NANO ENERGY SYSTEMS Available online at www.onecentralpress.com One Central Press journal homepage: www.onecentralpress.com/nano-energy-systems 42 ABSTRACT Yuto Kakinoki 1* and Naoki Ohtani 1* Fabrication of Molybdenum Trioxide Thin Films Using Precursors by Wet Process and Examination of Annealing Conditions Molybdenum trioxide (MoO 3 ) thin films were prepared by a spin-coating method using ammonium molybdate tetrahydrate (AMT, (NH 4 ) 6 Mo 7 O 24 4H 2 O) as a precursor. The detailed annealing condition was evaluated. For example, some samples were annealed twice; the first annealing was in vacuum, and the second was in air. We found that the transmission spectra and surface roughness clearly depended on the annealing condition. We also evaluated the electroluminescence (EL) and current-voltage properties of organic light-emitting diodes (OLEDs) that contained the fabricated MoO 3 thin films as a hole-transporting layer. 1 Department of Electronics, Doshisha University, 1-3 Tatara-Miyakodani, Kyotanabe-shi, Kyoto 610-0321, Japan. * corresponding author I. INTRODUCTION fabricated MoO 3 thin films by a spincoating method using ammonium molybdate tetrahydrate (AMT, (NH 4 ) 6 Mo 7 O 24 4H 2 O, Fig. 1) as a precursor [8, 9]. Figure 1 Molecular structure of ammonium molybdate tetrahydrate (AMT). The fabricated MoO 3 is a simple single layer that does not increase the number of fabrication processes. Moreover, some samples were annealed twice: in vacuum and in air. This is because the annealing of In solution-processable organic light-emitting diodes (OLEDs), poly(3,4-ethylenedioxythiophene): poly(styrene sulfonic acid) (PEDOT:PSS) is generally used as an hole-injection layer. However, its strong acidity leads to unstable device performance. Recently, molybdenum trioxide (MoO 3 ) has been studied to replace PEDOT:PSS in OLEDs because it has an important factor for selecting fine buffer layers between the electrodes and organic films for fabricating efficient and stable devices [1-4]. Two of these papers reported that solution-processed holeinjection layers containing MoO 3 and another material might improve solution-processed OLEDs: bilayer hole-injection layers containing C60 [3] and an anionic polyelectrolyte, poly(sodium 4-styrenesulfo-nate) (PSS-Na) [4]. On the other hand, MoO 3 thin films are currently fabricated using radio frequency (RF) magnetron sputtering [5-7] because it is of advantage in fabricating smooth and flat MoO 3 thin films than solution-process methods. On the other hand, however, the solution-process is advantageous in respect of a simple system that leads to low-cost production. In this research, we
Fabrication of Molybdenum Trioxide Thin Films Using Precursors by Wet Process and Examination of Annealing Conditions
Molybdenum trioxide (MoO3) thin films were prepared by a spin-coating method using ammonium molybdate tetrahydrate (AMT, (NH4)6Mo7O244H2O) as a precursor. The detailed annealing condition was evaluated. For example, some samples were annealed twice; the first annealing was in vacuum, and the second was in air. We found that the transmission spectra and surface roughness clearly depended on the annealing condition. We also evaluated the electroluminescence (EL) and current-voltage properties of organic light-emitting diodes (OLEDs) that contained the fabricated MoO3 thin films as a hole-transporting layer.
I. INTRODUCTION fabricated MoO3 thin films by a spincoating method using ammonium molybdate tetrahydrate (AMT, (NH4)6Mo7O244H2O, Fig. 1) as a precursor [8, 9].
Figure 1 Molecular structure of ammonium molybdate tetrahydrate (AMT).
The fabricated MoO3 is a simple single layer that does not increase the number of fabrication processes. Moreover, some samples were annealed twice: in vacuum and in air. This is because the annealing of
In solution-processable organic light-emitting diodes (OLEDs), poly(3,4-ethylenedioxythiophene): poly(styrene sulfonic acid) (PEDOT:PSS) is generally used as an hole-injection layer. However, its strong acidity leads to unstable device performance. Recently, molybdenum trioxide (MoO3) has been studied to replace PEDOT:PSS in OLEDs because it has an important factor for selecting fine buffer layers between the electrodes and organic films for fabricating efficient and stable devices [1-4]. Two of these papers reported that solution-processed holeinjection layers containing MoO3 and another material might improve solution-processed OLEDs: bilayer hole-injection layers containing C60 [3] and an anionic polyelectrolyte, poly(sodium 4-styrenesulfo-nate) (PSS-Na) [4]. On the other hand, MoO3 thin films are currently fabricated using radio frequency (RF) magnetron sputtering [5-7] because it is of advantage in fabricating smooth and flat MoO3 thin films than solution-process methods. On the other hand, however, the solution-process is advantageous in respect of a simple system that leads to low-cost production. In this research, we
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the precursor in vacuum reduces the oxygen atoms in the films. Thus, we evaluated the effect of a second annealing to recover oxygen atoms in the films. From these evaluations, we determined the optimal annealing condition for fabricating MoO3 layers. Finally, the OLEDs were solution-processed that contain a as a hole-transporting MoO3 layer that was fabricated using the optimal annealing condition.
