A Comparison Study of the Organic Small Molecular Thin Films

ARTICLE pubs.acs.org/JPCC

A Comparison Study of the Organic Small Molecular Thin Films Prepared by Solution Process and Vacuum Deposition: Roughness, Hydrophilicity, Absorption, Photoluminescence, Density, Mobility, and Electroluminescence Shu Feng, Lian Duan, Liudong Hou, Juan Qiao, Deqiang Zhang, Guifang Dong, Liduo Wang, and Yong Qiu* Key Lab of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, People's Republic of China ABSTRACT: In this work, we report a systematic study of the properties of the solution-processed organic small molecular thin films in order to get an in-depth understanding of the distinctive nature of solution-processed thin films. N,N0 -Di(3methylphenyl)-N,N0 -diphenyl-(1,10 -biphenyl)-4,4-diamine (TPD), a typical hole-transporting compound, was chosen to fabricate films by both solution process and vacuum deposition. The comparison study between the two kinds of TPD films shows that the surface of the solution-processed TPD films is more smooth and hydrophilic than that of the vacuum-deposited film. In addition, the density of the TPD films was measured by two different methods, and an increase in density was observed in solution-processed films in comparison with the vacuum-deposited film. The space-charge-limited current (SCLC) measurements indicate an increased hole transport mobility of the solutionprocessed TPD films over that of the vacuum-deposited film, which may result from the more compact intermolecular stacking in solution-processed TPD films. Finally, the light-emitting diodes with a solution-processed TPD layer as the hole-transport layer were fabricated and showed significantly enhanced injected current density and higher luminance relative to the device using the vacuum-deposited TPD film.

1. INTRODUCTION In the past decade, organic light-emitting diodes (OLEDs) based on vacuum-deposited small molecules have undergone significant progress since the vapor deposition technique allows for complicated multiple-layer devices and has secured excellent device performance.1
6 However, this vapor deposition process has critical drawbacks including fabrication complexity and low utilization of the expensive materials.6,7 Recently, solutionprocessed OLEDs based on small molecules have attracted increasing interest, and most of the efforts are focusing on synthesizing suitable small molecules and optimizing the device fabrication processes to improve the device efficiency.8
13 For instance, Kim et al.14 investigated a typical amorphous small molecular thin film of 2,20 ,200 -(1,3,5-benzenetriyl)-tris[1-phenyl1H-benzimidazole] (TPBI) and reported that similar average roughness values for vacuum-deposited and solution-processed films were obtained. Lee et al.15 chose the 2-(t-butyl)-9,10-bis(20naphthyl)anthracene (TBADN) doped with 4,40 -bis[2-{4-(N, N-diphenylamino)phenyl}vinyl] (DPAVBi) system to reveal the solvent effects on the film-forming property and device performance of the small molecular materials. The root-meansquare (rms) roughness values of films spun from toluene and chlorobenzene solutions were found to be similar and were quite similar to that of a vacuum-deposited film. They also investigated the film density through ellipsometry measurement and found that the densities of films processed from the toluene and r 2011 American Chemical Society

chlorobenzene solutions are much lower than that of the vacuum-deposited film.15 Ishihara et al.16 also investigated the effect of the preparation methods (vapor deposition or solution process) of organic thin films on charge injection from electrodes. They found that organic electroluminescent devices using spin-coated films of N,N0 -bis(3-methylphenyl)-N,N0 -diphenyl[1,10 -biphenyl]-4,40 -diamine (TPD) as a hole-transport layer and tris(8-quinolinolato)aluminum (Alq3) as an emitting layer exhibited higher injected current density and luminance than the corresponding devices using vacuum-deposited films of TPD. Although researchers have made efforts to reveal the nature of the films and devices obtained by solution process, many aspects for solution-processed small molecular thin films have still not been investigated. The carrier mobilities in solution-processed small molecular thin films have not been reported. Up to now, no systematic comparison between the films prepared via solution process and the films prepared via vacuum deposition has been reported. In this paper, we chose N,N0 -di(3-methylphenyl)-N, N0 -diphenyl-(1,10 -biphenyl)-4,4-diamine (TPD), a typical hole transporting compound, to prepare thin films. TPD has good solubility and film formation property, and its films can be prepared by both solution process and vacuum deposition methods. Received: April 20, 2011 Revised: June 9, 2011 Published: June 20, 2011 14278

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The morphological and photophysical properties of the TPD films were investigated by atomic force microscopy (AFM) and photoluminescent (PL) spectroscopy. The densities of the films have been measured by solution absorption method and X-ray reflectometry analysis. Hole mobilities of the TPD films were determined by using space-charge-limited current (SCLC) technique. A comparison between the properties of the solution-processed TPD films and the vacuum-deposited TPD films has also been carried out.

