Essential Differences of Organic Films at the Molecular Level via

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Essential Differences of Organic Films at the Molecular Level via Vacuum Deposition and Solution Processes for Organic LightEmitting Diodes Xing Xing,† Luwei Zhong,‡ Lipei Zhang,† Zhijian Chen,† Bo Qu,† Erqiang Chen,‡ Lixin Xiao,*,† and Qihuang Gong† †

State Key Laboratory for Mesoscopic Physics and Department of Physics and ‡Beijing National Laboratory for Molecular Sciences, Department of Polymer Science and Engineering and Key Laboratory of Polymer Chemistry and Physics of Ministry of Education and College of Chemistry, Peking University, Beijing 100871, People’s Republic of China ABSTRACT: This paper presents an in-depth understanding of the essential differences of organic small-molecule thin films at the molecular level via vacuum deposition and solution processes for organic light-emitting diodes (OLEDs). Synchrotron-based two-dimensional grazing incidence X-ray diffraction has been used to investigate the essential difference. The result reveals that tris(4-carbazoyl-9-ylphenyl)amine (TCTA) molecules show highly oriented arrangements, that is, face-to-face π−π stacking, in vacuum-deposited films, unlike the randomly arranged molecules in spin-coated films. The faceto-face π−π stacking behavior of the molecules in a vacuum-deposited TCTA film leads to higher hole-transport mobility, which is the essential reason for the higher efficiency of a vacuum-deposited OLED compared with that of a solution-processed counterpart, consistent with the calculation results.

1. INTRODUCTION Significant progress has been made in the development of organic light-emitting diodes (OLEDs), which can be fabricated via both vacuum deposition and solution processes. The vacuum deposition method has been used in the commercialized OLEDs due to the advantages of enabling the production of multilayer devices and ease in controlling the thickness of films. The solution processing method is a promising strategy for lowering the fabrication cost of OLEDs because it is easy to fabricate large-area devices. So far, few studies have been performed to compare the two types of OLEDs. Kim et al. chose 2-(t-butyl)-9,10-bis(2′-naphthyl)anthracene as the host for the emitting layer and found that solution-processed films had lower densities than vacuum-deposited films.1 Qiu et al. reported a systematic experimental study of N,N′-di(3methylphenyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4-diamine films fabricated by vacuum deposition and spin coating2 and found that the solution-processed film had more compact intermolecular stacking than its vacuum-deposited counterpart. These studies compared the behaviors and properties of films prepared via solution processes and vacuum deposition; however, no firm conclusions have been reached regarding the essential differences between vacuum-deposited and solution-processed organic films and OLEDs, and the internal mechanism is still unknown due to the lack of knowledge about the internal molecular arrangements in films.3 In this work, to obtain an in-depth understanding of the essential differences of organic small-molecule thin films at the molecular level via vacuum deposition and solution processes for OLEDs, we chose tris(4-carbazoyl-9-ylphenyl)amine (TCTA), a typical © 2013 American Chemical Society

hole-transporting compound, which can be prepared by both spin coating and vacuum deposition. Synchrotron-based twodimensional grazing-incidence X-ray diffraction (2D GI-XRD) was used to determine the molecular arrangements in films. We found that the essential reason for the higher efficiency of the device via vacuum deposition was the face-to-face π−π-stacking behavior of the TCTA molecules, which leads to higher hole mobility. This finding is consistent with the calculation results, which is the specific conclusion regarding the fundamental mechanism of vacuum-deposited organic films and devices compared with those solution-processed analogues.

2. EXPERIMENTAL DETAILS OLEDs were fabricated using the method described in our previous report.4 The ITO/glass substrate was cleaned by ultrasonication in deionized water, acetone, and ethanol, successively, for 20 min prior to device fabrication. All organic materials were then deposited by thermal evaporation under high vacuum (8 × 10−4 Pa) at a rate of 0.1−0.3 nm s−1, except the spin-coated TCTA film. After deposition of the organic layers, an ultrathin layer of LiF (0.8 nm) and an Al layer (100 nm) were evaporated in a vacuum as the cathode; the active areas of the devices were defined using a shadow mask with an opening circle 2 mm in diameter. The thickness of the evaporated layer was monitored using a quartz crystal Received: October 25, 2013 Revised: November 14, 2013 Published: November 18, 2013 25405

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Figure 1. AFM images of a vacuum-deposited TCTA film (a) and a spin-coated TCTA film (b). The image sizes are 5 × 5 μm2.