II. EXPERIMENTAL
Fabrication of MoO3 films. As a precursor solution, we mixed an AMT solution with pure water. The thin films of the precursor solution were formed by a spin-coating method on a substrate. Then the samples were annealed in air at various temperatures ranging from 260 to 500ºC. Some were annealed twice; the first annealing was in vacuum, and the second was in air. This is because the first annealing in vacuum reduced the oxygen atoms.Fabrication of OLEDs. We fabricated the following five kinds of OLEDs on an indium-tinoxide (ITO)-coated glass substrate containing MoO3 films as a hole-transporting layer:G1 & G2: ITO / MoO3 / MDMO-PPV / Al Annealing at 360ºC in air: AMT densities are 1wt% (G1) and 0.5wt% (G2).H1 & H2: ITO / MoO3 / MDMO-PPV / Al First annealing at 340ºC in vacuum and second annealing at 360ºC in air. The AMT densities are 1wt% (H1) and 0.5wt% (H2).I: ITO / PEDOT:PSS / MDMO-PPV / Al.Sample I was fabricated for comparison with the hole-transporting effect of the MoO3 layers with PEDOT:PSS.Measurement setup. The surface roughness of the MoO3 thin film was evaluated using an atomic force microscope (AFM, FS-150N, SII Nanotechnology). The transmission spectra were evaluated using a spectrophotometer (UV-2450, SHIMAZDU). The current-voltage characteristics and the electroluminescence (EL) spectra were measured using a combination system of a source meter (2400, Keithley), an integrating sphere, and a multi-channel monochrometer (PMA-12, Hamamatsu). Fourier transform infrared (FT-IR) spectroscopy was performed using an FT/IR-4100 (Jasco).
III. RESULTS AND DISCUSSION
Optimal annealing temperature. To find the optimal annealing temperature, we first evaluated the FT-IR spectra before and after the annealed samples at several temperatures. Figures 2 shows the FT-IR spectra of the four samples: not annealed, annealed at 260, 300, and 340ºC. The results clearly demonstrate that the FT-IR signals, which originated from the N-H stretching vibration of the NH4+ groups at around 3150-3380 cm-1 [10], disappeared after the annealing at over 340ºC. In addition, the CH2 bending vibration signal disappeared at around 1400 cm-1 [11]. These results mean that the precursor thin films became MoO3 films by annealing at over 340ºC. Thus we tried to find the optimal annealing temperature in the range from 340 to 500ºC by the observations using AFM. The AFM images in Fig. 3 clearly show that the surface roughness increased by raising the annealing temperature from 340 to 500ºC. In particular, some huge MoO3 islands were observed in the sample annealed at 500ºC. Consequently, the surface roughness annealed at 340ºC became minimum. Next we evaluated their transmission spectra. Figure 4 shows that the transmittance in the visible light regime was inversely reduced by increasing the annealing temperature. This is most likely caused by light scattering due to the surface roughness. The transmission spectra of the two samples (340 and 360ºC) were almost equal. On the other hand, the transmittance of the sample annealed at 380ºC was slightly smaller in the shorter wavelength regime. Therefore, we conclude that the optimal annealing temperature range is from 340 to 360ºC.
Figure 2 FT-IR spectra of four samples: Not annealed, annealed at 260, 300, and 340ºC.
Figure 3 AFM images of surface of MoO3 films annealed at four different temperatures. Their annealing temperatures and averaged surface roughnesses are denoted in figures.
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Figure 4 Transmission spectra of MoO3 films annealed at different five temperatures.
