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Utilizing Metal Oxide as Hole/Electron Injection layer in Hybrid Light Emitting Diode

Alehdaghi, Hassan | 2015

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 47143 (04)
  4. University: Sharif University of Technology
  5. Department: Physics
  6. Advisor(s): Taghavinia, Nima; Irajizad, Azam; Marandi, Maziar
  7. Abstract:
  8. Organic light emitting diodes (OLEDs) are one of the main subjects of research in the organic electronic advanced technology. There are many advantages for OLEDs like self-emitting, high efficiency, wide vision angle, high contrast, and rapid response time. The main challenge in this area is the long-time stability of performance in air ambiance. Although the encapsulation process could overcome the instability problem, it increases the final cost. In this project, metal oxides have been utilized for the injection of electrons and holes in hybrid light emitting diodes (HyLEDs). In the first stage, inverted structure of HyLEDs with ZnO and MgxZn1-xO layers was fabricated and characterized. These layers were applied as electron injection layers while the effects of cathode surface roughness and Mg/Zn ratio on the performance were investigated. Different thicknesses of ZnO layers in the range of 20-210 nm were deposited on the surface of FTO and polished-FTO substrates with RMS roughnesses of 9.2 nm and 2.2 nm, respectively. The deposition was carried out by standard spray-pyrolysis method. The results demonstrated that the low roughness of ZnO layers in lower thicknesses could improve the performance of the LEDs. Nevertheless, the thick ZnO layers with higher surface roughness resulted in lower operating voltage. Application of MgxZn1-xO thin films could also improve the external quantum efficiency and reduce the operating voltage. This could be attributed to the balancing of the energy levels. In the stage molybdenum oxide (MoOx) was used as a hole-injecting layer (HIL) in a conventional structure. The results clarified that the substitution of organic PEDOT:PSS hole injecting layer with MoOx could lead to the higher efficiencies. The thickness of MoOx layer in optimum situation was 8-18 nm and annealed in nitrogen atmosphere at 200 °C. The X-ray photo-electron spectroscopy (XPS) showed that molybdenum oxide has: a reduced structure of MoO2.95. The UPS analysis was also carried out and showed that there are sub-bandgap states which could be attributed to the reduced structure of molybdenum oxide. The lifetime experiments were also performed and demonstrated a 40 times improvement for the lifetime of the device with MoOx hole injecting layer compared to the similar PEDOT:PSS based LED
  9. Keywords:
  10. Roughness ; Hybridlight Emitting Diodes ; Zinc Oxide ; Molybdenum Oxide ; Metal Oxides ; Electron Injection ; Hole Injection ; Magnesium Zinc Oxide

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