The Electroluminescence Of ZnO Nanoparticle Film/organic Polymer Heterostructure Devices

As a wide direct band gap semiconductor, Zinc oxide is drawing more and more attention because of its unique physical, chemical, electrical and optical properties. Zinc oxide has a very attractive prospect in the field of ultra violet (UV) light emitting diodes and high density storage, because it has the advantage of blue and ultraviolet emission。 However, for ZnO nanostructural materials, low electroluminescence efficiency and poor p-type doping efficiency is always a bottleneck problem which obviously restrains the development and the wide application of ZnO. In recent years, with the development of organic semiconductor and the enrichment of ZnO nanostructure, many research groups begin to develop the ZnO ultraviolet light-emitting devices using ZnO nanostructure combined with organic materials. Our group has already developed a ZnO ultraviolet light-emitting device using ZnO nanorods array combined with organic materials, the ZnO nanorods array is growing vertically on the ITO substrate. But there are several problems:the surface roughness of ZnO nanorods array is very large, so we can’t form a good film on the top of the array, the MEH-PPV film is very easy breakdown and the device has poor stability. To solve these problems, we synthesized ZnO nanoparticle films by a sol-gel method on ITO glass, and the devices with ZnO nanoparticle film/organic heterojunction structure have been fabricated. The room temperature electroluminescence (EL) spectra have been studied.Firstly, we synthesized ZnO nanoparticle films by a sol-gel method on ITO glass and analyzed the preparation conditions and optical and electrical properties of ZnO nanoparticle films. The roughness of ZnO nanoparticle film is very low which is synthesized by a sol-gel method on ITO glass and sizes of the crystal particles are similar. SEM image of ZnO nanoparticle film shows that the average size of the crystal particles is about30nm and the crystal particles equally distributed which formed the smooth film..The smooth ZnO nanoparticle film is conducive to the preparation of organic thin film by spin-coating method. The photoluminescence (PL) spectrum of different nanoparticle films show the emission at390nm corresponding to the band edge emission of ZnO is enhanced and narrowed and the defect emissions are very weak.It is in favour of the electroluminescence of ZnO.Secondly, we fabricated light emitting diodes with ZnO nanoparticle film/MEH-PPV heterojunction structure. The influence of different thickness of the organic layers and ZnO nanoparticle films has been studied. The MEH-PPV organic thin film was prepared by spin-coating method and the devices with the structure of "ITO/ZnO nanoparticle film/MEH-PPV/LiF/Al have been fabricated.The experimental result shows that when the organic thin film was spined at the speed of3500r/min, the device has a strong near-ultraviolet emission and the electroluminescence of MEH-PPV is very weak. When the thickness of ZnO nanoparticle film is60nm, the deivce is very reliability and the defect-related emission of ZnO is very weak.Lastly, we tried to fabricate the devices with ZnO nanoparticle films/PMMA heterojunction structure for the first time, and the room temperature ultraviolet electroluminescence (EL) spectra have been studied. The experimental result shows that the deivce has a strong near-ultraviolet emission of ZnO. The luminous mechanisms of ZnO/organic polymer were discussed in terms of carrier tunneling process. The carrier tunneling process is the key to the near-ultraviolet emission of ZnO/organic polymer heterojunction structure.