Abstract:
Organic light emitting devices have been extensively investigated in many
promising applications. Surface potential (SP) is essential in characterizing the
fabrication process of Organic light emitting diodes (OLEDs). In the case of OLED
fabrication, some organic materials are spontaneously oriented and ordered in the
evaporation films in the OLED structure. Although, according to the literature, many
organic semiconductor films show such a spontaneous orientation polarization
(SOP), the formation mechanism of SOP has not been well understood. Thus, far
many studies have been dedicated to understand macroscopic properties of SOP in
the single component. However they have not been well characterized in terms of
SOP for mixed films which are often used in the OLEDs. Thus, it is essential to
characterise the surface potential and its local distribution on mixed films of polar
and nonpolar molecules which are commonly employed in OLED fabrication. As the
initial step, we have characterized TPBi (polar) - 2,2',2"- (1,3,5-Benzinetriyl)-tris(1-
phenyl-1-H-benzimidazole) with CBP (nonpolar) - 4,4′-Bis(N-carbazolyl) - 1,1′-
biphenyl via the Kelvin probe (KP) measurement technique in order to verify the
surface potential.Organic layers were deposited on half covered (using shadow
mask) Indium Tin Oxide (ITO) glass substrate via the vacuum evaporation
technique at UHV chamber with a base pressure of ̴10-4 Pa in the dark condition.
After carefully adjusting the evaporation condition of each molecule using thickness
monitor, two molecules were simultaneously deposited on the ITO substrate.
Several mixed films with different thicknesses were fabricated under the same
experimental conditions in order to identify the surface potential as a function of
mixed film thickness. KP measurement was performed for each film and after the
KP measurement, thickness of the films was estimated via profilemeter and atomic
force microscope measurements. The results of the surface potential against film
thickness were obtained for each and every film. It is clear from the results that the
surface potential of TPBi (polar) with CBP (nonpolar) mixed film increases with the
increment of film thicknesses which implies that the molecular interactions
decrease with the increment of film thickness. Further, we have compared the SP of
mixed film with single film of TPBi and found the values to be 74.75 mV/nm and
57.53 mV/nm for the mixed and the single films, respectively.
