| Colloidal quantum dots(CQDs) are superior to organic fluorescent dyes and inorganic phosphor owing to their outstanding advantages, such as wide range of absorption, high fluorescence quantum efficiency, excellent optical and chemical stability, narrow bandwidth, and easily tunable emissions. These impressive characteristics make QD a perspective candidate in applications of optoelectronics, photovoltaics and biolabels, especially in light-emitting devices, which may lead the development of next generation of displays and solid state lighting. During the fabrication of quantum dots light-emitting diodes(QLEDs), QDs with long organic ligands are always used as emitters. Unfortunately, these long chain ligands, on the one hand, form an organic insulating layer which will inevitably reduce the conductivity and increase the barrier of charge injection. On the other hand, they expand the space between quantum dots and charge transport materials, leading to the efficiency loss of exciton energy transfer. Both of the above two aspects can hinder the improvement of the performance of QLEDs. Moreover, organic semiconductor materials PEDOT:PSS is generally used in QLEDs as hole injection layer due to good conductivity, however, these materials can be easily affected with damp and oxidation, and may have corroding effect on ITO electrode, which will greatly affect the lifetime of QLEDs. This thesis is based on the ideas and perspectives of thinking on the issues mentioned above.In order to solve the problem of QDs with long chain surface ligands that affect the performance of QLEDs, blue ZnCdSe/ZnSe/Zn SeS/ZnS core-shell quantum dots with high quantum yields(QYs) were selected, and short-chain 1-octylmercaptan(OT) ligands were used as to replace the original long-chain ligands of oleic acid(OA) that coated on quantum dots through ligand exchange. After the exchange process, the results were verified with Fourier transform infrared spectrometer, transmission electron microscopy(TEM), and dynamic light scattering(DLS). Then, QLEDs using QDs with OT ligand as the emitting layers were fabricated and tested. The performance of the device showed the maximum brightness, peak current efficiency and external quantum efficiency(EQE) of 11600 cd/m2, 2.38 cd/A, and 4.6%, respectively. And the red shift and broadening of electroluminescence are not observed with the increase of bias voltage. In addition, the electroluminescence(EL) matches well with the photoluminescence(PL).To avoid the problems caused by the organic hole injection materials of PEDOT:PSS, molybdenum oxide(MoO3) with good electrical conductivity and stable properties were introduced as a hole injection layer materials. MoO3 synthesized by the solution method were characterized with X-ray powder diffraction(XRD), X-ray photoelectron spectrometer(XPS) and TEM. The results showed that the as-synthesized MoO3 particles were uniform with a narrow size distribution and good monodispersity. Next, we constructed QLEDs with MoO3 as hole injection layer. the devices with MoO3 as hole injection layer demonstrated maximum brightness of 21160 cd/m2, peak current efficiency of 2.45 cd/A and low turn-on voltage of 2.2 V, which are much superior to the control-devices fabricated with hole injection layer of PEDOT:PSS. In addition, the devices showed good stability under continuous bias and four times more operating lifetime than control devices. |
PDF Full Text Request