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UV-ozone-treated ultra-thin NaF film as anode buffer layer on organic light emitting devices

Yu-Cheng Chen, Po-Ching Kao, and Sheng-Yuan Chu

Open Access Open Access

Abstract

An ultra-thin NaF film was thermally deposited between ITO and NPB as the buffer layer and then treated with the ultraviolet (UV) ozone, in the fabrication of organic light emitting diodes (ITO/NaF/NPB/Alq3/LiF/Al) to study its effect on hole-injection properties. The treatment drastically transforms the role of NaF film from hole-blocking to hole-injecting. This transformation is elucidated using hole-only devices, energy band measurement, surface energy, surface polarity, and X-ray photoelectron spectra. With the optimal thickness (3 nm) of the UV-ozone-treated NaF layer, the device performance is significantly improved, with a turn-on voltage, maximum luminance, and maximum current efficiency of 2.5 V, 15700 cd/m2, and 4.9 cd/A, respectively. Results show that NaF film is not only a hole-blocking layer, but also a promising hole-injecting layer after UV-ozone treatment.

©2010 Optical Society of America

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Figures (4)
Fig. 1
Fig. 1 (a) Current density-voltage (b) luminance-voltage characteristics for OLEDs with a NaF ultra-thin buffer layer of various thicknesses (c) current density-voltage characteristics of the hole-only devices.

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Fig. 2
Fig. 2 (a) Ultraviolet photoelectron spectroscopy results of pristine NaF and UV-ozone treated NaF layers deposited on an ITO substrate; (b) Schematic energy band diagram.

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Fig. 3
Fig. 3 (a) Surface energy and (b) surface polarity of ITO substrate and NaF-covered ITO substrate.

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Fig. 4
Fig. 4 Na 1s X-ray photoelectron spectra for NaF film deposited on ITO substrate before and after UV-ozone treatment.

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Tables (3)

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Table 1 Surface Tension Components of Test Liquids a

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Table 2 Some selected properties of buffer layers coated onto ITO and the opto–electronic performances of OLEDs.

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Table 3 Contact angles of H2O and CH2I2 on ITO, NaF/ITO and UV-ozone treated NaF/ITO

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Equations (1)

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γ L . ( 1 + cos θ ) = 2 ( γ S p . γ L p ) 1 / 2 + 2 ( γ S d . γ L d ) 1 / 2 ,
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