**3.3 QLED device preparation**

*Quantum Dots - Fundamental and Applications*

enon for creating an exciton in the device.

**2.3 ZnO nanoparticles**

high-performance QLEDs.

introduced in Section 3.

**3.1 Experimental chemicals**

all from Aldrich.

**3. QLED fabrication by spin coating**

**3.2 Synthesis of ZnO nanoparticles**

to form an exciton; this is the QD layer in QLED.

• Hole injection layer (HIL)—for hole injection from the anode electrode.

• Emission layer (EML)—electron/hole transportation and their recombination

This direct injection of charge carriers is assumed as the most common phenom-

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.

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

Generally, solution-processed oxide CTLs can be deposited by two approaches,

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

The uncleation-dissolution-recrystallization growth method [16] was applied to synthesize the ZnO nanoparticles. Firstly 0.37 g potassium hydroxide was dissolved in 16.25 ml methyl alcohol. Then zinc acetate dihydrate solution was prepared by adding 0.74 g zinc acetate dihydrate in 31.25 ml methyl alcohol at 60°C under vigorous stirring. Then the potassium hydroxide solution was jetted into the zinc acetate dihydrate solution at the rate of 0.8 ml/min. The reaction takes around 1.7 h under N2 protection condition. After the reaction, the solution was allowed to sit for another 2 h to let the ZnO nanoparticles settle at the bottom of the reaction flask. The ZnO nanoparticles were washed twice by methyl alcohol. Then the ZnO

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

**72**

The patterned ITO glass was cleaned by detergent, methyl alcohol, acetone, and isopropyl alcohol in turn, each sonication for 20 minutes. After cleaning, UV ozone plasma was applied for surface energy modification. PEDOT:PSS was spin-coated on the cleaned ITO surface at 3000 rpm for 30 s and then baked under vacuum at 150°C for 30 min. Then hole transport layer (HTL) was prepared by spin coating poly-TPD on the annealed PEDOT:PSS surface at 3000 rpm for 60 s and baking at 100°C for 30 min under vacuum protection. Quantum dots were also deposited on the annealed poly-TPD surface by spin coating at 3000 rpm for 30 s. Then the synthesized ZnO nanoparticle solution was spin-coated at 1500 rpm for 60 s. The baking temperature is 60°C for 30 min. The cathode was deposited by vapor deposition method. A more detailed QLED device fabrication process was mentioned in previous work [2].
