Preparation And Characterization Of High Efficiency Quantum Dots Light - Emitting

Currently, colloidal quantum dots have the properties of size adjustability,narrow emission peak and high fluorescence quantum yield(QY). The quantum dot light-emitting diodes(QLEDs) based on colloidal quantum dots have the advantages of low turn-on voltage, high color purity, simple preparation process, long lifetime, good stability, emission wavelength tunability by the size of the quantum dots, which are very suitable for the next generation of wide color gamut flat panel display and high color rendering index lighting. After many researchers’ effort, the synthesis of quantum dots has many advances. With the core-shell structure, fluorescence quantum yield of quantum dots can reach more than 85%. The optical and electrical properties of QLEDs also have greatly improved, with the external quantum efficiency(EQE) from the initial 0.01% to 20.5%, the brightness up to thousands of candelas per square meter, the life time up to 10 million hours.The optoelectronic properties of QLEDs nearly reach that of organic light emitting diodes(OLEDs).But it has a long way to really go into commercial applications. There still needs more comprehensive studies in QLEDs, to clarify the light emission mechanism, improve the preparation process, and further enhance the performance. Thus, investigation of the QLEDs has great significance. In this paper, the main research work is the preparation and characterization of high efficiency QLEDs, including forward and inverted devices:1 、 In the forward device structure, we first prepared ITO/poly(3,4-ethylene dioxythiophene):poly(styrene sulphonate)(PEDOT:PSS)/poly(N,N-bis(4-butylphenyl)-N,N-bis(phenyl)benzidine(poly-T PD)/QD/1,3,5-Tri(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl(TPBi)/Li F/Al structure QLED.Through optimizing the concentration of red QD, the device can obtain a maximum current efficiency of 1.64 cd/A, a maximum external quantum efficiency(EQE) of 0.83%, and a maximum luminance of 4076cd/m2 when the QD concentration is 30 mg/ml. It is commonly known that there exists excitons concentration quenching effect. The appropriate concentration of QDs can reduce the excitons quenching effect, improving the efficiency of the device. In order to further improve the device performance,based on the concentration of 30 mg/ml, by doping QDs into polymer as luminescent layer and changing the mass ratio of QDs and polymer, the device can achieve a maximum current efficiency of 3.82 cd/A and a maximum EQE of 1.97% when the mass ratio of polymer(PVK:OXD-7) to QDs is 1:3, which is 2.3 times compared to the control device.2 、 In the inverted devices, in order to overcome the electron and hole injection unbalance problem in QLED devices, we adopted TCTA as the electron blocking layer(EBL). The device with TCTA can get a maximum current efficiency of 11.08 cd/A, a maximum EQE of 6.34%, and a maximum brightness of 16710 cd/m2, which is a 2.7 times improvement compared to the control device without TCTA. Besides, comparing to adopting CBP/NPB or CDBP/NPB as progressive hole transport layer, the device using TCTA/NPB as progressive hole transport layer show the highest efficiency, which means that the electron and hole injection unbalance problem was somewhat relieved.