Effect of second annealing. To improve the surface roughness, we evaluated the effect of the second annealing. To compare the surface roughnesses of the samples with and without a second annealing, we fabricated two samples (A and B) and annealed them once in vacuum at different temperatures. In addition, we attempted the second annealing for three samples in air (C, D, and E). Their annealing conditions
are listed in Table 1. Figure 5 shows the AFM images of the surfaces of the five samples. We found that the surface roughnesses of the samples from A to E were 1.00, 9.11, 0.398, 26.9, and 12.0 nm, indicating that sample C has the best annealing condition. The comparison of samples A and C clearly demonstrates that the secondannealing in air effectively to improved the surface roughness. Figure 5 shows the transmission spectra of the three samples. Sample C reveals fine transmittance which is larger than the sample annealed at 340ºC in air. This result shows the surface roughness of sample C (0.398 nm) is smaller than the sample at 340 ºC in air (0.61 nm, Fig. 3). This is caused by the smaller surface roughness of sample C. On the other hand, sample A reveals smaller transmittance in the visible light of the longer wavelength regime. Note that sample A looks blue, which is probably caused by the lack of oxygen atoms due to the annealing in vacuum. Sample C’s second annealing in air compensated for the lack of oxygen atoms in sample A, resulting in the improved transmittance.
Figure 5 AFM images of five samples listed in Table 1 and their surface roughnesses.
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Figure 6 Transmission spectra of three samples. Transmission spectrum of the sample annealed at 340 ºC in air equals to the spectrum shown in Fig. 4.
Application to OLEDs. To evaluate the hole-transporting effect of the fabricated MoO3 films, we fabricated the five OLEDs (G1, G2, H1, H2, and I) and evaluated their properties. Note that the transmission property of the MoO3 films contained in samples H1 and H2 matches sample C, which outperforms those of samples G1 and G2. Figure 7 shows both the current-voltage and voltage-luminance characteristics of the five OLEDs. The driving voltage of the OLEDs containing MoO3, annealed at 360ºC in air (sample G1), was reduced by about 44% compared to an OLED containing a PEDOT:PSS layer as a hole-transporting layer (sample I). The power efficiency at the maximum emission voltage also improved by about 2.17 times (sample G2). Thus, sample G2 outperformed sample G1. Sample H2’s driving voltage is lower than sample I. However, its maximum luminance is much smaller than that of sample I. The driving voltage of sample H1 almost equals sample I. In addition, its maximum luminance is much smaller than sample I. Although the transmission properties of samples H1 and H2 are better than those of G1 and G2, these results indicate that the EL properties of samples G1 and G2 are inversely better than H1 and H1. Thus, to improve the performance of samples H1 and H2, the sheet resistances of the MoO3 films must also be evaluated.
Figure 7 (a) Current-voltage characteristics and (b) voltage-luminance characteristics of five OLEDs.
III. SUMMARY
We evaluated the annealing conditions of wet-processed MoO3 films. MoO3 film that was annealed at 360ºC in air reveals good performance as a hole-transporting layer in OLEDs. The surface roughness was clearly improved by double annealing in vacuum and air (sample C). However, the EL properties that contain the double-annealed MoO3 layer must be optimized.
IV. REFERENCES
[1] T. Chiu and Y. Chuang, J. Phys. D: Appl. Phys. 48, 075101 (2015).
[2] M. Espinoza, V. Poblete, J. Bernede, L. Cattin, A. Godoy, F. Alzamora, and N. Gaumer, Polym. Bull. 70, 2801 (2013).
[3] J. Hou, J. Wu, Z. Xie, and L. Wang, Appl. Phys. Lett., 95, 203508 (2009).
[4] J. Peng, X. Xu, C. Yao, and L. Li, RSC Adv., 6, 100312 (2016).
[5] M. Morales-Luna, S. Tomas, M. Arvizu, M. Perez-Gonzalez, and E. Campos-Gonzalez, J. Alloys. Compound., 722, 938 (2017).
[6] S. Subbarayudu, V. Madhavi, and S. Uthanna, Adv. Mat. Lett. 4, 637 (2013).
[7] T. Vink, R. Verbeek, J. Snijders, and Y. Tamminga, J. Appl. Phys., 87, 7252 (2000).
[8] M. Torres-Luengo, H. Martinez, J. Torres, and L. Lopez-Carreno, J. Phys., 480, 012009 (2014).
[9] T. Chiang and H. Yeh, Materials, 6, 4609 (2013).[10] I. Sulistianti, Y. Krisnandi, and I. Moenandar,
IOP Conf. Series: Mat. Sci. and Eng., 188, 012041 (2017).
[11] D. Sajan, G. Sockalingum, M. Manfait. I. Hubert Joe, and V. Jayakumar, J. Raman Spectro., 39, 1772 (2008).
