Morphology and Performance of Carrier Transport Layers in Quantum Dots-Light Emitting Diode
|Keywords||QLED (Quantum Dots Light-Emitting Diode) Nickel Oxide Tungsten Oxide Hole transport layers Electron transport layers Colloid QuantumDots|
QLED is Quantum dots-Light Emitting Diode, which is a promising technology for creating large-area displays that could have applications for TVs, cell phones, and digital cameras. QLED is considered as a next generation display technology after OLED Displays which is another large-area LED technology. A typical QLED consists of three layers:one inner layer of quantum dots, one outer layer that transports electrons, and one outer layer that transports holes. The structure of a QLED is very similar to the OLED technology. But the difference is that the light emitting centers are quantum dots, especially colloidal quantum dots.Colloidal quantum dots, with their pure and saturated emission colors, narrow band width, good photostablity, size-tunable band-gaps and high photoluminescence quantum efficiencies, have become promising chromophores for use in QLED. QLED is especially appealing for their narrow bandwidth and simple color tunability, since changing the size of a quantum dot can changes its emission wavelength. QLED is anticipated to be more efficient than LCDs and OLED. It is cheaper to make. Samsung estimates that it costs less than half of what it costs to make LCDs or OLED panels. QLED is better than OLED. It is brighter, cheaper, and saves more energy.It can be seen that four types QLED with different carrier transport layers have been evolved nearly chronologically. The four types have:QLED with polymer carrier transport layers (CTLs), QLED with organic small molecule CTLs, QLED with inorganic CTLs and QLED with hybrid organic-inorganic CTLs. Among of the four types QLED, QLED with inorganic CTLs has great device densities in air and higher current densities due to inorganic CTLs. Therefore, our research will focus on QLED with inorganic CTLs. However, this type devices had a very low efficiency with an EQE (External Quantum Efficiency) of<0.1%. The inefficiency was mainly attribute to carrier imbalance due to a large hole injection barrier between the p-type metal oxide and the quantum dots. Our research is aimed to improve the hole mobility by changing the morphology of hole transport layers. The achieved results are stated as follows: (1) Thin films of NiO were grown directly on FTO-coated glass substrates using the solvothermal method and magnetron sputtering. Thin NiO films synthesized by the solvothermal method have different vertical microstructures of nanoplates and microflakes by changing solvothermal reaction time. Hall test shows that all NiO films are p-type semiconductor. As compared to magnetron-sputtering NiO films, the hole motilities of solvothermal NiO films increase several orders of magnitude.(2) Thin films of WO3were grown directly on FTO-coated glass substrates using the solvothermal method and magnetron sputtering. Thin WO3films synthesized by the solvothermal method have different vertical microstructures of nanoplates by changing solvothermal solvent and addition. Hall test shows that all WO3films are p-type semiconductor. As compared to magnetron-sputtering WO3films, the hole motilities of solvothermal WO3films increase several orders of magnitude.(3) Colloidal CdSe QDs were synthesized via microfluidic method. CdSe/ZnS core/shell structured QDs with significantly improved quantum efficiency were obtained by capping a shell in CdSe core. Changing the CdSe and CdSe/ZnS grain size offered an extended optical tunability from blue to red emission, while maintaining the excellent optical purity and brightness (FWHM17-30nm, QY up to90%).(4) Thin films of AZO were synthesized through magnetron sputtering. And the whole QLED device were manufactured. The I-E curves of two structures of NiO-CdSe/ZnS-AZO and WO3-CdSe/ZnS-AZO were tested. The structure of NiO-CdSe/ZnS-AZO shows good rectification characteristic.