Investigation of Techniques for Enhancement in Carrier Injection and Interfacial Electronic Structures in Organic Light Emitting Devices
Date Issued
2012
Date
2012
Author(s)
Wang, Po-Sheng
Abstract
Three techniques that enhance the carrier injection efficiency in organic light emitting diodes (OLEDs) are demonstrated in this dissertation. By proper treatments to common OLED materials, the injection current can be effectively improved in devices. The origins of the enhancement in device current and mechanisms regarding these treatments are investigated by ultra-violet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) to provide interpretations. The electron injection in inverted OLEDs is improved by activating the chemical reactions between Tris(8-hydroxyquinolato)aluminum (Alq3), lithium fluoride (LiF), and aluminum cathodes via post-process thermal annealing. By annealing the inverted OLEDs under proper temperatures a better electron injection can be achieved. UPS and XPS spectra reveal the evolution of valence band features as well as oxidation states of nitrogen in Alq3 layers, confirming chemical reactions in the inverted Alq3 tri-layers after proper thermal annealing. Two approaches that enhance the hole injection efficiency from indium tin oxide (ITO) anodes to hole transporting layers (HTLs) are provided with the incorporation of molybdenum oxide (MoO3) hole injecting layers (HILs). By either treating the surfaces of as-deposited MoO3 layers with argon ion sputtering or thermal annealing in high vacuum, efficient hole injection is provided by the modified MoO3 layers. Via UPS, formation of huge amounts of gap states is identified inside the band gap of MoO3 during ion sputtering or thermal annealing. Those gap states provide continuous transition paths for holes to hop through, resulting in superior hole injection efficiency from anodes to HTLs. XPS analysis shows the reduction of Mo atoms due to the removal of oxygen atoms after ion sputtering, providing changes in atomic concentration of the treated surfaces. During thermal annealing, topotactic decompositions of MoO3 release the oxygen inside the films and cause dimerization of Mo atoms, which generate gap states in the band gap. By treating the MoO3 layers, those hole transporting materials (HTMs) that cannot react with as-deposited MoO3 can now also achieve improved hole injection. 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) and N,N''-diphenyl-N,N''-bis(3-methylphenyl)-[1,1''-biphenyl]-4,4''-diamine (TPD) HTLs are demonstrated to have better hole injection current on the sputtered and annealed MoO3 layers.
Subjects
OLED
MoO3
LiF
UPS
XPS
Hole injection
electron injection
ion sputter
thermal annealing
Type
thesis
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