High-Performance Solution Processed Inorganic Quantum-Dot LEDs

Abstract

In this research paper, fabrication of novel CdSe\ZnS QD-LEDs with solution processing method is presented and the impact of trap energy levels in the electron and hole transport levels on these QD-LEDs brightness is investigated. Two types of QD-LEDs are fabricated with ITO as the transparent anode electrode, NiO nanoparticles as the hole transport layer (HTL), CdSe\ZnS QDs as the luminescent layer, ZnO: Ga as the electron transport layer (ETL), and Al as the cathode electrode. The NiO nanoparticles are synthesized by solgel or alternatively electrochemical method. Formation of different trap levels is observed in the crystal structures of the NiO nanoparticles synthesized by each of these methods. Considering the electrochemically prepared NiO nanoparticles, it is found that the density of trap levels is higher in the crystal structure of the NiO nanoparticles synthesized by sol-gel method and the device fabricated by the later material shows higher performance. Calculation of the electronic structure of ZnO: Ga by DFT methods (GGA-PBE) indicates that doping of Ga in the structure of crystalline ZnO creates new energy levels in conduction band and intermediate bands at the band gap of ZnO host. It facilitates electron injection from Al cathode to the ZnO:Ga ETL layer and from this layer to the QD luminescent layer. The fabricated devices show turn-on lower voltages than 5V in which a peak brightness of 500cdm -2 and 340cd m -2 is measured for the LEDs fabricated with sol-gel and electrochemically synthesized NiO nanoparticles respectively.Index Terms-Light emitting diode, Quantum dot, NiO, ZnO: Ga, Photo luminescence 1536-125X (c)

Switchable Multi-Color Solution-Processed QD-laser

“…For facile access to this structure, the simple and low-cost solution process method is suggested, which is called "wet, " and it provides the chemical system with controllable experimental conditions. The synthesis of colloidal QDs by chemical procedures in solution allows achieving tunable sizes of QDs 30,55 ; also the creation of suitable conditions for synthesis in this method is more convenient. The solution process is economical compared with the other expensive methods like epitaxial which needs a high-vacuum condition 30,31 .…”

“…Thus, the width of the QW is chosen as their energy state is the same with the GS energy of relevant QDs; then tunneling injection of carriers occurs via the barriers of QW into QDs, so they get localized in them. This process causes the fast transition of carriers 30 and improves the performance of QD-laser. Figure 1(B) exhibits the two-color QD-laser structure in which the active layer of QD-laser includes two different sizes of InGaAs QDs that is sandwiched by p and n-type AlGaAs as a carrier transition layer.…”

“…Basically, the size of QDs can be controlled by providing suitable experimental conditions such as the concentration of materials, temperature, pH and the speed of rotation of solvent 35,36 . In this technology, the diameter range of QDs can be tuned from 1 nm to 10 nm 30,31,37 . However, having the 0.1 nm size deviation for synthesized QD in the appropriate experimental condition is possible 37-39 .…”

High Throughput Quantum Dot Based LEDs

“…Because of the importance of light quality and lifetime of QD-LEDs, nowadays increasing efficiency of QD-LEDs is very considerable; therefore our group (OIC) by introducing new structures of QD-LEDs and using solution process method for fabrication of QD-LEDs has several works for improving of efficiency of this type of LEDs. In this way, crystal engineering and effects of FRET in QD-LED were investigated, which both of them improve quality of QD-LEDs [24].…”