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Title: Deposition and application of fluorocarbon coatings in organic light-emitting devices
Other Titles: Tan fu bo mo cai liao zhi zhi bei ji qi yu you ji fa guang qi jian zhong de ying yong
Authors: Tong, Shi Wun (唐詩韻)
Department: Dept. of Physics and Materials Science
Degree: Doctor of Philosophy
Issue Date: 2005
Publisher: City University of Hong Kong
Subjects: Electroluminescent devices
Light emitting diodes
Organic compounds
Notes: CityU Call Number: TK7871.89.L53 T66 2005
Includes bibliographical references.
Thesis (Ph.D.)--City University of Hong Kong, 2005
xxii, 210 leaves : ill. (some col.) ; 30 cm.
Type: Thesis
Abstract: Conducting fluorocarbon coatings (CFx) have recently been used as an anode buffer layer in Organic Light-Emitting Diodes (OLEDs) for enhancement of stability and carrier injection. The effectiveness of this buffer layer strongly depends on its resistivity. It was found here that the resistivity of poorly conducting (about 1010 Ω-cm or above) CFx coatings can be substantially decreased to 105 Ω-cm by either near UV or Ar ion irradiation. OLEDs were prepared on untreated and treated CFx for performance comparison. The UV treatment of CFx improved the device performance, while the Ar ion treatment led to deterioration. X-ray Photoelectron Spectroscopy (XPS) results revealed that Ar ion irradiation would break the CFx bonds to generate free radicals by fluorine depletion. The remaining dangling bonds of the carbon atoms previously bonded to hydrogen were free for cross linking, which explains the decrease of the resistivity. Energetic Ar ions would also cause migration of indium from the underlying indium-tin-oxide (ITO) substrate to the CFx surface region. Newly formed InF3 and SnF4 peaks in the surface and bulk regions of the sample reflect the vigorous interaction between CFx and ITO upon irradiation. Atomic Force Microscopy (AFM) results reveal undesirable bores features on the irradiated surface. These irregular features on the Ar-irradiated surface probably are likely to be resulted from the damaged ITO. These damages contributed to the detrimental performance of the electroluminescent device with Ar irradiated CFx/ITO anode. The XPS findings also show that the UV illumination leads to the increase of the fraction of carbon-carbon bonds, the reduction of the carbon-fluorine bonds and the F/C ratio (by the formation and release of HF molecules). C-C clusters (not observed upon argon bombardment) also formed due to the initial non-homogeneous distribution of the carbon atoms in the CFx film. Graphitic regions were created leading to the higher conductivity of the CFx layer while the underlying ITO anode remained intact. A comprehensive investigation of the correlation between device stability, hole-injection efficiency and surface morphology was performed. It was demonstrated that the UV-illuminated CFx (UV-CFx) surface was extremely stable even if it was exposed for days in atmospheric environment. Smallest energy barrier in the ITO/UV-CFx/NPB contact was achieved as shown by Ultraviolet Photoelectron Spectroscopy (UPS). AFM studies also show that UV-illuminated CFx layer has a smoothening effect on the subsequently deposited organic materials. It was further found that these beneficial effects of the UV-illuminated CFx coating are not sensitive to its deposition conditions such as excitation frequency, electrode spacing and input power. This makes the process easy for adaptation to manufacturing. In addition, a small organic molecule fluoro-material was newly synthesized as an anode buffer layer for the OLEDs. Insertion of this hydrophobic fluoro-material at the anode of ITO/organic interface can significantly reduce operating voltage, increase device efficiency, and operational stability. Such device exhibited a projected half-life time of 6620 hours; while that of a control device was only 3800 hours. UPS analysis suggested that the performance improvement was attributed to the reduction in hole-injection barrier. This fluoro-material also offers a significant advantage that the film can be simply prepared by thermal evaporation. Thus, it can be prepared at low substrate temperature and used in substrates sensitive to plasma damage.
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