The main goal of this study was to obtain an enhancement of the thermal durability of the OLEDs. Particular attention was given to the engineering of charge injecting contacts by introducing the epitaxial thin MgO(001) layer. Figure shows an important enhancement of thermal stability of the 20-nm-thick CuPc film grown the MgO(001) layer. Black color represents the result obtained before annealing, while red, green and blue colors correspond to the results for the sample after 1 h vacuum annealing at 250°C, 300°C and 350°C, respectively. The dashed lines represent CuPc, MgO and Fe peak positions from the powder diffraction file. After 1 h of vacuum annealing in the temperature range from 150°C to 350°C, a strong diffraction peak, corresponding to the (002) lattice plane of β-phase CuPc, persists at 2θ
= 7.15° which is quite close to the position expected from the reference data. The peak intensity increases as the annealing temperature increases up to 250°C, while the intensity decreases after the annealing at 350°C. These XRD patterns indicate that the Fe and MgO buffer layers are strongly (001) textured, only the (002) diffraction peaks of bcc Fe and fcc MgO are observed. Consequently, the CuPc films deposited on the epitaxial Fe/MgO(001) buffer layer are also predominantly (001) textured. Although several earlier works reported the occurrence of a phase transition between α and β phases by thermal treatment above 200°C [15
], but such a phase transition was not observed in this work.
X-ray diffraction θ-2θ scans with Cu Kα radiation for the 20-nm-thick CuPc films before and after thermal treatment.
Figure shows the surface morphology of 20-nm-thick CuPc films (Figure ) before and after annealing at (Figure ) 150°C, and (Figure ) 250°C, respectively. Relatively smooth surfaces with root mean square (RMS) roughness of less than 2 nm were observed after the annealing up to 250°C. However, the RMS roughness became larger by annealing at the temperature higher than 250°C (Table ). This is quite consistent with the strengthening of the  CuPc texture with increasing post-annealing temperature found by the XRD. This can simply reveal the enhancement of thermal stability and crystallinity of the CuPc layered structure due to the MgO(001) underlayer effect. From the AFM surface analysis, rather homogeneous roughness distributions for the vertical distance between the highest peak and the lowest valley were observed in the range from −4 to 4 nm.
AFM images of Si(001)/8 nm MgO/15 nm Fe/1.8 nm MgO/2 nm CuPc films. (a) as grown, post-annealed at (b) 150°C and (c) 250°C.
RMS roughness for CuPc films after post-annealing for 1 h under vacuum
Here, we report a hybrid system consisting of a highly qualified interface between the MgO/Fe/MgO(001) and the OSC CuPc. Figure corresponds to the TEM image for the 20-nm-thick CuPc film grown at RT. The image shows, from bottom to top, the Si substrate, the 8-nm-thick MgO buffer layer, and the 10-nm-thick metallic Fe epilayer covered with the 1.8-nm-thick MgO. Note that the MgO/Fe/MgO(001) multilayers are well-crystallized, but some layer roughness is observable.
Cross-sectional bright-field TEM image of Si(001)/8 nm MgO/15 nm Fe/1.8 nm MgO/20 nm CuPc film.
Figure shows the current density-voltage (J-V) (Figure ) and the luminance-voltage (L-V) (Figure ) characteristics for the TOLED devices with 10-nm-thick CuPc film prepared beyond the different anodes, such as Al (blue, S1), polycrystalline Fe (poly-Fe) (black, S4), Fe(001) with (green, S2) and without MgO(001) (red, S3). For more details of the TOLED structure, see Table .
Figure 5 Current density-voltage and luminance-voltage characteristics for TOLED devices with different anodes. Al (blue, S1), Fe(100)/MgO(100) (green, S2), Fe(100) (red, S3), and poly-Fe (black, S4) anodes. More detailed information about each sample structure (more ...)
When compared to the TOLED devices based on Al and poly-Fe and Fe(001) bottom electrodes, the threshold voltage for the TOLED based on the Fe/MgO(001) electrode increases significantly. More drastic increase in the driving voltage is also shown for that Fe/MgO(001)-based TOLED. The large driving voltage could be attributed to the larger work function of the MgO(001) layer (4.94 eV) [17
] than that of Al (4.1 eV) [18
] or poly-Fe (4.5 eV) [18
]. Additionally, the effect of surface potential at the MgO(001)/CuPc interface could not be negligible. Since it was reported that charge injection barriers at metal/organic or oxide/organic interfaces affect the charge injection and recombination significantly [19
], we also investigated the IV
(Figure ) and LV
(Figure ) characteristics for the Fe/MgO(001)-based TOLED with different CuPc thicknesses: 15 nm (red, S5), 5 nm (green, S6) and 1 nm (black, S7) as shown in Figure . The structure information of S5, 6 and 7 TOLED are given in Table . EL spectra of TOLED with different thickness of CuPc are shown in Figure : red for S5, green for S6, and black for S7. When the thickness decreases from 15 to 1 nm, no remarkable change appeared in the threshold and driving voltages, but current and EL intensity increased obviously. A largely enhanced EL was observed in the TOLED with 1-nm-thick CuPc layer. To improve the TOLED device performance, further studies to optimize the device structure and fabrication conditions are required; the EL efficiency is far from perfect. However, our results suggest a new possibility to integrate spintronics with organic electronics: The use of the epitaxial thin MgO(001) layer is proposed not only to improve the performance and the stability of OLED, but also to inject the fully polarized spin current from the Fe/MgO(001) interface to the OSC layers [9
]. Indeed, the enhanced thermal stability of a few-nanometer-thick CuPc films could result from the MgO(001) underlayer effect: The (002)-textured β phase of CuPc layer persists even after the vacuum annealing at 350°C. Significant work function alteration by inserting MgO(001) between ferromagnetic metal and the CuPc OSC layer could provide a wide versatility of device functionality. For example, polarized light could be generated by fully polarized spin injection through the Fe/MgO(001) interface. Thus, for future work, it is worth to study the magnetic field effect in this OLED device.
Current-voltage and luminance-voltage characteristics for TOLED devices.