Figure 2. Current density (a) and current efficiency (b) of OLEDs with vacuum-deposited and solution-processed TCTA films. The inset shows the energy level of the material used in the OLED.

vacuum-deposited one, which is consistent with the previous reports.1,2 To compare the performances of the device via different film formation processes, two devices were fabricated with structures of ITO (indium tin oxide)/TCTA (50 nm)/ aluminum tris(8-hydroxyquinoline)(Alq3) (10 nm)/2-(4-biphenylyl)-5-(4-t-butylphenyl)-1,3,4-oxadiazole (PBD) (40 nm)/ LiF (0.8 nm)/Al (150 nm). TCTA film was used as the holetransport layer by spin coating and by vacuum deposition. Alq3 was used as the emitting layer and PBD as the electrontransport layer; both were vacuum-deposited on the surface of TCTA layers. The current density of the vacuum-deposited OLED was higher than that of the spin-coated one. The current efficiency is much higher for the device via vacuum deposition, as shown in Figure 2. In general, a smoother film should result in higher efficiency for the device.5 However, we found that the device via vacuum deposition with a rougher film showed a higher efficiency than that of the solution-processed counterpart. To investigate the film properties via these two processes, the mobilities of TCTA films fabricated by vacuum deposition and spin coating (50 nm) were measured by using a hole-only device with the structure of ITO/TCTA (50 nm)/MoO3 (2 nm)/Au (100 nm), as shown in Figure 3. The mobility of the TCTA film via vacuum deposition was higher than that of the spin-coated one. This result is in accordance with the performances of the devices, where holes can accumulate at the interface of the emitting layer and the electron-transport layer due to the large gap between the highest occupied molecular orbitals of Alq3 (∼5.7 eV)6 and PBD (∼6.5 eV).7 The electron-transport layer of PBD can be used as a spacer to block hole/exciton diffusion. The hole accumulation results in more injection of electrons from the cathode and accelerates its

microbalance. The device performances were measured using a Keithley 2611 source meter and a spectrophotometer (Photo Research 650). All of the measurements were carried out in an ambient atmosphere at room temperature. Atomic force microscopy (AFM) images were acquired using a Nanoscope IIIA multimode microscope (Digital Instruments). A tapping mode was applied throughout this study using Veeco NanoProbe probes (model number: RTESP14; tip radius: ∼8 nm; force: ∼40 N m−1; frequency: ∼300 kHz). To properly reflect local stiffness variations in the film, the set point amplitude ratio (the ratio of the amplitude set point to the free oscillation amplitude) was ∼0.6 (moderate tapping). The height and phase images were collected simultaneously. Synchrotron-based 2D GI-XRD data were obtained using the BL14B1 beamline at the Shanghai Synchrotron Radiation Facility, with a wavelength of 1.2398 Å. A MarCCD detector was used for data collection. TCTA films with 50 nm thickness were prepared on cleaned ITO−glass substrates by both vacuum deposition and spin coating. Vacuum deposition was carried out under a high vacuum (8 × 10−4 Pa) at a rate of 0.1−0.3 nm s−1. A spincoated TCTA film of the same thickness was prepared from a solution with a concentration of 12 mg mL−1 in chloroform on the ITO−glass at 3000 rpm. After film formation, we tested the two films using AFM and 2D GI-XRD in an ambient atmosphere at room temperature.

3. RESULTS AND DISCUSSION To investigate the properties of the films via vacuum deposition and solution processes, we first compared their morphologies by AFM images, as shown in Figure 1. The roughnesses of the films via vacuum deposition and spin coating were 9.8 and 4.0 nm, respectively. The spin-coated film was smoother than the 25406

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flat-on orientation, as illustrated in Figure 5a. The d spacing for this diffraction is 4.0 Å, which is very close to the d space of

Figure 5. Diagram of the molecular arrangement in a TCTA film via vacuum deposition (a) and spin coating (b).

