The effects of ultra-thin cerium fluoride film as the anode buffer layer on the electrical characteristics of organic light emitting diodes
Introduction
Organic light-emitting diodes (OLEDs) have attracted much attention for application in solid-state lighting and flat-panel displays due to their low-voltage operation, wide viewing angles, high contrast, flexibility, low cost, low weight, fast response, and full color reproduction when compared to liquid-crystal displays (LCDs) and light-emitting diodes (LEDs) [1], [2], [3], [4], [5]. Tang and VanSlyke reported the first efficient OLEDs, which had a structure consisting of double-layer organic materials sandwiched between two electrodes [1]. Indium tin oxide (ITO) is the most commonly used anode material due to its high transparency (∼90% at 550 nm), low resistivity (∼2–4 × 10−4 Ω cm), and high work function (∼4.8 eV) in OLEDs [6], [7]. However, the ITO work function is insufficiently high to reduce the hole injection barrier, and so an energy barrier still exists at the ITO/organic interface despite it having been subjected to O2-plasma or ultraviolet (UV)-ozone treatment [7], [8], [9]. Therefore, improving the charge balance is necessary for increasing the efficiency of OLEDs. Because the mobility of holes is much higher than that of electrons in organic materials, hole/electron injection and transport ability must be improved to obtain a better charge balance in an OLED. In order to enhance the charge injection at the interface and ultimately reduce the driving voltage and improve the power efficiency of the device, one solution is to insert an anode buffer layer between the ITO and the hole transport layer (HTL), thereby reducing the energy barrier.
Recently, the insertion of metal-oxides with high work functions such as molybdenum oxide (MoO3) [10], [11], tungsten oxide (WO3) [12], [13], vanadium pentoxide (V2O5) [14], [15], and tantalum pentoxide (Ta2O5) [16], have been used as anode buffer layers for the injection of holes to improve both the charge injection at the interface and power efficiency. To this end, formations of various metal-doped ITO layers have been studied [3], [4], [5], [10], [11], [12], [13], [14], [15], [16]. However, very few studies have reported metal-fluorides, such as NaF [17], CuF2 [18], and AgF [19], as anode buffer layers to improve the charge injection at the interface and power efficiency of OLEDs. Moreover, to our knowledge no studies have attempted to improve the performance of OLEDs by using an ultra-thin CeF3 buffer layer (rare earth fluorides), which is the motivation of the present work. The rare earth fluorides are being extensively researched because of their mechanical and chemical stability. CeF3 is one of the potent rare earth fluorides attracting more attention because of its technological importance and superior properties. In our previous study, we used NaF, CuF2, and AgF layers as anode buffer layers (ABL) to improve the hole injection and power efficiency of OLEDs [17], [18], [19].
In this study, an ultra-thin CeF3 layer was used as an ABL to improve the hole injection and power efficiency of OLEDs. CeF3 layers of various thicknesses were grown via vacuum vapor evaporation on ITO/glass substrates, followed by UV-ozone treatment. The effects of the CeF3 layer on the electrical and optical properties of the OLEDs were investigated using contact angle measurements, X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), atomic force microscopy (AFM), and admittance spectroscopy analyses [3], [4], [5], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27].
Section snippets
Experimental procedures
Prior to film deposition, the glass/ITO substrates with a sheet resistance of 15 Ω/sq, were ultrasonically cleaned using a neutral detergent/deionized (DI) water (1:3 volume) mixture, DI water, isopropanol, and ethanol in sequence, and then subjected to UV-ozone treatment in a Jelight UVO-42 system for 20 min. The structure of the devices is ITO/CeF3 (X nm)/α-naphthylphenylbiphenyl diamine(NPB) (40 nm)/tris(8-hydroxyquinoline) aluminum (Alq3) (60 nm)/lithium fluoride (LiF) (1 nm)/Al (150 nm). High
Current density-voltage-luminance
Fig. 1(a) and (b) respectively shows the current density-voltage-luminance and current efficiency-current density characteristics of OLEDs with a pristine 0.5 nm CeF3 anode buffer layer and UV-ozone treated 0.5, 1, and 1.5 nm CeF3 anode buffer layers deposited onto the ITO substrate. Compared to the devices with the UV-ozone-treated CeF3 layers, the pristine CeF3 device had a lower current density, luminance and current efficiency. The optimal thickness for the UV-ozone treated CeF3 layer was
Conclusions
In this study, UV-ozone treated CeF3 films were used as anode buffer layers to improve the electro-optical properties of OLEDs, the optimal thickness of which was found to be a 0.5 nm. As such, when the UV-ozone treated 0.5-nm-thick CeF3 layer was inserted into the OLEDs, the turn-on voltage decreased from 4.2 to 3.6 V (at 1 mA/cm2), the maximum luminance increased from 7588 to 24760 cd/m2, and the maximum current efficiency increased from 3.2 to 3.8 cd/A compared with standard ITO devices. The XPS
Acknowledgement
This work was supported by the Industrial Technology Research Institute of Taiwan.
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