Improved Performance Of Organic Light-emitting Diodes With Ultra-thin Films As Modifying Layer

Tremendous progress has been made in the science and technology of organic light-emitting diode (OLED) due to its potential application in flat panel displays over the past few years. It is attractive owing to lots of merits such as high brightness, wide color range, low voltage operation and possibility for flexible display. At present, OLED efficiency and lifetime are the primary issues limiting the widespread commercialization. To solve the problem, looking for novel materials and employing new structure are usual methods. Based on the classical structure of double layer OLED (ITO/TPD/Alq₃/Al), this thesis mainly focuses on investigation of ultra-thin films inserted at electrode/organic interfaces as modifying layer to control carrier injection, and can be concluded to three aspects as follows:1. OLEDs using LiF ultra-thin films with different thickness as electron injection layer are fabricated by vapor thermal deposition. The diodes with structure of ITO/TPD/Alq₃/LiF/Al perform better electroluminescent properties compared with the diodes without LiF injection layer. The experimental results show that the insertion of the ultra-thin LiF layer enhances the electron injection, reduces the turn-on voltage and improves the luminance and efficiency of the diodes. The diode with a 0.5 nm LiF layer demonstrates the best performance and the reason why the performance of diode is improved by inserting LiF layer is explained by tunneling theory for a lower barrier height between Al and Alq₃.2. LiF is an excellent insulator with a large band gap of about 12 eV, which is usually used as a cathode modifying layer in OLED. The ultra-thin LiF layer inserted between anode and organic layer as hole buffer layer improves the photoelectric characteristics of the diodes directly, which blocks the hole injection and balances carrier injection. The mechanism for above phenomena can be obtained from the presence of the LiF layer exists a large voltage bias across it that steps the Highest Occupied Molecular Orbital (HOMO) of TPD up relative to the Fermi level of ITO. Furthermore, thick LiF buffer layer will reduce the characteristic of the diodes for too holes could be blocked and it is necessary to optimize the thickness of LiF buffer layer. Finally, it is found that the luminance and efficiency of the diode are maximize when 1 nm LiF hole buffer layer is inserted between ITO and TPD.3. A novel hole injection layer of niobium oxide (Nb₂O₅) deposited on the ITO anode by RF magnetron sputtering in organic light-emitting diode has been fabricated. Performances of the diode have been improved greatly with the structure of ITO/Nb₂O₅/TPD/Alq₃/Al. The structure of Nb₂O₅ ultra-thin film coated on ITO glass is amorphous observed by X-ray Diffraction (XRD), and the films exhibit high transmission in the visible region of spectrum detected using Ultra-Violet spectrophotometer (UV). Next, the insertion of this ultra-thin film results in a reduction of the hole injection barrier, which is related to the thickness of the film. That enhances the hole injection and improves the luminance and efficiency of the diode, since the buffer layer simultaneously blocks the diffusion of metal In into organic layers. The diode with a 2 nm Nb₂O₅ layer shows the best performance, which far precedes the diode with general CuPc hole injection layer.