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In this paper, we will present some studies of physics at the interfaces in the organic light emitting
devices. The paper can be separated into two parts. First part is the manipulation of interfacial energy
structures and electron transport properties of organic semiconductors. The second part is substitution
and dopant dependence of electronic structures in organic thin films
I will present an investigation of the energy structures and electrical doping mechanisms of the
organic semiconductor surface through current-voltage (I-V) characteristics and photoemission
spectroscopies. We found that both surface energy structures and transport properties can be
manipulated with mix of LiF or Cs2CO3. The I-V characteristics show that the current efficiency is
significantly improved with Cs2CO3 doped either at the surface or in the bulk Alq3. As Cs2CO3 doping
works efficiently with Al as well as other cathode metals, the interfacial chemistry and carrier injection
mechanisms of such cathode structures are compared to that of the conventional LiF thin layers.
To understand the mechanisms of the improvement on electron injection, the surface energy
levels of metal and organic materials were measured with ultraviolet photoemission spectroscopy
(UPS) and the interfacial chemistry was studied with X-ray photoemission spectroscopy (XPS). From
UPS spectra, we found that a thin layer of Cs2CO3, as thin as 0.5 A, at the metal and organic ETL
interface can bring the Fermi level of Alq3 from mid-gap to less than 0.2 eV below the lowest
unoccupied molecular orbital (LUMO), indicating that the Alq3 film at the interface is heavily n-type
doped with Cs2CO3 . The smaller gap between the Fermi level and LUMO with Cs2CO3 reduces the
electron injection barrier. Strong dipole fields are also found at the surface, which also affects the
electron injection considerably. The XPS data further show that Cs ions are dissociated at the interface
as soon as Cs2CO3 is deposited on Alq3. The result is different from the case of LiF, in which Al metal
is needed for releasing Li ions. With co-evaporation of Cs2CO3 with Alq3 in the bulk as n-doping ETL,
the current efficiency can be further improved, which is presumably attributed to the enhancement of
the electron transport in the Alq3 films.
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