Improvement Of The Near-ultraviolet Electroluminescence Characteristics Of ZnO Nanorods/MEH-PPV Heterostructures

At present, there are more requirements for short wave light emitting diodes. Low-dimensional ZnO nanostructural materials which could be applied in ultraviolet (UV) light emitting diodes have attracted great interest because of their unique physical, chemical, electrical and optical properties. However, for ZnO nanostructural materials, low electroluminescence efficiency and poor p-type doping efficiency is always a bottleneck in device applications. In this thesis, ZnO nanorod arrays which are highly vertical alignment are prepared by a simple hydrothermal method. Using as-grown ZnO nanorods, ZnO nanorods/MEH-PPV heterostructure devices for Near-UV (N-UV) light emitting diode (LED) are fabricated. Then the improvement of the N-UV electroluminescence of the devices has been studied.First, the effect of ZnO nanorods synthetizing condition and the thickness of organic layer on N-UV electroluminescence of heterostructures is discussed. ZnO nanorods array are grown on cleaned ITO substrates by two-step method. SEM image of as-grown ZnO nanorods shows that the ZnO nanorods grow perpendicular to the substrate orderly. The average diameter and the height of the nanorods are about 30 nm and 100 nm, respectively. XRD pattern indicates that ZnO nanorods have a typical wurtzite structure and grow along with the c-axis direction well. From the photoluminescence spectrum, there is a strong N-UV emission peak at 380 nm but no defect emissions. It indicates that the ZnO nanorods have a good quality. Then a series devices of ITO/ZnO/ZnO nanorods/MEH-PPV/Al are prepared. Under forward bias, the N-UV emission at 380 nm is detected. After discussing the effect of growing time of ZnO nanorods and the thickness of organic layer on the electroluminescence property of devices, the proper requirement of ZnO nanorods/MEH-PPV heterostructure devices for N-UV electroluminescence is concluded.After that, the improvement of N-UV electroluminescence of ZnO nanorods/MEH-PPV devices is investigated by using MoO3 or ZnS thin film as a hole-injecting buffer layer. MoO3 or ZnS thin film is deposited on the cleaned substrate by thermal evaporation. Then the ZnO nanorods are grown on the substrate with a hole-injecting buffer layer. And then devices ITO/ZnS(or Mo03)/ZnO/ZnO nanorods/MEH-PPV/Al are fabricated. Under forward bias, strong N-UV light emission at 380 nm is observed. The experimental results demonstrate that the suitable insertion of ZnS thin layer can enhance the N-UV electroluminescence mostly. Under the same voltage, the light intensity can be increased up to 10 times more than that of the device without ZnS layer. Moreover, the turn-on voltage is reduced remarkably. Thus, we get a simple and developmental approach to prepare a strong N-UV LED. Finally, we deeply investigate the effect of the ZnS hole-injecting buffer layer with different thickness (2 nm,4 nm,6 nm,8 nm) on the characteristic of devices. Experiment results show that the defect emissions of ZnO and the exciton emission of MEH-PPV are enhanced relatively. When ZnO nanorods are grown on ZnS layer, their crystallization and Orientation become bad according to their XRD and SEM.