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By Jungmyoung Ju, Yutaka Yamagata,* and Toshiro Higuchi

After observation of organic electroluminescence (EL) in the 1960s, the first organic light-emitting diode (OLED) device was successfully fabricated by Tang et al. in 1987.[1] They proposed a multilayer structure fabricated by thermal evaporation, which is still an effective method for materials of low molecular weight due to advantages in the thickness controllability and uniformity around 100nm as well as quality without pinholes.[2] Since then, development of polymer OLED materials,[3,4] as well as fabrication methods such as spin coating,[3,4] ink-jet printing,[5,6] screen printing,[7,8] roll-to-roll printing process,[9] laser-induced thermal imaging,[10,11] and the photolithographic method[12] have been proposed and studied.[13] In the organic full-color display industry, important parameters are patterning resolution, uniformity, material utilization yield, and fabrication costs. In the case of the thermal-evaporation method, a temperature rise may cause thermal deformation of the stencil mask and substrate. In addition, low material yield and the requirement of high-vacuum systems for every process may lead to high fabrication costs.[14] In the fabrication methods for polymeric materials, liquid-based methods have advantages of low temperature, low fabrication cost, and high material-utilization yield, with the exception of the spin-coating method. However, these patterning methods may cause a ring stain (coffee stain) effect through nonlinear evaporation, so that it may be difficult to fabricate uniform films.[15] To improve the uniformity, a considerable amount of additives to increase viscosity may be necessary. In addition, these liquid-based methods have problems in multilayer fabrication, because layers with the same solvent will intermix with each other. Electrospray deposition (ESD) is a method to fabricate thin films of micro/nanoparticles directly from solution. ESD was first introduced as a fabrication method of thin radioactive source in nuclear research in 1950s,[16–19] and was then used in various processes: sample preparation in mass spectrometry,[20,21] modification of silicon surfaces,[22] semi-conductive ceramics,[23] polymer coatings,[24] DNA and protein films for scanning

[*] Dr. Y. Yamagata VCAD System Research Program RIKEN (The Institute of Physical and Chemical Research) 2-1 Hirosawa Wako-shi Saitama, 351-0198 (Japan) E-mail: [email protected] J. Ju, Prof. T. Higuchi Department of Precision Engineering School of Engineering, The University of Tokyo 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656 (Japan)

DOI: 10.1002/adma.200900444

Adv. Mater. 2009, 21, 4343–4347

tunnelling microscopy,[25,26] and functionally and biologically active protein deposition.[27–30] The authors have also studied deposition/patterning methods using electrically charged nanoparticles generated by electrospray[31–33] and surface acoustic wave (SAW) atomizer.[34–36] The advantages of these methods are that they are performed under atmospheric pressure and room temperature, which may lead to low equipment cost. In addition, ring-stain effects do not occur due to deposition of almost dried nanoparticles by the electrostatic force. These nanoparticle-based deposition methods may solve problems of the OLED fabrication process such as material damage, yield, and thickness uniformity. Saf et al. have reported on application of electrospray to OLED thin-film fabrication,[37] but surface morphology depending on spray conditions have not been discussed in detail. First, we applied the normal electrospray deposition method to the polymer OLED application, as a preliminary test. However, it was not possible to fabricate a thin film of OLED material of acceptable quality, because the small gaps between the deposited nanoparticles functioned as pinholes. Consequently, after deposition of the metal electrode on the OLED layer, devices soon malfunctioned. In this communication, we propose a new method of fabricating thin and regular films for organic light-emitting devices using the electrospray deposition method. The basic concept of the proposed method is that nanoparticles are deposited on the target substrate just before they become completely dry. This is done by mixing in an additional solvent that has an evaporation speed relatively lower than that of the original solution. To investigate the morphology of the deposited layer, different concentrations of poly(2-methoxy-5-(2-ethylhexoxy)-1,4-phenylenevinylene (MEH-PPV) solutions were sprayed on and then measured using field-emission scanning electron microscopy (FE-SEM) and a white-light interferometer. Based on the morphology test, a small OLED pixel was fabricated, and current–voltage (I–V) curves and luminescence characteristics were also tested. Detailed experimental setups are given in the experimental sections. Five different concentrations of MEH-PPV were dissolved in toluene, and then mixed with dimethylformamide (DMF) solutions at six different concentrations respectively, as described in detail in the Experimental section. Finally, 30 combinations of samples were prepared and sprayed with an ESD device as shown in Figure 1a. The liquid sample was stored in a thin glass capillary with a tip size of approximately 20 mm. Then, the liquid sample was atomized into small charged droplets by applying a high voltage between a conductive wire and a conductive substrate. Atomized droplets started to evaporate under the atmospheric

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Thin-Film Fabrication Method for Organic Light-Emitting Diodes Using Electrospray Deposition

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Figure 1. Apparatus and devices used for the fabrication of OLED thin films using electrospray deposition (ESD). a) Apparatus configuration of electrospray deposition. b) Layout of substrate used for deposition (w ¼ 100 mm, s ¼ 100 mm, N ¼ 12 elements, metal electrode pad made of copper. Upper electrode width ¼ 340 mm). c) Photograph of deposited MEH-PPV and upper electrode.

