Growth and characterization of thin Cu-phthalocyanine films on MgO(001) layer for organic light-emitting diodes
© Bae et al.; licensee Springer. 2012
Received: 1 October 2012
Accepted: 20 November 2012
Published: 26 November 2012
Surface morphology and thermal stability of Cu-phthalocyanine (CuPc) films grown on an epitaxially grown MgO(001) layer were investigated by using atomic force microscope and X-ray diffractometer. The (002) textured β phase of CuPc films were prepared at room temperature beyond the epitaxial MgO/Fe/MgO(001) buffer layer by the vacuum deposition technique. The CuPc structure remained stable even after post-annealing at 350°C for 1 h under vacuum, which is an important advantage of device fabrication. In order to improve the device performance, we investigated also current-voltage-luminescence characteristics for the new top-emitting organic light-emitting diodes with different thicknesses of CuPc layer.
For charge injection into organic semiconductor (OSC) devices, oxide materials have been widely utilized. In general, indium thin oxide (ITO) is used as a transparent conducting electrode for optoelectronic devices which enable light to be emitted from the bottom of the structure. The work functions and electronic properties of ITO surfaces and, consequently, the interface electronic properties of contacts to OSC have become important issues to improve the organic light-emitting diode (OLED) device performance . Different chemical and physical treatments have been investigated to control the work function of an anode ITO electrode. In order to control efficiently the charge injection and transport in OSC devices, a novel approach by introducing a thin oxide layer between an anode and a hole-transport layer (HTL) in organic light-emitting diodes (OLEDs) has gotten considerable attentions recently [2–4]. After the first report of the role of ITO as a charge generation layer (CGL) , several other studies followed to demonstrate the effect of CGL based on oxide films, such as MoO3, V2O5, and WO3. The improved device efficiency was attributed to the generation of holes which could reduce the charge injection barriers at organic semiconductor interfaces upon application of an electric field.
Considering the monolithic integration of an OLED on silicon or organic thin film transistors as well as spin valves, a thin epitaxial MgO(001) layer as the CGL in OLEDs might be interesting to investigate for magneto-optoelectronic applications. The crystalline MgO(001) layer grown on Fe(001) is well known as a tunnel barrier for the fully epitaxial Fe(001)/MgO(001)/Fe(001) tunnel junctions with a huge magnetoresistance (MR) value at room temperature (RT) due to a band symmetry filtering effect [8, 9]. Remarkable improvements have been recently achieved in the field of organic spin valve devices [10, 11]. However, according to our knowledge, up to now the investigation on the MgO-based organic spin valves with a huge MR is not reported yet.
In order to understand the effect of an epitaxial MgO(001) layer on the charge generation mechanism of OLED devices, a preliminary investigation to fabricate the hybrid multilayer films with sharp amorphous or crystalline interfaces is required. However, the growth of organic thin films beyond the metal or oxide contact was challenging; and a number of problems had to be solved, especially to obtain smooth and ordered molecular monolayer of the organic materials suitable for applications of organic spintronic devices . Therefore, in this work, we investigated the growth of organic-inorganic hybrid multilayer structures for Si(001) substrate-based OLED devices. In particular, we focused on the growth of layer-structured HTL Cu-phthalocyanine (CuPc) and characterization of its surface morphology after heat-treatment under vacuum, since CuPc is one of the most popular OSC with high thermal and chemical stability suitable for thin film organic devices . Thus, it has been also widely used in optoelectronic devices .
Additionally, a new top-emitting OLED (TOLED) structure, which is formed on an opaque Si(001) substrate and an epitaxial MgO(001)/Fe(001)/MgO(001) bottom electrode so that light can emit from the thin Al top electrode, was investigated. Our TOLED design includes a semi-transparent cathode Al, a stack of conventional organic electroluminescent layers, and a thin CuPc film to enhance the hole injection into the electroluminescent layers.
We expect that our new approach could open up the door toward the development of multifunctional architectures for future device technology.
We used a ultra-high vacuum (UHV)-molecular beam epitaxy film evaporation system to stack successively inorganic elements beyond chemically etched p-type Si(001) wafers. The base pressure of the UHV-MBE chamber was lower than approximately 2 × 10−10 Torr. The epitaxial MgO(001)/Fe(001)/MgO(001) multilayers were formed at 250°C with low deposition rate of 0.003 nm/s. During the deposition the pressure was kept lower than 3 × 10−9 Torr.