2. EXPERIMENTAL METHODS Materials. N,N0 -Di(3-methylphenyl)-N,N0 -diphenyl-(1,1 0 -

biphenyl)-4,4-diamine (TPD) was purchased from Nichem Fine Technology Co. Ltd. The solvents chlorobenzene, dimethylbenzene, and chloroform were purchased from Beijing Chemical Reagent Co. Ltd., China, and were purified by reflux distillation in N2 for 12 h. 1,4,5,8,9,11-Hexaazatriphenylene-hexacarbonitrile (HAT-CN) was purchased from Nichem Fine Technology Co. Ltd. Surface Morphology and Hydrophilic Property. For the surface analysis, atomic force microscopy (AFM) was used. All the AFM images were recorded under ambient air with a multimode atomic force microscope (AFM, Seiko instrument SPA 400) operated in tapping mode. TPD film was prepared on a cleaned glass substrate by both vacuum deposition and spin coating. Vacuum deposition was carried out at a deposition rate of 2
3 Å s
1 at 10
5 Torr. Spin coating was carried out with various solvents containing TPD at different solution concentrations at a spinning rate of 1000 rpm to keep the same thickness, and then the film was baked at 50 °C for 25 min in nitrogen atmosphere to evaporate any residual solvent. After the film formation, we directly test the film surface using AFM. Contact angle measurement was used to evaluate the surface status of the substrate. Water contact angle values were acquired at room temperature under ambient conditions by using an optical contact angle measuring device (OCA 20, Dataphysics Instruments GmbH, Germany) by a sessile drop measuring method, which is a static contact angle assessment. Halogen lighting with continuous adjustable intensity without hysteresis for homogeneous back lighting was used to image the water droplet, whereas a 0.7
4.5-fold magnification charge-coupled device (CCD) camera video system with a resolution of 768  576 pixels was used to monitor and record the data (Figure 1). Ellipse fitting was selected as the default calculation method. The normal test liquid chosen to evaluate hydrophobicity was deionized water and glycol. In each measurement, an approximate 2 μL droplet was dispensed onto the substrates under investigation. Densities of Films. For the films’ density analysis, the spectrophotometric determination and X-ray reflectometry were used. After the AFM analysis, the TPD films were used directly to do spectrophotometric determination and X-ray reflectometry test. The TPD thin films had a width and length of 2.0 cm. The thickness was determined by using an AFM. A standard curve of organic material was plotted between the absorbance at maximum absorption wavelength (λmax) versus its corresponding concentration in organic solvent. To measure the mass of vacuum-deposited organic thin films, the substrate containing the thin films was dipped into the particular organic solvent (10
25 mL), was dissolved by sonication, and was diluted to appropriate concentration (absorbance in the range of 0.2
0.7), and their absorbances were measured by a UV
vis spectrophotometer (Agilent 8453). The unknown mass/concentration of an organic material was

Figure 1. A water drop on a vacuum-deposited film surface. θ is the contact angle.

