量子点发光二极管：结构、机理和制备

Quantum dot light-emitting diode (QLED) attracted much attention for the  next generation of display due to its advantages in high color saturation, tunable  color emission, and high stability. Compared with traditional LED display, QLED  display has advantages in flexible and robust application, which makes wearable and  stretchable display possible in the future. In addition, QLED display is a self-emissive  display, in which light is generated by individual subpixel, each subpixel can be individually controlled. Each subpixel in LED display is constituted by liquid crystal and  color filter, which make LED display have lower power efficiency and less enhanced  functionality. This chapter introduces the QLED based on the QLED structure and  light-emitting mechanism of QLED. Then, a novel method for fabricating QLEDs,  which is based on the ZnO nanoparticles (NPs) incorporated into QD nanoparticles,  will be introduced. The QLED device was fabricated by all-solution processes, which  make the QLED fabrication process more flexible and more suitable for industrialization. What is more, as QLED devices were planned to integrate into a display,  all-solution fabrication processes also make printing QLED display device possible in  the near future.

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).

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 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.

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

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).

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.

Fig1

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. ZnO NPs were synthesized by hydrolysis/ condensation reactions under basic conditions. The synthesis procedure will be  introduced in Section 3.

Detergent TFD4 was purchased from BioLab, PEDOT:PSS 4083 from Heraeus,  poly-TPD (LT-N149) from Luminescence Technology Corp Ltd., patterned ITO  glass from Xinyan Technology Ltd., green (CdZnSeS/ZnS) quantum dots from  Suzhou Mesolight Inc., and zinc acetate dihydrate powder, potassium hydroxide  flakes, acetone, isopropyl alcohol, methyl alcohol, chloroform, and chlorobenzene  all from Aldrich.