In order to investigate the thin CuPc film growth mode and its structural properties, 20-nm-thick CuPc plane films were deposited at RT beyond Si(001)/8.0 nm MgO/15 nm Fe/1.8 nm MgO by high vacuum thermal evaporation (base pressure at approximately 2 × 10−7 Torr). It should be noted that air-exposure on the MgO top surface is inevitable during the sample transfer from the UHV-MBE chamber to the HV-thermal evaporator.
Substrate and layers
Si(001)\SiO2\150 nm Al\10 nm CuPc\60 nm α-NPD\70 nm Alq3\20 nm Al
Si(001)\8 nm MgO\15 nm Fe\1.8 nm MgO\10 nm CuPc\60 nm α-NPD\70 nm Alq3\20 nm Al
Si(001)\8 nm MgO\15 nm Fe\10 nm CuPc\60 nm α-NPD\70 nm Alq3\20 nm Al
Si(001)\SiO2\15 nm poly-Fe\10 nm CuPc\60 nm α-NPD\70 nm Alq3\20 nm Al
Si(001)\8 nm MgO\15 nm Fe\1.8 nm MgO\15 nm CuPc\60 nm α-NPD\70 nm Alq3\20 nm Al
Si(001)\8 nm MgO\15 nm Fe\1.8 nm MgO\5 nm CuPc\60 nm α-NPD\70 nm Alq3\20 nm Al
Si(001)\8 nm MgO\15 nm Fe\1.8 nm MgO\1 nm CuPc23\60 nm α-NPD\70 nm Alq3\20 nm Al
For the Si(001)/MgO/Fe/MgO/CuPc hybrid multilayers before and after vacuum annealing for 1 h in the temperature range from 100°C to 250°C, the structural characteristics were examined by using X-ray powder diffraction technique (XRD) and atomic force microscope (AFM). In order to analyze the microstructure of these hybrid multilayer interfaces, cross-sectional samples for transmission electron microscope (TEM) were prepared using conventional mechanical polishing and dimpling techniques. All the images were obtained using a double aberration corrected JEOL FS2200 TEM (JEOL Ltd., Tokyo, Japan) with atomic resolution. A new specimen preparation process with minimum damage onto the organic layer was developed. We used mechanical thinning, followed by precision ion-beam polisher system or (PIPS) with very short time to clean the surface. We have also minimized the damage during the TEM observation by reducing the e-beam exposure time. In addition we have confirmed the presence of the CuPc by energy dispersive X-ray scan using the scanning TEM observation mode.
The current-voltage-luminance (I-V-L) characteristics were investigated by using a Keithley 236 source-measure unit and a Keithley 2000 multimeter equipped with a photomultiplier tube through an ARC 275 monochromator (Keithley Instruments Inc., Cleveland, OH, USA). The external quantum efficiency of the electroluminescence (EL), defined as the ratio of the emitted photons to the injected electric charges, was calculated from the EL intensity measured by using a calibrated Si photodiode placed at a normal angle to the device’s surface.
Results and discussion
RMS roughness for CuPc films after post-annealing for 1 h under vacuum
In order to control the operation of an OLEDs, we investigated a new possibility of the use of thin MgO(001) layer between the ferromagnetic metal Fe(001)-OSC CuPc interface as a organic surface modifier. Remarkably enhanced thermal stability of CuPc films with smooth surface morphology (RMS roughness ≤ approximately 2 nm) was obtained up to the temperature of 350°C. In this work, the use of appropriate oxide layers could represent a new interface engineering technique for improving reliability and functionality in OSC devices. Based on the reliable Si(001)/MgO(001)/Fe(001)/MgO(001)/CuPc hybrid stack, the new TOLED structure was investigated for future organic spintronic device applications.
This research is funded by the National Research Foundation of Korea (NRF) grants funded by the Ministry of Education, Science and Technology (MEST) (no. 2008-0062239, 2011-0017209, and 2010-0006749). YJB thanks the NRF (no. 2011-0001809) for the financial support.
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