extracted from its standard curve, and the density was calculated. The X-ray reflectometry (XRR) data were taken on an X-ray diffractometer (Bruker D8 Discover) at a wavelength of 0.154 nm (Cu KR radiation). The beam was focused on the sample using a system of different slits. An additional knife edge collimator was placed above the sample to reduce the background radiation and to limit the beam size. The scans were performed at a typical θ
2θ geometry with a covered spectrum of 0
6° for 2θ. For data evaluation, the recorded reflectivity curves were normalized and were fitted using the Parrat algorithm. From the corresponding fits, the film thickness and the electron density profile perpendicular to the surface can be obtained. Photophysical Characterization. Thin films for photophysical characterization with thickness of approximately 100 nm were deposited onto quartz substrates by spin coating from chloroform, dimethylbenzene, and chlorobenzene solutions. UV
vis absorption and photoluminescence (PL) spectra of TPD films were measured by a UV
vis spectrophotometer (Agilent 8453) and a fluorescence spectrophotometer (Jobin Yvon, FluoroMax-3), respectively. For easy comparison, the absorption spectra and the PL spectra were normalized. OLEDs Fabrication and Characterization. Devices are constructed in a sandwich geometry consisting of a bottom electrode, an organic small molecular thin film, and a top electrode. The thin films using TPD as a hole transport were prepared on a cleaned ITO-coated glass substrate by vacuum-deposition and spin-coating methods. Vacuum deposition was carried out at a deposition rate of 2
3 Å s
1 at 10
5 Torr. Spin coating was carried out with different solutions containing TPD at different solution concentrations at a spinning rate of 1000 rpm, and then the films were baked at 50 °C for 40 min in nitrogen atmosphere to evaporate any residual solvent. The different solvents used were chloroform, dimethylbenzene, and chlorobenzene. Then, a thin film of Alq3 (70 nm) was vacuum deposited onto the holetransport layer at a deposition rate of 2
3 Å s
1 at 10
5 Torr. Finally, LiF (0.5 nm) and Al (120 nm) were evaporated as the cathode. To investigate the hole mobility of TPD films prepared by vacuum-deposition and spin-coating methods, the hole-only devices with a structure of ITO/TPD (120 nm)/1,4,5,8,9, 11-hexaazatriphenylene-hexacarbonitrile (HAT-CN)(5 nm)/Al(120 nm) have been fabricated. The TPD film was prepared on a cleaned ITO-coated glass substrate by both vacuum deposition and spin coating under the same fabricated conditions as the above devices. A thin film of HAT-CN (5 nm) was vacuum deposited onto the TPD layer at a deposition rate of 20 Å min
1 at 10
5 Torr. Al thin film was prepared by thermal evaporation at a deposition rate of 6
10 Å s
1 at 10
5 Torr. The current density-luminancevoltage (J
L
V) characteristics were performed with a Keithley 14279

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Figure 2. AFM images (1 μm  1 μm) of TPD thin films with a thickness of about 100 nm prepared on the ITO substrate: (a) vacuum-deposited TPD film, (b) spin-coated film from chlorobenzene, (c) spin-coated film from dimethylbenzene, and (d) spin-coated film from chloroform solutions.

Table 1. Contact Angles (θ) and Surface Tension of TPD Films film

a

θ (H2O)

θ (glycol)

γd a(mJ/m2)

γp b(mJ/m2)

vacuum-deposited film spin-coated film from chlorobenzene

92.8° 90.4°

68.7° 69.2°

19.28 16.16

3.91 5.92

spin-coated film from dimethylbenzene

91.1°

69.8°

16.22

5.60

spin-coated film from chloroform

89.9°

68.9°

15.98

6.21

Dispersion of surface tension. b Polar component of surface tension.

4200 semiconductor characterization system. The electroluminescence (EL) spectra were collected on a spectrophotometer (Photo Research PR705). All the measurements were carried out in air at room temperature without further encapsulation.

3. RESULTS AND DISCUSSION 3.1. Surface Morphology and Hydrophilic Property of TPD Films. The TPD compound was dissolved into chloroform,

dimethylbenzene, and chlorobenzene solutions at different solution concentrations. After spin coating, the films were baked for 25 min (at 50 °C) on a hot plate under N2 atmosphere. The vacuum-deposited film was prepared at a deposition rate of 2
3 Å s
1 at 10
5 Torr. Figure 2 shows the AFM images of the vacuum-deposited and spin-coated thin films (film thickness ∼ 100 nm) of TPD on the ITO substrate. The root-mean-square (rms) roughness values of the spin-coated films processed from chlorobenzene, dimethylbenzene, and chloroform solutions are 0.46, 0.42, and 0.91 nm, respectively. These values are quite smaller than that of the vacuum-deposited film (1.8 nm) indicating that the spin-coated thin films are more smooth compared to the vacuum-deposited film. To realize the hydrophilicity and surface tension of thin films processed by vacuum deposition and spin coating, contact angle measurements were employed to observe the differences in wetting properties. The contact angles (θ) between ordinary liquids (H2O or glyol) and the TPD films processed from different solvents or from vacuum deposition were measured (Table 1). It can be seen from Table 1 that the contact angles between H2O and the spin-coated TPD films are smaller than that of a vacuum-deposited film, which indicates that these spin-coated films are less hydrophobic (or more hydrophilic). Since water is a highly polar solvent, the increase of hydrophilicity indicates that the surfaces of the spin-coated films are more polar compared to the vacuum-deposited film. In general, the polarity of a surface can be quantitatively characterized by the polar component (γp) of the surface tension, which can be calculated from the θ values according to the method of Wu.17