pressure at room temperature. Finally, these droplets or particles were deposited on the indium-tin-oxide (ITO) substrate by the electrostatic force. Figure 2a–d shows FE-SEM images of the deposited sample. Deposited film morphology can be classified into three categories: particle mode, wet mode, and thin-film mode. Figure 2a shows the typical particle mode. Figure 2b shows the typical film mode, where no particles or debris are observed. Figure 2c and d show the wet mode, which has a debris structure, similar to a collapsed bubble on the surface. One explanation for this may be that particles are deposited in a wet state, and liquid inside the deposit evaporates to form a bubble. After that, it is crushed by the tension force generated by the evaporation of the inside solvent.

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Figure 2. Morphology and deposition conditions. a–d) FE-SEM images of deposited samples. (a and b have scratched areas, for focusing purposes). Sample a) A-5, magnification: 2000; b) C-4, magnification: 2000; c) B-4, magnification: 100; d) enlarged image of B-4, magnification: 1000. e) Deposition mode with different samples, table showing particle mode (P), wet mode (W), film mode (F) and spray not possible (X). DMF concentration increases along column number from 1 to 6. MEH-PPV concentration decreases along rows from A to E. f) Surface roughness of different samples.

Thus, if these patterns were on the substrate, it would be classified as wet mode. Figure 2e shows a summary of the morphology test based on the FE-SEM images. Samples 1 and 2. except E-2, were not sprayed under the 5 kV applied voltage. The particle and wet modes were generated at relatively high MEH-PPV concentrations, such as A and B. In C, D, and E, almost all samples presented film mode. Figure 2f shows the average surface roughness (Ra) measured by white-light interferometry. The concentration of MEH-PPV

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measured in the same manner, and its thickness was about 126 nm. Using the pattern produced using sample C-4, the I–V characteristics were measured and compared with those of the spin-coated sample, as shown in Figure 3a. The voltage was applied up to the insulation breakdown point at which the current density suddenly dropped. Compared with the spin-coated sample, the current density of C-4 sample is higher at the lower applied voltage. The maximum current density as well as the breakdown voltage of the spin-coated sample is higher than those of the C-4 sample. Figure 3b shows the results of the relative pixel intensity with luminescence image measured using a cooled charge-coupled device (CCD) camera. The camera shows high linearity between pixel intensity output and actual luminescence. Comparing the two results, the C-4 sample shows higher pixel intensity than the spin-coated one in the lower-applied-voltage region. However, the spin-coated sample shows higher maximum pixel intensity. In the case of the electrosprayed sample, MEH-PPV was deposited only on the electrode, while the spin-coated sample covers the entire area, which may lead to the difference in breakdown voltage. The early current-density rise of the electrosprayed sample compared with the spin-coated one may be caused by the polystyrene added to increase viscosity. In conclusion, we proposed a new OLED fabrication method using electrospray deposition and employing two solvents with different evaporation temperatures. In the morphology test, it was discovered that three different deposition modes (particle, wet, and thin film) occur depending on the solvent mixture ratio and sample concentration. Based on this result, we successfully fabricated an OLED pixel, and its I–V and luminescence characteristics were measured, although the absolute performance is not high, probably due to its being composed of a single OLED layer. The effect of the charges caused by the electrospray have not been verified by experiments, but positive charges or hydrogen ions may have affected the electrical/electroluminescent characteristics of MEH-PPV, since electrospray capillary can have an effect similar to an electrolytic cell.[38] An OLED thin film fabricated by ESD showed better surface roughness than the spin-coated sample. The deposited MEH-PPV is concentrated on the ITO electrode pattern, and only little deposition was observed in places without an electrode. This suggests that the ESD method has a potentially higher material-utilization yield than other methods. A multilayered structure may be fabricated because the sample is quickly dried up after landing on the substrate. In this communication, we focused on film deposition morphology in various conditions, but in the future more detailed analysis of the characteristics of OLED materials, such as the luminescence spectrum, quantum efficiency, and the multilayered structure, should be tested.

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decreases as the sample number rises. If the size distribution of atomized droplets is the same, the size of the dried particles should only depend on the concentration of the sample. Thus, it is expected that surface roughness may decrease as the sample number increases. Compared with B, the surface roughness of C has dramatically decreased. It can be considered that this drastic decrease in roughness is due to the change in deposition mode as the concentration of MEH-PPV decrease. Comparing Figure2e and f, depositions with approximately 10 nm or less roughness are in film mode. Some of the samples in film mode showed even lower surface roughnesses than the spin-coated samples. In the morphology test, depositions were categorized into three modes by FE-SEM and interferometer profile. Based on these results, the OLED device was fabricated. We used C-4 concentration, since it has the highest concentration of MEH-PPV in film mode. Figure 1b shows the substrate configuration with patterned ITO electrode array. The ITO array was connected to the ground, and then the C-4 sample was electrosprayed. After ESD, a gold layer of approximately 100 nm was deposited using a thermal evaporator. Figure 1c shows a microscopy image of the OLED film fabricated. The vertical red stripes are deposited MEH-PPV on the 100 mm width ITO electrode. Charged droplets are attracted to the ITO electrode by the electrostatic force, so that MEH-PPV was only deposited on the electrodes. The horizontal dark line is a deposited gold layer on the MEH-PPV pattern. The thickness of ITO is 200 nm, and that of the deposited MEH-PPV is approximately 120 nm, according to the white-light interferometry measurements. A spin-coated sample was also fabricated and

