiched by n-type GaN and p-type GaN. **Figure 1(e)** shows the structure of QLED with GaN, while **Figure 1(f )** shows the corresponding electrical performance. Mueller et al. used metal-organic chemical vapor deposition (MOCVD) method to deposit n-type GaN and p-type GaN [4]. However, the deposition method is too harsh for QDs that make the luminescence efficiency lower than the first type of QLED device. Then in 2010, Bendall et al. demonstrated an all-inorganic QLED with metal oxide as the electron transport layer. The QLED had a layer-by-layer structure, which had three-layer emission layers as shown in **Figure 1(g)** and **(h)** [5]. The overall device performance was still poor which is caused by the degradation of QDs during the harsh deposition process of inorganic materials. However, these devices showed condescending stability under long-term

The organic materials had an advantage of high luminescence, while inorganic

According to the type of electron transport layers, the structure of QLEDs can be categorized into four different types (**Figure 2**): (a) organic/QD bilayer, (b) all-organic electron transport layer, (c) all-inorganic electron transport layer, and (d) organic/inorganic electron transport layer. The four different types of QLED

Among these four types of QLED structure, inorganic materials are one of the most important choices for electron transport layers owing to their high electrical conductivity and good stability against environmental factors such as oxygen and moisture. ZnO nanoparticles (NPs) applied in electron transport layer are a significant breakthrough in QLED development history, due to their excellent electron

*Four representative QLED structure types based on electron transport layers. (a) Organic polymer electron transport layer, (b) all-organic polymer electron transport layer, (c) all-inorganic electron transport layer, and* 

structure also represent the development history of QLEDs in sequence.

materials had an advantage of high reliability. Then the researcher combined the advantage of both organic materials and inorganic materials by using both organic materials and inorganic materials as the electron transport layers. MoS2, NiO, TiO2, and ZnO have been reported as the inorganic charge transport layers

*DOI: http://dx.doi.org/10.5772/intechopen.91162*

usage and high current density conditions.

(CTLs) [6–8].

## *Quantum Dot Light-Emitting Diode: Structure, Mechanism, and Preparation DOI: http://dx.doi.org/10.5772/intechopen.91162*

QLED in which QDs were sandwiched by n-type GaN and p-type GaN. **Figure 1(e)** shows the structure of QLED with GaN, while **Figure 1(f )** shows the corresponding electrical performance. Mueller et al. used metal-organic chemical vapor deposition (MOCVD) method to deposit n-type GaN and p-type GaN [4]. However, the deposition method is too harsh for QDs that make the luminescence efficiency lower than the first type of QLED device. Then in 2010, Bendall et al. demonstrated an all-inorganic QLED with metal oxide as the electron transport layer. The QLED had a layer-by-layer structure, which had three-layer emission layers as shown in **Figure 1(g)** and **(h)** [5]. The overall device performance was still poor which is caused by the degradation of QDs during the harsh deposition process of inorganic materials. However, these devices showed condescending stability under long-term usage and high current density conditions.

The organic materials had an advantage of high luminescence, while inorganic materials had an advantage of high reliability. Then the researcher combined the advantage of both organic materials and inorganic materials by using both organic materials and inorganic materials as the electron transport layers. MoS2, NiO, TiO2, and ZnO have been reported as the inorganic charge transport layers (CTLs) [6–8].

According to the type of electron transport layers, the structure of QLEDs can be categorized into four different types (**Figure 2**): (a) organic/QD bilayer, (b) all-organic electron transport layer, (c) all-inorganic electron transport layer, and (d) organic/inorganic electron transport layer. The four different types of QLED structure also represent the development history of QLEDs in sequence.

Among these four types of QLED structure, inorganic materials are one of the most important choices for electron transport layers owing to their high electrical conductivity and good stability against environmental factors such as oxygen and moisture. ZnO nanoparticles (NPs) applied in electron transport layer are a significant breakthrough in QLED development history, due to their excellent electron

#### **Figure 2.**

*Quantum Dots - Fundamental and Applications*

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**Figure 1.**

*Structure designs and material designs for efficient QLEDs. (a) Energy-level structure of the first QLED. (b) Electroluminescence of the first QLED [1]. (c) Electroluminescence spectra and structures for a 40-nm-thick film of Alq3, followed by a 75-nm-thick Mg:Ag cathode with a 50 nm Ag cap. (d) The corresponding external quantum efficiency [3]. (e) QLED structure consists of p-type GaN and n-type GaN. (f) Electroluminescence of QLED consists of p-type GaN and n-type GaN [4]. (g) QLED structure with metal oxide as the electron transport layer with layer-by-layer structure in emission layer. (h) Electroluminescence and photoluminescence of the QLED with metal oxide as the electron transport layer with layer-by-layer structure in emission layer [5].*

*Four representative QLED structure types based on electron transport layers. (a) Organic polymer electron transport layer, (b) all-organic polymer electron transport layer, (c) all-inorganic electron transport layer, and (d) organic/inorganic electron transport layer [1–10].*

mobility and no significant damage to the underlying QD layer during fabrication process. What's more, ZnO NPs are compatible with both polar solvent and nonpolar solvent, which makes the QLED fabrication process more flexible. More details about ZnO NPs will be introduced in Section 3.