where γ refers to surface tension (surface free energy), the subscripts l and s refer to liquid and solid, and the superscripts d and p refer to dispersive and polar components, respectively. The results for the dispersion (γd) and the polar (γp) components of the surface tension are also summarized in Table 1. It can be seen from the table that the vacuum-deposited film has a smaller polar term (γp = 3.91 mJ/m2) than the spincoated films. 3.2. Photophysical Properties. Figure 3 shows the absorption and photoluminescent (PL) spectra of vacuum-deposited TPD film and solution-processed TPD films. The absorption spectra of solution-processed TPD films consist of two distinct peaks at ∼315 nm and ∼360 nm. The band at higher wavelength is the dominant one regardless of the type of solvents. The PL spectra of solution-processed TPD films are all strongly redshifted compared to the vacuum-deposited one having a main peak at ∼410 nm. It is generally recognized that for polymers the aggregation of the polymer chains leads to a spectral red-shift in the photoluminescent spectra.18,19 Quantum mechanics calculations also suggest that a π
π stacking of the polymer backbones can red-shift the photoluminescent spectrum.20 Here, we suppose that the spectral red-shift in the PL spectra of solutionprocessed TPD films should be related to the enhanced aggregation of small molecules in solution-processed film compared to the vacuum-deposited film. It is reasonable to think that the enhanced aggregation might be reflected by the increased film density of the solution-processed films. 3.3. Densities of Films. To further study the aggregation state of the TPD molecules in vacuum-deposited and spin-coated films, the films’ density has been carefully measured and discussed. Xiang et al.21 have reported an efficient method to determine the tiny mass of an organic small molecule thin film. They used spectrophotometer and surface profiler to determine the mass and thickness of organic thin films. Thin-film density (F) was calculated according to A ¼ εlcM ¼ εl

p

γl ð1 þ cos θÞ ¼

4γds γdl 4γps γl þ p p γ s þ γl γds þ γdl

F¼ 14280

m εl m ¼ ¼ kcm MV 0 M V0

m m ðA
0:00110Þ  V ¼ ¼ V WLT WLT

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where A is the absorbance, ε is the molecular extinction coefficient, cm is the mass concentration, M is the molecular weight, V0 is the volume of the solution, l is the absorption path length, W is the width of the film, L is the length, and T is the film thickness. In this paper, we calculated the densities of organic thin films according to Xiang et al.’s method and X-ray reflectometry (XRR) analysis.21
24 The X-ray reflectometry (XRR) analysis has been used to investigate the properties of thin polymer films over recent decades.25,26 XRR enables the determination of film density, thickness, and surface roughness. However, the study about the application of the X-ray reflectivity technique in analyzing the density measurement of small molecular thin films is relatively scarce.27 The density data obtained via the two approaches are shown in Table 2. From this table, it is noted that the densities of spin-coated films are a little higher than that of the vacuum-deposited films.

According to Xiang et al.’s method,21 the densities of the spincoated films prepared from chlorobenzene, dimethylbenzene, and chloroform solutions are 1.18 ( 0.01, 1.17 ( 0.02, and 1.15 ( 0.01 g/cm3, respectively. These values are higher than that of the vacuum-deposited film (1.12 ( 0.01 g/cm3). Figure 4 shows the typical X-ray reflectometry spectra of vacuum-deposited TPD film and solution-processed films. A Kiessig oscillation feature is clearly seen. The film roughness is only several Å. According to the XRR experiments, the densities of the spin-coated films prepared from chlorobenzene, dimethylbenzene, and chloroform solutions are 1.23 ( 0.02, 1.20 ( 0.02, and 1.19 ( 0.02 g/cm3, respectively. These values are higher than that of the vacuum-deposited film (1.16 ( 0.01 g/cm3) indicating that the intermolecular stacking in spin-coated films is more compact compared to that in the vacuum-deposited film. Therefore, we have the same result according to two different density measurement methods. The increased density implies enhanced aggregation. It is, thus, supposed that for solution-processed small molecule films, the enhanced aggregation is responsible for the red-shift in the PL spectra of the films. 3.4. Hole Mobilities of TPD Films. The mobility evaluation is very important for studying the transport properties of organic thin films. As we all know, the thickness of small molecular films prepared by spin coating is commonly around 50
200 nm, which is too thin for using time-of-flight (TOF) technique to measure the mobility. For the TOF technique, the thickness of the organic layer is always required to be around 2
8 μm.28 In recent decades, the space-charge-limited current (SCLC) technique has been well established to evaluate carrier mobility in spin-coated polymer films and vacuum-deposited small molecular thin films.29
31 However, reports on the mobility measurement of solution-processed small molecular films by SCLC are not noted. In this paper, we carried out the carrier mobility

Figure 3. The normalized absorption spectra (a) and photoluminescent emission spectra (b) of the vacuum-deposited TPD film and solution-processed TPD films.

