*3.4.1 ZnO nanoparticle analysis*

The concentration of precursor, evaporation rate, and the time of reaction were all significant synthetic parameters, which affected the growth of ZnO nanoparticel dimensions and structures. **Figure 5** shows the TEM images of the as-synthesized

ZnO NPs from a well-dispersed ZnO colloidal solution carried out with the reference condition. To investigate the effect of reaction time on the growth morphology, reaction times of 80 and 105 min were carried out. The nanoparticle's size is smaller in the reaction of 105 min than in the reaction of 80 min. Moreover, the crystal lattice fringes are more clearly observed in the 105 min reaction sample rather than in the 80 min reaction sample. According to confinement effect, particles with smaller diameter would have higher energy. Therefore, the ZnO NPs used for QLED preparation are the smaller ZnO NPs.

In order to analyze the bandgap and quantum effects of the different ZnO NPs, their absorption and photoluminescence spectra need to be measured, which will be processed in future study. The energy bandgap (Eg) of the colloidal ZnO nanoparticles is determined from the intercept between the wavelength axis and the tangent to the linear section of the absorption band edge. The energy bandgap of ZnO NPs at 2.9 nm is 3.65 eV. The energy bandgap for the 5.5 nm ZnO NPs was 3.35 eV [17], while the energy bandgap of bulk ZnO is 3.2–3.3 eV [18], which is lower than the energy bandgap of ZnO NPs. It is found that the tendency of energy bandgap enlargement with decreasing size is consistent with the relationship based on effective mass approximation. Therefore, the reaction of 105 min can obtain smaller ZnO NPs than the ZnO NPs in the reaction of 80 min. In addition, the lattice fringes can be clearly observed in the TEM images, which suggests good crystallinity of the ZnO NPs.

### *3.4.2 QLED device performance and analysis*

**Figure 6(a)** shows the structure of the QLED device, while **Figure 6(b)** shows the energy band diagram of the QLED device. The QLED device is a multilayer structure, which consists of PEDOT:PSS, poly-TPD, QDs, ZnO NPs, and Al. The thickness of each layer was measured by the surface profile (Alpha-Step 200 Tencor). **Figure 7** shows the TEM image of quantum dots; the diameter of the quantum dots was around 7 nm.

**75**

**Figure 7.**

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

The energy-level diagram in **Figure 6(b)** illustrated that the electrons and holes can be easily recombined together in the emission layer. Because ZnO NPs have a wide energy bandgap, the holes can be stored in the quantum dot layer. At the same time, poly-TPD's energy bandgap is from −2.3 eV to −5.4 eV. −2.3 eV is larger than −3.8 eV, which can also restrict electrons in the quantum dot layer. What is more, adding PEDOT: PSS layer and poly-TPD layer reduces the energy gap for holes jumping into the emission layer (quantum dot layer). The introduction of ZnO NPs has a similar function as PEDOT:PSS and poly-TPD, which confine the excitation and recombination region, hence potentially improving the efficiency of photon generation. The thin layer structure of QLED device makes it a promising candidate

**Figure 8** shows the device performance analysis. The turn-on voltage is shown in **Figure 8(a)**, which is between 2 V and 3 V. The low turn-on voltage is due to the high electron mobility of the ZnO NPs and the design of the QLED structures. When there is a current applied to the device, the electrons can easily be injected into the emission layer, while the holes can also flow to the emission layer easily and be restricted by the ZnO NP layer. At the same time, the electrons accumulate at the interface of the poly-TPD/quantum dots due to the ~1.5 eV energy offset between the LUMO of poly-TPD and quantum dots. When one high-energy hole can be obtained after absorbing the energy released from the interfacial recombination of an electron/hole pair, the high-energy holes can cross the injection barrier at the poly-TPD interface and recombine with the electrons inside the QD layer and then emit photons. This is called the Auger-assisted hole injection [14, 19]. Therefore, the high electron mobility of ZnO nanoparticles and the band alignment structure can facilitate the hole transport and balance of the carrier injection of the device.

The electroluminance (EL) spectra of the QLED under different voltages are shown in **Figure 8(b)**. The inserted picture is the QLED device. The EL intensity of the QLED device increases as the applied voltage increases. The wavelength is 534 nm with the

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

for the next generation of flexible displays.

*The TEM image of green QDs from Mesolight.*

**Figure 6.** *(a) The QLED device structure and (b) the QLED device energy-level diagram.*

**Figure 7.** *The TEM image of green QDs from Mesolight.*

*Quantum Dots - Fundamental and Applications*

used for QLED preparation are the smaller ZnO NPs.

*3.4.2 QLED device performance and analysis*

*(a) The QLED device structure and (b) the QLED device energy-level diagram.*

quantum dots was around 7 nm.

