Elsevier

Organic Electronics

Volume 13, Issue 2, February 2012, Pages 222-229
Organic Electronics

Molecular orientation induced by high-speed substrate transfer during vacuum vapor deposition of organic films

https://doi.org/10.1016/j.orgel.2011.10.023Get rights and content

Abstract

The authors investigate a relationship between substrate transfer speeds during vacuum vapor deposition and orientation characteristics of organic molecules. Results show that rod-shaped molecules of alpha-sexithiophene (α-6T) are oriented in a substrate transfer direction and an absorption dichroic ratio of 1.44 is obtained from the oriented α-6T molecule film when a high substrate transfer speed of 4 m s−1 is used. By combining the substrate transfer technique with homoepitaxial growth of α-6T molecules on a rubbed surface, the absorption dichroic ratio further increases to 4.29. Polarized electroluminescence (EL) characteristics are investigated using rod-shaped molecules of 4,4′-bis[4-(di-p-tolylamino)styryl]biphenyl (DPAVBi) as a light-emitting hole-transport layer. An EL dichroic ratio of 2.12 is obtained due to an orientation of DPAVBi molecules caused by combining two techniques.

Highlights

► A long axis of organic molecules is oriented along a substrate transfer direction. ► Molecular orientation is enhanced as substrate transfer speeds are increased. ► Rod shapes of molecules are required to induce molecular orientation. ► Combination of substrate transfer with homoepitaxial growth enhances orientation. ► Polarized electroluminescence is observed from oriented molecule films.

Introduction

Recently, organic light-emitting diodes (OLEDs) have gained tremendous attention for use in full color flat panel displays and solid-state lighting. For OLEDs based on small molecules, multilayer structures are adopted to improve electron-to-photon conversion efficiencies and operational durability and are constructed by successive deposition of organic and metal layers from multiple deposition sources in a vacuum apparatus [1]. Organic layers embedded in OLEDs are generally vacuum-deposited on stationary substrates at deposition rates less than 0.1 nm s−1 unless substrates are moved to make layer thickness uniform. If the multilayer OLED structure can be constructed on a high-speed transferred substrate (for example, 1 m s−1) at an extremely high deposition rate (for example, 1 μm s−1) using a reel-to-reel manufacturing process, the overall cost (tact time) must be reduced to manufacture the OLEDs. The deposition rates and the substrate transfer speeds for the reel-to-reel manufacturing process are considered the key factors affecting performances of OLEDs. Indeed, it has been demonstrated that electron mobilities of tris(8-hydroxyquinoline)aluminum (Alq3) are reduced and power consumption of OLEDs is significantly increased by a small increase in deposition rate of Alq3 from 0.2 to 0.7 nm s−1 [2], [3]. In contrast, there has been still a lack of understanding of a relationship between substrate transfer speeds during vacuum deposition and characteristics of organic films and OLEDs. In this study, we discuss results for a change in these characteristics caused by the high-speed substrate transfer during the vacuum deposition.

The schematic structure of the vacuum evaporator used in this study is shown in Fig. 1(a). Our specially designed vacuum evaporator is equipped with a reel having a diameter of ≈12 cm, which can be rotated in a vacuum. Cleaned substrates are mechanically fixed onto the peripheral part of the reel. Organic films are vacuum-deposited on the substrates being transferred at a certain speed. The substrate transfer speeds can be controlled from 0 to 4 m s−1 by changing the reel rotation speeds from 0 to 666 rpm. In this study, we investigate how the substrate transfer speeds and molecular lengths affect polarized absorption, fluorescence, and electroluminescence (EL) characteristics of organic films using the above-mentioned evaporation system, which is modeled on the reel-to-reel manufacturing process. We demonstrate that rod-shaped molecules such as alpha-sexithiophene (α-6T) and 4,4′-bis[4-(di-p-tolylamino)styryl]biphenyl (DPAVBi) are oriented in the substrate transfer direction to some extent when a high substrate transfer speed of 4 m s−1 is used during the film deposition. Moreover, we find that the combination of the high-speed substrate transfer with homoepitaxial growth of organic molecules on a rubbed surface is a very effective technique used to obtain highly oriented organic molecule films. There have been many techniques to induce molecular orientations, such as mechanical orientations [4], [5], [6], [7], [8], liquid–crystalline self-organization [9], [10], [11], [12], [13], Langmuir–Blodgett deposition [14], [15], [16], and orientations on a specific substrate [17], [18], [19], [20], details of which are reviewed in Ref. [21]. Besides these techniques, the high-speed substrate transfer technique to induce a molecular orientation is believed to open an alternative way for unique anisotropic characteristics of various organic electronic devices, such as polarized EL [6], [11], [13], [15], [16], [17], [19] and enhanced charge-carrier conduction [5], [7], [12], [14], [18] when the reel-to-reel process is used to manufacture the devices.

Section snippets

Experimental

The chemical structures of organic molecules used in this study are shown in Fig. 1(a). Molecules of Alq3, N-N′-diphenyl-N-N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD), α-6T, and DPAVBi with different molecular lengths were used for comparison. High-purity source materials of Alq3 (Nippon Steel Chemical), α-NPD (Nippon Steel Chemical), and DPAVBi (Luminescence Technology) were purchased and used as-received. A source material of α-6T was purchased from Aldrich and purified twice using

Orientation of α-6T molecules by high-speed substrate transfer

The polarized absorption and fluorescence spectra of the 20 nm α-6T film prepared at the substrate transfer speed of 4 m s−1 are shown in Fig. 2(a). The shapes of the polarized spectra parallel and perpendicular to the substrate transfer direction are similar. However, the polarized spectrum parallel is larger in absorbance than that perpendicular, indicating that a long axis of α-6T molecules is aligned to the substrate transfer direction to some extent. The absorption and fluorescence dichroic

Conclusion

We investigate changes in molecular orientation characteristics caused by high-speed substrate transfer during film deposition using a special vacuum evaporator modeled on a reel-to-reel manufacturing process. From results obtained in this study, we find that: (1) a long axis of organic molecules is oriented along a substrate transfer direction, (2) the molecular orientation is enhanced as substrate transfer speeds are increased, (3) rod shapes of molecules are essential to induce the molecular

Acknowledgements

The authors are grateful to Tokyo Research Laboratory, TOSOH Corporation and Frontier Materials Chemistry Group, Sagami Chemical Research Center for providing us with MPT, an electron-transport material. This work is supported by Grants-in-Aid for Scientific Research (Grant Nos. 21760005, 20241034, and 20108012). Part of this work is based on “Development of the next generation large-scale organic EL display basic technology (Green IT Project)” contracted with New Energy and Industrial

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