Figure 4. Typical X-ray reflectometry spectra of vacuum-deposited TPD film and solution-processed TPD films. The thickness of films is kept the same (100 nm). The symbol line is the theoretical fit line.

Table 2. Densities of TPD Thin Films Measured by Two Methods film

vacuum-deposited film

spin-coated film from chlorobenzene

spin-coated film from dimethylbenzene

spin-coated film from chloroform

density (g/cm )

1.12 ( 0.01

1.18 ( 0.01

1.17 ( 0.02

1.15 ( 0.01

densityb (g/cm3)

1.16 ( 0.01

1.23 ( 0.02

1.20 ( 0.02

1.19 ( 0.02

a

a

3

The solution absorbance method. b The X-ray reflectivity method. 14281

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Figure 6. Space-charge-limited currents for the hole-only devices with vacuum-deposited and solution-processed TPD transport layers. The lines are the best fits to the space-charge-limited current behavior.

Figure 7. Mobility vs square root of the electric field for the hole-only devices with vacuum-deposited and solution-processed TPD transport layers.

Figure 5. Current density vs voltage (J
V) characteristics of the holeonly devices with a structure of ITO/TPD(120 nm)/HAT-CN(5 nm)/ Al(120 nm): (a) vacuum-deposited TPD layer, (b) spin-coated TPD layer from chlorobenzene solution, and (c) spin-coated TPD layer from chloroform solution.

evaluation by using the SCLC technique to study the transport properties of the solution-processed small molecular thin films. To test the TPD mobility, hole-only devices were prepared where a good Ohmic (or quasi-Ohmic) contact between the organic layer and the metal electrode is required. The hole-only devices with a structure of ITO/TPD (120 nm)/1,4,5,8,9, 11-hexaazatriphenylene-hexacarbonitrile (HAT-CN)(5 nm)/ Al(120 nm) have been fabricated. The TPD film was prepared

as a hole-transport layer on a cleaned ITO-coated glass substrate by both vacuum deposition and spin coating. HAT-CN and Al films were vacuum deposited. The HAT-CN film, as a hole injection layer between TPD and Al, has been demonstrated to induce a remarkable enhancement in hole current density making the contact between TPD and Al a good Ohmic contact.32 Figure 5 shows the J
V characteristics of the hole-only devices using solution-processed and vacuum-deposited TPD layers as the hole-transport layers. When the mobility, μ, is independent of the electric field, the current density, J, is described by the Mott
Gurney law:29 J ¼

pffiffiffi 9 E2 εε0 μ0 expðβ EÞ 8 L

where E is the electric field, ε and ε0 are the relative dielectric constant and the permittivity of the free space, respectively, L is the thickness of the organic layer, μ0 is the zero-field mobility, and β is the Poole
Frenkel factor. Figure 6 shows the logarithm of J/E2 versus the square root of the mean electric field. The fitted lines are in good agreement 14282

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Table 3. Hole Mobilities of TPD Films

a

vacuum-deposited

spin-coated film from

spin-coated film from

spin-coated film from

film

chloroform

dimethylbenzene

chlorobenzene

μ0(10
5 cm2/(V s))

1.5 ( 0.2

1.9 ( 0.3

2.3 ( 0.2

2.6 ( 0.3

μ(10
5 cm2/(V s))a

2.5 ( 0.2

2.9 ( 0.3

4.3 ( 0.2

5.4 ( 0.3

The hole mobility for the electric field at 0.5 MV/cm.

Table 4. Performance Data of the Bilayer OLEDs with the TPD Films Prepared via Vacuum Deposition or Solution Process

structures

turn-on

maximum

maximum

voltage

brightness

luminance

(V)

(cd/m2)

efficiency (cd/A)

3.2

7560

2.08

ITO/TPD(spin-coated from

2.8

14 500

3.30

chlorobenzene)/Alq3/LiF/Al ITO/TPD(spin-coated from

2.0

7620

3.67

3.1

10 520

2.52

ITO/TPD(vacuum-deposited)/ Alq3(70 nm)/LiF(0.5 nm)/Al

dimethylbenzene)/Alq3/LiF/Al ITO/TPD(spin-coated from chloroform)/Alq3/LiF/Al

Figure 8. Characteristics of the bilayer devices of ITO/TPD(70 nm)/ Alq3(70 nm)/LiF(0.5 nm)/Al with the TPD film prepared via vacuum deposition or solution process: (a) Current density versus voltage (empty symbols) and the luminance versus voltage (solid symbols); (b) current efficiency versus current density.