Figure 3. Current density versus voltage plot of hole-only device with the structure of ITO/TCTA (50 nm)/MoO3 (2 nm)/Au (100 nm), where TCTA transport layers were spin-coated and vacuum-deposited.

face-to-face π−π stacking, that is, 3.8 Å.16 It is reasonable to attribute this diffraction to loosely packed TCTA molecules in films. The preferential orientation in films has been found for many organic molecules.17−19 For the spin-coated film, shown in Figure 4b, the diffraction is displayed as a uniform ring, indicating no preferential orientation, and the molecular orientation is isotropically distributed (as shown in Figure 5b), similar to the amorphous phase. In general, face-to-face π−π stacking leads to higher charge mobility than random arrangement for molecules16,20,21 due to the formation of delocalized states by the overlapping of πorbitals in highly oriented molecules. Therefore, the mobility of the vacuum-deposited TCTA film was higher than that of the solution-processed counterpart, which results in higher efficiency of the device via vacuum deposition. This should be the essential difference of the film formed by different methods. To confirm this, Gaussian 09w was used to calculate the hole-transport properties of TCTA. The rate constant for charge transfer among molecules can be estimated according to Marcus theory22,23

transport because a large amount of holes accumulation can enhance the built-in electric field. Accordingly, more excitons can be generated in the device via vacuum deposition than the case of spin-coating by improving the quantities of holes and electron in the recombination zone. Therefore, the device via vacuum deposition has higher efficiencies than the device via solution processes.8,9 To find the essential reason why mobility is different via different film processes, the molecular arrangement was investigated via synchrotron-based 2D GI-XRD. Molecules are usually arranged randomly in films via both vacuum deposition and solution processes, resulting from its weak intermolecular interactions.10 However, a somewhat orientation for the molecular arrangement in OLEDs has recently been reported by Yokoyama et al.11−13 They used variable-angle spectroscopic ellipsometry to explore the molecular orientation by detecting the optical properties of amorphous organic films in different directions. However, the accurate molecular arrangement in OLEDs still remains unknown. To solve this problem, synchrotron-based 2D GI-XRD was used, which can provide information about the molecular structure and fine arrangement in thin/ultrathin films.14,15 The diffraction peaks are sharper and stronger for the vacuum-deposited film than that of the spin-coated film, as shown in Figure 4, implying more orientation and higher crystallinity in the former case. As indicated by the arrow in Figure 4a, dispersed diffraction was observed in the qz (out-of-plane) direction, which means that the planar TCTA structure (simulated by Gaussian 09w) has a

K = V2

⎛ π λ ⎞ exp⎜ − ⎟ ℏ kBTλ ⎝ 4kBT ⎠ 2

where V is the transfer integral, λ the reorganization energy, T the temperature (295 K, room temperature), kB Boltzmann’s constant, and ℏ the reduced Planck’s constant. The hole reorganization energy, λ, of TCTA was calculated to be 0.171 eV using Gaussian 09w with the B3LYP/6-31 +G**

Figure 4. Synchrotron-based 2D GI-XRD patterns of the TCTA film via vacuum deposition (a) and spin coating (b). 25407

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basis set. Using the ADF software package and SAOP/TZP theory, the hole-transfer integral, V, of TCTA for randomly arranged molecules is found to be Vrandom = 3.82 meV, and for strictly face-to-face π−π stacked molecules, Vordered = 7.44 meV. We can therefore obtain the rate constant of hole transfer among TCTA molecules for different arrangements, Krandom = 2.54 × 109 s−1 and Kordered = 9.62 × 109 s−1, that is, Kordered is larger than Krandom. Therefore, when TCTA molecules are arranged with π−π stacking, they have higher hole-transport mobility than those of randomly distributed TCTA molecules. These results agree very well with the experimental results on hole-transport mobility and the molecular orientations observed by synchrotron-based 2D GI-XRD.

4. CONCLUSION In summary, we achieved a comprehensive studying on organic films formed via vacuum deposition and solution processes. The spin-coated film was smoother than the vacuum-deposited one; however, the mobility of the film via the solution process was lower than that of the film via vacuum deposition. Synchrotron-based 2D GI-XRD was used to investigate the essential difference of the internal molecular arrangement. The vacuum deposition method leads to molecular face-to-face π−π stacking in vacuum-deposited TCTA films and results in higher hole-transport mobility and higher current efficiency for device, which is consistent with the calculation results, and should be the essential differences of organic films at the molecular level via vacuum deposition and solution processes.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (L.X.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSFC (Grants 61177020, 10934001, and 11121091), the National Basic Research Program of China (2009CB930504 and 2013CB328704), and the Beijing Municipal Science and Technology Project (Z101103050410002). The authors are grateful for use of beamlines BL14B1 (Shanghai Synchrotron Radiation Facility) and 1W1A (Beijing Synchrotron Radiation Facility).

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REFERENCES

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