Experimental Figure 3. Characteristics of fabricated OLED thin-film dot. a) V–I characteristics of spin-coated sample and C-4 sample. b) Relative intensity of pixel measured by cooled CCD camera (image of 100 mm width  340 mm height light-emitting area taken at 26 V, exposure time 10 s).

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Used Materials: MEH-PPV (Sigma–Aldrich 541443, Average Molecular Weight: 40,000–70,000) and Polystyrene (Sigma–Aldrich 330345, average molecular weight: 50,000) were used for the test sample. A certain amount of MEH-PPV is dissolved in toluene, then small amounts of DMF are added, according to Table 1.

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Table 1. Samples used for analysis of deposition characteristics of OLED on the electrospray. Sample number

A-1

A-2

A-3

A-4

A-5

A-6

Toluene þ MEH-PPV 0.5 wt % DMF Spray Voltage (kV)

500 mL 0 mL –

500 mL 50 mL –

500 mL 150 mL –

500 mL 250 mL 3.85

500 mL 350 mL 3.7

500 mL 450 mL 3.3

Sample number Toluene þ MEH-PPV 0.25 wt % DMF Spray Voltage (kV)

B-1 500 mL 0 mL –

B-2 500 mL 50 mL –

B-3 500 mL 150 mL 3.4

B-4 500 mL 250 mL 3.31

B-5 500 mL 350 mL 3.2

B-6 500 mL 450 mL 3.3

Sample number Toluene þ MEH-PPV 0.125 wt % DMF Spray Voltage (kV)

C-1 500 mL 0 mL –

C-2 500 mL 50 mL –

C-3 500 mL 150 mL 3.3

C-4 500 mL 250 mL 3.19

C-5 500 mL 350 mL 3.13

C-6 500 mL 450 mL 3.0

Sample number Toluene þ MEH-PPV 0.062 wt % DMF Spray Voltage (kV)

D-1 500 mL 0 mL –

D-2 500 mL 50 mL –

D-3 500 mL 150 mL 3.08

D-4 500 mL 250 mL 2.99

D-5 500 mL 350 mL 2.77

D-6 500 mL 450 mL 2.96

Sample number Toluene þ MEH-PPV 0.031 wt % DMF Spray Voltage (kV)

E-1 500 mL 0 mL –

E-2 500 mL 50 mL 5.0

E-3 500 mL 150 mL 2.86

E-4 500 mL 250 mL 2.45

E-5 500 mL 350 mL 2.9

E-6 500 mL 450 mL 3.0

ESD Setups: The tip diameter of the glass capillary was 20 mm. The distance between the glass capillary and the glass indium-tin-oxide (ITO) substrate was 4 cm. A Teflon shield (thickness 1 mm) was used for the insulation aperture, with a maximum 5 kV high-voltage power supply and an acrylic chamber (width: 20 cm square, height: 15 cm). The spray voltage in Table 1 indicates the applied voltage for each sample that enabled stable cone-jet mode spray. Morphology Test: 0.7 mL sample was used on an ITO glass plate (width: 16 mm, height: 30 mm, glass capillary approximately 1 mm). The hole size of the Teflon aperture was 5 mm square. After deposition, the samples were sputter-coated (JEOL, JFC-1600, Auto fine coater, 90 s, 30 mA) for white-light interferometry (ZYGO, Newview 5032). Fabrication Process of the Spin-Coated Sample: In 1 mL of 1 wt% MEH-PPV, 1 wt% polystyrene was added for viscosity control. An ITO glass plate (width: 76 mm, height: 26 mm) was used as the substrate. After four spin steps (500 rpm, 1 s- slope, 1 s- 2000 rpm, 20 s- stop), soft bake (50 8C, 60 s) was applied for evaporation of the solvent. The thickness of the film was 126 nm. Fabrication Condition of C-4: 0.7 mL of sample was used for a 10 mm aperture. The thickness of MEH-PPV was 120 nm. Anode and Cathode Information: After fabrication of the C-4 sample and the spin-coated sample, a thermal evaporator was used for deposition of the gold layer. The thickness of the ITO and the gold were 200 nm and 100 nm, respectively. Received: February 9, 2009 Revised: April 25, 2009 Published online: July 2, 2009

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Thin-film fabrication method for organic light-emitting diodes using electrospray deposition.

A new method for fabricating micropatterns of MEH-PPV thin films with surface roughnesses below 1nm is proposed, using electrospray deposition and a d...
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