with the experimental data, and the current in this region follows a field-dependent SCLC behavior. The slope and the intercept give β and zero-field mobility μ0, respectively. Figure 7 shows the field dependence of the hole mobility of the vacuum-deposited and solution-processed TPD films. At the electric field of 0.5 MV/cm, the estimated hole mobility of the vacuum-deposited TPD film is 2.5 ( 0.2  10
5 cm2 /V s, while in the solution-processed TPD films which were spin-coated from chloroform, dimethylbenzene, and chlorobenzene solutions, the hole mobilities are 2.9 ( 0.3  10
5, 4.3 ( 0.2  10
5, and 5.4 ( 0.3  10
5 cm2/V s, respectively. It can be seen that the mobilities of solution-processed TPD films are all slightly higher than that of the vacuum-deposited film. If we correlate these mobility data with the PL data and the density data, we might suppose that the improved mobilities in solution-processed TPD films are

attributed to the more compact intermolecular stacking in solution-processed TPD films (Table 3). 3.5. Electroluminescent Properties. To investigate the effect of the film formation method of TPD upon the properties of OLEDs, bilayer devices with a structure of ITO/TPD(70 nm)/ Alq3(70 nm)/LiF(0.5 nm)/Al were fabricated. The TPD films were prepared via vacuum deposition or solution process. The 8-hydroxyquinoline aluminum (Alq3) film was prepared by vacuum deposition. As shown in Figure 8, OLEDs with solution-processed TPD films as the hole-transport layer show higher current densities at the same voltage than the controlled device with vacuumdeposited TPD. The performance data of all the OLEDs are summarized in Table 4. The devices with spin-coated TPD exhibited lower turn-on voltages than the control device with vacuum-deposited TPD. The maximum luminance efficiencies of the devices using the spin-coated TPD film are all higher than that of the device using the vacuum-deposited film of TPD. A possible explanation is as follows: the more smooth surfaces of the spin-coated TPD films would benefit the carriers accumulated at the interface between the hole-transport layer of TPD and the emitting layer of vacuum-deposited Alq3, which may lead to improved electron injection. The maximum luminous efficiency was found from the device processed from dimethylbenzene because of its best carrier balance. Further increase in hole mobility would lead to a small reduction in efficiency as can be seen from chlorobenzene-based device. Ishihara et al.16 have reported previously that the OLEDs using TPD films which were spin-coated from tetrahydrofuran (THF) solution as a holetransport layer and Alq3 as an emitting layer exhibited higher current density and luminance than the corresponding devices using vacuum-deposited TPD. Lee et al.15 have also found that the solution-processed OLEDs using 2-(t-butyl)-9,10-bis (20 -naphthyl)anthracene (TBADN) doped with 4,40 -bis[2-{4(N,N-diphenylamino)phenyl}vinyl] achieved high luminous 14283

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The Journal of Physical Chemistry C efficiency because the solution-processed emitting layer possessed better hole-transporting capability. Though the density of the solution-processed TBADN is lower than the evaporated one because of some free volume between aggregates, improved hole transport in the solution-processed films was measured because of conduction pathway through the aggregates. Therefore, TPD is also a suitable molecule for solution processes. It is believed that high-performance OLEDs by solution processes can be expected by careful molecule modification and process optimization.

4. CONCLUSIONS We have investigated the distinctive nature of solutionprocessed small molecular TPD films with a comparison of the vacuum-deposited film. It was found that the solution-processed films are more compact than their vacuum-deposited counterpart showing enhanced film density. The hole mobility of TPD solution-processed films and vacuum-deposited counterpart has been measured by SCLC technique. The bilayer OLEDs fabricated using solution-processed TPD films as the hole-transport layer showed greater injected current density and higher luminance than the device with a vacuum-deposited TPD film. The results provide new insight into the nature of solution-processed small molecular thin films and will be helpful for estimating the potential application of solution-processed OLEDs. ’ AUTHOR INFORMATION Corresponding Author

*Phone: +86 10 62779988; fax: +86 10 62795137; e-mail: [email protected].

’ ACKNOWLEDGMENT This research was supported by the National Key Basic Research and Development Program of China (Grant No. 2009CB623604) and the National Natural Science Foundation of China (Grant No. 50873055 and 50990060).

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