**2.1 Electron molecular orbital**

Once a molecular orbital achieved the maximum electron energy, it is called the highest occupied molecular orbital (HOMO). Otherwise, if a molecular orbital has unfilled electrons, the molecular orbital is called the lowest unoccupied molecular orbital (LUMO). The energies of HOMO and LUMO affect the ionization potential and electron affinity of materials (**Figure 3**) [12].

Ionization potential energy is the minimum energy required to extract one electron from the HOMO, and electron affinity is the energy required to add one electron to the LUMO so that the system is stabilized [11, 12].

Before considering the light emission mechanism, it is important to understand the electron configuration in both the ground state and the excited state. Before excitation, when in ground state, the electrons are placed with both upward spin and downward spin (**Figure 3**). When excited, the electrons in the upper state are allocated with the same spin state, or the spin is reversed. The light emission is resulting from the energy transfer from the excitation state to the ground state.

### **2.2 Electron transfer and recombination**

Normal materials in QLED have high resistance at weak electric fields. Therefore, researchers introduced the thin film to create strong electric field and chose

**71**

**Figure 4.**

*The energy diagram of QLED.*

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

structures and materials suitable for charge injection [13]. The QLED performance is highly dependent on the choice of charge injection materials. Good charge injection materials should have high carrier mobility and balance the electron/hole injections well. The charge injection from electrodes follows the Schottky effect that means the injection barrier would be lowered according to the image force principle. When an electron is injected into the electrode, if all the HOMO orbitals are occupied and cannot accept the additional charge, the charge will be transferred into the LUMO. When electrons are transferred into the LUMO, they form an electric current. At the same time, there will be a hole injected from the anode electrode which will be transferred into the HOMO. However, when the amount of injected charge exceeds the internal charge amount, the conduction system changes from

If charge transfer by electric field and diffusion is taken into account and no trap

(1)

J = \_ 9 8 *V*<sup>2</sup> \_ *d*3

According to Eq. (1), the electric current is proportional to the square of the voltage. This is called the Mott-Gurney law, an extension of Child's law that takes

The recombination and generation of excitons of QLED are shown in **Figure 4**. When an electron and a hole recombined in the emission layer, the photons formed, whose wavelength corresponds to the energy bandgap of the quantum dots. The more electron and hole are recombined, the more photos will be generated, which corresponds to more light we could detect. Thus, people applied the hole transport layer (HTL) and electron transport layer (ETL) to restrict the electrons and holes in the emission layer, in order to improve the device efficiency. There are

• Electron transport layer (ETL)—for electron injection from the cathode and

• Electron injection layer (EIL)—for electron injection from the cathode

• Hole transport layer (HTL)—for hole transportation from HIL to EML.

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

ohmic to "space charge-limited current" [12–14].

collision into consideration [15].

five typical layers for a QLED structure:

transportation of the electron.

electrode.

is assumed, the electric current can be expressed as Eq. (1):

**Figure 3.**

*The orientation of the HOMO and LUMO [12].*

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

structures and materials suitable for charge injection [13]. The QLED performance is highly dependent on the choice of charge injection materials. Good charge injection materials should have high carrier mobility and balance the electron/hole injections well. The charge injection from electrodes follows the Schottky effect that means the injection barrier would be lowered according to the image force principle.

When an electron is injected into the electrode, if all the HOMO orbitals are occupied and cannot accept the additional charge, the charge will be transferred into the LUMO. When electrons are transferred into the LUMO, they form an electric current. At the same time, there will be a hole injected from the anode electrode which will be transferred into the HOMO. However, when the amount of injected charge exceeds the internal charge amount, the conduction system changes from ohmic to "space charge-limited current" [12–14].

If charge transfer by electric field and diffusion is taken into account and no trap is assumed, the electric current can be expressed as Eq. (1):

$$\text{se expressed as Eq. (1):}$$

$$\text{J} = \frac{9}{8} \frac{\varepsilon \mu V^2}{d^3} \tag{1}$$

According to Eq. (1), the electric current is proportional to the square of the voltage. This is called the Mott-Gurney law, an extension of Child's law that takes collision into consideration [15].

The recombination and generation of excitons of QLED are shown in **Figure 4**.

When an electron and a hole recombined in the emission layer, the photons formed, whose wavelength corresponds to the energy bandgap of the quantum dots. The more electron and hole are recombined, the more photos will be generated, which corresponds to more light we could detect. Thus, people applied the hole transport layer (HTL) and electron transport layer (ETL) to restrict the electrons and holes in the emission layer, in order to improve the device efficiency. There are five typical layers for a QLED structure:


**Figure 4.** *The energy diagram of QLED.*

*Quantum Dots - Fundamental and Applications*

about ZnO NPs will be introduced in Section 3.

**2. Light emission mechanism of QLED**

and electron affinity of materials (**Figure 3**) [12].

**2.2 Electron transfer and recombination**

electron to the LUMO so that the system is stabilized [11, 12].

**2.1 Electron molecular orbital**

the ground state.

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

The emission mechanism of QLED is discussed in this subsection. A QLED has a similar structure and behavior as an OLED. In the QLED, the emitter is a semicon-

Once a molecular orbital achieved the maximum electron energy, it is called the highest occupied molecular orbital (HOMO). Otherwise, if a molecular orbital has unfilled electrons, the molecular orbital is called the lowest unoccupied molecular orbital (LUMO). The energies of HOMO and LUMO affect the ionization potential

Ionization potential energy is the minimum energy required to extract one electron from the HOMO, and electron affinity is the energy required to add one

Before considering the light emission mechanism, it is important to understand the electron configuration in both the ground state and the excited state. Before excitation, when in ground state, the electrons are placed with both upward spin and downward spin (**Figure 3**). When excited, the electrons in the upper state are allocated with the same spin state, or the spin is reversed. The light emission is resulting from the energy transfer from the excitation state to

Normal materials in QLED have high resistance at weak electric fields. Therefore,

researchers introduced the thin film to create strong electric field and chose

ductor nanoparticle, while in the OLED, the emitter is an organic material.

**70**

**Figure 3.**

*The orientation of the HOMO and LUMO [12].*


This direct injection of charge carriers is assumed as the most common phenomenon for creating an exciton in the device.

In Section 1, the QLED structure types are elaborated. Thus, it is very important to design a QLED by factoring in the relationships between the work function of each layer. The QLED device fabrication process will be discussed in Section 3.

### **2.3 ZnO nanoparticles**

Behind the QLED structure, the ZnO nanoparticles (ZnO NPs) have gained substantial interest in the research community as the charge transport layer (CTL). In 2008, Janssen [8] and co-workers demonstrated all-solution-processed multilayer QLEDs by using ZnO NPs as ETLs and organic materials as HTLs. The colloidal ZnO NPs were dispersed in isopropanol, and the deposition of the ZnO NPs on the top of the QD layers did not dissolve the underlying layers. Since then, continuous efforts were made to improve the performance of QLED with solution-processed n-type oxides as CTLs. ZnO NPs are widely used as CTLs in the state of the art of high-performance QLEDs.

Generally, solution-processed oxide CTLs can be deposited by two approaches, the precursor approach and the nanocrystal approach. The molar ratio of zinc precursor to potassium hydroxide (KOH) played an important role in determining the shape of ZnO NPs and hence affected the conductivity and mobility of ZnO NP film prepared from ZnO NPs [15–17]. ZnO NPs were synthesized by hydrolysis/ condensation reactions under basic conditions. The synthesis procedure will be introduced in Section 3.