ZnO NPs from a well-dispersed ZnO colloidal solution carried out with the reference condition. To investigate the effect of reaction time on the growth morphology, reaction times of 80 and 105 min were carried out. The nanoparticle's size is smaller in the reaction of 105 min than in the reaction of 80 min. Moreover, the crystal lattice fringes are more clearly observed in the 105 min reaction sample rather than in the 80 min reaction sample. According to confinement effect, particles with smaller diameter would have higher energy. Therefore, the ZnO NPs

In order to analyze the bandgap and quantum effects of the different ZnO NPs, their absorption and photoluminescence spectra need to be measured, which will be processed in future study. The energy bandgap (Eg) of the colloidal ZnO nanoparticles is determined from the intercept between the wavelength axis and the tangent to the linear section of the absorption band edge. The energy bandgap of ZnO NPs at 2.9 nm is 3.65 eV. The energy bandgap for the 5.5 nm ZnO NPs was 3.35 eV [17], while the energy bandgap of bulk ZnO is 3.2–3.3 eV [18], which is lower than the energy bandgap of ZnO NPs. It is found that the tendency of energy bandgap enlargement with decreasing size is consistent with the relationship based on effective mass approximation. Therefore, the reaction of 105 min can obtain smaller ZnO NPs than the ZnO NPs in the reaction of 80 min. In addition, the lattice fringes can be clearly observed in the TEM images, which suggests good crystallinity of the ZnO NPs.

**Figure 6(a)** shows the structure of the QLED device, while **Figure 6(b)** shows the energy band diagram of the QLED device. The QLED device is a multilayer structure, which consists of PEDOT:PSS, poly-TPD, QDs, ZnO NPs, and Al. The thickness of each layer was measured by the surface profile (Alpha-Step 200 Tencor). **Figure 7** shows the TEM image of quantum dots; the diameter of the

**74**

**Figure 6.**

The energy-level diagram in **Figure 6(b)** illustrated that the electrons and holes can be easily recombined together in the emission layer. Because ZnO NPs have a wide energy bandgap, the holes can be stored in the quantum dot layer. At the same time, poly-TPD's energy bandgap is from −2.3 eV to −5.4 eV. −2.3 eV is larger than −3.8 eV, which can also restrict electrons in the quantum dot layer. What is more, adding PEDOT: PSS layer and poly-TPD layer reduces the energy gap for holes jumping into the emission layer (quantum dot layer). The introduction of ZnO NPs has a similar function as PEDOT:PSS and poly-TPD, which confine the excitation and recombination region, hence potentially improving the efficiency of photon generation. The thin layer structure of QLED device makes it a promising candidate for the next generation of flexible displays.

**Figure 8** shows the device performance analysis. The turn-on voltage is shown in **Figure 8(a)**, which is between 2 V and 3 V. The low turn-on voltage is due to the high electron mobility of the ZnO NPs and the design of the QLED structures. When there is a current applied to the device, the electrons can easily be injected into the emission layer, while the holes can also flow to the emission layer easily and be restricted by the ZnO NP layer. At the same time, the electrons accumulate at the interface of the poly-TPD/quantum dots due to the ~1.5 eV energy offset between the LUMO of poly-TPD and quantum dots. When one high-energy hole can be obtained after absorbing the energy released from the interfacial recombination of an electron/hole pair, the high-energy holes can cross the injection barrier at the poly-TPD interface and recombine with the electrons inside the QD layer and then emit photons. This is called the Auger-assisted hole injection [14, 19]. Therefore, the high electron mobility of ZnO nanoparticles and the band alignment structure can facilitate the hole transport and balance of the carrier injection of the device.

The electroluminance (EL) spectra of the QLED under different voltages are shown in **Figure 8(b)**. The inserted picture is the QLED device. The EL intensity of the QLED device increases as the applied voltage increases. The wavelength is 534 nm with the

**Figure 8.**

*(a) The QLED current density versus the voltage (J-V) curve, (b) the QLED electroluminance spectra as the applied voltage increased and (c) the 1931 CIE coordinate of QLED.*

full width at half maximum (FWHM) value is 44 nm through the spectrum. The peak wavelength of QLED electroluminance spectra is 2 nm red shift compared with the peak wavelength of QD solution, which might be because of the dot-to-dot interactions in the close-packed solid film. Moreover, the electric field induced the Stark effect [20]. The 1931 CIE coordinate after emission is (0.31, 0.66) as shown in **Figure 8(c)**.

**77**

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

This work discussed the structure and mechanism of QLED and demonstrated an all-solution process of QLED in the last section. The QLED has high luminance with low turn-on voltage. These properties, caused by the use of ZnO NPs, improve electron injection and enhance radiative recombination. The resulting QLED fabrication process also makes printing QLED a possible method in the future.

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

The authors declare no conflict of interest.

QLED quantum dot light-emitting diode

HDTV high-definition television

OLED organic light-emitting diode

ZnO NPs zinc oxide nanoparticles

EGBE ethylene glycol butyl ether

HOMO highest occupied molecular orbital LUMO lowest unoccupied molecular orbital

h Planck's constant h = 6.63 × 10<sup>−</sup>34 Js

k the magnitude of the wave vector L standing wave/the diameter of QDs

μ carrier mobility of materials

d the thickness of the thin film FWHM full width at half maximum

E the energy of a single particle in free space m mass of a single particle in free space v the velocity of a single particle in free space

TOPO trioctylphosphine oxide DMSO dimethyl sulfoxide HTL hole transport layer HIL hole injection layer EIL electron injection layer ETL electron transport layer

PEDOT:PSS poly(ethylenedioxythiophene):polystyrene sulphonate

Poly-TPD poly(N,N′-bis-4-butylphenyl-N,N′-bisphenyl)benzidine

**4. Conclusions**

**Conflict of interest**

**Nomenclature**

QD quantum dot DOD drop on demand

CRT cathode-ray tube LCD liquid crystal display LED light-emitting diode

ITO indium tin oxide

ODA octadecylamine

EML emission layer

λ wavelength

P particle momentum

J current density

ε dielectric constant

V voltage

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